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FEATURE ARTICLE Utilization of cyclodextrins in industrial products and processes Jo�zsef Szejtli Cyclolab L td., H-1525 Budapest, PO Box 435, Hungary During the last ten years the production of cyclodextrins has increased from several hundred to several thousand tons, and besides a-, b- and c-cyclodextrins, methylated and hydroxypropylated cyclodextrins are also produced on an industrial scale.For other purposes (chromatography, catalysis, reagents, diagnostics, specific drug formulations, etc.), about a hundred different cyclodextrin derivatives and complexes are commercially available. A dozen cyclodextrin-containing drugs have already been approved and marketed. Around 130–140 new cyclodextrin-related papers, patents (applications) and conference abstracts are published each month.The largest number of publications deals with actual or mainly potential pharmaceutical applications of cyclodextrins, but the largest amount of them is used in foods, cosmetics and toiletry products. A similar intense development is expected for the next decade. Supramolecular chemistry produces an astonishing variety of acceptable for most potential users, in most countries one or more CDs are approved in one specific product, or new and very spectacular ‘host’ molecules, which form inclusion-type associations with appropriate ‘guest’ species generally, for any purposes, and the number of products and processes which consumes the produced CDs is increasing (ions, radicals, molecules) called inclusion complexes, adducts, cryptands, etc.To most of these host–guest associations some continually. All this is supported by the remarkable increase of the CD potential practical utilization(s) is (are) attributed.1 literature. Some of these new synthetic hosts are highly specific, i.e. their molecular (or ionic) recognition capacity prefers a given ion or molecule. This type of host will deliver highly specific CD literature and sensitive sensors, or entrapping agents, sequestrators, for specific ions.This means that these hosts will be used only in While 25 years ago ca. 8–10 papers and patents were published rather restricted fields and amounts. Produced for a very on CDs per month, 5 years ago this number had increased to limited market, generally by complicated synthetic procedures, 20–25, and presently (1996) about 130 new papers, patents, from expensive starting substances, the majority of these hosts conference abstracts are dedicated to cyclodextrins (i.e.on will remain expensive speciality chemicals, further burdened average, 4 new publications per day). The number of CD with toxicological and environmental pollution problems. publications currently available (in July 1996) is ca. 13000. Of all inclusion complex forming hosts known, the cyclodex- The classification of CD papers (abstracted by CD-News2 trins (CDs), being of natural origin, organic, biocompatible in January–November 1994) according to their profile is substances, seem to have unique status: the availability of the illustrated in Fig. 1. Classification of the 197 lectures and raw material (starch) is not only unlimited but also cheap; the posters presented at the 8th International Cyclodextrin technology of their production is a relatively simple enzymic Symposium in April 1996 (Budapest) resulted in a practically conversion; the production is free from any environmental identical distribution pattern.3 pollution problems: practically, there is no unusable byproduct, Around 17% of all CD papers are dedicated to the fundaand no polluting substance escapes the apparatus; they are mentals of cyclodextrin chemistry and technology, where the non-toxic, biologically degradable substances (the main pri- really original important results are represented by papers on mary degradation product is glucose); their utility, of course the synthesis of new, chemically modified CDs and on the within well defined limits,seems to be inexhaustible: it is difficult biological effect of CDs (toxicology).The many papers on new to find any group of chemical products (drugs, cosmetics, food, sources of CTG-ase enzyme, its enzyme kinetics and methods plastics, paper, textiles, pesticides, photographic materials, etc.) of production of cyclodextrins are mainly reproductions of or processes (formulation, catalysis, separation, stabilization, earlier works.etc.) with no convincing examples for the use of CDs. The next group (ca. 20% of all CD papers) consists of all No wonder that, while at beginning of the 1970s CDs were fundamental inclusion phenomena studies which are not considered as rare, expensive, toxic, but very challenging directly practice-oriented: energetics and kinetics of inclusion, curiosities, ten years later, around 1980, several companies X-ray, NMR, EPR, circular dichroism and Raman specbegan simultaneously to offer industrially produced CDs and troscopy, thermal analysis, interaction of CDs with specific adequate toxicological studies documented the harmlessness guest types, enzyme modelling with CDs, preparation and of CDs when used as recommended.However, rapid develop- analysis of cyclodextrin complexes, etc. ment of the CD market had to face: lack of approval from the The largest group of CD papers is dedicated to the pharmaauthorities (for use in food, drugs, cosmetics, etc.); the potential ceutical application of CDs. The majority of drug molecules consumers did not know for what purposes and how they are poorly soluble in water, and consequently their biological should use CDs (to be a pioneer is risky, and costs a lot of absorption is slow and frequently far from complete, many money); the marketed amount was small, and consequently drug molecules are rather sensitive to oxidation, thermal the prices were high. decomposition, light, ions, etc.Many drug molecules are ideal Now, around the mid-1990s, CDs are produced in thousands complex-forming partners for cyclodextrins. This is a very productive field, and considering the lengthy development and of tons, their price is rapidly reaching the level which is J. Mater. Chem., 1997, 7(4), 575–587 575Fig. 1 Distribution of 1072 CD-related publications, abstracted by CD-News for January to November 1994 the strict requirements for approval of a new chemical entity The number of CD publications in the field of food and cosmetics (fourth group) is rather modest (ca. 6%) but in fact (a cyclodextrin complex of a well known drug molecule is always considered to be a new chemical entity) it must be more than 70% of all produced cyclodextrins are used in food and cosmetics (Fig. 2). If a drug substance is complexed with considered as a significant achievement that about a dozen drugs have already been approved and marketed in cyclodex- cyclodextrins and marketed in several countries it may need 20–40 tons of cyclodextrin per year (except for the successful trin-complexed form. This number, in the next few years, is expected to display an explosion-like increase.Nevertheless, prostaglandin E1–a-CD complex, the yearly production of which needs only several kilograms of a-CD because one vial the large number (>4000) of drug–CD-related papers and patents is a little misleading, because many authors publish contains only 20 mg PGE1 and 646 mg a-CD). However, only a single toiletry product, e.g.fragrance tissue, which needs no the same results in different journals under different titles, but with virtually identical content, re-discoveries are published authority approval, needs hundreds of tons of b-cyclodextrin every year! frequently, simply because the authors did not read the earlier literature and have discovered something that was published The application of cyclodextrins in pesticide formulation (fifth group) represents a very modest fragment (ca. 1%) of the ten or fifteen years earlier, but even if only about 30% of the published papers disclose really new and significant results, it CD literature, because the pesticide industry needs very cheap auxiliarymaterials.The price of cyclodextrins is not low enough is almost hopeless for a single scientist to read all the relant literature in this field.for the pesticide industry, but when one considers that practi- Fig. 2 Estimated segments of the CD market (1996) 576 J. Mater. Chem., 1997, 7(4), 575–587cally the same effects can be attained by formulating a pesticide molecule with CDs, as is the case for drug molecules, when the price of technical quality cyclodextrins drops below $3 per kg, the pesticide industry will use up many thousand of tons.At present, about 10% of the CD literature is dedicated to the application of cyclodextrins in chemical and biotechnological industries (sixth group); however, this number is expected to grow rapidly. This article attempts to survey mainly this section of the CD literature.The last group involves the application of cyclodextrins in analytical chemistry, and diagnostic preparations. The analytical application of CDs means mainly the application of cyclodextrins in chromatography. While ten years ago most papers in this field were on the subject of the application of CDs in gas chromatography, and five years ago in HPLC, nowadays the application of CDs in capillary zone electrophoresis dominates.Cyclodextrins display unprecedented potential for use in chiral separation on the chromatographic scale. For these purposes only small amounts of CDs or, more frequently, of specific chemically modified cyclodextrins, e.g. methylated, carboxymethylated or ionic derivatives (sulfated, carboxylated, aminated, etc.) are needed, and it is difficult to find a separation problem on the analytical scale which could not be solved by using CDs.In view of the overwhelmingly large number of CD publications, it is no wonder that about 400 reviews have been Fig. 3 Structure of b-cyclodextrin and the approximate cavity volumes published on CDs; the vast majority of them summarize only of a-, b- and c-cyclodextrins a specific section of the very broad field, and only a few dozen of them cite more than 100 references.Only a few monographs4 –6 attempted to give a well documented survey of all of these cylinders are ‘lined’ with H atoms and glycosidic O bridges, therefore they are relatively hydrophobic, while the the CD literature or, without claiming completeness, to summarize the essence of some or of a single specific subject outer surface, particularly the secondary side, is hydrophilic.The CDs (‘hosts’) are soluble in water, and when hydro- area.7–10 The development of all CD chemistry and technology is phobic molecules (‘guests’) which are compatible with the CDcavity dimensions (their geometrical dimensions, shape and reflected most spectacularly by the proceedings (or minutes) of the international CD symposia.The first such symposium was charge make a more or less tight fit possible) are added to an aqueous CD solution, a so-called inclusion complex is formed. held in Budapest in 1981, the second in 1984 in Tokyo, and since then they have been held every second year. The volumes Many well studied factors play roles in the formation and stabilization of these association-type host–guest complexes.containing the papers submitted for publication are 400–650 pages long, and as well as comprising the actual results and trends of CD research, they illustrate how the main research CD derivatives focus is shifting from fundamental research towards the colourful variety of industrial applications.11–17 For several reasons (price, availability, approval status, cavity dimensions, etc.) b-CD is the most widely used and represents Since 1985 a monthly abstracting service, CD-News2, has been publishing the abstracts of CD-related papers and patents, at least 95% of all produced and consumed CDs.It is used for many purposes; however, its anomalous low aqueous as well as any available information on CDs e.g.new CDcontaining products, conferences, approvals, actual trends. solubility (and the low solubility of most of its complexes) is a serious barrier to its wider utilization. Fortunately, by chemical or enzymatic modifications the solubilities of all CDs can be The Cyclodextrins improved markedly, and instead of the 18 g dm-3 aqueous b-CD solutions 500+g dm-3 aqueous b-CD derivative solu- Upon the addition of cyclodextrin glycosyl transferase enzyme (CGT-ase) to an aqueous solution of starch, every sixth or tions can be prepared easily.In cyclodextrins every glucopyranose unit has three free seventh of the eight a-1,4-glycosylic linkages is split, but the majority of the maltodextrinyl radicals formed, instead of hydroxy groups which differ in both function and reactivity.The relative reactivities of C(2) and C(3) secondary, and the reacting with a water molecule (hydrolysis), react with their own non-reducing end, resulting in a six-, seven- or eight- C(6) primary hydroxy groups depend on the reaction conditions (pH, temperature, reagents). In b-CD, 21 hydroxy membered macro-ring. These cyclic maltodextrins are called a-, b- and c-cyclodextrins (Schardinger dextrins, cyclomalto- groups can be modified by substituting the hydrogen atom or the hydroxy group with a wide variety of groups, e.g.alkyl, oligoses, cycloamyloses, etc.). Their IUPAC name is about five printed lines long, and consequently is never used. hydroxyalkyl, carboxyalkyl, amino, thio, tosyl, glucosyl, maltosyl, and thousands of ethers, esters, anhydro-, deoxy-, acidic, As well as these three ‘parent’ CDs, some minor CDs also exist: nine-, ten-, eleven- or twelve-membered rings are also basic, etc., derivatives can be prepared by chemical or enzymatic reactions.The aim of such derivatization may be: to formed in very small amounts. From the a-1,6-branched fragments of amylopectine ‘branched’ CDs are formed; moreover, improve the solubility of the CD derivative (and its complexes); to improve the fitting and/or the association between the CD recently the chemical synthesis of a five-membered (pre-a-) CD has been published. and its guest, with concomitant stabilization of the guest, reducing its reactivity and mobility; to attach specific (catalytic) In reality these ‘rings’ are empty ‘cylinders’ (Fig. 3). On one edge all the primary C(5)MCH2OH groups of the constituent groups to the binding site (e.g. in enzyme modelling); or to form insoluble, immobilized CD-containing structures and glycopyranose units are arranged, with all secondary C(2)M and C(3)MOH groups on the other edge. The internal cavity polymers, e.g. for chromatographic purposes. J. Mater. Chem., 1997, 7(4), 575–587 577From the thousands of CD derivatives described in hundreds in a chronic treatment with large doses, leaves the cholesterol in the kidneys depleted.of scientific papers and patents, only a few can be used for medicinal purposes. The first selecting factor is the availability A particular methylated b-CD, heptakis(2,6-di-O-methyl)-b- CD, (DIMEB) is a crystalline product.It is extremely soluble of such derivatives. Complicated multistep reactions, using expensive, toxic, environment-polluting reagents and purifi- in cold water but insoluble in hot water, therefore its purifi- cation and the isolation of its complexes are technically very cation of the products by chromatography are feasible for the preparation of derivatives only on the laboratory scale.To simple. For more than a decade it has been the subject of many studies, particularly as a solubilizing agent for poorly produce tons of CDs at an acceptable price, only about a dozen of the known CD derivatives can be considered. soluble drugs. Up to now no better solubilizer has been found among the CDs, but, apart from highest haemolysing capacity, The industrially produced, standardized and available (even in ton amounts) b-CD derivatives are the heterogeneous, its production cannot be realized at an acceptable price level.Large amounts of highly toxic wastes are the byproducts of amorphous, highly water soluble methylated b-CD and 2- hydroxypropylated b-CDs (Fig. 4). Owing to their heterogen- its synthesis, therefore nowadays the somewhat weaker solubilizer, but much cheaper, randomly methylated b-CD (RAMEB) eity, these products cannot be crystallized, which is important, e.g., for the production of liquid drug formulations.Much more is produced and marketed. DIMEB will remain, at least until an economic synthesis route is discovered, a relatively expen- important however, is, that these derivatives cannotform crystalline cholesterol complexes.b-CD has a particularly high affinity sive fine chemical, used in chromatography, diagnostics, etc., i.e. where only small amounts are needed. In almost all cases for cholesterol: the parenterally administered b-CD is not metabolized in the organism, but forms insoluble cholesterol RAMEB can be used instead of DIMEB. Reacting b-CD with starch in the presence of pullulanase complex crystals in the kidneys, resulting in nephrotoxicity.The first hydroxypropyl-b-CD containing drug formulations enzyme, one or two maltosyl or glycosyl groups will be attached to the primary side of the CD ring with a-1,6- are already approved and marketed in several countries. A methylated b-CD is more hydrophobic than b-CD itself, glycosidic linkages.The product is the so-called ‘branched’ CD (mono- or di-maltosyl, or mono- or di-glucosyl CD) which therefore it forms a more stable (but soluble) complex with cholesterol. Its affinity to cholesterol is so strong that it is is very soluble in water, being a heterogeneous, non-crystallizable substance. It is produced, and used in the food industry, capable of extracting cholesterol from blood cell membranes, resulting in haemolysis at around 1 mg cm-3 concentration.mainly in Japan, for example for the production of stable flavour powders. Hydroxypropyl-b-CD (HPBCD) is more hydrophilic than b-CD, therefore it forms a less stable complex with cholesterol. The interest in peracyl CDs is increasing. All acetylated CDs from peracetyl to peroctanoyl esters have been studied, partly Nevertheless, upon parenteral administration it collects cholesterol in the circulating blood and transfers it to the kidneys.as retard drug carriers, partly as bioadhesives, film-forming substances to be used in transdermal drug delivery systems. The HPBCD will be excreted, mainly in unchanged form, but, Very intensive work has been carried out on the development of heptakis(sulfobutyl)-b-CD, which is very soluble in water, non-crystallizable, and even at extremely high doses seems to be free from any toxic side-effects.At present it can be used as a chiral separating agent in capillary zone electrophoresis, but the aim of the intensive research is to develop it as a parenteral drug carrier for the preparation of aqueous injectable solutions of poorly soluble drugs.CD sulfates possess many similar properties as heparin without its anti-coagulating effect. The present study is focused on the anti-angiogenetic effect of the derivatives because they can apparently reduce the blood supply of tumour tissues through inhibiting the formation of new arteries. Monochloro-triazinyl b-CD is produced on an industrial scale from CDs and cyanuric chloride.It is reactive with cellulose fibres in alkaline media (see later). To elongate the actual CD cavity, substituents are attached to the primary or secondary side. This elongation may be hydrophilic, in which case hydroxyalkyl groups are attached to the ring, or hydrophobic, for example, by substituting the primary hydroxy groups with long fatty acid chains, ‘medusa’- like molecules can be prepared.These molecules behave as detergents, while retaining their complex-forming ability. The coming years will decide how these derivatives might be utilized. The chair conformation of the CD ring can be modified by inverting the positions of some hydroxy groups. For example, by removing the tosyl group in alkaline medium from a CDtosyl derivative, 2,3-anhydro derivatives can be prepared.Upon opening the anhydro ring one hydroxy group will take up an inverted position, and in this way cycloaltrins are formed. By eliminating an appropriate leaving group from the primary side, 3,6-anhydro-CDs are formed. Because of the twisted O OH O O CH2OMe O MeO O O CH2OMe O O O O CH 2OMe MeO OH MeO CH2OMe OH OH OH O CH2OMe MeO HO O O O CH2OMe CH2OMe MeO MeO HO HO O OH O O CH2OH O OH O O CH2OH O O O O CH2OCH2CHCH3 HO OH OH CH2OH OH O OH O CH2OH HO HO O O O CH2OCH2CHCH3 CH2OH HO O HO HO OH CH2CHCH3 OH OH CH2CHCH3 OCH2CHCH3 OH conformation of the anhydro glucopyranose unit the properties Fig. 4 Structure of crystalline heptakis(2,6-di-O-methyl)-b-cyclodex- of CDs (e.g. solubility) are increased strongly. trin (DIMEB) and of randomly hydroxypropylated b-cyclodextrin.It is possible to close one side of the CD cavity, for example The industrially produced non-crystallizable randomly methylated by over-bridging the primary or secondary side with an b-CD (RAMEB) also contains about 14 methoxy groups, but in a random distribution. appropriate bifunctional substituent. It is expected that these 578 J.Mater. Chem., 1997, 7(4), 575–587over-bridged CDs will form more stable complexes with certain guests. The essence of photodynamic tumour therapy is that such compounds must be delivered to the tumour tissues, which upon exposure to strong light become toxic through isomerization, splitting, etc. In this case, upon strong light irradiation the photosensitive molecules will become toxic just to the tumour cells.For such targeting of the drug very stable (105–107 dm3 mol-1) complexes are needed. The duplex homo- Fig. 6 Schematic representation of inclusion complex formation. Small or hetero-dimers of CDs (constructed only from one or two circles represent water molecules, which are repulsed both by the hydrophobic (potential guest) p-xylene, and the hydrophobic cavity of different CDs) form complexes which are more stable (by the truncated CD cylinder.The driving force for inclusion is mainly several orders of magnitude) than the singular CDs. By inter- the substitution of the polar–apolar interactions [e.g. between the connecting two CDs with appropriate bridges such duplex CD apolar CD cavity and polar water, or the apolar potential guest derivatives have been prepared (Fig. 5) which can form stable (p-xylene) and water] for apolar–apolar interactions (between the complexes with photosensitive porphyrinoid structures and to guest and the CD cavity). transport them to the target organs. Recently, ‘antennae’-bearing CDs have been reported. Such oligosaccharide units are attached to the CDs, which are oured (polar–apolar interaction) and therefore can be substi- receptor-specific, i.e.they will be bonded in the living organism tuted readily by appropriate ‘guest molecules’ which are less only on certain specific receptors. The aim of this work is to polar than water (Fig. 6). The dissolved cyclodextrin is the synthesize a receptor-targetting carrier, i.e.the drug complexed ‘host’ molecule, and the ‘driving force’ for complex formation with an antenna-bearing duplex CD would transport the is the substitution of the high-enthalpy water molecules by an specific drug just to the target organ. appropriate guest molecule. One, two or three cyclodextrin The most complicated CD derivatives are synthesized for molecules contain one or more entrapped guest molecules enzyme-modelling experiments. By dimerizing amino acid CD (most frequently the host5guest ratio is 151); this is the essence derivatives, hydrolase enzyme models have been prepared, of ‘molecular encapsulation’.which approximate the activity of natural enzymes. The formed inclusion complexes can be isolated as stable A dozen different CD derivatives are used in gas chro- crystalline substances.Upon dissolving these complexes an matography, liquid chromatography and capillary zone equilibrium is established between dissociated and associated electrophoresis. species, and this is expressed by the complex stability con- For other industrial purposes where toxicological demands stant, Ka. do not play a decisive role, epichlorohydrin cross-linked, The association of the CD and drug (D) molecules and the hydroxyethylated, or sometimes apparently rather fancy, but dissociation of the formed CD–drug complex is governed by by their uses justified, mixed ether–esters like heptakis(2,6-di- a thermodynamic equilibrium: O-methyl)-3-O-trifluoracetyl-b-CD and similar derivatives, are CD+D=CD·D produced and utilized. It is very probable that for drug-carrier purposes four or K1:1= [CD·D] [CD][D] five different CDs will be developed and produced in the future, because no one of them alone is able to fulfil all the very strict requirements which are usual in the case of a This is the simplest, and most common, case; however, 251, 152, 252, or even more complicated associations, and higher- parenteral drug carrier.order equilibria exist, almost always simultaneously. Recently, surprisingly high solubilization effects have been CD complex types reported upon the formation of so-called multicomponent complexes, which consist of a CD, a basic type guest and an In an aqueous solution the slightly apolar cyclodextrin cavity is occupied by water molecules which are energetically unfav- appropriate hydroxy acid.The hydroxy acid certainly forms a salt with the basic guest molecule, and moreover forms hydrogen bonds with the hydroxy groups of the CDs. Up to 20 000- fold solubility enhancement effects could be attained (Table 1). For example, the dose of the poorly water soluble Terfenadine (an anti-allergic drug) is 60 mg, which can be dissolved only in 6 dm3 water.In the form of a multicomponent CD complex, the 60 mg drug can be dissolved in <1 cm3 water, so it can be injected or applied as a nasal spray. Cyclodextrins belong to the most appropriate rotaxaneforming molecules. A long slim guest molecule can be threaded through the CD cavity, then both ends can be terminated by bulky groups or the terminal groups can be ionised and therefore the threaded molecule can not slip out from the cavity (Fig. 7). Various environmental effects (pH, irradiation, electric field, etc.) may cause this threaded molecule to rotate around its axis, otherwise its mobility is restricted. Similarly the CD ring’s mobility is also restricted, it can only move along the axis. By threading a long slim guest through a number of CD rings, a ‘molecular necklace’ can be prepared (Fig. 8). Recently a nylon–CD complex has been reported by reacting the b-CD complex of hexamethylenediamine with CD-complexed diacyl chloride. In this way new materials with quite interesting Fig. 5 ‘Duplex’ CD structures with appropriate guest molecules can form inclusion complexes of up to Kass#106–109 dm3 mol-1 properties can be produced. J.Mater. Chem., 1997, 7(4), 575–587 579Table 1 Amount of hydroxypropyl-b-cyclodextrin necessary to dissolve a single dose of Astemizol, Domperidon and Terfenadine in water ternary system amount drug–HP-b-CD–hydroxy acid HP-b-CD/g of 10% single HP-b-CD ternary in binary in ternary drug dose/mg solution/ml component molar ratio water/ml system system Astemizol 10 29 malic acid 1:1:2 0.1 2.9 0.03 Domperidon 10 105 tartaric acid 1:1:1 0.3 10.5 0.03 citric acid 1:1:3 0.4 10.5 0.03 Terfenadine 60 49 citric acid 1:2:2 0.8 4.9 0.35 tartaric acid 1:2:2 1.2 4.9 0.35 lactic acid 1:2:2 6.0 4.9 0.35 groups will split off and the long polymer chain will slip out from the polymeric tube (Fig. 9). Metal ions can be complexed with CDs in three ways: (i) the metal ion reacts with the hydroxy groups of the CD molecule; (ii) the metal ion forms a coordination complex with usual organic ligands and this coordination complex will be included in the CD cavity; (iii) the metal atom is bound covalently in an organometallic compound which will form a regular inclusion complex with a CD molecule.For case (i): a hydroxy complex is not an inclusion complex, it is more likely to be an outer-sphere complex, e.g.Cu2+ or Mn2+ ions in alkaline solutions form such CD-hydroxy–metal complexes. Case (ii) means the formation of ternary complexes: CD+organic ligand+metal ion. This is a real second-sphere coordination metal complex, e.g. a ferrocene is a coordination complex which consists of an iron ion sandwiched between two cyclopentadienyl molecules, forming a stable iron coordination complex.This coordination complex can form a true inclusion complex with CDs and this inclusion strongly modi- Fig. 7 Rotaxane: one a,v-diaminoalkane is threaded through a CD fies the physical and chemical properties of the included ring, then both terminal amino groups are converted to bulky groups (e.g. reacting with cobalt chloride–ethylenediamine).The ‘axis’ molecule coordination complex. cannot slip out from the CD ring, but can rotate freely within it. Case (iii), the complexation of organometallic compounds (this is a binary complex) also results in the modification of importantproperties of the includedcompound, e.g. in pharmaceutical preparations. Ferrocene complexes can be prepared easily, in crystalline form, in good yields.The excess of sublimable ferrocene can be removed easily by vacuum sublimation, while the CDbound ferrocene is stable up to the temperature of decomposition of the CDs (Fig. 10). Fig. 8 ‘Molecular necklace’ (polyrotaxane): a long slim poly(ethylene oxide) chain can be threaded through a series of CD rings; by terminating both ends of the chain with bulky substituents (e.g.reacting with 1-fluoro-2,4-dinitrobenzene) the structure is stabilized, with no covalent link ‘Molecular tubes’ can be prepared by complexing polyethylene glycol–bisamine with a-CD, then the formed polyrotaxane is reacted with 2,4-dinitrofluorobenzene. This way both ends of the long-chain guest are terminated by bulky groups. Upon reacting this polyrotaxane with epichlorohydrin the vicinal Fig. 9 ‘Molecular tube’: the CD rings in the molecular necklace can CD rings will be interconnected through glyceryl bridges be interconnected, e.g. by epichlorohydrine in alkaline solution. After between the primary and secondary sides of the CDs. Finally, removing the terminating bulky groups (by strong alkali) the axis molecule will slip out from the tube. upon exposure to strong alkali, the dinitrofluorobenzene 580 J.Mater. Chem., 1997, 7(4), 575–587escing molecule is transferred from the aqueous environment into an apolar surrounding, etc. (iii) The reactivity of the included molecule is modified. In most cases the reactivity decreases, i.e. the guest is stabilized, but in many cases the CD behaves as an artificial enzyme, accelerating various reactions and modifying the reaction pathway.(iv) The diffusion and volatility (in the case of volatile substances) of the included guest decrease strongly. (v) The formerly hydrophobic guest, upon complexation, becomes hydrophilic, therefore its chromatographic mobility is also modified. Fig. 10 A ferrocene molecule can be accommodated only sandwich- In the solid state, the important consequences are as follows.like between two a-CDs, horizontally and partially penetrates into (i) The complexed substance is molecularly dispersed in a one b-CD, while in c-CD it is fully incorporated, and its equatorial carbohydrate matrix forming a microcrystalline or amorphous axis coincides with the symmetry axis of the c-CD. powder, even with gaseous guest molecules. (ii) The complexed substance is effectively protected against any type of reaction, The orientation of an ionizable ferrocene within the CD except those with the CD hydroxy groups, or reactions cata- cavity depends on the pH of the solution (Fig. 11). On the lysed by them. (iii) Sublimation and volatility are reduced to basis of circular dichroism spectra the ferrocenecarboxylic acid a very low level.(iv) The complex is hydrophilic, easily wettable was assumed to orient itself inside the CD cavity parallel to and rapidly soluble. its axis, while the ionized carboxylate ions were perpendicular When in an aqueous system the formation of the CD to it (at pH 9 in water). inclusion complex can be detected, e.g. by NMR or circular In aqueous solution the antitumour carboplatin forms an dichroism, or through a catalytic effect, it does not mean that 151 complex with a-CD, but not with b- or c-CD (Fig. 12). a well defined crystalline inclusion complex can be isolated. The cyclobutane ring penetrates the CD cavity, with additional The two main components of the driving force of the inclusion stability arising from hydrogen bonds between the ammine process are the repulsive forces between the included water ligands and the hydroxy groups.Dimethyl-a-CD forms a molecules and the apolar CD cavity on the one hand, and similar complex with carboplatin. In contrast, the platinum between the bulk water and the apolar guest on the other phosphine complex trans-[Pt(PMe3)Cl2(NH3)] forms a 151 hand. This second factor does not exist in the crystalline (dry) complex with b-CD but not with a-CD, and the hydrophobic state. Therefore it is not uncommon for complex formation to trimethylphosphine ligand resides in the CD cavity.be convincingly proven in solution, but nevertheless the isolated product is nothing other than a very fine dispersion of Primary effects of inclusion on guest properties the CD and the guest.In the following section the versatile industrial utilization of The most important primary consequences of the interaction cyclodextrins will be illustrated. Considering the vast volume between a poorly soluble guest and a CD in aqueous solution of pertinent literature, even ahighly incomplete list of references are as follows. (i) The concentration of the guest in the would have to involve hundreds of them.The readers are dissolved phase increases significantly, while the concentration referred to the most detailed monographs5,6 and the most of the dissolved CD decreases. This latter is not always true, recent CD symposium volumes.3,14–17 however: ionized guests or hydrogen-bonding (e.g. phenolic) compounds may enhance the solubility of the CD.(ii) The spectral properties of the guest are modified. The chemical Cyclodextrins in drugs shifts of the anisotropically shielded atoms are modified in the The complexation of a drug molecule with a CD should be NMR spectra, when achiral guests are inserted into the chiral taken into consideration in the following cases:18 the drug is CD cavity they become optically active, and show strong poorly soluble, therefore its bioavailability (upon oral, dermal, induced Cotton effects on the circular dichroism spectra; pulmonar, mucosal, etc.) application is incomplete or irregular; sometimes the maxima of the UV spectra are shifted by because of the low dissolution rate, even in the case of complete several nm, fluorescence is greatly improved because the fluor- absorption the time for the orally administered drug to reach the effective blood level is too long, so that reduction of the lag time of the pharmacological effects is required; because of the low solubility no aqueous injectable solution (or other liquid formulation) can be prepared; the drug is chemically unstable: because of its autodecomposition, polymerization or degradation by atmospheric oxygen, absorbed humidity, light, etc., no marketable formulation with a satisfactory shelf-life can be produced; the drug is physically unstable: volatilization Fig. 11 Orientation of ferrocenecarboxylic acid in the CD cavity or sublimation result in losses, by migration the originally depends on the electrical charge of the guest homogeneous product becomes heterogeneous, by its hygroscopicity it liquifies in open air; the acceptability of the drug is bad, because of a bad smell, bitter or irritating taste; the drug is a liquid, but its preferred pharmaceutical form would be a stable tablet, powder, aqueous spray, etc.; the dose of the lipid(-like), barely homogenizable drug is extremely low, therefore content uniformity of the product is problematic; the drug is incompatible with the other components of the formulation; relief of serious side effects (throat, eye, skin or stomach irritation) is required; because of the extremely high biological Fig. 12 Structure of the antitumour carboplatin–a-CD complex and activity (in the case of drugs of extremely low doses), working of the trimethylphosphineplatinum–b-CD complex. In the former the with such powder is rather dangerous.cyclobutane is included in the a-CD cavity (forms no complex with The advantageous results of CD complexation of (CD- b-CD); in the latter the trimethylphosphine ligand is included in the b-CD-cavity (no complex is formed with a-CD). complexable) drugs are as follows: improved bioavailability J. Mater. Chem., 1997, 7(4), 575–587 581from solid or semi-solid formulations (Fig. 13); enhanced stab- formation of a medicinally useful CD complex of a drug molecule are as follows: more than five atoms (C, P, S, N) ility, increased shelf-life; reduced side-effects; uniform, easy to should form the skeleton of the drug molecule; the solubility handle powders, even from liquids; aqueous, injectable soluof the drug molecule in water should be less than 10 mg cm-3; tions from poorly soluble drugs can be prepared.the drug melting point temperature should be below 250 °C Speaking only of the numerous advantages of drug–CD (otherwise the cohesive forces between the molecules are too complexation can be very misleading, because there are just strong); the molecule should consist of less than five condensed as many limiting factors which restrict the applicability of CDs rings; its molecular mass should be between 100 and 400 (with to certain types of drugs, because not all drugs are suitable for smaller molecules the drug content of the complex is too low, CD complexation.Many compounds cannot be complexed, or large molecules do not fit the CD cavity). complexation results in no essential advantages.Inorganic Strongly hydrophilic, very small or very large molecules, e.g. compounds are generally not suitable for CD complexation. peptides, proteins, enzymes, sugars, polysaccharides, generally Exceptions are non-dissociated acids (HCl, HI, H3PO4, etc.) cannot be complexed. Nevertheless, when large, water-soluble halogens, gases (CO2, C2H4, Kr, Xe, etc.). Inorganic salts, e.g.molecules contain appropriate complex-forming side-chains, KCl, Fe salts, cannot be complexed. e.g. an aromatic amino acid in a polypeptide, they will react General preconditions (not without exceptions!) for the with CDs in aqueous solutions, resulting in modified solubility and stability (e.g. the stability of an aqueous solution of insulin, and of many other peptides, proteins, hormones and enzymes, is improved significantly in the presence of an appropriate CD).An insurmountable limiting factor in selecting the drug for complexation is the dose of the complex that has to be administered. A fundamental requirement is that the mass of a tablet should not exceed 500 mg. Since the drugs to be complexed have molecular masses between 100 and 400, and CDs have rather large molecular masses (972, 1132 and 1297 for a-, b- and c-CD, respectively), 100 mg of complex contains only about 5–25 mg active ingredient.If a single dose of a drug is not more than 25 mg then even a complex with an active substance content of 5% can carry the necessary dose in a single tablet of mass 500 mg, otherwise the possibility of a powder sachet or sparkling-tablet formulation has to be Fig. 13 Enhancement of bioavailability of Ketoconazole. When this taken into consideration. Thus, in the case of complex-forming drug is administered orally to rats in multicomponent complex form drugs, the relationship of the required dose and the molecular the bioavailability is improved significantly. In the absence of gastric acid (the achlorohydric state was provoked by Omeprazole treatment) mass determines the feasibility of oral administration in CD- no Ketoconazole can be detected in the blood.When the classic binary complexed form. complex is administered, only modest absorption is attainable, but Similarly, the volume of an injection should be less than administration of the multicomponent complex causes the blood level 5 cm3, or even better not more than 2–3 cm3, i.e.to dissolve to reach the required level. (AIDS patients require this drug to treat the necessary amount of the drug in 2–3 cm3 of 40% HPBCD their mycotic infections, but usually they have low gastric acidity, solution, 800–1200 mg HPBCD can be used. In liquid formu- therefore the drug cannot be absorbed from the usual formulations.) *: gastric pH 6.5–8; **: gastric pH 1.7.lations the use of CD derivatives in large excess is possible, Fig. 14 Structures of various CD complexes. Toluene fits well into a b-CD cavity, for diphenylamine two b-CDs form a ‘capsule’. A long-chain fatty acid can be accommodated by three or more a-CDs. In case of prostaglandin E1 the a-CD accommodates only the aliphatic chain of the unsaturated cyclic hydroxy fatty acid, but this is enough to convert it into a water-soluble complex.b-CD can accommodate the cyclopentane moiety, but c-CD is too wide for this guest. 582 J. Mater. Chem., 1997, 7(4), 575–587Table 2 Some approved and marketed CD-containing pharmaceutical products drug trade name formulation indication company/country Prostavasin intraarterial chronic arterial occlusive PGE1–a-CD Ono, J.Schwarz, D. 20 mg/amp. disease, etc. PGE1–a-CD Prostandin 500 infusion controlled hypotension during Ono, J. 500 mg/amp. surgery PGE1–b-CD Prostarmon E sublingual tablet induction of labour Ono, J. OP-1206–c-CD Opalmon tablet Buerger’s disease Ono, J. piroxicam–b-CD Brexin, Cicladol tablet, sachet and suppository Analgesic, anti-inflammatory Chiesi, I, Masterpharma, I.D., B., NL., stb. garlic oil–b-CD Xund, Tegra, drage�es anti-atherosclerotic Bipharm, Hermes, Allidex, D., Garlessence Pharmafontana, H., D., USA benexate–b-CD Ulgut, Lonmiel capsules anti-ulcerant Teikoku, J., Shionogi, J. iodine–b-CD Mena-Gargle gargling throat disinfectant Kyushin, J. Dexamethasone, Glymesason ointment analgesic, anti-inflammatory Fujinaga, J.Glyteer–b-CD nitroglycerin–b-CD Nitropen sublingual tablet coronary dilator Nippon Kayaku, J. Cefotiam-hexetil–a-CD Pansporin T tablet antibiotic Takeda, J. new oral cephalosporin Meiact tablet antibiotic Meiji Seika, J. (ME 1207)–b-CD thyaprofenic acid–b-CD Surgamyl tablet analgesic Roussel-Maestrelli, I. chlordiazepoxide–b-CD Transillium tablet tranquilizer Gador, Ar.hydrocortisone–HPbCD Dexacort liquid mouthwash against aphta, Icelandic Pharm., Isl. gingivitis, etc. itraconazol–HPbCD Sporanox liquid AIDS, oesophagal candidiosis Janssen, B. e.g. in the case of a Prostavasin injection the molar ratio of pane derivatives) form stable complexes with CDs, and in dry complexed form remain stable for long periods, without any prostaglandin E1 to a-CD is 1:11 (20 mg PGE1+646 mg a-CD per dose) (Fig. 14). further protection, at room temperature. Such powder flavours are approved, produced and used in several countries, e.g. A 3000 I.U. vitamin D3 tablet contains only 0.075 mg cholecalciferol, a Prostarmon-E tablet contains only 0.5 mg PGE2, France, Japan, Hungary; for example, a lemon-peel oil–b-CD complex mixed with powdered sugar is used in pastries, spice- the active ingredient content of a nitroglycerin tablet is 0.5–4 mg; these and similar drugs are ideal for CD com- flavour mixtures complexed with CDs are used in the preparation of canned meat, sausages, etc.In Germany the garlic plexation, but even the 20 mg piroxicam-containing Brexin tablet is a widely marketed successful product (Table 2).oil–b-CD complex is marketed as odourless drage� es (to substitute the garlic, and a number of various unstable garlic prep- If the Ka stability constant of a complex is low (<102 mol-1) the existence of the complex can be evidenced in solution, but arations, consumed to reduce the blood cholesterol level). In USA the FDA have not yet generally approved the consumption upon removing the water the obtained product is often only a intimate mixture (e.g.a coprecipitate) which contains the host of any CDs in drugs or foods, but the garlic oil–b-CD complex is already approved, and is available on the US market. and guest in a fine dispersion. Removing the water also results in the elimination of an important component of the driving Similarly, the number of cosmetic products which contain CDs is on the increase.Suntan lotions, long-lasting perfumes force for complexation: the repulsive forces between water and the hydrophobic drug. Upon contact with water complex and dermocosmetic products of leading cosmetic companies often contain CDs, to eliminate the unpleasant odour of some formation is an instantaneous process, i.e.in solution the guest is really includedin theCD cavity, and dissociation–association vital component, or just to decelerate the perfume release rate, leading to long-lasting effects, etc. equilibrium is reached within seconds. In such cases the guest is not protected against external In the USA, at present the largest amount of b-CD is used in dryer-added perfumed fabric softener sheets.The non-woven destructive factors, like oxygen or humidity, but if the guest is stable enough, only its low solubility causes problems; such tissue impregnated with a mixture of a waxy fabric softener and a perfume–b-CD complex is added to the laundered intimate mixtures can be utilized for preparations e.g. of solid formulations of improved bioavailability. If, however, the guest textiles after washing but prior to drying, providing a longlasting fabric freshness and an agreeable scent. is unstable then only full complexation (also prevailing in the anhydrous state) can help.In Belgium low-cholesterol butter is produced. The molten butter is mixed with b-CD, which does not react with triglycer- In cases of extremely high complex stability constants (>ca. 104 dm3 mol-1) the bioavailability can even be reduced. ides but forms complexes with cholesterol, and the b-CD complex is easily removable from the butter. More than 90% The complex is practically not absorbed; only the released, molecularly dispersed (dissolved) drug molecules are absorbed. of the cholesterol can be removed in one step. The butter does not retain any CD. Other low-cholesterol milk products, like In such cases the co-administration of an even better complexforming competitor molecule (e.g.phenylalanine) can help. cheese and even low-cholesterol egg, are produced by this technology. Hundreds of published examples illustrate the stabilizing, solubility- and bioavailability-enhancing, side-effect-reducing and advantageous technological effects of CD complexation of instable, poorly soluble, locally irritating drugs.CDs in textiles, fibres and papers Binding CD to fibres chemically or by adsorption opens new CDs in foods and cosmetics ways for the preparation of perfumed textiles, with slow release of the perfume. Even the opposite, i.e. binding distasteful smells Flavour substances are generally volatile substances which deteriorate readily.Most of them (e.g. terpenoids, phenylpro- (e.g. of sweat), can be performed. b-CD bound by dimethylol- J. Mater. Chem., 1997, 7(4), 575–587 583carbamide to viscose or polyamide rayon can absorb butyric auxiliary substances. While the chemical oxygen demands (in mg g-1) for the widely used tensides NP10, Uniperol O acid from aqueous solution.Monochlorotriazinyl-b-CD (MCT) is the first reactive cyclo- and Gisapon 1555 are 2020, 1930 and 2290, respectively, for b-CD this value is only 1060. dextrin derivative manufactured on an industrial scale. The monochlorotriazinyl group is used widely in reactive textile The b-CD complex of o-methoxycinnamaldehyde has been incorporated into shoe insoles to inhibit microbial growth and dyes as a reactive anchor.This derivative is able to form stable covalent bonds with nucleophilic groups and can be prepared foul odours. Cotton fabric was immersed into the o-methoxycinnamaldehyde and a b-CD-containing ethanol–water mix- easily in water in an effective one-pot synthesis from cyanuric chloride and b-CD in a yield of ca. 90% based on the triazinyl ture to attain a loading of 10 g active ingredient per m2.This fabric was placed between two chlorovinylidene sheets. group. The optimized degree of substitution, DS=per anhydroglucose unit, assures a good complexing capacity even Symptoms such as athlete’s foot, rashes, blisters and dry skin were effectively controlled. when the derivative is fixed to surfaces like textiles. This cyclodextrin derivative containing 2–3 reactive groups in the An adsorbent composed from carboxymethylcellulose, CD and hexamethylolmelamine adsorbed non-ionic surfactants but ring can be used as a building block for new CD derivatives, as a cross-linking agent or as an excellent material for surface did not adsorb anionic dyes. By treating dyeing waste waters with this adsorbent, the waste water could be recycled for modification.The immobilized (wash-fast) cyclodextrin can be loaded with perfumes, or insect repellents, which are released dyeing with a further addition of dyes. The anti-foaming capacity of b-CD can be utilised in laun- only slowly by the effect of body-heat and released humidity (perspiration), and simultaneously can bind the distasteful- dries and also in the flotation of ores, e.g.limonite. Fragrant paper or paper containing protective substances smelling components of perspiration (deodorizing effect) (Table 3). can be prepared using CD complexes of perfumes, insecticides, rust inhibitors, mould- and mildew-proofing agents, fungicides Wash-fast insect-resistant fibres were prepared by treating fibres with a composition containing an organic insect-proofing and bactericides.These complexes have to be mixed with the pulp and water before drying. The retention time of these agent, a cyclodextrin or a low molecular mass cyclodextrin polymer, and a siloxane. For example, treating acrylic fibres active ingredients is extended greatly. For example, a fenitrothion b-CD complex sprayed on a wet paper web, passed with a solution containing 0.34% b-CD and 0.14% isobornylthiocyanoacetate (based on the fibre mass), produced a woven between drying rollers heated to 100°C, and wound to give insecticide containing paper, has been shown to be effective fabric which even after 20 washings showed an insecticide property.for more than six months. By fixing CD or CD polymer fragrance complexes to the melt mixture of synthetic fibre polymers (e.g.polyester) and CDs in adhesives and coatings weaving fabric from such fibres, wash-fast fragrant fabrics can be produced. Epoxy resin adhesives are produced and stored as separated components mixed just before utilisation. By complexing the b-CD modifies the mechanism of interaction between cotton fibres and direct dyes used for trichromatic dyeing of cotton.curing agent (polymerization catalyst) with b-CD a onepackage composition can be prepared. Layering such a com- 4-Aminoazobenzene is incorporated into the cavity of b-CD with its monosubstituted phenyl group. This is why CD acts position between metal plates and heating to 130 °C for 5 min will cause binding to take place. as a retarder in dyeing processes in the ‘finishing’ bath, diminishing the rate of dyeing.This retarding effect increases The properties of cyanacrylate adhesives can be improved significantly by heptakis(2,6-di-O-butyl-3-O-acetyl)-b-CD. The the affinity of the dyestuff for the textile, but decreases the rate of diffusion into the fabric. ethyl-2-cyanacrylate monomer is stabilized with 20 ppm phosphoric acid, 20 ppm SO2 and 100 ppm hydrochinon.Various For colouration of polyester fibres, so-called dispersionsdyes are used, which are very poorly soluble in water amounts of dibutylacetyl-b-CD are added to this mixture, and using as an adhesive, e.g. to bind hard cartoon papers by quick (0.1–10 mg dm-3). Without using solubility-enhancing agents (tensides), uniform dyeing is not possible.CDs, however, can heating to 200 °C, the polymerisation (binding) of cyanacrylate has been accelerated, and the tear strength of the binding substitute the tensides, e.g. a 0.3 g dm-3 conversion mixture (which contains all three cyclic and non-cyclic dextrins) has increased significantly (Table 4). Emulsion-type coatings (paints) contain emulsion polymer been shown to be approximately equivalent to 1 g dm-3 Levegal HTN (a non-ionic tenside). Both these solubility- binders, to give after drying a resistant, continuous protecting film on the coated surface.To ensure the formation of a good enhancing agents resulted in acceptable dyeing homogeneity, while without them the colouration was very heterogeneous, film, the applied layer must contain various compatible components, like solvent, pigment, thickener and binder.The rheolog- both with Resolin Orange RL and Resolin Rot FB. By treating textile materials with CD-containing finishers, ical properties of the paint are determined by the thickener, which is usually a hydrophobically modified polymer, like the physically fixed CDs allow easy removal of sweat or sweat degradation products from the textile by prevention of their polyurethane, polyacrylamides, cellulose ethers, etc.To avoid a concomitant too high viscosity (which makes the formation penetration to the fibre interior. CDs represent a new class of auxiliary substances for the of a uniform surface coating difficult), viscosity suppressors must be added to the emulsion. textile industry.It is very important that their chemical oxygen demand (in waste water) is lower than that of the usual textile Table 4 Effect of heptakis(2,6-di-O-butyl-3-O-acetyl)-b-CD on the binding time and tear strength at sticking hard cartoon paper Table 3 Possible applications of MCT-finished textiles19 (150 g m2) with ethyl-2-cyanacrylate adhesive20 application examples dibutylacetyl-b-CD tear strength after added (% v/w) binding time/s 30 s/N cm-1 laundry perfuming fragrance release odour absorption sweat absorption controlled release antimicrobial (hospital) 0 >180 5 0.05 80 7 textiles insect-repellent textiles 0.10 30 10 0.20 20 15 stabilisation active ingredients 584 J.Mater. Chem., 1997, 7(4), 575–587The viscosity can be reduced by adding organic solvents to ant membranes can separate enantiomers of racemic mixtures, e.g.DL-tryptophan. Coating a thermoplastic resin sheet with a such emulsions, but the use of organic solvents must be avoided because of safety, health-damaging and environmental pollut- thin layer of CD or a CD derivative, and vacuum forming for 25 s at 150 °C gave container lids with good mosaic patterns.ing effects. Surfactants can strongly reduce the viscosity of such emulsions, but their use results in the formation of a less UV-curable ink-printed cards (e.g. telephone cards) can be produced with a perfume–b-CD complex mixed into the resistant coating. The viscosity-enhancing effect of hydrophobically modified printing inks. A calendar page printed with fruit design was coated with an orange fragrance containing b-CD complex macromolecules in aqueous emulsions is based on the hydrophobic –hydrophobic interactions between these molecules.and it emitted the fragrance for about three months. Rubber compositions with improved resistance to ozone, Upon adding CDs to this emulsion the CD molecules will associate with the hydrophobic sites and, being strongly ageing and discolouration contain CD complexes of various antioxidants.hydrated, inhibit the association of the macromolecules, resulting in a strong reduction of the viscosity. The cord strength of polyester fibres used for reinforcement of rubbers can be improved by CDs. Their resistance to heat As can be seen from Table 5, RAMEB was the most effective viscosity suppressor.and degradation is better after treatment with a CD solution. For example, the tear strengths of c-CD-treated cord before and after vulcanization were 15.6 and 14.7 kg, respectively, vs. CDs in plastics and rubber 15.6 and 12.4 kg. respectively, without c-CD treatment. Upon complexing the vulcanizing agents with CD, no vul- CD complexes of NCO-containing compounds can be utilised as cross-linking agents, e.g.for foamed polyurethane sheet canization occurs during working or kneading the rubber but only after vulcanizing agent is released by heating at the production. The cross-linking agent–b-CD complex is stable at ambient temperature, it only becomes reactive on heating. moulding temperature. CD complexes are compatible with thermoplastic resins. Mixing a dry pulverised CD complex of a perfume, for example a geraniol–a-CD complex, with a thermoplastic resin (poly- CDs in photographic and recording materials ethylene) and moulding it yielded plastic products with long Important properties such as the relative sensitivity and fog of lasting (at least six months) fragrance.Rapid loss of the perfume silver halide-containing photographic materials can be by volatility and thermal decomposition can be avoided in improved by adding CDs to light-sensitive photographic gela- this way.tin layers. Upon mixing CD complexes of thymol, eugeneol, isobu- Additives, dyes, stabilizers and fog inhibitors used in the tylquinoline, etc., with molten PVC a natural leather odour photographic industry should be fixed to a certain layer of the emitting (leather-like) material was prepared, e.g.for auto- film or photopaper. This can be achieved by using derivatives mobile door internal coverings. with ‘heavy’ side chains. It seems to be more convenient to By complexing dyes with CDs, which are used for colouring prepare the water-soluble polymer complexes of these sub- plastics, a more homogeneous colouration can be obtained. stances.In complexed form their mobility is reduced markedly, CDs can be used, for different purposes, in plastic laminates, and they become fixed to the layer required. Diminished films and membranes. Biodegradable plastics have been pre- diffusion can be observed on preparing, processing or storing pared by blending not more than 10% b-CD complex with the film.Another advantage of the soluble polymer complex the plastic substance. The role of the CD is to protect the is that poorly soluble, or even water-insoluble, stabilizers can plastic from the action of deteriorating agents during the useful be applied to the film in aqueous solutions. A CD–gelatin life of the article. However, when the plastic article is discarded composition as the photographic layer shows lower water and microbial action begins, the b-CD is degraded and the absorption and accelerated diffusion of the developing agents.deteriorating agent is released. This agent is chosen from the A thermal recording sheet which contains a colourless group of substances well known to cause embrittlement, crack- benzylleucomethylene blue–b-CD complex and an acidic devel- ing or other physical degradation of the plastic, e.g.surfactants. oper [Ni(NO3)2] exhibits high sensitivity and good light The entire piece of plastic erodes, leaving behind small frag- resistance. Photochromic materials can be produced by ments of the former article. complexing a spiropyran, a dithizone metal complex, a tri- The Fe2SO4–c-CD combination has been shown to be phenylmethane dye or a fulgide with c-CD.Complexation effective for the inhibition of the permeation of oxygen through increases the stability of the light-produced colour and polypropylene laminates. Membranes for ultrafiltration have decreases the colour density of the unirradiated materials. been prepared from mixtures of aromatic poly(ether sulfones) Complexing sodium-1,4-dihydroxyanthraquinone-2-sulfonate and 1–34% CD or CD derivatives.The filtering effectiveness with a-CD, dissolving it in water and poly(vinyl alcohol) then and permeability of the CD-containing membranes were higher coating onto a support gave a laser-recording medium. The than those of equivalent conventional products. Homogeneous included guest is photoisomerisable.By dissolving CD in a transparent amorphous membranes with good physicomechan- photosensitive solution of a diazonium salt, more stable photo- ical properties and more than 15 vol% pores of equal dimen- sensitive diazo-type copying materials can be produced. sions were prepared by dispersion of dimethyl-b-CD or triacetyl-b-CD in cellulose acetate. CD-containing water-resist- CDs in catalysis Table 5 17.5 g ACRYSOL RM-8 (a hydrophobically modified polyure- Heating an aqueous solution of RhCl3 and a- or b-CD at thane thickener) was emulsified in 77.6 g water then 4.9 g of each CD reflux, followed by further refluxing in the presence of EtOH, was added to the emulsion.21 The viscosity was determined by a gives a black colloidal dispersion of Rh particles of diameter Brookfield viscometer 28 A° .The colloidal dispersion is an effective catalyst in the hydrogenation of alkenes at 30°C under atmospheric pressure. CD viscosity/mPa s Platinum dispersions can be stabilised similarly by CD. HP-c-CD >100000 Aggregation is probably retarded by the CDs. As the catalyst HP-a-CD 19200 activity, e.g. in systems used for solar-energy conversion, is HP-b-CD 5200 related to the size of Pt particles, this colloid stabilising effect random methylated-b-CD (RAMEB) 802 of CDs may eventually be utilised. J.Mater. Chem., 1997, 7(4), 575–587 585The benzoin-isopropyl ether–b-CD complex is an effective catalyst for photopolymerization of vinyl polymers. The following examples have been selected to illustrate the potentials of CDs in detoxification of dangerous substances by catalysing their decomposition. Trichlorfon is a crystalline, contact stomach poison insecticide.At its production after isolating the crystalline Trichlorfon, the remaining mother liquor contains a considerable amount of very toxic non-crystallizable Trichlorfon. In alkaline solution its decomposition proceeds via the elimination of one molecule of HCl, whereby the unsaturated reaction product rearranges to the stable Dichlorovos (DDVP), which is a very toxic volatile liquid.b-CD accelerates this process and, if enough b-CD is present in the system, the crystalline DDVP–b-CD complex is immediately formed, and can be isolated as a poorly Fig. 15 Principle of intensification of the enzymatic (microbial) trans- soluble, stable microcrystalline product.After CD treatment formationof poorly soluble lipophilic substrates. TheCD complexation the disposition of the much less toxic waste is easier and the of the substrates improves their wettability and solubility, i.e. enhances DDVP–b-CD complex itself is a useful insecticide. A similar their concentration in the aqueous phase where the reaction takes detoxification of chlorobiphenyls, by eliminating a molecule of place.In many cases the reaction is accelerated through continuous hydrochloric acid through direct photolysis in aqueous malto- removal of the inhibiting products by CD complexation. syl-CD, has also been reported. The very dangerous acetylcholin esterase-inhibitor neurotoxic agents, soman and sarin, can be deactivated by b-CD. Until recently, the Leprae bacillus (Mycobacterium leprae) was considered to be uncultivatable under in vitro conditions.The isopropyl methyl phosphonofluoridate is hydrolysed by alkali, but by using b-CD even at pH 7.4 and 25°C a The most important energy source for the bacillus is palmitic (or stearic) acid which, however, cannot penetrate the thick, considerable detoxification is achieved. This process appears to be as fast in human plasma, in vitro, as in tris buffer.This strongly hydrophilic shell of the mycobacterium. By solubilizing the fatty-acids (or fatty alcohols), however, with dimethyl- may eventually be used to improve emergency medicinal therapy, and can certainly be used for detoxification of the b-cyclodextrin, the mycobacterium can be cultivated in vitro, on a synthetic medium.This discovery will facilitate the environment when soman is spilled accidentally. screening of drugs against similarly difficult microorganisms. CDs in biotechnology CDs in environmental protection The application of cyclodextrins in biotechnology began only in the 1980s, but rapid development is expected in this field.Biological waste water treatment means that dissolved organic and frequently toxic substances are oxidised, hydrolysed, The majority of biotechnology processes mean an enzymecatalysed transformation of a substrate in an aqueous medium. degraded by a large number and variety of yeast and bacteria which are present in the biological sludge. The waste waters The main difficulties which used to arise are as follows: the substrate is hydrophobic, sparingly (or hardly at all) soluble of the food industries are generally liable to biological degradation, but those of the organic chemical industry, containing in water; the enzyme or the enzyme-producing microbial cells are sensitive to the toxic effects of the substrate or to inhibitors e.g. pesticides, drugs, their intermediaries, which are really ‘hard’ environment polluting agents, are usually more or less which can even be the product of the transformation; the substrate or the product is unstable under the conditions of resistant to biological degradation, and are often devastating for the detoxifying microorganisms.These chemicals can be the enzymatic transformation; isolation of the product from the very heterogeneous system is difficult. tolerated and metabolized by the microbial flora of the activated sludge system only up to a certain level.When the toxic Cyclodextrins and their derivatives enhance the solubility of complexed substrates in aqueous media, and reduce their concentration level is exceeded the microbial flora are proportionally paralysed, and the biological activity of the sludge toxicity, but they do not damage the microbial cells or the enzymes.As a result, the enzymatic conversion of lipophilic decreases more or less irreversibly. To avoid this unwanted effect, an alternative is the partial and temporary masking of substrates can be intensified (accelerated, or performed at higher substrate concentrations) both in industrial processes the toxic substances by converting them to non-toxic CD inclusion complexes.Swolleninsoluble CDpolymers (e.g. cross- and in diagnostic reagents, the yield of product-inhibited fermentation can be improved, organic toxic compounds are linked with epichlorohydrin) can be used to remove polychlorinated biphenyls or detergents like lithium dodecylsulfate20 and tolerated and metabolized by microbial cells at higher concentrations, and compounds in small amounts can be isolated naphthalene-2-carboxylate from water.Tributylphosphate can be removed from water by converting it into an insoluble simply and economically from complicated mixtures (Fig. 15). Some examples illustrating the rapidly growing and promis- b-CD complex.The method is recommended for treatment of waste water from nuclear fuel reprocessing plants. Cross-linked ing uses of cyclodextrins in various operations are: the intensi- fication of the conversion of hydrocortisone to prednisolone, b-CD polymers containing polymer membranes can be used to remove volatile halogenated organic compounds from tap the improvement in the yield of fermentation of lankacidine and podophyllotoxin, the stereoselective reduction of water.It is estimated that industry emits about 2% of the 200 benzaldehyde to L-phenylacetyl carbinol, and the reduction in toxicity of vanillin to yeast, or organic toxic substances to million tons of solvents produced annually by the organic chemical industry. Recovery of a small fraction of such sub- detoxificating microorganisms.In the presence of an appropriate cyclodextrin derivative (e.g. 2,6-dimethyl-b-cyclodextrin), stances is performed by solid or liquid absorbers.An alternative method to those used normally, i.e. cooled condensers or lipid-like inhibitor substances are complexed. The propagation of Bordatella pertussis and the production of pertussis toxin absorbers filled with actived carbon or silica gel, is the application of CD solutions.CDs can also react with appropriate therefore increases up to 100-fold. Cyclodextrins and their fatty acid complexes can substitute for mammalian serum in tissue guest molecules in the gas or vapour phase. Upon bubbling a large volume of air containing solvent cultures. 586 J. Mater. Chem., 1997, 7(4), 575–5872 Cyclodextrin News, Cyclolab, Budapest. through a CD solution, below the temperature at which the 3 Proc.Eighth Int. Cyclodextrin Symp., Budapest, 1996, ed. J. Szejtli complex crystallizes, an immediate precipitation of the crystal- and L. Szente, Kluwer, Dordrecht, 1996. line complex is observed. The molar ratio of guest molecule: 4 J. Szejtli, Cyclodextrins and T heir Inclusion Complexes, Akade�miai CD is generally 0.4–151. With increasing temperature the Kiado�, Budapest, 1982.solubility of CDs increases, but the complex stability decreases 5 J. Szejtli, Cyclodextrin T echnology, Kluwer, Dordrecht, 1988. strongly. Using epichlorohydrin-modified highly soluble b-CD, 6 J. Szejtli and T. Osa, Comprehensive Supramolecular Chemistry, 80–95% of 1,2-dichloroethane could be removed from vol. 3: Cyclodextrins, Pergamon, Oxford, 1996. 7 Cyclodextrins and T heir Industrial Uses, ed. D. Duchene, Editions 35–80 mg solvent per dm3 air in pilot experiments. Recovery de Sante�, Paris, 1987, p. 448. of chlorinated organic compounds like CHCl3, CCl4, C2HCl3, 8 New T rends in Cyclodextrins and Derivatives, ed. D. Duchene, C2Cl4 and of hydrocarbons has been reported using various Editions de Sante�, Paris, 1991. CDs. b-CD solution can be used to remove bromine and 9 Inclusion Compounds, ed. J. L. Atwood, J. E. D. Davies and iodine, not only from air, but even from gaseous chlorine, D. D. MacNicol, Academic Press, London, 1984, vol. 1–3. during electrolysis of NaCl. 10 K. H. Fro� mming and J. Szejtli,Cyclodextrins in Pharmacy, Kluwer, Odorous gases (e.g. at treatment of industrial wastes, fecal Dordrecht, 1993. sewage, or slaughterhouse effluents) can be deodorized by CD 11 Proc. 1st Int. Symp. on Cyclodextrins, Budapest, 1981, ed. J. Szejtli, Reidel, Dordrecht, 1982. solutions, by bubbling the gases through a CD solution, or by 12 J. L. Atwood, J. E. D. Davies and T. Osa, in Proc. T hird Int. Symp. atomizing the CD solution with compressed air to form a mist on Clathrate Compounds and Molecular Inclusion and the Second curtain and passing the waste gases through this mist curtain. Int. Symp. on Cyclodextrins, T okyo, 1981, Reidel, Dordrecht, 1985. Ozone can be removed from waste gases by using the CD 13 Proc. Fourth Int. Symp. on Inclusion Phenomena and the T hird Int. complexes of terpenoids (e.g. limonene) in the corona discharge Sympt. on Cyclodextrins, L ancaster, 1986, ed. J. L. Atwood and part of the electrical appliance. E. D. Davies, Reidel, Dordrecht, 1987, p. 455. It is not reasonable to wash toxic organic substances out of 14 Proc. Fourth Int. Symp. on Cyclodextrins, Munich, 1988, ed. soil using organic solvents (costs, additional pollution, danger O. Huber and J. Szejtli, Kluwer, Dordrecht, 1988. 15 Minutes of the Fifth Int. Symp. on Cyclodextrins, Paris, 1990, ed. of explosion, etc.); only aqueous systems can be taken into D. Duchene, Editions de Sante�, Paris, 1990. account. Detergents, however, also have strong effects on the 16 Minutes of the Sixth Int. Symp. on Cyclodextrins, Chicago, 1992, ed. environment. CDs could probably be used successfully to wash A. R. Hedges, Editions de Sante�, Paris, 1992. poorly soluble toxic substances out from the upper layers of 17 Proc. Seventh Int. Cyclodextrin Symposium, T okyo, 1994, ed. soil. CDs will be metabolized without causing any problem, T. Osa, Business Center for Academic Societies, Japan, 1994. and the solubilized toxic substances are certainly more avail- 18 J. Szejtli,Med. Res. Rev., 1994, 14, 353. able for the soil microorganisms. Promising experiments for 19 Consortium fu�r electrochemische Industrie GmbH, Eur. Pat. Appl., mobilization and microbial degradation of polyaromatic EP 0697415 A1, 1996; Ger. Offen.,DE 4429229 A1, 1996. 20 G. Wenz, K. Engelskirchen, H. Fischer, H. C. Nicolaisen and hydrocarbons in polluted soils are in progress. S. Harris (Henkel), Ger. Pat., DE 4009621 A1, 1990. 21 W. Lau and V. M. Shah, (Rohm and Haas), Eur. Pat. Appl., EP References 0614 950 A1, 1994. 1 Comprehensive Supramolecular Chemistry, ed. J. M. Lehn, J. L. Atwood, J. E. D. Davies, D. D. MacNicol and F. Vogtle, Pergamon, Oxford, 1996, vol. 1–11. Paper 6/05235E; Received 26th July, 1996 J. Mater. Chem., 1997, 7(4), 5

ISSN:0959-9428    

DOI:10.1039/a605235e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Synthesis of bulky bis(ether anhydride)s and poly(ether imide)s with bulky mainchain units† Geoffrey C. Eastmond* and Jerzy Paprotny Donnan L aboratories, Department of Chemistry, University of L iverpool, PO Box 147, L iverpool, UK L 69 3BX A series of bis(ether anhydride)s has been synthesized from very bulky bisphenols. Poly(ether imide)s have been synthesized from these bis(ether anhydride)s and two diamines in order to provide a series of poly(ether imide)s with bulky main-chain units.The polymers have been characterized in terms of molecular weight, solubility, glass-transition temperature and thermal stability in order to identify a series of high-performance polymers which are processable and potentially useful for applications which require soluble poly(ether imide)s for fabrication into, for example, gas separation membranes.Polyimides are now well established as high-performance inherently rigid, bulky main-chain units into poly(ether imide)s. To this end we have identified a number of available bulky polymers with a number of applications including microchip encapsulation, films and membranes.1 Their lack of solubility diols 2a–e and have investigated their abilities to undergo nitro displacement reactions with 4-nitrophthalodinitrile and requires that they are normally applied as poly(amic acid) intermediates which are subsequently imidized.There have thus be converted into bis(ether anhydride)s 5a–e (Scheme 1). We have also undertaken a preliminary assessment of the been many attempts to produce more soluble and processable alternatives.A successful development was the commercial abilities of the anhydrides to undergo polymerization with diamines (Scheme 2) to produce processable poly(ether imide)s poly(ether imide) Ultem;2 the incorporation of main-chain ether linkages imparts solubility and melt processability. which might have useful properties for specific applications.Here we describe the results of these studies and report on the Subsequently, several groups have atttempted to produce other members of this family (1) in order to further enhance pro- glass-transition temperatures, molecular weights, solubilities, thermal stabilities and colour of these polymers. cessability and tailor properties for specific applications; in 1, Ar is an aromatic unit derived from an aromatic diol and There has, over many years, been considerable interest in polymers containing moieties derived from adamantane and species 1 are usually derived from a bis(ether anhydride), formed by a reaction sequence involving nitro displacement the literature has been reviewed.9 Recent papers have reported polyimides,10 poly(ether ketone)s,11 polyphenylenes12 and between the diol and a suitable nitrophthalic acid derivative (often a nitrophthalodinitrile), and a diamine. Approaches polybenzoxazoles.13 One reason for interest in such polymers is the inherent thermal and chemical stability of the ada- which have been used to enhance solubility include incorporating substituent groups on Ar and/or Ar¾ groups and changing mantane unit.This report includes the synthesis of poly(ether imide)s with main-chain adamantane-1,3-diyl units and their the susbstitution patterns of aromatic residues.3–6 preliminary characterization.More recently we prepared a series of poly(ether imide)s with pendant adamantyl units, and the properties of these polymers and a comparison with polymers with main-chain units will be reported separately.14 Experimental Diols (and their sources) used to prepare bis(ether anhydride)s One of the main potential applications of poly(ether imide)s were: 4,4¾-(adamantane-1,3-diyl)diphenol 2a (Aldrich), 6,6¾- is as membranes for gas separation, which requires the poly- dihydroxy-4,4,4¾,4¾,7,7¾-hexamethyl-2,2¾-spirobichromane 2b mers to be soluble for fabrication into hollow-fibre, asymmetric (TCI), 1,1¾-bi-2-naphthol 2c (Aldrich), 4,4¾-bicyclo[2.2.1]hep- membranes.Two factors are important in producing optimum tane-2,2-diyldiphenol 2d and 4,4¾-tricyclo[5.2.1.02,6 ]decane- membrane properties. The polymers must show good per- 8,8-diyldiphenol 2e; the latter two diols were gifts from meability of gases and good selectivity in gas separation. It Kodak Ltd. 4-Nitrophthalodinitrile was obtained from TCI, has long been recognised that bulky groups enhance gas 4,4¾-oxydianiline was an ultrapure sample from BP. m- permeability.7 Amongst other studies by various groups, we Phenylenediamine (Fluka) was sublimed prior to use. Other have demonstrated a pattern of substituent type on selec- solventsand reagents used were obtained from sources specified tivity.5,8 We found that groups which were bulky enhanced previously and were used as supplied.gas permeability while substituents which reduced main-chain Diols were reacted with 4-nitrophthalodinitrile in dimethyl rotation enhanced selectivity. These factors together probably sulfoxide (DMSO) solution in the presence of anhydrous decrease the efficiency of chain packing and bulky, stiff units, potassium carbonate at room temperature to produce bis(ether which are probably difficult to pack in an efficient manner, dinitrile)s 3a–e,15 exactly as described in previous publi- appear to impart favourable properties.cations.5,6 Reactions were typically performed using 10 mmol Because a combination of bulky units and chain rigidity of diol and 20 mmol of 4-nitrophthalodinitrile and were con- might enhance both permeability and selectivity in gas tinued for 24 h.Products were isolated by pouring reaction separation, we have investigated the possibilities of introducing mixtures into water and washing the products with methanol. Similarly, the bis(ether dinitrile)s were hydrolysed to bis(ether diacid)s 4a–e with potassium hydroxide, isolated and then † This paper was presented, in part, at the International Polymer Symposium, Gliwice, Poland, September 1995.dehydrated to bis(ether anhydride)s 5a–e with glacial acetic J. Mater. Chem., 1997, 7(4), 589–592 589Scheme 1 Elmer Series 7 instruments. Measurements were made under nitrogen at The Leverhulme Centre for Innovative Catalysis, University of Liverpool, with a heating rate of 40°C min-1, samples were annealed for 6 h at 170°C in vacuo, and in air at The University of Sussex with a heating rate of 10°C min-1. Results and Discussion All diols 2a–e satisfactorily underwent nitro displacement reaction with 4-nitrophthalodinitrile according to Scheme 1 to give pure bis(ether dinitrile)s 3a–e in good yield.The elemental analysis data for the bis(ether dinitrile)s are given in Table 1.Each bis(ether dinitrile) was readily hydrolysed by potassium hydroxide to its corresponding bis(ether acid) 4 which was isolated. The bis(ether acid)s were not characterized but were directly dehydrated to the bis(ether anhydride). The bis(ether anhydride)s 5a–e were characterized by elemental analyses and melting points, supported by spectrocopy; details of elemental analyses, yields and melting points, together with the solvents used for recrystallization, are reported in Table 2.Scheme 2 In order to test the feasibility of preparing poly(ether imide)s from the several bis(ether anhydride)s, each bis(ether anhydride) was reacted with one or two aromatic diamines [usually acid and acetic anhydride as described previously; these procedures are summarized in Scheme 1.Elemental analysis data m-phenylenediamine (MPD) and 4,4¾-oxydianiline (ODA)] in NMP or DMAC, to prepare poly(amic acid)s which were then for the bis(ether dinitrile)s and bis(ether anhydride)s, together with their melting points and yields, are given in Tables 1 and chemically imidized with an acetic anhydride–pyridine mixture.After precipitation into methanol, the poly(ether imide)s were 2, respectively. Poly(ether imide)s were prepared from the bis(ether anhy- boiled with methanol in order to remove residual NMP. Where soluble in NMP–1 M LiCl, the molecular weights of the dride)s in a two-stage process involving initial formation of poly(amic acid) by reaction with diamine in N-methylpyr- poly(ether imide)s were determined and in all cases the glasstransition temperatures of the polymers were measured by rolidone (NMP) or N,N-dimethylacetamide (DMAC) and subsequent chemical imidization with an acetic anhydride– differential scanning calorimetry.The results are reported in Table 3. pyridine mixture (151, v/v) as described in previous publications and summarized in Scheme 2.5,6 It is noticeable that the glass-transition temperatures (Tg) of the polymers are almost independent of the diamine used Molecular weights were determined by gel-permeation chromatography using DMF–1 M LiCl as eluent, PL-gel and are dominated by the bis(ether anhydride) structure; in several series of poly(ether imide)s, derived from various polystyrene columns and polystyrene standards (both from Polymer Laboratories) with refractive index detection (Knauer diphenols or dihydroxyphenylenes, those based on ODA have glass-transition temperatures about 10°C lower than those detector).Glass-transition temperatures were determined using a Perkin-Elmer DSC2 differential scanning calorimeter. based on MPD. We previously noted a reduced dependance of Tg on diamine structure in poly(ether imide)s based on Thermogravimetric analysis (TGA) was undertaken on Perkin- 590 J.Mater. Chem., 1997, 7(4), 589–592Table 1 Synthesis of bis(ether dinitrile)s elemental analysis yield (%) bis(ether recrystallization dinitrile) C H N solvent pure crude mp/°C 3a Calc. 79.72 4.89 9.79 MeCN 87 96 188–189 Found 79.68 4.86 9.84 3b Calc. 75.46 5.19 9.02 MeCN 87 98 276–277 Found 74.60 5.30 9.02 3c Calc. 80.28 3.36 10.03 MeCN–MeOH 79 99 253–254 Found 80.18 3.36 10.08 3d Calc. 78.93 4.54 10.51 MeCN–MeOH(152) 78 91 197–198 Found 79.00 4.51 10.56 3e Calc. 79.60 4.92 9.78 MeCN 95 — 143–145 Found 79.66 4.92 9.75 Table 2 Synthesis of bis(ether anhydride)s elemental analysis bis(ether recystallization anhydride) C H solvent yield (%) mp/°C 5a Calc. 74.50 4.60 Ac2O 73 184–186 Found 74.53 4.59 5b Calc. 70.90 4.88 Ac2O–MeCN 87 290–291 Found 70.27 4.80 5c Calc. 74.73 3.13 Ac2O 88 221–222 Found 73.64 3.05 5d Calc. 73.42 4.54 Ac2O 95 111–112 Found 73.33 4.22 5e Calc. 74.50 4.60 Ac2O–MeCN 85 168–169 Found 74.60 4.59 Table 3 Properties of polymers Ar Ar¾ Mw/kg mol-1 Tg/°C 119 255 106 253 83 271 122 268 — 254 — 250 98 248 52 255 70 253 J.Mater. Chem., 1997, 7(4), 589–592 591naphthalene-derived bis(ether anhydride)s.16 When used for poly(ether imide)s. Loss of 30% does not correspond to loss of the total diol unit or to its central aliphatic structure. practical gas separation membranes it is necessary for polymers to operate at high temperatures and the glass- Rather, the weight loss corresponds approximately to loss of one substituted chromane unit per repeat, which might be transition temperatures of the polymers based on bulky anhydrides are consistent with use for this purpose.It is eliminated following scission of an aryl ether linkage adjacent to the phthalimide unit. anticipated that polymers with significantly higher glasstransition temperatures could be prepared by polymerization of the bis(ether anhydride)s with rigid diamines, especially Conclusions those with hindering substituents ortho to the phthalimide unit; we anticipate that glass-transition temperatures in excess It has been demonstrated that 4,4¾-(adamantane-1,3- diyl)diphenol 2a, 6,6¾-dihydroxy-4,4,4¾,4¾,7,7¾-hexamethyl-2,2¾- of 300 °C can readily be achieved.In general the polymers had little colour.Those based on spirobichromane 2b, 1,1¾-bi-2-naphthol 2c, 4,4¾-bicyclo-[2.2.1]- heptane-2,2-diyldiphenol 2d and 4,4¾-tricyclo- [5.2.1.02,6]- MPD were essentially colourless and did not discolour when heated in air to temperatures up to 250°C. In addition, all decane-8,8-diyldiphenol 2e all undergo nitro displacement with 4-nitrophthalodinitrile to give bis(ether dinitrile)s which can polymers were soluble in chloroform and, when prepared from such solutions, solvent-cast films were creasable and did not be converted to bis(ether anhydride)s.The bis(ether anhydride) s react with aromatic diamines to give poly(ether imide)s fracture and hence had useful mechanical properties; mechanical properties have not yet been determined.The low colour with bulky main-chain units which are soluble in chloroform and are processable from solution in chloroform or aprotic is consistent with reduced interchain interactions between Nphenylphthalimide units which, in turn, is indicative of less solvents. Where soluble in DMF–1 M LiCl, the molecular weights of the polymers were determined and the polymers efficient chain packing, due to the bulky nature of the units close to the phthalimide residues.The units could prevent were shown to have high molecular weight; molecular weights of polymers based on 5c and ODA and 5d and MPD were close approach of phthalimide units and could give rise to high permeability for gas separation membranes. not determined because of their insolubility in the medium used.The polymers have little colour and those based on Of relevance to potential applications is the thermal stabilities of the polymers. Thermal stabilities of the polymers pre- MPD are virtually colourless. Glass-transition temperatures are in the range 240–270 °C and the polymers based on MPD pared from the bis(ether anhydride)s 5a, b, d and e with MPD by DTA have been examined under a nitrogen atmosphere by show good thermal stability.TGA. The results are presented in Fig. 1 which shows that all the polymers have good thermal stability. The data in Fig. 1 The authors wish to thank Dr M. R. H. Siddiqui of the Leverhulme Centre for Innovative Catalysis, University of exaggerate the true stabilities of the polymers beause the heating rate used was 40°C min-1.In comparison, data Liverpool, for undertaking the TGA measurements under nitrogen, Dr N. C. Billingham, University of Sussex, for those obtained with the same polymer based on 2a (in air) at a heating rate of 10°C min-1 showed the same shape of thermo- undertaken in air, Kodak Ltd for the gift of two diols and the EPSRC for providing financial support. gram but initial decomposition started at 405 °C.Nevertheless, the data in Fig. 1 provide comparative data for the samples investigated and show the patterns of behaviour on thermal References decomposition. Initial decomposition of all samples occurs at a similar temperature with decomposition of the polymer based 1 See, for example, Polyimides, ed. D. Wilson, H. D. Stenzenberger and P.M. Hergenrother, Blackie & Sons, Glasgow, 1990. on adamantane starting to decompose at a slightly lower 2 R. O. Johnson and H. S. Burhlis, J. Polym. Sci., Polym. Symp., 1983, temperature; similar initial decomposition temperatures prob- 70, 129. ably indicate a common mode of chain scission, e.g. at the 3 F.W. Harris andL. H. Lanier,in Structure–Solubility Relationships ether linkage to the phthalimide; polyimides which do not in Polymers, ed.F. W. Harris and R. B. Seymour, Academic Press, have such ether linkages, e.g. Kapton, are more stable. All NY, 1977, p. 183. 4 T. L. St.Clair, A. K. St.Clair and E. N. Smith, Polym. Prepr. Am. samples show decomposition in two stages, in common with Chem. Soc., Div. Polym. Chem., 1976, 17(2), 359. many other data on related polymers.Decomposition of the 5 G. C. Eastmond, P. C. B. Page, J. Paprotny, R. E. Richards and polymer based on spirobichromane is somewhat distinctive in R. Shaunak, Polymer, 1994, 35, 4215. showing a rapid loss of 30 wt% over a relatively small tempera- 6 G. C. Eastmond and J. Paprotny, Polymer, 1994, 35, 5149; ture rise; this step is probably indicative of a mode of decompo- Macromolecules, 1995, 28, 2140.sition associated with the spirobichromane moiety rather than 7 W. J. Koros, G. K. Fleming, S. M. Jordan, T. H. Kim and H. H. Hoehn, Prog. Polym. Sci., 1988, 13, 339. the aromatic structures common to all the polymers and other 8 G. C. Eastmond, J. Paprotny and I. Webster, Polymer, 1993, 34, 2865. 9 A. P. Khardin and S. S. Radchenko, Russ. Chem. Rev., 1982, 51, 272. 10 Y. T. Chern and W. H. Chung, J.Polym. Sci., Part A: Polym. Chem., 1996, 34, 117. 11 L. J. Mathias and C. L. Lewis, Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem., 1995, 36(2), 140. 12 G. L. Tullos and L. J. Mathias, Polym. Prepr., Am. Chem. Soc. Div. Polym. Chem., 1995, 36(2), 316. 13 T. L. Grub and L. J. Mathias, Polym. Prepr., Am. Chem. Soc. Div. Polym. Chem., 1996, 37(1), 551. 14 G. C. Eastmond and J. Paprotny, paper presented at 4th European Technical Symposium on Polyimides and High-performance Polymers, Montpellier, 1996. 15 D. R. Heath and J. G. Wirth, US Pat. 3 730 946 (1973); 3 787 475 (1974). 16 G. C. Eastmond and J. Paprotny, J.Mater. Chem., 1996 6, 1459. Fig. 1 Thermogravimetric analysis data for poly(ether imide)s based on bis(ether anhydride)s (a) 5a, (b) 5b, (c) 5d and (d) 5e with MPD, Paper 6/07709I; Received 13th November, 1996 heating rate 40 °C min-1, nitrogen atmosphere 592 J. Mater. Chem., 1997, 7(4), 589–592

ISSN:0959-9428    

DOI:10.1039/a607709i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Synthesis and characterization of poly[3-(butylthio)thiophene]: a regioregular head-to-tail polymer Francesca Goldoni,a Dario Iarossi,a Adele Mucci,a Luisa Schenetti*a and Massimo Zambianchib aDipartimento di Chimica, Universita` diModena, via Campi 183, I-41100Modena Italy bIstituto dei Composti del Carbonio Contenenti Eteroatomi e loro applicazioni, Area di Ricerca CNR, via Gobetti 101, I-40129 Bologna Italy Poly[3-(butylthio)thiophene] was obtained from 2,5-dibromo-3-(butylthio)thiophene according to the method of Kobayashi.The polymer was characterized as having regioregular head-to-tail (HT) connections with significant extended conjugation length and optical properties, such as solvatochromism and photoluminescence. The polymer is soluble in common organic solvents and can easily form films.Poly(3-substituted thiophenes)1–9 with a variety of substituents 3-(Butylthio)thiophene such as alkyl, alkoxy and alkyl heteroatom-functionalized10–13 3-(Butylthio)thiophene21 was prepared in high yield (96%) side chains have been extensively investigated. Polythiophenes according to the method described in the literature.19 carrying electron-donating alkylthio groups, on the other hand, dH 2-H 7.11, 4-H 7.02, 5-H 7.31, CH2(a) 2.85, CH2(b) 1.63, are less well documented.Although poly[3-(alkylthio)thio- CH2(c) 1.43 and CH3 0.92. dC C-2 122.87, C-3 132.35, phenes] are generally obtained only by chemical methods,14,15 C-4 129.65, C-5 125.96, CH2(a) 35.01, CH2(b) 31.48, CH2(c) papers dealing with the electropolymerization of 3-(methyl- 21.81 and CH3 13.60.thio)thiophene16 and of some (oligothio)thiophenes have appeared.17 Polymers of 3-(ethylthio)- and 3,4-bis(ethylthio)- 2,5-Dibromo-3-(butylthio)thiophene thiophene have been chemically synthesized and characterized. 14,15 An attempt to polymerize 2,2¾55¾,2-terthiophenes Bromination of 3-(butylthio)thiophene was carried out accord- 3¾-substituted with the MSR group, where R is a long alkyl ing to the method of Taylor et al.22 A solution of 3-(butyl- chain, has been reported,18 but with unsatisfactory results.thio)thiophene (5.16 g, 30 mmol) in dichloromethane (55 ml) Recently, a paper about the synthesis of a series of regioregular was stirred while N-bromosuccinimide (NBS) (10.7 g, poly[3-(alkylthio)thiophene]s19 reported the poor solubility 60 mmol) was added portionwise.The temperature of the and the uncharacterizability of the poly[3-(butylthio)thio- reaction mixture was raised to 40°C and maintained at this phene] obtained. temperature for 16 h. The oily crude product was purified by We here report the polymerization, based on the method of vacuum distillation to give 2,5-dibromo-3-(butylthio)thiophene Kobayashi,14,20 and the characterization of poly[3-(butyl- [7.4 g, 83% yield, bp 112 °C (0.3 torr), nD21 1.6129]. thio)thiophene] (PBTT).The polymer is soluble in common dH 4-H 6.90, CH2(a) 2.83, CH2(b) 1.57, CH2 (c) 1.43 and CH3 organic solvents such as CHCl3, CCl4, toluene, benzene, THF 0.92. dC C-2 110.92, C-3 134.05, C-4 132.33, C-5 112.82, CH2(a) and CS2, and shows a highly HT-HT regioregularity. 35.98, CH2(b) 31.57, CH2(c) 21.68 and CH3 13.58. Polymerization Experimental The polymer was prepared by catalysed polymerization of a Synthesis and polymerization Grignard reagent according to methods described in the litera- Scheme 1 outlines the experimental route from monomer to ture.14,20 Magnesium metal (0.20 g, 8.2 mmol) was reacted, polymer. under nitrogen, with 2,5-dibromo-3-(butylthio)thiophene (2.8 g, 8.5 mmol) in dry 2-methyltetrahydrofuran (2MeTHF) (8 ml).After removing nearly all the solvent, dry anisole (7 ml) and nickel(II) bis(diphenylphosphino)propane dichloride [Ni(dppp)Cl2] (0.02 g, 0.04 mmol) were added. The mixture was heated at 135°C for 80 h and poured into a solution of HCl-acidic methanol (100 ml). The dark precipitate was filtered, washed with HCl-acidic methanol (20 ml) and purified by Soxhlet extraction with methanol (48 h).Extensive (24 h) extraction with dichloromethane afforded 0.6 g (41% yield) of polymer soluble in CHCl3, THF, toluene, benzene and CS2 at room temp. The polymer was purified by reprecipitation from hexane into toluene. Measurements UV–VIS spectra were recorded on a Varian-Cary 3spectrophotometer.The fluorescence spectrum was recorded on a Jobin- Yvon JY3CS spectrofluorophotometer with CHCl3 solution of Scheme 1 Reagents and conditions: i, NBS, CH2Cl2, 40°C; ii, Mg, 2-MeTHF, reflux; iii, Ni(dppp)Cl2 , anisole, 135 °C polymer. Gel permeation chromatography (GPC) was carried J. Mater. Chem., 1997, 7(4), 593–596 593out using an Alltech system equipped with a Shodex KF-804L column and a Shodex RI-71 refractive index detector, with THF as the eluent, at room temperature. The average molecular masses were calculated using a calibration curve of monodisperse polystyrenic standards. 1H and 13C NMR measurements were performed on a CDCl3 solution of PBTT with a Bruker AMX-400 WB operating at 400.13 and 100.61 MHz, respectively.The 1H and 13C chemical shifts (d, ppm) are quoted with respect to the CHCl3 signal at 7.26 (for 1H) and 77.0 (for 13C). HMQC23 parameters for aromatic and aliphatic region: spectral width ( f 2)=1–5 ppm, 2048 complex points; spectral width ( f1)=10–50 ppm, 256 t1 increments with 16 scans per t1 value; relaxation and evolution delays=0.5–1 s and 2.78–4.00 ms, respectively. Zero filling in f 1 and f2, sine function in f1 were applied before Fourier transformation.HMBC24 parameters: spectral width ( f 2)=8 ppm, spectral width ( f1 )=150 ppm, 256 t1 increments with 64 scans per t1 Fig. 1 1H NMR spectrum of PBTT. Expanded region corresponds to 4-H proton. The symbols denote CHCl3 satellites (*) and an H2O value; relaxation delay=0.5 s and delay for long-range coup- signal (#), respectively.ling constant evolution=70 ms. Zero filling in f 1 and f 2, sine function in f1 were applied before Fourier transformation. DSC (differential scanning calorimeter) was performed with These signals do not seem indicative of the presence of minor a Perkin-Elmer DCS-4, at a scanning rate of 10°C min-1. configurational triads but of b- and terminal protons of shorter FTIR: an IR spectrum (KBr disk) was recorded using an oligomers. The aliphatic region of the 1H NMR spectrum Infrared Fourier Spectrometer (Bruker IFS 113 v).displays four groups of signals centred at d 2.95, 1.65, 1.44 and 0.94 and assigned to CH2(a), CH2(b), CH2(c) and CH3 protons, respectively. Results The deshielding of the 4-H and CH2(a) protons in PBTT Characterization of the polymer with respect to the 3-(butylthio)thiophene (0.35 and 0.10 ppm, respectively) points to the presence of a prevalent HT regio- PBTT was characterized by GPC, UV–VIS and IR and 1H polymer, in accordance with previous data on dimers of and 13C NMR spectroscopy.Thermal stability was tested 3-(methylthio)thiophene.25 The regiochemistry assignment by DSC.deduced on the basis of the 1H chemical shifts was confirmed The molecular mass, determined by GPC [relative to a by the 13C NMR data of the aromatic carbons. poly(styrene) standard, with THF as eluent, at room temp.] The 13C NMR data of PBTT were obtained and assigned was: weight-average molecular mass Mw=5049 and number- through inverse-detected heteronuclear multiple-quantum average molecular mass Mn=3772 with a polydispersivity (HMQC)23 and multiple-bond (HMBC)24 correlation experi- index=1.34.The Mn of PBTT corresponds to 22 repeating ments, as previously applied to poly(3-alkylthiophenes).26–28 units per chain. The HMQC and HMBC spectra are shown in Fig. 2 and 3. The UV–VIS spectrum, in dilute CHCl3 solution displays The HMQC spectrum of PBTT shows the presence of a an absorption at lmax=502 nm.major correlation between 4-H (d 7.37) and C-4 (d 130.3) with The IR spectrum of PBTT is characteristic of 2,5-coupled a coupling constant 1JH,C of 169 Hz, typical of a b-CH thio- 3-substituted polythiophenes: one aromatic CMH stretching phene fragment.29 Another very low-correlation of the same of weak intensity at 3067 cm-1, assignable to the CMHb b-H, C-b type, between a minor proton signal at d 7.30 and a stretching, and one out-of-plane deformation of aromatic CMH carbon signal at d 130.4 was found and assigned to a shorter bonds at 818 cm-1 (due to the trisubstituted ring) are detected.oligomer with the same regiochemistry rather than to a struc- The butylthio alkyl chain gives rise to stretching vibrations in tural defect.the region 2954–2854 cm-1 and to deformation modes below 1466 cm-1. Thermal analysis by DSC did not detect any glass transition temperature Tg, but did reveal a decomposition process at Td of 310 °C. Regiochemistry assignment of PBTT The regiochemistry of poly(3-alkylthiophene)s is commonly evaluated from inspection of the aromatic region. In fact, this region provides the distribution of the four configurational triads HT–HT (head to tail-head to tail), HT–HH (head to tail-head to head), TT–HT (tail to tail-head to tail), and HH–TT (head to head-tail to tail), due to all the possible couplings.The 1H NMRspectrum of PBTT, in CDCl3, displays (Fig. 1) a major singlet at d 7.37. This signal is strongly deshielded with respect to the b-protons in 3-(butylthio)thiophene (d 7.02), in 3,3¾-di(butylthio)-2,2¾-bithiophene (d 7.08) and 4,4¾-di(butylthio)- 2,2¾-bithiophene (d 7.06).21 Similar behaviour was observed on passing from 3-(methylthio)thiophene (d 6.97) to 3,4¾-di(methylthio)-2,2¾-bithiophene (d 7.20 for 3¾-H).25 Other low intensity signals, whose relative intensities change with the Fig. 2 Coupled HMQC spectrum of the aromatic region of PBTT purification procedure, are present in the aromatic region. 594 J. Mater. Chem., 1997, 7(4), 593–596allows us to overcome the configurational trimers. Only the 13C chemical shifts of the aromatic carbons of the monomer are needed. Its reliability lies in the fact that the four sets of data depend mainly on the different type of adjacent connectivities, the 3-substituent effect being subtracted by using the 13Cchemical shifts of the monomer.Regiochemistry assignment of PBTT can be performed by comparing the differences between the 13C chemical shifts of the aromatic carbons of polymerand monomer and the connectivity parameters derived for the trimers of 3-hexylthiophene26 and the triads of PHET reported in Table 1. As was previously underlined,28 Dd(C-3) and Dd(C-4) are more convenient for the configurational assignment of the polymer, mainly due to their easy detection and assignment.The Dd values found for PBTT [Dd(C-2)=10.6 ppm, Dd(C- 3)=-3.3 ppm, Dd(C-4)=0.6 ppm and Dd(C-5)=8.5 ppm] are very close to those of the HT-HT trimer or triad and furnish a further confirmation of the regiochemistry assignment. UV–VIS and fluorescence spectroscopy PBTT displays a strong solvatochromic effect.Fig. 4 shows Fig. 3 HMBC spectrum of PBTT performed with an evolution delay the UV–VIS spectra of PBTT in mixtures of CHCl3 and of 70 ms. Three carbon signals were detected through the 4-H proton CH3OH. The increased addition of the poor solvent methanol and one of these (C-3) was also detected through the CH2(a) protons.to a chloroform solution shifts the maximum absorption to a longer wavelength, with a change from pink to violet. The Only three aromatic carbon signals at d 133.5, 129.1 and maximum shift observed, on passing from CHCl3 solution 134.5 were detected in the HMBC experiment. These are (100/0) to CHCl3–CH3OH (20/80), is represented by the assigned to C-2, C-3 and C-5, respectively, on the basis of the presence of a shoulder at 599 nm.When the ratio is further presence of multiple-bond coupling constants with the 4-H increased, a precipitate of polymer is formed. proton of the aromatic region and with the CH2(a) protons. The solid-state UV–VIS absorption spectra of the film of The aliphatic region displays four signals at d 36.0, 31.7, 21.9 PBTT formed by solvent evaporation on glass from CHCl3 and 13.6 assignable to CH2(a), CH2(b), CH2(c) and CH3 and CS2 solutions are reported in Fig. 5. The film from CHCl3 carbons, respectively. solution displays absorptions at a longer wavelength (three The detection of four aromatic carbon signals, coming from peaks at 515, 555 and 596 nm) with respect to that obtained a single triad, is indicative of high regioregularity.In fact, in from CS2 solution and the spectrum is similar to that obtained the case of poly(3-hexylthiophene) (PHT) 85% HT regioregu- in CHCl3–CH3OH solution. The changes observed in the lar, almost all the carbons of the four configurational triads UV–VIS absorption spectra of film and of CHCl3/CH3OH were detected in the same conditions.26 A similar situation was solution are probably due to a more extensive conjugation found for poly(3-hexanoyloxyethylthiophene) (PHET) 45% HT regioregular.28 The assignment of 13C signals of the four configurational triads of PHT was made by direct comparison with the 13C signals of the central units of the four corresponding trimers.26 The two sets of data match within 0.5 ppm.In the case of PHET, it was shown that, if the differences between the 13C chemical shift of the four carbons of a triad and the corresponding carbons of (3-hexanoyloxyethylthiophene) monomer were considered, four sets of data (Dd), one for each triad, were obtained (Table 1, rows b). These sets were very similar (within 1 ppm) to those derived, in the same way, from the central units of the four configurational trimers of 3- hexylthiophene (Table 1, rows a).The comparison of rows a and rows b of Table 1 enabled the regiochemistry assignment Fig. 4 UV–VIS spectra of PBTT in (a) CHCl3, (b) CHCl3–MeOH of PHET to be made.28 This second triad-based28 approach (352), (c) CHCl3–MeOH (253) and (d) CHCl3–MeOH (154) Table 1 Calculated Dd values (in ppm) for the carbons of the central unit of each configuration trimers of 3-hexylthiophene (rows a) and of the triads of PHET (rows b) with respect to the corresponding monomer a=trimer of PHT b=triad of PHET C-2 C-3 C-4 C-5 HT–HT a 11.0 -3.7 0.4 8.8 b 10.8 -2.9 1.0 9.1 TT–HT a 10.1 -3.3 -2.0 10.2 b 9.6 -2.7 -1.5 10.3 HT–HH a 8.9 -0.6 -0.9 10.9 b 8.1 0.4 -0.4 10.3 TT–HH a 7.8 -0.1 -3.3 12.3 Fig. 5 UV–VIS spectra of PBTT films: (%) from CS2 solution, b 6.6 0.7 -2.8 11.9 (#) from CHCl3 solution J. Mater. Chem., 1997, 7(4), 593–596 5957 P. Ba� uerle, U. Segelbacher, K-U. Gaudl, D. Huttenlocher and length of the polymer chain or to a more abundant presence M. Mehring, Angew. Chem., Int. Ed. Engl., 1993, 32, 76. of conjugated chains of polymer. The solvatochromic behav- 8 S.Li, C. W. Macosko and H. S. White, Science, 1993, 259, 957. iour of PBTT and the UV–VIS spectra are in line with those 9 T-A. Chen and R. D. Rieke, J. Am. Chem. Soc., 1992, 114, 10087. reported for poly(3-alkylthiophene)s.30 10 R. D. McCullough and S. P. Williams, J. Am. Chem. Soc., 1993, The fluorescence spectrum of a chloroform solution of PBTT 115, 11608. 11 P. Ba�uerle and S.Scheib, Adv. Mater., 1993, 5, 848. obtained with an excitation wavelength of 490 nm gives a 12 G. Daoust and M. Leclerc,Macromolecules, 1991, 24, 455. maximum emission wavelength at 603 nm with a yield Wf 13 C. Della Casa, F. Andreani, P. Costa Bizzarri and E. Salatelli, greater than 10-2. J. Mater. Chem., 1994, 1035. 14 J. P. Ruiz, K. Nayak, D. S. Marynick and J. R. Reynolds, Macromolecules, 1989, 22, 1231.Conclusions 15 P. Ruiz, M. B. Gieselman, K. Nayak, D. S. Marynick and J. R. Reynolds, Synth. Met., 1989, 28, C481. The polymerization method based on Kobayashi’s procedure 16 M. Sato and H. Morii, Polym. Commun., 1991, 32, 42; affords a regioregular HT poly[3-(butylthio)thiophene] sol- Macromolecules, 1991, 24, 1196. uble in all the common organic solvents.This synthetic route 17 P. Ba�uerle, G. Go�tz, A. Synowczyk and J. Heinze, L iebigs Ann., enables a soluble and characterizable polymer to be obtained, 1996, 279. and it seems preferable to that proposed by Rieke. 18 A. R. Sørensen, L. Overgaard and I. Johannsen, Synth. Met., 1993, Further studies on the physical properties of PBTT are 55&n 1626. 19 X. Wu,T-A. Chen and R.D. Rieke, Macromolecules, 1995, 28, 2101. in progress. 20 M. Kobayashi, J. Chen, T.-C. Chung, F. Moraes, A. J. Heeger and F.Wudl, Synth.Met., 1984, 9, 77. We are grateful to Dr G. Ponterini for the fluorescence 21 U. Folli, F. Goldoni, D. Iarossi, A. Mucci and L. Schenetti, spectrum and to the Centro Interdipartimentale Grandi J. Chem. Res., 1996, (S) 69;M 0552–0569. Strumenti (CIGS) of Modena University for the use of the 22 E.C. Taylor and D. E. Vogel, J. Org. Chem., 1985, 50, 1002. Bruker AMX-400 WB. 23 A. Bax, R. H. Griffey and B. L. Hawkins, J. Magn. Reson., 1983, 55, 301. This work was supported by Consiglio Nazionale delle 24 A. Bax and M. F. Summers, J. Am. Chem. Soc., 1986, 108, 2093. Ricerche (CNR). 25 U. Folli, D. Iarossi, M. Montorsi, A. Mucci and L. Schenetti, J. Chem. Soc., Perkin T rans. 1, 1995, 537. 26 A. Mucci and L. Schenetti, Macromol. Chem. Phys., 1995, 196, References 2687. 27 M. Ferrari, A. Mucci, L. Schenetti and L. Malmusi, Magn. Reson. 1 J. Roncali, Chem. Rev., 1992, 92, 711. Chem., 1995, 33, 657. 2 R. D. McCullough, R. D. Lowe, M. Jayaraman and D. L. 28 F. Goldoni, D. Iarossi, A. Mucci, L. Schenetti, P. Costa Bizzarri, Anderson, J. Org. Chem., 1993, 58, 904. C. Della Casa and M. Lanzi, Polymer, in the press. 3 M. Zagorska, L. Firlej, P. Bernier, I. Kulszewicz-Bajer and 29 F. W. Wehrli and T. Wirthlin, Interpretation of Carbon-13 NMR A. Pron, J. Polym. Sci. Polym. Chem., 1992, 30, 1761. Spectra, Heyden, Bristol, 1978. 4 G. Barbarella, A. Bongini and M. Zambianchi, Macromolecules, 30 A. Chen, X. Wu and R. D. Rieke, J. Am. Chem. Soc., 1995, 117, 233. 1994, 27, 30. 5 H. Mao and S. Holdcroft, Macromolecules, 1992, 25, 554. 6 H. Mao, B. Xu and S. Holdcroft, Macromolecules, 1993, 26, 1163. Paper 6/06771I; Received 3rd October 1996 596 J. Mater. Chem., 1997, 7(4), 593–596

ISSN:0959-9428    

DOI:10.1039/a606771i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Ionomer-like behaviour of protonated polyaniline: effect of ionic strength on the optical spectra Soumyadeb Ghosh, Graham A. Bowmaker and Ralph P. Cooney* Department of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand The absorbances of polyaniline (PAni) at 850 nm due to the electronic transition from a polaron band and at 130 cm-1 due to vibrational excitation of the hydrogen bond are observed to change non-linearly with the degree of protonation.The extent of non-linearity for these changes is dependent on the ionic strength of the protonating medium. The effect of ionic strength on the bands originating from the two different types of transitions (electronic and vibrational) are found to be similar. A model has been proposed relating PAni with ionomers to account for the observations.This also provides an explanation for the formation of metallic islands in the polymer. It seems that the distribution of the dopants in the polymer matrix and the interchain interactions have a combined influence on the electronic structure of protonated PAni. Properties of conducting polymers are known to be dependent In the present work, a band at 130 cm-1, tentatively assigned to the NH+,N hydrogen bond in the protonated emeraldine on the type of dopant counter ion, the molecular environment of the polymer and other factors that determine the intermol- form of PAni,2 is reported.The change in the absorbance of this band, along with the change in the near-IR band with the ecular interactions in the system.Polyaniline (PAni) has been found to be the most interesting polymer in this respect.1 Apart degree of protonation of the polymer, have also been studied. Though the effect of ionic strength on the degree of protonation from being dependent on the protonation of the polymer, known as acid doping,2 the electrical conductivity, optical of PAni is well known,10,13 in the present paper the influence of the ionic strength on the vibrational as well as electronic properties and other electronic properties of PAni are highly sensitive to factors such as the nature of the dopant, the solvent (near-IR) absorbance spectra of PAni, at a particular degree of protonation, has been investigated.Based on the results it from which the PAni film is cast, and the presence of any additive.1,3 The change in the properties may be so great that is suggested that ionic interactions and hydrogen bonding play vital roles in formation of the metallic islands4 in PAni.the process of modification of conductivity and other properties by using appropriate additives has been termed secondary doping3 and its mechanism has been related to the structure Experimental of the polymer on a scale larger than the molecular level (e.g.conformation of the polymer chain, crystallinity, etc.).3 Other PAni in the emeraldine form was prepared by the standard larger scale features reported are the metallic islands4 in chemical method.2 A fraction, soluble in dimethyl sulfoxide protonated PAni and physical cross-linking of the polymer (DMSO) or N-methylpyrrolidone (NMP), was extracted from chains.5 Spectroscopic studies in the near-IR region also the undoped PAni powder and was used for all the following indicate the presence of interchain interactions in solutions of spectroscopic studies.For UV–VIS spectroscopic studies, a protonated PAni.6,7 thin film of PAni was cast on the inside wall of a cuvette and Protonation of PAni has been followed by elemental analy- equilibrated for at least 5 h with aqueous media of different sis,2,8 potentiometric titration9,10 and various spectroscopic ionic strengths (different concentrations of KCl) and of different methods.11,12 As most spectroscopic parameters do not yield pH (adjusted by the addition of aqueous HCl).The absorbance a quantitative measure of the degree of protonation, the former spectra of the polymer films in the region 280–875 nm were two methods8–10 are employed for measuring the degree of recorded using a Hitachi (U-3400) spectrophotometer, for each protonation of the polymer.It has been shown that the set of pH and ionic strength under in situ conditions, with the absorbance at ca. 800 nm, due to the polaronic band of PAni, cuvette filled with the aqueous medium.The change in the does not change proportionally with the degree of pro- absorbance of the film at 850 nm relative to that at pH 7 was tonation.7 On the other hand, the degree of protonation of measured at various pH values. For FTIR studies, a free- PAni is known to be dependent on the ionic strength of the standing film of PAni cast from the solution was studied under protonating medium.This has been attributed to the Donnan ex situ conditions. The film was equilibrated with aqueous potential10,13 between the solid polymer phase and the aqueous KCl solutions of different concentrations and pH. After at phase owing to the presence of fixed positive charges on the least 5 h of equilibration with each solution, the film was taken protonated PAni.Protonation of PAni has also been followed out, the excess solution was removed by pressing the film using vibrational spectroscopy. A large shift of the mid-IR between filter papers and was then dried in vacuum at band for the NMH stretching mode is observed,14 which has ca. 70°C. PAni is known to be stable at this temperature.15 been ascribed to strong interchain hydrogen bonding between The same film was used for each set of spectra relating to a the positively charged amine groups and the imine nitrogens particular ionic strength.The FTIR spectra of the PAni films in protonated PAni. However, the shifted band appears only were recorded using a Digilab FTS-60 spectrometer. Though as a broad hump from ca. 1700 to 900 cm-1 and it is difficult in the FTIR experiment equilibrium conditions were not to obtain quantitative information on hydrogen bonding in maintained, the Donnan effect is expected to be small at the PAni from this band.Hence, it would be interesting to study high ionic strength of the medium (1 mol dm-3).13 Therefore, the spectra of PAni in the far-IR region where the correspond- the change in the Donnan potential and, hence, in the degree ing band due to the NH+,N hydrogen bond is expected to of protonation of PAni, due to the increase in the concentration appear.Studies of hydrogen bonding would yield important of the aqueous KCl solution absorbed in the polymer phase during the drying process, is assumed to be negligible. information about the interchain interactions in PAni systems.J. Mater. Chem., 1997, 7(4), 597–600 597Fig. 1 UV–VIS spectra of PAni films in 1 mol dm-3 KCl solutions of different pH Fig. 2 Plot of the change in the relative absorbance (x) at 850 nm from the UV–VIS spectra (,) and at 130 cm-1 from the far-IR spectra (—) vs. degree of protonation of PAni equilibrated with aqueous KCl solutions of different concentrations: +, ', 1 mol dm-3; % 0.1 mol Results dm-3; $, #, 0.0 mol dm-3 KCl.The closed and open symbols correspond to the experimental points from the far-IR spectra and the UV–VIS spectroscopy UV–VIS spectra, respectively. UV–VIS spectra of PAni films equilibrated at different pH values are shown in Fig. 1. Upon protonation of PAni with decreasing pH, a band appears in the near-IR region due to the formation of positively charged polarons on the polymer chain.16 The peak position of this band is above the limit up to which the spectra could be recorded for the present studies (Fig. 1). However, from the shape of the spectra it is evident that the absorbance due to the polaron band attains almost the highest value above 850 nm. Also, since 850 nm seems to be not too far from the peak position, it is reasonable to assume that absorbances at these two positions result from the same species and they would be proportional to each other.Therefore, the absorbance at 850 nm (Ap) is used as a measure of concentration of the charge carriers and, hence, of the amount of conducting phase in the system. The change in the absorbance at 850 nm upon protonation (Ap-Ai) was measured with respect to the absorbance at pH 7 (Ai).The relative change in the absorbance (x) was calculated as x=(Ap-Ai)/(Af-Ai) (1) where Af is the maximum absorbance at 850 nm resulting from the protonation of the polymer. The degree of protonation of PAni at different pH values and ionic strengths can be obtained from the results of potentiometric titrations.10 When x is plotted against the degree of protonation of PAni (Fig. 2), it is observed that x does not vary in a linear fashion. That is, the appearance of the polaron band is not directly related to the degree of protonation of the polymer. Further, the variation of x is also dependent on the ionic strength of the medium. For the film equilibrated with lower ionic strengths, the polaron Fig. 3 Mid-IR spectra of PAni, with a degree of protonation of 0.27, band appears even at low degrees of protonation and the equilibrated in aqueous KCl solutions at concentration of (a) 1 mol change in x is gradual, whereas at high ionic strength, the dm-3 (pH 4.2), (b) 0 mol dm-3 (pH 2.1); (c) spectrum obtained from band appears only above a critical value of the degree of subtraction of spectrum (a) from spectrum (b) after normalisation with protonation and x changes sharply thereafter.respect to the peak at 830 cm-1 [(c)=(b)-1.14(a)] Mid-IR spectroscopy from the difference spectrum [Fig. 3(c)], that this band is stronger when the PAni film is equilibrated with aqueous The mid-IR spectra of PAni, at the same degree of protonation of 0.27, but equilibrated with media of different ionic strengths, media of lower ionic strength.The bandwidths of the other peaks in the spectrum are also increased (leading to the are shown in Fig. 3. The appearance of the broad hump in the baseline from ca. 1700 to 900 cm-1 has been attributed to appearance of the sharp peaks in the difference spectra), possibly owing to the higher interchain hydrogen bonding the band for the NMH stretching mode, shifted owing to the hydrogen bonding in PAni.14 It may be observed, particularly interactions in the latter sample.It is also interesting to note 598 J. Mater. Chem., 1997, 7(4), 597–600that the broad absorbance above 1800 cm-1, due to the tail Discussion of the electronic band of the free carriers associated with the Upon protonation of PAni, localized charged sites are formed conducting state of PAni,14 is much stronger for the film on the polymer backbone. As the degree of protonation is equilibrated at lower ionic strength.This conforms with the increased, the charged sites overlap to form the polaron band, above results from the UV–VIS spectra, which show that the leadingto the appearance of the near-IR peak.The conductivity near-IR band of the partially protonated PAni is stronger in and the intensity of the EPR signal of the polymer change low ionic strength media. Though the studies in the mid-IR concomitantly with the near-IR band upon protonation.19 region give some qualitative evidence of the hydrogen bonding Therefore, the absorbance of PAni in the near-IR region is in PAni and its dependence on the ionic strength of the related to the conducting state of the polymer.However, an protonating medium, the quality of information is not good ideal polaron lattice in PAni requires a degree of protonation enough for any quantitative analysis. of 0.5, whereas in the present study, as also observed earlier,2,4 the conducting state in PAni appears at much lower degrees Far-IR spectroscopy of protonation.This is due to the fact that the protonation of In the far-IR (450–75 cm-1) spectra of PAni (Fig. 4), a peak PAni is not homogeneous throughout the bulk—it occurs in at 410 cm-1, which is present in the spectrum of unprotonated clusters, leading to the formation of conducting domains in PAni, increases in intensity and a new broad band at ca.the polymer matrix, known as metallic islands.4 The formation 130 cm-1 appears on protonation. The 410 cm-1 peak, which of such domains may occur due to phase segregation of the is also observed in Raman spectra of PAni,17 has not been ionic species—the protonated portion of the polymer and the assigned. The 130 cm-1 band is assigned to hydrogen bonding counter anions, in the low dielectric medium of the unpro- between the charged amine groups and the imine nitrogens tonated polymer matrix.Such phase segregation is well known (vide inf ra). Owing to a better signal-to-noise ratio in the lower in ionomeric systems.20 At low ionic strength, owing to the energy region, the 130 cm-1 peak was used to quantify the phase segregation, the local degree of protonation of the effect of protonation of PAni on its vibrational spectrum.The polymer in the ionic phase is high even at lower degrees of absorbance at 130 cm-1 was measured after correcting for the protonation of the bulk. This leads to electronic overlap of the upward shift in the baseline of the spectra (Fig. 4) with charged sites on the polymer with the formation of the protonation.The shift is probably caused by an increase in polaronic band and, hence, to an increase in the absorbance the reflectance of the polymer in the far-IR region upon at 850 nm, whereas at high ionic strength, the charged sites on formation of the conducting state with protonation, as reported the polymer may be stabilised by the free ions present in the earlier.18 The relative change in the absorbance (x) at 130 cm-1 medium, preventing phase segregation at low degrees of pro- is calculated using eqn.(1) and plotted against the degree of tonation. Only at higher degrees of protonation do the charged protonation of the polymer in Fig. 3. This shows that the sites come close together to interact forming the polaronic change in absorbance of PAni in the far-IR region is also not band.This seems to explain the observation that for high ionic directly proportional to its degree of protonation and is strength of the doping medium, the 850 nm band appears only dependent on the ionic strength of the doping media. However, above a critical value of the degree of protonation (ca. 0.24). the dependence of x on the ionic strength is found to be less The broad band from 1700 to 900 cm-1, observed in the for the 130 cm-1 band than for the 850 nm band. This may be mid-IR spectrum of protonated PAni, has been interpreted as due to the fact that in the far-IR experiment, the PAni film an NMH stretching band, shifted owing to strong interchain was dried before recording the spectra whereas the UV–VIS hydrogen bonding between protonated sites and the imine spectra were recorded in situ, keeping the film in equilibrium nitrogens (NH+,N).14 Also, for imidazole, a shift in the with the aqueous medium.Drying of the film in the former NMH stretching frequency is observed, accompanied by the case changes the dielectric environment in the polymer phase, appearance of a band in the far-IR region at 142 cm-1, which which may obscure the effect of the ionic strength on the far- is assigned to the stretching mode of the NH,N hydrogen IR absorbance.On drying, the absorbance at 850 nm was also bond.21 The broad far-IR band for PAni observed at found to decrease by a factor of three for the PAni film with ca. 130 cm-1 may therefore be tentatively assigned to the a degree of protonation of 0.25.NH+,N vibration. This is supported by the fact that the peak position remains unaffected on changing the acid for protonation of PAni from hydrochloric acid to sulfuric acid or acetic acid. However, no appreciable change in the peak position could be observed on exchanging the protons with deuterium ions by equilibrating PAni with HCl solution in D2O. This may be due to fact that only a small change in the peak position is expected on deuterium exchange; the decrease in vibrational frequency due to the increase in the mass is partly compensated by the decrease in the zero-point energy due to a large anharmonicity in the potential-energy well of the hydrogen bond.22 Also, since in the 130 cm-1 vibration mode the protonated imine group moves as a whole, the effect of the change in the mass, due to the isotope exchange, on the vibration frequency should be small.In the case of imidazole, a shift of only 9 cm-1 was observed.21 In the present experiment, such a small change in the peak may be obscured by the broadness of the 130 cm-1 band. In polymeric systems, interchain hydrogen bonding is often a cooperative phenomenon.23 That is, the extent of hydrogen bonding depends non-linearly on the number of hydrogen bonding sites; the formation of a hydrogen bond occurs only when a critical number of consecutive hydrogen-bonding sites are present.This may explain the non-linear dependence of the Fig. 4 Far-IR spectra of PAni films equilibrated in 1 mol dm-3 KCl solutions of different pH absorbance of the 130 cm-1 band on the degree of protonation.J. Mater. Chem., 1997, 7(4), 597–600 599At low ionic strength, owing to phase segregation of the ionic polymers such as polyacetylene.25 It may be valuable to review earlier interpretations of results on doping of conducting groups, the local concentration of protonated nitrogens, which polymers in the light of the present study.are also the hydrogen-bonding sites, is high in the ionic phase. Hence, in the ionic phase the critical number of consecutive We would like to thank Professor S. K. Rangarajan for hydrogen-bonding sites may be reached even at low degrees suggesting the similarity between the metallic clusters in PAni of protonation, leading to formation of the hydrogen bonds.and ionic clusters in ionomers. The financial support from the At high ionic strength, owing to the decreased phase segre- Public Good Science Fund (PGSF) of the Foundation for gation, the extent of hydrogen bonding is decreased, lowering Research Science and Technology, New Zealand (Contract no. the intensity of the absorbance at 130 cm-1. It should be noted UOA402), is also gratefully acknowledged.however, that the present studies do not give conclusive evidence for assignment of the 130 cm-1 band to hydrogen bonding. Nevertheless, from the nature of its variation with References the degree of protonation, the 130 cm-1 band seems to be 1 A. J. Heeger, Synth. Met., 1993, 55–57, 3471. associated with certain cooperative interactions in the system. 2 J-C. Chiang and A. G. MacDiarmid, Synth.Met., 1986, 13, 193. Such interactions would also lead to phase segregation and 3 A. G. MacDiarmid and A. J. Epstein, Synth.Met., 1994, 65, 103. the above discussion would hold good even if the band is due 4 F. Zuo, M. Angelopoulos, A. G. MacDiarmid and A. J. Epstein, to any other cooperative intermolecular bonding in PAni. Phys. Rev. B, 1987, 36, 3475. 5 A. G. MacDiarmid, Y. Min, J. M. Wiesinger, E. J. Oh, E. M. Scherr UV–VIS spectroscopy therefore shows that the electronic and A. J. Epstein, Synth.Met., 1993, 55–57, 753. properties of PAni are dependent on the ionic environment of 6 Y. Cao, P. Smith and A. J. Heeger, Synth.Met., 1989, 32, 263. the polymer chain. On the other hand, examination of the 7 S. Ghosh, Doctoral Thesis, Indian Institute of Science, Bangalore, FTIR spectra indicate that the interchain interactions through India, 1995.the hydrogen bonding in PAni are also dependent on similar 8 A. G. MacDiarmid, J-C. Chiang, A. F. Richter and A. J. Epstein, factors. Earlier studies6,7 on PAni solutions in acidic solvents Synth. Met., 1987, 18, 285. 9 C. Menardo, M. Nechtschein, A. Rousseau, J.P. Travers and suggest that the electronic states pertaining to the near-IR P. Hany, Synth.Met., 1988, 25, 311. transition extend in three dimensions and involve interchain 10 S. Ghosh,Macromolecules, 1995, 28, 4729. electronic coupling. It is possible that the interchain electronic 11 V. M. Geskin, Ya. A. Letuchii and Ye. A. Katsman, Synth. Met., interaction is reinforced by the interchain hydrogen bonding. 1992, 48, 241. The observations in the present studies are in accord with the 12 E. T. Kang, K. G. Neoh and K. L. Tan, Adv. Polym. Sci., 1993, earlier observations that the optical spectra and other elec- 106, 135. 13 P. Chartier, B. Mattes and H. Reiss, J. Phys. Chem., 1992, 96, 3556. tronic properties of PAni are sensitive to the nature of the 14 Ph. Colomban, A.Gruger, A. Novak and A. Regis, J. Mol. Struct., counter anion.1,3 This dependence may be caused by the 1994, 317, 261. modification of the metallic islands (i.e. the segregated ionic 15 E. M. Genies, A. Boyle, M. Lapkowski and C. Tsintavis, Synth. clusters) by the surfactant anions, which determines the surface Met., 1990, 36, 139. energy at the interphase boundaries.Therefore, examples 16 S. Stafstrom, J. L. Bredas, A. J. Epstein, H. S. Woo, D. B. Tanner, W. S. Huang and A. G. MacDiarmid, Phys. Rev. L ett., 1987, 59, showing the effect of intermolecular interactions on various 1464. properties of PAni are already available. However, the present 17 I. Harada and Y. Furukawa, V ibr. Spectra Struct., 1991, 19, 369. investigations show the importance of ionic interactions and 18 K.Lee, A. J. Heeger and Y. Cao, Synth. Met., 1995, 72, 25. the involvement of interchain hydrogen bonding in PAni 19 M. Wan, W. Zhou, Y. Li and J. Liu, Solid State Commun., 1992, systems. The ionic strength not only determines the degree of 81, 313. protonation due to the Donnan effect, it also affects the 20 L. Holliday, Ionic Polymers, Material Science Series, Applied Science Publishers, London, 1975.distribution of ions and, hence, the electronic structure of 21 C. Perchard and A. Novak, J. Chem. Phys., 1968, 48, 3079. protonated PAni. 22 L. J. Bellamy, Advances in Inf rared Group Frequencies, Methuen, In previous studies, the ionic strength of the medium for London, 1968, ch. 8. protonation of PAni has seldom been controlled. In a recent 23 A. B. Scranton, J. Klier and C. L. Aronson, in Polyelectrolyte Gels, electrochemical study,24 the mechanism of ion exchange during ed. R. S. Harland and R. K. Prud’homme, American Chemical redox reaction on PAni-coated electrodes has been shown to Society, Washington DC, 1992, p. 171; I. Iliopoulos and R. Audebert, Macromolecules, 1991, 24, 2566. be dependent on the ionic strength of the electrolyte solution. 24 M. H. Troise Frank and G. Denuault, J. Electroanal. Chem., 1994, In industrial applications, since PAni may be used in conditions 379, 399. of different ionic strengths, knowledge of the dependence of 25 S. Pekker and A. Janossy, in Handbook of Conducting Polymers, the conducting state of the polymer on the ionic environment ed. T. A. Skotheim, Marcel Dekker, New York, 1986, vol. 1, ch. 2. is of great importance. Apart from PAni, the formation of metallic clusters on doping is also observed in other conducting Paper 6/05565F; Received 9th August, 1996 600 J. Mater. Chem., 1997, 7(4), 597–600

ISSN:0959-9428    

DOI:10.1039/a605565f

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

a-Halogenation of triphenylene-based discotic liquid crystals: towards a chiral nucleus Neville Boden, Richard J. Bushby,* Andrew N. Cammidge, Sarah Duckworth and Gareth Headdock School of Chemistry and Centre for Self-Organising Molecular Systems, University of L eeds, L eeds L S2 9JT , UK In an attempt to prepare chiral discotic liquid crystals based on a helically twisted triphenylene nucleus a route has been developed for the introduction of a-fluoro, -chloro and -bromo substituents and it is shown that multiple a-halogenation is also possible.The monosubstituted derivatives all show enhanced mesophase stability whilst formation of the mesophase is suppressed for the polyhalogenated derivatives. Rather surprisingly, reaction of 2,3,6,7,10,11-hexahexyloxytriphenylene (HAT6) with iodine monochloride results in chlorination rather than iodination. Ferroelectric discotic liquid crystals1–3 are potentially attractive AM1 method they were 12.8 (a-F), 19.6 (a-Cl) and 25.8° (a- Br); and by the PM3 method they were 2.5 (a-F), 20.7 (a-Cl), for device applications.1 Most effort in the search for these has concentrated on the creation of chiral discogens1,3 and has 24.5 (a-Br) and 25.5° (a-NO2 ).A feature of all of these (and related) calculations is the way in which the introduction of relied on the relatively weak chirality imparted by the introduction of a chiral centre into the highly disordered side-chain the a-substituent and twisting of the nucleus induces an updown- up-down orientation of the alkoxy groups at positions region3 or by adding chiral ‘dopants’.4 Much stronger chirality would presumably be imparted if a helical twist could be 2,3,6 and 7.The effect on position 2 is purely steric and that on the remaining positions presumably minimises local dipolar imparted to the central aromatic core. Given the known chirality of 4,5-disubstituted phenanthrenes5–7 it seems reason- interactions.able to assume that suitable a-substitution of a triphenylene nucleus would convert it from a planar to a propeller-like Synthesis geometry.† Indeed this has already been demonstrated in the case of 1,12-diiodotriphenylene, where the X-ray crystal struc- Potentially the greatest twist in the triphenylene nucleus would be imparted by introducing the largest possible halogen, an a- ture shows that the nucleus possesses a strong helical twist.6 Electrophilic substitution of triphenylenes in the a position is iodo substituent.6 Hence our initial experiment was to treat 2,3,6,7,10,11-hexahexyloxytriphenylene (HAT6) 1 with iodine difficult because of steric hindrance.However, we recently reported that, in those electron-rich derivatives in which all of monochloride.10 Toour surprise we obtained not the monoiodo derivative but a mixture of 1-chloro-2,3,6,7,10,11-hexahexyl- the b-positions are blocked by alkoxy groups, the a-positions can be nitrated.8,9 Here we show that these a positions can oxytriphenylene 2, 1,8-dichloro-2,3,6,7,10,11-hexahexyloxytriphenylene 3 and 1,5-dichloro-2,3,6,7,10,11-hexahexyloxy- also be halogenated9,10 and explore the effect of a-substituents on the geometry of triphenylene-based discotic mesogens.triphenylene 4 (Scheme 1). Exhaustive treatment with iodine monochloride gave a tetrachloro derivative with two equally Because of the size of the halogeno substituents and the fact that polyhalogenated derivatives can be made this seemed the intense aryl hydrogen singlets in the 1H NMR spectrum, which must therefore be 1,4,5,9-tetrachloro-2,3,6,7,10,11-hexahexyl- best path to discogens with chiral nuclei.oxytriphenylene 5 (Scheme 2). Although iodine monochloride normally acts as an iodinating agent12 it is known that in Calculations some circumstances it can act as a chlorinating agent,13,14 particularly if the substrate is one that is readily oxidised. Whereas simple model-building suggests that the a-nitro group in 1-nitro-2,3,6,7,10,11-hexahexyloxytriphenylene could be These reactions probably involve an initial one-electron oxidation step and proceed through the aryl radical cation.13 In accommodated without upsetting the planarity of the aromatic core, provided it was orthogonal to the p-system,8 MNDO support of this notion, we noted that an intense green coloration, typical of the (HAT6)V+ radical cation,15 developed in and PM3 calculations on 1-nitro-2,3,6,7,10,11-hexamethoxytriphenylene suggest that there is a lower energy minimum in the course of our reaction.We suggest that the most reasonable mechanism is one in which the iodine monochloride oxidises which the nitro group remains conjugated with the p-system of the aromatic nucleus but in which the nucleus develops a the HAT6 to the radical cation (HAT6)V+, which then undergoes nucleophilic attack by chloride.10 Further support helical twist (Plate 1)11 with the nitro group lying below the opposing peri-hydrogen.Similar calculations in which the for this mechanism is provided by an alternative synthesis of geometries of the a-fluoro, a-chloro and a-bromo derivatives of hexamethoxy triphenylene were optimised all show a similar chiral twist (Plate 2) increasing in magnitude along the series a-F>a-Cl>a-Br>a-NO2.The calculated dihedral angles C(12)MC(12a)MC(12b)MC(1) for the chloro, bromo and nitro derivatives proved fairly independent of the MO method employed but for the flouro derivative the calculated twist was significantly method-dependent.The calculated dihedral angles C(12)MC(12a)MC(12b)MC(1) by the MNDO method were 23.9 (a-F), 25.4 (a-Cl), 25.9 (a-Br) and 28.5° (a-NO2 ); by the † Professor K. Praefcke has independently concluded that a-halogenation provides a potential route to chiral triphenylene-based discotics Plate 1 Energy minimised structure of 1-nitro-2,3,6,7,10,11-hexa- and we thank him for helpful discussions and communication of unpublished results.methoxytriphenylene J. Mater. Chem., 1997, 7(4), 601–605 601Scheme 1 Scheme 2 using bromine in carbon tetrachloride at low temperature.11 If HAT6 1 is treated with excess bromine in dichloromethane at room temperature a tribromide is obtained showing three equal aromatic hydrogen singlets in the 1H NMR spectrum and which is assigned the structure 7 (Scheme 3).The alternative structures, the 1,4,5- and 1,8,9-tribromides, which would also formally account for the NMR spectrum, seem highly improbable on steric grounds. The fact that this tribromo compound 7 and not the symmetrical 1,5,9-tribromo derivative was formed seems surprising. It may well be that the Plate 2 Energy minimised structure of (a) 1-nitro-2,3,6,7,10,11-hexamethoxytriphenylene, (b ) 1-chloro-2,3,6,7,10,11-hexamethoxytriphenylene and (c ) 1-bromo-2,3,6,7,10,11-hexamethoxytriphenylene the monochloride 2 in which the reaction is deliberately carried out in a two-step manner.HAT6 1 is oxidised to (HAT6)V+ with [bis(trifluoroacetoxy)iodo]benzene and then the reaction worked up with tetrabutylammonium chloride.16 Praefcke et al.have now reported a more conventional synthesis of the monochloride 2 using aluminium trichloride and sulfuryl chloride in 1,2-dichlorobenzene and of the corresponding monobromide (1-bromo-2,3,6,7,10,11-hexahexyloxytriphenylene 6) Scheme 3 602 J. Mater. Chem., 1997, 7(4), 601–605Scheme 5 Plate 3 Energy minimised structure of 1,4,8-tribromo-2,3,6,7,10,11- hexamethoxytriphenylene dibromo compound 13 (Scheme 5).The preference for attack in the b-position is not surprising and presumably reflects ‘steric control’. Discussion The phase behaviour of the new compounds reported in this paper and of related systems is summarised in Table 1. The polyhalogeno derivatives 5 and 7 proved to be oils at room temperature and the derivatives 12 and 13 showed no liquid crystal behaviour.The monofluoro compound 10, monochloro compound 2 and monobromo compound 6 are crystalline solids which show enantiotropic behaviour, exhibiting a columnar liquid crystal phase with a polarising microscopy texture typical of that of a Dh phase. Miscibility studies on the Scheme 4 monochloro and monobromo derivatives 2 and 6 confirm this assignment.11 It is interesting to note that, despite the presumed non- regiochemistry of some of these reactions is determined by planarityof the nucleus, most of these a-substituted compounds steric rather than electronic factors.MNDO calculations on still show an enhanced mesophase range as compared to the 1,4,8-tribromo-2,3,6,7,10,11-hexamethoxytriphenylene pro- parent compound HAT6 1.Within the series of halogenated duced two energy minima. The absolute minimum corre- compounds, the K–D transition is essentially constant, whereas sponds to the normal propeller-shaped conformation, and is the D–I transition temperature steadily decreases. The effect shown in Plate 3. The second local minimum is one in which of substituents on the stability of the Dh phase is complex and the ‘stacking’ of the bromine at C(8) and the hydrogen at not fully understood but, in general, electron-withdrawing C(9) are reversed.On the assumption that the reversal of this groups tend to exert a stabilising effect.11 Local dipolar inter- arrangement has a reasonably high energybarrier it ispossible actions may also be important in stabilising a columnar that formulae 7a and 7b represent isolable isomers.This is arrangement.8 Hence, relative to HAT6 1, the mononitro potentially an unusual form of isomerism. compound 10 shows a higher clearing point and this may be We have not attempted to prepare 1-fluoro-2,3,6,7,10,11- attributed to the stabilisation of the columnar stacks through hexahexyloxytriphenylene 10 by direct substitution, but it is strong local (opposed) dipolar interactions or alternatively to easy to prepare using the iron(III)-mediated coupling of the fact that this is the substituent with the strongest electron- 3,3¾,4,4¾-tetrahexyloxybiphenyl 8 and 1-fluoro-2,3-dihexylox- withdrawing effect.In the cases of the monofluoro compound ybenzene 9 (Scheme 4).17 The 1H NMR spectrum of this 10, the monochloro compound 2 and the monobromo com- product showed one low field aromatic hydrogen at d 8.51 pound 6, the stability of the columnar arrangement decreases with a splitting of 8 Hz.This splitting could plausibly be as the steric bulk of the substituent increases, the strength of interpreted as the result of an ortho coupling and raises the the local dipolar interactions decreases and the electron- possibility that the product did not have the desired structure, withdrawing effect of the substituent decreases.An a-methyl but instead was the alternative coupling product 3-fluoro- substituent destroys the liquid crystal behaviour altogether.17 1,2,6,7,10,11-hexahexyloxytriphenylene. However, this seems This weak donor substituent is large enough to destroy the unlikely in view of the absence of the peak at ca.d 9 which planarity of the nucleus but not polar enough to give a is characteristic of triphenylenes containing an a-alkoxy sub- significant dipole. stituent.17 The correctness of the assigned structure was Direct proof of the chirality of these systems has yet to be confirmed by NOE experiments.‡ The signal at d 8.51 proved achieved although it is clear from our calculations that they to be that for the hydrogen at the 12-position and the splitting cannot be planar.This is further supported by the X-ray crystal to be due to a through-space coupling between the fluorine structure of 1,12-diiodotriphenylene.6 In terms of a practicable, and hydrogen.18 resolved chiral system it is important to note that the 4,5- We have also investigated the substitution reactions of disubstituted phenanthrenes all racemise at relatively low derivatives without the symmetrical 2,3,6,7,10,11-substitution temperatures.5,7 The calculated barrier11 to racemisation for pattern.17 In the case of 1,4,6,7,10,11-hexahexyloxytripheny- 1,12-dichloro-2,3,6,7,10,11-hexamethoxytriphenylene is 179 kJ lene 11 treatment with iodine monochloride gives the monoch- mol-1, suggesting that it may be possible to resolve compounds loro compound 12 and treatment with bromine gives the such as the tetrachloro derivative 5 at room temperature.For a practical chiral discotic liquid crystal based on these prin- ‡ These assignments have been independently verified by more ciples it seems important to introduce large substituents and extensive NMR studies of our product by Dr J. Jakupovic and Professor K.Praefcke, Technische Universita�t, Berlin. preferably multiple large substituents that do not destroy the J. Mater. Chem., 1997, 7(4), 601–605 603Table 1 Transition temperatures of compounds as determined by optical microscopy (K=crystal phase, Dh=discotic hexagonal liquid crystal phase and I=isotropic liquid phase) compound K–Dh Dh–I K–I 2,3,6,7,10,11-hexahexyloxytriphenylene 1 67 100 — 1-nitro-2,3,6,7,10,11-hexahexyloxytriphenylenea <25b 136 1-fluoro-2,3,6,7,10,11-hexahexyloxytriphenylene 10 39 116 — 1-chloro-2,3,6,7,10,11-hexahexyloxytriphenylene 2 37c 98 — 1-bromo-2,3,6,7,10,11-hexahexyloxytriphenylene 6d 37 83 — 1-methyl-2,3,6,7,10,11-hexahexyloxytriphenylenee — — 60 1,4,5,9-Tetrachloro-2,3,6,7,10,11-hexahexyloxytriphenylene 5 — — <25 1,4,8-Tribromo-2,3,6,7,10,11-hexahexyloxytriphenylene 7 — — <25 2-chloro-1,4,6,7,10,11-hexahexyloxytriphenylene 12 — — 52 2,3-dibromo-1,4,6,7,10,11-hexahexyloxytriphenylene 13 — — 70 aRef. 8. bRef. 11 gives 43°C. cRef. 11 gives 31 °C.dRef. 11. eRef. 17. liquid crystal behaviour. Unfortunately, the tetrachloro and t, J 7, CH3 ), 1.4–1.6 (36H, m, CH2), 1.95 (12H, m, CH2 ), 4.1–4.3 (12H, m, OCH2), 7.70 (2H, s, ArH), 7.71 (2H, s, ArH), tribromo derivatives 5 and 6 have proved to be oils. The discovery of other polybrominated and polychlorinated deriva- 8.65 (2H, s, Ar-H 9 and 12 of 3), 8.80 (1H, s, Ar-H), 8.83 (1H, s, Ar-H).tives that are liquid crystalline seems to offer the most promising way forward. Synthesis of 2 using [bis(trifluoroacetoxy) iodo]benzene Experimental 2,3,6,7,10,11-Hexahexyloxytriphenylene 1 (1.0 g, 1.2 mmol) was added to dichloromethane (45 cm3) and cooled to 0°C before Phase behaviour was determined on an Olympus BH-2 micro- [bis(trifluoroacetoxy)iodo]benzene (0.6 g, 1.4 mmol) was scope with a Mettler FP82HT hotstage.Samples for combus- added and stirred for 15 min. Whilst maintaining the tempera- tion analyses were routinely dried at 25°C and 0.3 mmHg. ture below 3°C, tetrabutylammonium chloride (0.67 g, NMR Spectra were recorded on a General Electric QE300 2.4 mmol) was added and stirred for 15 min before being or a Bruker AM400 instrument.Chemical shifts (d) are given allowed to warm to room temperature and stirred for a further relative to tetramethylsilane, coupling constants (J) are given 20 min. The reaction mixture was poured onto methanol in Hz and the solvent used was CDCl3 (0.03% SiMe4). (100 cm3) and the solvents removed in vacuo. The residual Mass spectra were obtained on a VG Autospec instrument solid was purified by column chromatography on silica eluting and were recorded by the mass spectrometry staff of the School with 7% v/v diethyl ether–light petroleum and recrystallised of Chemistry, University of Leeds.All peaks >20% are from ethanol to give l-chloro-2,3,6,7,10,11-hexahexyloxytriphen- reported. ylene 2 (0.42 g, 40%) as a white solid, which exhibited spectro- Column chromatography on silica refers to the use of Merck scopic data identical to those of a previously prepared sample Kieselgel Type 60.of 2. Except for the dichloro derivatives 3 and 4, 1H NMR and thin layer chromatography (TLC) showed that all of the 1,4,5,9-Tetrachloro-2,3,6,7,10,11-hexahexyloxytriphenylene 5 halogenation products were obtained as single isomers. Light petroleum refers to the 40–60 °C boiling fraction. 2,3,6,7,10,11-Hexahexyloxytriphenylene 1 (1.0 g, 1.2 mmol) was dissolved in dichloromethane (40 cm3) and stirred whilst iodine Molecular orbital calculations monochloride (excess) was added. The reaction was followed by TLC eluting with 40% v/v dichloromethane–light pet- The geometries shown in Plates 1–3 were optimised at the roleum. Once the reaction had been driv to one product (as PM3 level.shown by TLC), saturated aqueous sodium metabisulfite (40 cm3) was added and stirred for 20 min. After separation of 1-Chloro-2,3,6,7,10,11-hexahexyloxytriphenylene 2 the organic phase and extraction of the aqueous phase with 2,3,6,7,10,11-Hexahexyloxytriphenylene 1 (1.0 g, 1.2 mmol) was dichloromethane (3×25 cm3), the combined dichloromethane dissolved in dichloromethane (60 cm3) after which iodine fractions were dried (MgSO4) and the solvent evaporated to monochloride (0.30 g, 1.8 mmol) was added in one portion. give a dark oil.This oil was purified by column chromatogra- The mixture was stirred for 20 min before saturated aqueous phy on silica eluting with 25% v/v dichloromethane–light sodium metabisulfite (60 cm3) was added and stirred for a petroleum to give the title compound 5 (0.51 g, 44%) as a further 20 min. The organic phase was separated and the colourless oil; dH 0.95 (18H, t, J 7, CH3 ), 1.3–1.6 (36H, m, aqueous layer extracted with dichloromethane (3×20 cm3), CH2), 1.90 (12H, m, CH2), 3.9–4.3 (12H, m, OCH2), 8.53 (1H, the combined organic phase and extracts then being dried with s, Ar-H), 9.01 (1H, s, Ar-H); m/z 966 (M+, 100%).magnesium sulfate. After removal of solvents in vacuo the residual oily solid was purified by column chromatography on 1,4,8-Tribromo-2,3,6,7,10,11-hexahexyloxytriphenylene 7 silica eluting with 7% v/v diethyl ether–light petroleum and recrystallised from ethanol to give the title compound 2 (0.26 g, 2,3,6,7,10,11-Hexahexyloxytriphenylene 1 (0.4 g, 0.48 mmol) was stirred in dichloromethane (20 cm3) whilst bromine (6 25%) as a white solid, K–D 37°C, D–I 98°C (Found: C, 75.05; H, 9.6.C54H83ClO6 requires C, 75.1; H, 9.7%); dH 0.95 (18H, drops) was added carefully, and then stirred for a further 1.5 h. The solvent was removed in vacuo and the residue purified by t, J 7, CH3), 1.4–1.6 (36H, m, CH2 ), 1.95 (12H, m, CH2 ), 4.15 (2H, t, J 6.5, OCH2), 4.25 (10H, m, OCH2), 7.8 (4H, s, Ar-H), column chromatography on silica eluting with 50% v/v dichloromethane–light petroleum to give the title compound 9.05 (1H, s, Ar-H); m/z 862 (M+, 100%).An inseparable mixture of 1,8-dichloro-2,3,6,7,10,11-hexahexyl- 7 (0.15 g, 29%) as a pale yellow oil (Found: C, 60.75; H, 7.95. C54H81Br3O6 requires: C, 60.85; H, 7.66%); dH 0.95 (18H, t, J oxytriphenylene 3 and 1,5-dichloro-2,3,6,7,10,11-hexahexyloxytriphenylene 4 (0.13 g, 12%) was also isolated as the first 7, CH3), 1.3–1.6 (36H, m, CH2), 1.9 (12H, m, CH2), 4.05–4.25 (12H, m, OCH2), 8.4 (1H, s, Ar-H), 8.48 (1H, s, Ar-H), 8.64 fraction off the column (Found: C, 72.35; H, 9.15; Cl, 8.15.C54H82Cl2O6 requires C, 72.21; H, 9.2; Cl, 7.9%); dH 0.95 (18H, (1H, s, Ar-H); m/z 1066 (M+, 100%). 604 J. Mater. Chem., 1997, 7(4), 601–6051-Fluoro-2,3-dihexyloxybenzene 9 The solvents were removed in vacuo and the resulting crude product purified by column chromatography on silica eluting 3-Fluorocatechol (5 g, 0.039 mol), 1-bromohexane (14.2 g, with 30% v/v dichloromethane–light petroleum. Recrystal- 0.086 mol) and potassium carbonate (11.8 g, 0.086 mol) were lisation from ethanol gave the title compound 12 (0.19 g, 28%) added to ethanol (100 cm3) and heated under reflux for 3 days.as a white solid, mp 70°C (Found: C, 65.5; H, 8.55; Br, 15.9. After cooling, dichloromethane (100 cm3) was added and the C54H82Br2O6 requires: C, 65.7; H, 8.37, Br, 16.2%); dH 0.9 solid residues removed by filtration through Celite.Once the (18H, t, J 7, CH3), 1.3–1.7 (36H, m, CH2 ), 1.85 (12H, m, CH2 ), solvents had been removed in vacuo, the oily residue was 3.73 (4H, t, J 6.5, OCH2 ), 4.2 (8H, m, OCH2), 7.7 (2H, s, Ar-H purified by column chromatography on silica eluting with 30% 8 and 9), 8.98 (2H, s, Ar-H 5 and 12); m/z 986 (M+, 100%). v/v dichloromethane–light petroleum to give the title compound (10.5 g, 91%) as a colourless oil (HRMS: found M+, We thank the EPSRC for financial support. 296.2149. C18H29FO2 requires M, 296.2152); dH 0.9 (6H, t, J 7, CH3), 1.3–1.5 (12H, m, CH2 ), 1.8 (4H, m, CH2), 4.0 (4H, m, OCH2), 6.6–6.7 (2H, m, Ar-H), 6.91 (1H, m, Ar-H); m/z 296 References (M+, 100%). 1 (a) K. Bock and W. Helfrich, L iq. Cryst., 1992, 12, 697; (b) L iq.Cryst., 1995, 18, 387; (c) X. H. Chen and G. Scherowsky, J. Mater. 1-Fluoro-2,3,6,7,10,11-hexahexyloxytriphenylene 10 Chem., 1995, 5, 417. 2 (a) H. Zimmermann, R. Poupko, Z. Luz and J. Billard, 3,3¾,4,4¾-Tetrahexyloxybiphenyl 8 (1.0 g, 1.8 mmol) and 1- Z. Naturforsch., 1986, 41A, 1137; (b) J. Malthete and A. Collet, fluoro-2,3-dihexyloxybenzene 9 (2.14 g, 7.2 mmol) were added J.Am. Chem. Soc., 1987, 109, 7544. to a stirred suspension of ferric chloride (2.35 g, 14.4 mmol) in 3 (a) J. Malthe�te, J. Jacques, N. H. Tinh and C. Destrade, Nature, dichloromethane (60 cm3). After stirring for 2 h the crude 1982, 298, 46;(b) C. F. Van Nostrum, A. W.Bosman, G. H. Gelinck, P. G. Schouten, J. M. Warman, A. P. M. Kentgens, reaction product was precipitated by adding the reaction M.A. C. Devilliers, A. Meijerink, S. J. Picken, U. Sohling, mixture to methanol (200 cm3). This precipitate was filtered A. J. Schouten and R. J. M. Nolte, Chem. Eur. J., 1995, 1, 171. and washed with methanol before being allowed to dry and 4 (a) N. Usol’tseva, K. Praefcke, D. Singer and B. Gu�ndogan, L iq. purified by column chromatography on silica eluting with 8% Cryst., 1994, 16, 617; (b) K.Praefcke, D. Singer and A. Eckert, L iq. v/v diethyl ether–light petroleum. Subsequent recrystallisation Cryst., 1994, 16, 53. from ethanol afforded the title compound 10 (1.05 g, 69%) as 5 M. S. Newman and A. S. Hussey, J. Am. Chem. Soc., 1947, 69, 3023. 6 A. J. Ashe, J. W. Kampf and P. M. Savla, J. Org. Chem., 1990, a white solid, K–D 39°C, D–I 116°C (Found: C, 76.8; H, 9.95; 55, 5558. F, 2.3.C54H83FO6 requires: C, 76.55; H, 9.87; F, 2.24%); dH 7 A. H. A. Tinnemans and W. H. Laarhoven, T etrahedron, 1979, 0.95 (18H, t, J 7, CH3), 1.3–1.64 (36H, m, CH2 ), 1.95 (12H, 35, 1535. m, CH2), 4.15–4.3 (12H, m, OCH2), 7.67 (1H, s, Ar-H), 7.8 8 (a) N. Boden, R. J. Bushby and A. N. Cammidge, Mol. Cryst. L iq. (2H, s, Ar-H), 7.85 (1H, s, Ar-H), 8.51 (1H, d, J 8, Ar-H); m/z Cryst., 1995, 260, 307; (b) N.Boden, R. J. Bushby and 846 (M+, 100%). A. N. Cammidge, L iq. Cryst., 1995, 18, 673; (c) N. Boden, R. J. Bushby, A. N. Cammidge and G. Headdock, J. Mater. Chem., 1995, 5, 2275. 2-Chloro-1,4,6,7,10,11-hexahexyloxytriphenylene 12 9 N. Boden, R. J. Bushby, A. N. Cammidge and G. Headdock, UK 1,4,6,7,10,11-Hexahexyloxytriphenylene 11 (0.5 g, 0.6 mmol) Pat.GB 9505940.8, 1995. 10 N. Boden, R. J. Bushby, A. N. Cammidge and G. Headdock, was dissolved in dichloromethane (30 cm3) and cooled to 0°C T etrahedron L ett., 1995, 8685. (ice–salt slurry). Iodine monochloride (0.12 g, 7.4 mmol) was 11 K. Praefcke, A. Eckert and D. Blunk, L iq. Cryst., in the press. added dropwise in dichloromethane solution (ca. 2 cm3) and 12 (a) R. B. Sandin, W. V. Drake and F. Leger, Org. Synth., 1943, Coll. stirred for 20 min at 0°C before being allowed to warm to Vol. II, 196; (b) G. H. Woollet and W. W. Johnson, Org. Synth., room temperature. Saturated aqueous sodium metabisulfite 1943, Coll. Vol. II, 343. (35 cm3) was added and the organic phase separated and 13 D. E. Turner, R. F. O’Malley, D. J.Sardella, L. S. Barinelli and P. Kaul, J. Org. Chem., 1994, 59, 7335. washed with water before the solvents were removed in vacuo. 14 (a) P. R. Birkett, A. G. Avent, A. D. Darwish, H. W. Kroto, The resulting crude product was purified by column chroma- R. Taylor and D. R. M. Walton, J. Chem. Soc., Chem. Commun., tography on silica eluting with 20% v/v dichloromethane– 1993, 1230; (b) J.Chem. Soc., Chem. Commun., 1995, 683. light petroleum and recrystallised from ethanol to give the title 15 (a) N. Boden, R. C. Borner, D. R. Brown, R. J. Bushby and compound 12 (0.10 g, 19%) as a white solid, mp 52°C (Found: J. Clements, L iq. Cryst., 1992, 11, 325; (b) N. Boden, R. J. Bushby, C, 74.9; H, 9.7; Cl, 4.05. C54H83ClO6 ires: C, 75.1; H, 9.7, J. Clements and R. Luo, J.Mater. Chem., 1995, 5, 1741. 16 Y. Kita, H. Tohma, K. Hatanaka, T. Takada, S. Fujita, S. Mitoh, Cl, 4.10%); dH 0.91 (18H, t, J 7, CH3), 1.4–1.6 (36H, m, CH2), H. Sakurai and S. Oka, J. Am. Chem. Soc., 1994, 116, 3684. 1.80 (12H, m, CH2), 4.1–4.3 (12H, m, OCH2), 7.1 (1H, s, Ar- 17 (a) N. Boden, R. J. Bushby and A. N. Cammidge, J. Chem. Soc., H), 7.7 (1H, s, Ar-H), 7.8 (1H, s, Ar-H), 9.1 (1H, s, Ar-H), 9.17 Chem. Commun., 1994, 465; (b) N. Boden, R. J. Bushby, (1H, s, Ar-H); m/z 862 (M+, 100%). A. N. Cammidge and G. Headdock, Synthesis, 1995, 31. 18 (a) G. W. Gribble and W. J. Kelly, T etrahedron L ett., 1985, 26, 2,3-Dibromo-1,4-6,7,10,11-hexahexyloxytriphenylene 13 3779; (b) I. D. Rae, A. Staffa, A. C. Diz, C. B. Giribet, M. C. Ruiz de Azua and R. H. Contreras, Aust. J. Chem., 1987, 40, 1923; 1,4,6,7,10,11-Hexahexyloxytriphenylene 11 (0.57 g, 0.7 mmol) (c) I. D. Rae, J. A. Weigold, R. H. Contreras and G. Yamamoto, was added to dichloromethane (30 cm3) and cooled to 0°C. Magn. Reson. Chem., 1992, 30, 1047. Bromine (ca. 0.5 cm3) was added dropwise and the reaction stirred for 3 h before being poured onto methanol (150 cm3). Paper 6/06447G; Received 18th September, 1996 J. Mater. Chem., 1997, 7(4), 601–605 605

ISSN:0959-9428    

DOI:10.1039/a606447g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Liquid crystalline derivatives of oligoethylene-amines and -amino ethers with amide, ester, urea or urethane functions Uwe Stebani,a Gu�nter Lattermann,*a Michael Wittenbergb and Joachim Heinz Wendorffb aMakromolekulare Chemie I, Universita�t Bayreuth, D-95440 Bayreuth, Germany bInstitut fu� r Physikalische Chemie und Wissenschaftliches Zentrum fu� r Materialwissenschaften, Philipps-Universita�t Marburg, D-35032 Marburg, Germany The mesomorphism of diethylenetriamine and triethylenetetramine derivatives, substituted with the 3,4-bis(decyloxy)benzoyl group (‘two chain’ substituent) via amide, ester, urea or urethane moieties, is described.Furthermore, different examples of related linear and cyclic oligoethyleneamino ethers are investigated and compared with the mesomorphism of the first group.Both lamellar smectic A and hexagonal columnar mesophases can be observed in linear compounds, depending on the length of the linear unit. A cyclic derivative displays a cubic phase. The conclusion is emphasized that the mesomorphism of these classes of compounds is caused by microphase separation. Several groups have recently described liquid crystalline 4 reveals that with a decreasing number of hydrogen bonds, the melting temperatures decrease.In 4, the crystalline phases cyclic,1–14 linear12,13,15–17 and branched13,18 oligo- or polyalkyleneamides, which are an interesting class of mesogens disappear after the first heating. Whereas 2,2,2-tet 2 exhibits the highest melting temperature of all the compounds shown because of several reasons: (i) they do not fulfil the criteria of conventional thermotropic mesomorphism, i.e.they do not in Table 1, crystalline phases are no longer detectable in the analogous 2,2,2-diester 5. Although the clearing temperatures exhibit a rigid, anisometric architecture of classical rod-like or disc-like single molecules (molecular mesogens); (ii) they cannot follow the same pattern, the mesophases do not disappear in compounds 3, 4 and 5, but instead become dominating in be classified in general as supramolecular mesogens, forming aggregates of non-mesogenic single molecules via strong inter- ‘pseudo’ enantiotropic 4 and enantiotropic 5.Under the polarizing microscope, for compounds 3 and 4 action forces, e.g. hydrogen bonding, ionic or charge transfer forces; (iii) derivatives of the considered group of mesogenic broken fan-shaped textures can be observed. The 2,2,2-diester 5 exhibits a broken spherulithic texture.oligo- or poly-alkyleneamines can either be converted to the corresponding ionic liquid crystals18,19 or can be used as ligands in a variety of new groups of metallomesogens.14,20–24 Here we describe materials which give evidence for (i) and (ii). Results Ester endgroup derivatives To investigate the influence of terminal hydrogen bonding on the mesophase structure of the 3,4-bis(decyloxy)benzoyl (‘two chain’) substituted diethylenetriamine 1 (2,2-tri)15 and on a postulated ring closure12,13 of the triethylenetetramine derivative 2 (2,2,2-tet),16 we synthesized analogues of 1, i.e.the monoester 3 (2,2-monoester), the diester 4 (2,2-diester) and the analogue of 2 with two ester endgroups 5 (2,2,2-diester). Thermal behaviour.Polarizing microscopy and differential scanning calorimetry (DSC) measurements revealed monotropic mesophases for 2,2-monoester 3 and 2,2-diester 4 and an enantiotropic mesophase for 2,2,2-diester 5. Their transition temperatures together with those of 2,2-tri 115 and 2,2,2-tet 216 are summarized in Table 1.The monotropic mesophase of 2,2-monoester 3, followed by a rapid crystallization, is detectable only under the polarizing microscope. In DSC measurements, only a slight shoulder on the crystallization peak appears (even at different heating rates). The 2,2-diester 4 exhibits crystalline phases only on the first heating, which do not reappear on further heating, i.e.the ‘pseudo enantiotropic’ mesophase can be frozen in below its glass temperature at Tg=5 °C (DCp=0.98 kJ mol-1 K-1). 2,2,2-Diester 5 exhibits no crystalline phase from the beginning. It shows a glass transition temperature at Tg=18 °C (DCp= 1.29 kJ mol-1 K-1). A comparison of the transition temperatures of 1 with 3 or J.Mater. Chem., 1997, 7(4), 607–614 607Table 1 Transition temperatures, DCp values and DH values of compounds 1, 2, 3, 4 and 5; Tg, glass transition temperature; K, crystalline phases; M, mesophase; I, isotropic phase transition temp/°C (DH/kJ mol-1) compound Tg/°C (DCp/kJ mol-1 K-1) K1 K2 K3 M I 1a — $ 90.6 (65.9) $ 85.6e (2.4) $ 2b 52 (0.33) $ 59.0c (25.2) $ 93.5 (21.1) $ 104.0 (3.0) $ 3 — $ 52.0 (26.5) $ 83.0 (38.5) $ 66d,e $ 4 5 (0.98) $ 39.5c $ 53.5c (59.3)f $ 60.5c (0.8) $ 35.5e (2.6) $ 5 18 (1.29) $ 62.5 (5.1) $ aRef. 15. bRef. 16. cOnly on first heating. dDetectable only under the polarizing microscope. eMonotropic. fDH for K1 and K2. Table 2 X-Ray data for compounds 1, 2, 4 and 5 distancea/A° lattice constant compound d100 d110 d200 d210 d220 d300 dlayer (T /°C) ahex (T/°C) 1b 30.1 15.1 10.1 30.2 (85) 2b 28.4 16.8 14.6 11.2 8.4 9.7 33.6 (90) 4 30.6 15.4 10.2 30.7 (25) 5 30.5 18.4 15.6 12.2 10.6 36.9 (20) adhk0, lattice spacings; dlayer, layer distance.bRef. 16, 25. X-Ray investigations. The monotropic phase behaviour did Thermal behaviour. Polarizing microscopy and DSC measurements revealed enantiotropic mesophases for both 6 not allow the determination of the mesophase structure of compound 3.For 4 and 5 the results are summarized in and 7. Their transition temperatures are summarized in Table 3. With respect to 2, compound 6 shows crystalline phases Table 2, together with the values for 1 and 2.16,25 Likewise to the analogous 2,2-tri 1,16 the presence of a only on first heating, with a lower melting temperature. This behaviour is contrary to the expectation that a larger number lamellar smectic A mesophase in 2,2-diester 4, which was deduced from the broken fan shaped texture, is supported by of possible hydrogen bondings would favour crystallinity in 6 with respect to 2.The clearing temperature of the mesophase, the first- to third-order reflections (d100, d200 and d300) in the diffractogram. In analogy to 2,2,2-tri 2,16 the 2,2,2-diester 5 which can be frozen in at room temperature, is slightly raised by about 4°C.The value of Tm for 7 is increased not only with displays a hexagonal columnar phase (Colh), characterized by the additional d110 and d210 reflections. respect to 2 but also to 6, although the number of possible hydrogen bonds is decreased with respect to 6. Although this Except for the influence on the transition temperatures, the formal exchange of amide versus ester endgroups, i.e.the behaviour would indicate a stabilized crystalline phase, it appears only on first heating. As a consequence, the mesophase impossibility for the formation of hydrogen bonding, has no further influence on the mesophase. can be frozen in at room temperature too. The clearing temperature for 7 is increased with respect to 2 and 6.Apparently, the number of possible hydrogen bonds does not Derivatives with urea and urethane substituents relate in a simple way to the thermal behaviour of linear A second possibility for the investigation of the role of hydrogen oligoethylene amine derivatives. bonding is not to reduce but to enhance the number of proton Under the polarizing microscope both compounds exhibit donors and acceptors. Therefore, we synthesized compounds spherulithic textures, as shown in Plates 1 and 2. 6 (2,2,2-urea) and 7 (2,2,2-urea/urethane), whose ‘two chain’ substituents are linked to the triethylenetetramine backbone X-Ray investigations. Compounds 6 and 7 display hexagonal via urea or urethane moieties instead of amide groups.columnar mesophases (Colh) with lattice parameters given in Table 4. With respect to 2,2,2-diester 5, ahex for 6 and 7 is found to be in the same range. Apparently, the formation of a hexagonal columnar mesophase depends only on the number of substituents, i.e. four in 2, 5, 6 and 7, independent of the nature of the linking groups betwekbone and the ‘two chain’ substituents and, in consequence, independent of the number of possible hydrogen bonds.In the following, we investigate the consequence of further variations of the molecular structure of this type of mesogen. Derivatives with oxobridges We synthesized the linear analogues of 2,2-tri, 2,2,2-tet and 2,2,2-urea with oxobridges, i.e. compounds 8, 9 and 11.Furthermore, the cyclic ethyleneamino ethers 12 and 14 with a different number of amide groups and 13 with urea groups were investigated. To compare the influence of the number of alkoxy chains, a ‘three chain’ compound 10, related to 9, was synthesized. 608 J. Mater. Chem., 1997, 7(4), 607–614Table 3 Transition temperatures, DCp values and DH values in 2,2,2-urea 6 and 2,2,2-urea/urethane 7; Tg, glass transition temperature; K, crystalline phases; M, mesophase; I, isotropic phase transition temp/°C (DH/kJ mol-1) compound Tg/°C (DCp/kJ mol-1 K-1) K1 K2 Colh I 6 44 (0.57) $ 57.0a $ 75.0a (38.9)b $ 108.0 (1.6) $ 7 39 (0.56) $ 109a $ 115.0 (2.7) $ aOnly on first heating.bDH for K1 and K2. Plate 1 Optical texture of the mesophase after cooling from the isotropic phase for 2,2,2-urea 6, T=106 °C Plate 2 Optical texture of the mesophase after cooling from the isotropic phase for 2,2,2-urea/urethane 7, T=100°C Thermal behaviour.None of the linear oligoethyleneamino peratures. Going from 8 to 9 and maintaining a constant number of substituents and possible hydrogen bonds, increas- ethers 8–11 displays mesomorphism, as shown in Table 5.Comparing the linear compounds 8 with 1 and 9 with 2, we ing the number of oxobridges in 9 with respect to 8 increases the melting temperature. Comparing cyclic derivatives 12 and observe that with an equal number of possible hydrogen bonds, the compounds with oxobridges exhibit higher melting tem- 14 with 15, we observe that, contrary to the linear compounds, J.Mater. Chem., 1997, 7(4), 607–614 609Table 4 X-Ray data for compounds 6 and 7 of possible hydrogen bondings going from 9 to 11 is favourable for the formation of a mesophase. distancea/A° Among the cyclic derivatives, compounds 12 and 13 are not lattice constant liquid crystalline. Apparently, in 12 compared to the enanti- compound d100 d110 d200 d210 d300 ahex (T/°C) otropic compounds 14 and 15, the number of ‘two chain’ 6 32.10 18.90 16.10 12.34 10.92 36.9 (85) substituents is too small to induce mesomorphism, i.e.the 7 32.30 18.08 15.86 — — 37.0 (100) minimum number is three substituents, as shown for compound 14. Again, as with the linear compound 11, increasing the adhk0, lattice spacings. number of possible hydrogen bonds going from 12 to 13 does not induce the formation of a mesophase.Under a polarizing microscope, 14 exhibits an isotropic, viscous phase. At the ‘clearing’ temperature of 67°C, which is indicated by a sharp peak in DSC measurements, a strong decrease in its viscosity can be observed under the microscope. These observations are typical for a cubic phase. Preliminary X-ray measurements in the very small mesophase range gave no information on the space group of the cubic phase.The tetrasubstituted cyclen derivative 15 has been shown to exhibit a hexagonal columnar mesophase (Colh).12 The thermal behaviour of the derivatives with oxobridges demonstrates that a too small number of substituents with alkoxy chains does not favour the formation of a liquid crystalline phase, presumeably due to insufficient space filling in the outer sphere of the molecule.This factor seems to be dominant with respect to the influence of hydrogen bonds. Discussion and Conclusion Our results concerning oligoethylene-amine and -amino ether derivatives with decyloxy sidechains can be summarized as follows. (i) In linear derivatives, the presence of oxobridges (compare 8 with 1, 9 with 2 and 11 with 6) increases the melting temperature, contrary to the behaviour of cyclic compounds (compare 12 and 14 with 15).In both classes, an increasing number of oxobridges (compare 9 with 8 and 12 with 14) increases the melting temperature. (ii) In linear derivatives, changing from amide to urea groups (comparing 2 with 6 and 9 with 11) decreases the melting temperature (in the case of 6 only on first heating), contrary to the behaviour of cyclic compounds (compare 12 with 13). On first heating, compound 7 with urea/urethane functions exhibits the highest melting temperature with respect to 2 and 6.(iii) Comparing linear derivatives 1 with 4 and 2 with 5 it is evident that hydrogen bonding influences the transition temperatures, but is not essential for the appearance of a mesophase. (iv) The existence of mesomorphism for the linear 2,2-tri compound 1 but not for analogous 8, and for the cyclic analogue 14 but not for 12, demonstrates that a minimum number of three ‘two chain’ substituents is necessary to obtain liquid crystallinity.Even additional hydrogen bonds in compound 11 with respect to 9 and in 13 with respect to 12, or a larger number of decyl side chains at the endgroups in the ‘three chain’ compound 10, are apparently not able to compen- the presence of oxobridges in 12 and 14 decreases the melting sate for the two small number of side chains along the molecule, temperatures.i.e. in this class of related compounds a minimum number of Changing from amide to urea groups in linear compounds, three ‘two chain’ substituents is the dominating factor for the i.e.from 2 to 6 or from 9 to 11, decreases the melting existence of mesomorphism. The absence of mesomorphism in temperatures, though the number of possible hydrogen bonds related sulfur-containing macrocyclic compounds with only increases. Unlike linear derivatives, changing from amide ‘two chain’ substituents26 seems to support these findings.groups in cyclic 12 to urea groups in 13 increases the melting (v) By comparing the linear derivatives 1, 3 and 4 (lamellar temperature. smectic A mesophases) with compounds 2, 5, 6 and 7 (hexa- Introducing ‘three chain’ substituents in 10 instead of ‘two gonal columnar mesophases), we can conclude that the type chain’ groups in 9 decreases the melting temperature.of mesophase is apparently not influenced by the number of For 8 compared to 1 and in 9 compared to 2, the presence possible hydrogen bonds, which varied with the nature of the of only two ‘two chain’ substituents is apparently insufficient linkage groups (amide, ester, urea, urethane), but only by the for mesophase formation. Neither increasing the number of decyl side chains going from 9 to 10 nor increasing the number number of substituents.Likewise, variation of the linkage 610 J. Mater. Chem., 1997, 7(4), 607–614Table 5 Transition temperatures DCp values and DH values of linear and cyclic oligoethyleneaminoether derivatives 8–14 and the cyclen derivative 15; Tg glass transition temperature; K, crystalline phases; M, mesophase; I, isotropic phase transition temp/°C (DH/kJ mol-1) compound Tg/°C (DCp/kJ mol-1 K-1) K1 K2 K3 K4 M I 8 — $ 52.5a (4.5) $ 115.0 (68.5) $ 9 — $ 111.0 (57.5) $ 128.0 (45.4) $ 10 — $ 20.0 (46.8) $ 47.5 (33.7) $ 101 (61.5) $ 11 — $ 98.0 (60.6) $ 12 — $ 13.5b (15.7) $ 49.5a (20.8) $ 68.5a (1.4) $ 80.5a (55.1) $ 13 — $ 80.0 (3.6) $ 97.0a (25.9) $ 124.0 (73.8) $ 14 33c (0.03) $ 59.5b (5.8) $ 64.9a (45.6) $ 67.0 (3.3) $ 15d — $ 108 $ 154 $ aOnly on first heating.bOn second and further heatings. cDetermined only on fast heating with 15 K min-1 without preceeding recrystallization. dRef. 12. groups does not influence to a larger extent the lattice spacings formation via intermolecular hydrogen bonding is essential for the formation of a columnar arrangement.in the different mesophase types. A smectic A phase is verified when only three apolar ‘two In other words, neither a role as ‘molecular mesogens’, in terms of classical molecular anisometry, nor a role as ‘supra- chain’ substituents occupy a space, which would be too small to surround entirely the polar part of the molecule, leading molecular mesogens’ via hydrogen bonding is responsible for the observed liquid crystallinity.then to a lamellar structure. Two possible lamellar arrangements of 1 and 4, assumed also for 3 as can be deduced from What then would be the driving force for the mesomorphism in this class of compounds? We conclude that microphase polarizing microscopy, are shown schematically in Fig. 1. A minimum number of four ‘two chain’ groups is necessary separation of polar and apolar parts plays the dominant role.With four substituents in 2, 5, 6 and 7, the apolar parts of the for the formation of a hexagonal columnar mesophase (Colh) in linear oligoethyleneamine derivatives 2, 5, 6 and 7, as well molecule fill the space around the polar backbone, leading thus to the columnar arrangement. The column core is formed as in the cyclic derivative 15.(vi) Contrary to the influence on the melting temperature by the backbone of the oligoethyleneamine derivative. Without the above discussed role of hydrogen bonds and with respect of the linear derivatives, changing from amide to urea groups (comparing 2 with 6) increases the clearing temperature of the to the conformational flexibility of the ethylene bridges, instead of a regular helix a more or less irregular twisting of the Colh phase.The highest Tc is found in compound 7 with urea/ urethane linking groups. backbone of the polar core, radially surrounded by the apolar alkyl sidechains (Fig. 2), would then be the third alternative (vii) With respect to the linear derivatives with three substituents, the mesomorphism of the cyclic compound 14 appar- to the models discussed above for 2.12,13 Due to its conformational flexibility, the cyclic derivative 15 ently plays an intermediate role between the lamellar and columnar state, resulting in a cubic phase.Such cubic phases with four ‘two chain’ substituents is likewise assumed to fill up the inner volume of a column, not in the conventional likewise exist with other members of oligoethyleneamine derivatives, i.e.linear 2,2-tri15 and 2,2,2-tet16 derivatives. ‘discotic’ manner, but in a more or less flexible fashion with its microphase-separated polar and apolar parts. The discussion12,13 of two possible mechanisms for the formation of columnar structures of low molecular, linear N- With respect to the formation of hexagonal columnar phases of N-acylated linear poly(ethyleneimine)s or substituted acylated oligoamine 2 involves the formation of columnar aggregates by intramolecular or intermolecular hydrogen poly(oxazoline)s,17 two models have been discussed.13 In the first model, the column core is described as being formed by bonds.The meaning is that a ‘discoid geometry could perhaps be achieved by means of intramolecular hydrogen bonding a single polymer chain with a helical conformation, while in the second model it is formed by several polymer chains, more between terminal amide groups’, leading to a ‘stacking of these cyclic subunits into a columnar arrangement.Alternatively, or less stretched along the central core axis. The first model (helix model) is favoured over the second one.13 Because of the formation of intermolecular hydrogen bonds between’ the terminal amide groups of ‘the oligomeric subunits’ was ‘con- the absence of hydrogen bonding, not only in 5 but also along the backbone of the ‘two chain’ substituted poly(ethylene- sidered: the column would then be formed by a helically folded chain of hydrogen-bonded oligoamide molecules’.imine)s, and with respect to the flexibility of the ethylene bridges in the backbone, again the microphase separation or The existence of a hexagonal columnar (Colh) mesophase in compound 5, which in contrast to 2 does not possess the core/shell structure of a column with polar core and apolar shell should be taken into account as a third alternative to the possibility of forming hydrogen bonds, demonstrates that neither a ring closure via intramolecular hydrogen bonding two models discussed above for the ‘two chain’ substituted poly(ethyleneimine)s.13 This possibility would also explain very and thus the formation of a disc-like structure nor a helix simply that, with the same backbone, the related N-benzoyl Fig. 1 Possible schematic arrangements of linear oligoethyleneamine Fig. 2 Schematic arrangement of linear oligoethyleneamine derivatives with a minimum number of four ‘two chain’ substituents to give the derivatives with three ‘two chain’ substituents to give lamellar structures polar core and apolar shell of a column in a hexagonal array J.Mater. Chem., 1997, 7(4), 607–614 611substituted poly(ethyleneimine) exhibits a lamellar crystalline Materials structure.27 In this case, the absence of alkyl chains is appar- Argon was dried over molecular sieves and potassium on ently mainly responsible for the lamellar morphology: the aluminum oxide.Dioxane was refluxed over potassium and interface curvature between two separated phases depends, distilled under inert gas. The relevant oligoethylene-amines among other factors, on the space required of one of the and -amino ethers are commercially available in high purity phases.28 This is also valid for such different cases as micellar grade.systems29 or block copolymers.30 In the field of liquid crystallinity this point of view plays a role in amphiphilic mesogens,31 Synthesis or in compounds which contain a polar macrocyclic core and The purity of all new compounds was checked by IR, 1H ‘wedge’- or ‘V’-shaped apolar groups, filling the space in the NMR, 13C NMR and mass spectroscopy, SEC (size exclusion outer sphere of a columnar arrangment.32,33 Likewise in ‘tubuchromatography, GPC) and partially by elemental analysis.lar’ architectures with ‘taper shaped’ sidegroups, the columnar Yields, MS data and the elution volume of the obtained SEC core/shell structure (‘endo/exo’ structure) is caused by the single peaks are given in Table 6. 3,4-Bis(decyloxy)benzoyl ‘microsegregation of polar groups’ [e.g. flexible oligooxyethy- chloride was synthesized using previously described methods.46 lene segments, poly(methacrylate) backbones] ‘from the nonpo- 3,4-Bis(decyloxy)phenyl isocyanate was obtained in high yield lar aliphatic and aromatic groups at the column periphery’.34 from 3,4-bis(decyloxy)benzoic acid by standard reaction with Of course, if other interactions like supramolecular (hydro- the corresponding azide,47 which was obtained in high purity gen bonding, ionic or charge transfer forces) or sterical forces after precipitation from toluene solution in the freezer, filtration (e.g.incorporation of classical anisometric molecular units) are and subsequent column chromatography on silica gel with additionally present, they contribute in a specific weighting to toluene. The subsequent Curtius rearrangement47 yielded, after the morphological structures observed. 3 h and evaporation of toluene, the pure isocyanate, which The different requirements for the formation of columnar was stored under inert gas. mesophases, described hitherto in the literature, are (i) classical, more or less stiff, disc-like (discoid) molecular structures Ester endgroup derivatives. The acylation of compounds 3, 4 (‘discotic phases’), (ii) supramolecular arrangements (self- and 5 was performed using the method for the synthesis of assembling) of single molecules (without classical discoid aniso- related amindes,15 with a reaction time of 12 h instead of 8 h, metry) to columnar aggregates via hydrogen bonding, ionic at 80°C.forces, charge transfer or because of sterical reasons, (iii) micro- Compound 3: n(KBr)/cm-1 3364, 2956, 2924, 2854, 1717, phase separation of incompatible parts, assumed to play a 1632, 1600, 1583, 1510, 1467, 1431, 1272, 1224, 1139, 762; role also in the mesomorphism of poly(organophospha- dH(CDCl3) 7.55 (d, 2H, aromatic), 7.45 (d, 1H, aromatic), 7.30 zene)s35,36 and perhaps in poly(dialkylsiloxane)s,37,38 and (br m, 2H, aromatic, NH), 6.65–6.85 (m, 5H, aromatic), 4.40 (iv) regular helical arrangements of stiff polymeric back- (br t, 2H, CO2CH2), 3.60–4.10 (m, 18H, OCH2, CH2N), 1.8 bones.39–42 Taking these into account, it is obvious that the (m, 12H, OCH2CH2), 1.1–1.6 (m, 84H, CH2 ), 0.85 (t, 18H, denomination ‘discotic’ (Dh etc.) for columnarphases in general CH3); dC(CDCl3), 173.7 (NCO), 167.3 (NHCO), 166.0 (CO2 ), is no longer meaningful, or in other words ‘the terms disc to 153.5, 151.6, 150.2, 149.1, 148.7, 148.6, 128.0, 126.4, 123.6, 121.6, describe slices through the column and discotic, to describe 119.6, 119.5, 114.1, 112.9, 112.5, 112.2, 111.8 (aromatic), 69.2, the mesophase type, become meaningless expressions’.33 69.1, 69.0 (OCH2), 61.9 (CO2CH2), 48.0, 44.7, 39.1 (CH2N), Futhermore, it is clear that a number of other denominations, 31.9–22.4 (CH2), 14.0 (CH3) (Calc.for C85H144N2O10: C, 75.40; such as e.g. pyramidal, bowl-like, phasmidic, tubular, and H, 10.72; N, 2.07.Found: C, 75.79; H, 10.73; N 2.14%). abbreviations, like wh (which denotes originally only phasmidic Compound 4: n(KBr)/cm-1 2956, 2924, 2855, 1714, 1635, phases after the initial of the greek root wasma43 ), Oh, Bh, Hl 1600, 1515, 1467, 1431, 1271, 1214, 1139, 1019, 7562; dH(CDCl3) or HCl, are used in the literature by different authors for 7.65 (d, 2H, aromatic), 7.45 (s, 2H, aromatic), 6.70–6.90 (m, different columnar phases of one and the same type with 5H, aromatic), 4.40 (br t, 4H, CO2CH2 ), 3.60–4.10 (m, 16H, respect to X-ray findings.OCH2, CH2N), 1.8 (m, 12H, OCH2CH2), 1.1–1.6 (m, 84H, To avoid this somewhat confusing situation, we16 and CH2), 0.85 (t, 18H, CH3 ); dC(CDCl3), 173.7 (NCO), 166.1 others44,45 began to use the abbreviations Colh, for the general (CO2), 153.8, 151.3, 149.1, 148.7, 128.0, 126.4, 123.6, 121.6, ‘liquid-crystallographic’ relevant term hexagonal columnar 119.6, 114.1, 112.2, 111.8 (aromatic), 69.2, 69.1, 69.0 (OCH2 ), mesophase, or Colr, for rectangular columnar phases etc., 61.9 (CO2CH2), 49.0 (CH2N), 31.9–22.6 (CH2), 14.1 (CH3) taking into account only the X-ray characteristics, independent (Calc.for C85H143NO11: C, 75.34; H, 10.64; N, 1.03. Found: C, of a specific molecular structure and the different mechanisms 75.48; H, 10.52; N, 1.03%). of formation. Compound 5: n(KBr)/cm-1 identical to that for 4; dH(CDCl3) Table 6 Yields, mass spectra values and elution volumes (Ve) of size Experimental exclusion chromatograms (SEC, GPC) of compounds 3–14 Instruments mass spectrum SEC compound yield (%) m/z (intensity, %) Ve/ml IR: BioRad/Digilab FTS 40.NMR: Bruker AC 250, 250 MHz. MS: Varian 312. SEC (size exclusion chromatography, GPC): 3 71 1353 (M·+, 0.2%), 153 (100%) 25.9 Waters ALC 200, RI-Detector Melz LCD 201, eluent THF, 4 29 1354 (M·+, 1.8%), 153 (100%) 25.9 5 62 1812 (M·+, 0.9%), 153 (100%) 25.9 elution rate 0.5 ml/min-1; 2×60 cm PL columns, 5 mm particle 6 72 151 (100%) 24.6 size, 100 and 500 A° pore width.Elemental analysis: 7 74 151 (100%) 23.7 Mikroanalytisches Labor Ilse Beetz, Kronach. Polarizing 8 55 936 (M·+, 18%), 179 (100%) 25.5 microscope: Leitz Laboluz 12 pol, hot stage Mettler FP 82, 9 59 980 (M·+, 18%), 178 (100%) 25.8 control unit Mettler FP80, photoautomat Wild MPS 45/51 S; 10 47 1461 (M·+. 0.3%) 24.8 11 29 151 (100%) 25.3 DSC: Perkin-Elmer DSC 7, standard heating rate 10 K min-1. 12 41 982 (M·+, 1.8%), 417 (100%) 25.6 X-Ray measurements were performed with a WAXS- 13 23 151 (100%) 25.5 Goniometer Siemens D 5000, h/2h, Cu-Ka: 1.5418 A° , in the 14 58 1422 (M·+, 0.2%), 43 (100%) 24.7 mesophase, after cooling from the isotropic phase. 612 J.Mater. Chem., 1997, 7(4), 607–6147.65 (d, 2H, aromatic), 7.45 (s, 2H, aromatic), 6.70–6.90 (m, Compound 10: n(KBr)/cm-1 identical to that for 8; dH(CDCl3) 7.0 (s, 4H, aromatic), 6.5–6.6 (br t, 2H, NH), 4.0 8H, aromatic), 4.40 (br t, 4H, CO2CH2), 3.60–4.10 (m, 24H, OCH2, CH2N), 1.8 (m, 16H, OCH2CH2), 1.1–1.6 (m, 112H, (m, 12H, OCH2), 3.6 (m, 12H, OCH2CH2O, OCH2CH2N), 1.8 (m, 12H, OCH2CH2 ), 1.1–1.6 (m, 84H, CH2), 0.85 (t, 18H, CH2), 0.85 (t, 24H, CH3); dC(CDCl3), 173.4 (NCO), 166.2 (CO2 ), 154.2, 151.8, 149.5, 148.9, 128.1, 126.4, 123.3, 121.8, CH3); dC(CDCl3), 167.3 (NHCO), 153.0, 141.2, 129.3, 119.4, 105.8 (aromatic), 73.4, 69.8, 69.3, (OCH2), 39.8 (NCH2 ), 119.9, 113.8, 112.0, 111.8 (aromatic), 69.3, 69.1, 69.0 (OCH2), 61.7 (CO2CH2), 49.1 (CH2N), 31.9–22.6 (CH2 ), 14.0 (CH3) 31.9–22.6 (CH2), 14.0 (CH3).Compound 11: n(KBr)/cm-1 3314, 3142, 2956, 2923, 2852, (Calc. for C114H192N2O14: C, 75.45; H, 10.66; N, 1.54. Found: C, 75.58; H, 10.66; N, 1.50%). 1643, 1602, 1583, 1542, 1514, 1468, 1317, 1273, 1225, 1135, 1067, 763, 720; dH(CDCl3 ) 7.4 (s, 2H, NH), 7.1 (d, 2H, aromatic), 6.6–6.8 (d, 2H, dd, 2H, aromatic), 5.5 (t, 2H, NH), 3.9 (t, 8H, Derivatives with urea and urethane substituents.For compounds 6 and 7, 1 mmol of the relevant amine or the compound OCH2), 3.5–3.7 (br m, 8H, OCH2CH2N), 3.3–3.4 (m, 4H, CONHCH2), 1.80 (m, 8H, OCH2CH2), 1.10–1.60 (m, 56H, with terminal hydroxy groups was added under inert gas to 3,4-bis(decyloxy)phenyl isocyanate (4.4 mmol) in 80 ml dry CH2), 0.85 (t, 12H, CH3); dC(CDCl3), 156.9 (NHCONH), 149.6, 144.9, 132.9, 115.0, 112.3, 107.2, (aromatic), 70.5, 70.2, dioxane and stirred 6 h at 80°C.The conversion was followed by the decrease of the isocyanate peak in the IR spectrum of 70.0, 69.0 (OCH2), 40.1 (NCH2), 31.9–22.6 (CH2 ), 14.0 (CH3 ). Compound 12: n(KBr)/cm-1 2956, 2924, 2855, 1635, 1600, samples taken from the reaction mixture under inert gas at different times.At the end of the reaction, the solvent was 1582, 1515, 1467, 1431, 1271, 1214, 1139, 1019, 762; dH(CDCl3) 7.0–7.3 (m, 4H, aromatic), 6.8 (d, 2H, aromatic), 3.4–4.1 (m, evaporated and the white residue recrystallized from ethyl acetate, followed by a column chromatography on silica gel 24H,OCH2, OCH2CH2N), 1.80 (m, 8H, OCH2CH2 ), 1.10–1.60 (m, 56H, CH2 ), 0.85 (t, 12H, CH3); dC(CDCl3 ), 172.4 (NCO), 60 with ethyl acetate and a second recrystallization from hexane–ethyl acetate–ethanol (65351).Finally, the products 150.2, 148.6, 128.5, 120.9, 113.4, 112.7 (aromatic), 69.1 (OCH2 ), 52.2 (OCH2CH2N), 48.4 (NCH2), 31.9–22.6 (CH2), 14.1 (CH3) were lyophilized from benzene solution. Compound 6: n(KBr)/cm-1 3308, 3142, 2956, 2924, 2855, (Calc.for C60H106N2O8 : C, 73.27; H, 10.86; N, 2.85. Found: C, 73.77; H, 10.58; N, 2.86%). 1714, 1647, 1608, 1556, 1515, 1469, 1426, 1263, 1228, 1134, 1019, 802, 722; dH(CDCl3 ) 8.5–8.6 (br s, 2H, NH), 6.9–7.3 (m, Compound 13: n(KBr)/cm-1 3311, 3140, 2954, 2922, 2854, 1646, 1602, 1586, 1542, 1514, 1464, 1321, 1274, 1225, 1139, 8H, aromatic, NH), 6.5–6.8 (m, 6H, aromatic), 5.6–5.8 (br t, 2H, NH), 3.7–4.0 (m, 16H, OCH2), 3.1–3.5 (br m, 12H, CH2N), 1067, 761, 722; dH(CDCl3) 7.6 (s, 2H, NH), 7.15 (d, 2H, aromatic), 6.75 (d, 2H, aromatic), 6.55 (dd, 2H, aromatic), 1.8 (m, 16H, OCH2CH2), 1.1–1.6 (m, 112H, CH2), 0.85 (t, 24H, CH3); dC(CHCl3), 157.1 (NHCON), 156.7 (NHCONH), 149.6, 3.7–4.1 (m, 16H, OCH2, OCH2CH2N), 3.3–3.6 (br m, 8H, CH2N), 1.80 (m, 8H, OCH2CH2), 1.10–1.60 (m, 56H, CH2 ), 149.4, 145.1, 144.7, 133.6, 132.8, 132.6, 114.9, 114.8, 112.1, 111.9, 106.9 (aromatic), 70.0, 69.1, 69.0 (OCH2), 49.6, 48.0, 47.3, 0.85 (t, 12H, CH3); dC(CDCl3), 156.7 (NHCON), 149.9, 144.5, 133.7, 115.6, 110.6, 106.1 (aromatic), 70.3, 69.8, 69.0 (OCH2, 39.5 (CH2N), 31.9–22.7 (CH2), 14.1 (CH3) (Calc.for C114H198N8O112: C, 73.11; H, 10.66; N, 5.98. Found: C, 73.39; OCH2CH2N), 52.2 (NCH2), 31.6–22.6 (CH2 ), 14.0 (CH3).Compound 14: n(KBr)/cm-1 identical to that for 12; H, 10.26; N, 5.59%). Compound 7: n(KBr)/cm-1 3316, 3143, 2954, 2926, 2855, dH(CDCl3) 6.6–7.1 (br m, 9H, aromatic), 3.4–4.1 (br m, 28H, OCH2, OCH2, CH2N), 1.80 (m, 12H, OCH2CH2), 1.10–1.60 1710, 1651, 1606, 1592, 1518, 1467, 1431, 1259, 1226, 1134, 1019, 799, 722; dH(CDCl3 ) 7.8–8.0 (br s, 2H, NH), 7.3 (d, 2H, (m, 84H, CH2 ), 0.85 (t, 18H, CH3); dC(CDCl3) 172.1 (NCO), 150.2, 149.1, 148.8, 128.6, 128.2, 119.3, 118.9, 112.9, 112.5, 112.2, aromatic), 7.0–7.1 (br s, 2H, NH), 6.9 (dd, 2H, aromatic), 6.6–6.8 (m, 8H, aromatic), 4.4 (br m, 4H, CO2CH2), 3.8–4.0 112.0 (aromatic), 69.8, 69.1 (OCH2), 52.5 (OCH2CH2N), 49.7, 47.8, 42.9 (NCH2), 31.8–22.6 (CH2), 14.0 (CH3) (Calc.for (m, 16H, OCH2), 3.4–3.7 (br m, 8H, CH2N), 1.8 (m, 16H, OCH2CH2), 1.1–1.6 (m, 112H, CH2 ), 0.85 (t, 24H, CH3); C89H151N3O10: C, 75.11; H, 10.69; N, 2.95. Found: C, 75.04; H, 10.90; N, 3.00%). dC(CDCl3), 155.8 (NHCON), 154.0 (NHCO2), 149.7, 149.6, 145.6, 144.8, 133.4, 131.2, 114.9, 114.7, 111.7, 111.2, 106.8, 106.0 (aromatic), 70.1, 69.9, 69.1, 69.0 (OCH2), 63.2 (CO2CH2), 47.7 U.S. and G. L. gratefully acknowledge the financial support (CH2N), 31.9–22.7 (CH2), 14.1 (CH3) (Calc. for of the Deutsche Forschungsgemeinschaft (DFG La 662/1–2). C114H196N6O14: C, 73.03; H, 10.54; N, 4.48. Found: C, 73.58; H, 10.67; N, 4.79%). References Derivatives with oxobridges. The acylation of compounds 1 J.-M. Lehn, J. Malthe�te and A.-M. Levelut, J. Chem.Soc., Chem. 8–10, 12 and 14 was performed using the method for synthesis Commun., 1985, 1794. 2 C. Mertesdorf and H. Ringsdorf, L iq. Cryst., 1989, 5, 1757. of related amides,15 while compounds 11 and 13 were synthe- 3 G. Lattermann, L iq. Cryst., 1989, 6, 619. sized using 3,4-bis(decyloxy)phenyl isocyanate (2.2 mmol) fol- 4 J. Malthe�te, D. Poupinet, R. Vilanove and J.-M. 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Commun., 1992, 1434. 10 S. H. J. Idziak, N. C. Maliszewskyj, G. B. M. Vaughan, Compound 9: n(KBr)/cm-1 identicalto that for 8; dH(CDCl3) P. A. Heiney, C. Mertesdorf, H. Ringsdorf, J.P. McCauley, Jr. and 7.4 (d, 2H, aromatic), 7.2 (dd, 2H, aromatic), 6.8 (d, 2H, A. B. Smith, III, J. Chem. Soc., Chem. Commun., 1992, 98. aromatic), 6.5–6.6 (br t, 2 H, NH), 4.0 (m, 8H, OCH2), 3.5–3.8 11 M. Zhao, W. T. Ford, S. H. J. Idziak, N. C. Maliszweskyj and (br m, 12H, OCH2CH2N), 1.8 (m, 8H, OCH2CH2 ), 1.1–1.6 (m, P. A. Heiney, L iq. Cryst., 1994, 16, 583. 56H, CH2), 0.85 (t, 12H, CH3); dC(CDCl3), 167.1 (NHCO), 12 H.Fischer, S. S. Ghosh, P. A. Heiney, N. C. Maliszewskyj, 151.9, 148.9, 126.8, 119.4, 112.9, 112.1 (aromatic), 70.2, 69.9, T. Plesnivy, H. Ringsdorf and M. Seitz, Angew. Chem., 1995, 107, 879; Angew. Chem., Int. Ed. Engl., 1995, 34, 795. 69.3, 69.0 (OCH2 ), 39.7 (NCH2), 31.8–22.6 (CH2), 14.0 (CH3). J. Mater. Chem., 1997, 7(4), 607–614 61313 M.Seitz, T. Plesnivy, K. Schimossek, M. 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ISSN:0959-9428    

DOI:10.1039/a607819b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Self-assembling properties of non-ionic tetraphenylporphyrins and discotic phthalocyanines carrying oligo(ethylene oxide) alkyl or alkoxy units Johannes M. Kroon,a Robert B. M. Koehorst,b Marinus van Dijk,a Georgine M. Sandersa and Ernst J. R. Sudho�lter*, a aDepartment of Organic Chemistry, Wageningen Agricultural University, Dreijenplein 8, 6703 HB Wageningen, T he Netherlands bDepartment ofMolecular Physics, Wageningen Agricultural University, Dreijenlaan 5, 6703 HA Wageningen, T he Netherlands The thermotropic phase behaviour and self-assembling features of some non-ionic tetraphenylporphyrins and phthalocyanines containing oligo(ethylene oxide) alkoxy or alkyl units have been investigated. From DSC measurements and polarization microscopy it was concluded that none of the tetraphenylporphyrins was mesomorphic while the phthalocyanines displayed discotic hexagonal phases even at room temperature.The aggregation of the compounds in aqueous media was studied by means of UV–VIS and fluorescence spectroscopy and it has been found that in water the tetraphenylporphyrins form J- or head-to-tail type of aggregates while phthalocyanines form H- or face-to-face type of aggregates.The luminescence properties of the tetraphenylporphyrin and phthalocyanine aggregates are explained on the basis of the molecular exciton approximation. Steric constraints and orientational disorder in the tetraphenylporphyrin aggregates determine the luminescence yield relative to the monomeric species. The cofacial arrangement of the macrocycles in phthalocyanine aggregates results in a forbidden S1–S0 transition and thus in a complete disappearance of the luminescence.Molecular electronics, photonics and iono-electronics are cur- similar and ordered J-type aggregates in solution, at the air– rently receiving great interest. A lot of ongoing research deals water interface and on solid substrates. with the construction of nanometre-sized, self-organizing sys- In this paper we describe the synthesis, thermotropic behavtems capable of transporting electrons, holes, excitons and/or iour and self-organizing properties of three structurally related ions.The combination of electronic and ionic transport pro- tetraphenylporphyrins (1–3) and two phthalocyanine comcesses in supramolecular devices could be of great importance pounds 4a,4b.In the TPPs 1 and 2 the polar units and the in the development of future nano-electronic devices. TPP cores are separated by an alkoxy and an alkyl spacer, In this respect, molecular assemblies of porphyrins and respectively. This was done since it has been found that the phthalocyanines (Pcs) have been the subject of intensive presence of ether linkages in the side chains directly attached research in view of their fascinating electronic and optical to the phenyl substituent dramatically affects the thermotropic properties.The spontaneous arrangement of large aromatic phase behaviour.2 In TPP 3, which is an isomer of 2, the macrocycles is facilitated by peripheral substitution with long sequence of the polar and non-polar parts is inverted.Pcs 4a alkyl or alkoxy chains. For several porphyrins1–2 and phthalo- and 4b contain side-chains which are identical to those of TPP cyanines,3–8 discotic thermotropic mesomorphic behaviour 1; they are designed in order to compare the physicochemical was observed, resulting in long linear stacks of aromatic moi- behaviour of TPP and Pc-aggregates. The self-assembling eties surrounded by insulating hydrocarbons, which might features of these compounds have been studied by optical enhance the one-dimensional nature of electronic transport spectroscopic methods (absorption, fluorescence).processes.9–11 Recently, it has been found that introduction of hydrophilic groups at the peripheral sites of the Pcs results in the formation of lyotropic mesophases.For example, attachment of oligo- (ethylene oxide) moieties results in columnar lyotropic mesophases in addition to thermotropic columnar phases.12–13 Furthermore, the attachment of polar side chains such as oligo(ethylene oxide) moieties and crown ether rings to large aromatic macrocycles is also of interest in the construction of supramolecular wires and ion conducting channels.4,13a Several porphyrins and phthalocyanines of these types have been designed in order to investigate complexation behaviour and the potential for electronic and ionic conduction.14–19 In our efforts to construct molecular assemblies which combine the functions of electronic and ionic conduction, we have recently reported20 the thermotropic phase behaviour and aggregation properties of a series of tetraphenylporphyrin (TPP) derivatives in which the TPP macrocycle and a polar ethylene oxide part are separated by isolating hydrocarbon spacers (e.g.TPP 1). It appeared that these compounds do not show mesomorphic behaviour but they tend to organize into J. Mater. Chem., 1997, 7(4), 615–624 615Results Scheme 3 Reagents: i,K2CO3 , DMF; ii, CuCN, DMF; iii, DBN, EtOH Synthesis TPP 1 was synthesized as described previously.20 of base, a Rosenmund-von Braun reaction yielding the dicyan- The synthetic procedure for the TPPs 2,3 and Pcs 4a,b is ides and the conversion of the phthalodinitrile into 4a by presented in Scheme 1 and 2.refluxing in absolute ethanol in the presence of DBN (1,5- 5-Phenyl-1-hydroxypentane was brominated using PBr3.diazabicyclo[4.3.0]non-5-ene). During the preparation of the Substitution of the bromine by triethylene glycol monomethyl dicyanides a substantial amount of 4b was formed. ether was carried out in the presence of sodium. An aldehyde group was introduced by adding hexamethylenetetraamine Thermotropic phase behaviour (HMTA) in trifluoroacetic acid (TFA).21 Porphyrins.The phase transition temperatures as determined TPP 2 was synthesized using the Adler method22 by refluxing by differential scanning calorimetry (DSC) and polarization the substituted benzaldehyde and pyrrole in propionic acid. microscopy of the compounds 1–4 are compiled in Table 1. The benzaldehyde, leading to TPP 3, was synthesized as The calorimetric data for TPP 1 have already been follows. reported20 and are included in Table 1.Upon heating from The hydroxy group in triethylene glycol monohexyl ether -50°C, a phase transition with a small enthalpy change has was replaced by chlorine using thionyl chloride. 4- been found at -22 °C (2.1 kJ mol-1). Melting to the isotropic Hydroxybenzaldehyde was then substituted by the chloro- phase (clearing point) occurred at 67°C (37 kJ mol-1).The derivative in the presence of a base and potassium iodide. transitionshave found to be reversible although some hysteresis The steps leading to Pc 4a, were carried out using known occurred during the cooling cycle. procedures23 (Scheme 3). Bromination of catechol using mol- For TPP 2 no distinct transitions were observed with DSC ecular bromine, alkylation of dibromocatechol in the presence during the first heating cycle starting at -40 °C but a broad endothermic transition centred at ca. 75°C (ca. 23 kJ mol-1) which corresponds to the clearing temperature. Subsequent cooling to -60°C revealed no phase transitions. However, polarization microscopy showed the appearance of an anisotropic texture at 60–50°C upon cooling from the isotropic phase.In the case of TPP 3 a complex phase behaviour was observed. During the first heating three transitions have been found at -1, 30 and 47°C, while isotropization occurred at 55°C. Large hysteresis effects were observed during the first cooling cycle. In the second heating cycle two transitions (0 and 41°C) have been observed before isotropization occurred.Inspection of this sample by polarization microscopy showed that during the heating no textural changes could be detected by passing through the lower phase transition temperatures. Upon cooling from the isotropic phase the material becomes Scheme 1 Reagents: i, PBr3; ii, HO(C2H4O)3Me, Na; iii, HMTA, anisotropic between 30 and 40°C. CF3CO2H; iv, pyrrole, propionic acid Phthalocyanines.Both Pcs 4a,b are viscous oi at room temperature. During the first heating cycle of these compounds from room temperature to higher temperatures no phase transitions have been observed. Subsequent cooling to-50 °C shows an exothermic peak at -20 °C (DH=21 kJ mol-1) for 4a. A second heating cycle from -50 °C shows a broad exothermic peak at -20°C (DH=21 kJ mol-1) and an endothermic peak centred at ca. 5°C (DH=42 kJ mol-1). The clearing temperatures, which were not observed with DSC, could be determined with polarization microscopy. The Scheme 2 Reagents: i, HO(C2H4O)3H, Na; ii, SOCl2 ; iii, 4-hydroxybenzaldehyde, K2CO3, butanone, KI; iv, pyrrole, propionic acid absence of the DSC peaks could probably be explained by the 616 J. Mater. Chem., 1997, 7(4), 615–624Table 1 Phase transition temperatures (°C) and enthalpy changes DH (kJ mol-1) (between brackets) for the investigated compounds 1–4 compound second heating cycle first cooling cycle 1 C1 -22 (2.1) C2 67 (37) I I 50 C2 -40 C1 2 C 70–75 (23) I I 60–50 Cb 3 C1 0 (18) C2 41 (24) C3 55 (10) I I 40–30a C2 -10 C1 4a Cc -20 (-21), 5 (42) Dho 140–160b I I 160–140b Dho -20 (-21) Cc 4b Dho 210–225b I I 220–210b Dho aC is the crystalline phase, Dho the hexagonal ordered discotic phase and I the isotropic phase.bThese temperatures are determined with polarization microscopy, since DSC revealed no transitions. cC: partly crystallized, see text. fact that the temperature range for isotropization for both Pcs is rather broad, 140–160 °C for 4a and 210–225 °C for 4b.A mosaic texture appears upon heating the compounds from room temperature until just before the clearing temperature (Plate 1). After cooling the isotropic melt there is a strong tendency to homeotropic alignment, except at the birefringent defects which appear as intense linear lines in the dark background (Plate 2). Between parallel analyser and polarizer digitate stars are observed (see Plate 3).Aggregation in solution: UV–VIS absorption spectroscopy Porphyrins. The presence of the polar ethylene oxide units in the hydrophobic side chains of the TPPs results in an increased solubility in polar solvents like acetone and methanol. For the investigated TPPs 1–3 it was found that they exist as monomers in these solvents at concentrations of Plate 3 Digitate stars observed in the mesophase of 4a at 85 °C upon cooling from the isotropic phase (parallel polarizers) 10-7–10-4 M as they show a strong Soret or B-absorption band centred at ca. 420 nm and four characteristic Q-bands at 500–650 nm. For TPP 1, it has recently been found20 that in binary acetone–water mixtures aggregation of the TPPs into ordered aggregates was induced by increasing the volume fraction of water.The volume fraction at which aggregation occurs depends on the porphyrin concentration. In this section we report on the aggregation properties of the TPPs 1–3 in methanol–water (v/v 1/1) and in pure water at concentrations of 10-5–10-6 M. The spectra of 1–3 in a number of solvents are shown in Fig. 1 for the Soret band region at a concentration of 5×10-6 M.The three strucurally related TPPs display rather different aggregation behaviour in aqueous media. In Fig. 1(a), Plate 1 Micrograph of the mesophase of 4a at 130°C (crossed polarizers) it can be seen that in methanol, 1 is present as a monomer as it shows an intense Soret band at 418 nm (e=4.5×105 dm3 mol-1 cm-1, FWHM=750 cm-1). However, a relatively broad red-shifted aborption band (FWHM=2080 cm-1) is found at 428 nm directly after addition of 15 ml 1mM acetone solution of 1 into 3 ml of water.After several hours, this broad band splits into a relatively intense band at 440 nm (e= 3.3×105 dm3 mol-1 cm-1, FWHM=540 cm-1) and a relatively weak band at 400 nm (e=7×104 dm3 mol-1 cm-1). The aggregation process is accelerated considerably, upon addition of a non-ionic amphiphile, polyoxyethylene(23) lauryl ether (CMC=6–9×10-5 M, abbreviated as C12E23). The spontaneous formation of aggregates was indicated by similar changes in the optical spectrum as in the former case (e440= 5×105 dm3 mol-1 cm-1, FWHM=250 cm-1).In Fig. 1(b), it is shown that in methanol–water (v/v 1/1) a broad, asymmetric absorption band is initiallyfound at430 nm.In this solvent mixture, spontaneous aggregation is thus hindered. The aggregation process can again be accelerated by addition of C12E23, resulting in the same spectral features as observed for 1 in water. However, in contrast to pure water Plate 2 Micrograph of the mesophase of 4b at room temperature after cooling from the isotropic phase (crossed polarizers) the aggregates are not stable in methanol–water, since the J.Mater. Chem., 1997, 7(4), 615–624 617Fig. 1 Visible absorption spectra of (a) TPP 1 in (i) MeOH, (ii) H2O (+0.5% acetone) directly after preparation of the solution, (iii) H2O (+0.5% acetone) after 3 h, (iv) after addition of C12E23 ; (b) TPP 1 in (i) MeOH–H2O (151 v/v) directly after preparation of the solution, (ii) after addition of C12E23, (iii) after 1 d; (c) TPP 2 in (i) MeOH, (ii) H2O; (d) TPP 3 in (i) MeOH, (ii) H2O.Measurements were performed at room temperature (concentration: 5×10-6 M). spectrum changes into a split band centred at 440 nm while coefficient of 2×104 dm3 mol-1 cm-1. This spectral behaviour is characteristic of columnar aggregated Pcs.5b It can also be the blue-shifted band disappears.For TPP 2, spontaneous aggregation is found in water seen in Fig. 2(a), that the spectrum in methanol contains a minor contribution of the monomeric species as was indicated [Fig. 1(c)]. The spectral features of these aggregates are somewhat different from those of 1 in water. The red-shifted band by the weak shoulders at 660 and 770 nm. It should be noted that for 4b, the spectral behaviour is essentially the same is now located at 446 nm (e=2.8×105 dm3 mol-1 cm-1) and somewhat broader (FWHM=600 cm-1 cf. 250–500 cm-1 for (spectra not shown). The tendency of the Pcs 4a and b to form aggregates 1). Moreover, an additional band appears at the blue side (375 nm) of the spectrum. Addition of C12E23 does not have increases in the order toluene (er=2.4), chloroform (er= 4.8)<acetone (er=21)<acetonitrile (er=37)<ethanol (er= any significant effect on the spectral behaviour.Identical spectral behaviour was observed in a methanol–water (v/v 1/1) 24)<methanol (er=34)<water (er=80), indicating that aggregation of the Pcs is not facilitated simply by increasing the mixture (spectra not shown). The spectrum of TPP 3 in pure water is characterized by a bulk relative permittivity (er). This is further confirmed by the fact that in an apolar solvent, such as pentane (er=1.8), relatively broad absorption band (FWHM=1800 cm-1) centred at ca. 425 nm directly after preparation of the solution.aggregation also occurs already at very low concentrations. This is shown in Fig. 1(d ) together with the monomeric spectrum of 3 in methanol.In addition, the spectrum in methanol– Aggregation in solution: fluorescence spectroscopy water (v/v 1/1) is essentially the same (not shown). No further red-shift and sharpening of the bands were observed on a Porphyrins. The fluorescence characteristics of the TPPs 1–3 are compiled in Table 2. longer timescale. For 1 in water, it was found that directly after preparation of the solution, the fluorescence is quenched by ca. 75% Phthalocyanines.Both Pcs are highly soluble in a large variety of solvents except for alkanes like pentane and hexane. relative to the monomeric species in methanol. Addition of C12E23 leads to a twofold enhancement of the quantum yield The optical absorption spectra of 4a and 4b were measured in a range of solvents and are displayed in Fig. 2(a) for 4a; 4a and a small blue shift of the fluorescence bands. For TPP 2, spontaneous aggregation occurs in methanol– shows two intense Qx and Qy bands at 660 (e=1.25×105 dm3 mol-1 cm-1) and 700 nm (e=1.6×105 dm3 mol-1 cm-1) in water (v/v51/1) and water (see former section). In methanol– water a small decrease of the quantum yield has been found chloroform, while for 4b one intense Q-band at 680 nm (e= 2×105 dm3 mol-1 cm-1) is observed (spectrum not shown).as compared to the monomer in pure methanol, while in water a 70% reduction of the fluorescence yield was observed. These bands are characteristic for the monomeric species. In water, the spectrum is blue-shifted and broadened. The maxi- It was also shown in the former section that no spontaneous aggregation occurred for TPP 3 in water and methanol–water. mum of the Q-band is located at 620 nm with an extinction 618 J.Mater. Chem., 1997, 7(4), 615–624reference compounds such as linear alkoxy and alkyl phenyl porphyrins. If 1 is compared with tetrahexadecyloxyporphyrin (R=OC16H33), which has a chain of comparable length (17 cf. 18 atoms),20 a decrease of 51°C of the clearing temperature is observed.An even more remarkable difference in phase behaviour is observed if 2 (side chain contains 16 atoms) is compared with tetra(hexadecylphenyl)porphyrin (R= C16H33). The latter TPP exhibits two phases between the crystal and isotropic phase which were assigned to be discotic lamellar (DL) mesophases.2 The clearing temperature has been found at 129 °C which is ca. 55°C higher than for 2. For Pc 4a a similar comparison can be made. For H2Pc[OC18]8 (R=OC18H37), it has been reported9b that a C�Dh transition occurs at 98°C (DH=239 kJ mol-1) while the clearing temperature has been found at 247 °C (DH= 18 kJ mol-1). The clearing temperature for 4a is considerably lower and found between 140 and 160°C.The mosaic textures which are observed for 4a (Plate 1) and the digitate stars between parallel analyser and polariser (Plate 3) are indicative of a hexagonal discotic columnar mesophase.6a–c Similar textures were earlier observed for H2Pc[OC12]8 (R=OC12H25) and the nature of the columnar phase of the latter compound was identified as a discotic hexagonal ordered mesophase Dho by means of X-ray diffraction analysis.We conclude therefore that 4a displays a Dho mesophase. From our DSC data, we cannot make a definite conclusion about the nature of the phase transitions between -20 and +5°C. The observed exothermic peaks at -20 °C during cooling and subsequent heating can be due to a strongly retarded crystallization of the side-chains because of the disordered ethylene oxide units, while the endothermic peak at 5°C can be attributed to melting of the side-chains.Detailed X-ray diffraction analysis, however, should be used to identify Fig. 2 Visible absorption spectra of 4a (a) in (i) CHCl3 , (ii) MeOH these transitions. It is apparent, however, that by introduction and (iii) H2O at room temperature (concentration: 5×10-6 M) and (b) of the ethylene oxide units, it is possible to reduce the C�Dh in (i) H2O and (ii) of spin-coated film on a glass substrate transition temperature to such an extent that a columnar mesophase is observed even at room temperature.Comparable This resulted in lower luminescence yields of resp. 0.13 and phase behaviour has recently been found by Clarkson for octa- 0.18 vs. methanol (see Table 2).substituted Pcs containing a combination of oligo(ethylene oxide) and alkyl substituents.13a Phthalocyanines. For 4a, monomeric Q-emission bands are Finally, we come to the conclusion that liquid crystalline observed in chloroform at 710 and 790 nm. The fluorescence behaviour is only observed for the Pcs while the investigated yield of the Pc is 3–4 times higher than for the TPPs.In TPPs are all non-mesomorphic. The presence of the meso- methanol, a weak fluorescence is observed which is attributed phenyl groups, which are twisted with respect to the porphyrin to residual monomeric Pcs in the solution as was confirmed core25,26 prevents a close packing of the rings resulting in direct by fluorescence excitation spectra. In water the luminescence melting from the solid to the isotropic phase.Phthalocyanines, is totally quenched. No emission is observed for 4b since however, show very strong stacking of the flat rings, so that luminescence comes from a tripdoublet state with a very low columnar order is preserved during melting of the side chains. quantum yield at room temperature.24 The substitution of the alkyl or alkoxy chains with ethylene oxide units leads to more conformational disorder in the side chains, which results in a marked reduction of the melting and Discussion clearing temperatures.Thermotropic phase behaviour Aggregation in solution: UV–VIS absorption spectra The calorimetric data of the investigated TPPs show that incorporation of the ethylene oxide fragments in the side chains Theory. The large spectral changes which occur upon aggregation are generally interpreted by the molecular exciton markedly affects the phase behaviour compared to appropriate Table 2 Relative fluorescence yieldsa and wavelength maxima [in brackets, nm] of the porphyrins 1–3 in solution at room temp. 1 2 3 solvent I/Imethanol (lmax/nm) I/Imethanol (lmax/nm) I/Imethanol (lmax/nm) chloroform 0.94 [659, 724] 0.95 [654, 720] 1 [657, 723] methanol 1 [656, 721] 1 [651, 717] 1 [655, 721] methanol–water (151 v/v) 0.48 [657, 723] 0.70 [653, 721] 0.18 [659, 725] waterb 0.25 [655, 719] 0.33 [653, 721] 0.13 [659, 725] water (+C12E23)c 0.50 [651, 717] aThe yields I are relative to the yields measured in methanol. Excitation was performed at 515 nm (concentration=5×10-6 M).bDirect after preparation of the solution and degassing.cAfter addition of excess of C12E23. J. Mater. Chem., 1997, 7(4), 615–624 619approximation developed by Kasha.27 This model, which relative to the monomeric species. In 3, the polar fragments are substituted directly at the phenyl ring. The strong inter- neglects electronic overlap of the p-systems, is based on the interaction between localized transition dipole moments.The action of the ethylene oxide fragments with water molecules prevents a close packing of the porphyrin macrocycles and coupling results in a splitting of the monomer state into a higher energy and a lower energy contribution. The resulting hinders the formation of ordered assemblies. In order to deduce a structural model for the arrangement transition energy (E±) is related to the energy of the monomer, Eo eqn.(1) of the porphyrin macrocycles, the absorption spectra can be interpreted in terms of a displacement along the x and E±=Eo+D±V (1) y axes (Fig. 3). in which D is a dispersion energy term and reflects the change For aggregates of 120 and related tetraalkoxyphenyl porphyin environment from monomer to oligomer, while V is the rins,28–30 a splitting of the Soret band into a blue- and a redexciton splitting energy.For a cofacial array of N chromoph- shifted component relative to monomeric species has been oric units this term is related to the magnitude of the transition observed. A stack-of-cards configuration has been proposed28 moment (M) and the geometry of the aggregate as given by in which the porphyrin macrocycles are oriented edge-on-edge eqn.(2). with a separation distance of approximately 4–5 A° [Fig. 3]. The large difference in intensity between the two bands at 400 and 440 nm remains, however, unclear and cannot be explained V#2TAN-1 N BM2 R3 (1-3 cos2 a)U (2) by the exciton theory [see Fig. 1(a)]. In an alternative, symmetrical model, a head-to-tail type In eqn.(2), R is the centre-to-centre distance of two chromo- arrangement could be formulated in which the degenerated phores in the aggregate and a is the angle between the centre- transition dipole moments give rise to one excitonic splitting31 to-centre vector and the transition moment, which are assumed [Fig. 3(d)]. The high energy transition, which is normally to be parallel in this case.The theory predicts, from the forbidden in a parallel coplanar situation, gains some transition selection rule, that when a<54.7° the absorption band of the probability due to a small deviation of the parallel alignment aggregate will be red-shifted ahen a>54.7°, the band will of the transition dipole moments. This explains the presence be blue shifted.of the low-intensity blue shifted band. The assembling number, N, in the stacks can be evaluated A similar phenomenon has been found in polymeric cofaci- semiempirically by eliminating the unkwown variables in eqn. ally stacked O-linked Si-Pc(OC12H25)8 aggregates6d [see (2) which leads to eqn. (3) Fig. 3(b)]. The strong cofacialinteraction of the Pc-macrocycles 1/N=1-[DE(N)/DE(N�2)] (3) results in a blue shifted band of high intensity (a=90°) and a weaker red-shifted band.This splitting might be indicative of where DE(N) is the exciton shift with assembling number N a non-linear SiMOMSi angle, resulting in a deviation from and DE(N�2) for an infinite stack. It can be seen from eqns. zero of the S0–S1 transition moment. (1–3) that the energy shift doubles going from a dimer (N=2) For 2, a larger red-shift of the Soret band has been found to extended aggregates (N=2).(446 nm) than for 1, while a blue-shifted absorption band is centred at 375 nm. These features can be interpreted in a Porphyrins. In the metal free-base TPPs, the intense Soret or B-band has two degenerate perpendicular transition dipole similar way as for 1, so that the most plausible structural arrangement is head-to-tail with a small deviation of a parallel components x and y.The lowest excited state is split up into two components, giving the four characteristic Qx (0–1), Qx alignmentof the transition dipoles. Minor structural differences in the aggregates exist, however, between 1 and 2 due to the (0–0), Qy (0–1), Qy (0–0) absorptions. The largest energy shifts are observed in the Soret band region which is a direct presence of the ether linkages attached to the phenyl ring in 1.We have not performed experiments in order to determine indication of strong exciton coupling. The magnitude of transition moment M for the Q-bands is very low, as compared the assembling number of the aggregates yet.In the literature, attempts have been made to calculate the aggregation number with the Soret band, and only minor shifts are observed in the spectra. In the context of the exciton model we can qualitatively interpret the observed spectral changes of the aggregated TPPs relative to the monomers. For TPP 1, a broad Soret absorption band is found at 422 nm after injecting the acetone solution in 3 ml of water. If there is a strong variation of the mutual distance R and torsion angle between the transition moments, it will lead to substantial broadening of the absorption band.The spectral changes observed on a longer time scale are explained by an increasing interaction of the porphyrin rings (smaller R). The red-shifted peak is rather narrow as compared to the monomeric species and indicates a homogeneous environment of the macrocycles.The peaks sharpen even more when an excess of non-ionic surfactant is added. At these concentrations micelles of C12E23 are formed and the porphyrin aggregates are probably embedded in the hydrophobic part of the micelle resulting in more order and an enhanced stability of the aggregate. The spontaneous aggregation of 1 in water into ordered assemblies is retarded since water molecules are able to interact with the oxygen atoms directly linked to the phenyl rings.In Fig. 3 Structural arrangements of porphyrins or phthalocyanines in 2, no oxygen atoms are linked to the phenyl groups, providing aggregates (a) a the angle between the transition dipole moment (M) a clear separation of the hydrophobic macrocycles and the and the centre-to-centre vector (R), (b) face-to-face x,y : a=90°, blue surrounding hydrophilic ethylene oxide groups.This separa- shift (side view), (c) edge-to-edge, x: a<54.7°, y: a>54.7° red and blue tion results in a spontaneous formation of ordered aggregates shift (top view) and (d ) head-to-tail, x,y : a<54.7° red shift (top view) [ref. 25(b)] as was indicated by a substantial red shift of the Soret band 620 J. Mater. Chem., 1997, 7(4), 615–624of stacks by means of fluorescence quenching studies. Different Aggregation in solution: fluorescence spectroscopy numbers for tetraalkoxyphenylporphyrin assemblies have been The different fluorescence properties of the TPP aggregates as extracted for domain aggregates in LB-films (N7)28 and compared to the Pc-aggregates can be understood in the aggregates in vesicles (N#4).29 The reported maxima (400 and framework of the exciton theory and is schematically depicted 440 nm) in the absorption spectrum for these TPP aggregates in Fig. 4. It has been demonstrated that large exciton coupling are, however, identical despite of the different aggregation occurs in the Soret band of the TPPs while relatively minor number.Interestingly, a dilute aqueous solution of 1 displays coupling effects were observed in the Q-bands. In general, an identical absorption spectrum. It is expected that the fluorescence from TPPs in solution occurs primarily from solution consists mainly of dimers.Upon increasing the concen- the Q-state (S1), although B-state (S2) fluorescence has been tration no spectral changes are observed.Furthermore, the observed for some monolayer assemblies.26 Q-State fluores- absorption spectrum of a spin-coated film has been found to cence in TPP aggregates is thus in principle an allowed be the same as that of an aqueous solution.20 It is expected transition from a relatively unperturbed Q-state [Fig. 4(a)]. that larger aggregates are formed in solid films, although the It has often been stressed that the fluorescence of aggregates film could consist of repeating aggregates of dimers and trimers. is quenched relative to monomeric species as a result of The exciton model predicts an increasing energy shift with interchromophoric interactions35,36 but in other studies1b,37 it increasing assembling number [eqns.(2) and (3)]. The absorp- has been found that the fluorescence quantum yields in porphy- tion spectra of aggregated tetraalkoxyphenyl porphyrins rin dimers or solid films could be as high as for the monomers. derivatives seem to be identical, irrespective of the different Important factors that determine the competition of internal assembling numbers of the stacks.It is not exactly clear why conversion and radiative decay are the steric geometry and the exciton approximation cannot be applied in this particular the orientational order of the aggregates.1b,17 case, or the numbers extracted from the fluorescence quenching The fluorescence data presented in this paper reveal a studies are incorrect. It seems useful to have a polymeric array reduction of the fluorescence yield for all the TPPs relative to of TPP molecules in order to determine DE(N�2), while the monomers indicating that a significant amount of self- more direct methods for the determination of aggregation quenching occurs in the aggregates.The lowest fluorescence number are recommended. yields were found in water for the TPPs 1 directly after preparation of the solution and for 3 in water and methanol– water (v/v 1/1).The UV–VIS spectra displayed relatively broad Phthalocyanines. The lowest excited state of a mononuclear Soret bands which are explained by the existence of aggregates metal-free Pc is split into two components Qx and Qy, giving with an inhomogeneous environment. The orientational dis- two main sharp absorptions at 660 and 700 nm.In a metallo- order results in enhanced rates of internal conversion thereby Pc (e.g. 4b) the excited state is doubly degenerate, giving rise diminishing the fluorescence quantum yield. Interestingly, to one absorption band at 680 nm. Upon aggregation, the Q- additionof C12E23 leads to well-ordered aggregates (see Plate 1) states split into a higher and a lower energy contribution.and an enhancement of the fluorescence yield. Moreover, 2 From the absorption spectra in Fig. 1 and 2, it becomes forms ordered aggregates in methanol–water of which the clear that the tendency for aggregation of the Pcs 4a/b is larger fluorescence yield has been found to be nearly identical to that as comparthe TPPs 1–3. This difference can be explained of the monomer.This supports our interpretation that the by a larger p-system for the Pcs, resulting in a stronger p–p fluorescence yields of TPP aggregates are strongly determined interaction of the aromatic rings, while the flatness of the Pc- by the degree of orientational order in the aggregates. core provides for a closer packing of the rings. The situation for the Pcs is reversed as compared to the It has already been pointed out by Schutte32 that aggregation TPPs [Fig. 4(b)], supported by the spectral shifts upon aggre- in polar solvents is caused by the fact that there is a strong gation, which are in the opposite direction to those for the solvent–solvent interaction excluding the Pc molecules from TPPs. The strongest coupling now occurs in the Q-band region solution which causes them to aggregate.This explains the where the transition probability is larger than in the Soret fact that aggregation in methanol is stronger than in aceto- band region. Since the lowest energy transition is formally nitrile, despite the higher relative permittivity for acetonitrile (emethanol=34 vs. eacetonitrile=37), since there is a much stronger tendency for hydrogen bond formation in methanol.The aggregation effect in solvents with low relative permittivities (e.g. pentane) is explained by minimization of the screening of the Pc–Pc interaction. In contrast to the TPPs described above, which form typically head-to-tail type of aggregates, the Pcs display a broad blue-shifted absorption band as compared to the monomer case.This is indicative of the formation of face-to-face or H-type of aggregates in which the rings are cofacially stacked (Fig. 3(b)]. The maximum of the absorption band lies at approximately 620 nm. In other studies of Pc-aggregates,32,33 it has been found that assembling numbers could succesfully be calculated by using the modified exciton theory [eqn. (3)].Dimers of Pcs octasubstituted by branched alkoxy chains32 or crown ethers15a display energy shifts which are identical as for the aggregates of 4a in water. For the latter species, the mutual distance, R, as well as the other parameters in the exciton approximation, M and a, are expected to be similar to those reported for the former Pc dimers. Thus, we come to the Fig. 4 Schematic representation of the relative energy levels for (a) a conclusion that in a dilute solution (c=5×10-6 M), 4a mainly head-to-tail type of porphyrin dimer and (b) a H- or cofacially stacked consists of dimers.The absorption spectrum of a spin-coated dimer. hnA Refers to allowed absorptions to higher levels (plain film of 4a [Fig. 2(b)] display the same wavelength maximum arrows), hnE is the dimer fluorescence (open arrow), ic=internal as for water (lmax=620 nm) suggesting that the film consists conversion, isc=intersystem crossing and the dashed arrows represent forbidden radiative transitions.of an assembly of dimers [Fig. 3(b)].34 J. Mater. Chem., 1997, 7(4), 615–624 621forbidden in cofacially arranged (H-) aggregates no fluores- energy migration it is important to construct films of highly ordered assemblies consisting of large aggregates.On the other cence and an enhanced intersystem crossing to triplet states is expected.27b This has indeed already been observed for Pc- hand, the mobility of ions through polar channels is found to be much higher in amorphous regions than in crystalline dimers in solution38 and for crown ether Pcs16 which form perfectly eclipsed cofacial dimers in the presence of ions, regions.These requirements seem to be partly fulfilled in the discotic phase of Pc 4a, in which the columnar order is resulting in a relatively narrow blue-shifted Q-band and complete disappearance of the fluorescence. preserved while the hydrophilic ethylene oxide units are in a liquid-like state.It should be noted, however, that there is The relatively broad blue-shifted Q-band in our Pc-dimers indicates a more flexible mutual arrangement of the Pc- an increased positional disorder of the aromatic units in the discotic phase, resulting in less efficient exciton/charge macrocycles in the aggregate resulting in a mixture of staggered and eclipsed dimers. In contrast to the TPP-aggregates, we transport.Materials in which the discs are strictly confined to a one- conclude that it is not the disordered environment which explains the total quenching of the fluorescence in Pc- dimensional spine surrounded by a liquid-like hydrocarbon mantle could be of great interest in the construction of aggregates but can be fully interpreted in terms of the exciton approximation.molecular wires capable of transporting ions and electrons. Polymeric O-linked Si–Pc 4a will be a promising candidate Luminescence studies have also been carried out for the discotic Pc[OC12]8 as a function of temperature.39 At the for combining the functions of efficient electronic and ionic conduction in one system and future research will be planned solid-to-mesophase transition, a sharp drop in luminescence intensity occurred.This was explained in terms of a change in in order to design such materials. the rate of energy migration to quenching sites. However, the phase transition is accompanied by a structural change from Experimental Section a tilted to a cofacial arrangement of the macrocycles which in General methods our opinion perfectly accounts for the disappearance of the fluorescence.NMR spectra were acquired using a 200 MHz Bruker AC200 Finally, some Pcs have shown a high efficiency in photodyn- spectrometer at 298 K. amic therapy since they absorb at longer wavelengths than Differential scanning calorimetry (DSC) measurements were TPPs, especially in a region where human tissues are trans- performed with a DSC 7 (Perkin Elmer) (heating/cooling rate parent.In general, aggregation of Pcs leads to a reduction of 10°C min-1). the luminescence yield, which is a serious drawback in possible Melting points and textures were measured using a Mettler biological applications. It is therefore interesting that very hot-stage attached to an Olympus polarizing microscope. recently a Pc has been synthesized40 with anti-aggregative Heating/cooling rates were 10°C min-1 and near the transition properties by introduction of 16 polyoxyethylene side chains temperatures 2°C min-1.at the periphery of the Pc core. This was the first example of The electronic absorption spectra were recorded with a a water-soluble, non-ionic Pc which exhibits fluorescence in Perkin Elmer Lambda 18 spectrophotometer.Fluorescence aqueous media.41 spectra were recorded on a Perkin Elmer LS5. The aqueous solutions for absorption and fluorescence measurements were Conclusions prepared by injecting a small volume (10–20 ml) of 1mM solution of the compounds in acetone via a microsyringe into We have reported upon the thermotropic phase behaviour and 3 ml solvent. the aggregation properties of three tetraphenylporphyrin and Fluorescence yields for the TPPs were measured relative to two phthalocyanine derivatives containing oligo(ethylene the monomeric TPPs in methanol.All solutions were made in oxide) hydrocarbon units. None of the TPPs was found to be 1 cm cuvettes with an absorption at the excitation wavelength mesomorphic while the Pcs display discotic phases at room below 0.2.temperature. The incorporation of ethylene oxide units at the The relative yields were then calculated by using: Yield= side chains results in a remarkable lowering of the melting and [(AsIun2)/AuIsn02] where: u subscript refers to the TPP in the clearing temperatures as compared to derivatives with unknown solvent and s to the TPP in methanol. A is the unbranched linear alkyl chain derivatives of similar length.absorbance at the excitation wavelength, I the integrated The tendency of Pcs to aggregate in solution has been found fluorescence intensity across the band and n, n0 the refraction to be stronger than for the TPPs. The stronger p–p interaction index of the unknown solvent and methanol. and the flatness of the aromatic cores in the Pcs cause them to aggregate more strongly than the TPPs.The presence of Synthesis of tetraphenylporphyrins 2 and 3 meso-phenyl groups prevents a close packing of the rings. The TPPs form J- or head-to-tail type of aggregates, while Pcs Tetrakis-(6,9,12,15-tetraoxahexadecyl) phenylporphyrin (2). 5-Phenyl-1-hydroxypentane (24.0 g, 0.15 mol) was added care- tend to organize into H- or cofacially stacked aggregates.Despite the structural similarities of the investigated TPPs, fully to 14 ml of PBr3 in 15 min while cooling the solution. The mixture was stirred at 100 °C for 1.5 h and then poured they display a strikingly different aggregation behavior in water. The velocity of organization into ordered assemblies onto ice. After extraction with dichloromethane, the organic fractions were washed with water and NaHCO3 solution and depends on the number of ether linkages substituted at the meso-phenyl group and increases in the order 3<1<2.dried on MgSO4. Vacuum distillation yielded 20.5 g (62%) of 5-phenyl-1-bromopentane. Bp: 146–148 °C (14 mmHg). The luminescence properties of the TPP and Pc aggregates can be explained on the basis of the exciton approximation.Sodium (0.50 g, 22 mmol) was dissolved in 25 ml of triethylene glycol monomethyl ether at 100 °C and cooled to room The degree of self-quenching in the TPP aggregates relative to monomeric species proved to be a function of steric geometry temp. 5-Phenyl-1-bromopentane (4.54 g, 20 mmol) was then added and the mixture was stirred at 100 °C for 3 h.The and orientational order. The cofacial arrangement of the rings in Pc-aggregates results in a forbidden S1–S0 transition and a reaction mixture was poured onto ice and extracted with diethyl ether. The organic fractions were washed with water complete disappearance of the luminescence. Comparison of the solution spectra of the investigated and the solvent was evaporated under reduced pressure.The crude product was purified on silica gel [eluent light compounds with the spectra of spincoated films have indicated that the films consist of a repeating number of small aggregates petroleum (40–60)–ethylacetate 154] yielding 4.78 g (77%) of 6,9,12,15-tetraoxahexadecylbenzene. (dimers). For practical applications in terms of charge and 622 J.Mater. Chem., 1997, 7(4), 615–624A mixture of the latter compound (2.98 g, 9.6 mmol) with Synthesis of the phthalocyanines 4a,b 15 ml of trifluoroacetic acid was stirred at room temperature 1,2-Bis( 1,8,11,14,17-pentaoxaoctadecyl)-4,5- and then 1.54 g (11 mmol) hexamethylenetetraamine (HMTA) dibromobenzene. A mixture of 1,2-dibromocatechol (5 g, was added. The resulting mixture was stirred overnight at 18 mmol), 1-bromo-7,10,13,16-tetraoxaheptadecane, (13.1 g, 90°C.After evaporation of trifluoroacetic acid the residue was 40 mmol) and K2CO3 (16 g, 116 mmol) in 200 ml of DMF dissolved in water and stirred for 1 h. A solution of Na2CO3 was stirred and heated to 100 °C for 24 h and then poured was added until pH=8. The solution was extracted with into water and extracted with CHCl3 (3×150 ml).The organic dichloromethane. The organic fractions were washed with layers were washed with water (3×100 ml), dried with MgSO4 NaCl solution, dried over MgSO4 and the solvent was evapor- and the solvent was evaporated to give a brownish oil. The ated under reduced pressure. The crude product was puri- crude product was purified on silica gel (0.04–0.063 mm; eluent fied on silica gel (eluent ethyl acetate–dichloromethane 151) ethyl acetate) yielding the title compound as a yellow oil (8.2 g, yielding 1.53 g (47%) of 4-(6,9,12,15-tetraoxahexadecyl)- 60%). dH (200 MHz, CDCl3), 1.40 (m, 8H), 1.55 (q, 4H), 1.75 benzaldehyde.(q, 4H), 3.34 (s, 6H), 3.4 (t, 4H), 3.6 (m, 24H), 3.89 (t, 4H), The title compound was then synthesized by refluxing a 7.02 (s, 2H).n/cm-1 (CHCl3) 650 Ar–Br. mixture of 1 equiv. of benzaldehyde together with 1 equiv. pyrrole in propionic acid for 0.5 h. After evaporation of the 1,2-Bis( 1,8,11,14,17-pentaoxaoctadecyl)-4,5-dicyanobenzene. propionic acid, the crude mixture was purified on silica gel A mixture of the preceding compound (3.9 g, 5 mmol) and (eluent dichloromethane with gradual addition of methanol) CuICN (1.25 g, 15 mmol) in 50 ml of DMF was refluxed for and finally on Al2O3 (act.II), eluent dichloromethane with 24 h under a nitrogen atmosphere.The cooled mixture was gradual addition of methanol, yielding of the desired TPP 2 poured in 150 ml of aqueous ammonia (25%) and air was in a yield of 10%. Elemental analysis, Found: C, 71.43; H, 8.42; bubbled through the solution for 24 h.The desired dinitrile N, 3.55%. C92H126N4O16 requires C, 71.56; H, 8.23; N, 3.63%. was extracted with dichloromethane–methanol (4/1). The green dH (200 MHz, CDCl3): -2.76 (s, 2H), 1.5–2.0 (m, 24H), 2.96 organic phase was washed with water and dried over MgSO4. (t, 8H), 3.37 (s, 12H), 3.5–3.7 (m, 68H), 7.55 (d, 8H), 8.11 (d, Chromatography (silica gel, eluent ethyl acetate–methanol, 8H), 8.86 (s, 8H).UV–VIS, lmax/nm (log e/dm3 mol-1 cm-1), gradient 1–3%) afforded 1.8 g (55%) of the title compound. CHCl3: 421(5.68), 522 (4.25), 554 (4.01), 593 (3.75), 649 (3.76). dH (200 MHz, CDCl3), 1.50 (m, 8H), 1.58 (q, 4H), 1.90 (q, 4H), 3.38 (s, 6H), 3.50 (t, 4H), 3.6 (m, 24 H), 4.05 (t, 4H), 7.11 (s, 2H). n/cm-1 (CHCl3): 2210 (CN).Tetrakis(1,4,7,11-tetraoxahexadecyl )phenylporphyrin (3). A residual green fraction was isolated by elution with ethyl Sodium (14 g, 0.61 mol) was dissolved in 364 g of triethylene acetate–methanol (1/1) and identified as the copper–phthalocy- glycol (2.43 mol) at 100 °C and cooled to room temp. 100 g anine 4b (70 mg). Elemental analysis, Found: C, 61.03; H, 8.52; (0.61 mol) of 1-bromohexane was added and the mixture was N, 4.06%.C136H224N8O40Cu requires C, 61.07; H, 8.44; N, stirred for 5 h at 200°C. The mixture was poured into water 4.19%. UV–VIS, lmax/nm (log e/dm3 mol-1 cm-1), CHCl3: (500 ml) and light petroleum bp 100–120°C (400 ml) and 613(4.55), 680 (5.30). heated to 70°C. After separation of the water layer, the organic fraction was washed with hot water (3×100 ml) 2,3,9,10,16,17,23,24-Octa (1,8,11,14,17-pentaoxaoctadecyl) and the solvent was evaporated. Vacuum distillation yielded phthalocyanine ( 4a).The dinitrile (1.6 g, 2.5 mmol) and DBN 62.4 g (44%) triethylene glycol monohexyl ether (bp (0.37 g, 2.5 mmol) were dissolved in 8 ml of absolute ethanol 125–130 °C/0.1 mmHg). and refluxed for 48 h. The reaction mixture turned dark green.A mixture of 58.5 g (0.25 mol) triethylene glycol monohexyl After evaparation of ethanol, the crude oil was chromato- ether and 19.75 g of pyridine (0.25 mol) was stirred and cooled graphed on silica gel (eluent ethyl acetate–methanol 2–10%), in ice. 44.6 g (0.38 mol) Thionyl chloride was slowly added alumina (act.II/III, eluent 2% MeOH in CH2Cl2).The last and the solution was refluxed for 1.5 h. After cooling to room impurities were removed by soxhlet extraction with light temp., the mixture was poured into ice–water. A 10% (m/m) petroleum (60–80) yielding the green sticky phthalocyanine 4a NaCl solution was then added and the resulting mixture was (200 mg, 10%). Elemental analysis, Found: C, 62.67; H, 8.85; extracted with diethyl ether.The organic layers were washed N, 3.93%. C136H226N8O40 requires C, 62.50; H, 8.72; N, 4.29%. with 10% (m/m) NaCl solution, dried and evaporated to dH (200 MHz, CDCl3): 1.5–1.8 (m, 8H), (q, 16H), 3.34 (s, 24H), dryness. Vacuum distillation yielded 52.7 g (84%) of 1-chloro- 3.5–3.7 (m, 108H), 4.63 (t, 16H), 8.95 (s, 8H). UV–VIS: lmax/nm (3,6,9-trioxa)pentadecane (bp 125–130 °C/0.1 mmHg).(log e/dm3 mol-1 cm-1) CHCl3: 602 (4.41), 646 (sh, 4.67), 665 1-Chloro-(3,6,9-trioxa)pentadecane (38.3 g, 0.15 mol), 4- (5.11), 702 (5.21). hydroxybenzaldehyde (12.2 g, 0.1 mol), K2CO3 (27.6 g, 0.2 mol) and 5.0 g of potassium iodide were stirred in 200 ml This work is supported financially by the Dutch Agency for butanone and refluxed for 3 d. The precipitate was filtered off Energy and Environment (Novem).We are indebted to A. van and washed with dichloromethane. The filtrate was concen- Veldhuizen for performing the NMR measurements. trated and a solution of 30 g NaHSO3 in 63 ml of water and 38 ml of ethanol was added and the resulting mixture stirred for 2 h. Extraction was then carried out with diethyl ether and References the organic fractions were washed with water, dried over 1 (a) B.A. Gregg, M. A. Fox and A. J. Bard, J. Am. Chem. Soc., 1989, MgSO4 and evaporated under reduced pressure. 111, 3024; (b) B. A. Gregg, M. A. Fox and A. J. Bard, J. Phys. Yield 7.6 g (23%) of 4-(1,4,7,10-tetraoxahexadecyl)- Chem., 1989, 93, 4227. benzaldehyde. 2 Y. Shimizu, M. Miya, A. Nagata, K. Ohta, I. Yamamoto and The title compound 3 was then synthesized in ca. 10% yield S. Kusabayashi, L iq. Cryst., 1993, 14, 795. using the same procedure as described above. Elemental analy- 3 (a) M. J. Cook, J. Mater. Sci.: Mater. Electron., 1994, 5, 117; (b) sis, Found: C, 71.66; H, 8.53; N, 3.51%. C92H126N4O16 requires M. J. Cook, Adv. Mater., 1995, 7, 877; (c) G. C. Bryant, M. J. Cook, T. G. Ryan and A. J. Thorne, T etrahedron, 1996, 52, 809.C, 71.56; H, 8.23; N, 3.63%. dH (200 MHz, CDCl3), -2.70 (s, 4 C. F. van Nostrum, S. J. Picken, A. J. Schouten and R. J. M. Nolte, 2H), 0.9 (t, 12H), 1.3 (m, 24H), 1.62 (q, 8H), 3.50 (t, 8H), J. Am. Chem. Soc., 1995, 117, 9957. 3.7–3.9 (m, 32H), 4.0 (t, 8H), 4.36 (t, 8H), 7.25 (d, 8H), 8.10 5 (a) C. Piechowski, J. Simon, A. Skoulios, D. Guillon and P.Weber, (d, 8H), 8.89 (s, 8H).UV–VIS, lmax/nm (log e/dm3 J. Am. Chem. Soc., 1982, 104, 5245; (b) C. Piechowski and J. Simon, mol-1 cm-1), CHCl3: 422(5.66), 519 (4.19), 556 (4.01), 593 Nouv. J. Chim., 1985, 3, 159; (c) P. Weber, D. Guillon and A. Skoulios, L iq. Cryst., 1991, 9, 369. (3.67), 650 (3.74). J. Mater. Chem., 1997, 7(4), 615–624 6236 (a) J. F. van der Pol, E. Neeleman, J.W. Zwikker, R. J. M. Nolte 22 (a) A. D. Adler, F. R. Longo and W. Shergalis, J. Am. Chem. Soc., and W. Drenth, Recl. T rav. Chim. Pays-Bas, 1988, 107, 615; (b) J. 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ISSN:0959-9428    

DOI:10.1039/a605328i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Effect of media polarity on the photoisomerisation of substituted stilbene, azobenzene and imine chromophores N. R. King, Eric A. Whale, Fred J. Davis,* Andrew Gilbert and Geoffrey R. Mitchell Polymer Science Centre, T he University of Reading, Whiteknights, Reading, Berkshire, UK RG6 2AD The influence of substituents and media polarity on the photoinduced E�Z geometrical isomerisation of the stilbene, azobenzene and N-benzylideneaniline chromophores has been compared and assessed.The efficiency of the process in all three systems is markedly dependent on the presence and characteristics of electron-donor and electron-acceptor substituents at the 4- and 4¾- positions. The results are discussed in terms of relaxation of the E-excited singlet state. In the absence of a nitro substituent, relaxation to the S1 orthogonal state competes effectively with non-productive intramolecular electron transfer; in the presence of a nitro substituent, the T1 orthogonal state is formed from inter-system crossing.For systems with a 4-nitro and a 4¾-electron-donor substituent, access to the triplet state is inhibited by polar solvents promoting formation of the inactive charge-transfer state from the S1 state, and no isomerisation is observed.Similar effects are observed in both solution and polymer films. Such variations in behaviour have important implications for the utilisation of the chromophores in nonlinear optical phenomena including photorefractivity. There is currently considerable interest in initiating and controlling changes in polymer properties by the use of photoinduced molecular rearrangements of chromophores which are incorporated either as tethered side chains or in a guest–host system.1,2 The E–Z photointerconversions of aryl-substituted 2p systems has attracted particular attention for this application, and the volume of literature describing the use of azobenzene-based dyes and of arylpropenoates for this purpose is considerable.2,3,4 The photoinduced geometrical changes of N-benzylideneanilines and stilbenes also have potential in this area.Azobenzene and stilbene and their derivatives have been extensively studied in the area of nonlinear optical properties, including second-order phenomena such as photoinduced poling5 and frequency doubling6 and in third-order phenomena such as degenerate four-wave mixing7 and photorefractivity.8 The trends found in these investigations may have important implications in the exploitation of stilbenes and azobenzenes in these emerging technologies.While photoinduced geometrical isomerisations can be useful to tailor polymer properties for specified purposes,4,5 they can also be detrimental for other applications; for example isomerisation may lead to an undesirable reduction in the secondorder nonlinear optical response of a material5 or in photorefractivity, a feature which is particularly important with azobenzene-based chromophores.8 Furthermore, the type of photoactivated devices for which these systems may find use is markedly dependent on the time response of the chromophores both in the forward photochemical process and in the back thermal relaxation.9 It is therefore of importance to determine the features of the chromophore and the influence of the media which promote or inhibit the photoisomerisation and its thermal reversion.10 We have observed with a number of these chromophores that their photoresponses and thermal relaxation depend markedly on the nature and position of substituents. Kikuchi et al.have recently described solventdependent anomalous photochemical behaviour of a nitrostyrylpyrene.11 Thus the direct photoconversion of trans-1-[2-(4- nitrophenyl)ethenyl]pyrene 1 to the cis isomer observed in hexane solution with 436 nm light (F=0.11), is reported to be completely suppressed in acetonitrile.A similar, although less marked, influence of solvent polarity is noted with the corresponding nitrostyrylnaphthalene derivative. The influence of solvent on the efficiency of photoisomerisation of the nitrostyrylarenes may be rationalised in terms of the increase in polarity enhancing the relaxation of the excited singlet state to a lowlying intramolecular charge-transfer state and/or promoting a higher degree of electron transfer in this state (as shown in Scheme 1).The resulting dipolar species 2 has low energy and undergoes neither intersystem crossing to the triplet state nor conversion to the orthogonal excited state from which either geometrical isomer may be formed.Thus no, or very inefficient, E–Z photoconversion is observed. Here we evaluate the way in which substituents can influence the solvent polarity-sensitive photoisomerisation of stilbenes, and consider whether such sensitivity to the polarity of the media is relevant to the azobenzenes and to benzylideneaniline derivatives: the former chromophores are commonly used for nonlinear optical phenomena and as chromophores for inducing photochemical changes in polymer systems.Stilbenes and arylpropenoates isomerise by internal rotation about the ethene bond in the p,p* excited state, but in contrast azobenzenes and benzylideneanilines may also undergo such geometrical change by a mechanism which involves a semi-linear excited transition state with inversion of the bond angle at one of the nitrogen atoms.12 max.625 Results and Discussion The absorption data obtained from UV–VIS spectroscopy of selected stilbenes, azobenzenes and benzylidine anilines are summarised in Table 1. The long wavelength absorption maxima are given for both methylcyclohexane and acetonitrile solutions together with the absorbance observed before and after irradiation (to a steady-state absorbance) at the l These data are discussed below.Stilbenes It has been known for many years that, upon direct irradiation, geometrical isomerisation of unsubstituted stilbene and the J. Mater. Chem., 1997, 7(4), 625–630Scheme 1 Decay of the S1 state of 2p systems and the associated geometrical conversions for diaryl systems 4-cyanostilbenes occurs from the singlet excited state,13,14,15 whereas nitrostilbenes undergo the conversion from the triplet excited state.16,17 Increasing the solvent polarity lowers the efficiency of the E to Z photoconversion of 4-nitrostilbene (W=0.5 in benzene and 0.39 in methanol) but this effect is little different from that reported for 4,4¾-dinitrostilbene.16 However, in agreement with the above rationale for the influence of solvent polarity, an increase in the electron-donor properties of the 4¾-substituent increases this effect and the quantum yield for the E to Z isomerisation for 4-nitro-4¾- dimethylaminostilbene 3 decreases from 0.28 in cyclohexane18 to essentially zero in acetonitrile.19 Solvent effects on the photophysical behaviour of 320 and of electron-rich stilbenes have been discussed in terms of a twisted intramolecular charge-transfer species.21 We were interested in electron-donor–electron-acceptor 2p systems for their use in nonlinear optical applications and in photorefractive polymers, and have examined a number of stilbenes in this context.Essentially, the influence of solvent on the photoinduced E–Z isomerisation, as measured by absorption spectroscopy, was predictable and directly related to the potential for intramolecular electron-transfer in the excited state of the stilbene. For example, while the efficiency of geometrical isomerisation of 4-nitro-4¾-alkoxystilbenes decreased markedly as the solvent polarity was increased, the results from irradiation of the more polar derivatives 3 and 4 were in agreement with literature reports.11,18,19 Thus in cyclohexane, irradiation at the lmax of 415 and 454 nm of 3 and 4, but with a low potential for excited state intramolecular respectively, induced E to Z isomerisation, but in acetonitrile neither stilbene showed any spectral change.Furthermore, while luminescence was observed in the non-polar solvent from both stilbenes, no emission was detected in either case in acetonitrile solution consistent with very rapid decay of the S1 state to a non-emissive intramolecular charge transfer stat20 4-Cyanostilbene 5 produced no solvent-dependent photochemistry but it is to be noted that for cyanostilbenes the geometric isomerisation arises from the singlet rather than the triplet Azobenzenes For azobenzenes, photoinduced Z–E isomerisation is a more complex process since there are two pathways of rotation involving an orthogonal p-state (as for stilbene) and inversion about the nitrogen.12 Either mechanism may be operating, and which is favoured would seem to depend upon the character of the excited state (n,p* or p,p*); furthermore, it is likely that this is influenced by the nature and position of the substituents.Despiteconsiderable interest in the photochemistry of azobenzenes, there is only scant information concerning the mechanism of the interconversion of derivatives having both electrondonor and electron-acceptor substituents in a conjugative relationship (frequently termed ‘pseudo stilbenes’).Since in the system the two excited states have similar energies, in such derivatives both inversion (n,p*) and rotation (p,p*) pathways are likely; furthermore, the Z to E thermal reversion, probably by an inversion process, may be expected to be very rapid.12 From our studies, it is apparent that at room temperature azobenzene derivatives having either (i) a 4-nitro substituent electron-transfer, as in 6, or (ii) significant electron-donor– electron-acceptor properties but with a cyano in place of the 4-nitro group as the electron-acceptor moiety, as in 7, undergo E to Z isomerisation in polar and non-polar solvents with closelysimilar efficiencies. Thus isomerisation, probably following intersystem crossing due to the presence of the nitro group in the former case, occurs in preference to relaxation to the intramolecular charge-transfer state.The situation can, however, be readily perturbed and azobenzenes such as Disperse 626 J. Mater. Chem., 1997, 7(4), 625–630 state.15 These results are consistent with the proposal that while relaxation of the E-excited singlet state of the stilbene to the common orthogonal state can compete effectively with intramolecular electron-transfer, intersystem crossing and access to the triplet state is inhibited by polar solvents for systems having a 4-nitro and 4¾-electron-donor substituents.Table 1 UV–VIS spectra of chromophores 3–16 in methylcyclohexane and acetonitrile l Chromophore max/nm 415 454 315 332 364 371 450 410 436 340 375 methylcyclohexane effect of irradiationa % cis o Aps >40b 0.68 >38b 0.74 >56b 0.58 >37c 0.55 >88b 0.23 0.96 0.27 0.45 1.16 0.84 A 1.14 1.20 1.17 0.87 1.83 1.05 0.39 0.54 1.29 0.95 1.36 0.77 >9b >31b >17b >10d >11e >43e J.Mater. Chem., 1997, 7(4), 625–630 lmax/nm 435 464 315 335 364 375 475 430 439 357 374 acetonitrile effect of irradiationa % cis o Aps ~0b 0.905 1.45 0.31 0.36 0.27 ~0b >59b >33c >87b >13b 0.93 ~0b 0.47 ~0b 1.10 0.59 1.02 A 0.91 1.45 0.75 0.54 2.10 1.05 0.47 1.13 0.59 1.25 1.31 0.76 ~0d >18e >43e 627Table 1 (continued ) methylcyclohexane 380 374 355 Red 1 9 and Disperse Orange 3 10, having the potential both for significant intramolecular charge-transfer through the presence of donor and acceptor groups in aconjugative relationship and for reactivity from the triplet state by the mediation of the nitro group, again showed solvent polarity-dependent photochemistry.Thus, as with the stilbene systems,the presence of a 4¾-alkoxy group on the 4-nitroazobenzene chromophore is sufficient to reduce markedly (ca. two-fold) the E–Z photoisomerisation efficiency on changing of the solvent from methylcyclohexane to acetonitrile. Furthermore, the magnitude of this influence is temperature-independent between room temperature and-35 °C indicating that the phenomenon does not arise from a rapid thermal relaxation of the Z isomer in the more polar solvent.In contrast, it is again evident that more powerful electron-donor 4¾-substituents, as on the two Disperse dyes 9 and 10, promote the intramolecular electrontransfer in acetonitrile solution to the exclusion of intersystem crossing as no photoisomerisation was observed in these systems down to -35 °C.Photoinduced E–Z isomerisation could be detected in both 9 and 10 at room temperature in methylcyclohexane by simultaneous monitoring of their absorption spectra during irradiation, but total reversion occurred rapidly in the dark.The half-lives of the Z isomers of 9 and 10 at 293 °C were determined as 0.65±0.05 and 1.00±0.07 s, respectively. The Z isomers of both dyes were stable at -35°C and the spectra were fully restored to those of the E isomers on warming the solutions to room temperature. From these results, it is apparent that although the geometric conversion of azobenzenes may be accomplished by inversion at nitrogen and bond rotation, the influence of substituents and solvent polarity dominates the E–Z isomerisation process.Thus, as for the stilbenes, the efficiency of intramolecular electron-transfer compared to isomerisation in the E-azobenzenes is dependent upon the nature of the electron-acceptor group (cyano or nitro) at the 4-position and the strength of the 4¾-donor substituent in conjunction with the polarity of the solvent.aReduction in Amax reflects minimum concentration of cis isomer at the photostationary state; a spectral subtraction procedure shows that the actual concentration of the unstable isomer is generally a few percent higher than this value. bAt 298 K. cAt 283 K. dAt 238 K.eAt 243 K. fAt 248 K. shown in the use of the conversions of the conjugated diarylazomethine systems for the manipulation of polymer properties, possibly because of their hydrolytic liability. However, since the thermal barrier between the two isomers is low, the relaxation from a photoinduced geometrical change in these imino compounds is extremely rapid and this feature may offer advantages for some light-driven devices.In 1977 Maeda and Fischer reported the photoinduced E–Z isomerisation of a number of N-benzylideneanilines and from experiments at -70°C they were able to characterise the thermally labile Z isomers.24 These authors also note that of the many derivatives of diarylazomethines studied, only the 4-nitro-4¾-dimethylamino derivative 11 did not exhibit the photoinduced changes in non-polar solvents, but the reasons for this apparent anomaly were not discussed.In view of the results from the present and previous studies with the similarly substituted stilbenes and azobenzenes, this observation by Maeda and Fischer is unexpected and implies that even in aliphatic hydrocarbon solvents, photoinduced electron-transfer to yield the inactive intramolecular charge-transfer state can be exclusive for diarylazomethine systems substituted with both powerful electronacceptor and -donor groups in a conjugative relationship.However, further investigations into the influence of electrondonor–electron-acceptor substituents to mediate either the E to Z isomer formation photochemically or the thermal retroprocess appear to have attracted very limited attention.By modifying the electron-donor–electron-acceptor properties of the substituents, we have been able to observe E–Z photoinduced isomerisation in a range of polarisedN-benzylideneanilines, and, furthermore, with these systems, there appears to be the potential to tailor the photoresponse for particular applications. Thus irradiation at the wavelength maximum of solutions of N-(4-nitrobenzylidene)aniline 12, and azomethine derivatives such as 13, which have a cyano group as the 4- position electron-acceptor substituent and a weaker electrondonor group than amino in the 4¾-position, produced the change in the absorption spectrum associated with E–Z isomerisation.As for the respective corresponding azo compounds 6 and 7, the photoconversions of 12 and 13 occurred with approximately equal efficiencies in both polar and non-polar solvents. Combining the influences of the nitro group with the weaker alkoxy donor substituent did, however, have a considerable effect on the photochemistry of these diarylazomethines.Thus in marked contrast to the photoinactive derivative 11, ~0f >23f Imines The mechanism of E–Z photoisomerisation of the imino chromophore may again involve either or both bond rotation and linear inversion. There is an appreciable amount of literature on the photoinduced geometrical change of oximes and hydrazones,22,23 but little interest seems to have been 628 J.Mater. Chem., 1997, 7(4), 625–630 effect of irradiationa >21d 0.98 1.24 >62f 0.35 0.92 >22f 0.28 0.36 acetonitrile effect of irradiationa >19d 0.66 0.82 380 1.41 1.43 380 0.36 0.47 359Fig. 1 UV Spectra obtained as a function of time on irradiation of N-(4-nitrobenzylidene)-4-(6-hydroxyhexyl)aniline 15 at -25°C in methylcyclohexane (spectra recorded every 120 s) Fig. 2 UV Spectra obtained as a function of time on irradiation of N-(4-nitrobenzylidene)-4-(6-hydroxyhexyl)aniline 15 at -25°C in acetonitrile (spectra recorded every 120 s) 14 underwent photoisomerisation at -10 °C in methylcyclohexane and at a rate three times faster than in acetonitrile. At this temperature, the Z isomer was stable but reverted rapidly to the E isomer on warming to 25°C.As we wished to use such derivatives as 14 for pendent chromophores on polyacrylate backbones, the N-benzylideneaniline 15 was synthesised. This azomethine underwent a smooth E toZ isomer conversion at -25°C in methylcyclohexane (Fig. 1), but under the same conditions in acetonitrile solution little change was observed in the absorption spectrum (Fig. 2) of 15. Similar results were observed with 15 at 0°C, but at this temperature the thermal reversion to the E isomer is rapid and there was less than 20% photoconversion in the stationary state from irradiation in the non-polar solvent. Polymer matricies We have carried out a preliminary assessment of the influence of apolymer matrix on the photochemistry of selected examples of the three types of chromphores.Thin films of poly(methyl methacrylate), polystyrene or polyacrylonitrile (glass transition temperatures well in excess of room temperature) incorporating 5% w/w of the stilbene 3, the azobenzenes 6, 7 and 9, or the N-benzylideneanilines 12 and 13 were prepared by dip coating. In most cases, the films had poor optical characteristics when measured at low temperature; as a consequence, the occurrence of photoinduced E to Z isomerisations was assessed from birefringence measurements of the 18 samples before and after their irradiation with plane polarised light at the wavelength maximum of the chromophores. This technique relies on the biasing of the optical axis of the chromophores in a direction perpendicular to the plane of polarisation of the light source.2 With the exception of the stilbene, all chromophores in poly- (methyl methacrylate) and polystyrene displayed birefringence changes, although the magnitude of the effect was variable between samples.Chromophores 3, 6 and 9 showed no change in polyacrylonitrile and the effect of radiation on the other four samples was only small.Not surprisingly, it appears that the environment in the polymer matrix has a similar effect to a change in the solvent polarity on the photochemistry of the chromophores. Conclusion 1 In summary, the present study shows that the tendency for E to Z photoisomerisation in stilbene, azobenzene and diarylazomethine chromophores is markedly influenced by the electronacceptor–electron-donor characteristics of the 4- and 4¾- substituents, and yet further mediation of the process results from a change in the polarity of the media.The stilbenes and azobenzenes exhibit closely parallel behaviour which may be rationalised by isomerisation from the excited singlet state for those derivatives having a cyano group as the electron-acceptor substituent, but for systems having a 4-nitro group a competition occurs between intersystem crossing to the reactive excited triplet state and intramolecular electron transfer in the S state to the inactive charge-transfer state.For the latter type of chromophores, the competition is markedly influenced by the power of the electron-donor substituent at the 4¾-position and the polarity of the solvent or polymer matrix.For azobenzene systems it is also possible that the low yields of the Z isomer on irradiation can be attributed to a rapid thermal back-reaction; indeed the rate of this process appears to increase markedly with increasing solvent polarity.9,25,26 Clearly, such a mechanism could not apply to stilbene systems. Overall, although the photochemical behaviour of the diarylazomethine system is similar to that of the other two chromophores, the asymmetry in the 2p unit appears to facilitate the intramolecular electron-transfer and this evidently occurs for the 4-nitro-4¾-dimethylamino derivative even in a non-polar solvent.629 Experimental Photochemical measurements The photochemical E–Z geometric isomerisation and the thermal back reaction were measured by absorption spectroscopy supported by HPLC analysis for systems showing change at room temperature.Fig. 3 depicts a schematic diagram of the UV–VIS spectrometer arrangement used for the measurement of the change in UV–VIS absorption spectra of the chromophores whilst simultaneously being irradiated. A 1024 pixel photodiode array (Jobin Yvon) coupled with a monochromator allowed for rapid accumulation of spectra (with a minimum interval of 14 ms).The data described in this presentation used an acquisition time of 250 ms and the absorption spectra were recorded every 20 s. Typically, the absorption spectra were obtained while the sample under investigation was simultaneously irradiated by a 150W xenon arc lamp, the light from which passed through a monochromator set at a wavelength close to the absorption maximum of the chromophore.A purpose-built sample chamber allowed the temperature to be kept constant within the range 173–473 K. The apparatus for monitoring changes in the birefringence of thin polymer films is described elsewhere.27 Polymer films of thickness ca. 1.5 mm were prepared by dip-coating.28 J. Mater. Chem., 1997, 7(4), 625–630Fig. 3 Schematic of the apparatus used for simultaneous irradiation and monitoring of stilbene, imine and azo chromophores at variable temperatures Materials 4¾-(N,N-Dimethylamino)-4-nitrostilbene was obtained commercially and used without further purification.Cyanostilbene was prepared according to a literature procedure; mp 117–118 °C (lit.,29 115°C). The procedure for the preparation of 4-nitro-2-cyano-4¾-(N,N-dimethylamino)stilbene 4 is described below. 4-Nitroazobenzene 6, Disperse Red 1 9 and Disperse Orange 3 10 (Aldrich)were used as supplied, following chromatographic assurance of purity. 4-Cyano-4¾-hydroxyazobenzene and 4-nitro-4¾-hydroxyazobenzene were prepared from cyanoaniline and nitroaniline by diazotisation followed by coupling with phenol in the usual way;30 both compounds gave satisfactory analytical data. The pentyloxy-substituted azobenzenes 7 and 8 were prepared from these compounds via their reaction with bromopentane in DMF containing suf- ficient (>1 equiv.) potassium carbonate to ionize the phenol.Subsequent purification using column chromatography and recrystallisation gave the desired compounds in yields of 50% or greater. The imine chromophores were prepared using standard procedures. Alkoxy and v-hydroxy alkoxy imines were prepared from the corresponding hydroxy-terminated imine by reaction with the appropriate alkyl halide.The imine precursor was added to a solution of sodium methoxide (formed in situ from Na and methanol); this was stirred for approximately 20 min before dry toluene was added. The solvent was then removed by vacuum distillation. Dimethylformamide and the appropriate alkylhalide were then added and the solution was heated at 110 °C for at least 12 h.The solution was added to water and the organics separated by washing with diethyl ether. The diethyl ether was then dried using MgSO4. The solvent was removed by rotary evaporation leaving an oil. This was purified by column chromatography (silica using 151 diethyl ether–CH2Cl2 as the eluent). 4-Nitro-2-cyano-4¾-(N,N-dimethylamino)stilbene 4. A mixture of 2-methyl-5-nitrobenzonitrile (40 mmol, 6.5 g) and 4- (N,N-dimethylamino)benzaldehyde (50 mmol, 5.5 g) with 10 drops of piperidine was heated for 2 h at 140°C.A red solid cake formed during the reaction. The solid product was dissolved in 30 ml of hot ethanol and the solution was refrigerated. Red crystals formed over 12 h, which were removed by filtration and then recrystallised from dioxane; mp 218 °C; 630 J.Mater. Chem., 1997, 7(4), 625–630 (H nmax/cm-1 2224; 1H NMR (400 MHz, CDCl3): d 8.42 [1H 3 ), d, J5-3 2.2], 8.31 [1H (H5), dd, J3-5 2.0, J6-5 9.0], 7.91 6), d, J5-6 9.2], 7.49 [2H (H2¾,6¾) d, J3¾2¾=J5¾6¾=9.0], a), d, Jb-a 15.8], 7.21 [1H (Hb), d, Ja-b 16], 6.69 3¾,5¾) d, J2¾-3¾=J6¾-5¾=9.0], 3.10 [6H (NMe2 ), s]; [1H (H 7.42 [1H (H [2H (H 13C NMR (100 MHz, CDCl3): d 151.55, 147.42, 144.62, 138.51, 129.42 (C2¾,6¾), 128.62, 127.14, 124.85, 122.88, 116.50, 116.42, 111.89 (C3¾,5¾), 110.30, 40.09 (NMe2).This work was funded by the EPSRC through GR/H66891 (21st Century Materials initiative) and through a studentship (to N. R. K.). References 1 J.L. R. Williams and R. C. Daly, Prog. Polym. Sci., 1977, 5, 61. 2 S. Xie, A. Natansohn and P. Rochon, Chem.Mater., 1993, 5, 403. 3 J. Stumpe, O. Zaplo and D. Kreysig, Macromol. Chem., 1992, 193, 4 S. H. Barley, A. Gilbert and G. R. Mitchell, Macromol. Chem., 5 P. M. Blanchard and G. R. Mitchell, Appl. Phys. L ett., 1993, 63, 6 Molecular Non-L inear Optics, Materials, Physics, and Devices, ed.1567. 1991, 192, 2801. 2038. J. Zyss, Academic Press, 1994. 7 P. N. Butcher and D. Cotter, T he Elements of Nonlinear Optics, 8 K. Meerholz, B. L. Volodin, Sandalphon, B. Kippelen and 9 T. Fischer, R. Ruhmann and A. Seeboth, J. Chem. Soc., Perkin 10 T. Husain, G. R. Mitchell and A. Gilbert, Mol. Cryst., L iq. Cryst., 11 Y. Kikuchi, H.Okamoto, T. Arai and Q. K. Tokumaru, Chem. Cambridge University Press, Cambridge, 1990. N. Peyghambarian, Nature, 1994, 371, 497. T rans 2, 1996, 1087. 1994, 246, 311. L ett., 1993, 1811. 12 For a review of the photoinduced isomerisation of azobenzenes, see H. Rau in Photochemistry and Photophysics, ed. J. F. Rabek, CRC Press, Boca Raton, 1990, vol. 2, ch. 4. 13 J.Satiel, A. Marinari, D. W-L. Chang, J. C. Hitchener and E. D. Megarity, J. Am. Chem. Soc., 1979, 101, 2982; J. Satiel and J. L. Charlton, in Rearrangements in Ground and Excited States, ed. P. DeMayo, Academic Press, New York, 1980, vol. 3, p. 25. 14 D. H.Waldeck, Chem. Rev., 1991, 91, 415. 15 D. Schulte-Frohlinde and D. V. Bent, Mol. Photochem., 1974, 6, 16 D. Schulte-Frohlinde and H. Go�rner, Pure Appl. Chem., 1979, 51, 17 H. Go�rner and D. Schulte-Frohlinde, Ber. Bunsen-Ges. Phys. 315; H. Go�rner, J. Photochem., 1980, 13, 269. 279. Chem., 1984, 88, 1208. 18 H. Go�rner, J. Photochem. Photobiol., A, 1987, 40, 325. 19 H. Gruen and H. Go�rner, J. Phys. Chem., 1989, 93, 7144. 20 R. Lapouyade, A. Kuhn, J-F. Le�tard and W. Rettig, Chem. Phys. L ett., 1993, 208, 48. 21 J-F. Le�tard, R. Lapouyade and W. Rettig, Chem. Phys. L ett., 1994, 222, 209. 22 A. Padwa, Chem. Rev., 1977, 77, 37; A. C. Pratt, Chem. Soc. Rev., 1977, 6, 63. 23 See S. T. Reid, Photochemistry, Specialist Periodical reports of the Royal Society of Chemistry, 1979–1995, vols. 10–26 inclusive for later reviews in this area. 24 K. Maeda and E. Fischer, Israel J. Chem., 1977, 16, 294. 25 P. D. Wildes, J. G. Pacifici, G. Irick, Jr. and D. G. Whitten, J. Am. Chem. Soc., 1971, 93, 2005. 26 N. Nishimura, T. Sueyoshi, H. Yamanaka, E. Imai, S. Yamamoto and S. Hasegawa, Bull. Chem. Soc. Jpn., 1976, 49, 1381. 27 N. R. King and G. R. Mitchell, unpublished data. 28 P. M. Blanchard, A. Gilbert and G. R. Mitchell, J. Mater. Chem., 1993, 3, 1015. 29 G. Riezebos and E. Havinga, Recl. T rav. Chim. Pays-Bas., 1961, 80, 446. 30 I. M. Vogel, T extbook of Practical Organic Chemistry, 5th edn., revised B. S. Furniss, A. J. Hannaford, P. W. G. Smith and A. R. Tatchell.Paper 6/07980F; Received 25th

ISSN:0959-9428    

DOI:10.1039/a607980f

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Investigation of the photoelectric conversion of a novel molecule (E)-N-methyl-4-{2- [4-(dihexadecylamino)phenyl]ethenyl}pyridazinium iodide, in LB films fabricated on an SnO2 electrode T-R. Cheng, C-H. Huang* and L-B. Gan State Key L aboratory of Rare EarthMaterial Chemistry and Applications, Peking University, Beijing, 100871, People’s Republic of China An investigation of cathodic photocurrent response from a novel molecule with strong non-linear optical properties (second harmonic generation), (E)-N-methyl-4-{2-[4-(dihexadecylamino)phenyl]ethenyl}pyridazinium iodide (MHPd) in LB films on transparent SnO2 glass has been carried out.A quantum yield of 0.3% was obtained under irradiation with 450 nm light in 0.5 mol dm-3 KCl electrolyte solution and ambient conditions. The studies of the relationship between the light intensity and the cathodic photocurrent indicate that MHPd assemblies perform a unimolecular process in charge recombination.The effects of bias voltage, oxygen and nitrogen, electron donors and acceptors such as methyl viologen diiodide, ascorbic acid and hydroquinone were examined and have provided supporting evidence for the proposed electron-transfer mechanism.Photoconductivity of Langmuir–Blodgett (LB) films has ethanol solution using piperidine as the catalyst.25 The product was purified by column chromatography at 1 atm pressure attracted considerable attention in the field of photoelectronics for their variety of applications, such as molecular switches,1 (eluent: chloroform–methanol, 1051).Elemental analysis and NMR spectroscopic results were satisfactory. The water was LB film rectifiers,2 information storage and processing.3 Several reports have been made on transient photoelectric responses deionized in-house and purified by passing through a EASY pure RF compact ultrapure water system (Barnstead Co., in LB films.4–16 A photocurrent response in dye molecules fabricated as LB films has also been reported.17,18 Key events USA).Chloroform, used as the spreading solvent, KCl, used as the electrolyte for the electrochemical experiment, and occurring after photoexcitation of the aggregate are postulated to be: (1) photogeneration of electron–hole pairs within a dye ascorbic acid (AA) were of analytical grade and were used without further purification.Hydroquinone (HQ) was recrys- aggregate; (2) electron and hole migration within a dye aggregate; and (3) electron transfer between the excited dye aggregate tallized from water before use. Methyl viologen diiodide (MV2+) was synthesized by the reaction of 4,4¾-dipyridyl with and an electron donor or acceptor.19 Basic studies of the molecular details of the photoinduced excess methyl iodide in refluxing ethanol for 6 h.The product was filtered and washed with ethanol at least four times and electron-transfer process are of particular value not only in terms of their photoconductivity but also for many other man- gave satisfactory NMR results. made molecular effects in devices. The results obtained can be helpful in the design of novel molecular structures with optimal Preparation and characterization of LB films photoelectrical properties in the future.Monolayers of MHPd were obtained by spreading a chloro- Recently, researchers have found that dye molecules like form solution of MHPd (ca. 0.5 mg ml-1) onto a pure water squarine,19,20 stilbenes,21 azastilbenes22 and azopyridinium23,24 subphase (25±1 °C, pH 5.6) in a British NIMA Technology have both non-linear optical and photoelectric conversion Langmuir–Blodgett Model 622 trough.The substrate was properties. They act as sensitisers of semiconductor and elec- transparent SnO2 glass and was hydrophilically pretreated.† tron donors and acceptors, at the same time as photocurrent Its lateral resistance was ca. 50 V. The LB films were made by generators.The novel molecule (E)-N-methyl-4-{2-[4-(dihexa- dipping the precleaned substrate into the aqueous subphase decylamino)phenyl]ethenyl}pyridazinium iodide (MHPd) is a and raised at a rate of 5 mm min-1 with a surface pressure of dye molecule. It acts as a good non-linear optical material 35 mN m-1. Transfer ratios were ca. 1.0. The MHPd–SnO2 (second harmonic generation25). In the present work, the dye assemblies used in the photoelectrochemical experiments con- MHPd was transferred as highly ordered molecules onto a sisted of a single layer of MHPd on an SnO2 electrode.transparent SnO2 electrode from a water/air interface and its The characterization of the aggregation of MHPd in LB photoelectrochemical properties were investigated so that a films was carried out by UV–VIS absorption spectroscopy.fundamental understanding of the photoelectrochemical process could be obtained. Photoelectrochemical experiments In all the photoelectrochemical experiments, the measurements were carried out in a conventional three-electrode cell which consisted of a polished platinum wire as the counter electrode, an Ag/AgCl reference electrode and the MHPd LB film- (C16H33)2N N N CH3 MHPd + I– modified SnO2 glass as the working electrode the effective area of which was 0.8 cm2.The supporting electrolyte used was 0.5 mol dm-3 KCl aqueous solution. In preparing samples with Experimental Materials † In cleaning the substrate, the SnO2 glass was immersed in 50% MHPd was synthesized by condensing 1,4-dimethylpyridazine sulfuric acid below 70 °C for less than 10 min, then rinsed with ultrapure water at least three times.with one equivalent of N,N-dihexadecylaminobenzaldehyde in J. Mater. Chem., 1997, 7(4), 631–635 631different concentrations of donor and acceptor, solid samples KCl solution for two days were within this range. Fig. 2 shows the action spectra of the cathodic photocurrent with no bias were added directly to the 0.5 mol dm-3 KCl electrolyte solution.All measurements were made at 25±1 °C on a Model voltage. Its spectral response coincided well with the absorption spectra of LB films (Fig. 1), suggesting that the aggregation of 600 voltammetric analyser (CH Instruments, USA). In the photochemical experiment, a 500W xenon lamp (Ushio MHPd in LB films is responsible for photocurrent generation.Under the same working conditions as those used to obtain Electric, Japan) was used as the excitation source. The incident white light was perpendicular to the contact area of the the action spectrum, the photocurrent obtained at 450 nm (light intensity 1.8×1016 photons cm-2 s-1) ranged from 57 working electrode. The intensity of the incident beam was checked using a model LM-91 Photopower meter (National to 63 nA.This photocurrent means that the quantum yield, which is cited per absorbed photon, is ca. 0.3% (the absorbency Institute of Metrology, Beijing, China). Different wavelengths were obtained by using filters (Toshiba, Japan) with certain of the LB film at 450 nm is ca. 0.92%). bandpasses. For instance, 450 nm light was obtained by using a filter the transmission of which was centred at 450 nm with Effect of bias voltage and light intensity a bandpass of 15 nm.IR light was filtered with a Toshiba IRA- The effect of bias voltage was investigated to explore the 25S (Japan) filter in all the experiments. electron transfer process between the MHPd LB film and the SnO2 electrode. A linear relationship was found between the Spectroscopic measurements cathodic photocurrent generation and the bias voltage applied to the working cell under ambient conditions.The equation All the UV–VIS spectra were obtained from a Shimadzu UV- 3100 spectrometer (Japan). for this line is iph=-1.96 V+406 (deviating coefficient R= 0.9957, Fig. 3), where iph is the photocurrent and V is the bias voltage applied.The negative slope (-1.96 nA mV-1) of the Results and Discussion line implies that the polarity of the electrical field caused by the applied negative voltage is the same as the polarity of the Characterization of the MHPd LB film modified SnO2 electrode inner electrical field from which the cathodic photocurrent is The quality of the MHPd monolayer on the hydrophilically produced.This provides circumstantial evidence for the expla- pretreated SnO2 electrode was controlled by the surface press- nation that the cathodic photocurrent is produced by a flow ure (35 mN m-1) and the transfer ratio (1.0±0.1). At a surface of electrons from the working electrode through the LB film pressure of 35 mN m-1, the film was in the solid phase and to the electrolyte solution.the p–A isotherm in this range is steep and smooth. The high To probe the recombination pathway MHPd may take, the collapse pressure (60 mN m-1) ensures that the film at 35 mN m-1 is stable. The unity transfer ratio ensures that the obtained film on the substrate was 100% transferred and retained the packing pattern as on water/air interface. The area per molecule was ca. 60 A° 2. In the UV–VIS absorption measurements (Fig. 1), a large blue shift from 578 nm in chloroform solution as the monomer to 524 nm for the LB films on the SnO2 substrate was found. This shift indicates that H-aggregates formed in films. It is possible that the aggregation of the molecules resulted from the interactions between the chromophores. Photocurrent generation of the MHPd–SnO2 electrode A cathodic photocurrent was observed from the MHPd–SnO2 electrode.From ten parallel MHPd–SnO2 electrodes, a factor of ca. 2.6 variation in the photocurrent data was found. Samples in 0.5 mol dm-3 KCl generated 150–400 nA photocurrent when they were irradiated with 240 mW cm-2 of white Fig. 2 Action spectrum (electrolyte solution 0.5 mol dm-3 KCl, light intensity 1.8×1016 photons cm-2 s-1, 450 min) light.Photocurrents generated from samples which were stored in a refrigerator (4°C) for two weeks and in 0.5 mol dm-3 Fig. 3 Effect of bias voltage on photocurrent generation (0.5 mol dm-3 Fig. 1 UV spectra of MHPd in chloroform solution (dashed line) and in LB films on SnO2 substrate (solid line) KCl, light intensity 240 mW cm-2, ambient conditions) 632 J.Mater. Chem., 1997, 7(4), 631–635Fig. 4 Effect of light intensity on photocurrent generation (0.5 mol dm-3 KCl, ambient conditions) effect of the irradiation light intensity was investigated. A good linear relationship between the light intensity (240 mW cm-2) and the photocurrent was observed. The equation for this line is iph=1.55I (deviation coefficient 0.997, Fig. 4), where I is the light intensity. Comparing this equation with the generally used form, iph=kIm, one can see that, in our case, k=1.55 and m=1. According to Donovan et al.9 m= 1 is the characteristic of unimolecular recombination. Thus, MHPd underwent unimolecular recombination in the charge loss process. Fig. 5 Effect of oxygen and nitrogen on photocurrent generation Effect of donor and acceptor [0.5 mol dm-3 KCl, degassed conditions for (a), ambient conditions for (b); light intensity 240 mW cm-2] Oxygen and nitrogen.Unless deliberately degassed, oxygen exists in aqueous solutions. Since oxygen itself is a good electron acceptor,19 the photocurrents generated under ambient, nitrogen-degassed and oxygen-saturated conditions were studied.At the beginning, the photocurrent under ambient conditions (0.5 mol dm-3 KCl, 25°C, pure water) was 250 nA. Then, as shown in Fig. 5, upon degassing with nitrogen the cathodic photocurrent decreased gradually to ca. 100 nA and was unchanged thereafter. Regarding the nitrogen-degassed solution as an oxygen-free system, oxygen was bubbled into it. As expected, cathodic photocurrent enhancement was observed. The photocurrent increased from ca. 100 nA to ca. 250 nA and remained constant thereafter. These results indicate that oxygen acts as potential electron acceptor and the ambient oxygen concentration (ca. 2.7×10-4 mol dm-3)26 is the most effective one. Oxygen effectively accepted electrons from the MHPd film and thus promoted electron transfer between the SnO2 electrode, the MHPd film and the electrolyte.Although Fig. 6 Effect of hydroquinone on photocurrent generation. (0.5 mol oxygen is a major factor, it is not the only one which affects dm-3 KCl, nitrogen degassed conditions,light intensity 240 mW cm-2) the photocurrent in the electrolyte solution, because only 60% of the photocurrent was attributed to it. transfer from the SnO2 electrode to MHPd films, but also redirect the flow of electrons.Other donors and acceptors. The effects of other donors and acceptors were studied for further elucidation of the conclusion For the electron acceptor methyl viologen diiodide, the opposite effect was found (Fig. 7). Methyl viologen diiodide obtained above, and also to gain a better view of the probable mechanism of photocurrent generation in the MHPd–SnO2 (EA/A-=-0.23 vs.Ag/AgCl) increased the cathodic photocurrent markedly and stabilized the photocurrent at the system. In this approach, the cathodic photocurrent was generated by irradiation with 240 mW cm-2 white light in 0.5 mol enhanced value. The photocurrent increased gradually with increasing concentration of MV2+. This means that MV2+ dm-3 KCl solution under nitrogen-degassed conditions. Hydroquinone (HQ), methyl viologen diiodide (MV2+) and acted as a supersensitizer in accepting electrons from the MHPd assemblies and therefore increased the concentration ascorbic acid (AA) were used as donors and acceptors. Hydroquinone, whose redox potential is +0.13 vs.Ag/AgCl of electrons involved in the electron transfer process.The levelling off of the increasing effect began at 10 mmol dm-3 (red.) quenched the cathodic photocurrent markedly (Fig. 6) when added to the MHPd–SnO2 system. If the quantity of where the photocurrent was ca. 1300 nA. The effects of both hydroquinone and methyl viologen diiodide have provided HQ added is greater than 0.042 mmol dm-3 (calculated according to the data obtained), it will reverse the photocurrent from further support for our conclusions.However, ascorbic acid which we supposed to be an electron cathodic to anodic. This greater attenuation and reversal of the cathodic photocurrent suggested that by donating electrons donor [ED+/D=+0.10 vs. Ag/AgCl (red.)], resulted in an unexpected increase of the cathodic photocurrent from to MHPd LB assemblies, HQ can not only inhibit electron J.Mater. Chem., 1997, 7(4), 631–635 633Fig. 7 Effect of methyl viologen diiodide on photocurrent generation. (0.5 mol dm-3 KCl, degassed conditions, light intensity 240mW cm-2) Scheme 1 Fig. 8 Effect of ascorbic acid on photocurrent generation. (0.5 mol Scheme 2 dm-3 KCl, degassed conditions, light intensity 240 mW cm-2) aminostilbene has enhanced double-bond character of the ca. 150 nA to ca. 580 nA (Fig. 8). We attribute this unusual central ethenic bond in the first excited singlet state compared observation to the acidity of ascorbic acid whose pKa is ca. with the ground state, whereas the reverse was found for the 4.3.27 Evidence supporting this is as follows: (1) calculations unsubstituted stilbene. Therefore, for dihexadecyl-substituted show that 0.4 mmol dm-3 of AA brings the pH of the solution amino MHPd, the central ethenic bond may have enhanced to 3.93; (2) pH variations caused by >0.4 mmol dm-3 AA are double-bond character. Thus the electron redistribution may much smaller than those caused by <0.4 mmol dm-3 AA; lead to the charge separated state (a) but not to valence (3) in a Britton–Robinson electrolyte solution, the peak photo- tautomerization state (b) which results in reduced double-bond current increased from ca. 300 nA at pH 6.5 to 1278 nA at character in the central ethenic bond, as shown in Scheme 1.pH 4.0; (4) our previous work had proven that pH can affect The proposed mechanism for the electron transfer process the wfb (flat band potential) of SnO2 and the acidic environment can be described as shown in Scheme 2.MHPd was trans- is thus beneficial to cathodic photocurrent generation. The formed to MHPd* (excited state) after irradiation with white levelling off of photocurrent, which begins at 0.4 mmol dm-3 light. The higher energy level of the excited state allowed the AA, may result from the combination of positive AA effect overlap of the energy of MHPd* and ECB (energy of the and the negative acidic effect, which was slowed by the smaller conducting band) of SnO2. According to the Frank–Condon pH difference.principle, the only possibility under these circumstances is electron transfer between the SnO2 substrate and MHPd* in Mechanism of photocurrent generation the LB films.As shown in Scheme 2, electrons flow from the SnO2 semiconductor to the excited state of MHPd* in the LB In terms of the stilbene compounds, there are two possible states they may adopt after illumination with light or absorp- films and the excited state loses electrons to the electrolyte. The MHPd* acts like an electron transporter. Whatever factors tion of energy: a geometry change, such as trans–cis isomerization, 22 and electron redistribution, e.g.charge separation or cause this to occur, as long as they can improve the electron transporting ability, they may enhance the photocurrent valence tautomerization. In the case of facile trans–cis isomerization, a marked decrease of the trans–cis quantum yield (wt–c) MV2+, for example, can accept electrons from the exited state, and it increases the cathodic photocurrent.HQ, which donates was found when the amino group was dimethyl substituted (wt–c was <0.001, and the cis component was <2%). It can be electrons to the ground state of MHPd, prohibits the flow of electrons from SnO2 to MHPd*, thus resulting in a reduced reasonably assumed that when the amino group is dihexadecyl substituted, like in MHPd, the large hindrance can result in photocurrent and even reversing the electron flow.AA, which increased wfb to a higher energy level, caused electrons to flow an even smaller wt–c and a smaller cis component. Therefore facile trans–cis isomerization may not be the pathway taken from SnO2 more easily. Thus, an enhanced photocurrent was observed with the use of AA.by MHPd. Calculations22 show that the dimethyl-substituted 634 J. Mater. Chem., 1997, 7(4), 631–6358 P. E. Burrows, K. J. Donovan and E. G. Wilson, T hin Solid Films, Conclusions 1989, 179, 129. 9 K. J. Donovan, R. V. Sudiwala and E. G. Wilson, Mol. Cryst. L iq. A cathodic photocurrent was obtained from MHPd LB films Cryst., 1991, 194, 337. fabricated on an SnO2 electrode.The quantum yield for 10 T. Nagamura, K. Toyozawa, S. Kamata and T. Ogawa, T hin Solid photocurrent generation upon irradiation of the electrode with Films, 1992, 210, 332. 450 nm light in 0.5 mol dm-3 KCl solution and under ambient 11 K. J. Donovan, R. Paradiso, K. Scott, R. V. Sudiwala, conditions is 0.3%. When the MHPd–SnO2 electrode was E. G. Wilson, R. Bonnett, R.F. Wilkins, D. A. Batzel, T. R. Clark and M. E. Kenny, T hin Solid Films, 1992, 210, 253. subjected to more favourable conditions, such as in an electron 12 K. J. Donovan, R. V. Sudiwala and E. G. Wilson, T hin Solid Films, acceptor solution of methyl viologen diiodide (10 mmol dm-3) 1992, 210, 271. under ambient conditions and with an applied negative bias 13 T. Nagamura, S.Kamata, K. Toyozawa and T. Ogawa, Mol. Cryst. voltage (-100 mV), the quantum yield increased to ca. L iq. Cryst., 1993, 227, 171. 0.8–0.85%, because the cathodic photocurrent under these 14 K. J. Donovan, K. Scott, R. V. Sudiwala, E. G. Wilson, R. Bonnett, conditions is stronger. The effects of methyl viologen diiodide, R. F. Wilkins, T. R. Clark, D. A. Batzel and M. E. Kenny, T hin Solid Films, 1993, 232, 110.ascorbic acid, hydroquinone and the bias voltage support the 15 K. J. Donovan, K. Scott, R. V. Sudiwala, E. G. Wilson, R. Bonnett, conclusion that the cathodic photocurrent was produced by R. F. Wilkins, R. Paradiso, T. R. Clark, D. A. Batzel and the flow of electrons from the SnO2 electrode through the M. E. Kenny, T hin Solid Films, 1994, 244, 923.MHPd assemblies to the electrolyte solution. An electron- 16 K. J. Donovan, J. Elliott, K. Scott, R. V. Sudiwala, E. G. Wilson, separated excited state was considered to be involved in the T. R. Clark, D. A. Batzel and M. E. Kenny, T hin Solid Films, 1994, 244, 928. proposed mechanism for the cathodic photocurrent generation. 17 K. Saito and H. Yokoyama, T hin Solid Films, 1994, 243, 526. 18 H. Yanagi, Y. Toda and T. Noguchi, Jpn. J. Appl. Phys., 1995, The authors wish to thank the National Climbing Plan A and 34, 3808. the National Natural Science Foundation of China for financial 19 Y-S. Kim, K. Liang, K-Y. Law and D. G. Whitten, J. Phys. Chem., support of this project. 1994, 98, 984. 20 P. V. Kamat, S. Das, K. G. Thomas and M. V. George, J. Phys. Chem., 1992, 96, 195. 21 W. S. Xia, C. H. Huang, L. B. Gan and H. Li, J. Chem. Soc., References Faraday T rans., 1996, 92, 3131. 1 S. Nespurek, Int. J. Electronics, 1994, 76, 777. 22 H. Garner and H. Gruen, J. Photochem., 1985, 28, 329. 2 M. Sugi, K. Sakai, M. Saito, Y. Kawabata and S. Hzima, 23 W. S. Xia, C. H. Huang, X. Z. Ye, C. P. Luo, L. B. Gan and Electronics Optics, 1986, 69. Z. F. Liu, J. Phys. Chem., 1996, 100, 2244. 3 S. Kamata and T. Nagamura, Colloids Surf. A, 1995, 103, 257. 24 W. S. Xia, C. H. Huang, L. B. Gan, H. Li and X. S. Zhao, J. Chem. Soc., Faraday T rans., 1996, 92, 769. 4 T. Nagamura, K. Matano and T. Ogawa, J. Phys. Chem., 1987, 25 To be published. 91, 2019. 26 S. L. Morov, Handbook of Photochemistry, Marcel Dekker, New 5 F. Willig, R. Eichberger, K. Bitterling, W. S.Durfee, W. Storck and York, 1973, p. 89. M. Van der Auweraer, Ber. Bunsen-Ges. Phys. Chem., 1987, 91, 869. 27 S. C. Lin and Y. H. Zeng, Principles of Acid–Base T itration, 6 J. K. Severn, R. V. Sudiwala and E. G. Wilson, T hin Solid Films, Advanced Education Press, Beijing, 1989, p. 489. 1988, 160, 171. 7 T. Nagamura, S. Kamata, K. Sakai, K. Natano and T. Ogawa, T hin Solid Films, 1989, 179, 293. Paper 6/06721B; Received 2nd October, 1996 J. Mater. Chem., 1997, 7(4), 631–635 635

ISSN:0959-9428    

DOI:10.1039/a606721b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Si/C phases from the IR laser-induced decomposition of 1,4-disilabutane Elvira A. Volnina,a Jaroslav Kupc¢§©¥¢¥ k,b Zdene¢§k Bastl,c Jan S ¢§ ubrt,d Leonid E. Gusel¡�nikova and Josef Pola*b aInstitute of Petrochemical Synthesis, Russian Academy of Sciences, 117912 Moscow, Russia bInstitute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 16502 Prague, Czech Republic cJ.Heyrovsky¢¥ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, 18223 Prague 8, Czech Republic dInstitute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 25068 R¢§ ez¢§ near Prague, Czech Republic CO2 laser-induced decomposition of 1,4-disilabutane (DSB) in the gas phase yields gaseous C1.2 hydrocarbons and RSiH3 compounds (R=H, CH3, C2H5 and C2H3), and it represents a convenient process for chemical vapour deposition of thin films composed of Si/C/H, Si/C and Si components.Hydrogenated amorphous Si1.xCx materials, prepared by Samples of the solid deposits were produced on different substrates (KBr, Cu) housed in the cell before the irradiation. chemical vapour deposition (CVD) via conventional pyrolysis, photolysis or plasma-assisted decomposition of various volatile For XPS, SEM and TEM measurements the samples had to be transported from the cell and exposed to ambient precursors, have recently attracted much attention due to their importance in photovoltaic and optoelectronic applications atmosphere.Properties of the deposit were measured on the FTIR and in high-temperature ceramics.Properties of the Si/C films can be improved by the choice of the CVD conditions and by spectrometer, a VG ESCA 3 Mk II electron spectrometer and a SEM Tesla BS ultrahigh vacuum instrument equipped with using specially designed hydridocarbosilanes which offer better control of the Si/C stoichiometry and lower deposition an energy dispersive X-ray analyser (EDXA). Transmission electron microscopy measurements of the solid precipitates temperatures (see, e.g., refs. 1.9). Of these precursors, silacyclobutane, 6,10,11 1,3-disilacyclobutane,7,10,11 disilylmethane,5,12 were carried out using a Philips 201 microscope. X-Ray photolelectron spectra and X-ray excited Auger elec- trisilylmethane,12 1,3-disilabutane,13 tetramethylsilane14 and monoorganylsilanes15,16 have been studied.The synthetic tron spectra were obtained using Al-Ka and Bremsstrahlung radiation, repectively. The energy resolution expressed by the approach to another interesting class of Si/C precursor, a,vdisilaalkanes, has been solved,17 but the potential of these fwhm of the Au 4F7/2 line was 1.2 eV. The energy scale of the spectrometer was calibrated with the Au 4f7/2 binding energy compounds in the CVD of Si/C materials has not yet been assessed.fixed at 84.0 eV. Detailed spectral scans were taken over Si 2p, Si (KL23L23) and C 1s regions. The overlapping peaks were Following our previous studies on various Si/C phases prepared from single precursors using excimer-laser11,16,18 or resolved into individual components using a Shirley-type23 background, Gaussian.Lorentzian lineshapes and the damped IR-laser10,15,19.21 radiation, we report in this paper on the CO2 laser-induced decomposition of 1,4-disilabutane (DSB) and non-linear least-squares technique.24 Quantification of the element surface concentration ratios was accomplished by properties of the Si/C phases deposited in this process. correcting the integral intensities of the photoelectron peaks for their cross-sections25 and accounting for the dependence of the analyser transmission26 and electron mean free paths on Experimental kinetic energy.27 The results obtained did not depend (within The experiments were carried out with a tunable TEA CO2 the experimental error) on angle of electron detection, thus laser (Plovdiv University) operating on the P(38) line of the showing the absence of measurable surface concentration 0001.1000 transition (927.0 cm.1).The wavelength and flu- gradients. ence were checked by a model 16-A spectrum analyser (Optical DSB (>99.5% purity) was obtained using the procedure Eng. Co.) and by a pyroelectric detector (ml-1 JU Charles described in ref. 28 and was distilled prior to use.University). Gaseous DSB was irradiated with a number of pulses at a fluence of 0.3 J cm.2 in a glass cell (45 mm i.d., Results and Discussion 10 cm length) equipped with two NaCl windows and a valve ended by a septum. The irradiation of the d(Si.H) mode of DSB [the absorption Changes in the composition of the cell content were moni- band centred at 928 cm.1, Fig. 1(a)] leads to a strong visible tored by gas chromatography (gas chromatograph Shimadzu luminescence which is most intense during the first ten pulses, 14A equipped with FID, and coupled with Chromatopac C- the formation of volatile hydrocarbons (ethene, ethyne and R5A computing integrator; columns packed with Porapak P methane) and organosilanes (silane, vinylsilane, methylsilane, and OV-1 silicon elastomer, temperature programmed) and by ethylsilane), and also to the deposition of a brown material.FTIR (Nicolet, model Impact 400 spectrometer) spectroscopy. The production of this deposit was observed after a single Reaction progress was estimated by using a diagnostic band pulse and was most effective within the first 40 pulses. of DSB at 927 cm.1. The volatile products of the DSB decomposition were identified by GC.MS (Schimadzu QP Volatile compounds and the gas-phase chemistry 1000 quadrupole mass spectrometer).The quantitativeanalyses are based on the knowledgeof response factors for the identified Regarding the volatile products (Fig. 2) and their alteration with the decomposition progress (Fig. 3), the relative yield of products which were determined or taken from ref. 22. J. Mater. Chem., 1997, 7(4), 637.640 637silane (5–60%) increases and those of ethene (ca. 30–60%) and vinylsilane (20–10%) decrease. The other volatile products are formed in only very small quantities: ethyne (2–4%), methane (ca. 1%), ethylsilane (<1%), methylsilane (1–2%) and 1,2,5- trisilapentane (ca. 0.5%). The residual pressure in the reactor after freezing the volatile products in a trap indicated the presence of hydrogen. The identified volatile products were estimated to be formed by less than 20% of the DSB, which implies that the parent compound is mostly utilized for the formation of the solid deposit.The volatile products indicate that the DSB decomposition is initiated by at least four primary reactions which are 1,1-H2 (1), 1,2-H2 (2), alkane (ethylsilane, 3) and alkene (vinylsilane, 4) elimination pathways (Scheme 1).These reaction prototypes are commonly known to operate in the decompositions of alkylsilanes (see, e.g., refs. 29–32). Fig. 1 FTIR spectra of DSB (1.3 kPa) (a) and of the deposit before (b) and after (c) exposure to air H3SiCH2CH2SiH3 H3SiCH2CHNSiH2 :SiH(CH2)2SiH3 :SiH2+C2H5SiH3 :SiH4+H2CNCHSiH3 d (1, 1-H2) d (1, 2-H2) d (alkane) d (alkane) (1) (2) (3) (4) Scheme 1 Other plausible pathways are 1,4-H2 elimination or intramolecular b-CMH insertion into :SiH(CH2)2SiH3 leading to H2SiCH2CHSiH3 and further to :SiH2+H2C=CHSiH3 (ref. 33). The small amounts of vinylsilane and ethylsilane indicate that these compounds are intermediary products; it can be inferred34,35 that they decompose into C2H4, C2H5(H)Si:, H2Si: and CH3CH=SiH2, and H2Si: and C2H4, respectively. The depletion of ethene in the course of the DSB decomposition indicates that ethene is removed by reactions with silylenes.The intermediacy of the simplest H2Si: silylene is revealed (i) by the presence of 1,2,5-trisilapentane formed via insertion of :SiH2 into DSB, and (ii) by the presence of silane which can only be formed by the reaction of silylene with molecular hydrogen. The accumulation of silane as the DSB decomposition progresses (Fig. 2) serves as evidence of significant amounts of dihydrogen in the decomposition mixture produced Fig. 2 Typical GLC–MS trace of the mixture obtained on laser by H2 elimination. irradiation of DSB. Column: Porapak Peak identification: 1, air; 2, SiH4; 3, C2H4; 4, CH3SiH3; 5, H2CNCHSiH3; 6, H5C2SiH3; 7, DSB; Properties of the deposit 8, H3SiSiH2CH2CH2SiH3 . The solid films show [Fig. 1(b)] IR absorption at 809 [n(SiMC)], 1260 [d(CH3MSi)], 2120 [n(SiMH)], 2900 and 2960 [n(CMH)] cm-1 and they develop [Fig. 1(c)], upon standing in air, an absorption at ca. 1050 cm-1 which is assignable to the n(SiMO) mode.The first three bands constitute the typical pattern of a-SiC5H films36,37 and reveal characteristic absorptions of SiMH, SiMC and CMH in saturated moieties. Comparison of the FTIR spectra of the deposit and of DSB (Fig. 1) reveals that the solid material is much poorer in hydrogen. The relative contents of the SiMH and CMH bonds can be estimated38,39 by using the SiMH and CMH per-bond oscillator strength in SiH4 and CH4.The absorptivity [normalized to that of the n(SiMC) band] of n(SiMC), n(SiMH) and n(CMH), in the given order 1.0, 0.28 and 0.02, are compatible with ca. 2.5 times more H at Si than at C. The best fit of the Si 2p (Fig. 4) core level spectrum is obtained using four components of the same widths. The presence of four components is more clearly seen in the Si (KL23L23) Auger electron spectrum (Fig. 5) of the deposit. Fig. 3 Major product distribution (mol%) in irradiated DSB vs. They are consistent with presence of the four different chemical decomposition progress. 2, C2H4; ', C2H2; &, H2CNCHSiH3; 1, SiH4. states of Si, namely elemental silicon, silicon in SiC, silicon in 638 J. Mater. Chem., 1997, 7(4), 637–640Fig. 6 Fitted photoelectron spectra of C 1s electrons Fig. 4 Fitted photoelectron spectra of Si 2p electrons Fig. 7 SEM image of the deposit Fig. 5 Fitted photoelectron spectra of Si (KL23L23) Auger electrons Si/C/H polymer, and an oxidized form of silicon. The Si 2p core level binding energies, obtained by fitting the measured spectrum, and their assignments based on comparison of the measured values with the literature data40–42 are given in Table 1.The C 1s core level spectra (Fig. 6) reveal the presence of two chemical states of carbon, namely carbidic carbon and carbon belonging to organosilicon polymer. Their concentration and the corresponding binding energies are also displayed in Table 1. The FTIR and XPS analyses show that the gas-phase chemistry results in the formation of three types of materials which reveal the operation of three types of final reaction steps.The formation of a ‘polymeric’ saturated Si/C/H material is due to polymerization reactions of a number of unsaturated species formed in the gas phase by routes (1)–(4) and consecutive reactions. The occurrence of the elemental silicon is in line Fig. 8 TEM image of the deposit (magnification 70000×) with complete dehydrogenation of some gaseous :SiH2 and SixHy intermediates. The presence of silicon carbide is compatible with dehydrogenation of intermediate silenes (possessing strong SiNC bonds); this reaction has been observed under similar conditions upon IR laser irradiation of silacyclobutane and 1,3-disilacyclobutane.10 We stress that the dehydrogenations leading to Si and SiC are feasible due to direct absorp- Table 1 The Si 2p and C 1s core level binding energies (Eb) of the fitted photoemission lines (in eV), their assignment, and the calculated tion of the laser radiation in the transients possessing SiMH atomic concentrations (c) of Si and C present in the individual chemical bonds.The development of the SiMO type component can be states (normalized to S Si=1) explained by a small amount of unsaturated SiNC bonds (see also refs. 10,11,15,16) in the superficial layers and by reaction Eb (Si 2p) Eb (C 1s) assignment c of these bonds with atmospheric oxygen.43 We note that the XPS analysis of the deposit (ca. 5 nm layers) indicates that the 99.6 — Si0 0.30 100.8 — Si carbide 0.34 oxygen cannot be removed by ion sputtering, which implies 102.1 — Si polymer 0.27 that oxygen can penetrate to some depth of the superficial 103.2 — Si oxide 0.09 layers and there react with residual SiNC bonds.— 283.1 carbidic C 0.35 SEM images of the deposits (Fig. 7) show that the deposits — 284.4 C in polymer 0.29 have particulate structure and consist of agglomerates. 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ISSN:0959-9428    

DOI:10.1039/a606524d

出版商:RSC    

年代:1997

数据来源: RSC