摘要:

Synthesis and liquid crystal properties of phthalocyanine derivatives containing both alkyl and readily oxidised phenolic substituents? Paul Humberstone, Guy J. Clarkson," Neil B. McKeown and Kevin E. Treacher Department of Chemistry, University of Manchester, Manchester, UK M13 9PL The synthesis, using a mixed phthalonitrile cyclotetramisation, of phthalocyanine derivatives containing both long alkyl side- chains and redox-active, sterically hindered phenolic (3,5-di-tert-butyl-4-hydroxyphenyl)substituents is described. A detailed structural characterisation, including high resolution NMR spectroscopy, revealed that the statistically predicted amounts of regioisomers were prepared for each compound. Some isomers could be isolated using a combination of column chromatography and HPLC.A detailed investigation of their thermotropic mesophase behaviour is reported, including X-ray diffraction structure determination. These compounds are designed to combine interesting oxidative behaviour with liquid crystalline properties. Since their discovery in 1977,' columnar mesophases derived OR from aromatic cores substituted with flexible side-chains have been of great interest due to the possibility of producing materials with anisotropic electronic conductivity.' For example, recent work with aligned triphenylene mesophases *YBUtdoped with strong oxidants have revealed that conductivity along the axes of the columns is at least three orders of magnitude greater than that across the col~mns.~ In addition, the photoconductivity of a related system was shown to be remarkably effi~ient.~ Columnar mesogens with phthalocyan- ine (Pc) as the aromatic core5 are particularly relevant to this field of study due to the well-known electronic conductivity of this macrocycle when doped with oxidants6 and its widespread use as a photoconductor in ~erography.~ However, Pcs require strong oxidants, such as iodine, to produce the conductive state and it would be desirable to produce more easily oxidisable Pc mesogens for anisotropic conductivity measurements.Recently, we have explored the concept of using redox-active, sterically hindered phenols, specifically 3,5-di- But BU'(tert-butyl-4-hydroxyphenyl)(DTBHP) substituents, in order to modify the electronic properties of the Pc ring.8 This OR research was prompted by previous studies on meso-PC1, R=H tetra( DTBHP)-porphyrin which revealed a rich oxidative Pc 2, R = H, all Pc benzo sites substituted with CI chemistry.'-'' Our initial investigations indicate that DTBHP PC3, R = CH20CH&H20CH3 substituents can also cooperate with the Pc macrocycle in a number of interesting oxidative processes.For example, non-mesogenic tetra( DTBHP) phthalocyanine (Pc 1) oxidises R' R' in aerated and basified toluene solution to form stable and delocalised radical species and ultimately, benzoquinone containing Pc derivatives.' The exciting redox behaviour of Pc 1 is incontrast to that observed for dodecachlo-rotetra(DTBHP)phthalocyanine (Pc 2), previously studied by Milaeva et in which the steric effects of the adjacent chlorine atoms effectively hinders coplanarity, and thus strong n--7c orbital interactions, between the Pc ring and DTBHP substituents.This also explains the reported absence of a significant bathochromic shift of the visible absorption band (Q-band) of Pc 2 on deprotonation of the DTBHP moieties. A large bathochromic shift of the Q-band is observed for a solution of Pc 1 on addition of a base (Fig. 2), illustrating the R' k' strong electronic interaction between the deprotonated DTBHP substituents and the Pc core-a prerequisite for cooperative oxidative processes. The aim of the work reported in this paper was to prepareun- symmetrically substituted Pc derivatives (Pcs 5, 7,9 and ll), we hoped would combine interesting oxidative behaviour with containing both DTBHP and flexible alkyl substituents, which liquid crystalline properties. Previous studies have shown that octa-alkyl substitution (e.g.Pc 4) leads to columnar mesophase and that unsymmetrical substitution patterns t Presented at the Second International Conference on Materials forming Pcs'~-'~ Chemistry, MC', University of Kent at Canterbury, 17-21 July 1995. need not preclude liquid J. Mater. Chem., 1996, 6(3), 315-322 315 R' R' R' R' RO R' R' But But RO Synthesis of phthalocyanines Symmetrically substituted Pcs 1 and 4 have been synthesised previously by the lithium pentyloxide catalysed cyclotetramis- ation of 4-(3,5-di-tert-butyl-4-hydroxyphenyl)phthalonitrile l513 and 4,5-bis( hexadecy1)phthalonitrile 14, respectively Similarly, non-uniformly substituted Pcs can be prepared by mixed phthalonitrile cyclotetramisations, although a complex Pc product mixture is obtained which requires separation by chromatography l5 l9 21 Unfortunately, both Pcs 1 and 4 have 14 13 15 R' R' R' k' similar chromatographic properties as exemplified by their TLC Rf values 0 8 and 0 7, respectively, using toluene as eluent and silica gel as substrate Therefore, it was envisaged that a mixed cyclotetramisation reaction between phthalonitriles 13 and 14 would result in an inseparable product mixture of Pcs 1, 4,57,9,11 Recent work in our laboratory has shown that the combined use of oligo(ethy1eneoxy) and alkyl substituents allows excellent separation of complex Pc mixtures, including opposite and adjacent regioisomers, by the use of simple column chromatography l5 Generally, the greater the number of polar oligo(ethy1eneoxy) side-chains attached to the Pc, the longer the compound is retained on the silica column Thus, we predicted that an oligo(ethy1eneoxy) substituent attached to the DTBHP moiety of phthalonitrile 13,prior to the mixed cyclotetramisation with phthalonitrile 14, would allow sub- sequent separation of the resultant Pc products The methoxy- d Pcs3and4 Separation by chromatography R = --CH@CH,CHZOCH3 Scheme 1 Reagents and conditions 1, NaH, MEM-Cl, THF, 48 h, 11, LiOC,Hl,-C5H110H, 135 "C, 4 h, 111, H,O, iv, pyndinium toluene-p- sulfonate, CSH,,OH, 135 "C, 4 h 316 J Muter Chem, 1996, 6(3), 315-322 (ethoxy)methylene (MEM) group, an often used protecting group for hydroxy functionality,22 was chosen as the oligo- (ethyleneoxy) side-chain due to its stability to strongly basic conditions and ease of removal.Thus, the target Pcs 5, 7, 9 and 11 were prepared by the route shown in Scheme 1. An efficient reaction between I 8 I I 600 700 600 700A./nm Fig. 1 Visible absorption spectra of Pcs 9(a) and 7(b) m 650 0 1 2 3 4 no.of DTBHP substituents Fig. 2 Position of Q-band absorption (nm) on addition of excess base (tetrabutylammonium hydroxide) for Pcs with 0 (Pc 4), 1 (Pc 5), 2 (Pc 7),3 (Pc 11) and 4 (Pc 1) DTBHP substituents 8.8 4 E 9.0-5 9.2 9.4 9.6 W -I.'.-9.8 916 9.4 phthalonitrile 13 and MEM chloride prepared 4-[3,s-di-tert-butyl-4-(1,3,6-trioxaheptyl)phenyl]phthalonitrile 15. Phthalo-nitriles 14 and 15 reacted together successfully in a mixed cyclotetramisation reaction, catalysed by lithium pentyloxide, in refluxing pentanol. Simple column chromatography separ- ated the Pcs 3, 4, 6, 8 (the opposite di-DTBHP precursor), 10 (the adjacent di-DTBHP precursor) and 12.In addition, HPLC chromatography was successful in isolating, on a small scale, each of the three isomers of the adjacent di-DTBHP substitued Pc 10. The structural assignment, using high resolution 'H NMR, of these isolated compounds to the expected three regioisomers (lOa, lob, 1Oc) will be discussed in the next section.Removal of the MEM groups from 6, 8, 10 and 12 to reveal Pcs 5, 7, 9 and 11, respectively, was achieved in high yield by the action of pyridinium toluene-p-sulfonate.22 Final purification of Pcs 3-12 involved recrystallisation from an appropriate solvent. Structural characterisation of phthalocyanines Each of the isolated Pcs gave elemental analyses and fast atom bombardment (FAB) mass spectra consistent with their pro- posed structures (with the exception of Pcs 5 and 6 which did not display molecular ion peaks). Visible region absorption spectra of dilute toluene solutions of Pcs 5, 9, 11 and 6, 10, 12 display the characteristic split Q-band of non-aggregated metal-free Pcs [e.g.Fig. l(u)]. However, Pcs 7 and 8 have a very different appearance [e.g. Fig. 1(b)],analogous to similar Q-band splitting effects observed in other Pc derivatives with opposite substitution Addition of base (tetra- butylammonium hydroxide) to toluene solutions of Pcs 5, 7, 9, 11 produces a bathochromic shift of the Q-band which is proportional to the number of DTBHP substituents (Fig. 2). This illustrates that, as expected, the strong electronic inter- action between the phenolic groups and the Pc ring, necessary for cooperative oxidative chemistry, is not affected by the presence of the hexadecyl side-chains. No such shift is seen with the analogous visible spectra of the MEM precursors, Pcs 6, 8, 10, 12. Excellent quality NMR spectra, with no evidence of broaden-a--9:2 9.0 8.8 8.6 8.4 I 2 F1/ppm Fig.3 COSY 'H NMR spectrum of the aromatic region of Pc 6. The assignments, labelled Ha-Hd, refer to the partial general structure shown. The ortho coupling between Ha and Hb, and the meta coupling between Ha and Hc are clearly discernible. J. Muter. Chem., 1996, 6(3), 315-322 317 ing due to intermolecular aggregation, could be obtained for each Pc product from a dilute solution (1 mg per cm3, in ['H6] benzene), at elevated temperature (60 "C), using a high resolution spectrometer (500 MHz). The NMR spectra are, in each case, consistent with the proposed structures. A COSY analysis of the aromatic region of the spectrum for Pc 6 (Fig. 3) helped to assign the protons attached to the same benzo subunit as the DTBHP moiety (see partial structure).Proton Ha (6 8.51) is relatively shielded because of its peripheral location23 and proton Hb (6 9.61) can be assigned due to its strong ortho coupling (JHa-Hb 10 Hz) with Ha. The metu coupling (JHa-Hc 2Hz) between protons Ha and Hc allows us to assign Hc (6 10.00). The six other benzo protons, adjacent to the hexadecyl side-chains, give rise to six discrete singlets between 6 9.0 and 9.5 and the two aromatic protons of the DTBHP substituent (Hd) resonate at 6 8.35 ppm. The aromatic region of Pc 5 is very similar to that of its MEM derivative Pc 6, however, the higher field region has a sharp singlet (6 5.32, 1 H) originating from the phenolic hydroxy group whereas that of Pc 6 has peaks consistent with the MEM functionality (6 5.41, singlet, 2 H; 4.12, triplet, 2 H; 3.63, triplet, 2 H; 3.37, singlet, 3 H).The 'H NMR spectra of opposite di-DTBHP substituted Pcs 7 and 8 show clearly that they are both composed of two regioisomers present in roughly equal quantities. In particular there are two separate resonances associated with each of the protons (Ha, Hb and Hc) of the Pc benzo subunit attached to the DTBHP moiety. There are also two separate peaks of equal intensity for the aromatic protons, Hd, on the DTBHP substituent of Pc 7 (6 8.30 and 8.35) and for the phenolic hydroxy proton (6 5.31 and 5.30). HPLC analysis could not separate the two isomers of Pc 7 or 8. Three possible isomers are expected for adjacent di-DTBHP substituted Pcs 9 and 10 and three separate fractions were isolated, using column chromatography followed by prepara- tive HPLC, from the mixed phthalonitrile reaction.Each of these fractions gave FAB mass spectra consistent with the molecular formula of Pc 10.'H NMR of the aromatic region of each of the fractions of Pc 10 is given in Fig. 4 labelled with our tentative assignments. For the least symmetrical isomer, lob, the DTBHP moieties are not equivalent and this allows assignment to the middle fraction obtained by HPLC which has a 'H NMR containing two well resolved sets of peaks for the protons Hd as well as the neighbouring benzo protons Hb and Hc. The correlation of isomer structures 10a and 1Oc with the NMR spectra of the two remaining fractions is more difficult as both show only the expected single set of peaks for these protons.Our assignment is based on the greater molecu- lar dipole moment of lOc, due to the similar relative orientation of the DTBHP substituents, which would account for the longer HPLC retention time observed for the third fraction. The shorter retention time of the first fraction and identical chemical shift (6 9.24) for the four benzo protons adjacent to the hexadecyl side-chains are consistent with the expected low dipole moment of isomer 10a.A comparison of the intensities of the peaks for each of the pure isomers isolated by HPLC, as compared with the isomeric mixture obtained only by column chromatography, indicates that 10a, 10b and 1Oc were prepared in the statistically predicted ratio of 1:2 :1, respect- ively.The liquid crystal transition temperature of the isomeric mixture, Pc 10,appears to be dominated by the major isomer 10b (Table 1)which possesses the highest clearing temperature of the three pure isomers. Due to the labourious separation of the isomers by preparative HPLC, it was decided to use only the isomerically mixed precursor, Pc 10,to prepare the redox- active Pc 9. The tri-DTBHP substituted Pcs 11 and 12 gave, in each case, 'H NMR spectra consistent with a mixture of four inseparable regioisomers. 318 J. Mater. Chem., 1996, 6(3), 315-322 R' R' Pc 10a R' R' '8" Pc 10b pBU1 But RO Mesomorphic properties As characterised by polarised optical microscopy and differen- tial scanning calorimetry (DSC), all of the Pcs 5, 7,9,11 and their precursors, Pcs 6, 8, 10, 12, possess at least a single mesophase over a broad temperature range [Table 1 and Fig. 5(u) and (b)].Each compound, on cooling from its isotropic phase, forms a fluid mesophase which is mainly homeotropic when viewed using a polarising microscope but displays bire- fringent fan-like defects (Fig.6). This texture is characteristic of a columnar mesophase which has a two-dimensional lattice of hexagonal symmetry but in which there is no periodicity of Hd Pc €id 1O:O 9.8 9.6 9.4 9.2 9.0 8h 8.6 8.4 8.2 ppm Fig.4 'H NMR spectra of the aromatic region of the three isomers of the adjacent di-DTBHP substituted Pc 10, isolated by HPLC.The labels Ha-Hd refer to the partial general structure shown in Fig. 3. Tentative assignment of these three fractions to the expected regioisomers 10a, 10b and 1Oc is discussed in the text. Table 1 transition temperaturesa/"C (AH/J g-') 4 108 (41.9) 170 (<1) 196 (2.6) 5 72 (46.5) 96 (") 215 (3.0) 6 61 (37.3) --135 r) 228 (3.8) 7 --17 -255 (2.2) 8 -30 (13.1) --265 (2.5) 9 --20 -195 (1.3) 10 -37 (8.3) --249 (3.3) ---223load -lobd --250 10cd --209 11 --214 (0.9) 12 --287 (2.3) Transition temperatures quoted are for heating cycle. Significant supercooling is observed for solid to mesophase transitions, on cooling, but only a small amount (<5 "C) occurs for the isotropic to Dhd transition. Onset of glass transition, observed by DSC, on heating sample.'Transition not observed by DSC. Transition temperatures measured only by optical microscopy due to insufficient material. Solid to mesophase transition is difficult to observe optically. molecular spacing along the columnar axes (Dhd). The Dhd mesophase is commonly encountered for Pc mesogens especi- ally those substituted by alkyl side-chains, such as the sym- metrical by-product of the mixed phthalonitrile reaction, Pc 4.'3-15X-Ray diffraction studies of this mesophase were carried out for Pcs 5 and 11 which confirm its structyre as Dhd with intercolumnar distances of 32.4 and 29.4 A, respectively (Table 2). However, the lower degree of two-dimensional hex- agonal ordering within the mesophase formed by Pc 11, as compared with that of Pc 5, is evident from the appearance of only lower order diffraction rings.However, the diffraction ring assfciated with the molecular ordering within the columns (d-3.6 A) is sharper for the mesophase of Pc 11 than for Pc 5. The mono-DTBHP containing Pc 5 and its precursor Pc 6 also display a second mesophase at lower temperature which develops as radial striations on the fan defects of the Dhd mesophase (Fig. 7). This mesophase behaviour is also shown by the symmetrical Pc 4. However, no associated thermal transition can be detected by DSC and it gives an identical diffraction pattern as the Dhd mesophase (Table 2). A similar mesophase has been reported for some 1,4,8,11,15,18,22,25-octaalkyl Pcs and was classified as Dhd on the basis of a detailed X-ray diffraction st~dy,'~ although the optical texture is reminiscent of a mesophase with rectangular symmetry.24 This mesophase is labelled as D, in Table 1and Fig.5, however it is possible that it is simply a textural variation of the Dhd 350- (4 250- 200- 8 8 rn 8 150- A 100- 0 A 50- 0 0 0 0 8 2oj A A 4 6 8 10 12 Pc Fig. 5 Plot of the transition temperatures of (a)the DTBHP substituted Pcs 5, 7, 9 and 11 and (b)of the MEM-DTBHP substituted Pcs 6, 8, 10 and 12. In both cases the symmetrical octahexadecyl derivative (Pc 4) is included for comparison. 0 indicates a solid to mesophase transition (crystal to D, for Pcs 4, 5 and 6, glass to Dhd for Pcs 7, 9, 11 and 12 and crystal to Dhd for Pcs 8 and 10.A indicates a D, to Dhd transition. indicates a Dhd mesophase to isotropic liquid transition. Fig.6 Optical texture of the Dhd mesophase of Pc 5 (20Ox magnification, 190"C) mesophase. A high resolution X-ray diffraction study is planned to clarify the structure of the D, mesophase. Noting the unsymmetrical substitution patterns, the inflex- ible and bulky nature of the DTBHP substituent, and the different regioisomers present in most of the Pcs, it is quite remarkable that the DTBHP containing Pcs possess liquid crystallinity over such broad temperature ranges. These factors do seem to destroy the solid state crystallinity of Pcs 7, 9, 11, 12 as they display only glass transitions (Tg) rather than true melting points.However, the glassy state exhibited by Pcs 7, 9, 11, 12 may be a useful material property as it could be used to freeze the structure of the columnar mesophase into the solid state. Despite the obvious destabilisation of the J. Mater. Chem., 1996,6(3), 315-322 319 Table 2Powder X-ray diffraction data Pc meso phase D"/A T/"C 5 Dhd 32.3 150 5 Dx 32.3 85 11 Dhd 29.4 150 'Intercolumnar distances (D)calculated from 1,O diffraction ring. Fig.7 Optical texture of the D, mesophase of Pc 5 (2OOx magnification, 85 "C) crystalline state by the DTBHP moieties, the well defined clearing points (mesophase to isotropic transition) of Pcs 5, 7, 9, 11 are at least as high as that of the symmetrical Pc 4which contains no DTBHP functionality.An interesting comparison can be made between the clearing temperatures of Pc 7 with that of the isomeric Pc 9 showing that the mesophase formed by the opposite isomer is stable up to 60°C higher than that of the adjacent isomer. Possible oxidation of Pcs 5, 7, 9, 11 during heating was investigated by visible absorption spectroscopy of the Pcs subsequent to mesophase characterisation. No change in the spectrum was observed. In conclusion, the combination of alkyl and redox-active DTBHP substitution of Pc is compatible with the formation of columnar mesophases over a broad range. More generally, this work suggests that the Pc core can tolerate a wide range of functional moieties in various substitution patterns and still maintain mesogenic potential.The redox properties of Pcs 5, 7, 9, 11 are currently being investigated and will be presented in a future paper. Experimental Equipment and materials Routine 'H NMR spectra were measured at 200 MHz using a Varian Gemini 200 spectrometer. High resolution (500 MHz) 'H NMR spectra were recorded using a Varian Unity 500 spectrometer. UV-VIS spectra were recorded on a Shimadzu Bragg spacing (d) intensity hexagonal lattice assignment (h,k) 28.0 16.1 strong medium 14.0 weak 10.4 4.8 very weak broad, weak 3.61 broad, weak 28.0 strong 16.1 medium 13.8 weak 10.4 4.6 very weak broad 3.61 broad, weak 25.5 15.4 4.8 strong very weak broad 3.6 medium UV-260 spectrophotometer from toluene or dichloromethane solutions using cells of pathlength 10 mm.Elemental analyses were obtained using a Carlo Erba Instruments CHNS-0 EA 108 Elemental Analyser. Routine low resolution chemical ionisation (CI) were obtained using a Fisons Instruments Trio 2000. Pc fast atom bombardment (FAB) spectra were recorded on a Kratos Concept spectrometer. Routine melting point determinations were carried out with a Gallenkemp melting point apparatus and are uncorrected. Transition temperatures were obtained by optical microscopy using a Nikon Optiphot- 2 microscope in conjunction with a Mettler FP82HT hot stage and were confirmed by differential scanning calorimetry (DSC) using a Seiko DSC 220C instrument.Temperature variable, small angle X-ray diffraction studies were carried out on a Philips PW1130/00 X-ray generator, using Cu-Ka radiation as the source, and the data collected on a photographic plate placed 60mm away from the sample. Analytical HPLC was carried out using a silica Resolve cartridge (8Si05p) fitted with Perkin-Elmer Diode array LC-480 UV detector. Preparative HPLC was carried out on a Gilson unit using a Rainin Dynamax 60A silica column (2 1.4 x 250 mm) and a 115 Gilson UV detector at 255 nm. All solvents were dried and purified as described in Perrin and Armarego.2s Silica gel (60 Merck 9385) was used in the separation and purification of Pcs by column chromatography.Preparation of 4-[ 3,5di-tevt-butyl-4-( 1,3,6-trioxaheptyl) phenyl] phthalonitrile 15 To a stirred solution of 4-(3,5-di-tert-butyl-4-hydroxyphe-nyl)phthalonitrile,* 13, (2 g, 6.0 mmol) in dry tetrahydrofuran (THF) (20 cm3), maintained at 0 "C, was added sodium hydride (0.28 g, 11.8 mmol). To this red solution was added MEM chloride (0.82 cm3, 7.2 mmol) and the reaction was stirred under a nitrogen atmosphere for 48 h. The reaction mixture was then added to aqueous ammonia (30%,. 150 cm3) and the product extracted with THF (3 x 50 cm3). The combined THF extracts were washed with water (2 x 100 cm3), dried with anhydrous magnesium sulfate and filtered. The THF was removed, under reduced pressure, and the resultant solid was passed through a silica column using dichloromethane as eluent.Evaporation of the solvent, followed by recrystallisation from ethanol, yielded pure 4-1:3,5-di-tert-butyl-4-( 1,3,6-trioxu- heptyl)phenyl]phthalonitr~leas white plates ( 1.9 g, 75%), mp, 130-131 "C; v (evap. film)/cm-' 2958,2231 (CN), 1598 (Found: C, 74.4; H, 7.7; N, 6.5. C,,H,,N,O, requires C, 74.3; H, 7.7; N, 6.7%); rn/z (CI) 438 (M+NH4+); 6,(200 MHz; CDC1,) 1.5 320 J. Muter. Chern., 1996, 6(3), 315-322 (18 H, s), 3.45 (3 H, s), 3.65 (2 H, t), 4.02 (2 H, t), 5.05 (2 H, s), 7.45 (2 H, s), 7.8-8.0 (3 H, m). Phthalocyanine preparation Note: all Pc yields are based upon initial total weight of phthalonitrile precursors. Phthalocyanines 6, 8, 10, 12. To a rapidly stirred mixture of 4-[3,5-di-tert- butyl-4- ( 1,3,6- trioxaheptyl ) phenyl ]phthaloni-trile, 15, (0.72 g, 1.71 mmol) and 4,5-bis( hexadecy1)phthaloni- trile, 14, (1.0 g, 1.73 mmol) in refluxing pentanol(5 cm3), under a nitrogen atmosphere, was added excess lithium metal (0.2 g). Heating and stirring were continued for 6 h.On cooling, water (30 cm3) was added, and the reaction mixture heated to ensure complete Pc demetallisation. Evaporation of the water, under reduced pressure, left a green product mixture. The resultant solid was dissolved in toluene and passed through a silica column, at 50°C, using an eluent composed of an increasing amount of THF relative to toluene. The first fraction (100 mg, 5.8%) (Rf=0.9, hot toluene) proved to be identical to a previously prepared sample of 2,3,9,10,16,17,23,24-octakis(hex-adecyl )phthalocyanine 4." The second fraction was collected and applied to a fresh silica column (eluent: toluene-heptane, 1: 1, 50 "C, Rf=0.4) and recrystallised from hot toluene to afford 2-( 3,5-di-tert-butyl-4-( 1,3,6-trioxuheptyl)phenyl]-9,10,16,17,23,24-hexakis(hexadecyl)phthaEocyanine 6 as a blue solid (140mg, 8.lY0) (Found: C, 81.5; H, 11.15; N, 5.3.C146H238N803 requires C, 81.43; H, 11.14; N, 5.20%); A,,,(toluene)/nm 703, 671, 647, 610, 348; 6,(500 MHz; C,D6; 60°C) -1.3 (2 H, br s), 1.01 (18 H, br t), 1.22-1.7 (144 H, br m), 1.8 (12 H, br m), 1.92 (18 H, s), 2.1 (12 H, br m), 3.28 (12H, br m), 3.38 (3 H, s), 3.65 (2H, t), 4.12 (2H, t), 5.37 (2H, s), 8.35 (2H, s) 8.51 (1 H, d), 9.15 (1 H, s), 9.23 (1H, s), 9.29 (1H, s), 9.33 (1H, s), 9.4 (1H, s), 9.45 (1 H, s), 9.62 (1H, s), 10.0 (1 H, s). The third fraction was collected and applied to a fresh silica column (eluent: toluene-heptane, 9: 1, 50 "C, Rf=0.25) and recrystallised from dichloromethane into ethanol (9 :1) to afford 2,16( 17)-bis [3,5-di-tert-butyl-4-( 1,3,6-trioxahep- tyl)phenyl]-9,10,23,24-tetrakis(hexadecyl)phthalocyanine8 as a mixture of two isomers (60mg, 3.5%) (Found: C, 79.2; H, 10.2; N, 5.6%. C13&2~,N8O6 requires c, 79.38; H, 10.20; N, 5.61%); A,,,(toluene)/nm 720, 695, 681, 651, 623, 387, 345; dH(500MHZ; C6D6; 60°C) -1.41 (2H, br S), 1.0 (12 H, br t), 1.2-1.85 (104 H, br m), 1.89 (36 H, s), 2.02-2.24 (8 H, br m), 3.1-3.33 (8 H, br m), 3.35 (3 H, s), 3.36 (4 H, m), 4.12 (4H, m), 5.35 (2 H, s), 5.36 (2 H, s), 8.30 (2H, s), 8.31 (2 H, s), 8.41 (1 H, d), 8.45 (1 H, d), 9.16 (1 H, br s), 9.28 (1 H, br s), 9.4 (1 H, br s), 9.46 (1H, br s), 9.49 (1 H, br d), 9.59 (1 H, d), 9.89 (1 H, s), 9.95 (1 H, s) [Found: m/z 1998. 13C2Cl,oH202N806(M+H+) requires 1997).The fourth frac- tion was collected and applied to a fresh silica column (eluent: toluene-THF, 40 :1, 20 "C, Rf =0.2) and recrystallised from ethanol-dichloromethane (9: 1) to afford, as a mixture of three isomers, 2( 3),9( 10)-bis[ 3,5-di-tert-butyl-4-( 1,3,6-trioxaheptyl)- phen yl] -1 6,17,23,24-tetrakis (hexadec y l)phthalocyanine 10 as a blue-green waxy solid (120 mg, 7.0%) (Found C, 79.4; H, 10.1; N, 5.6.C132H202N806 requires C, 79.38; H, 10.20; N, 5.61%); A,,,(dichloromethane)/nm 707, 674, 645, 612, 345; hH(500MHZ; C6D6; 60°C) -1.2 (2H, br S), 1.0 (12H, m), 1.2-1.84 (104 H, br m), 1.89 (9 H, s), 1.90 (9 H, s), 1.92 (9 H, s), 1.95 (9 H, s), 2.0-2.2 (8 H, br m), 3.1-3.31 (8 H, br m), 3.35 (3 H, s), 3.36 (3 H, s), 3.64 (4 H, m), 4.11 (4 H, m), 5.35 (1 H, s), 5.36 (1 H, s), 5.37 (1 H, s), 5.38 (1 H, s), 8.31, 8.32, 8.38, 8.41 (4 H, s), 8.52 (2 H, m), 9.0-9.48 (4 H, br m), 9.5-9.76 (2 H, br m), 9.84-10.14 (2 H, m) [Found: W/Z 1998. 13CzC130H~02N806 (A4+H+) requires, 19971. These three isomers were separated by preparative HPLC, to afford firstly 2,10-bis[3,5-di-tert-butyl-4-( 1,3,6-trioxaheptyl)-phenyl]-16,17,23,24-tetrakis(hexadecyl)phthalocyanine 10a, HPLC retention time (t,)=244 s (eluent: hexane-ethyl acetate, 8: 1, 20°C); 6,(500 MHZ; C6D6; 60°C) -1.7 (2 H, br s), 1.0 (12H,m), 1.2-1.86(104H, brm), 1.93(36H, brs),2.11(8H, m), 3.26 (8 H, m), 3.37 (6 H, s), 3.66 (4 H, t), 4.13 (4 H, t), 5.39 (4 H, s), 8.33 (4 H, s), 8.46 (2 H, d), 9.24 (4 H, br s), 9.6 (2 H, br d), 9.86 (2 H, br s).Secondly, 2,9-bis(3,5-di-tert-butyl-4-[1,3,6-trioxaheptyl)phenyl]-16,17,23,24-tetrakis(hexadecyl) phthalocyanine lob, HPLC retention time (t,) =429 s (eluent: hexane-ethyl acetate, 8 :1, 20 "C); 6,( 500 MHz; C6D6; 60 "C) -1.74 (2 H, br s), 1.0 (12 H, m), 1.2-1.88 (104 H, br m), 1.94 (18 H, s), 1.96 (18 H, s), 2.12 (8 H, m), 3.1-3.3 (8H, br m), 3.364, 3.366, 3.373, 3.375 (6 H, s), 3.66 (4 H, m), 4.14 (4 H, m), 5.4 (4H, s), 8.35 (2 H, s), 8.41 (2 H, s), 8.45 (2 H, br d), 8.94 (1 H, br s), 9.07 (2H, br d), 9.23 (1 H, br s), 9.42 (1 H, br d), 9.57 (1H, br d), 9.90 (1 H, s), 9.94 (1 H, s) and finally 3,9- bis [3,5-di-tert-butyl-4-( 1,3,6-trioxaheptyl)phenyl]-16,17,23,24-tetrakis(hexadecyl)phthalocyaninelOc, HPLC retention time (t,) =596 s (eluent: hexane-ethyl acetate, 8 : 1, 20 "C); 6,(500 MHZ; C6D6; 60°C) -1.7 (2 H, br S), 1.0 (12 H, t), 1.2-1.86(104H, br m), 1.98 (36H, s), 2.13 (8H, m), 3.08-3.28 (8 H, br m), 3.37 (6 H, s), 3.66 (4 H, t), 4.14 (4 H, t), 5.40 (4 H, s), 8.43 (4 H, s), 8.54 (2 H, br d), 8.97 (2 H, br s), 9.19 (2 H, br s), 9.54 (2 H, br s), 10.0 (2 H, br s).The fifth fraction was collected and applied to a fresh silica column (eluent :toluene-THF, 20: 1,20 "C, Rf= 0.1) and recrys- tallised from ethanol-dichloromethane (9: 1) to afford, as a mixture of four isomers, 2(3),9( 10),16( 17)-tris[ 3,5-di-tert- butyl-4-( 1,3,6-trioxaheptyl)phenyl]-23,24-bis (hexadec y1)phthalo- cyanine 12 as a blue-green solid (80 mg, 4.7%) (Found: C, 77.1; H, 9.0; N, 6.2.Cl,,H,66N,09 requires C, 77.00 H, 9.10; N, 6.10%); A,,,(toluene)/nm 707, 676, 648, 614, 355; dH(500 MHz; C6D6; 60 "C) -1.4 (2 H, br s), 0.99 (6 H, m), 1.28-1.82 (52 H, br m), 1.91 (54 H, m), 2.05 (4 H, m), 3.1-3.31 (4H, br m), 3.35 (9 H, br s), 3.64 (6H, br t), 4.12 (6H, br t), 5.3-5.42 (6 H, br m), 8.1-8.62 (9 H, br m), 8.84-10.1 (8 H, br m) [Found: m/Z 1841. 13CC117H166N809 (M+ H+) requires 18411. The sixth fraction was collected and applied to a fresh silica column (eluent: toluene-THF, 10: 1,20 "C, Rf=0.05) and reprecipitated from ethanol-dichloromethane (9:1) to afford, as a mixture of four isomers, 2,9( 10),16( 17),23,(24)-tetra- kis[3,5-di-tert-butyl-4-( 1,3,6- trioxaheptyl)phenyl]phthalocyan-ine 3 as a green solid.(75 mg, 4.4%) (Found: C, 73.9; H, 8.1; N, 6.7. C104H130N8012 requires C, 74.16; H, 7.78; N, 6.65%); A,,,(toluene)/nm 712, 678, 651, 614, 418, 360; 6,(500 MHz; C6D6; 60°C) -1.2 (2 H, br S), 1.91 (72 H, br S), 3.35 (12 H, br s), 3.64 (8 H, br t), 4.11 (8 H, br t), 5.36 (8 H, br s), 8.14-8.64 (12 H, br m), 9.1-10.04 (8 H, br m) [Found: m/z 1685. 13CClo3H,30N~0,,(M+H+) requires, 16841. Preparation of phthalocyanines 5,7,9,11 General procedure for the removal of the MEM group.2-(3,5-di-tert-butyl-4-hydroxypheny1)-9,10,16,17,23,24-hex-akis(hexadecy1)phthalocyanine 5. A mixture of 2-( 3,5-di- tert-butyl-4-[1,3,6 -trioxaheptyl ) phenyl] -9, 10, 16, 17,23,24- hexakis(hexadecy1)phthalocyanine 6 (57 mg, 0.03 mmol) and pyridinium toluene-p-sulfonate (38 mg, 0.15 mmol) in pentanol (1 cm3) was stirred at reflux under a nitrogen atmosphere for 4 h. The pentanol was removed, under reduced pressure, to give a blue solid which was recrystallised from toluene to give 2-( 3, 5-di-tert-butyl-4-hydroxyphenyl)-9,10, 16, 17, 23, 24-hexa- kis(hexadecy1)phthalocyanine5 as a blue solid (46 mg, 84%) (Rf= 1.0, hot toluene) (Found: C, 82.6; H, 11.25; N, 5.5. C142H230N80 requires C, 82.57; H, 11.23; N, 5.43%); A,,,(toluene)/nm 705, 672, 647, 610, 346; 6,(500 MHz; C6D6; 60°C) -1.28 (2 H, br s), 1.01 (18 H, br t), 1.3-1.9 (156 H, br m), 1.77 (18H, s), 2.14 (12H, br m), 3.27 (12H, br m), 5.32 (1 H, s), 8.23 (2H, s), 8.51 (1 H, d), 9.15 (1 H, s), 9.25 (1 H, s), J.Muter. Chem., 1996, 6(3), 315-322 321 9.26 ( 1 H, s), 9.33 (1 H, s), 9 37 ( 1 H, s), 9 41 (1 H, s), 9.6 1 ( 1 H, References d), 9.99 (1 H, s). The following Pcs were prepared by the same methodology 1 S Chandrasekhar, B K Sadashiva and K A Suresh, Pramana, 1977,7,471 2 S Chandrasekhar, Liq Cryst, 1993,14,3 2,16( 17)-Bis( 3, 5-di-tert-butyl-4-hydroxyphenyl)-9,10, 23, 24- 3 tetraku(hexadecy1)phthalocyanme 7 (from Pc 8). Reprecipitated from toluene into ethanol to give a green waxy solid (87%) 4 (R,=0.85; toluene-hexane, 1.2,20 "C) (Found C, 81.5; H, 9.95, N, 6 1. C124H186N802 requires c, 81.79; H, 1030; N, 6 16%), 5 A,,,(toluene)/nm 721,700,681,650,617,388,353; 6,(500 MHz; C6D6,6OoC) -1.6(2H,brS), I.O(12H,t), 1.2-18(104H, br m), 175 (18 H, s), 176 (18 H, s), 2.09 (8 H, m), 3.2 (8 H, m), 6 5.31 (1H, s), 5.32 (1H, s), 8.18 (4H, br s), 8.40 (1H, d), 8.44 7(1 H, d), 9.08 (1 H, br s), 9.17 (1 H, br s), 927 (1 H, br s), 933 8 (1 H, br s), 9.46 (1 H, br s), 9.56 (1 H, br s), 9.84 (1H, br s), 9.91 (1 H, br s) [Found: m/z 1821 13CC123H186N802 (M+H+) 9 requires 18201 10 112( 3),9( 10)-Bzs( 3,5-di-tert-butyl-4-hydroxphenyl)-l6,17,23,24-12 tetrakis(hexadecyl)phthalocyunine 9 (from Pc 10) Rep-recipitated from toluene into ethanol to gwe a green waxy solid (79%) (R, = 0.48; toluene-hexane, 1:2, 20 "C) (Found.13 C, 82.0; H, 10.5, N, 6.0. C124H186N802 requires C, 81 79; H, 10.30; N, 6.16%); &,,,(toluene)/nm 709, 674, 643, 611, 350, 146,(500 MHZ; C6D6; 60°C) -1.3 (2 H, br S), 1.0 (12 H, m), 1.2-1.7 (104 H, br m), 1.75, 1.76, 178, 1.81 (36 H, s), 2 08 (8 H, 15 m), 3.23 (8 H, m), 5.32, 5.33, 5.34 (2 H, s), 8 19, 8 20, 8.27, 8.30 (4H, s), 8.4-8.6 (2H, br m), 8.9-9.78 (6 H, br m), 9 86-10 1 16 (2H, br m) [Found. m/z 1821. 13CC123H186N802 (M+Hf) 17requires 18201. 18 2(3),9(lo),16( 17) -Trzs( 3,5 -dz -tert -butyl-4-hydroxyphenyl-23,24-bis(hexadecyl)phthalocyanine11 (from Pc 12) Rep-19 recipitated from toluene into ethanol to give a green waxy 20solid (85%) (Found.C, 80 5; H, 9.45, N, 7 0 C106H142N803 requires C, 80.76; H, 9.08; N, 7.11%); Lax(toluene)/nm 710, 21 679, 653, 616, 348; 6,(500 MHZ; C6D6, 60°C) -1 5 (2 H, br 22 s), 0.99 (6H, t), 1.2-1.65 (52H, br m), 1.76 (54H, m), 18-22 (4 H, br m), 3.3 (4 H, br m), 5 24-5.4 (6 H, br m), 8 0-8.6 (9 H, 23 br m), 8.8-101 (8H, br m) [Found: m/z 1576. 2413CClosH,42N803(M +H ) requires 15761+ G.J.C. is supported by an EPSRC grant (G/H 57233) awarded 25 under the Innovative Polymer Synthesis Initiative We would also like to thank the DRA, Malvern and EPSRC for a CASE studentships (K.E.T and P.H.). N Boden, R J Bushby and J Clements, J Chem Phys, 1993, 98,5920 D Adam, P Schuhmacher, J Simmerer, L Haussling,K Siemensmeyer, K H Etzbach, H Ringsdorf and D Haarer, Nature, 1994,371, 141 J Simon and P Bassoul, in Phthalocyanines-Properties and Applications, eds C C Leznoff and A B P Lever, VCH, NY, 1992, vol 2, p 223 See, C C Leznoff and A B P Lever, Phthalocyanines-Properties and Applications, VCH, NY, 1989, vol 1, 1992, vol 2 K Y Law, Chem Rev, 1993,93,449 P Humberstone, G J Clarkson and N B McKeown, manuscript in preparation A V Melezhik, V D Pokhodenko, Zh Org Khim ,1982,18,1054 T G Traylor, K B Nolan, R Hildreth and T A Evans, J Am Chem SOC , 1983,105,6149 L R Milgrom, Tetrahedron, 1983,39,3895 E R Milaeva, V M Mamav, I P Gloriozov, I N Chechuhna, A I Prokof'ev and Y G Bundel, Dokl Akad Nauk SSSR, 1989, 306,1387 A S Cherodian, A N Davies, R M Richardson, M J Cook, N B McKeown, A J Thomson, J Feijoo, G Ungar and K J Harrison, Mol Cryst Liq Cryst, 1991,196, 103 M K Engel, P Bassoul, L Bosio, H Lehmann, M Hanack and J Simon, Liq Cryst, 1993,15,709 G J Clarkson, N B McKeown and K E Treacher, J Chem SOC, Perkin Trans I, 1995,1817 C Piechocki and J Simon, J Chem SOC, Chem Commun , 1985, 259 J F Van der Pol, E Neeleman, R J M Nolte, Z W Zwikker and W Drenth, Makromol Chem ,1989,190,2727 I Chambier, M J Cook, S J Cracknell and J McMurdo, J Muter Chem, 1993,3,841 N B McKeown, I Chambier and M J Cook, J Chem SOC, Perkin Trans I, 1990, 1169 Y Ikeda, H Konami, M Hatano and K Mochizuki, Chem Lett, 1992,763 T G LinDen and M Hanack, Chem Ber , 1994,127,2151 H Monti, G Leandn, M Klos-Ringuet and C Corriol, Synth Commun, 1983,13,1021 T R Jansen and J J Katz, in The Porphyrins, ed D Dolphin, Academic Press, New York, 1979, vol 2, p 13 C Destrade, P Foucher, H Gasparoux and N Huu Yinh, A M Levelut and J Malthete, Mol Cryst Liq Cryst, 1984, 106, 121 D D Perm and W L F Armarego, Purlfication of Laboratory Chemicals, Pergamon, Oxford, 3rd edn ,1988 Paper 5/04830C, Received 21st July, 1995 322 J.Muter. Chem.,1996,6(3), 315-322

ISSN:0959-9428    

DOI:10.1039/JM9960600315

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

~~~~ Langmuir-Blodgett multilayers of six compact porphyrin amphiphiles' Colin L. Honeybourne and Kevin J. Barrel1 Department of Chemical and Physical Sciences, The University of the West of England, Frenchay Campus, Bristol, UK BS16 1QY By a modification of the Jackson-MacDonald condensation between two symmetrically substituted dipyrrylmethanes, we have synthesised two porphyrin free-base amphiphiles and four metalloporphyrin amphiphiles all of which serve as models for the naturally derived product mesoporphyrin IX dimethyl ester. These compounds have different substituents along their hydrophobic edge to those found in mesoporphyrin IX, which has the sequence Me, Et, Me, Et. Our materials have the sequences (Me), or (Et)4 thus making the hydrophobic edge slightly larger, or slightly smaller than that contained in mesoporphyrin IX.Upon an aqueous subphase, Langmuir films yere obtained at pressures in the range 14-50 mN dm3 mol-', with, at full compression, molecular areas of the order of 65 A2 and angles-of-tilt of the macrocyclic plane to the subphase of some 80". LAXRD data were taken on Langmuir-Blodgett multilayer films of the (Et), compounds which proved to have the threshold minimum magnitude of hydrophobic edge for multilayer formation, with Y-type bilayers being given by the zinc and silver complexes. In research upon Langmuir-Blodgett thin films of porphyrins Tredgold and co-worker~l-~ have utilised the naturally derived mesoporphyrin IX (and related diesters, diols and metal com- plexes) whereas Honeybourne and co-worker~~-~ have syn- thesised a range of models for mesoporphyrin IX.In the latter, emphasis has been placed upon varying the size of the hydro- phobic edge (rings C and D in Fig. 1) for use in a range of studies such as ferrochelatase kinetic^,^ concentration depen- dent NMR' and the formation of Langmuir films and Langmuir-Blodgett multilayers.6 Mesoporphyrin IX has a hydrophobic edge consisting of R1=R3=methyl and R2=R4=ethyl. Models for mesoporphyrin IX include4.' R' =R2=R3=R4=H, methyl or ethyl and R1 =R2 =propyl, R2=R3=ethyl.6 In this work we report the synthesis, character- isation and thin film deposition of the all-methyl and all-ethyl variants mentioned above. Koga et uL7 have very recently reported the aggregation of mesoporphyrin IX in mixed ?Presented at the Second International Conference on Materials Chemistry, MC', University of Kent at Canterbury, 17-21 July 1995.R' R2 Langmuir-Blodgett films with arachidic acid. In contrast to these compact porphyrins, which lack the long alkyl chains characteristic of the molecules usually employed in the forma- tion of Langmuir-Blodgett films, a number of groups of workers are using the readily-synthesised meso-substituted porphyrins in which meso-substitution at the four CHC bridges by aryl groups has taken place. At least one of these four aryl groups carries a long-chain alkyl s~bstituent.~-'~These molecules include sulfonamidoporphyrins8 and nitro-phenyl-amid~phenylporphyrins.~Other workers have used either the tetra-4-pyridyl meso-substituted porphyrin free base (in conjunction with a novel inter-layer spacing technique)" or the new stellular pentaporphyrins with a,w-dioxyalkyl (C-3, C-4) spacer groups.12 Chou et uL9 have remarked that, in the absence of steric constraints from multiple aliphatic chains, porphyrin-porphy- rin stacking permits close packing of the porphyrin rings.However, without any aliphatic chains, the Langmuir films will be very rigid and their flow properties will be adverse as the substrate breaks and re-breaks the surface, giving either no multilayers, or multilayers of poor quality. We seek to 43 R3 R4 M Fig. 1 Structure and labelling of new porphyrins and metalloporphyrins J.Muter. Chem., 1996, 6(3), 323-329 323 identify the minimum size of hydrophobic edge that allows Langmuir-Blodgett multilayer deposition of a model for the free base of mesoporphynn IX dimethyl ester and its complexes without recourse to the use of additives" (such as arachidic acid, mesitylene, etc) so that the packing density of the porphynn nngs is maximised within a layer and the inter-layer spacing is minimised (thereby facilitating the Mott hopping of charge carriers between layers) Experimental Synthetic procedures Porphyrin free base diesters. The final stages of the complex and lengthy synthetic method, the full details of which will be submitted elsewhere (and are available from the author by 11 request) are shown in Scheme 1 In this paper R1=R2=R3= R4=either methyl or ethyl The key reaction was the MacDonald conden~ation'~ between two dipyrrylmethanes, one being a dialdehyde (7) that exhibits the valuable property of long-term chemical stability (at least 10 years when kept in the dark at <4 "C) The second dipyrrylmethane that featured in the condensation reaction was a relatively unstable di- carboxylic acid (8) formed by de-esterification (under dry nitrogen) of a benzyl diester (9) The dicarboxylic acid (8) was utilised as soon as possible without purification of the initial product The success of the condensation reaction, to produce a diprotonated porphodimethene (10) was indicated by the development of a deep-red burgundy colouration, at which stage sufficient sodium acetate was added to inhibit any further chemical reactions induced by the high acidity of the medium Rapid oxidation by atmospheric oxygen produced the porphy- Scheme 1 Flow chart of final stages of the synthesis of porphynn free bases (Jackson-McDonald condensation) 324 J Muter Chem , 1996, 6(3), 323-329 rin diacid (11) which was easily converted into the methyl diester which was then extracted into chloroform.Purification was achieved by utilisation of an alumina column and recrystal- lisation from methanol. The complete reaction sequence depicted in Scheme 1 proceeded with 10-15% overall yield. Zinc complexes. Quantitative yields were given by main- taining a mixture of equimolar quantities of zinc acetate hydrate and the porphyrin free base at reflux for lOmin, utilising a solvent consisting of a 9 :1 (v/v) mixture of chloro- form and methanol.Silver complexes. The inorganic starting material was a salt containing monovalent silver, as either the acetate or the nitrate. During the course of the reaction between equimolar quantities of silver salt and porphyrin free base, at elevated temperature in acetic acid [tetraethyl compound (6)] or dimethylformamide [hexamethyl compound (3)], the silver is oxidised'' to the divalent state, with the complexes of Ag" being produced in virtually quantitative yield. Confirmation of structures by spectroscopy The absorption spectra in the UV and visible regions were consistent with those expected either from porphyrin free bases (1and 4) exhibiting an etio-type distribution of relative intensit- ies or from their corresponding divalent metal complexes.The complexation process increases the symmetry of the chromo- phore from D,h to D&. Thus, the vibrational interactions due to the two N-H bonds cause four absorption bands (Qk, Qlx, QOy, Q1,) to occur in the visible region, which are reduced to two bands upon complexation (Fig. 2). The N-H stretching 0.0 t0.0 0. -0.6 .0.4 0. -0.2 0. .o.o 400 500 600 700 0.8, ,2.4 400 500 600 700 A/nm Fig. 2 (a)Absorption spectra of LB multilayers of compound 4 (4, 8, 18, 28 and 38 layers). (b) Absorption spectra of compound 6 as a 40-layer LB film (-) and a solution (1.4 x lop5 mol dmP3) in chloroform (---).frequency (a weak band at 3330 cm-') and the high-field NH proton NMR signal (ca. 6 -4 at higher concentrations) also disappear (as expected) upon successful completion of the complexation reaction. Porphyrins and metalloporphyrins display a large diamag- netic ring current which moves the internal (NH) protons to high applied fields, and the peripheral protons (especially the four bridging CH protons) to low applied field. The tendency of porphyrins to aggregate is so strong that one molecule in a dimer is influenced by the ring current of its paired neighbour. The free bases in this work exhibited large concentration- dependent effects similar in kind to our earlier published results' for R1=R2=R3=R4=H.Thus the results given in Table 1have been obtained by extrapolating our proton NMR data to zero concentration. Molecular masses, corresponding to molecular structural formulae were confirmed using the molecular ion peak pro- duced by electron impact16 mass spectrometry (Hewlett- Packard 5995), and supported by CHN microanalytical data. Preparation of thin films Sample preparation. An appropriately small quantity of each porphyrin was weighed, using a Cahn Electronic Microbalance, and dissolved in Aristar-grade chloroform to give an accurately Itknown concentration in the region of 1mg ~rn-~. was essential to use these solutions as soon as possible, storing them in tightly sealed containers in the dark at a temperature of ,<4"C.Substrate preparation. The substrates chosen were glass microscope slides (Richardson), optically flat quartz (UQG Ltd) and silicon wafer (University of Bristol and Plessey, Caswell). The surfaces of these substrates were rendered hydro- phobic as described later. The surfaces of glass or quartz slides were first treated with chromic acid and then washed thoroughly with dust-free water (Millipore) followed by washing with HPLC-grade isopropyl alcohol (IPA). They were then further cleaned by the rinsing action of IPA using a specially designed Soxhlet extractor to facilitate cleaning a large number of substrates at one time. After drying in dust-free air, the substrates were rinsed with a 2% (v/v) solution of dichlorodimethylsilane in l,l,l-trichloro- ethane, using the same bulk Soxhlet technique. The substrates were then washed with water (Millipore) to remove any residual hydrochloric acid.The hydrophobic nature of the substrates was checked by pulling them through the surface of pure water. A completely dry surface indicated that the substrates were uniformly hydrophobic. The preparation of hydrophobic silicon surfaces was carried out on a small scale, with each sample being treated individu- ally. The slice of silicon wafer was cleaned in a Soxhlet Table 1 Proton NMR chemical shifts (6, ppm) of the new porphyrin free bases 1 and 4 extrapolated to zero concentration Me, hydrophobic edge 1 Et, hydrophobic edge 4 NH -3.41 -3.38 -CH2CH3 - 1.92 -CH2CH2C02CH3 3.34 3.30 -CH, 3.60 3.66 -(CH2)2C02CH3-CH2CH3 3.65 - 3.66 4.10 -CH2CH2C02CH3 4.37 4.44 HI% 9.88 10.11 HB 9.94 10.11 HY H6 9.94 10.01 10.11 10.11 -CH3 3.50 J.Mater. Chem., 1996, 6(3), 323-329 325 extractor with high-purity chloroform, and then by HPLC- grade IPA After drying in a stream of dust-free air, the sample was subjected to further cleaning in an ultrasonic bath contain- ing pure water (Millipore) The surface of the water was removed by suction (to avoid contamination) The sample was then removed from the water, and emerged completely dry if the cleaning process had been successful Using plastic utensils, the cleaned silicon was chemically etched in 5% hydrofluoric acid for approximately 25 s Copious rinsing with pure water then produced a totally hydrophobic silicon substrate without the use of hydrophobizing chemical agents The hydrophobic nature of freshly etched silicon has been a point of contention because silicon forms a new oxide layer by atmosphenc oxidation of the freshly etched surface, thereby rendering it hydrophilic To overcome the effect of this oxide layer, hydrophobizing silylating agents are usually employed However, we wish to emphasise that the formation of the oxide layer on freshly etched silicon proceeds sufficiently slowly to permit the use of this matenal as a hydrophobic substrate Using the technique of X-ray photoelectron spectroscopy (XPS), the extent of oxidation approximately 10 min after chemical etching was determined The spectrum obtained of the chemical shift of the Si(2p) electrons contains only a single signal (from elemental silicon) The extent of oxidation slowly increases over a period of 11 days as shown by the gradual appearance of a much smaller peak corresponding to the chemical shift of Si(2p) electrons in silicon dioxide The Langmuir-Blodgett trough.The apparatus used was supplied and designed by Nima Technology Ltd It consisted of a circular boro-silicate glass trough, with a tnangular-sector barner constructed of polytetrafluoroethylene Control of the equipment was by Nima Technology software implemented on a BBC microcomputer Changes in the surface pressure (surface tension) of a floating Langmuir monolayer were monitored by a feedback loop consisting of a linear voltage differential transducer and a Wilhelmy plate comprising a piece of filter paper 10mm in width The temperature of the subphase was maintained by a thermostatically controlled water bath, and the pH monitored by a pH probe The dipper mechanism was equipped with controls for selecting dipping speed and length of dipping stroke Between each experiment, the trough and barrier were scrupulously cleaned This was carried out by drying, rinsing with chloroform and then with IPA, followed by drying in dust-free air Finally, the equipment was thoroughly rinsed with pure water (Millipore) To ensure that contamination by air-borne particles was kept to a minimum, the equipment was housed in a microbio- logical cabinet (Hepaire) This latter was fitted with a fan such that a positive internal air-pressure could be maintained, with air being drawn through a microbiological filter Preparation of Langmuir and Langmuir-Blodgett films. The subphase for the spreading of floating monolayers (1 e Langmuir layers) consisted of pure water (Milli-Q grade) at 20°C The pH was 5 8 due to the buffering action of dissolved carbon dioxide The porphynn molecules were spread from a solution in chloroform onto the clean surface of the subphase The solution was added dropwise in close proximity to the surface of the subphase Sufficient time was allowed for each drop to evaporate before adding the next drop Usually 25 mm3 of solution, added using a graduated syringe, was adequate The molecules were then compressed between the fixed and moveable barriers the rate of movement of the latter reduced the surface area by 100 cm2 min-' Good quality pressure-area (PA) isotherms were produced during the formation of Langmuir monolayers The stability of the Langmuir monolayers at a predetermined surface press- 326 J Muter Chem ,1996,6(3), 323-329 ure was monitored with respect to time a dipping pressure of 35 mN m-' was thereby found to be the most suitable To obtain LB films, layers were transferred consecutively on to solid substrates at dipping speeds rangmg from 5 to 10mm min-l Multilayer formation proceeded via a modified Y-type deposition with material being deposited on to the solid substrate on both the up-stroke and the down-stroke Deposition was executed without intervening pauses for the films to drain because the films emerged completely dry when pulled from the aqueous subphase Low-angle X-ray diffraction (LAXRD).The application of LAXRD to the study of the structure of LB films has been described by Tredgold l7 The X-ray measurements were taken on a Philips Diffractometer PW1700 (by courtesy of Rolls Royce, Bristol) The quasi-chromatic source was a chromium target X-ray tube equipped with a vanadium filter Divergence and scatter slits were used that possessed angular apertures of 025" The divergence slit ensured that small angles could be scanned without saturation of the signal by the X-ray beam A receiving slit (02 mm) defined the width of the reflected beam The sample was rotated by 8 and the counter tube was rotated by 28 To obtain maximum resolution, a 0 25", 10 s step scan was used The substrate used for these measurements was hydrophobic silicon the LB film facing the moveable barrier was chosen because this was of higher visual quality Results of Thin Layer Formation Pressure-area (PA) isotherms of Langmuir monolayers Hexamethylporphyrin compounds.Fig 3 shows a PA iso- therm for the silver complex (3) on a subphase of pure water The plot was obtained by compressing the monolayer at 100cm2 min- I, whilst monitoring surface pressure and area This is a very good quality isotherm showing the solid, liquid and gas phases The semicondensed or liquid phase occurs in the region of 0-14 mN dm3 mol-' The condensed or solid phase lies in the region of 14-50 mN dm3 mol-' The region above 50 mN dm3 mol-' represents the onset of collapse within the porphyrin monolayer If the condensed (solid) portion of the isotherm is extrapolated to zero pressure, the moiecular area for this porphyrin molecule is found to be 68 A2 This value indicates that the molecules were orientated at 20" to the normal to the subphase, with the methyl ester groups in the subphase and the porphyrin nuclei projecting above the subphase If the porphyrin nngs had been lying parallel to th? water surface then a molecular area in the region of 238 A2 would have been expected These conclusions 70.01 .o area/A2 molecule-1 Fig.3 Pressure-area (PA) isotherm of a floating Langmuir monolayer of compound 3 (see Fig 1) on a subphase of pure water 40.0 80.0 120.0 wed2 molecule-' Fig. 4 Pressure-area (PA) isotherms of a floating Langmuir monolayer of compound 3 (see Fig. 1) after a number of compressions and decompressions were arrived at with the aid of a space filling model of the porphyrin. Fig.4 demonstrates the reproducible nature of this PA isotherm after several expansions and compressions. This characteristic indicated that the film was robust and not subject to collapse. The reproducibility of these plots confirmed that the monolayer was a true Langmuir film and not a system of aggregated molecules that formed crystalline islands on the surface of the subphase.In addition, it was also concluded that the monolayer was contamination free due to the uniform- ity of the isotherm. The free base hexamethylporphyrin derivative (1) also produced reproducible isotherms on a subphase of water with the three phases of solid, liquid and gasebeing clearly visible. A molecular area of approximately 63 A2 indicates that the porphyrin planes subtend an angle of ca. 18" to the normal to the subphase. The phase transitions occurred at different surface pressure values to those given by the compound 3. The semicondensed region was generally between the values of 0 and 7.5 mN dm3 mol-', whereas the condensed portion was in the range of 7.5-22 mN dm3 mol-'.Above this range collapse occurred. Changing the compression rate from 100 to 50 cm2 min-' had no effect on the position of these phases. The zinc complex (2) did not form the same reproducible isotherms that were described in the previous two cases. The first compression of the monolayer on a subphase of water produced a PA curve with distinctive phases of solid, liquid and gas. However, after two or three further expansions and compressions the isotherms only exhibited a solid and gas phase. This effect was unchanged whether the monolayer was on a surface of pure water or aqueous cadmium chloride. This behaviour may be attributed to aggregation of the molecules within the porphyrin monolayer. On the first compression the molecules were free to move and align themselves relative to one another.However, eventually the molecules aggregated together such that when the film was expanded the molecules were unable to respread. This was observed as a loss of the liquid or semicondensed region in the isotherm. This effect was further substantiated by utilisation of an experiment similar to that performed by Baker et ul.'* This was carried out by modifying the spreading mixture to chloro- form and mesitylene, in the ratios of 4: 1 respectively. The mesitylene was less volatile than the chloroform and therefore evaporated at a slower rate. This provided the required 'lubri- cation' of the molecule such that reproducible isotherms could be obtained (Fig. 5). Tetraethylporphyrin compounds.These compounds (the free- base, 4, the zinc complex, 5, and the silver complex, 6) all 25.0 50.0 75.0 100.0 area/A2molecule-' Fig. 5 Pressure-area (PA) isotherms for a floating Langmuir mono- layer of compound 2 (see Fig. 1) on a subphase of cadmium dichloride (2.5 x lop4mol dm3) utilising a chloroform-mesitylene mixture as the spreading phase produced PA isotherms of high quality on the aqueous sub- phase. The isotherms displayed the characteristic features of 'solid', 'liquid' and 'gaseous' phases, and became reproducible after only two compression-decompression cycles; the zinc compound did not exhibit behaviour similar to that of its hexamethyl counterpart (2). Angles subtended to the normal to the subphase of 18-22' were exhibited in all cases. The collapse pressure for all three compounds occurred at 45 mN dm3 mol-l. Deposition of Langmuir-Blodgett multilayers Hexamethylporphyrin compounds. The zinc (2) and silver hexamethylporphyrins (3)produced stable floating monolayers at surface pressures of 30-40 mN dm3 mol-' on subphases of aqueous cadmium chloride and pure water.The free base derivative (1) also demonstrated stability but at the lower surface pressure of 15 mN dm3 mol- '. Molecular area us. time plots indicate that there was negligible area loss after a time period of approximately 10 min in all three cases. Once stability had been achieved, multilayer formation was attempted using substrates of hydrophobic glass and quartz. Monolayer and multilayer deposition on to these substrates did not take place.In all three cases, glass or quartz was passed through the monolayer at a rate of 5-10 mm min-', and turbulence could be seen at the three-phase contact point (air/film/subphase). The film appeared to attach to the substrate and then break away sending a shock wave through the monolayer. As a result little or no deposition took place. When the same experiment was carried out using hydrophilic quartz, a monolayer could be deposited by drawing the substrate up through the surface. High deposition ratios were recorded (90-100%) for the zinc and silver derivatives. The free base porphyrin also deposited a monolayer in the same way but only gave 65% coverage. Attempts to produce multi- layer films proved fruitless. The same turbulence effect took place similar to that previously described. Drainage times of between 1 and 2 h were employed for the monolayers but this had no effect in furthering the number of layers deposited.This lack of deposition was first believed to be a function of the substrate. However, the alteration of substrate prep- aration conditions and the use of hydrophilic and hydrophobic solids discounted this theory. The rigidity of the porphyrin monolayers was deemed to be the main cause of the lack of deposition because, even when a spreading mixture of chloro- form and mesitylene (4: 1) was used (such that the molecules could flow to a greater extent due to the lubricating effect of the mesitylene) no multilayer deposition took place.J. Muter. Chem., 1996, 6(3), 323-329 327 Tetraethylporphyrin compounds. The tetraethylporphyrin compounds all produced stable floating monolayers, from which multilayer films could be produced, using monolayers on the subphase surfaces of water or aqueous cadmium chlonde The optimum dipping pressures were determined from PA isotherms and the area-time plots For the zinc, free base and silver tetraethylporphyrins these were found to be 40, 35 and 30 mN dm3 mol-' respectively Multilayers were then deposited on to hydrophobic glass, quartz and silicon sub- strates at dipping rates of 5-10 mm m1n-l Although the extent of deposition was slightly greater on the up-stroke than on the down-stroke in the case of the metalloporphyrins (5,6), both gave good-quality Y-type films However, in contrast, the free base gave a mode of deposition intermediate between Y-type and X-type The down-stroke gave an average of 61% deposition whereas the upstroke gave 97% deposition This discrepancy between the deposition ratios may be explained by the poor flow qualities of the Langmuir mono- layer of compound 4 It was noticed that a slightly inferior film was obtained for the substrate surface adjacent to the fixed barrier A visually perfect film was obtained on the surface facing the movable barrier This phenomenon was further investigated by a simple experiment Fine talcum powder was sprinkled over the surface of a condensed porphyrin monolayer A glass substrate was then passed through the surface of the subphase It was observed that the powder moved to the substrate surface nearest the movable barner with great ease The flow of powder to the reverse side of the substrate appeared to be hindered This was very noticeable in the small area between the substrate edge and the outer perimeter fixed barrier This observation demon- strated the need for slow speeds for the deposition of this compound, so that recovery of uniformity within the floating Langmuir monolayer can take place A similar experiment was carried out using stearic acid In this case the powder flowed freely around the edges of the substrate giving an even distribution across the surface during deposition This experiment demonstrated how the shape of a molecule can affect the rheology of a floating monolayer The stearic acid molecules are long and slender and therefore move freely under compression However, the porphyrins are bulky molecules and therefore would not be expected to flow so efficiently It is clear that the hydrophobic edge in the tetraethylporphy- rin free base (4) is not quite large enough to give high quality multilayers by Y-type deposition, although reasonable quality multilayers are obtained for the zinc and silver complexes of 4 The hydrophobic edge in the hexamethyl compounds (1-3) is of inadequate size for any type of multilayer deposition on a solid substrate Characterisationof Langmuir-Blodgett multilayers The satisfactory quality" of the LB films obtained by multi- layer deposition on hydrophobic glass was confirmed by the linearity of the plot of absorbance against number of layers The principal optical absorption band (the Soret band) was used to obtain these results In accord with the work of Luk," the Soret band was broadened by solid-state interactions from the 'full width at half height' value obtained from solution spectra A further change, characteristic of solid state interactions, is the red shift of the bands in the 500-600 nm region If diffraction maxima occur in the LAXRD of LB films, then it is possible to obtain a value for the d spacing because of the ordering perpendicular to the surface of the substrate In the case of films that deposit in the Y-type mode, the d spacing will provide an estimate of the thickness of the bilayer If the 328 J Mater Chem, 1996,6(3), 323-329 4.001 I 3.504 I 1.50 2e/degrees Fig.6 LAXRD plot for a Langmuir-Blodgett multilayer (60 layers) of compound 5 (see Fig 1) on a fresh, chemically etched silicon wafer ( Plessey-Caswell), using a Phillips Diffractometer PW 1700 (Rolls Royce, Bristol) number of layers is known then the thickness of the film can be calculated The diffraction data from the tetraethylporphyrin free base (4) was very noisy, although some evidence of long- range order could be seen However, a multilayer of 60 layers of the zinc compound (5) gave good first and second prder peaks (Fig 6), corresponding to a bilayer spacing of 34 A and a tilt-angle of 31" to the normal to the substrate Conclusions We have shown that the R1 =R2=R3=R4 =methyl compound (1) and its zinc and silver complexes (2 and 3 respectively) do not have a sufficiently large hydrophobic edge to permit the formation of Langmuir-Blodgett multilayers due to the poor flow properties of the Langmuir monolayers These compounds (1-3) have a hydrophobic edge smaller than that in the naturally derived analogue mesoporphyrin IX dimethyl ester The R1=R2=R3=R4= ethyl compound (4) and its zinc and silver complexes (5 and 6 respectively), all of which have a hydrophobic edge slightly larger than mesoporphyrin IX dimethyl ester, give Langmuir-Blodgett multilayers by Y-type deposition (for 5 and 6) or by a mode of deposition intermedi- ate between Y-type and X-type for the free base (4) The Langmuir monolayer of 4 has been shown to exhibit inferior flow properties when compared with compounds 5 and 6 In common with the work of Tredg~ld,'~ we have found that the use of metalloporphyrins (of Zn and Ag) improves the prospect for Y-type deposition Tredgold's work also suggests that further improvements in the quality of multilayers would be obtained by utilising the diol analogues of 5 and 6 [say, 5 (diol) and 6 (diol)] The use of 5 and 6 or 5 (diol) and 6 (diol) would yield a higher density of packing of porphyrin moieties within a layer, bilayers that were more compact and a reduced repeat-spacing between bilayers than the heavily substituted l3systems currently being used by many workers If a Langmuir-Blodgett multilayer of a pure free base porphyrin is required, then recourse must be taken to the R1 = R4=propyl, R2 =R3=ethyl analogue (see Fig 1) reported else- where,6 if a high packing density of porphyrin moieties is required References 1 R Jones, R H Tredgold and P Hodge, Thrn Solid Films, 1983, 99,25 2 R H Tredgold, A J Vickers, A Hoorfar, P Hodge and E Khoshdel, J Phys D Appl Phys ,1985,18,1139 3 R.H. Tredgold, S. D. Evans, P. Hodge and A. Hoorfar, Thin Solid Films, 1988, 160,99. 4 C. L. Honeybourne, J. T. Jackson and 0.T. G. Jones, FEBS Lett., 1979,98,207. 5 C. L. Honeybourne, J. T. Jackson, D. J. Simmonds and 0.T. G. Jones, Tetrahedron, 1980,36, 1833.6 C. L. Honeybourne and K. J. Barrell, J. Phys. Condens. Mutter, 1991,3SA, S35. 7 T. Koga, T. Nagamura and T. Ogawa, Thin Solid Films, 1994, 243, 606. 8 A. J. Hudson, T. Richardson, J. P. Thirtle, G. G. Roberts, R. A. W. Johnstone and A. J. F. N. Sobral, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A. 1993,234, 385. 9 H. Chou, C. T. Chen, K. F. Stork, P. W. Bohn and K. S. Suslick, J. Phys. Chem., 1994,98, 383. 10 F. Bonosi, G. Ricciardi, F. Lelj and G. Martini, Thin Solid Films, 1994,243,335. 11 D. Q. Li, C. T. Buscher and B. I. Swanson, Chem. Muter., 1994, 6, 803. 12 R. Bonnett, S. Ioannou, C. Pearson, M. C. Petty, M. Rogers-Evans and R. F. Wilkins, J. Muter. Chem., 1995,5,237. 13 S. Yamada, K. Kuwata, H. Yonemura and T. Matsuo, J. Photochem. Photobiol. A, 1995,87, 115. 14 G. P. Arsenhault, E. Bullock and S. F. MacDonald, J. Am. Chem. SOC.,1960,82,4384. 15 G.D. Dorraigh, J. R. Miller and F. M. Huennekens, J. Am. Chem. Soc., 1959,73,4315. 16 A. H. Jackson, G. W. Kenner and K. M. Smith, Tetrahedron, 1965, 21,2913. 17 R. H. Tredgold, Rep. Prog. Phys., 1987,50, 1609. 18 S. Baker, M. C. Petty, G. G. Roberts and M. V. Twigg, Thin Solid Films, 1983,99, 53. 19 S. Y. Luk, F. R. Mayers and J. 0.Williams, Thin Solid Films, 1988, 157, 69. Paper 5/05612H; Received 23rd August 1995 J. Mater. Chem., 1996, 6(3), 323-329 329

ISSN:0959-9428    

DOI:10.1039/JM9960600323

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

New oxide ion conducting solid electrolytes, Bi4V2011: M; M =B, Al, Cr, Y, LaP Chnoong Kheng Lee,' Boon Hong Bay" andAnthony R. Westb "Chemistry Department, Universiti Pertanian Malaysia, 43400 Serdang, Selangor, Malaysia bChemistry Department, University of Aberdeen, Meston Walk, Aberdeen, UK AB9 2UE The compositional ranges of Bi4V2011 solid solutions containing trivalent cations: B, Al, Cr, Y and La have been determined by means of a phase diagram study at solidus temperatures, ca. 850 "C. At least three mechanisms for accommodating variable cation contents are required. These are nominally V+Bi, V+M, Bi-+M; the first two also involve the formation of anion vacancies. The detailed mechanisms by which ions as different in size as B and La can enter the Bi,V2011 structure cannot, however, be inferred from the phase diagrams.Phase diagrams at 500 "C have much smaller solid-solution areas, indicating the stability and extent of the solid solutions to be very temperature-dependent. Conductivity studies show that La-, Y-and Al-doped materials, with a composition around Bi4Vl~8Mo~2010~8, are the best conductors at 300 "C with a conductivity value of up to 1.4 x Q-' cm-'. At 600 "C, Al-doped materials have the highest conductivity, ca. 1 x lo-' R-' cm-'. Conductivity Arrhenius plots show changes of slope associated with the phase transitions: a+P+y for B, Al, Cr and P3y for Y; data for La (y-polymorph only) are almost linear. Solid electrolytes obtained by doping Bi4V2OI1 are a new family of oxide ion conductors known as BIMEVOX.1-3 The highest conductivity has been reported for Cu and Ni doped materials with values as high as 3.2~lop3 R-' an-' at 300 "C., The compound Bi,V201, exhibits three crystallo- graphic polymorphs, a, p and y, in the temperature range 25-895"C.3,5 The structure of the different phases has been described as consisting of alternating sheets of constitution Bi2022+ and vo3.5Uo.52-, where 0 refers to oxide ion vacancies.' High levels of oxide ion conductivity occur in the high-temperature y-form, which can be stabilised to room temperature by the partial substitution of V by Cu or Ni.Doped Bi,V,O,, materials have been under intensive study in several laboratories because of their attractive electrical properties.In addition to being good oxide ion conductors, they have also been shown to exhibit ferroelectric and pyro- electric properties.6-8 Most of the studies concentrated on partial substitution of V by lower-valent cations such as Co, Ni, Cu, Zn, Al, Ti19299*10 and Ge," resulting in the stabilisation of the y-polymorph to room temperature. Substitution of Mo leads to stabilisation of the P-polymorph.12 The possibility of other substitution mechanisms involving Bi sites as well as V sites has been less studied. Following on from the phase diagram study of the Bi203-V205 join showing a range of Bi-rich solid solution^,^ similar solid solutions were found in Pb-,I3 Ge-I' and divalent cation-doped material^.'^ In our previous study of bismuth vanadate solid solutions doped with a series of divalent cations, it was shown, with the help of phase diagrams, that at least three mechanisms were necessary to accommodate variable cation content in the solid solutions.These are: V+Bi, V+M, Bi+M and possibly also the forma- tion of interstitial M. There also appeared to be a clear correlation between dopant M2+ size and the locus of the solid-solution area. With increasing ionic size, the solid solu- tions extend to progressively more Bi-deficient compositions, indicating a greater readiness for the larger ions to substitute directly into Bi sites. Conductivity studies indicated that the best oxide ion conductors were obtained for the composi- tions with x=0.2 and y=O in the general formula Bi4+yV2-y-xMxOll-y-3x/2, where M is the divalent cation." Apart from a single composition with A13+ as the dopant, no systematic studies of trivalent doping of Bi,V,O,, have ?Presented at the Second International Conference on Materials Chemistry, MC2, University of Kent at Canterbury, 17-21 July 1995.been reported. We therefore report here the results of an overview into doping Bi4V2011 with a selection of trivalent cations, ranging from the smallest, B to the large La; remark- ably, extensive doping occurs for all cation sizes. Experimental Bismuth vanadate solid solutions doped with trivalent cations were prepared by solid-state reaction in gold foil boats. Reagents used were Bi203 (99.9Y0, Aldrich), V205 (99.8%, Aldrich), B203 (99.98YO,Aldrich), Al,03 (99.99%, Johnson Matthey), Cr203 (99.999%, Johnson Matthey), La203 (99.9%, Aldrich) and Y203 (99.99Y0, Aldrich).They were dried at 300 "C prior to weighing. Compositions were weighed (ca. 3 g total), mixed with acetone in an agate mortar, dried, fired at temperatures in the range of 820-880°C for 20 h, depending on composition, and air-cooled to room temperature (ca. 1-2 min). Samples were analysed by X-ray powder diffraction (XRD, Philips diffractometer, Cu-Ka, radiation). For determi- nation of the phase diagram at 500"C, selected samples were heated at 800°C for 2 h, cooled to 500°C at 50°C h-', annealed at 500 "C overnight, and air-quenched. For differential thermal analysis (DTA), a DuPont 991 instrument with a 1200°C cell and a heating rate of 10°C min-l was used.Pellets for electrical property measurement were cold-pressed and sintered at 820-880 "C overnight; Au paste electrodes were then fired on at 200-600°C. In order to ensure that the materials were in the y-form, the pellets, with electrodes attached, were refired at 820-850 "C overnight and air quenched, immediately before conductivity measurements. Ac impedance measurements were made over the range 150-800 "C using a Hewlett-Packard 4192A impedance analyser over the frequency range 10-lo6 Hz. Samples were equilibrated at constant temperature for 30 min prior to each set of measurements. Results and Discussion Solid solutions of Bi4V2OI1with M3+ For each trivalent cation, the relevant region of the phase diagram Bi203-V205-M,03 was investigated in order to establish the locus and compositional extent of the single- phase Bi,V,O,, solid solutions.In each case, the effect of temperature had to be considered since the solid solutions may be more extensive at high temperatures, as found on the J. Muter. Chem., 1996, 6(3), 331-335 331 Bi203-V205 join Cooling rate was also important, since sometimes the various y-+P+a transitions may be suppressed by rapid cooling The phase diagrams presented here (Fig 2-6, later) refer to final reaction temperatures in the range 820-900 "C, depending on composition (these temperatures were found by trial and error to be close to the solidus), after which samples were removed from the furnace and allowed to cool naturally in air In addition, the phase diagrams were determined at an arbitrary and lower temperature, 500°C, as described in the Experimental The solid-solution limits at 500°C are indicated using dotted lines in Fig 2-6 In order to facilitate description and discussion of the results, and in particular to assess the likely substitution mechanism(s), the directions of possible solid-solution formation for Bi4VZOli doped in various ways are shown in Fig 1 Since the trivalent dopant is of different charge to V, additional compensation mechanisms are required other than for (7) which would involve M substituting directly for Bi The various schemes shown in Fig 1 cover the range of possible ionic compensation mechanisms, including formation of interstitial cations (3) or anion vacancies ( 1, 2, 4-6) It is important to recognize that proper crystallographic studies are required to determine the precise structural details of each substitution as these cannot be inferred from the phase diagram The general formula of the solid solutions may be written as Bi4+yV2--y--xMx01~ The phase diagram results -x-y are presented on a triangular compositional grid with x and y as the variables We refer to mechanism (2) (I e y=O, variable x) in which M substitutes for V as the 'stoichiometric join' In B-doped materials (Fig 2) the solid solution is quite extensive in direction (2), corresponding to the mechanism V+B [z e for y=O, x(max)=O 31 However, the maximum solid-solution extent is displaced towards negative values of y, with the V-rich limit of the solid-solution area running roughly parallel to direction (5) All the B-doped materials give the a-polymorph, it was not possible to stabilise the y-polymorph to room temperature with B as a dopant In Al- and Cr-doped materials (Fig 3 and 4) the solid- solution area was much more extensive than for B substitution with, for instance, a limit of x=O 5 at y=O compared with x= 0 3 at y=O for B Maximum x values of ca 0 7 and 0 6, respectively, were obtained at negative values of y for Al- and Cr-doped phases In contrast to the B-doped materials, most compositions gave the y-polymorph Thus, at y =0, y-phase was obtained for all solid solutions with x >O 2, the a-poly- morph was obtained only for lower values of x and a few compositions with negative values of y Sharma et a1 loreported that Bi4Vl 20108 was an a-polymorph at the preparation temperature of 650 "C In this study, composition x=0, y =0 2 gave the a-polymorph at 500°C (see later) and so our results are consistent with those in ref 10 In Y-and La-doped materials (Fig 5 and 6) the solid- solution areas are much smaller than those obtained for Al- and Cr-substitution, but tend to extend to more negative values of y The difference between Y-and La-doped solid solutions is that for Y-doped materials the P-polymorph was most commonly obtained while the a-and y-polymorphs were obtained for La-doped materials Stabilization of the 1-poly- -04 -02 0 02 yl" B14+y"2-yoll -y Fig.3 Composition range of the Bi4+,,VZ yAlxO,, '2'5 0 20 40 *'2'3100 tlon, (A)a, (A)a+, (0)Y,(0)Y+ x (mol %) Fig.1 Possible doping mechanisms for trivalent cations in bismuth vanadate (BI~VZO,,) solid solution solu-solid ,, -02 -01 0 01 02 43-02-01 0 01 02 03 y'" B14+ yv2 -yol 1 -y Y'nB'4+yV2-y011-y Fig.2 Composition range of the Bi4+,,V2 ,,BxOll sohsolid ,, Y+ Y+tion, (A)a, (A)a+, (0) tlon, (A)a, (A)a+, (0)Y, (0) 332 J Mater Chem, 1996, 6(3), 331-335 Fig.4 Composition range of the BI~+,,VZ-~ ,,CrXOll ,, solid solu- -0.2-0.1 0 0.1 0.2 Yin Bi, + yv2-yol,-y -0.3 -0.2 4.1 0 0.1 0.2 0.3 Yi"Bi~+yV2-y01,-y Fig. 6 Composition range of the Bi4+yV2-x-yLaxOll-x-y solid solu-tion; (A)a, (a)a+, (0)Y,(0)Y+ morph is not common but it has been reported previously in Bi4V2Ol1 doped with Mo.12 The phase diagrams at 500°C give very much smaller solid solution areas for all the materials (Fig.2-6, dotted curves). In addition, all the solid solutions give a-polymorphs on cooling from 500°C to room temperature. These results show that the stability of the trivalent-doped materials is very temperature dependent, as has been noted in the parent bismuth vanadate solid solution^.^ In the divalent cation- doped materials, however, the solid solution areas at 500"C, though smaller,16 are not reduced in size by nearly as much as those found here with the trivalent dopants. The phase diagrams for the trivalent cation-doped systems show that several mechanisms are required to account for the wide range of single-phase compositions that form, as is the case also for the divalent-doped systems.14 The three most simple mechanisms are: V+Bi (1); V-+M (2); Bi+M (7).The phase diagrams indicate, however, that neither of mechanisms (2) or (7) is particularly dominant for any of the trivalent dopants, in contrast to the divalent systems in which mechan- ism (2) predominates for small dopants and mechanism (7) for larger ones.The trivalent systems are generally most extensive in directions (4)-( 6) (Fig. 1) which could indicate that some kind of double substitution mechanism, involving substitution of M for both Bi and V, is preferred.Since the range of trivalent dopants studied varies greatly in size (Table 1) it is difficult to imagine that atoms as small as B could substitute directly for Bi in the same way that La could. The distorted nature of both Bi and V sites in the crystal Table 1 Octahedral ionic radii" ionic radius/pm ionic radius/pm Bi3+ 103 v5+ 54 Y3+ 90 ~13+ 53.5 La3+ 103.2 Cr3+ 61.5 B3+ 27 a From ref. 18. structure of Bi4V201, may allow considerable flexibility of the structure towards atoms of different size; thus B could perhaps occupy off-centre positions on substituting for Bi, in the same way that Li is able to substitute for Ca in the perovskites (Ca, -xLix)(Zrl -xTax)03.17 Clearly, structural studies to eluci- date the doping mechanisms are required since these cannot be deduced from the loci of the solid solutions in the phase diagrams.Conductivity of doped Bi4V2OI1 Conductivity measurements were carried out for single-phase materials obtained on the stoichiometric join (y=O) as well as for selected compositions with varying y and x (with reference to the general formula Bi4+yV2--y-xMxOll -x-y). Conductivity values were extracted from impedance complex plane plots. In general, for temperatures below 400 "C, a broadened semicircle with a low-frequency spike was obtained; at higher tempera- tures the spike became more pronounced. Typical impedance data are shown in Fig. 7 for an La-doped sample (Bi4.10V1~70La0.20010~70);the associated capacitance of the semi- circle [Fig.7(u)] has a value of 3.9 x F cm-' after correcting for jig capacitance, which is typical of a bulk component. There was no sign of any lower frequency, second semicircle that might be attributable to grain boundary impedances. Grain boundary resistances appear to be negligible in comparison with bulk resistances, therefore. At higher temperatures, the low-frequency spike inclined at ca. 45" to the horizontal axis was the predominant feature [Fig. 7(b)]; its associated capacitance of ca. F cm-' is characteristic of ionic polarization phenomena at the blocking electrodes, and a diffusion-limited Warburg impedance, thus supporting the idea that conduction was purely or predominantly ionic. There was no sign of partial collapse of the low-frequency spike, such as would occur if significant levels of electronic conduction were present. As in the case of Bi4V2Ol1 solid solutions5 and other doped Bi4V201, phase^,^,^," for some of which transport number2 and dc polarisation measurement^^-^ have been carried out, the main conducting species in these materials appears to be oxide ions.Arrhenius plots of the conductivity of representative doped materials are shown in Fig. 8. All have the same overall composition, x=O.2, y=O, which is the composition close to that of highest conductivity with dopants such as Co, Ni and Ti. The pellets for the Al- and Cr-doped Bi4V2OI1 were initially the y-polymorph but the Arrhenius plots show three different &O 470 490 510 530 ~'1105n Fig.7 Impedance data for Bi4~,oV,.70Lao.20010.70 at (a) 200 "C and (b) 500°C J. Muter. Chem., 1996, 6(3), 331-335 333 0 * B-doped --1 --2 h p-r 1-C \ b 4-vm -. --5 -6 -7 08 1 12 14 16 18 2 22 24 26 lo3KIT Fig. 8 Arrhenius plots of Bi,V1 8Mo 20i08 M =B, Al, Cr, Y,La slopes corresponding to a-, #I-and y-polymorphs and similar to that of the B-doped pellet which was the a-polymorph at the beginning of the conductivity measurements This further indicates that the y-polymorph in Al- and Cr-doped solid solutions is metastable at low temperatures and readily reverts to the a-polymorph during the heating cycle of conductivity measurements The p-and y-polymorphs were present in the Y-and La- doped pellets and their conductivity behaviour was as expected with one and zero changes of slope, respectively (Fig 8) There was, however, some evidence of slight curvature in the plot for the La-doped sample at high temperature Companng the different materials in Fig 8, conductivity at eg 300°C is highest with La as the dopant, closely followed by Y, that of A1 is somewhat lower and that of B, Cr is much lower In terms of possible applications, the La-doped materials in particular are attractive because their conductivity data are reversible on cooling and do not show the same scatter and hysteresis which is often seen in materials that undergo the a+j 3y transitions The effect on conductivity of varying composition was investigated For A1 and Cry the conductivity was measured for samples with different x at y=O, both sets showed a conductivity maximum at x=0 20, as shown for the Al-doped materials in Fig 9 Solid solutions on this stoichiometric join are much less extensive for B, Y and La dopants but neverthe- less the conductivity increased to a maximum at x z02 The effect of varying y at a constant x value of 0 2 was also studied For Al- and La-dopants, the conductivity decreased by a factor of 2 to 3 between y=O and y=O 1, for both systems (Table 2) This trend is similar to that observed in the parent, undoped materials5 and divalent-doped materials Is Conclusions Bi4V2OI1 is an extremely versatile host structure for doping and is able to accept large amounts of trivalent ions, indepen- dent of their size Thus, remarkably, ions as different in size as B and La are able to enter the Bi4V2OI1 structure in large amounts The mechanism of doping is complex but can nominally be readily interpreted in terms of the three simple cation substi- tution mechanisms V+Bi, Bi+M and V+M Some dopants 334 J Mater Chern, 1996,6(3), 331-335 *> 2-600"C h F 3-Y I C \ g -4-400"c -0 5-3CO "C 6-250 OC 7' I I I I I I 01 015 02 025 03 035 04 045 x in BI& xAlxO,, Fig.9 Conductivity isotherms for Al-doped materials with varying x M=Al M=La T/"C y=o y=o 1 y=o y=o 1 200 199x10 832x10 576x10 215x10 250 129x10 ' 543x10 303x10 ' 106x10 ' 300 648x10 294x10 138x10 473x10 350 269x10 134x10 487x10 165x10 400 967x10 520x10 138x10 466x10 450 327x10 246x10 333x10 108x10 500 180x10 590x10 664x10 207x10 550 800x10 229x10 124x10 363x10 600 111x10 406x10 238x10 575x10 favour retention of the a-structure (B), whereas others more readily stabilise the y-structure (eg La) The conductivity for all five dopants studied appears to be a maximum around the composition x=0 2, y =0 that appears also to be favoured by other dopants Impedance data indicate the conduction mechanism to be ionic, and presumably there- fore due to 02-ions, similar to the conclusions reached for other doped y-structures The highest conductivity at 300°C was obtained for La as the dopant, 14 x R-' cm-I Although this value is about one order of magnitude lower than for the best BIMEVOX matenals, the La-doped material may be attractive for appli- cations due to the reproducibility and reversibility on heat/cool cycles of its conductivity data We thank Mr Azali Md Sab, Soil Science Department, UPM for assistance with the X-ray diffraction analysis CKL is grateful to the Majlis Penyelidikan Kemajuan Sains Negara for financial support, grant no 2-07-05-009 References 1 F Abraham, J C Boivin, G Mairesse and G Nowogrocki, Solid State Ionccs 1990,40/41,934 2 T Jharada, A Hammouche J Fouletier, M Kleitz, J C Boivin and G Mairesse, Solid State Ionics, 1991,48,257 3 F Abraham, M F Debreuille-Gresse, G Mairesse and G Nowogrocki, Solid State Ionccs, 1988 28-30,529 4 E Pernot M Anne M Bacmann P Strobe1 J Fouletier, R N Vannier, G.Mairesse, F. Abraham and G. Nowogrocki, Solid State 11 C. K. Lee, M. P. Tan and A. R. West, J. Muter. Chem., 1994,4,525. Ionics, 1994,70171,259. 12 R. N. Vannier, G. Mairesse, F. Abraham and G. Nowogrocki, 5 C. K. Lee, D. C. Sinclair and A. R. West, Solid State Ionics, 1993, J. Solid State Chem., 1993,103,441. 62, 193. 13 R. N. Vannier, G. Mairesse, G. Nowogrocki, F. Abraham and J. C. 6 K. V. R. Prasad and K. D. R. Varma, J. Phys. D: Appl. Phys., 1991, Boivin, Solid State Ionics, 1992,53-56, 713. 24, 1858. 14 C. K. Lee, G. S. Lim and A. R. West, J. Muter. Chem., 1994,4,1441. 7 K. V. R. Prasad, A. R. Raju and K. D. R. Varma, J. Muter. Sci., 15 C. K. Lee, G. S. Lim, K. S. Low and A. R. West, Solid State Ionic 1994,29,2691. Materials, World Scientific Pub. Co., 1994, p. 211. 8 K. V. R. Prasad and K. D. R. Varma, Ferroelectrics, 1995,preprint. 16 C. K. Lee, G. S. Lim and A. R. West, unpublished results. 9 R. Essalim, B. Tanouti, J. P. Bonnet and J. M. Reau, Muter. Lett., 17 R. I. Smith and A. R. West, J. Solid State Chem., 1994,108,29. 1992, 13, 382. 18 R. D. Shannon, Acta Crystallogr., Sect. A, 1976,32,751. 10 V. Sharma, A. K. Shukla and J. Gopalakrishnan, Solid State Ionics, 1992,58, 359. Paper5/04832J; Received 21st July, 1995 J. Mater. Chem., 1996,6(3), 331-335 335

ISSN:0959-9428    

DOI:10.1039/JM9960600331

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Role of titanium in TiO, :SiO, sol-gels: an X-ray diffraction study? Jane S. Rigden,"" Robert J. Newport," Mark E. Smith," Peter J. Dirken" and Graham Bushnell-Wyeb "Physics Laboratory, University of Kent at Canterbury, Canterbury, Kent, UK CT2 7NR bCCRL Daresbury Laboratory, Daresbury, Warrington, UK WA4 4AD Transmission X-ray diffraction has been used to study a series of powdered silica :titania sol-gel glasses with titania contents ranging from a 'pure' silica sample through to high titania levels where phase separation is predicted to occur. Analysis of the data reveals a change in second- and third-neighbour coordination numbers with increasing Ti content and confirms that for low titanium contents the sol-gels are atomically mixed. The lower titanium content sol-gels have also been studied as spun thin films using shallow-angle X-ray diffraction.Comparison with the transmission studies shows an increase in disorder in the silica network when the material is in the form of a coating rather than in the bulk. An increase in the number of Si- 0-H bonds is also suggested. Mixed silica :titania materials are of significant technological importance. Silica glasses containing a few mol% TiO, are used as ultra-low thermal expansion (ULE)glasses' and mixed titanium :silicon oxides are important as catalysts and catalytic support materials.' In the optical industry they can be pro- duced as anti-reflective thin film coatings, or with tailored or graduated refractive indices.' The properties of titania :silica binaries, however, are strongly dependent on their chemical composition, homogeneity and texture; homogeneity at the atomic level is especially important.The different rates of hydrolysis of Ti alkoxides and Si alkoxides can mean that phase separation occurs as Ti-rich and Si-rich regions are formed: this significantly reduces the usefulness of the material. Sol-gel synthesis, based on hydrolysis and subsequent conden- sation of metal alkoxide precursors, is a relatively new method that combines atomic level mixing with a high degree of porosity .' Although SiO, shows four-fold Foordination in the bulk with an Si-0 distance of ca. 1.6; A, TiOz is six-coordinated with Ti-0 distance of ca. 1.93 A. A short Ti-0 distance of 1.82 A has been observed in EXAFS studies of silica gels and glasses with low TiOz levels, suggesting that the titanium is in four-fold coordination with oxygen.170NMR spectroscopy has confirmed that atomic mixing occurs in such glasses by revealing the presence of Ti-0-Si bond^;^.^ in contrast, OTi, and OTi4 features in the NMR spectra' of glasses with higher TiOz content (ca. 41 mol%) indicate that they are phase separated. Although silica and silica :titania binaries have been studied in their crystalline phases using X-ray diffracti~n,~.~ little work has been completed on the gels in their amorphous state. Transmission X-ray or neutron diffraction can reveal structural information averaged over an entire sample, and is therefore a useful method for studying bulk materials.However, conven- tional techniques cannot be used to study thin films or coatings, owing to the difficulty in separating properties of the film from those of the substrate. Shallow-angle X-ray diffraction, where sample penetration depths are lessened by reducing the incident angle of radiation onto the film, is a relatively new technique which can enable scattering from a thin film, and therefore structural information, to be isolated. The results presented herein represent an X-ray diffraction study of four SiOz :TiO, sol-gel glasses, with titanium content varying from 0 to ca. ?Presented at the Second International Conference on Materials Chemistry, MC', University of Kent at Canterbury, 17-21 July 1995.5 atom%. The samples are studied in bulk form using trans- mission diffraction and as thin films using shallow angle diffraction. Samples Four sol-gel glasses were prepared by hydrolysis of alkoxides with water and ethanol mixtures in the approximate ratios 1:2: 7.5, with varying titania contents., Sample 1, labelled 'pure silica', contained no titania; samples 2 and 3 contained small amounts of titania and were labelled 'low titania' and 'high titania', respectively; sample 4 contained very high titania levels and was in the domain in which phase separation was predicted to occur, and this sample was therefore labelled 'phase separated'. Note that all four samples contained substan- tial residual amounts of volatiles such as ethanol.Table 1 shows the compositional information, including mass and electron densities. It is immediately clear that the sample labelled 'phase separated' was compositionally very different from the samples which were expected to be atomically mixed. Whilst the other samples maintained a hydrogen :carbon ratio of ca. 2.7 : 1, the H:C ratio for sample 4 has risen to ca. 6: 1, with much less carbon but much more oxygen held in the system. The silica sol-gel and the two lowest titanium content mixed gels, were used to produce thin films by the spin coating method;' the 'phase separated' sample was not used to create a thin film as it was not possible to produce a uniform coating which adheres to the substrate with a non-atomically mixed sol.An excess of liquid was dispensed onto the surface of the substrate, and then the sample was rotated at low speed so that the liquid flowed radially outwards, driven by centripetal force. Any surplus liquid flowed to the edge of the substrate and dripped off. As the film thinned, the rate of removal of liquid slowed as the viscosity increased; in the final stages most of the thinning occurred by evaporation of volatiles.* The method produced a very uniformly thin coating, and the process could be repeated several times to build up a thicker film or, for example, to produce layers with slightly differing qualities. In the samples used herein, six layers of film were deposited to produce a film ca. 1 pm thick on a polished silicon wafer.Although the underlying physics and chemistry that govern polymer growth and gelation are the same for films and bulk sol-gels, several factors in the evolution of thin films mean that, structurally, the two forms can be quite different.' In bulk J. Muter. Chem., 1996, 6(3), 337-342 337 Table 1 Compositional information for the four SiO, :Ti02 samples sample composition (atom%) no. label Ti Si 0 1 pure silica 0.0 4.0 16.2 2 low titania 0.4 4.7 19.7 3 high titania 0.84 3.8 17.0 4 phase separated 4.9 7.0 38.2 systems evaporation usually occurs after gelation, whereas in thin films the deposition and evaporation processes happen simultaneously, and this results in a competition between compaction of the structure caused by evaporation and the stiffening (and therefore resistance to compaction) of the mate- rial caused by the condensation process.The short duration of deposition and evaporation/drying in thin films means that considerably less cross-linking occurs than in bulk gels, which generally results in more compact, dried structures.' Also, thin films are constrained by their geometry, and the continued shrinking causes stresses; this is particularly true for films made by spinning methods. It is likely, therefore, that there is a marked difference between the structure of sol-gels in thin films and in the bulk; it is expected that the rapid gelation of thin films will result in a more disordered material than in the bulk with a lower concentration of volatiles.Transmission Diffraction and the Shallow-angle Technique Both transmission and shallow-angle X-ray diffraction measurements were carried out on Station 9.1 at the Synchrotron Radiation Source at the CCRL Daresbury Laboratory, UK. The intrinsically highly parallel nature of the beam provided by a synchrotron source is of advantage over conventional focused laboratory X-ray sources for the shallow- angle technique in that the associated serious geometric aber- ration effects are avoided. Further, the high intensity beam provided by a synchrotron source is necessary for the relatively weak scattering from the small volume of amorphous material sampled in the shallow-angle geometry; also the availability of relativ5ly hard X-rays allows a wide dynamic range (up to ca. 18A-').The shallow-angle technique was first developed by Lim and Ortizg in 1987, who studied polycrystalline iron oxide layers on glass substrates, i.e. sharp Bragg peaks on a diffuse Debye Scherrer background. The method and analysis has recently been developed and used to study a variety of both amorphous and crystalline thin films. The refractive index of materials at X-ray wavelengths is less than unity; consequently, at incident angles below a critical value, ac, total external reflection occurs. Below a, limited penetration is achieved uiathe evanescent mode, and is exponentiallx damped: in principle sampling depths of ca. 10 to ca. 1000A may be achieved. Above a, the penetration depth increases rapidly with incident angle, inversely with the wavelength of the radiation, and is limited by photoelectric absorption; it is this region where shallow-angle diffraction can be used in a practi- vertxal crystal horizontal monochromator kapton shts 1 f0ll wtute beam -C H densitylg cm-3 electrons per A3 21.6 58.6 2.25 0.736 20.2 55.5 2.45 0.789 21.0 57.3 2.65 0.856 7.0 42.7 3.10 0.954 cal way to isolate scattering from a thin film mounted on a substrate. The conventional (transmission) X-ray diffraction arrange- ment12 is modified to produce the shallow-angle configuration, as shown schematically in Fig.1. The white beam from the synthrotron source is monochromated to a wavelength of 0.7 A by a channel cut crystal and proceeds through a pair of slits which define the incident-beam profile; a narrow slit profile of 100 pm x 10 mm is used in shallow-angle work to limit off-sample contamination scattering from the straight through beam at the lowest incident angles, where the beam's 'footprint' will be at its largest.The sample is set at a fixed, small angle a, to the incident X-rays. An iterative procedure of height and angle adjustment is used to define the zero-angle for the sample;fo~'' this procedure is very important given the small angles used in data collection. It is also essential that the sample is smooth and flat: any significant irregularity in the film thickness or sample curvature will produce a high uncertainty in a, and hence in the collected scattering profile.The scattered radiation passes through an arrangement of horizontal and vertical slits to the detector. A long slit package limits the viewed area and reduces the angular spread of scattered radiation incident on the detector and results in a resolution of ca. 0.07'. Data is collected sequentially at angles 28=2-130"; this is later converted to the scattering vector Q = 4n/A sin 8. X-Ray Diffraction Theory and Data Analysis Transmission diffraction Preliminary data reduction for transmission diffraction is carried out in the usual way,13 and includes correction for dead-time losses, the polarisation of the incident X-ray beam, and removal of the 8 dependence resulting from the changing sample volume ill~minated.~ The sample container and back- ground scattering is subtracted using a suitably corrected 'empty cell' data set; correction for sample absorption is then carried out.For a system of N identical atoms the scattered intensity (in electron units), is given by:" wherefis atomic form factor, Q is the scattering vector with magnitude lQl= (4n/A)sin 8 (for a scattering angle 28 and incident X-ray wavelength A), and ramis the distance vector between the positions of atoms n and m. This equation represents both the intra-atomic (self scattering) and the inter- atomic scattering (interference term) of the system. In the experimental data a third term is also collected which includes the inelastic scattering produced by the system; this can be calculated using tables14 and removed.When there is more than one atom type, an approximate monitor method can be used to calculate the scattered intensity by Fig. 1 Experimental configuration for the shallow-angle diffraction choosing a convenient 'unit of composition', uc, for the arrangement material. We can then define an average scattering factor per 338 J. Muter. Chem., 1996,6( 3), 337-342 electran? c fm UCf,=-c2, UC where the sum over uc represents the weighted sum over the atoms of atomic number 2,. The form factor for each atom type can then be approximated by fm=Kmfe,where K, will be approximately equal to 2,. In each case K, will vary with scattering vector Q,and the validity of this treatment depends on the error in treating K, as an average over the entire Q range involved.For any fixed displacement Y, the electron density averaged over all directions is given by pj(r) where the subscript j represents the atom type; this shows fluctuations from the average electron density of the sample, pe. For an amorphous material with no preferred orientation, spherical symmetry can be assumed and the integral over volume can be reduced to: The left-hand side of the equation is readily obtainable from experiment and is known as the structure factor S(Q).We then define an interference function i(Q)by rm Qi(Q)=4.n I Kjr[pj(r)-pel sin Qr dr (4)Jo uc Inverting this by Fourier transformation we obtain the com- bined radial distribution function, which may be rearranged to give the total pair distribution function g(r)= uc=1+ JV 2.n2rpec Kj (5)pe CKj uc uc In practice, it is not possible to measure X-ray diffraction data directly in electron units; an absolute intensity measurement is therefore obtained by scaling the data to oscillate about the theoretical self scattering term, and hence producing the S(Q).Shallow-angle diffraction It is not possible to treat shallow-angle X-ray diffraction data in the same quantitative manner as transmission diffraction data: basic data reduction accounts for detector dead-time, changes in incident-beam current and beam polarisation effects. A further correction is needed to account for the fact that the collected X-ray beam is actually scattered from the refracted beam within the sample; this produces a small shift in the measured scattering angle, 28." More sample-specific correc- tions such as sample absorption and multiple scattering are not included in the reduction procedure for the shallow-angle technique; these corrections are complicated by unknown factors in the sample geometry which make it difficult to determine the actual spread of penetration depths into the sample and/or substrate and the contributions from each.This situation could be clarified somewhat if the incident X-rays did not penetrate the substrate at all; this can be achieved by using thicker films, reducing the incident angle, or increasing the incident X-ray wavelength; however, increasing 1decreases the Q-range and therefore the real-space resolution, and there is little to be gained by reducing cri much below the detector slit resolution.Progress in the longer term is likely to depend on the use of indirect data reduction tools based on Monte Carlo methods. Subtraction of the background scattering in the shallow- angle geometry is also problematic, as there is no direct method of removing the sample and measuring the 'background' scatter. It must therefore be assumed that the background scattering may be approximated by a smooth curve, and can therefore be removed, along with the atomic form factor, by fitting a Chebyshev polynomial through the data. While this method produces an 'interference function' which shows the same peak positions as would be derived by following standard procedures for transmission geometry data,g there is no practi- cable method of converting the data to electron units, and therefore it is not possible to produce absolute coordination numbers from the real-space information.Transmission diffraction Fig. 2 shows the experimental S(Q) data for. the four silica: titania samples, confined to a Q-range <10 A-' for clarity. It is clear that the general shape of all four curves is the same, with only the 'phase separated' curve showing a significant increase in the intensity of the oscillations. Since X-ray scat- tering results from interaction with electrons, scattering will be dominated by correlations involving heavier atoms, i.e.Si, Ti and 0.It can be seen from Table 1 that samples 1-3 show very similar compositions and only a very small amount of titanium is present; sample 4, however, contains significantly more titanium which has resulted in much stronger scattering. Fig. 3 shows the corresponding g(r) curves: despite the similarity between these data and the S(Q) data for the lower titanium content samples, small changes are highlighted when corrections for sample composition are included in the trans- ition to g(r)and the data is normalised to the number of bond pairs. Clear differences in the shape of the curves for the four samples are evident, particularly in the region of theosecond and third neighbours. The first main peak at ca.1.61 A shows 12 3 4 5 6 7 8 9 10 CUB-' Fig. 2 Experimental S(Q) data for the four silica: titania samples measured in transmission geometry I I I 1 0.4 ;L. I I 1 I I 1 2 3 4 5 6 7 r/a Fig. 3 Experimental g(r) data for the four samples in transmission geometry J. Mater. Chem., 1996,6(3), 337-342 339 slight differences in position, but the peaks are similar in width and height, indicating that all four samples have a very similar first-neighbour coordination number Since the S(Q) data covered relatively short dynamic range in this case (0 45-14 A-’), and a heavy windowing function16 was used in the Fourier transform to avoid termination errors, the reso- lution of the real-space tats is relatively low, all correlations between ca 14 and 1 9 A, therefore, are contained within the first peak Thi! region is expected to include correlations from Si-Oo at 1 61 A, the short four-coordinate Ti-0 distance at 182 A (where present), and the C-0 and C-C correlations at 1 43 and 1 53 A, respectively, of any residual ethanol present in the sample The low resolution of the data means that it is not possible to distinguish between first-neighbour Si-0 and Ti-0 distances if the network remains four-coordinated, how- ever, if phase separation occurred and regions of pure six- coordinated tit?nia were formed, the longer Ti-0 correlation length of 193 A should be visible in the g(r), resulting in a wider first peak for the ‘phase separated‘ sample Although there is a slight widening of the peak in the ‘phase separated’ sample, it is not evident whether this is statistically significant Clear differences are visible in the g(r) data between ca 22 and 34A, however, corresponding to the 0-Si-0, 0-Ti-0, Si-0-Si, Si-0-Ti or Ti-0-Ti correlations (Table 2) Although the interatomic distances for four-coordinated silica and six- coordinated titania are known, there is some uncertainty concerning the distances within an atomically mixed silica tit- ania amorphous network The distortion which will be pro- duced when a titanium atom with a long Ti-0 distance is substituted into the silica network may result in a slight shortening of the Si-0 distance or, more probably, a distortion in the bond-angle distribution The distances in Table 2 were calculated assuming bond angles and distances are the same in the atomically mixed case as in pure silica, the values therefore will be only an indication of the possible interatomic distances The g(r) dtta for the ‘pure silica’ sample show! a strong peak at 3 12 A with a distinct shoulder at ca 2 65 A, as is the case for the first peak, the silica data appear to show interatomic distances which are slightly larger than those expected this may arise from the relatively poor statistical quality of the original, exploratory data for this sample, which limits the real-space resolution when a severe window function is applied Despite this problem, the data show clearly that correlations in this region result from two separate main interatomic distances, those of 0-Si-0 and Si-0-Si The introduction of a very small amount of titanium in the ‘low titania’ sample results in an immediate change in the shape of the g(r)curve with the two peaks becoming of similar intensity, but with a plateau formed between them This observation suggests that the network has become more complex, with a range of correlations forming This is consistent with a small number of 0-Ti-0 and/or Si-0-Ti bonds forming in the network, indicating the presence of atomically mixed SiO, Ti0, regions The findings for the ‘high titania’ sample follow this trend, with more correlations occurring at longfr distances corresponding to mixed corredations at or above 3 A, and less at the 0-Si-0 distance of 2 6 A The ‘phase separated’ sample does not appear, on averaging over the entire sample, to be structurally different from the lower titania samples The same trend of a slight shift to longer distances is observed, but there is no clear direct evidence for 0-Ti-0 correlations at 2 46 and 2 79 A,as might be expected if phase separation had occurred (although the peaks do appear to be slightly wider, which would be consistent with phase separation) It is a!so interesting to observe the formation of a peak at ca 0 95 A in the ‘phase separated’ sample, which is also visible in the ‘high titania’ sample although it is less intense This peak may result from the 0-H bond distance, being present in either volatiles such as ethanol, or in the main silica titania network Since there is little carbon in the ‘phase separated’ sample (see Table 1) it is likely that the 0-H groups are forming within the silica titania network, and are increasing in number as the amount of titanium in the sample is increased This might suggest that 0-H groups are becoming network terminators to reduce the stresses formed by the distortion of the network at titanium sites, or are forming at the boundary between regions of phase separated silica or titania Shallow-angle diffraction Fig 4 shows the corrected data for the three sol-gel samples after fitting and subtracting a polynomial from the data The scattering from the samples containing titania look very similar, but the ‘pure silica’ sample shows a much stronger scattenng across the whole Q-range Small Bragg peaks due to the $icon substrate are visible at approximately 6 0, 9 5 and 13 5 A-’ in the ‘pure silict’ and ‘low titania’ samples, although only the peak at ca 6A-’ appears to be present in the ‘high titania’ sample, this is likely to be due to the fact that the higher titania film is more electron-dense and therefore the incident beam is attenuated more quickly This observation, coupled with the fact that the silicon peaks are very small, suggests that the penetration depth covered by the incident X-rays is only just greater than the thickness of the films and so penetration into the silicon wafer is very small The sharpness of t>e first sol-gel peak in the ‘pure silica’ data at ca 1 9A-1 may, in addition, indicate contamination from an underlying residual silicon Bragg reflection The interference functions reveal the similanties between the scattering from all three samples after the first major peak It is $ear that the visible Bragg peaks, particularly the one at ca 6 A-l, represent a significant problem if analysis were to continue by way of conventional direct Founer transform to a pair distribution function, their presence could lead to strong pure silica -low htwa ----Oo6 hgh titama0 05I1fl -0 04Y5 003 n a 002 v h9 0010 000 -0 01 -0 02 Fig.4 Experimental S(Q) data for the three lower titania sol-gel samples using the shallow-angle technique Table 2 Interatomic distances for different types of correlations SIO, SIO, Ti02 Ti0, four-coordinate atomically mixed six-coordinate correlatiop 0-Si-0 SI-0-s1 0-Ti-0 Si- 0-Ti 0-Ti-0 TI-0-Ti dist ance/A 2 62 3 06 ca 30 ca 33 2 46, 2 19 3 03 340 J Muter Chem , 1996,6(3), 337-342 silicon correlations in g(r).However, the rapid decay of the dat? to the asymptotic value, after the firstosharp peak at ca. 1.8 A-l and a small second peak at ca. 4.5 A-l, indicates that all three samples show a high degree of disorder. This is demonstrated further in Fig. 5 where scattering from the ‘high titania’ sample in thin-film form (shallow-angle geometry) is compared to scattering from the bulk (transmission geometry); both data sets are at a similar stage of data reduction.Both curves are dominated by a first sharp peak primarily associated with Si-0 first-neighbour correlations, but the bulk sample also shows definite second and third peaks; for the thin-film sample it is very difficult to determine any distinct higher-order correlations, although some evidence of residual structure in that region is visible. Owing to the contamination by silicon Bragg reflections, and the large amount of statistical noise in the data produced by scattering from very small volumes, the information available from a Fourier transform-ation into real-space is limited; Fig. 6 shows the Fourier transform of the S(Q) function for the ‘high titania’ sample as an example of the r-space information obtainable.The S(Q) data is initially weighted by a sharpening function, and then a heavy windowing function is used in the Fourier transform to reduce ripples resulting from statistical noise or termination effects. The strong correlation visible at ca. 1.5 A is associated with the Si-0 distante; in bulk silica the Si-0 first-neighbour distance is 1.61,A; however, the silicon-oxygen distance is reduced to 1.50A when taken out of the confines of the silica network, for example when part of an Si(OH)4 unit. This may be further evidence that the silica network has become more disordered when in a thin film, and, contrary to the case for the bulk material, there are few long silicon-oxygen chains I N1 shallow angle -I-‘I 0.1s I transmission----I A .-v) c3 $ 0.10 v h9-0.05 -----------_---_____ -.--I I I I I I0.000 2 4 6 8101214 0iA-’ Fig.5 A comparison of I(Q) data for the ‘high titania’ sample, meas-ured in both transmission and shallow-angle geometries 0.95 I ,w, II I 1 1 I 012345678 r/A Fig. 6 Example Fourier transform of the shallow-angle S(Q) data for the ‘high titania’ sample and more hydrogen atoms terminating the network. There is little order apparent in the g(r) after the first main peak; in particular, interatomic distances resulting from 0-Si-0 (2.6 A) and Si-0-Si (3.0 A) bonds which am prominent in g(r) data from the bulk silica :titania sol-gels are not visible here. Discussion and Conclusions Preliminary X-ray diffraction data on four silica :titania sol-gel glasses, using a conventional 8:28 ‘powder’ transmission geometry, with compositions ranging from 0 to 42mol% titanium, has provided evidence to suggest that for lower levels of Ti, up to ca.20 mol%, the silica and titania are atomically mixed in fourfold coordination, in agreement with NMR data.3 For higher atomic percentages of titanium, there is no direct evidence for the existence of phase separated areas of six-coordinated Ti, although this cannot be discounted; the increas-ing O-H coordination number in the higher titanium samples does, however, indicate that phase separation has indeed occurred at these elevated Ti concentrations. The bulk pair correlation function data shown was produced using a heavy windowing function in the Fourier transform, which results in lower resolution real-space data.The method used to calculate g(r)also assumes that the self-scattering term for each of the atom types does not vary far from that produced for one ‘atomic unit’ of the material; when atoms of very differing atomic number are present in a sample this is a poorer approximation, and this may cause some error in the relative intensities of the peaks in g(r). Nevertheless, we are able to conclude that: (i) there is an increase in the number of 0-Ti-0 and Ti-O-Si bonds with increasing Ti content below ca. 20mol%; this indicates that the material is atomically mixed; (ii) for higher Ti contents (above ca. 40 mol%) there is no direct evidence for Ti-O-Ti bonds or phase separation, although there is a slight increase in the average first neighbour distance; and (iii) the number of O-H bonds increases with increasing Ti content suggesting that there is more network termination; this may be indicative of phase separation.In addition, the more novel shallow-angle X-ray diffraction method has been used to examine three silica: titania sol-gel thin films. Although difficulties arising from contamination from the substrate can reduce the quantitative nature of the final data, it is still possible to make clear qualitative statements about the structure of the films in comparison with their bulk counterparts. There is evidence for a higher degree of disorder in the silica network of the thin films, with many Si-0 bonds but a reduction in the number of rigid Si-0-Si chains com-pared to the bulk.An apparent shortening of the Si-0 first-neighbour distance may indicate that there is a tendency for more Si-O-H bonds to form, possibly associated with cracks in the stressed films; this is consistent with observations of the properties of sol-gel thin films discussed earlier.8 It was not possible to measure quantitatively the small differences in structure between the ‘pure silica’ sample and those containing titania; this might be expected considering the low titania content and the fact that it bonds substi-tutionally into the silicon network at these low titanium levels. Moreover, it is unlikely that anything other than major differences could be observed with the shallow-angle technique until a method is devised of allowing an exact background subtraction process, and this remains a limitation to the usefulness of the technique when applied to the study of amorphous materials.The penetration depth of the X-rays into the sample/substrate assembly would also have to be reduced, using either thicker samples (in which case the films might be structurally different from those used in real appli-cations), or higher wavelengths. The method also collects data from a range of scattering volumes, dependent on the penetra-tion and film thickness; this results in a range of absorption and multiple scattering effects and therefore a blurring of the J.Mater. Chem., 1996, 6(3), 337-342 341 data The method does, however, have some potential in the study of amorphous thin films and coatings where sample processing or treatment induces more substantial structural effects (eg crystallisatiam, phase separation or the loss of volatiles from a single sample) that can be followed zn sztu We acknowledge the CCRL Daresbury Laboratory for the provision of Synchrotron beamtime on Station 9 1 References C J Bnnker and G W Scherer, Sol-Gel Science, Academic Press, Boston, 1990 M Itoh, H Hatton, and K J Tanabe, J Catal, 1974,35,225 M E Smith and H J Whitfield, J Chem SOC Chem Commun, 1994,723 P J Dirken, M E Smith and H J Whitfield, J Phys Chem, 1995, 99,395 5 T J Bastow, A F Moodie, M E Smith and H J Whitfield, J Mater Chem ,1993,3,697 6 J-J Cheng and D-W Wang, J Non-Cryst Solids, 1988,100,288 7 M Emih, L Incoccia, S Mobiho, G Fagherazzi and M Guglielmi, J Non-Cryst Solids, 1985,74, 129 8 D E Bornside, C W Macosko and L E Scnven, J Imaging Technol, 1987,13,122 9 W P G Lim and C Ortiz, J Mater Res, 1987,2,471 10 T M Burke, D W Huxley, R J Newport and R Cernik, Rev Sci Instrum ,1992,63, 1150 11 T M Burke, PhD Theszs, University of Kent, 1994 12 G Bushnell-Wye and R Cernik, Rev Scz Instrum, 1992,63,999 13 D W Huxley, PhD Thesis, University of Kent, 1991 14 International Tables for Crystallography Vol Ill, ed C H Macgillavry and G D Rieck, Kynoch Press, Birmingham, vol 3,1968 15 B E Warren, X-ray diffraction, Dover Publications, New York, 1990 16 J Waser, Rev Mod Phys ,1953,25,671 Paper 5/05332C, Received 9th August, 1995 342 J Mater Chem , 1996, 6(3), 337-342

ISSN:0959-9428    

DOI:10.1039/JM9960600337

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Synthesis of CdS and CdSe nanoparticles by thermolysis of diethyldithio-or diethyldiseleno-carbamates of cadmium? Tito Trindade and Paul O'Brien*S Department of Chemistry, Queen Mary and Westjield College, Mile End Road, London, UK El 4NS Cadmium sulfide and cadmium selenide nanoparticles have been synthesised by a novel route involving the thermal decomposition of the bisdiethyldithio- or bisdiethyldiseleno-carbamates of cadmium in refluxing 4-ethylpyridine solutions. The nanodispersed materials were studied by electronic spectroscopy and bandgaps were blue shifted. Transmission electron microscopy of the samples showed material to be in the nanosize range and crystalline. There has been considerable interest in the synthesis and characterisation of semiconductor nanoparticle~.'-~ Nanoparticles, also known as nanocrystallites, Q-particles or quantum dots, are particles with a high surface : volume ratio and diameters of up to 10-20 nm, their opto-electronic proper- ties are different from the bulk counterparts, and new techno- logical applications have been proposed for this type of The prospects for devices are now more immediate and a number of recent papers have reported on either the photoluminescent properties of nanodispersed II-VI materials' or photoluminescent devices based on such II-VI materiakg Nanoparticles are also important in fundamental research because they represent a state of matter in which the transition from molecular to the bulk (macrocrystalline) level can be investigated e~perimentally.'-~ The preparations of nanoparticles of many semiconductors have been reported, these include: PbS,lo,l' CdS,',1'-'6 CdSe,16-'9 CdTe,16 ZnS,20-22 ZnO,', Ti02,24 InP,25 G~As,'~.'~ Zn3PZ2' and Cd3P2.20*21 More recently, the synthesis of nano- composites has been subject of intense research as well.Some examples of nanocomposite materials described in the literature are ZnS/CdSe," CdS/PbS,'1*2' SiO,/CdSZ9 and CdS/ZnS.30 There are other reports of studies concerning the preparation of nanoparticulate systems including elemental Ag,31 Ge,32 Pd33 and Pt34or metal halides such as HgIZ3' and PbI,.36 Theoretical models predicting the optical properties of semi- conductor nanoparticles are a~ailable,~-~' but the properties of nanoparticles obtained by any new synthetic procedure are hard to anticipate. The following characteristics are desirable in the final nanodispersed system: high purity, monodispersity and an ability to control surface derivatization. Nanoparticles with these properties have been prepared by several synthetic methods and/or separation technique^.^^^ The chemical methods used for the preparation of semiconductor nanopart- icles involve reactions in various media including: aqueous solution, microemulsions, zeolites, gels, polymers and glasses.Steigerwald et al." prepared CdSe nanoparticles using the solution-phase thermolysis of Cd [Se(C,H,)], . The method involved" refluxing the precursor in 4-ethylpyridine, a high boiling point solvent, to give optically clear solutions contain- ing the nanoparticles.The high reflux temperature promotes the decomposition of the precursor producing the semicon- ductor nanoparticulate material. Such single-molecule precur- sors contain the metal and non-metal (chalcogenide) within the same molecule and are therefore attractive sources for ?Presented at the Second International Conference on Materials Chemistry, MC', University of Kent at Canterbury, 17-21 July 1995. 1Present address: Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London, UK SW7 2AZ. the one-step preparations of nanoparticles containing those elements. Such an approach avoids the use of toxic and pyrophoric compounds such as Cd( CH,), , which is commonly used for preparing nanodispersed cadmium chalcogens.l6 Solid cadmium diethyldithiocarbamate (Cddtc) and cad- mium diethyldiselenocarbamate (Cddsc) are dimeric com-pounds of molecular formula (Cd [E,CN(C,H,),], ), (E = S, Se). Their crystal structures have been rep~rted~',~~ and show distorted square-pyramidal coordination at the metal. The bisdiethyldithio- or bisdiethyldiseleno-carbamates of cadmium have been used in chemical vapour deposition experiments to prepare II-VI semiconductor film^.^',^^ In this work, solutions of these compounds in 4-ethylpyridine were used to produce CdS and CdSe nanoparticles. This solvent has a high boiling point (168 "C, 1 atm) and dilute solutions of Cddtc and Cddsc in 4-ethylpyridine remain optically clear for more than 24 h.Nanoparticulate material with a derivatized surface has pre- viously been obtained by using 4-ethylpyridine as the solvent;I8 however, metal thiocarbamates/selenocarbamates have never been used as precursors for semiconductor nanoparticles. Experimental Chemicals CdC1,(99 + %, Aldrich), NaS2CN(C,HS),.3H,0 (98%, Aldrich), 4-ethylpyridine (98Y0, Aldrich), pyridine (99 + YO, Aldrich, CH2Cl, (99Y0, BDH) and light petroleum (bp 6O-8O0C, BDH) were all used as received except 4-eothylpyri- dine which was dried with molecular sieves (type 3 A, BDH) and deoxygenated under a nitrogen flow. Synthesis of molecular precursors Cddtc was synthesized by adding stoichiometric quantities of aqueous equimolar (0.1mol drn-,) solutions of CdC1, and NaS2CN(C,H,),~3H,0.The white solid obtained was filtered off and washed thoroughly with deionised water. This solid was purified by recrystallization from hot CH2Clz. Cddsc was synthesized by the method described in the literature4' by treating N,N-diethyldiselenocarbamate, as the diethylam-monium salt, with an aqueous solution containing a stoichio- metric amount of CdC1,. The compounds were identified by 'H NMR (CDC1,) and IR spectroscopy. Synthesis of the CdS and CdSe nanoparticles Solutions (5-50 mmol dm-, in the precursor) were prepared by dissolving the required amount of the compound in 4- ethylpyridine at room temperature. The solutions were filtered and then heated at the reflux temperature of 4-ethylpyridine J.Mater. Chem., 1996, 6(3),343-347 343 (168°C). The reflux was performed both under the ambient atmosphere and an N2 atmosphere. The formation of the nanoparticles as a function of the time of heating was moni- tored by extracting an aliquot of the refluxing solutions and transferring it to a vial immersed in an ice-bath and recording the optical absorption spectrum. The addition of light pet- roleum to the final cooled and optically clear solutions resulted in the flocculation of a precipitate which was collected by centrifugation. These solids were washed with dichloro-methane-light petroleum and then dried under vacuum to give powders which were stored under N,. The material gave optically clear solutions when redissolved in either pyridine or 4-ethylpyridine; any insoluble material in these redispersions was isolated by centrifugation and discarded.Material characterisation and instrumentation IR spectroscopy of the powders was performed using CsI (99.9%, Aldrich) pellets and a Perkin-Elmer 1720X FTIR spectrometer. Optical absorption spectra were recorded at room temperature with a Philips PU 8710 spectrophotometer. Silica cells (1 cm) were used and the starting solution for each precursor was used as reference. The pyridine solutions were analysed using pure pyridine as reference. The 'H NMR spectra were recorded in a Bruker 250 AM pulsed Fourier transform instrument. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDAX) spectroscopy were performed with a JEOL JSM35CF microscope operating at 25 kV.The samples for analysis were prepared by putting one drop of the sample solution onto pure aluminium plates and letting the solvent evaporate at room temperature. X-Ray powder diffraction (XRPD) patterns were measured using a Guinier camera and a Philips 1130 X-ray generator (Cu-Kol radiation). Samples for X-ray diffraction were prepared by placing the powder onto adhesive tape. Conventional transmission electron microscopy (TEM) results were obtained using a JEOL-JEM 1200 EX I1 scanning and transmission electron microscope operating at 100 kV; high resolution transmission electron microscopy (HRTEM) was performed with a JEOL 2000 FX electron microscope operating at 200kV.A sample for TEM was prepared by placing an aliquot of pyridine solution containing the nanopar- ticles onto an amorphous carbon surface on a copper grid and wicking away the solvent with a paper tip. Particle sizes were determined by measuring the diameter of around one thousand particles on the TEM images. Results and Discussion Optical properties of nanodispersed CdS and CdSe The optical properties of solutions of Cddtc and Cddsc in 4- ethylpyridine change with heating. Typical changes, as a func- tion of time of heating, using the starting solution as reference, are reported in Fig. 1 and 2 for the Cddtc and Cddsc precursors, respectively. With longer heating times the optical homogeneity of the solutions is not maintained and scattering perturbs the absorption spectra.In both cases the absorption edge is blue- shifted in relation to the bulk bandgap value, (the absorption edge is taken as the intersection of the base line with the tangent drawn to the band shoulder). Such shifts in the absorption edge of semiconducting materials have been associ- ated'-5 with particles having sizes comparable to the de Broglie wavelength of the electron and hole. The alterations observed in the band profiles (Fig. 1 and 2) are associated with a chemical transformation of the molecular precursors, since for both cases the starting solution was used as reference. A blank consisting of pure 4-ethylpyridine, refluxed over 6 h under ambient atmosphere, showed an increase in the intensity of the absorption band at 325 nm.If 344 J. Muter. Chem., 1996, 6(3), 343-347 0.30 w avelength/nm 0.40 r 0.30-,Q> \ \0 \ \c \(d \ 0.20 -\\\ \% \ bulk band gap\ \n 6 \ 0.10--.-.------._.-0.00 Fig. 2 Optical absorption spectra of Cddsc in 4-ethylpyridine solutions at different reflux times: ---, 2; ......, 4;-, 6 h the reflux is carried out under N2 the 4-ethylpyridine shows no such changes in its optical characteristics. However, the changes associated with the formation of nanodispersed mate- rial are similar if heating is performed in the presence or absence of N,. The absorption characteristics for CdS (Fig. 1) and CdSe (Fig. 2) are in agreement with the initial formation of CdS or CdSe nanoparticles which agglomerate to form particles of larger dimensions.Other authors'' have reported similar optical spectra for CdSe particles grown from Cd [Se(C6H5)I2 in 4-ethylpyridine solutions. The band at 420 nm was assigned'* to electronic transitions occurring in small CdSe clusters dispersed in 4-ethylpyridine. In this work an absorption band at around 413 nm was also observed in both 4-ethylpyridine and pyridine solutions containing the CdSe species (Fig. 3). The optical absorption spectrum of CdS nanoparticles have been p~blished'~*~~ and are similar to those shown in Fig. 1, even though media as diverse as ze01ites'~ and water" have been used in the preparation. The species responsible for the absorption were isolated as powders by addition of light petroleum to the cooled solutions followed by centrifugation.The solids obtained are readily dispersed in pure 4-ethylpyridine or pyridine. The optical absorption spectra for the pyridine solutions were recorded, and showed that the shifts in the edges to higher energies compared to the bulk values are still observed (Fig. 3). The 1.60 4.501 / ... 400 450 500 550 600 650 700 750 800 wavelengthlnm Fig. 3 Optical absorption spectra of the pyridine solutions containing the soluble powders obtained from Cddsc in 4-ethylpyridine at different reflux times: ---,0.25; ---, 0.5; ---, 2; .-.-.+,4; -.-.-, 5; -, 6 h observed band shifts, on refluxing, suggest an increase in mean particle diameters for the CdSe clusters with time.I6 It was found that the pyridine solutions containing the nanoparticles were unstable and the optical properties changed with time.A broadening of the sharp band at 413nm, after 48 h, can be seen clearly (Fig. 4) for the sample obtained from Cddsc after 15 min reflux. The 4-ethylpyridine growth solutions also shows broadening of the absorption bands when kept standing. Such broadening is to be expected if an agglomeration process occurs because the polydispersity of the particulate system will be increased. The spectrum of a pyridine solution containing the powder obtained from the addition of light petroleum to the 4- ethylpyridine solution of Cddtc after 6 h reflux is shown in Fig.5. The 'bandgap' of the CdS nanoparticles was determined using the direct transition method4' by fitting the absorption data to eqn. (1) (Fig. 5, inset): a(hv)cc (hv -E,)1'2 (1) where a is the absorption coefficient of the semiconductor material, hv is the photon energy and E, is the optical bandgap. The optical bandgap obtained by using this method is 2.63 eV, which is slightly blue-shifted from that of bulk CdS (2.53 eV). CdS particles begin to present4 the charactoeristic bandgap of bulk material at a diameter of around 80A, i.e. for particles within the nanosize range. 420 470 520 570 620 670 720 waveleng t h/nm Fig. 4 Optical absorption spectrum of the pyridine solution after 48 h (t=0.25 h in Fig. 3) 350 400 450 500 550 wavelength/nm Fig. 5 Optical absorption spectrum of the pyridine solution containing the soluble powder obtained from Cddtc in 4-ethylpyridine (6 h reflux).Inset shows the fit of the absorption edge by the direct transition method. Characterization of solid phases Prolonged times of heating led to 4-ethylpyridine solutions containing solid material in suspension and/or fixed to the walls of the flask. With the dithiocarbamate the suspended material is dark yellow and the XRPD pattern consists of broad lines typical of hexagonal CdS. The bulk material obtained from the selenocarbamate precursor is brown-grey and adhered firmly to the walls of the flask forming a specular film. The XRPD pattern showed evidence for elemental hexag- onal Se and hexagonal CdSe; for the latter case the SEM showed well defined spherical particles within the submicro- metric range (Fig.6). EDAX on a single particle showed the presence of both Se and Cd. These results suggest that Cddtc and Cddsc are thermally decomposed in 4-ethylpyridine solu- tions. Prolonged heating times lead to bulk material but the solutions still contain nanosized particles of CdS and CdSe, as indicated by their optical absorption spectra. This hypothesis is also supported by the results obtained from the characteris- ation of the solid phases obtained from the syntheses. The powders isolated from the solutions during the growth of nanodispersed material were characterised by IR, EDAX and XRPD. The IR spectra of the powders do not show the characteristic bands of the molecular precursors.The ease of dissolution of these powders in pyridine and 4-ethylpyridine suggests the binding of solvent molecules to the nanoparticles surface. However, the characteristic bands of the 4-ethylpyri- dine were not found in the IR spectra (e.g. the sharp and strong bands due to the ring stretching of 4-ethylpyridine around 1602 and 1560cm-'). It is probable that surface coverage has occurred to an extent below the detection limits of the IR experiment. The low surface coverage of the nanopart- Fig. 6 SEM image of CdSe particles J. Mater. Chem., 1996, 6(3), 343-347 345 icles by the solvent molecules could also explain their relatively Fig. 7 CdSe nanoparticles obtained from Cddsc in 4-ethylpyndine after refluxing for 5min (0)conventional TEM image.(b) HRTEM" \I "l\, image (bar =10 nm) facile agglomeration The elements detected by EDAX were Cd and the chalcogen- ide element (S or Se) Si and C1 were also detected as contaminants, probably from vacuum grease and/or precursor Unlike the bulk solids obtained in both syntheses these powders did not show XRPD patterns in our equipment, a result that does not preclude some crystallinity in the samples Particle size assessment The material contained in the optically clear pyridine solutions containing CdSe was subjected to further analysis by TEM The TEM analysis was performed for samples obtained after 15min [Fig 7(4, Fig 7(b) shows the HRTEM image] and 360min reflux The particle size distribution for the shorty time is shown in Fig 8, in which the mean diameter is 48 A The TEM of a sample refluxed for longer time shows larger particles with some agglomeration having occurred The results obtained by TEM confirm that the agglomer- ation is a favourable process in the pyridine solutions contain- ing the CdSe nanoparticles The agglomeration leads to some spread on the particle size distribution (Fig 8) In ,Fig 7(b)the lattice fringes of particles with diameters up to 50 A are clearly observed, confirming the presence of dispersed nanocrystallites in the pyndine solution Analysis of the patterns for several different particles was most consistent with a predominance of the hexagonal phase This result shows that the CdSe nanopart- icles have the bulk unit-cell structure, despite their markedly different optical properties, in agreement with reports made by other authors This work showed that single-molecule precursors such as Cddtc and Cddsc can be used for the preparation of soluble nanosized CdS and CdSe particles, respectively Work is in progress in our laboratories on the use of alk~l of the precursors used in the work described here Our main concern is to overcome some of the limitations found in this work, such as the practical manipulation of significant quantities of semiconductor nanoparticles and the inherent Fig.8 Particle size distnbution of the CdSe nanoparticles obtained from Cddsc in 4-ethylpyndine after refluxing for 15min and redispersion in pyndine 346 J Muter Chem , 1996,6(3), 343-347 instability of the solutions containing the nanoparticulate materials.21 22 H. Weller, A. Fojtik and A. Henglein, Chem. Phys. Lett., 1985, 117,485. V. Sankaran, J. Yue, R. E. Cohen, R. R. Schrock and R. J. Silbey, Tito Trindade thanks JNICT (Portugal) for a grant. We thank 23 Chem. Mater., 1993,5, 1133. A. J. Hoffman, H. Yee, G. Mills and M. R. Hoffmann, J. Phys. Mr. K. Pel1 (QMW College) and Dr. X. Zhang (IRC for Semiconductors, Imperial College) for technical assistance with the microscopy work. 24 25 Chem., 1992,96,5540. D. Duoghong, J. Ramsden and M. Griitzel, J. Am. Chem. SOC., 1982,104,2977. T. Douglas and K. H. Theopold, Inorg.Chem., 1991,30,594. 26 M. A. Olshavsky, A. N. Goldstein and A. P. Alivisatos, J. Am. References 27 Chem. SOC., 1990,112,9438. P. C. Sercel, W. A. Saunders, H. A. Atwater, K. J. Vahala and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 R. Rosseti, J. L. Ellison, J. M. Gibson and L. E. Brus, J. Chem. Phys., 1984,80,4464. A. Henglein, Chem. Rev., 1989,89, 1861. M. L. Steigerwald and L. Brus, Acc. Chem. Res., 1990,23,183. H. Weller, Angew. Chem., Int. Ed. Engl., 1993,32,41. H. Weller, Adv. Mater., 1993,5, 88. L. Brus, J. Phys. Chem., 1994,98,3575. A. Fojtik and A. Henglein, Chem. Phys. Lett., 1994,221, 363. B. 0.Dabbousi, C. B. Murray, M. F. Rubner and M. G.Bawendi, Chem. Mater., 1994,6,216. V. L. Colvin, M. C. Schlamp and A. P. Alivisatos, Nature, 1994, 370, 354.A. J. Nozik, F. Williams, M. Nenadovic, T. Rajh and 0.I. Micic, J. Phys. Chem., 1985,89,397. H. S. Zhou, I. Honma, H. Kumiyama and J. W. Haus, J. Phys. Chem., 1993,97,895. A. Fojtik, H. Weller, U. Koch and A. Henglein, Ber. Bunsenges. Phys. Chem., 1984,88,969. L. Spanhel, M. Haase, H. Weller and A. Henglein, J. Am. Chem. SOC.,1987,109, 5649. Y. Wang and N. Herron, J. Phys. Chem., 1987,91,257. H. J. Watzke and J. H. Fendler, J. Phys. Chem., 1987,91,857. C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. SOC., 1993,115,8706. M. L. Steigerwald, A. P. Alivisatos, J. M. Gibson, T. D. Harris, R. Kortan, A. J. Muller, A. M. Thayer, T. M. Duncan, D. C. Douglas and L. E. Brus, J. Am. Chem. SOC., 1988,110,3046. J. G. Brenman, T. Siegrist, P. J. Carroll, M.Stuczynski, L. E. Brus 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 R. C. Flagan, Appl. Phys. Lett., 1992,61, 696. H. S. Zhou, H. Sasahara, I. Honna, H. Komiyama and J. W. Haus, Chem. Mater., 1994,6, 1534. S. Chang, L. Liu and S. A. Asher, J. Am. Chem. SOC.,1994, 116, 6739. T. Cassagneau, G. B. Hix, D. J. Jones, P. Meireles-Torres, M. Rhomi and J. Roziere, J. Mater. Chem., 1994,4, 189. A. Henglein, P. Mulvaney and T. Linnert, Faraday Discuss., 1991, 92, 31. A. Kornowski, M. Giersig, R. Vogel, A. Chemseddine and H. Weller, Adv. Mater., 1993,5, 634. C. Amiens, D. Caro, B. Chautret and J. S. Bradley, J. Am. Chem. SOC.,1993,115, 11638. M. T. Reetz and W. Helbig, J. Am. Chem. SOC., 1994,116,7401. M. W. Peterson, 0.I. Micic and A. J. Nozik, J. Phys. Chem., 1988, 92,4160. 0.I. Micic, L. Zonggnem, G. Mills, J. C. Sullivan and D. Meisel, J. Phys. Chem., 1987,91,6221. L. E. Brus, J. Chem. Phys., 1983,79,5560. L. E. Brus, J. Chem. Phys., 1984,80,4403. P. E. Lippens and M. Lannoo, Phys. Rev. B, 1989,39,10395. Y. Wang and N. Herron, J. Phys. Chem., 1991,95525. Y. Nosaka, J. Phys. Chem., 1991,955054. M. B. Hursthouse, M. Azad Malik, M. Motevalli and P. O’Brien, Polyhedron, 1992,11,45. M. Bonamico, G. Mazzone, A. Vaciago and L. Zambonelli, Acta Crystallogr.,1965, 19, 898. D. M. Frigo, 0. F. Z. Khan and P. OBrien, J. Crystal Growth, 1989,96,989. 19 and M. L. Steigerwald, J. Am. Chem. SOC., 1989,111,4141. A. R. Kortan, R. Hull, R. L. Opila, M. G. Bawendi, M. L. Steigerwald, P. J. Carrol and L. E. Brus, J. Am. Chem. SOC.,1990, 45 46 I. Pankove, Optical Processes in Semiconductors, Dover Publications, New York, 1970,p. 36. M. Azad Malik and P. OBrien, Ado. Mater. Opt. Electron., 1994, 112,1327. 3, 171. 20 S. Baral, A. Fojtik, H. Weller and A. Henglein, J. Am. Chem. SOC., 1986,108,375. Paper 5/05017K; Received 28th July, 1995 J. Mater. Chem., 1996, 6(3), 343-347 347

ISSN:0959-9428    

DOI:10.1039/JM9960600343

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Synthesis and properties of copper (11) and oxovanadium( IV) complexes derived from polar Schiff's bases Eduardo Campillos, Mercedes Marcos, Ana Omenat and Jose Luis Serrano Quimica Orgbnica, Instituto de Ciencia de Materiales de Aragbn, Universidad de Zaragoza-CSIC, 50009-Zaragoza, Spain The synthesis and mesogenic behaviour of copper(I1) and oxovanadium(1v) complexes derived from Schiff 's bases substituted with polar groups and of their corresponding ligands are reported. The polar groups studied are F, CF,, CN, and CH2CN in the 3 or 4 position of the aniline moiety. The ligands exhibited smectic C, smectic A and nematic phases, whereas the complexes showed smectic A and nematic phases, and in some cases oxovanadium(1v) complexes exhibited a smectic crystal phase.Metal complexes of Schiff's bases have played an important role in the development of metallomesogens.' The particular advantage of the salicylaldimine ligand system is the consider- able flexibility of the synthetic procedure which has allowed the preparation of a wide variety of complexes whose properties are strongly dependent on the ligand structure and on the metal used.2 However, little is known about the relationship between the molecular structure and the mesophase behaviour of these compounds. Ovchinnikov et al. have carried out some systematic studies on these kinds of c~mplexes,~-~ while we have described the mesomorphic properties of several series of copper (II), nickel (11) and oxovanadium(1v) complexes derived from N-alkyl and N-aryl salicylaldimines6 and have studied the relationship between mesogenic behaviour and molecular str~cture.~ In these studies we found that the best liquid-crystal properties are obtained with systems derived from 2,4-dihydroxybenzal- dehyde.To improve our knowledge of the relationship between molecular structure and mesogenic properties, we present in this paper the synthesis and the mesogenic properties of several new families of copper(I1) and oxovanadium (~v) complexes derived from imines with lateral and/or terminal polar groups in the amine part with general structure A. Experimental Synthesis The preparation of the ligands and chelates was carried out according to Scheme 1 (series A) and Scheme 2 (series B and C). Synthesis of the Schiff's bases.The free ligands L were synthesized using a well known method' by mixing an ethanolic solution of 1 mmol of the appropriate aldehyde (3 or 7) with 1 mmol of the appropriate amine (4) and 2 drops of acetic acid as catalyst. The precipitated product was purified by recrystallization from ethanol (yields 60-78 %). Synthesis of aldehyde 3.4-Hexyloxy-2-hydroxybenzaldehyde was synthesized as described previouslyg using Williamson's method by reaction of hexyl bromide (2; 1 mmol), with 2,4- dihydroxybenzaldehyde (1) in the presence of KHCO, (1 mmol) as base in acetone. The product was purified by flash chromatography using hexane-thy1 acetate (96 :4) as eluent. Synthesis of aldehyde 7. 4-(4-Decyloxybenzoyloxy)-2-hydroxybenzaldehyde was synthesized by reaction of 4-decyl- oxybenzoyl chloride (6; 1 mmol; synthesized by refluxing for 2 h a solution of 4-alkoxybenzoic acid (5) in SOCl2 and two Y XH \---I A Series A : R = HI3C6O-M=CU Series B : R = C~H,~O-(=J-coo-M = cu, vo Series c : R = c,~H,,o -(J-coo-M = cu, vo X Y Z ligand Cu" complexes V"O complexes F H F 1A 1A-Cu H F H 2A 2A-c~ H CF, H 3A 3A-c~ H CN H 4A 4A-c~ HH CN 5A 5A-c~ HH CH2CN 6A 6A-c~ F H F 1B 1B-Cu 1B-VO H F H 2B 2B-c~ 1B-VO H CF, H 3B 3B-c~ 3B-VO H CN H 4B 4B-c~ 4B-VO HH CN 5B 5B-c~ 5B-VO HH CH2CN 6B 6B-c~ 6B-VO F H F 1c 1c-cu 1c-vo H F H 2c 2c-cu 2c-vo H CF, H 3c 3c-cu 3C-VO H CN H 4c 4c-cu 4C-VO H H CN 5c 5c-cu 5C-VO H H CH2CN 6C 6C-CU 6C-VO drops of DMF] with 2,4-dihydroxybenzaldehyde (1; 1 mmol) in CH2C12 and triethylamine as a base.The product was purified by recrystallization in ethanol. The Schiff bases L and the intermediates were characterized by elemental analysis and IR spectroscopy. Elemental analysis showed that the structures of all the materials are consistent with those expected (Table 1). J. Muter. Chem., 1996, 6(3), 349-355 349 series A series A-Cu Scheme 1 Reagents and conditions (I) KHCO,, acetone, reflux, (11) ethanol, AcOH, reflux, (111) ethanol, reflux 7+ Ln (iv) n= 10 (series C)Ln + Cu(CH3CG) 2H20 -Wbh n= 6 (series 6) series B-Cu n = 6 series C-Cu n = 10 series B-VO n = 6 series C-VO n = 10 Scheme 2 Reagents and conditions (I) SOCl,, DMF, reflux, 3 h, (11) CH,Cl,, Et,N, room temp, (111) EtOH, AcOH, reflux, (iv) EtOH, (v) MeOH, Et,N Preparation of the metal complexes.The synthesis of cop- per@) complexes was carried out as described previously4b by the addition of an ethanolic solution (20 ml) containing copper(I1) acetate [Cu(OAc), -HzO](1mmol) to a hot solution of the appropriate imine (2 mmol) in ethanol (50 ml) Oxovanadium(1v) complexes were synthesized by the addition of a methanolic solution (20 ml) containing vanadium(1v) oxide sulfate (VOS04 5H,O) (1 mmol) in the presence of triethylamine to a hot solution of the appropriate imine (2 mmol) in methanol (50 ml) In both cases the solution was refluxed for 1-2 h After cooling the precipitate was collected by filtration and recrystallized from a mixture of ethyl acetate and ethanol (1 3) The crystals are green for the oxovanadium(1v) complexes and brown for the copper(I1) complexes The elemental analyses and yields of the copper(I1) complexes 350 J Mater Chem , 1996, 6(3), 349-355 are gathered in Table 2, those for the oxovanadium(1v) com- plexes are in Table 3 Techniques Microanalysis was performed with a Perkin-Elmer 2400 mic-roanalyser IR spectra were obtained using a Perkin-Elmer 1600 (series FTIR) spectrophotometer over the 400-4000 cm -' spectral range The textures of the mesophases were studied with an optical microscope (Nikon) equipped wth polarlzed light, a Mettler FP82 hotstage and a Mettler central processor Measurements of transition temperatures were made using a Perkin-Elmer DSC-2 differential scanning calorimeter with a heating or cool- ing rate of 10°C min-' The apparatus was calibrated with indium (156 6 "C,28 44 J g-') and tin (232 1 "C, 60 5 J g-') Table 1 Elemental analytical data (calculated values in parentheses) and yields of the ligands molecular ligand formula C(%) H(%) N(%) yield (YO) 1A 2A 3A 4A 5A 6A 1B 2B 3B 4B 5B 6B 1c 2c 3c 4c5c 6C 68.9 (68.7) 72.1 (72.4) 65.8 (65.8) 74.6 (74.5) 74.2 (74.5) 75.3 (75.0) 69.3 (68.9) 71.9 (71.6) 66.9 (66.8) 73.5 (73.3) 73.4 (73.3) 74.0 (73.7) 70.7 (70.7) 73.1 (73.2) 69.1 (68.8) 75.1 (74.7) 74.7 (74.7) 75.3 (75.0) 6.5 (6.3) 4.3 (4.2) 7.2 (7.0) 4.6 (4.4) 6.2 (6.0) 3.7 (3.8) 7.0 (6.8) 8.6 (8.7) 6.9 (6.8) 8.7 (8.7) 6.9 (7.1) 8.1 (8.3) 5.8 (5.5) 2.9 (3.1) 6.3 (6.0) 2.7 (2.9) 5.7 (5.4) 2.7 (2.9) 6.2 (5.8) 6.1 (6.3) 6.2 (5.8) 6.3 (6.3) 6.5 (6.1) 6.0 (6.1) 6.7 (6.5) 2.8 (2.8) 7.1 (6.9) 2.8 (2.8) 6.7 (6.3) 2.5 (2.6) 7.2 (6.8) 5.7 (5.6) 7.0 (6.8) 5.4 (5.6) 7.2 (7.0) 5.4 (5.5) 64 61 80 71 72 95 70 73 70 69 72 77 91 88 91 89 89 94 Table 2 Elemental analytical data (calculated values in parentheses) and yields of the copper(n) complexes molecular yield complex formula C(Yo) H(%) N(Yo) (Yo) 1A-Cu 62.9 (62.7) 5.7 (5.5) 3.9 (3.9) 38 2A-c~ 65.6 (65.9) 6.4 (6.1) 4.0 (4.1) 40 3A-c~ 65.5 (65.9) 5.4 (5.3) 3.5 (3.5) 50 4A-c~ 67.7 (68.0) 6.3 (6.0) 7.9 (7.7) 67 5A-c~ 67.6 (68.0) 6.2 (6.0) 7.8 (7.9) 73 6A-c~ 68.4 (68.7) 6.7 (6.3) 7.4 (7.6) 61 1B-Cu 66.6 (67.0) 5.1 (5.0) 2.7 (2.9) 38 2B-c~ 66.8 (66.8) 5.6 (5.4) 2.9 (3.0) 78 3B-c~ 62.6 (62.8) 5.0 (4.9) 2.9 (2.7) 47 4B-c~ 68.9 (68.5) 5.6 (5.3) 5.8 (5.8) 78 SB-CU 68.7 (68.5) 5.6 (5.3) 6.0 (5.9) 62 6B-c~ 68.9 (69.0) 6.0 (5.6) 5.4 (5.8) 30 1c-cu 67.0 (66.7) 6.2 (5.9) 2.4 (2.6) 64 2c-cu 68.9 (68.9) 6.3 (6.3) 2.6 (2.7) 54 3c-cu 65.4 (65.1) 6.0 (5.8) 2.2 (2.5) 49 4c-cu 70.1 (70.4) 6.2 (6.2) 4.9 (5.3) 40 5c-cu 70.1 (70.3) 6.3 (6.2) 5.1 (5.3) 59 6C-CU 70.6 (70.8) 6.9 (6.5) 4.9 (5.2) 53 Table 3 Elemental analytical data (calculated values in parentheses) and yields of the oxovanadium(rv) complexes molecular yield complex formula C(Yo) H(%) N(%) (Yo) 1B-VO C26H24F2N05V 63.9 (64.3) 5.7 (5.5) 2.7 (2.9) 35 2B-VO C26H25FNOsV 67.0 (66.6) 5.7 (5.3) 2.9 (3.0) 52 3B-VO C2,H2,F3N05V 62.9 (62.6) 5.0 (4.8) 2.7 (2.7) 50 4B-VO C2,H2,N20SV 68.7 (68.3) 5.7 (5.3) 5.5 (5.9) 65 5B-VO C27H25N,05V 68.1 (68.3) 5.4 (5.3) 5.8 (5.9) 30 6B-VO C28H27N205V 69.2 (68.8) 5.7 (5.5) 5.8 (5.7) 351c-vo C3,H32F2N05V 66.9 (66.5) 6.0 (5.9) 2.4 (2.6) 53 2c-vo C30H33FN05V 68.7 (68.6) 6.7 (6.3) 2.3 (2.7) 73 3C-VO C31H33F3NOSV 65.1 (64.9) 6.1 (5.8) 2.2 (2.4) 35 4C-VO C31H33N205V 70.3 (70.1) 6.0 (6.2) 5.1 (5.3) 45 5C-VO C31H33N20SV 69.9 (70.1) 6.0 (6.2) 5.1 (5.3) 25 6C-VO C32H35N205V 70.3 (70.5) 6.8 (6.4) 5.2 (5.1) 60 Results and Discussion Synthesis and characterization Ligands.The ligands of series A, B and C were synthesized by condensation in warm ethanol using the appropriate aniline with the aldehyde obtained by alkylation of 2,4-dihydroxy- benzaldehyde (series A) and by esterification of 2,4-dihydroxy- benzaldehyde with the acid-chloride derivative of 4-decyloxybenzoic acid (series B and C).The C=N stretching vibration in the ligands is located in the 1619-1630 cm-' (series A) and 1613-1625 an-' (series B and C) range. Compounds in series B and C also show a stretching band between 1718 and 1740 cm-', assigned to the ester group v (C=O). The CN stretching band is located in the 2246-2251 cm-I range for ligands with the group CH,CN, 2228-2232 cm-' for ligands with the CN group in position 3 and 2220-2223 cm-I for ligands with the CN group in position 4.As can be observed, there is a shift to lower frequencies: 4-CH2CN>3-CN >4-CN due to increasing conjugation. Complexes. The copper(r1) complexes were prepared by reacting the appropriate imine with copper(I1) acetate mono- hydrate in warm ethanol. The oxovanadium complexes were prepared by reacting the appropriate imine with vanadyl sulfate pentahydrate and triethylamine in warm methanol. The complexes are soluble in toluene, chloroform, dichloro- methane and insoluble in hexane, diethyl ether and ethanol. Elemental analyses of the complexes were consistent with their proposed structures (Tables 2 and 3). The C=N stretching vibration is shifted to lower frequencies for the complexes (copper, 1611-1620 cm-'; oxovanadium, 1607-1620 cm-') compared to that of the free ligands (1613-1632 cm-I).This indicates that the azomethine N atom is involved in metal- nitrogen bond formation. The oxovanadium complexes also exhibit a stretching band at around 983-987 cm-' assigned to v (V-0); this suggests that these complexes have a monomeric structure." Mesogenic behaviour The optical, thermal and thermodynamic data of the ligands and the complexes are summarized in Tables 4 and 5, respect-ively, and Fig. 1-7 represent the mesophase ranges for the ligands and complexes. Mesogenic properties of the ligands. As can be observed in Table 4 and Fig. 1-3, among the ligands of series A only the compound with the CN group in position 4 (X=Y=H, Z= CN) exhibits mesomorphism.The absence of mesomorphism in most of the ligands in series A can be accounted for by the low molecular length :width ratio. However, the presence of the CN group in position 4 makes nematic mesomorphism possible, probably due to the increase in electronic polariz- ability along the molecular axis. On the other hand, all the compounds of series B and C exhibit liquid-crystal properties The type of mesophase is influenced by the length of the alkoxybenzoyloxy group and by the polar group. Thus, the 150 100 9R RN oc 50 0 1A 2A 3A 4A 5A cornDound Fig. 1 Transition temperatures for the ligands of series A J. Muter. Chem., 1996,6(3), 349-355 351 Table 4 Optical, thermal and thermodynamic data for the ligands ligand X Y Z transition T/"C AHlkJ mol 1A F H F c-I 74 0 33 5 2A H F H c-I 55 2 35 3 3A H CF3 H c-I 48 8 28 8 4A H CN H c-I 69 2 29 0 5A H H CN C-N 83 5 40 5 N-I 125 3 14 6A H H CH2CN c-I 100 4 25 8 1B F H F C-N 96 7 23 24 N-I 214 5 191 2B H F H C-N 90 5 26 7 N-I 155 8 0 71 3B H CF3 H c-I 99 6 I-N" 87 6 N-Sc" 70 6 4B H CN H c-I 127 0 33 9 I-N" 120 9 0 38 5B H H CN C-SA 104 1 34 24 SA-N 127 Xb N-I 248 9' 6B H H CH,CN C-N 140 6 37 2 N-I 234 6 06 1c F H F C-N 86 5 33 2 N-I 184 2 16 2c H F H c1-c2 59 4 54 C2-N 80 7 23 9 N-I 127 7 09 N-Sc" 81 9 04 3c H CF3 H c-I 97 9 37 8 I-SA" 80 3 21 4c H CN H c-I 123 3 25 4 I-N" 115 4 09 5c H H CN C-SA 106 8 42 7 SA-I 240 4b 6C H H CHZCN C-SA 136 9 40 7 SA-N 191 3 02 N-I 213 7 11 SA-SC" 136b " Monotropic transition Optical data 200 200 EdN EZIN0P SA SA fJ sc fJ sc 100 oc 100 oc n 0 18 20 38 38' 48 48' 58 68 1C 2C 2C' 3C 3C' 4C 4C' 5C 6C 6C' corn pou nd cornpound Fig.2 Transition temperatures for the ligands of series B Fig. 3 Transition temperatures for the ligands of series C fluoro derivatives [2,4-difluoro (X=Z =F, Y =H) or 3-flU01-0 those of series A due to the presence of an aromatic ring joined (X=Z=H, Y=F)] exhibit a nematic phase in both series B by an ester bond to the structure derived from 2-hydroxy- and C, whereas the 4-cyano denvatives exhibit a nematic benzylideneaniline This involves important structural changes, mesophase in ligands of series B and mainly a smectic A phase in particular an increase in the molecular length with little or in ligands of series C Both derivatives 3-CF3 (X=Z=H, Y= no change in the width, and an extension of the conjugation CF3) and 3-CN (X =Z =H, Y =CN) in series B exhibit mono- through the ester bond (which means an increase of the tropic mesomorphism The 3-CF3 denvatives exhibit nematic electronic polarizability of the molecule) These two factors and smectic C mesophases, whereas the 3-CN derivatives only allow an increase in the anisotropy of the polarizability for show nematic mesomorphism molecules with three aromatic rings, which favours molecular The absence of enantiotropic mesomorphism in these com- interactions and liquid-crystal properties for the ligands in pounds can be explained by the steric hindrance of a big group series B and C such as CF, or CN in position 3, which is unfavourable for By comparing the melting temperatures of the ligands of mesophase stability Ligands of series B and C are different to series A, B and C, the following sequencies can be established 352 J Muter Chem , 1996,6(3),349-355 series A: 4-CN >4-CH2CN>2,4-diF >3-CN >3-F >3-CF3 series B: 4-CN >4-CH2CN>2,4-diF >3-F >3-CN >3-CF, series C: 4-CN >4-CH2CN>2,4-diF >3-F >3-CN >3-CF3 As can be seen, in each of the three series the nature of the group and its position have a similar influence on the melting temperature.However, the effect of the group on the appear- ance of mesomorphism and on the type of mesophases formed is very different for the three series.Mesogenic properties of the copper(1r) complexes. The com- plexes of series A-Cu are not liquid crystalline, with the exception of the complex with two fluoro atoms in positions 2 and 4 (X=Z=F, Y=H) which exhibits a smectic A meso- phase (Table 5, Fig. 4). The complexes of series B-Cu and C- Cu exhibit smectic A and nematic phases (Table 5, Fig. 5 and 6). The nematic phase of the complexes shows textures that are typical of this type of mesophase, the marbled texture on heating and the schlieren texture on cooling. The smectic A phase of the complexes was identified by the appearance on heating of both the mielinic and homeotropic texture. Homeo tropic and focal-conic textures were observed on cooling from the isotropic liquid.The type of mesophase formed is affected by the polar group and by the length of the 4-alkoxybenzoyloxy group in series B-Cu and C-Cu. It is observed that the CN group gives rise to a smectic A mesophase in both series B-Cu and C-Cu, with the exception of the 4-CN derivative with n =6 (series B-Cu) which exhibits a nematic mesophase. The complexes with fluorine atoms in the structure (2,4-diF, 3-F and 3-CF3) exhibit nematic meso- morphism in both series. This phase is monotropic for n=6 with a CF, group due to the volume of this group in position 3 which destabilizes the mesophase. The highest melting temperatures are exhibited by the 4-CN 300-SA oc16A-CU 1A-c~ PA-CU 4A-c~ 5A-CU compound Fig.4 Transition temperatures for the complexes of series A-Cu 300-I BN SA oc " 16-Cu 26-CU 3B-CU 36-CU' 4B-c~ 58-CU 6ECu compound Fig. 5 Transition temperatures for the complexes of series B-Cu 3001 200 ON9F H SA oc 100 IC-Cu 2C-CU 3C-CU 4C-CU 5C-CU 6C-CU compound Fig. 6 Transition temperatures for the complexes of series C-Cu derivatives. The mesomorphic stability of these complexes is due to the fact that the substituent is parallel to the main axis of the molecule and gives rise to less steric hindrance and higher anisotropy of polarizability, thus favouring intermolecu- lar interactions. A comparative study of the melting temperatures allows us to establish the following sequences: series A-Cu: 4-CN >4-CH2CN>2,4-diF >3-CF, >3-CN >3-F series B-Cu: 4-CN >3-F >4-CH2CN>2,4-diF >3-CN >3-CF3 Series C-Cu: 4-CH2CN>3-CN >2,4-diF >3-F >3-CN >3-CF3 In contrast to the uncomplexed ligands, in these complexes the groups and their position have a different influence on the mesomorphism in series A-Cu, B-Cu and C-Cu.The steric effect of the lateral groups decreases the stability of the mesophase, and the sequences of clearing temperatures for the complexes of series B-Cu and C-Cu are: series B-Cu: 4-CN >2,4-diF >3-F >3-CN >4-CH2CN>3-CF, series C-Cu: 4-CN >2,4-diF >4-CH2CN>3-CN >3-F >3-CF3 The negative influence of the lateral substituents is similar to that observed for the ligands, but less marked.Such a lateral- group effect has also been observed by another research group." Mesogenic properties of the oxovanadium (rv) complexes. As can be observed in Table 5 and Fig. 7 and 8, mesogenic behaviour is not favoured for these complexes, in contrast to their ligands and to the homologous copper(I1) complexes. Only three complexes in series B-VO (2,4-diF, 3-F and 4- CH,CN) and four complexes in series C-VO (2,4-diF, 3-F, 4-CN, 4-CH2CN) exhibit liquid-crystal properties. The fluoro derivatives (2,4-diF, 3-F) exhibit a nematic mesophase, but the 3-F derivative exhibits a monotropic mesophase in both series. The absence of enantiotropic meso- morphism in these compounds is probably due to the greater steric hindrance of the VO group compared with the Cu group.In general, the melting and clearing temperatures are higher for the complexes of series B-VO than for the complexes of series C-VO, as was also seen for the copper(I1) complexes. The sequence for the melting temperatures is: series B-VO: 3-CN >3-F >4-CH2CN>4-CN >3-CF3>2,4-diF series C-VO: 3-F >4-CN >3-CN >3-CF3>2,4-diF >4-CH2CN The effect of the volume of the VO group has a greater J. Mater. Chem., 1996, 6(3), 349-355 353 Table 5 Optical, thermal and thermodynamic data for copper@) and oxovanadium(1v) complexes complex X Y Z transition T/"C AH/kJ mol-' 1A-CU F H F C-SAs*-I 136.9 150.3 45.2 4.2 2A-c~ H F H c1-c2 100.4 5.8 c2-I 167.6 50.1 3A-c~ 4A-c~ H H CF3 CN H H c-I c-I 124.9 122.6 37.4 57.3 5A-c~ H H CN c-I 201.4 41.0 6A-c~ H H CH2CN c-I 180.1 57.8 1B-Cu F H F c1-c2 193.8 38.1 C2-N 204.5 22.0 N-I 279.5 1.3 2B-c~ H F H C-N 218.1 39.0 N-I 230.5 2.0 3B-c~ H CF3 H c-I I-N" 174.8 146.0' 58.6 4B-c~ H CN H C,-C2C2-N 203.9 21 1.4 2.1 43.8 N-I 230.4 1.6 5B-c~ H H CN C-SA 252.2 61.9 SA-I 286.5' 6B-c~ H H CH2CN CrC2 170.2 29.1 c2-sA 205.2 41.2 SA-N 227.0 N-I 230' 1.1 1c-cu F H F c1-c2 161.7 59.3 C2-N 170.8 54.2 N-I 265.3 1.48 2c-cu H F H C1-C2 C2-( N +C) (N+C)-NN-I 80.7 166.3 171.9 195.5 12.0 57.2 6.2 1.6 3c-cu H CF3 H c1-c2 58.5 28.0 C2-N 146.7 46.7 N-I 150.1 1.5 4c-cu H CN H C-SAs*-I 165.1 205.7 43.5 5.0 5c-cu H H CN c1-c2 176.1b c2-c3 186.0 65.20 C3-SA 193.5 17.1 SA-I 276'9' 6C-CU H H CHzCN C-SA 199.8 66.6 SA-I 231.4' 1B-VO F H F C-N 21 1 30.4 N-I 249.4 0.9 2B-VO H F H c,-c2c2-I 222.7 246.7 1.55 50.4 I-N" 190' 3B-VO 4B-VO H H CF3 CN H H c-I c-I 182.9 249.3 62.2 76.1 5B-VO H H CN c-I 195.8 90.2 6B-VO H H CH2CN c1-c2 131.3 6.02 1c-vo F H F c24, c42 SA-I 245 286.2b*' 58.8 43.2 14.9 C2-N 144.6 41.4 N-I 198.1 0.5 2c-vo H F H c-I 217.3 60.7 I-N 174.5 0.6 3C-VO 4C-VO 5C-VO H H H CF3 CN H H H CN c-I C-Sc,, Sclys-1c-I 181.7 188.5 21@ 203.9',' 76.4 10.1 34.1 6C-VO H H CH2CN C-Sc*ys SclY*-I 141.0 231.9 22.6 31.7 ~~~~ ~ Monotropic transition.Microscopy data. 'Complex partially decomposed. influence than the volume effect of the polar groups on the C,-C,-mesophase or isotropic liquid transitions while, com- appearance of liquid-crystal properties. Complexes 3-CN and plex SC-Cu (4-CN) exhibits a C1-C,-C,-mesophase transition 4-CH2CN of series C-VO exhibit a smectic crystal phase. sequence. Crystalline polymorphism. It is observed that most of the Conc,usionscomplexes exhibit crystalline polymorphism. This is typical for mesogenic compounds. Complexes 2A-Cu, 1B-Cu, 4B-Cu, The 4-alkoxybenzoyloxy group favours the appearance of 6B-Cu, 2B-VO, 6B-VO, 1C-Cu, 3C-Cu7 1C-VO exhibit mesogenic properties with respect to the 4-alkoxy group.The 354 J. Muter. Chem., 1996,6( 3), 349-355 200 0 BNk SA oc 100 0 1B-VO 28-VO 28-VO' 36-VO 4B-VO 58-VO 68-VO compound Fig. 7 Transition temperatures for the complexes of series B-VO 200 g 100 n.. IC-VO 2C-VO 2C-VO' 3C-VO 4C-VO 5C-VO 6C-VO cornpound Fig. 8 Transition temperatures for the complexes of series C-VO complexes with the cyano group yield mainly a smectic A mesophase whereas the corresponding ligands, in addition to a smectic A phase, also exhibit a nematic phase. The presence of a fluorine atom favours the nematic mesophase in both ligands and complexes.The mesophase ranges are wider in the ligands than in the complexes, which might be explained by the greater volume and the lower length: width ratio of the complexes, which decrease the stability of the mesophase. The copper(1r) com- plexes exhibit better mesogenic behaviour than the analogous oxovanadium(1v) complexes, which may be explained on the basis of geometric factors: copper(I1) complexes have a square- planar coordination, whereas the oxovanadium(1v) complexes have a square-pyramidal coordination. It is observed that for the copper@) complexes the polar group has a large influence on the transition temperatures owing to the square-planar geometry of these complexes around the metal centre, whereas in oxovanadium(1v) complexes it is the VO group that has the greater influence, with the polar groups playing a secondary role.This work was supported by the CICYT (Spain), project nos. MAT93-0104 and MAT94-07 17-CO2-01. References 1 A. M. Giroud-Godquin and P. M. Maitlis, Angew. Chem., Znt. Ed. Engl., 1991, 30, 375; P. Espinet, M. A. Esteruelas, L. A. Oro, J. L. Serrano and E. Sola, Coord. Chem. Rev., 1992, 117, 215; D. W. Bruce, in Inorganic Materials, ed. D. Bruce and D. O'Hare, Wiley, Chichester, 1992; S. A. Hudson and P. M. Maitlis, Chem. Rev., 1993,93, 861; D. Bruce, J. Chem. SOC., Dalton Trans., 1993,2983. 2 R. H. Holm and M. J. O'Connor, Prog. Znorg. Chem., 1971,14,477. 3 I. V. Ovchinnikov, Yu. G. Galyametdinov, G. I. Ivanova and L. M.Yagfarova, Dokl. Akad. Nauk SSSR, 1984,276,126. 4 A. P. Polishchuk, M. Yu. Antipin, R. G. Gerr, T. V. Timofeeva, Yu. T. Struchkov, Yu.G. Galyametdinov and I. V. Ochinnikov, Kristallograjiya, 1989,34, 122. 5 A. N. Polishchuk, M. Yu. Antipin, T. V. Timofeeva, Yu. T. Struchkov, Yu.G. Galyametdinov and I. G. Bikchantaev, Kristallograjiya, 1991,36, 389. 6 M. Marcos, P. Romero and J. L. Serrano, J. Chem. Soc., Chem. Commun., 1989, 1641; Chem. Muter., 1990, 2, 495; J. L. Serrano, P. Romero, M. Marcos and P. J. Alonso, J. Chem. SOC., Chem. Commun., 1990,859. 7 M. Marcos, P. Romero, J. L. Serrano, C. Bueno, J. A. Cabeza and L. A. Oro, Mol. Cryst. Liq. Cryst., 1989, 167, 123; M. Marcos, P. Romero, J. L. Serrano, J. Barbera and A. M. Levelut, Liq. Cryst., 1990, 7, 251; E. Campillos, M. Marcos, L. T. Oriol and J. L. Serrano, Mol. Cryst. Liq. Cryst., 1992,215, 127. 8 P. Keller and L. Liebert, Solid State Phys. Suppl., 1978,14, 19. 9 M. Artigas, M. Marcos, E. Melendez and J. L. Serrano, Mol. Cryst. Liq. Cryst., 1985, 130, 337. 10 R. L. Farmer and F. L. Urbach, Inorg. Chem., 1974, 13, 587; M. Pasquali, F. Marchetti and S. Merlino, J. Chem. Soc., Dalton Trans., 1977, 139; A. Serrete, J. Carroll and T. M. Swager, J. Am. Chem. SOC., 1992,114,1887. 11 E. Bui, J. P. Bayle, F. Perez, L. Liebert and J. Courtieu, Liq. Cryst., 1990,8, 513. Paper 5104649A; Received 14th July, 1995 J. Muter. Chem., 1996, 6(3), 349-355 355

ISSN:0959-9428    

DOI:10.1039/JM9960600349

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Thermotropic phase transitions in 5,15=bis (4-alkoxyphenyl )octaalkylporphyrins Ernst J. R. Sudholter,*" Marinus van Dijk," Cees J. Teunis," Georgine M. Sanders,"Sybolt Harkema,b Gerrit M. H. van de Velde,' Pieter G. Schoutend and John M. Warmand a Wageningen Agricultural University, Laboratory of Organic Chemistry, Dreijenplein 8, 6703 HB Wageningen, The Netherlands bUniversity of Twente, Laboratory of Chemical Physics, PO Box 21 7, 7500 AE Enschede, The Netherlands 'University of Twente, Laboratory of Inorganic Materials Science, PO Box 217, 7500 AE Enschede, The Netherlands dDelft University of Technology, IRI, Radiation Chemistry Department, Mekelweg 15, 2629 JB Delft, The Netherlands Nineteen novel alkyl substituted porphyrins have been synthesized and their thermal phase behaviour has been investigated in detail.Twelve compounds showed a reversible phase transition below the isotropization temperature. From time resolved microwave conductivity (TRMC) measurements and powder X-ray diffractometry it was concluded that the molecular packing does not change significantly at the lower phase transition temperature and that the porphyrin cores occupy isolated positions. Single X-ray diffraction measurements showed that the porphyrins are arranged in a layered structure and that the space between the layers is occupied by the alkyl substituents of the pyrrole units. The phase transitions at the lower temperature were therefore identified as changes in the crystal ordering of the porphyrins. Self-organized systems of porphyrin molecules are of prominent interest in the study of energy-transfer and electron-transfer processes, for improving our understanding of the photosystem in plants and for the development of new advanced molecular materials with special photophysical properties.Our current research is directed towards the fundamental and more applied problems of photovoltaic organic solar cells based on porphyrin dye molecules, which are deposited on wide bandgap semiconductor materials, like Ti02 and SnO,. In our approach, the self-assembly process of the porphyrin molecules is directed in different ways, i.e. (1) by using amphi- philic porphyrins, orientation can be obtained at an air-water interface and the organized porphyrins can be transferred to the substrate by the Langmuir-Blodgett technique;' (2) by introduction of alkyl substituents at the periphery of the porphyrin core, it might well be possible to introduce calamitic or discotic liquid crystalline properties in a given temperature range;2 (3) by introduction of additional ligating groups to the porphyrins, able to complex transition-metal ions, coordi- nation type dimers, trimers etc.may be ~btained.~ In this contribution we describe our study on the synthesis and phase characterization of two series of alkyl substituted porphyrins. The aim of the research is to correlate phase behaviour and molecular structure. In contrast to phthalocyan- ines, little work has been performed on the mesomorphic properties of porphyrins.' The first report on mesomorphic properties of porphyrins came from Goodby et aL4 In a very narrow temperature traject of only 0.1 "Ca monotropic discotic phase was identified for bis( hydrochlorides) of uroporphyrins.In addition copper containing 5,15-bis( 4-alkoxypheny1)octa- methylporphyrins have been investigated5 and these com-pounds do not possess mesomorphic properties. This is in contrast to a report from Bruce et ~l.,~~who identified a crystal smectic B phase for zinc containing 5,15-bis(4-alkoxyphenyl)-porphyrins. For some tetraalkoxyphenylporphyrins meso-morphic phases were reported on account of calorimetric data; these phases were not identified.6b Gregg et al. reported an extensive study on octaester-substituted porphyrins and their zinc derivative^^^ and on metal containing octakis(alkoxyethy1) p~rphyrins.~"These compounds do form discotic liquid crystalline phase^.^ Recently, Shimizu et aL8 reported that on model to predict mesophase formation. In this model separate disordering temperatures of alkyl side chains and rigid cores determine the existence of mesophases in alkyl-substituted disk-like molecules.Results Synthesis Two series of alkyl-substituted porphyrins have been synthe- sized and characterized by their phase behaviour as a function of temperature. In one series (compounds 1-9) the number of carbon atoms n of the alkoxy chain has been varied between i, PTS, MeOH-CH&12 ii, DW, THF R R Scheme 1 Compound number (n,m): 1 (1,4) 5 (9,4) 9 (18, 4) 13 (8, 5) 17 2 (6,4) 6 (10,4) 10 (8, 1) 14 (8, 6) 18replacing the alkyl substituent in 5,10,15,20-tetrakis(4-alkyl-pheny1)porphyrins by an alkoxy chain, the discotic lamellar 3 (7, 4) 7 (12,4) 11 (8, 2) 15 (8,8) 19 phases (DL)disappeared.Collard and Lillyag propose a simple 4 (8,4) 8 (16,4) 12 (8, 3) 16 (8, 12) J. Muter. Chem., 1996,6(3), 357-363 4-Zn 5-Zn 6-Zn 1 and 18 with a fixed substitution pattern on the pyrrole nngs consisting of a methyl and a butyl group (rn=4) In the other series (compounds 10-16) the length of the alkoxy chain (n=8) has been kept constant and rn, the number of carbon atoms on the pyrrole alkyl group, has been varied between 1 and 12 In addition three Zn-metallated porphynns (compounds 17-19) have been prepared and Characterized The new compounds have been prepared from the correspond- ing 2,2’-methylene dipyrroles and 4-alkoxybenzaldehydes, according to the method described before lo The compounds have been purified by chromatography and characterized by mass spectra, ‘H NMR spectra and elemental analysis Differential scanning calorimetry and hot-stage polarization microscopy The phase transition temperatures and enthalpy changes have been determined by differential scanning calorimetry (DSC) and the data obtained are displayed together with the calcu- lated entropy changes (ASIR)in Table 1 Increasing the length of the alkoxy substituent from n=l to 18 with constant m=4 shows a decrease of the phase transition temperature to the isotropic phase (Fig 1, Table 1) For several compounds (1, 2, 4, 5, 6, 8) additional phase transitions at lower temper- ature have been observed All these transitions are rever-sible Inspection of these samples by hot-stage polarization microscopy did not show significant textural changes on passing through this lower phase transition temperature and the appearance of the material remained solid Increasing the length of the pyrrole alkyl substituent from rn =1 to 12 with constant n =8 also shows a decrease of the phase transition temperature to the isotropic phase (Fig 2, Table 1) In this senes the additional phase transitions at lower temperature (compounds 4, 10-13) are again reversible, no textural changes could be detected by hot-stage polarization microscopy, and the material remained solid (Plate 1) Metallation of some porphynns with Zn2 (compounds+ 17-19) shows a small increase of the phase transition tempera- ture to the isotropic phase For 17 and 18 no lower phase transition temperature was observed, while for 19 the lower phase transition temperature was found at 60°C The phases present between the lower phase transition and Table 1 Phase transition temperatures, enthalpies and entropies of 5,15-bis(4-alkoxyphenyl)-2,8,12,18-tetraalkyl-3,7,13,17-tetramethylporphyrins crystal-isotropic crystal-cry stal compound n m C-tI T/”C AH/kJ mol-’ AS/R c-+c T/”C AH1 kJ mol AS/R metal-free 1 1 4 308 42 87 233 6 14 2 6 4 234 53 12 4 51 3 13 3 7 4 234 52 12 2 - - - 4 8 4 214 52 12 9 77 12 42 5 9 4 192 55 14 2 19 15 60 6 10 4 170 38 102 147 22 62 7 12 4 161 51 140 - - - 8 16 4 120 27 81 70 9 31 113 41 12 7 9 18 4 115 82 25 2 - - - 10 8 1 286 45 96 202 20 50 11 8 2 285 64 13 8 8 6 24 12 8 3 246 63 14 6 83 19 64 4 8 4 214 52 12 9 77 12 42 13 8 5 191 32 82 180 19 50 14 8 6 187 70 18 4 - - - 15 8 8 170 63 170 - - - 16 8 12 143 83 23 9 - - - Zn-metallated 17 8 4 220 46 11 2 - - - 18 9 4 213 41 10 2 - - - 19 10 4 197 39 100 60 5 18 400, 1 200 2 200 yI-* * 100 0 I 0 2 4 6 8 10 12 14 m Fig.2 Phase transition temperatures of 5,15-bis(4-octyloxyphenyl)-octaalkylporphynns (n=8) as a function of alkyl chain length (m) 0,to the isotropic liquid phase, 4,lower phase transition temperature 358 J Muter Chem, 1996, 6(3), 357-363 Plate 1 Textures of compound 4 between crossed polarizers at (a)50 "C and (b) 150"C the phase transition to the isotropic state have been investi- gated in more detail using powder X-ray diffraction measure- ments and time resolved microwave conductivity measurements (TRMC). Powder X-ray diffraction measurements Compounds 4, 6, 8, 10, 12 and 13 have been investigated by X-ray diffractometry at different temperatures. Typical diffractograms are given for 4 at 50 and 95 "C(well below and above the lower phase transition temperature, which is observed at 77 "C; Fig.3). No drastic changes or broadening of the diffraction lines could be observed on passing the lower phase transition temperature, either for this compound or for the other compounds investigated. Special attention was paid to the tegion around 28=20", corresponding to distances of d z4.5 A (corresponding to porphyrin-porphyrin stacking dis- tances)," but no significant changes could be monitored. These observations strongly indicate that, on passing through the lower phase transition temperature, little change in the molecu- lar packing occurs and the ordering in the intermediate phase is very similar to the ordering in the original solid state.Time resolved microwave conductivity measurements (TRMC ) In the TRMC experiments, a small uniform concentration (<lOpmol dm-3) of electron-hole pairs is produced by a nanosecond duration pulse of high-energy (3 MeV) electron radiation. A fraction of the initial electron-hole pairs escape rapid geminate recombination and if they are mobile they result in a reduction in the microwave power reflected by the Fig. 3 Powder X-ray diffractogram of 4 at (a) 50 "C and (b)95 "C sample. This reduction in power is directly related to the radiation induced conductivity, AD." For compounds 1 and 4 the time resolved conductivities per unit absorbed dose (Ao/D) are shown in Fig.4 with the result obtained from octakis(2- nony1oxyethyl)porphyrin.l' It is clear that for the former compounds the observed signal is about 30 times smaller than for the latter and the lifetimes of the transients are very much shorter.The end-of-pulse conductivity per unit absorbed energy (Aa/D), is 9.8 x and 4.2 x low9S m2 J-l for compounds 1 and 4, respectively. The (Aa/D),, values are related to the sum of mobilities of the hole and electron charge carriers [Cp=p(-)+p(+)] and to the average energy required to produce one electron-hole pair E, by (AC/D)~=Cp/E,,." Using a value of E,=25 eV yields a lower limit to the sum of xmobilities: for 1 Xp~2.45 m2 V-' s-' and for 4 Zp> 1.05 x lo-' m2 V-' s-'. This is to be compared with Zp>9 x m2 V-' s-' for the octaalkoxyethyl compound. Upon increasing the temperature of compound 4 and passing its lower phase transition temperature at 77"C, the Aa/D transient does not change significantly.This suggests that only minor changes in the packing of the porphyrin molecules occur. This is consistent with our observations on the tempera- J. Mater. Chem., 1996, 6(3),357-363 359 5 I I I 11' 4t 11 1 n 0 50 loo 150 200 250 timdns Fig. 4 Dose-normalised radiation induced conductivity transients observed on pulsed ionization of solid samples A pulse width of 10 ns was used for the octaalkoxyethyl compound (0)and of 50 ns for the other two samples, compounds 1(0)and 4 (i-t),whose signals have been multiplied by a factor of 10 ture dependent powder X-ray diffraction on this sample The small values observed for (AcT/D)~and Xp for compounds 1 and 4 compared with the much higher values observed for octakis(2-nonyloxyethy1)porphyrinare probably due to the large centre-to-centre distance !etween the porphyrin mol- ecules For 4 a distance of 10 3 A within a layer was observed (vide znfra) In octakis( 2-nonyloxyethy1)porphyrin the macrocycles are arranged ino tilted stacks with a cofacial distance of approxi- mately 34A and a Sentre-to-centre distance in the direction of the stacks of 4 3 A Rapid charge migration has also been observed in stacks of 2,5-didecyloxy- 1,4-be?zoquinone, having a quinone centre-to-centre distance of 4 2 A l2 Single crystal X-ray diffraction For compounds 4 and 2 the single crystal X-ray structures have been resolved (Fig 5 and 6, respectively) Both structures contain half a molecule in the unit cell, the other half being generated by a centre of symmetry The alkoxy chain of compound 4 is in the extended zig-zag conformation The alkoxy chain of compound 2 also shows a (less perfect) extended conformation, with some disorder as is evident from the high values of the thermal parameters in this part of the molecule Adjacent butyl groups of compound 4 show an alternating orientation with respect to the nngs, in contrast with compound 2 where a non-alternating orientation is found The porphyrins are packed in a layered structure [see Fig 5(c) and 6(c)] and the space between the layers is filled with the alkyl chains connected to the pyrrole units Within a given layer the porphynns are Fot in close contact The centFe-to- centre distance is 103A for compound 4 and 9 8A for compound 2 The space above and below the porphyrin core within a layer is occupied by the alkoxyphenyl units, and the phenyl groups interact perpendicularly with the neighbouring porphyrin core Such an interaction is quite common in porphynn lattices l3 Discussion In the series of alkyl-substituted porphyrins investigated, we have shown that increasing the alkyl chain length decreases the phase transition temperature to the isotropic phase Many of the compounds studied also show a reversible lower phase transition temperature The TRMC measurements indicate that charge migration is not affected by passing this phase transition temperature and that it is much lower than the charge migration observed in stacks of octakis( 2-nonyloxy- 360 J Muter Chem, 1996, 6(3), 357-363 Fig. 5 Single crystal X-ray structure of 4 (a) individual molecule, (b) side view within a layer, (c) top view Fig.6 Single crystal X-ray structure of 2 (a) individual molecule, (b)side view within a layer, (c) top view ethy1)porphynn This indicates that in our system the porphy- rin cores are not in cofacial contact and occupy a more isolated position both above and below the lower phase transition temperature Such a structure is supported by the results obtained from single crystal X-ray diffraction measurements, Table 2 Yields, elemental analyses and parent peaks in the FD mass spectra for compounds 1-19 c (Yo) H (Yo) N (Yo) compound yield (TOT calc.found calc. found calc. found mlz 1 67 80.75 80.56 8.28 8.42 6.97 6.94 802 2 49 8 1.47 8 1.47 9.18 9.31 5.93 5.88 942 3 50 81.59 81.42 9.33 9.46 5.76 5.94 97 1 4 54 81.71 81.66 9.48 9.67 5.60 5.53 998 5 35 81.81 81.98 9.61 9.80 5.45 5.52 1027 6 62 81.91 81.86 9.74 9.78 5.30 5.17 1054 7 34 82.10 82.13 9.97 10.10 5.04 4.89 1110 8 71 82.42 82.32 10.37 10.44 4.57 4.42 1224 9 53 82.57 82.30 10.55 10.39 4.38 4.16 1280 10 23 79.26' 79.22 8.34' 8.38 6.57b 6.54 83 1 11 37 81.22 81.33 8.86 9.09 6.32 6.33 887 12 38 81.48 81.53 9.19 9.42 5.94 5.94 942 13 49 81.92 81.64 9.74 9.91 5.31 5.26 1054 14 73 82.11 81.99 9.97 10.12 5.04 4.91 1110 15 77 82.43 82.66 10.38 10.68 4.58 4.56 1224 16 13 82.93 82.92 11.00 11.18 3.87 3.84 1447 17 75.61' 75.56 8.60' 8.79 5.17' 4.96 1061 18 74.26d 74.20 8.58d 8.80 4.91d 4.64 1089 19 74.97" 74.80 8.77" 9.07 4.82" 4.62 1117 a The yields have not been optimized. With 0.25 mol of dichloromethane.'With 0.25 mol of dichloromethane. With 0.6 mol of dichloromethane. " With 0.5 mol of dichloromethane. supplemented by the results from powder X-ray diffraction. The single crystal X-ray experiments showed that the porphy- rins are located in layers and that the space between the layers is filled by the more disordered butyl chains and methyl groups connected to the pyrrole units.It might be expected that by passing the lower phase transition temperature this disorder increases. However, since the powder X-ray diffractometry results showed only very small changes and no broadening of the diffraction lines we come to the final interpretation that in the alkyl substituted porphyrins investigated reversible crystal- crystal and crystal-isotropic liquid transitions occur. Conclusions (1)Increasing the number of carbon atoms in the alkoxy or alkyl-substituted porphyrins, decreases the phase transition temperature to the isotropic liquid phase. (2) Subsequent incorporation of zinc in the porphyrin raises this phase trans- ition temperature.(3) Passing the lower phase transition temperature does not significantly change the molecular pack- ing of the porphyrins, as deduced from TRMC measurements on 4 and powder X-ray diffractometry on 4, 6, 8, 10, 12 and 13. (4) The porphyrin cores occupy isolated positions, as deduced from the absence of rapid charge migration in 1 and 4. (5) The porphyrin molecules in the solid state (2 and 4) are arranged in a layered structure. The space between these layers is occupied by the methyl and butyl groups connected to th? pyrrole unit. The porphyrin centre-to-centre distance is 10.3A for 4 and 9.8 A for 2. (6)Reversible crystakrystal and crystal- isotropic liquid phase transitions have been identified. Experimental The 'H NMR spectra were recorded on a 200MHz Bruker AC 200E spectrometer. All spectra were measured as CDCl, solutions.Field desorption mass spectra were measured on a MS 902equipped with a VG ZAB console. General procedure for the synthesis of the 4-alk ylox ybenzaldehydes A mixture of 4-hydroxybenzaldehyde (0.2 mol), l-bromoalkane (0.25 mol), anhydrous potassium carbonate (0.3mol; dried at 140 "C) and butan-2-one (150cm3) was refluxed with stirring for 20 h. To the cooled reaction mixture diethyl ether (200 cm3) was added and the inorganic salts were filtered off and washed with diethyl ether. The washings were combined with the filtrate and the solvent was evaporated in uucuo. The residue was purified: the aldehydes with alkyl chains up to n= 12 by distillation at 0.1-0.01 mmHg and the aldehydes with n= 16 and n= 18 by chromatography over silica gel, eluent light petroleum (bp 60-80"C)with gradual addition of ethyl ace- tate up to 5%.Average yield 70%. All aldehydes have been described in the 1iterat~re.I~ Synthesis of the 3,3'-dialkyl-4,4-dimethyl-2,2'-meth ylenedipyrroles The synthesis of the 2,2'-methylenedipyrroles (previously denoted as bispyrrolylmethanes) with methyl, ethyl and butyl as the variable alkyl substituent has been described in a previous publication."" The other 2,2'-methylenedipyrroles were prepared in a completely analogous way. The dipropyl- and the dihexyl-methylenedipyrroles have been described in the literat~re.~~?~~ The dipentyl-, dioctyl- and the didodecyl- methylenedipyrroles are rather unstable; they were charac-terized by their 'H NMR and mass spectra and used without further purification for the synthesis of the porphyrins.General procedure for the synthesis of the 2,8,12,18-tetraalkyl- 5,15-bis(4-alkyloxypheny1)-3,7,13,17-tetramethylporphyrins To a solution of 3,3'-dialkyl-4,4-dimethyl-2,2'-methylenedipyr-role (10mmol) and 4-alkyloxybenzaldehyde (10mmol) in methanol (200 cm3) and dichloromethane (50 cm3), toluene-p- sulfonic acid (0.5 g) was added. The mixture was stirred for 4h at room temperature and left overnight in the refrigerator. The solvent was removed below 40°C and the residue was dissolved in tetrahydrofuran (250 cm'). A solution of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone(4.2 g) (DDQ) was added over a period of 5 min and the mixture was stirred for 1 h at room temperature.After evaporation of the solvent in U~CUOthe residue was dissolved in dichloromethane (50 cm3), the insoluble material was filtered off and the filtrate was diluted with methanol-triethylamine, 3 : 1 (250cm3). Upon cooling to 0°C a precipitate was formed, which was filtered off and washed with methanol-dichloromethane, 9 : 1. The crude product thus obtained was purified by chromatography over silica gel (Merck Kieselgel 60,0.040-0.063 mm), eluent J. Mater. Chem., 1996, 6(3), 357-363 361 dichloromethane with gradual addition of 1-2% methanol. The yields varied between 7 and 73%, average yield 45%.The conversion of free base porphyrins into their zinc derivatives'* is quantitative (100%). The yields, elemental analyses and mass spectral data are shown in Table 2.f 'H NMR spectra of the compounds 1-19. A typical example is the 'H NMR spectrum of compound 4: &-2.45 (2 H, brs, NH), 0.93 (6 H, t, CH3 $-Octyl), 1.07 (12 H, t, CH3 a-butyl), 1.37 (16 H, m, CH, G,&,(,q-octyl), 1.70 (12 H, m, CH, y-butyl, y-octyl), 1.98 (4 H, m, CH2 p-octyl), 2.14 (8 H, m, CH2 p-butyl), 2.52 (12 H, S, CH3), 3.96 (8 H, t, CH2 a-butyl), 4.24 (4 H, t, CH2 a-octyl), 7.24 (4 H, d, 3 H-, 5 H-phenyl), 7.90 (4 H, d, 2 H-, 6 H- phenyl), 10.20 (2 H, s, 10 H-, 20 H). The other compounds had similar spectra differing only in the alkyl part of the spectrum.The integration values were in agreement with the expected values. Phase transition measurements DSC measurements were performed on a Perkin-Elmer DSC- 7 system. Optical inspection of the samples was carried out between crossed polarizers of an Olympus BH-2 polarization microscope which was equipped with a Mettler FP82 HT hot- stage, controlled by a Mettler FP 80 HT central processor. Pulse radiolysis time resolved microwave conductivity The powder samples were contained in a microwave cell consisting of a piece of rectangular Q-band wave guide of cross section 7.1 x 3.55 mm2 closed at one end with a metal plate and fitted with a wave guide flange at the other. Approximately 200mg of material was compressed by hand into the cell using a close-fitting Teflon plunger.The length and mass of the sample were measured. The samples were ionized by irradiation with a pulse of 3 MeV electrons from a Van de Graaf accelerator. The pulse duration could be varied from 0.2 to 50ns. The precise conditions of irradiation and energy deposition have been presented before." The energy deposition is close to uniform throughout the sample and accurately known. Changes in the conductivity of the sample on irradiation are monitored as changes in the microwave power reflected with nanosecond time resolution. By monitoring the signals over the frequency band available of 26.5 to 38 GHz the absolute value of the radiation-induced conductivity could be determined and related to the total dose (in J m-3) absorbed by the sample during the pulse to give the dose normalised conductivity, Ao/D, which is plotted as a function of time in Fig.4. Powder X-ray diffraction Samples of about 20mg of the porphyrin compounds were put into a small (0.5 cm diameter) depression of a Pt-Rh sample carrier and smoothed to a flat surface. The platinum carrier strip also functioned as a connection between anode and cathode for heating purposes. The high temperature chamber surrounding the carrier was a Biihler HDK 1.4, mounted on the 8 axis of a Philips goniometer PW1050, automated with a stepping motor, graphite monochromator, counter and Windows operating and analysing software of Sietronics Sie Ray 122D. Divergence and receiving slits were both 1.0 mm.Scans were done in air, usually between 10 and 60" 28, with a scan velocity of 2 or 1" min-l and scan step of 0.02". t For the zinc derivatives of the porphyrins and for compound 10 we had to assume the presence of 0.25-0.6 mol of CH,Cl, per mol of porphyrin. The tendency of porphyrins to include solvent molecules, which are difficult to remove, has been described before (see ref. 10). 362 J. Muter. Chem., 1996, 6(3), 357-363 Single crystal X-ray diffraction The most important crystallographic data are collected in Table 3. Only small crystals could be obtained in the form of thin needles. Therefore only a limited number of significant reflections could be measured, resulting in a rather low obser- vation to parameter ratio.Data were collected in the 01-28 scan mode [scan width (w): 1.0+0.4 tan SO], using graphite monochromated Mo-Ka radiation. The intensity data were corrected for Lorentz and polarization effects and for long time scale variation. No absorption was applied. The structure was solved with MULTAN17 and refined by full-matrix least-squares. Weights for each reflection in the refinement (on F) were w= 4FO2/a(Fo2), with a(Fo2)=02(1)+(PF,~)~;the value of the instability factor p was determined as 0.04. All calculations were done with SDP." Atomic scattering factors were taken from International Tables for X-ray Crystal10graphy.l~ In both structures the asymmetric unit contains one half molecule; the other half is generated by a centre of symmetry.Positions and thermal parameters of the non-hydrogens were refined anisotropically. To keep the number of variables in the refinement small, hydrogen atoms were put in calculated positions and treated as riding atoms. Positions for hydrogens of the methyl groups attached to the rings, which could not be calculated, were found from difference Fourier syntheses and were subsequently refined. In both structures the N-H hydrogen atom could neither be found from difference Fourier synthesis, nor could the position be deduced from the geometry around the nitrogen atom. Consequently the N-H hydrogen atom has not been included in the calculations. Both structures show disorder in some part of the molecule, Table 3 Crystallographic data compound 4 2 C68H94N402 C64H86N402 Mr 999.5 943.4 crystal system triclinic t riclinic sp$ce group Pi Pi 44 10.302 (4) 9.828 (2) b/+ 11.609 (5) 11.490 (2) c/A 14.451 (6) 13.805 (6) aldegrees 106.53 (4) 105.35 (2) PJdegrees 105.77 (4) 104.60 (2) y/degrees 100.12 (4) 100.54 (3) V/A3 1533 (3) 1401 (2) z 1 1 1.082 1.117DX/wyP3P/mm -0.60 0.62 T/K 293 (1) 293 (1) crystal size/mm3 0.50 x 0.10 x 0.02 0.40 x 0.15 x 0.02 data collection: radiation Mo-Ka Mo-Ka /degrees 22.5 25.0emax refl.measured 3989 491 1 refl. obs I >3a(I) 1696 1375 h range -13+13 -11411 k range -1444 -13+13 1 range -17+0 0416 refinement: final R 0.065 0.060 Rw 0.074 0.059 S 2.17 1.34 observations 1696 1375 parameters 359 341 0.02 0.20(A/4max APmaxle 4-33 0.19 0.27 Aprnin/e A--0.26 -0.17 as evidenced by the large thermal parameters of some atoms.In compound 2 disorder is found in one of the two independent butyl groups. In compound 4 the alkoxy chain shows some disorder. In both cases bond lengths and angles are affected by the disorder. Atomic coordinates, bond lengths and angles and thermal parameters have been deposited at the Cambridge Crystallographic Data Centre. For details of the deposition scheme, see ‘Information for Authors’, J. Muter. Chem., 1996, Issue 1. 8 9 10 111, 3024; (b) B. A. Gregg, M. A. Fox and A. J. Bard, J. Phys. Chem., 1989, 93,4227; (c) B. A. Gregg, M. A. Fox and A. J. Bard, J. Phys. Chem., 1990, 94, 1586; (d) B. A. Gregg, M. A. Fox and A. J. Bard, J.Chem. SOC., Chem. Commun., 1987,1134. (a) Y. Shimizu, M. Miya, A. Nagata, K. Ohta, I. Yamamoto and S. Kusabayashi, Liq. Cryst., 1993,14,795;(b)Y. Shimizu, M. Miya, A. Nagata, K. Ohta, I. Yamamoto and S. Kusabayashi, Chem. Lett., 1991, 25; (c) Y. Shimizu, A. Ishikawa, S. Kusabayashi, M. Miya and A. Nagata, J. Chem. SOC., Chem. Commun., 1993,656. D. M. Collard and C. P. Lillya, J. Am. Chem. SOC., 1991,113,8577. G. M. Sanders, M. van Dijk, A. van Veldhuizen, H. C. van der Plas, U. Hofstra and T. J. Schaafsma, J. Org. Chem., 1988,53, 5272. The authors wish to thank Mr. H. Jongejan for performing the elemental analyses and Mr. A. van Veldhuizen for the NMR measurements. 11 12 (a)P. G. Schouten, J. M. Warman, M. P. de Haas, M. A. Fox and H. L. Pan, Nature, 1991, 353, 736; (b) P. G.Schouten, J. M. Warman and M. P. de Haas, J. Phys. Chem., 1993,97,9863. E. M. D. Keegstra, P. G. Schouten, A. Schouten, H. Kooijman, A. L. Spek, M. P. de Haas, J. W. Zwikker, J. M. Warman and L. W. Jenneskens, Red. Trau. Chim. Pays-Bas, 1993,112,423. References 13 (a) M. P. Bym, C. J. Curtis, Y. Hsiou, S. I. Khan, P. A. Sawin, S. K. Tendick, A. Terzis and C. E. Strouse, J. Am. Chem. SOC., 1993, J. M. Kroon, E. J. R. Sudholter, A. P. H. J. Schenning and R. J. M. Nolte, Langmuir, 1995,11,214. (a) D. W. Bruce, in Inorganic Materials, ed. D. W. Bruce and D. OHare, Wiley, New York, 1992, ch. 8; (b) J. M. Kroon, P. S. Schenkels, M. van Dijk and E. J. R. Sudholter, J. Mater. Chem., 1995,5, 1309. 14 15 115, 9480; (b) M. P. Byrn, C. J. Curtis, I.Goldberg, Y. Hsiou, S. I. Khan, P. A. Sawin, S. K. Tendick and C. E. Strouse, J. Am. Chem. SOC., 1991,113,6549. G. W. Gray and B. Jones, J. Chem. SOC.,1954,1467 and references cited therein. J. L. Sessler, J. Hugdahl and M. R. Johnson, J. Org. Chem., 1986, (a) A. M. Brun, S. J. Atherton, A. Harriman, V. Heitz and J.-P. Sauvage, J.Am. Chem. SOC., 1992, 114,4632; (b)R. Schrijvers, M. van Dijk, G. M. Sanders and E. J. R. Sudhblter, Red. Trau. Chim. Pays-Bas, 1994,113,351. J. W. Goodby, P. S. Robinson, B.-K. Teo and P. E. Cladis, Mol. Cryst. Liq. Cryst., 1980,56, 303. S. Gaspard, P. Maillard and J. Billard, Mol. Cryst. Liq. Cryst., 1985,123,369. 16 17 18 51,2838. T. Nagata, A. Osuka and K. Maruyama, J. Am. Chem. SOC., 1990, 112,3054. G. Germain, P. Main and M. M. Woofson, Acta Crystallogr., Sect. A, 1971,27,368. B. A. Frenz and Associates Inc. (1983), Structure Determination Package, College Station, Texas, and Enraf-Nonius, Delft, The Netherlands. (a) D. W. Bruce, D. A. Dunmur, L. S. Santa and M. A. Wall, J. Mater. Chem., 1992, 2, 363; (b)S. Kugimiya and M. Takemura, 19 International Tables for X-ray Crystallography, Kynoch Press, Birmingham, England, 1974,vol. IV. Tetrahedron Lett., 1990,31, 3157. (a)B. A. Gregg, M. A. Fox and A. J. Bard, J. Am. Chem. SOC., 1989, Paper 5/04588F;Received 12th July, 1995 J. Mater. Chem., 1996, 6(3), 357-363 363

ISSN:0959-9428    

DOI:10.1039/JM9960600357

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Self-assembly of non-linear optical chromophores through ionic interactions Joong Ho Moon, Jung Up Choi, Jin Ho Kim, Hoon Chung, Jong Hoon Hahn, Seung Bin Kim and Joon Won Park* Department of Chemistry, Pohang University of Science and Technology, Center for Biofunctional Molecules, San 32 Hyoja-dong, Pohang, 790-784, Korea Ionic attractions were applied to the construction of non-linear optical (NLO) chromophoric monolayers. In order to fully utilize such an interaction, stilbene-type NLO chromophores having a long alkyl chain and an anionic group at one end of the chain were designed; of this family of chromophores, sodium 11-[4-(trans-4’-pyridylstyryl)oxy]undecan-1-yl sulfate (3) was synthesized. By allowing a few minutes, this anionic chromophore self-assembles successfully on a cationically charged surface, which is prepared by treating clean fused silica with 3-aminopropyltriethoxysilane and then with iodomethane.The pyridine moiety of the self- assembled chromophore can be methylated to augment the molecular hyperpolarizability, p. The characteristics of the monolayer were examined via contact angle measurements, UV-VIS spectroscopy, grazing-angle FTIR spectroscopy, and NLO property measurements. Materials with desirable non-linear optical (NLO) properties have attracted enormous interest in recent years owing to their wide applicability.’ In addition to inorganic materials (e.g. LiNbO,, KH2P04, P-BaB204), organic materials with conju- gated n-electron systems offer great promise. The advantage of organic materials over inorganic ones lies in their inherent tunability, fast non-linear responses in the VIS-IR region, and the possibility of better fabrication, processing, and molecular architecture.In addition, organic materials have larger hyper- polarizabilities, lower relative permittivities, and higher laser- damage thresholds. Second-order NLO phenomena include processes such as second harmonic generation (SHG), linear electro-optic (Pockels) effect, optical rectification, and sum and difference frequency mixing. These processes find many poten- tial applications in optical processing and the storage of data or images. Highly conjugated aromatic organic molecules bearing an electron-donating group at one end and an electron-accepting group at the other end exhibit large values of the first molecular hyperpolarizability, p.However, these molecules should be stacked non-centrosymmetrically in bulk media to gain second order non-linear susceptibility, x(’). Most organic materials stack randomly in bulky media to cancel out dipole moments. In order to asymmetrically align NLO molecules on the surface of a substrate, several methods have been developed. Recent studies’ have demonstrated that self-assembly (SA) is a very useful method for generating ordered ultra-thin films, especially for NLO materials. Thin NLO films have been reported in which the sequential construction of covalently self-assembled chromophores forms a multilayer ~tructure.~Alternatively, transition-metal ions, e.g.zirconium(Iv), can be positioned between layers to stabilize the ~tructure.~Decher and co-workers demonstrated that the application of ionic attraction is an attractive alternative method with inherent advantages.’ Very recently, this approach was successfully applied to the building of multilayers of porphyrin derivatives,6 and a monolayer of an NLO chr~rnophore.~ We report here that the chromophore 3, which contains an anionic functional group and long alkyl chain, forms a mono- layer on a cationically charged surface within minutes at room temperature. 3 Experimental All chemicals used were reagent grade from Aldrich Chemical Co. All solvents for the self-assembly processes were of HPLC grade from Mallinckrodt Chemical Co.Elemental analyses were performed at Galbraith Laboratories, Inc. The fused quartz plates were purchased from Wale Apparatus Co. ‘H and I3C NMR spectra were recorded on a Bruker AM 300 spectrometer operating at 300 and 75.4 MHz, respectively. IR spectra were recorded on a Bomem MB-102 FTIR spec-trometer. UV-VIS spectra were recorded on a Hewlett-Packard diode-array 8452A spectrophotometer. Melting points were measured with a Thomas Hoover capillary melting-point apparatus and were uncorrected. Contact angles were meas- ured with face contact angle meters (Model CA-D and CA-DT) from the Kyowa Interface Science Co. Synthesis Compound 1 was prepared using methods described elsewhere.* 11-[ 4-(tvans-4-Pyridylstyryl)oxy] undecan-1-01 2.Ethanol (32 cm3) was added to a mixture of trans-4-hydroxystyryl- pyridine 1 ( 1.00 g, 5.05 mmol), potassium carbonate (0.84 g, 6.06 mmol) and a trace amount of potassium iodide; then 11- bromoundecan-1-01 (1.90 g, 7.57 mmol) in ethanol (10 cm3) was added slowly at room temperature. The solution was heated for 16 h using an oil bath, the temperature of which was maintained at 80 “C. The resulting solution was evaporated to dryness, redissolved in THF, and filtered off. The product was eluted through a column packed with silica gel (eluent chloroform-acetone, 7 : 1-4 :1 v/v). The crude product could be purified further by recrystallization from ethyl acetate to give 1.06 g (57%) of a yellow powder (2).‘H NMR [(CD,),SOj 6: 8.50 (d, 2 H), 7.58 (d, 2 H), 7.50 (d, 2 H), 7.47 (d, 1 H, J= 16.8 Hz, vinylic), 7.06 (d, 1 H, J=16.8 Hz, vinylic), 6.96 (d, 2 H), 4.40 [t, 2 H, CH,(CH2),,OH], 4.27 (t, 1 H, OH), 3.37 [m, 2 H, (CH,),,CH,OH], 1.71, 1.40, 1.26 [m, 18 H, CH,(CH,),CH,OH]; I3C NMR [(CD,),SO] 6: 159, 150, 145, 133, 128.5, 128.4, 123, 120, 115, 67.4, 60.6, 32.41, 32.37, 28.94, 28.87, 28.82, 28.76, 28.64, 28.52, 25.38; UV-VIS (methanol)A,,, 330 nm; IR (KBr) vlcm-’: 3366br, 3029,2921,2851, 1594, 1512, 1472, 1419, 1289, 1259, 1176, 1057, 1014, 970, 832; mp 128-131 “C (decomp). Elemental analysis, Calc.: C, 78.43; H, 9.05; N, 3.81.Found: C, 78.45; H, 9.25; N, 3.81%. J. Muter. Chem., 1996, 6(3), 365-368 365 Sodium 11-[ 4-( trans-4 -Pyridylstyryl) oxy] undecan-1-yl sulfate 3' Compound 2 (0 76 g, 2 07 mmol) was dissolved in anhydrous pyridine (4 0 cm3) under a nitrogen atmosphere To the solution cooled in an ice bath, chlorosulfonic acid (0 156 cm3, 2 351 mmol) was slowly added After 3 h at room temperature, the resulting solution was neutralized by adding sodium carbonate (3 g) and water (5 cm3) Filtration and evaporation to dryness gave a yellow powder Diethyl ether was added cautiously onto a methanolic solution (30 cm3) of this powder to produce a double layer Placing in an freezer (-20 "C) gave the analytically pure product ( 100 g, 97%) [Corrosive chlorosulfonic acid can be replaced by dicyclohexyl- carbodiimide (DCC) and sulfuric acid,5b but this results in longer, laborious purification steps ] 'H NMR [(CD,),SO] 6 850 (d, 2 H), 758 (d, 2 H), 751 (d, 2 H), 747 (d, 1 H, J= 16 2 Hz, vinylic), 7 07 (d, 1 H, J= 16 2 Hz, vinylic), 6 96 (d, 2 H), 399 [t, 2 H, CH2(CH,),oS0,Na], 368 [t, 2 H, (CH,)loCH,S04Na], 170, 145, 126 [m, 18 H, CH,(CH2)9CH,S04Na], I3C(lH} NMR [(CD,),SO] 6 159, 150, 145, 132, 129, 128, 123, 120, 115, 674, 654, 2898, 2888, 28 85, 28 82, 28 66, 28 63, 28 52, 25 41, 25 35, UV-VIS (meth-anol) Amax/nm 236 (&/drn3 mol-' cm-l 117 x lo4), 330 (E 2 82 x lo4), (DMF) E.,,,/nm 328 (c/dm3 mol-' cm-' 2 81 x lo4), IR (KBr) v/cm-l 3366br, 3026, 2923, 2852, 1597, 1512, 1473, 1242, 1175, 1069, 1041, 999, 820, mp 210°C (decomp ) Elemental analysis, Calc C, 61 38, H, 6 87, N, 2 98 Found C, 61 03, H, 6 93, N, 2 93% Self-assem bl y Clean plates of fused silica were sonicated in piranha solution" (H,SO,-30% H202, 7 3 v/v) for 1h For further cleaning, the H20-30% H,O,-conc NH3 (5 1 1) and H,O-30% H,O,-conc HCl (6 1 1) steps of the RCA (SC-1 and SC-2) procedure" were applied The substrates were washed with copious amounts of deionized water and subsequently with acetone, and dried in a vacuum The thus-prepared substrates were immersed into a toluene solution (24 cm3) containing 3- aminopropyltriethoxysilane'2 (4 cm3) under a nitrogen atmos- phere for 24 h at 25 "C The substrates were then washed with toluene, sonicated in a tolueneacetone mixture (1 1 v/v) for 2 min Finally, the substrates were sonicated again in acetone for 2min, washed with fresh acetone, and air-dried The aminosilylated surfaces were quaternized at room temperature by reaction with iodomethane (1 cm3) dissolved in toluene (16 cm3) for 24 h in the presence of 1,8-bis(dimethylamino)- naphthalene (34 mg) After this, the substrates were washed thoroughly with toluene and air-dried The substrates were immersed into a DMF solution of the chromophore ( low3mol dm-3) at 0°C Typically 1 min was allowed for the self- assembly of the chromophore on the surface Finally, the substrates were immersed for 3 h at 25°C in toluene solution (16 cm3) containing iodomethane (1cm3) for quaternization of the pyridine moiety Grazing-angle FTIR spectroscopy All grazing-angle IR measurements were made with an evacu- able BOMEM 2 26 FTIR interferometer equipped with an MCT wide-band detector, and at a grazing incidence angle of 80-85" using the grazing incidence reflection technique (SPECAC P/N 19650 monolayer/grazing angle accessory) The spectra were recorded at a resolution of 4 cm-' with 7000 co- added scans Reference spectra for the films were obtained on the freshly cleaned fused silica substrates Second harmonic generation (SHG) The 1064nm, 6ns output of a Q-switched Nd YAG laser, operating with a repetition rate of 10 Hz, was attenuated to ca 25 mJ by using a half-wave plate and a polarizer A visible- 366 J Muter Chem , 1996,6(3), 365-368 blocking filter was employed to eliminate any visible radiation emitted by the laser flashlamps The horizontally polarized laser beam was focused onto a sample with a lens of focal length 20 cm The beam diameter on the sample was approxi- mately 0 5 mm The sample was mounted on a rotation stage, of which the resolution was ca 05" The p-polarization was selected for the light to be detected by a Glan-Thompson linear polarizer The second harmonic radiation was separated from the fundamental by using IR-blocking filters and band pass filters (532 O& 1 5 nm), and was detected by a photomul- tiplier tube, the output of which was amplified and integrated by a boxcar averager with a 30ns gate-width The level of output signal from the boxcar integrator was monitored by an oscilloscope A 3 mm thick, y-cut quartz single crystal (dll = 1 1 x lo-' esu) was used as a reference to determine the NLO coefficients of the sample films Results and Discussion A chromophore with a sulfonate anion on the long alkyl chain can be synthesized as shown in Scheme 1 The adopted syn- thetic scheme allows easy variation of the alkyl chain length, and this advantage will be helpful in the study of the effect of the chain length on the self-assembly process The chromo- phore itself shows a relatively small molecular hyperpolariz- ability, p,but quaternization of the pyridine moiety increases the p value dramatically For successful self-assembly, the substrate was treated as shown in Scheme 2 Treatment with 3-aminopropyltriethoxysil- ane (3-APTS) gave substrates coated with organic layers of various thicknesses, depending upon the reaction conditions OH 6-or H2SOJDCC 1 2 3 Scheme 1 Synthesis of the chromophore 3-m MeIfuseddamsubstrate d -3, A B C 3 w 4-Scheme 2 Self-assembly of the NLO chromophore on the cationally charged surface Table 1 The measured advancing contact angles of water after each step of the chemical modification substrate contact angle, BJdegrees A clean fused silica 8-13 B aminosilylated surface 46-5 1 C quaternized surface 33-38 D self-assembled surface 59-63 E pyridinium surface 28-33 E '1 b',03 d4 \ -00 .. 2 00 308 4 00 500 A /nm Fig. 1 UV VIS absorption spectra of self-assembled monolayers -, before, after methylation Upon methylation of the pyridine moiety, the absorption maximum shifts from 308 to 364nm Both values are shifted to shorter wavelengths in comparison to those in solution, and confirm the interaction between the neighbouring chromophores Through ellipsometric measuremepts, it was found that the thickness of the layer was ca 100 A when the substrates were treated with 3-APTS in toluene solution for 1 day The primary amine groups on the surface were methylated to give quaternary amine groups, in which the presence of the cation is independent of the pH of the environment Adsorption studies showed that the self-assembly of the chromophore through ionic exchange was successful and complete in only a few minutes under ambient conditions Only the methylattd substrates with the appropriate thickness (in this case ca 100A) produced a self-assembled chromophoric layer of high surface density The self-assembled chromophore was further exposed to methyl iodide for methylation, which enhanced the efficiency for SHG Contact-angle measurement IS one of the most sensitive methods for examining the chemical nature of surfaces Changes in hydrophilicity (or hydrophobicity), which is greatly influenced by the kinds of functional groups present on the surface, can be monitored easily using the method Advancing contact angles between water and the surfaces were measured after each chemical treatment (Table 1) The observed values agreed with the expected ones, and the observation partly confirms the success of each building step Self-assembly of the chromophore can also be monitored by UV-VIS absorption spectroscopy (Fig 1) The measured absorption maximum of the chromophoric layer is 308 nm, this shifted to 364 nm by derivatizing the pyridine moiety with iodomethane These values are blue-shifted in comparison with the values for the chromophore (A,,, =330 nm in MeOH, 328 nm in DMF) and the methylated chromophore (A,,,= 292, 386 nm in MeOH, 370 nm in DMF) in the solution The blue shift of the absorption confirms that the chromophore is packed in a parallel polar arrangement l3 The surface density of the chromophore is calculated from the absorption coefficient (~=2 8 x lo4dm3 mol-l cm-I in MeOH and DMF) and measured absorbance (A=0 04 +_ 0 01) The calculated t In Me,SO solution the chromophore 3 can be completely methylated by the addition of an excess of methyl iodide at room temperature I 1 3000 2950 2900 2850 2800 wavenum berkm-' Fig.2 Grazing-angle FTIR spectra (a) aminosilylated substrate (state C in Scheme 2, 6 0 cm resolution, 7000 scans), (b) methylated chromophoric layer (state E in Scheme 2, 8= 83" 4 0 cm resolution 7000 scans) surface density (4k1 molecules per 100 A') is comparable to those of similar self-assembled molecules 1 Grazing-angle FTIR spectroscopy is a very useful analytical method that provides valuable information about the direction and packing mode of chromophores in the molecular layers For alkyl chains on the surface, the tilt angle of the chain can be deduced from the intensity of the methylene vibrations [v,,,(CH,), vasym(CH2)]in the spectra It is also observed that C-H stretching frequency of the methylene group for the crystalline samples shows a red-shift relative to that for liquid- like samples [2918 us 2924cm-' for vaSym(CH2), 2851 us 2855 cm-' for v,,,(CH,)] ''As shown in Fig 2, the IR spec-trum of the self-assembled layer shows C-H stretches at 2919 and 2851 cm-l The stretching frequency of the methylene group shows that the packing of the alkyl chains is more like that of crystalline states Significant SHG intensity was detected when the methylated chromophoric layer was irradiated with light (1 064 pm) from the Nd YAG laser The measured macroscopic hyperpolariz- ability [x'')] of esu is comparable to those obtained from other SA and Langmuir-Blodgett (LB) methods Meanwhile, the value is about 10 times larger than that for the same type of chromophore containing a short alkyl chain Therefore, it can be said that the long alkyl chain assists the polar orien- tation of the chromophore in the layer Conclusion The ionic interaction has been successfully applied for the self- assembly of an NLO chromophore with a long dkyl chain and a sulfonate functional group at the end of the chain Because of the short time required for the assembly, this method may be an attractive alternative for the assembly of non-centrosymmetric NLO chromophoric layers This work is supported in part by the Basic Science Research Institute Program, Ministry of Education, 1995, Project No BSRI-95-3436 $ Self-assembled monolayers of 6-( 4-pheny1azophenoxy)hexane 1 thiol on gold have been studied with AFM and STM, an4 it was found that the unit cell, with lattice dimensions of 6 1 and 7 9 A, is populated with two molecules See H Wolf, H Ringsdorf, E Delamarche, T Takami, H Kang, B Michel, C Gerber, M Jaschke, H-J Butt and E Bamberg, J Phys Chem, 1995, 99, 7102 J Mater Chem , 1996, 6(3), 365-368 367 References 1 (a) P N Prasad and D J Williams, Introduction to Nonlinear Optical Effects in Molecules and Polymers, Why, New York, 1991, (b) Y R Shen, The Principles of Nonlinear Optics, Wiley, New York, 1984, (c) Materials for Nonlinear Optics, ed S R Marder, J E Sohn and G D Stucky, ACS Symp Ser 455, American Chemical Society, Washington DC, 1991, (d) Organic Materials for Non-linear Optics IZI, ed G L Ashwell and D Bloor, Royal Society of Chemistry, Cambridge, 1993 2 (a)H Sellers, A Ulman,Y Shnidman and J E Eilers,J Am Chem SOC, 1993, 115, 9389, (b) J P Folkers, J A Zerkowski, P E Laibinis, C Seto and G M Whitesides, in Supramolecular Architecture, ed T Bein, ACS Symp Ser 499, American Chemical Society, Washington, DC, 1992, pp 10-23, (c)A Ulman, Ultrathin Organic Films, Academic, Boston, 1991,(d) A Ulman, Adv Mater , 1990, 2, 573, (e) M D Porter, T B Bright, D L Allara and C E D Chidsey, J Am Chem Soc, 1987,109,3559 3 (a)S B Roscoe, S Yitzchaik, A K Kakkar, T J Marks, W Lin and G K Wong, Langmuir, 1994, 10, 1337, (b) S Yitzchaik, A K Kakkar, Y Orihashi, T J Marks, W Lin and G K Wong, Mol Cryst Liq Cryst, 1994, 240, 9, (c) S Yitzchaik, S B Roscoe, A K Kakkar, D S Allan, T J Marks, Z Xu, T Zhang, W Lin and G K Wong, J Phys Chem, 1993, 97, 6958, (d) D Li, M A Ratner and T J Marks, J Am Chem SOC, 1990,112,7389 4 (a)H E Katz, W L Wilson and G Scheller, J Am Chem Soc, 1994, 116, 6636, (b) H Byrd, S Whipps, J K Pike, J Ma, S E Nagler and D R Talham, J Am Chem SOC,1994,116,295, (c)M E Thompson, Chem Muter, 1994, 6, 1168, (d) H E Katz and M L Schilling, Chem Muter, 1993, 5, 1162, (e) H Byrd, J K Pike and D R Talham, Chem Mater, 1993, 5, 709, (f)H C Yang, K Aoki, H Hong, D D Sackett, M F Arendt, S Yau,C M BellandT E Mallouk, J Am Chem SOC, 1993,115, 11855, (8) D Li, S C Huckett, T Frankcom, M T Paffett, J D Farr, M E Hawley, S Gottesfeld, J D Thompson, C J Burns and B I Swanson, in Supramolecular Architecture, ed T Bein, ACS Symp Ser 499, American Chemical Society, Washington, DC, 1992, pp 33-45 5 (a) G Decher, J D Hong and J Schmit, Thin Solid Films, 1992, 211, 831, (b) G Decher and J D Hong, Makromol Chem Makromol Symp, 1991,46,321, (c)G Decher and J D Hong, Ber Bunsen-Ges Phys Chem , 1991,95,1430 6 X Zhang, M Gao, X Kong, Y Sun and Y Shen, J Chem Soc Chem Commun ,1994,1055 7 J U Choi, C B Lim, J H Kim, T Y Chung, J H Moon, J H Hahn, S B Kim and J W Park, Synth Met, 1995,71,1729 8 D Papa, E Schwenk and E Khngsberg, J Am Chem Soc, 1951, 73,253 9 C Lee, E A O’Rear, J H Harwell and J A Sheffield, J Colloid Interface Sci ,1990,137,296 10 A Ulman, Ultrathin Organic Films, Academic, Boston, 1991, p 108 11 (a) W Kern, Semiconductor International, 1984, 94, (b)W Kern, J Electrochem Soc, 1990,137, 1887 12 (a)K Chen, W B Caldwell and C A Mirkin, J Am Chem Soc, 1993, 115, 1193, (b) H Lee, L J Kepley, H Hong and T E Mallouk, J Am Chem Soc, 1988,110,618 13 (a)R Steinhoff, L F Shi, G Marowsky and D Mobius, J Opt SOC Am B, 1989, 6, 843, (b) G H Wagniere and J B Hutter, J Opt Soc Am B, 1989, 6, 693, (c) M Orrit, D Mobius, U Lehmann and H Meyer, J Chem Phys , 1986,85,4966 Paper 5/02982A, Received 10th May, 1995 368 J Muter Chem , 1996,6(3), 365-368

ISSN:0959-9428    

DOI:10.1039/JM9960600365

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Photo-active and electro-active protein films prepared by reconstitution with metalloporphyrins self-assembled on gold Liang-Hong GUO,+~ George McLendon,*b Haja Razafitrimo' and Yongli Gao' aDepartment of Chemistry, University of Rochester, Rochester, N Y 14627, USA bDepartment of Chemistry, Princeton University, Princeton, NJ 08544, USA 'Department of Physics and Astronomy, University of Rochester, Rochester, N Y 14627, USA Long-chain thiol derivatives of metalloporphyrins (M-protoporphyrinate IX di [12-sulfanyldodecyl] ester; M =Fe3 ,Zn2+) were + synthesized and self-assembled onto a gold substrate. X-Ray photoelectron spectroscopy (XPS), fluorescence spectroscopy and electrochemistry were employed to characterize the surface-immobilized species. XP spectra of the Fe-porphyrin showed the characteristic core level signals for C Is, 0 Is, S 2p, N 1s and, in particular, Fe 2p.The compound also displayed chemically reversible and stable electrochemical reactivity in an aqueous electrolyte, and its voltammetric response was manipulated by co-adsorbing with a diluent component, sulfanylpropionic acid. Fluorescence excitation and emission spectra of the Zn derivative adsorbed on Au closely resembled those of Zn-protoporphyrins in solution. After immersion in a dilute solution of apomyoglobin, significant changes in the voltammetric response of the adsorbed Fe-porphyrin and fluorescence excitation spectra of the Zn derivative were observed, and were attributed to the formation of the respective Fe- and Zn-myoglobin protein at the interface.A control experiment using native myoglobin instead of the apoprotein in the reaction ruled out the possibility of non-specific protein binding as the cause for the observed changes. We report in this paper a simple and yet potentially versatile strategy to prepare immobilized protein films that are coval- ently attached to solid surfaces and are fully active. The strategy relies on the self-assembly of the thiol derivative of a protein's prosthetic group on a gold surface, and on its ability to subsequently react with the apo-protein to form a complex which is functionally equivalent to the native protein. The latter reaction is termed reconstitution and is also a self-assembly process. The combination of these two self-assembly processes provides a convenient means to produce two-dimensionally (2D) ordered protein films on solid substrates, but it remains largely unexploited.' Surface-immobilized proteins are highly desirable in such applications as biosensors, bioseparations and bioelectronics, and active research has been carried out to improve the methods of immobilization.2 A generally employed approach in covalent immobilization is using a bifunctional linker that possesses a reactive head group, an end group and a spacing chain between the two groups.Two types of head group frequently used are thiols and silanes, which react readily with noble metal and oxide surfaces, respectively. Proteins are then immobilized through covalent binding of their surface amino acid residues with the exposed end groups of the linker.3 The end groups are usually organic functionalities which react readily with amino-, carboxy- or thiol-containing amino acids.An interesting and useful alteration of this scheme involves using guest molecules as end groups to recognize and bind the corresponding protein hosts.4 In our approach, long-chain thiol derivatives of metallopor- phyrins (Scheme 1) were used as the linkers for protein immobilization. Long-chain thiol compounds have been found to self-assemble on gold surfaces and form 2D ordered mono- layer^,^ which can serve as a good starting structure for the preparation of organized protein monolayers. Metalloporphyrins were chosen because they are the active groups in a number of proteins carrying out a variety of vital biological functions such as oxygen transport, electron transfer, t Present address: IGEN Inc., 16020 Industrial Drive, Gaithersburg, MD 20877, USA.la lb M=Fe& IC M=Zn2+ Scheme 1 catalysis and photosynthesis. In many of these proteins, the porphyrins are not covalently attached to any amino acids and can thus be removed easily. The resulting apoprotein is able to refold back to its original structure by reconstituting with either native or, perhaps more importantly, modified porphyrin groups.6 Furthermore, the redox and optical proper- ties of metalloporphyrins can be fine-tuned by metal substi- tution and ring modification, making them good materials for electronic and optical devices.In this paper, we have used myoglobin as an example to illustrate how to prepare surface-immobilized photoactive and electroactive protein films by reconstitution at an interface, i.e. a reaction between an apoprotein in solution and a metallopor- phyrin thiol self-assembled on a gold surface. Experimental Syntheses Protoporphyrin IX di( thioacetalidodecyl ) ester, la. Protoporphyrin IX dimethyl ester (Sigma; 500 mg) was dis- solved in distilled CH2C12 (50 cm3), benzene (100 cm3) and J. Muter. Chem., 1996, 6(3), 369-374 369 toluene (100 cm3) A ten-fold excess of bromododecanol (Aldrich) and toluene-p-sulfonic acid (Aldrich) were added and the mixture heated at reflux under N, for 5 h The solvents were removed under reduced pressure and the solid redissolved in CH,Cl, and purified with an alumina (EM AX 0612-1) column, using CH,C1, as the eluent One equivalent of the resulting product was dissolved in 10 cm3 absolute ethanol Thioacetic acid was added, followed by a solution of 22 equivalents of Na in 10 cm3 ethanol The mixture was heated at reflux under N, for 7 h, and the product was passed through an alumina column 'H NMR (300 MHz, CDCl,) 6 9 4 (s x 4, 4H, CH), 782 (9x2, 2H, CH=C), 61 (m, 4H, C=CH2), 4 1-4 3 (m, 8 H, OCH2, CH,CO), 3 3 (m, 12 H, CH,), 2 9 (m, 4 H, CH,S), 2 4 (s, 6 H, SCOCH,), 1 8 (t, 4 H, CH2CH2S), 1 1 (br, 32 H, CH,), -2 2 (s, 2 H, NH) Iron(m) protoporphyrinate di (1Zsulfanyldodecyl) ester chlor- ide, lb.Anhydrous iron(I1) chloride (300 mg) and la (200 mg) were dissolved in 20 cm3 dry DMF under N, and reacted at 55 "C with stirring for 7 h The product was brought to dryness and hydrolysed heating at reflux under N, in a mixture of 25 cm3 THF, 25 (3111, H20 and 17 g Na,CO, for 8 h The THF was then removed and CHCl, was added to the residue The organic layer was separated from the water layer and washed three times with water, then dried with Na,SO, The solid was dissolved in CHC1, and passed through an alumina column The second, dark brown band was collected and dried UV-VIS (CHCI,), A/nm 399, IR (KBr) vC-&m-' 2928, 2856, vS--H/cm-l 2360, v,,,/cm 1726, 1685 Zinc(I1) protoporphyrinate di( 1Zsulfanyldodecyl) ester, lc. Unmetallated porphyrin la was first hydrolysed as described above The purified thiol compound was dissolved in 20 cm3 CHC1, and mixed with an excess of zinc acetate saturated in methanol The mixture was stirred at room temperature for 4 h, completion of the reaction was confirmed by the appear- ance of characteristic Zn-porphyrin absorption bands The product was washed three times with H20 and dried over Na2S0, The solvents were removed and TLC of the product (alumina plate) showed only one band 10-Hydroxydecanethiol. 10-Bromodecanol was mixed7 with an excess of thiourea in CH,CH,OH-H,O (90 lo), followed by the addition of 12 equivalents of NaOH The mixture heated at reflux under N, for 30min, extracted with CHCl,, and purified on silica TLC plates 'H NMR (300 MHz, CDC1,) 6 362 (t, 2H, OCH,), 25 (9, 2H, CH,S), 10-1 4 (br, 12H, CCH,C), 1 6 (m, 4 H, OCH,CH,, SCH,CH,) Apomyoglobin. The acid butan-2-one method' was used to prepare apomyoglobin from haemmyoglobin (Sigma) Monolayer assembly Metalloporphyrin-substituted thiols were dissolved in deaer- ated chloroform in micromolar concentrations, and either sulfanylpropionic acid (Aldrich) or 10-hydroxydecanethiol was added to the desired mole ratio Gold wafers (Polishing Corporation of America, CA) were cleaned by etching in H,O,-H,SO, (1 3), followed by electrochemical cycling in 1 mol dmP3 H2S04 Prior to the same treatment, gold disk electrodes (Bioanalytical Service, IN, USA) were polished with alumina slurry and sonicated briefly The cleaned gold was rinsed successively with water, ethanol and chloroform, and then kept immersed in the thiol solution overnight Instruments All XPS measurements were performed in an ultra-high vacuum (UHV) system with a base pressure of 7 x lo-'' Torr The spectrometer consisted of a VG XR2E2 high-power X-ray 370 J Muter Chem , 1996, 6(3), 369-374 source operated at the Mg-Ka line (hv= 1253 6 eV) and a VG ADES 500 angle-resolved electron energy analyser The energy resolution of the XP spectrometer was 1 2 eV, resulting from the convolution of the natural width of 0 68 eV of the Mg-Ka line and the resolution of 10eV chosen for the hemispherical energy analyser A detailed description of the UHV system has been presented elsewhere UV-VIS absorp- tion spectra were recorded on a Perkin-Elmer Lambda 6 spectrophotometer, and fluorescence excitation and absorption spectra were collected on a SPEX Fluorolog-2 fluorimeter Cyclic voltammetric (CV) experiments were performed using a Cypress potentiostat and software The geometric area of the electrode was 003 cm2, and the potential is reported with respect to an Ag/AgCl reference electrode Results and Discussion Metalloporphyrin films self-assernbled on gold Metalloporphyrin derivatives of dodecanethiols were found to self-assemble on clean gold substrates after overnight immer- sion Spectroscopic and electrochemical measurements, as described below, provided experimental evidence which sug- gests the existence of metalloporphyrins adsorbed on gold A representative full XP spectrum of the as-prepared sample of FePPC12SH adsorbed on a gold/silicon wafer is presented in Fig 1, along with three insets The full spectrum clearly shows the expected C 1s and 0 1s core levels as well as the Au features, which are still visible as the adsorbed film was not thick enough to fully attenuate the Au signals lo Core levels for S 2p, N 1s and, in particular, Fe 2p core levels with its characteristic, ca 13 eV, splitting between the 2p, and 2p4 level," were also detected, as shown in the insets of Fig 1 The peak positions for C Is, 0 Is, S 2p, N 1s and Fe 2p, are estimated to be at 2845, 5319, 161 5, 3999 and 7098eV, respectively, as aligned with the Au 4f; peak The XPS results confirm the strong adsorption of FePPC12SH on gold, as is expected for a thiol compound The Fe-porphyrin monolayer was also characterized by electrochemical techniques Owing to their significance in biological systems, the redox chemistry of Fe-porphyrins has been investigated extensively in both aqueous and non-aqueous solutions using electrochemical methods Although their chemistry in aqueous solutions resembles more closely what occurs in a biological environment, the study was hindered by low solubility and aggregation effects One way to overcome the problem of low solubility is to adsorb irreversibly a thin film of an iron-porphyrin onto an electrode (typically a pyrolytic graphite) from a non-aqueous solution, and then transfer it to an aqueous solution This approach, however, lacks control over the film thickness and the orientation and lateral distribution of the porphyrin, which may have a signifi- cant effect on its interfacial redox chemistry In contrast, the thiol derivatives are expected to form a covalently attached monolayer on gold with a fixed orientation and through-chain porphyrin-to-electrode distance Furthermore, by co-adsorbing with unsubstituted thiol molecules at different mole ratios, the intermolecular spacing between the porphyrin groups can be varied and a fairly ordered monolayer can thus be assembled Fig 2 shows two cyclic voltammograms of Fe-porphyrin modified electrodes prepared from chloroform solutions of 1 20 and 1 60 FePPCI2SH-COOHC2SH, respectively For the former sample, the potential cycling was initiated at 100 mV and scanned in the positive direction to 500 mV then back to 100mV No redox waves were observed in this potential region Presumably, the Fe-porphyrin, as prepared, is in the chloride form, and thus is not electroactive in this potential range However, if the potential was first scanned negatively to a lower limit of -600 mV and back to an upper limit of .* .Fe 2p 158 162 166 700 710 720 730 Au 4d c 1s I I I I I I I 0 200 400 600 800 bindingenergylev Fig. 1 A full range XP spectrum of the Fe-protoporphyrin IX derivative of dodecanethiol self-assembled on gold-coated silicon. Core levels for S 2p, N 1s and Fe 2p are shown in the insets.-4--5-800300 0 300800 600 -300 0 300 poten tial/mV Fig. 2 Cyclic voltammograms of gold disk electrodes measured in 0.1 mol dmP3 NaCl-20 mmol dm-3 Na,HPO, after adsorption in a mixture solution of FePPC12SH and HOOCC,SH at a molar ratio of (a) 1 : 20, electrolyte pH 6.9, and (b) 1 : 60, pH 8.3. Scan rate: 400 mV s-'. 500 mV before finishing at 100 mV, two couples of redox waves appeared in the voltammogram. When the cycle was repeated continuously several times, the current of the two peaks at the negative end decreased whereas the ones at the positive end grew [Fig. 2(a)]. The first couple, with an average peak potential of -162 mV (us. Ag/AgCl, pH 6.9), are believed to be associated with the reduction and re-oxidation process of Fe"'-protoporphyrin IX chloride. The symmetric shape of the peaks and linear dependence of the peak current as a function of scan rates are both characteristic of surface-confined elec- troactive species.Integration of the cathodic peak in the first cycle gave a charge of 1.21 pC, corresponding to a surface coverage of 1.25 x 10-mol. If one assumes that the protopor- phyrin plane is perpendicula? to the electrode surface, and its dimensions are13d ca. 14 x 3 A, full monolayer coverage on the electrode (geometric area 3.14 mm2) would be 1.24 x mol. The excellent agreement between the predicted and exper- imental values indicates a full monolayer of FePPCI2SH adsorbed on gold. The voltammogram of the electrode adsorbed in 1:60 FePPC12SH-COOHC2SH is markedly different from the other one in that there is only one cathodic and one anodic wave, and the peak currents are steady regardless of the potential cycling [Fig.2( b)]. The average peak potential is estimated at -246 mV when measured in an electrolyte of pH 8.3. The shift of the potential as a function of pH, indicative of proton involvement in the electrochemical reaction, is consistent with the notion that reduction of Fe"'-protoporphyrin IX chloride leads to the dissociation of the chloride ion and its replacement by hydr0~ide.l~" The absence of any redox waves at the positive end for the 1:60 electrode after repeated cycling seems to suggest that the waves observed with the 1: 20 electrode are due to the cross-linking of the Fe-porphyrins, such as the formation of p-0x0 dimers.14 Such a process would be inhibited if the intermolecular spacing in the monolayer is large enough, as would be the case for a 1 : 60 electrode.The exact nature of this cross-linked species is not clear at present, and must be investigated in combination with spectroscopic methods. Zn-protoporphyrin-substituted dodecanethiols were also synthesized and adsorbed on clean gold wafers, and were characterized by fluorescence spectroscopy. Fig. 3 shows the excitation and emission spectra of a ZnPPC12SH monolayer on gold. The excitation spectrum [Fig. 3(a)] exhibits one intense peak at 412 nm and two small peaks in the range 50s 600 nm, and the emission spectrum [Fig. 3(b)] consists of two maxima at 590 and 645 nm.Both the excitation and emission spectra closely resemble those of Zn-protoporphyrin in solu- tion.15 In addition, the excitation spectrum of the adsorbed porphyrin looks similar to the absorption spectrum of the solution species. Based on these observations, it is concluded J. Muter. Chem., 1996, 6(3), 369-374 371 1000 I I 300 400 500 6w 500 800 700 800 wavelengthhm Fig. 3 Fluorescence excitation (a) and emission (b) spectra of a gold- coated silicon wafer after adsorption in a mixture solution of 1 100 ZnPPC,,SH-HOOCC,SH that ZnPPC,,SH adsorbs on a gold substrate with no pertur- bation to its steady-state fluorescence properties Myoglobin films reconstituted with self-assembled metalloporphyrins It has been found previously that, in addition to protohaem, many other haem derivatives such as mesohaem and proto- haem dimethyl ester, combine with apomyoglobin to form 1 1 complexes which carry out similar biological activities as the native myoglobin Of particular interest to the present study is a report in which a myoglobin reconstituted from a monoal- kylated protohaem derivative and the apoprotein shows UV-VIS absorption spectra similar to those of the native protein The results suggest that attaching a long alkyl chain to the protohaem does not inhibit its reactivity with apomyoglobin To find out if the alkylthiol derivatives of the protohaem also have the ability to bind to myoglobin, a reconstitution experiment was performed in the solution phase The reconsti- tution was carried out by first dissolving an excess of ZnPPC,,SH in pyridine and then mixing with 14 pmol dmP3 apomyoglobin in an aqueous buffer (pH 3) The mixture was stirred at 4°C overnight and then passed through a Bio-Rad G25 gel column to remove the low molecular mass pyridine and porphyrin A UV-VIS absorption spectrum of the complex formed between apomyoglobin and ZnPPC,,SH after gel filtration is shown in Fig 4 Two absorption maxima at 410 and 427 nm are present in the Soret band region, characteristic of apomyoglobin and Zn-myoglobin, respectively The incom- plete, low yield reaction can be attributed to the insolubility ’\n I 8 01!OJIP O°F -0 1 -0 2 300 350 400 450 500 550 600 650 700 750 wavelengthhm Fig.4 A UV-VIS absorption spectrum of the reaction product between 14 pmol dm apomyoglobin and an excess of ZnPPCI2SH The product was passed through a gel filtration column with a phosphate buffer before the spectrum was taken of ZnPPC,,SH in water, which precipitated immediately after addition to the protein solution Nevertheless, the results strongly suggest that ZnPPC,,SH is capable of combining with apomyoglobin The solubility problem can be overcome by carrying out the reconstitution in the solid phase, z e ,by reacting apomyoglobin with either ZnPPC,,SH or FePPC12SH pre-adsorbed on a gold substrate However, to design a surface structure that would react with myoglobin efficiently, two factors need to be considered Firstly, proteins have a strong tendency to adsorb non-specifically and irreversibly on metal surfaces, accompanied by loss of their native tertiary structures and, consequently, of their activities A general approach to inhibit this process is the chemical modification of a gold surface with organosulfur compounds containing polar end groups, a method which has been used successfully in the investigation of the direct electrochemistry of electron-transport proteins and redox enzymes l7 Secondly, to immobilize all the porphyrin groups on a surface accessible to proteins in solution and to avoid possible steric hindrance, the intermolecular spacing between the porphyrins should be at least as large as the dimensions of the protein A two-component monolayer structure is designed to fulfil the requirements discussed above The monolayer is prepared by the overnight immersion of a clean Au substrate in a solution mixture composed of MPPC,,SH and HOOCC2SH The ratio of the mixture can be varied to control the spacing betyeen the porphyrins Assyming a molecular size of 20A2 for HOOCC,SH and 30A for the dimensions of myo- globin, a rough estimate suggests the mole ratio MPPCI2SH-HOOCC2SH should be lower than 1 35 Since HOOCC,SH is the major component of the monolayer, its carboxy group should help to inhibit non-specific protein adsorption on the surface An additional consideration for using HOOCC,SH instead of longer thiols is that it leaves some space in the vertical direction between the porphyrin and the monolayer surface In this arrangement, a protein molecule will not ‘hit the wall’ when it approaches the porphyrin The Soret band of ZnPPC12SH in its fluorescence excitation spectra is used to monitor the progress of its binding to apomyoglobin In Fig 5(a), the excitation spectrum of an Au wafer adsorbed in 1 100 ZnPPC12SH-HOOCC,SH was first recorded The substrate was then immersed in an aqueous solution of 3 pmol dmP3 apomyoglobin for a certain period of time, rinsed with water, followed immediately by fluorescence measurement As illustrated in the figure, the intensity of the Soret band increased progressively with longer immersion times, accompanied by a red shift of the peak position that eventually amounts to 6-7 nm A previous fluorescence study on the binding between apohaemoglobin and Zn-protoporphyrin IX in aqueous solution noted an increase in the molar absorption coefficient and a small red shift, and attributed these changes to protein bindingi5 We have seen essentially the same effects when apomyoglobin and Zn-protoporphyrin IX adsorbed on gold were used As discussed earlier, proteins might also bind to a surface in a non-specific manner and cause some changes to the spectra of the fluorophore To rule out this possibility, a control experiment was performed, in which a native myoglo- bin (already bound to an Fe-haem) was used instead of the apoprotein As shown in Fig 5(b), there was no significant change in either intensity or peak position, even after an immersion time of 1 day In a control experiment, a hydroxy- terminated alkanethiol with a much longer chain (HOC,,SH) was co-adsorbed with the Zn-porphyrin thiol on Au at a ratio of 1 200 and was then immersed in the same apomyoglobin solution No change in the fluorescence spectrum was detected, regardless of the immersion time in the protein solution This experiment revealed the effect of steric hindrance on protein 372 J Muter Chem , 1996, 6(3), 369-374 400 r I c a, 360 400 440 480 I 1 I 1 I I 360 400 440 480 wavelengthhm Fig.5 (a) Fluorescence excitation spectra of 1:100 ZnPPC,,SH-HOOCC,SH adsorbed gold substrate before and after immersion in an apomyoglobin solution (3 pmol dmP3 in 0.1 mol dmP3 NaCl-20 mmol dm-3 Na,HPO,, pH 8.3) for various periods of time.(b) Spectra of 1:100 ZnPPC,,SH-HOOCC,SH adsorbed gold substrate before (solid line) and after (broken line) immersion in a haem-myoglobin solution (3 pmol dmp3 in 0.1 mol dm-3 NaC1-20 mmol dm-3 Na,HPO,, pH 8.3) for 24 h. interaction with surface-immobilized porphyrins. We can con- clude from these experimental results that apomyoglobin was bound to the Zn-porphyrin groups immobilized on gold co- adsorbed with sulfanylpropionic acid. The reaction between apomyoglobin and FePPClzSH self- assembled on Au was assessed by the electrochemical response of the Fe-porphyrin group. In Fig. 6(a), a gold disk electrode modified with 1:200 FePPClzSH-HOOCCzSH showed a cathodic peak at -303 mV and an anodic peak at -175 mV.Immersion of the electrode in a 3 pmol dm-3 apomyoglobin solution for 10min induced a shift of the cathodic peak to -316 mV, and of the anodic peak to -154 mV, resulting in an increase of the peak separation from 128 to 162 mV. Longer immersion in apomyoglobin, up to 1 h, widened the peak separation further but to a lesser extent. The observed change in peak separation probably indicates that the Fe-porphyrins on the electrode react with the apomyoglobin in solution. Another indication is from the electrochemical experimental result using an electrode adsorbed with 1:40 FePPC12SH-HOOCCzSH, as illustrated in Fig. 6(b). As described earlier, at low mixing ratios the voltammetric response of the Fe-porphyrin is characterized by two redox couples, one of which is supposed to be the product of intermolecular cross-linking.Such a response was observed when a gold electrode modified with 1:40 FePPC,,SH-HOOCCzSH was measured in a pH 6.9 electro- lyte. However, if an electrode was transferred into an apomyog- lobin solution immediately after modification with thiols and immersed for 45 min, the voltammetric waves attributed to the cross-linked species were not observed. Presumably, binding between the porphyrin groups and the protein molecules prevented the former from interacting among themselves. 4-2-0---2 -4-4 C -600 -400 -200 0 200h 3 0' -3 I I I-6I -600 -300 0 300 600 potential/mV Fig.6 (a) Cyclic voltammograms of 1 :200 FePPCl2SH-HOOCC2SH adsorbed gold disc electrodes in 0.1 mol dmP3 NaCl-20 mmol dmP3 Na,HPO, (pH 8.3) before (-), after 10 min (---), and after 1 h (---) immersion in a 3 pmol dm-3 apomyoglobin solution. (b) Cyclic voltammograms of 1:40 FePPC12SH-HOOCC2SH adsorbed gold disc electrodes in 0.1 mol dm-3 NaC1-20 mmol dm-3 Na,HPO, (pH 6.9) after 45 min immersion in the electrolyte (-), and in a 4 pmol dmp3 apomyoglobin solution (-.-.-). Scan rate: 400 mV s-l. Conclusion We have demonstrated, with spectroscopic and electrochemical evidence, that both Zn- and Fe-protoporphyrin derivatives of dodecanethiol self-assemble and adsorb irreversibly on gold substrates, and reported their respective fluorescent and inter- facial redox properties. Furthermore, photoactive and electro- active myoglobin protein monolayers are assembled at a solution/monolayer interface by reconstitution of apomyo- globin in solution with the corresponding metalloporphyrin monolayer self-assembled on the gold surface.We acknowledge financial support from the National Science Foundation (Grant CHEM-9120001 for G. M. and DMR- 9303019 for Y. G.) and an African Graduate Fellowship from the African-American Institute for H. R. Technical assistance in organic syntheses by Cristina Geiger and in protein preparation by Dr. Robin Henderson is also acknowledged. References 1 E. Katz, D. D. Schlereth and H.-L. Schmidt, J. Electroanal. Chem., 1994,368, 165; E.Katz, A. Ya, Shkuropatov, 0.I. Vagabova and V. A. Shuvalov, Biochim. Biophys. Acta, 1989,976, 121. 2 S. A. Barker, in Biosensors: Fundamentals and Applications, ed. A. P. T. Turner, I. Karube and G. S. Wilson, Oxford University Press, New York, 1987, p. 85. 3 S. M. Amador, J. M. Pachence, R. Fischetti, J. P. McCauley, Jr., A. B. Smith, I11 and J. K. Blasie, Langmuir, 1993, 9, 812; M. Collinson, E. F. Bowden and M. J. Tarlov, Langmuir, 1992, 8, 1247; H.-G. Hong, M. Jiang, S. G. Sligar and P. W. Bohn, Langmuir, 1994, 10, 153. J. Muter. Chem., 1996, 6(3), 369-374 373 4 L Haussling, B Michel, H Ringsdorf and H Rohrer, Angew Chem, Int Ed Engl, 1991,30,569 Press, New York, 1977, R H Felton, in The Porphyrzns, ed D Dolphin, Academic Press, New York, vol V, 1978, p 53, 5 G M Whitesides and P E Laibinis, Langmuzr, 1990, 6, 87, A Ulman, An Introduction to Ultrathzn Organic Films, Academic D G Davis, in The Porphyrzns, ed D Dolphin, Academic Press, New York, vol V, 1978, p 127, K M Kadish, Prog Inorg Chem, Press, Boston, 1991, L H Dubois and R G NUZZO,in Annu Rev 1986,34,435 Phys Chem, ed H L Strauss, G T Babcock and S R Leone, 13 (a)K Shigehara and F C Anson, J Phys Chem, 1982, 86, 2776, 6 Annual Review Inc ,Palo Alto, CA, vol 43,1992, p 437 S Sano, in The Porphyrzns, ed D Dolphin, Academic Press, New (b)K A Macor and T G Spiro, J Electroanal Chem, 1984, 163, 223, (c) P Bianco, J Haladjian and K Draoui, J Electroanal York, vol VII, 1978, p 378 Chem, 1990, 279, 305, (d) S R Snyder and H S White, J Phys 7 C M Miller, P Cuendet and M Gratzel, J Phys Chem, 1991, Chem, 1995,99,5626 95,877 14 W I White, in The Porphyrzns, ed D Dolphin, Academic Press, 8 9 F W J Teale, Bzochzm Bzophys Acta, 1959,35, 543 Y Gao and J Cao, Phys Rev B, 1991,43,9692 15 New York, vol V, 1978, p 303 J J Leonard, T Yonetani and J B Callis, Bzochemzstry, 1974 10 H Razafitrimo, Y Gao, L -H Guo and G McLendon, Appl Phys 13,1460 Lett, submitted 16 T Hamachi, K Nakamura, A Fujita and T Kunitake, J Am 11 C D Wagner, W M Riggs, L E Davis and J F Moulder, in Chem Soc, 1993,115,4966 Handbook of X-Ray Photoelectron Spectroscopy, ed G E 17 L -H Guo and H A 0 Hill, Ado Inorg Chem, 1991,36,341 12 Mullenberg, Perkin-Elmer Corporation, 1979 G Dryhurst, Electrochemistry of Biological Molecules, Academic Paper 5/03808A, Received 13th June, 1995 374 J Mater Chem ,1996, 6(3), 369-374

ISSN:0959-9428    

DOI:10.1039/JM9960600369

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Reorganisation of layer structures formed from amphiphilic molecules Maria Bardosova,a~b Philip Hodge," Richard H. Tredgold*" and Martin Woolley" "Departmentof Chemistry, University of Manchester, Oxford Road, Manchester, UK MI 3 9PL bInstituteof Computer Systems, Slovak Academy of Sciences, Bratislava, Slovak Republic Two closely related amphiphilic compounds have been deposited as multilayers by evaporation in uucuo, the structures of which have been studied as a function of time using low-angle X-ray diffraction and polarising microscopy. The material consisting of the shorter of the two molecules forms a structure in which the repeat unit corresponds to a monolayer but, initially, there are two phases present with slightly different d spacings. At room temperature this material reorganises over a period of 1 week so that it consists entirely of one of these phases.The material consisting of the longer molecule forms a structure in which the repeat unit is a bilayer (a Y structure). On storing for 1week at room temperature the first-order Bragg peak disappears but the second- and third-order Bragg peaks remain. A possible explanation for this remarkable behaviour is proposed. Recently we published a study of the formation of ordered multilayers of some materials capable of existing in a smectic liquid-crystal phase.' Of particular interest were compounds 1 and 2. We were able to form multilayers of these compounds by thermal evaporation in oucuo and also by the Langmuir- Blodgett (LB) method.These two compounds can just barely be deposited by this latter technique and thus the isopropyl ester group represents the least hydrophobic terminal group which can facilitate the deposition of multilayers by the LB method. The LB layers were, however, of poor quality and no further reference will be made to them in this paper. Here we present the results of a study of the spontaneous reorganisation of multilayers of these compounds which take place as a function of time and which we have monitored by low-angle X-ray diffraction. 1 n=6 2 n=9 Experimenta1 The synthetic methods, apparatus for thermal evaporation and for X-ray measurements have been described elsewhere.' The substrates were clean glass rendered hydrophobic by exposure to hexamethyl disilazane vapour.Evaporation was carried out at an average pressure of <1Op5Torr. The sub- strate temperature was controlled by a Peltier effect device and the deposition rate was monitored by a commercial thickness monitor. The films that were stored at room temperature (23 "C) were at normal room humidity and those that were stored at -30 "C were in a deep freeze and therefore the humidity was extremely low. In both cases the atmosphere was air. Results Compound 1 forms multilayers by evaporation whose d spacing lies between 2.13 and 2.23 nm and thus are definitely not in a Y configuration. Initially two distinct phases are present but the one corresponding to a d spacing of 2.23 nm disappears if the material is stored at room temperature for a week.The phase corresponding to a d spacing of 2.13 nm persists (see Fig. 1). However, if materials are stored at 4°C or lower, both t-----1 I I 0 2 c 6 8 10 12 14 2blldegrees Fig. 1 X-Ray diffractograms obtained from multilayers of compound 1 deposited by thermal evaporation. The total film thickness was 150 nm and it was deposited on a glass hydrophobic substrate held at -30 "C. (a) Newly deposited film, (b)film stored at room temperature for 1 week. phases survive over a similar period. The films which showed the best order as evidenced by X-ray diffraction were deposited on substrates maintained at temperatures in the range -10 to + 10"C. Compound 2 forms Y layers when evaporated on to sub- strates held at temperatures between -30 and +35 "C.The d spacing obtained by X-ray diffraction is 4.89 nm as compared with a molecular length of 2.83 nm. Examination of multilayers of this compound (deposited on glass) by means of a polarizing microscope indicate that it forms a two-dimensional polycrys- talline film, each crystallite having its own optical axes. Such a behaviour would be consistent with a uniform tilt angle, each crystallite having its own tilt direction. A tilt of 30" would reconcile the molecular length with the existence of a Y structure having the d spacing quoted. If multilayers of compound 2 are stored for 1 week at room temperature the first Bragg peak disappears but the second- and third-order peaks remain.The disappearance of the first- order peak could be explained if it is supposed that the material rearranges itself so that the repeat distance becomes equivalent to one monolayer. However, in these circumstances, the third- order peak would also disappear, which is not the case here. As this result is rather surprising we repeated this experiment three times and, initially the first-order peak is present but, on each occasion, it disappears after storing the specimen for 1 week at room temperature, whereas the third-order peak J. Muter. Chem., 1996, 6(3), 375-376 375 0 2 6 6 10 12 14 2Megrees Fig. 2 X-Ray diffractograms obtained from multilayers of compound 2 deposited by thermal evaporation on a glass hydrophobic substrate The total film thickness was 150nm and the substrate was held at 25°C (a) Newly deposited film, (b) film stored at room temperature for 1 week survives This behaviour is illustrated in Fig 2 In our earlier study' we found that films of this material examined under the polarising microscope became isotropic at 48 "C and recovered their original form on cooling below 46°C Accordingly, we used X-ray diffraction to examine films of this material which had been heated to 56°C and then cooled to room temperature and found that the second-, third- and some higher-order peaks were still present, but that the first-order peak had disappeared Newly formed films that had been stored at -30 "C for 1 week and then examined retained the first-order peak Throughout these studies there was no evi- dence that the basic d spacing had changed Discussion The multilayer structure of compound 1 involves a monolayer repeat unit This is not surprising as many smectic liquid- crystal materials whose molecules are not centrosymmetric nevertheless form structures with a monolayer repeat unit As far as the two phases of this material are concerned we can, at this stage, do no more than make a plausible conjecture It seems probable that the two phases correspond to the well known liquid expanded and liquid condensed phases (see for example ref 2) observed in the case of Langmuir films of carboxylic acids, and which correspond to films in which the molecular tilt is towards the nearest and next-nearest neigh- bours respectively In this case the phase with the smaller repeat distance would correspond to the more tightly packed and more stable condensed phase Initially we believed that the behaviour of compound 2 could be accounted for by a process of reorganisation into a structure in which the repeat distance became equivalent to the thickness of a monolayer, but the obstinate survival of the third-order Bragg peak proves that this cannot be the case We now suggest that the only reasonable explanation of the results which we observe is as follows The film as initially formed consists of a Y layer structure in which one set of monolayers are tightly packed and the other set of monolayers is less tightly packed We have discussed the X-ray diffraction pattern expected to arise from such a multilayer elsewhere3 We suggest that annealing at room temperature brings about a rearrangement to a structure in which both sets of monolayers are equally tightly packed It is possible that such a structure could have a variation of electron density in the direction normal to the planes whose first-order Fourier component is negligible This mechanism would be consistent with the fact that the rearrangement which we observe does not change the basic d spacing of the multilayer For this model to explain all the facts it would be necessary for the more tightly packed monolayers to bridge voids in the less tightly packed mono- layers However, this must surely happen in the many cases of LB films reported in the literature where the deposition ratios in the upward and the downward directions are substantially different We thank the Engineering and Physical Science Research Council (EPSRC) the Royal Society and the British Council for financial support References 1 M Woolley, R H Tredgold and P Hodge, Langmuzr, 1995,11,683 2 R H Tredgold, Order in Thin Organzc Films, Cambridge University Press, Cambridge, 1994, pp 50-57 3 M Bardosova, R H Tredgold and Z Ah-Adib, Langmuir, 1995, 11,1273 Paper 5/05734E, Received 30th August, 1995 376 J Mater Chem , 1996, 6(3), 375-376

ISSN:0959-9428    

DOI:10.1039/JM9960600375

出版商:RSC    

年代:1996

数据来源: RSC