Chemical behaviour of oxotitanium( IV ) phthalocyanine (OTiPc) solutions associated with the preparation of OTiPc monolayers and multilayers Kimiya Ogawa, Jiachang Yao, Hisatomo Yonehara and Chyongjin Pac" Kawamura Institute of Chemical Research, 631 Sakado, Sakura-city, Chiba 285, Japan The chemical behaviour of oxotitanium(1v) phthalocyanine (OTiPc) in mixtures of dichloromethane and trihalogenoacetic acids was studied by means of spectroscopic measurements. In dichloromethane-trifluoroacetic acid mixtures, OTiPc was relatively unstable, undergoing gradual decomposition via the radical cation of OTiPc; phthalimide and 3-iminoisoindolin- 1-one were isolated. In dichloromethane-trichloroacetic acid mixed solvent, on the other hand, a stable solution of OTiPc was obtained with no formation of the radical cation.A monolayer of OTiPc was formed upon spreading this solution onto the water surface and was deposited on substrates with a horizontal lifting method to form a multilayer film. Polarized visible and near-IR absorption spectra and X-ray diffraction of the film indicated a specific order of molecular orientation. Phthalocyanine (Pc) compounds are widely known as diverse- functional materials, among which oxotitanium(1v) phthalo- cyanine (OTiPc, Fig. 1) is industrially utilized as a highly photoconductive material for photoreceptor devices in copiers and laser printers.' Preparation of thin solid films of OTiPc is of importance in view of spectroscopic, photoelectric and photoelectrochemical interests, and of potential device appli- cations.Depending on the method of film preparation, the molecular ordering and orientation in the film, which should affect its optical and photoelectric properties, may be con-trolled. Vacuum evaporation of thin films of OTiPc has been studied by several Evidence for preferential molecu- lar orientation is seen in some case^,^.^.^ though it may well be substrate-de~endent.~ On the other hand, a polymer-disper- sion coating method has been popularly employed in photo- receptor production.' However, a preferential orientation of molecules cannot be expected with this method. The Langmuir-Blodgett (LB) technique is a promising method for the fabrication of highly ordered ultrathin films of organic materials.8 The first example of LB films of Pc com- pounds was presented by Baker et al.for metal-free phthalo- cyanine and its tetra-tert-butyl-substituted deri~ative,~and since then numerous studies on LB films of Pcs have been reported." Because the preparation of sample solutions in volatile organic solvents is a requisite for the LB technique, the majority of the previous studies employed soluble Pcs with appropriate substituents at their peripheral benzene rings. On the other hand, the solubilities of most of the unsubstituted Pcs in common organic solvents are too low to apply the LB method. Exceptionally, LB films of metal-free PCS~*''*~~can be obtained by the hydrolysis of soluble dilithium Pc, whereas Fig. 1 Structural formula of OTiPc zinc Pc 13-' and rare-eart h-me tal bisp h t halocyanines "-" are known to be soluble in certain solvents.We previously reported a new method for the fabrication of LB films of OTiPc as mixtures with a soluble Pc derivative, based on the finding that the solubilities of OTiPc in chloroform are increased upon admixing with the Pc derivative." However, this method is not applicable, in principle, to the preparation of LB films of neat OTiPc, without any additives or matrices. A more general technique is therefore required for easy access to the fabrication of Langmuir monolayers and multilayers of various unsubsti- tuted Pcs. In a few patent specification^,^^^^^ it was reported that mixtures of trifluoroacetic acid (TFAA) with some common organic solvents, such as dichloromethane (DCM), can dissolve OTiPc in amounts large enough for crystal phase transform- ations by recrystallization.However, it is still left unexplored whether or not such mixtures of strong organic acids and common organic solvents can be used as good spreading solvents for the monolayer study of OTiPc and other unsubsti- tuted Pcs.'~ The present paper reports spectroscopic studies on the chemical behaviour of OTiPc solutions in mixtures of DCM with TFAA or trichloroacetic acid (TCAA), in connec- tion with their applicability as spreading solutions. We also report here the successful preparation of an OTiPc monolayer using an appropriate mixed solvent, and the characterization of a deposited multilayer.Experimenta1 General Pure OTiPc was synthesized according to a new method developed in our lab~ratory,'~ and sublimed twice prior to use. Trifluoroacetic acid (TFAA, > 98.0%), trichloroacetic acid (TCAA, > 99.0%), and dichloromethane (DCM, > 99.0%) were commercially obtained (Wako Pure Chemicals) and used as received. Solutions for spectroscopy and monolayer experi- ments were prepared by dissolving OTiPc in DCM containing various amounts of TFAA or TCAA. Stabilities of the solutions were examined by monitoring changes in visible and near-IR absorption spectra. Gas chromatography (GC) was performed on a Shimadzu GC-8A instrument equipped with an SE-54 capillary column. 'H NMR spectra were measured on a JEOL GSX-400 spectrometer, field desorption mass spectrometry (FDMS) on a Shimadzu GCMS 9100-MK instrument, and visible and near-IR (VIS-NIR) absorption spectra on a multichannel photodetector MCPD-1000 system (Otsuka Electronics).X-Ray diffraction (XRD) patterns were obtained J. Muter. Chem., 1996,6(2), 143-148 143 with an RAD-IIA diffractometer (Rigaku Co.) using Cu-Ka X-radiation. Reaction of OTiPc in DCM-TFAA To a mixture of DCM (20 ml) and TFAA (5 ml) was added OTiPc (OSOg) under nitrogen, and then the homogeneous solution thus obtained was allowed to stand in the dark for 9 days. After evaporation followed by the addition of 20ml of ethanol, the mixture was stirred for 2 h at room temperature and then filtered. The blue solid thus obtained was dried and subjected to FDMS measurement.The filtrate was condensed to dryness to give 0.2 g of colourless solid, which was analysed by GC and 'H NMR spectroscopy. arise from the protonation of OTiPc in equilibrium depending on TFAA concentrations [eqn. ( 1)]. OTiPc +H+ $OTiPc-H + (1) A further increase of the TFAA content (220%, Fig. 2b) brought about small blue shifts, ultimately giving a spectrum with almost unresolved maxima at 710-725 nm in neat TFAA. This change is not likely to be due to diprotonation of OTiPc, because each protonation step to Pcs usually causes red shifts of the Q-band.27 We tentatively suggest that this blue shift is caused by the greater solvation energy for polar 0TiPc.H' in its ground state in solvents with larger polarity.28 The spectrum of the OTiPc solution in conc.sulfuric acid showed a largely red-shifted%,,,at 815 nm (Fig. 3), which might be attributable Fabrication of monolayers and multilayers For the fabrication of monolayers and multilayers of OTiPc, DCM containing 0.1 mol dm-3 TCAA was preferably used as the spreading solvent, as will be described later. The sample solutions (typically cu. 2 x mol dm-3) were spread onto doubly distilled pure water (20°C) to form monolayers of OTiPc. Surface pressure (n)-area (A)isotherms were recorded with a commercial LB trough system (KSV-5000LB). Multilayers of OTiPc were deposited at a surface pressure of 9.5 mN m-' by means of a horizontal lifting method.26 Quartz or glass plates were used as substrates after their surfaces had been made hydrophobic with dimethyldichlorosilane.Results and Discussion Chemical behaviour of DCM-TFAA or TCAA solutions VIS absorption spectra of OTiPc in DCM-TFAA solvents at various mixing ratios are shown in Fig. 2. The solutions were prepared by adding TFAA, DCM or TFAA-DCM mixtures to a solution of OTiPc in neat TFAA. At very low TFAA contents up to 1% in volume, the absorption maximum (Amax) occurs at 700 nm (Fig. 2a), which is at slightly longer wave- length than that of a neat DCM solution (690 nm). At higher TFAA contents, the spectra showed a decrease in absorbance at 700 nm accompanied by a further red shift of Amax and the appearance of a new absorption peak at ca.740nm with an isosbestic point at 707 nm (Fig. 2a), giving a split spectral shape with the maxima at 711 nm and 743 nm at 5% TFAA in DCM. The separation (827 cm-') between 700nm and 743nm corresponds to an average red shift of the lowest energy Q-band (700+ 300 cm-') reported for the monoproton- ation of metal Pcs.~~ Therefore, the spectral changes should 0.2 s a42 n2 0.1 600 700 800 600 700 800 Alnm Fig. 2 Visible absorption spectra of OTiPc in TFAA-DCM mixed solvent at various mixing ratios: a, low TFAA contents of 0.5% (-), 1.0% (...), 2.0% (---) and 5.0% (---) v/v; b, higher TFAA contents of 20% (-), 50% (-..) and 100% (---) v/v ([OTiPc] z 3 x mol dm-3) 144 J. Mater. Chern., 1996, 6(2), 143-148 to the tetraprotonated form (OTiPc-4H+), based on the simi- larity to the case of a cobalt Pc deri~ative.~' The protonation equilibrium of OTiPc in DCM-TFAA solvents was also suggested from 'H NMR spectra for a solution of OTiPc in 4: 1 (v/v) CD,Cl2-CF,CO2D mixed solvent at various temperatures (Fig.4). At -30 "C, two 400 600 800 1000 A./nm Fig. 3 Visible and near IR absorption spectra of OTiPc for saturated DCM solution (-), for solutions (2x rnol dmP3) in TFAA(l%-DCM mixed solvent (...) and in neat TFAA (---), and for concentrated H,SO, solution (----, 1.5 x lop5mol dmp3) -20 I I 9 8 6 Fig. 4 'H NMR spectra of OTiPc in 1 :4 CF3C02D-CD2C12 measured at various temperatures resonances appear at 6 8.3 and 9.2, which are attributable to the protons at the 4,5 and 3,6 positions of the peripheral benzene rings, respectively.Upon raising the temperature, however, the signals become broader and coalesce at >0 "C; no signal can be observed at room temperature. This obser- vation strongly suggests that the protonation of OTiPc should be nearly complete in 4 : 1 DCM-TFAA at -30 "C but occurs in equilibrium at higher temperatures on the timescale of the nuclear magnetic transition. In D,S04, on the other hand, the distinct peaks at 6 8.7 and 9.7 can be observed even at room temperature, showing that the protonation equilibrium should be extremely biased toward the tetraprotonated form. Note that the VIS-NIR absorption spectra discussed above (Fig. 2 and 3) were recorded immediately after the preparation of the solutions.The VIS-NIR absorption spectra of OTiPc in 4 :1 (v/v) DCM-TFAA revealed interesting features follow- ing various storage periods. When OTiPc was dissolved (cu. mol dm-3) in a freshly prepared solvent, the absorp- tion bands at 650-740 nm attributable to OTiPc/OTiPcH+, which are green in colour, gradually disappeared with isosbestic points accompanied by the appearance of the absorption maxima of a purple species at 560nm and 880nm, as shown in Fig. 5(a). The same spectral change was observed also when the solution was kept in the dark. The spectrum of the purple species is typical for the monomeric cation radicals of metal Pcs.~~,~'The one-electron oxidation of bis-cyano( phthalocyan- inato)ferrates(~~~)~~and some aromatic molecule^^^.^^ is known to occur in DCM-TFAA.Thus, the spectral change in Fig. 5(a) indicates a stoichiometric transformation of OTiPc to its radical cation species [eqn. (2)]. OTiPc/OTiPc-H+ 2OTiPc' (2)+ 3 400 GOO 800 1000 400 600 800 1000 hlnm Fig. 5 Changes in visible and near IR absorption spectra of OTiPc in (a) freshly prepared and (b) aged 1:4 TFAA-DCM solvent, following various storage periods: immediately after dissolution (-) and after 1 h (...), 2 h (---) and 3 h (---); (a) [OTiPc]z2x lop5mol drnp3,(b) [OTiPc]z7 x lo-' mol dm-3 The one-electron oxidation of substituted terthiophenes in this solvent was discussed in terms of the photooxidation by molecular oxygen with the aid of catalysis by TFAA.32 In the case of OTiPc, however, this mechanism cannot reasonably explain the following observations.In contrast with the case of the fresh solvent [Fig. 5(a)], the purple species was almost instantaneously formed upon dissolving OTiPc into an 'aged' DCM-TFAA solvent that had been stored for 13 days after mixing [Fig. 5(b)]: the oxidation of OTiPc occurs much faster in aged solvents than in fresh solvents. This was again true with the aged solvent that had been bubbled with nitrogen for 20 min just before dissolution of OTiPc and also with the aged solvent stored for 13 days under an N, atmosphere. Presumably, an oxidant might be gradually generated and accumulated after the mixing of DCM and TFAA to induce the facile one-electron oxidation of OTiPc in the aged solvent.The bleaching of the OTiPc radical cation occurs within a few hours, as shown in Fig. 5(b), and was confirmed to be accelerated upon the addition of a few drops of water to the purple solution. This observation can be easily explained in terms of facile hydrolysis of the OTiPc radical cation, in agreement with the well known facts that radical cation species are unstable in the presence of water or methan01.~~'~' On the other hand, for a solution of OTiPc in a fresh DCM-TFAA (4: 1) solvent containing 1% (by volume) of water, the radical cation species was only slightly visible even after 3 h, and the consumption of OTiPc was suppressed to a significant extent compared with a solution in the absence of water.Thus, water added to fresh DCM-TFAA would retard the generation of the presumed 'oxidant' and/or the formation of the OTiPc radical cation, though it readily hydrolyses the radical cation once formed. These effects of water suggest that the radical cation formation should be a major pathway for the decomposition of OTiPc in DCM-TFAA solvents. Possible final products from the decomposition of OTiPc were analysed. After a solution of OTiPc in DCM-TFAA had been kept in the dark at room temperature for 9 days, ethanol- insoluble blue solids and ethanol-soluble colourless solids were separated. An FDMS for the blue solid showed an exclusive peak attributable to OTiPc (m/z 576) and a few small noises, but no signal of H,Pc at m/z 514, demonstrating that H,Pc contamination should be negligible or only minor.This was further confirmed by analysis of the IR absorption bands at 965 cm-' (OTiPc) and at 1007 cm-' (H,Pc); H,Pc would be present in d3%, if at all. Although the demetallation of OTiPc was reported to occur in TFAA,33 this reaction should be only a minor pathway in the present case. The colourless solid was confirmed by 'H NMR and GC to be a mixture of 3-iminoisoindolin-1-one (64%) and phthalimide (36%). Thus, the major pathway of the decomposition of OTiPc in DCM-TFAA solvent can be summarized as in eqn. (3). +[OTiPc +H $OTiPc.H + ]5OTiPc' + NH 0 From technical viewpoints of Langmuir monolayer study, TFAA-DCM mixtures could provide a possible spreading solvent for OTiPc, if preparation conditions can be carefully set to minimize the decomposition of OTiPc.We previously reported that monolayers of copper phthalocyanine (CuPc) can be successfully fabricated by the use of 10: 1 DCM-TFAA as spreading solvent, since the chemical change of CuPc is negligible in this solvent.24 However, with this solvent, minor decomposition of OTiPc is unavoidable before spreading or upon contact with water, and is clearly unfavourable for the fabrication of pure OTiPc monolayers. In contrast, a stable J. Muter. Chem., 1996,6(2), 143-148 145 and convenient spreading solution of OTiPc for the monolayer study was obtained by employing the weaker organic acid, TCAA, instead of TFAA. A sufficient amount of OTiPc (ca.2 x mol dmW3) is soluble in DCM containing 0.1 mol dm-3 TCAA. The solution showed a VIS absorption spectrum with Laxat 696nm, which is very similar to that of unpro- tonated OTiPc or to those in the presence of <1% TFAA shown in Fig. 2 and 3. Presumably, the protonation equilibrium of OTiPc is largely biased toward the unprotonated form in this solution. On standing the solution for 2 days under an ambient atmosphere, only a 5.6% decrease of the maximum absorbance was observed without any change in shape of the spectrum: the colour of the solution remained unchanged. This solution was used as the spreading solution within 24 h after preparation. Fabrication of monolayers and multilayers of OTiPc A surface pressure-area isotherm (20 "C) for the monolayer of OTiPc, formed on a pure water surface, i! shown in Fig.6. The limiting area for the isotherm was 62 A2, which suggests that the plane of the Pc macrocycle substantially stands up from the plane of the water surface, i.e., the 'edge-on' style molecular orientation of OTiPc. Hysteresis was observed on a compression-decompression (0-9.5 mN m -I) experiment, pre- sumably because of some irreversible aggregation that is likely to occur for monolayers of Pc compounds. However, the monolayer was fairly stable under the deposition conditions: the decrease of surface area at a constant surface pressure of 9.5 mN m-' was 5.3% in the first 10 min after compression, and was 2.4% in the next 10min.Fig. 7(a) shows polarized VIS-NIR absorption spectra for an as-deposited multilayer film of OTiPc (40 layers) at various incident angles of the p-polarized light beam with respect to the film plane. The solid line spectrum in Fig. 7(a) (incident angle 0") shows an absorption maximum at 720nm with a slight shoulder around 650 nm, a spectral feature characteristic of amorphous OT~PC.~,~ Nevertheless, molecular orientation in this film cannot be considered as entirely random as the term 'amorphous' may imply, because there is a clear depen- dence of the spectra [Fig. 7(a)] on the incident angle: the increase of the incident angle results in an increase of the absorbance at ca. 650 nm accompanied by a concomitant decrease of the absorbance at 720 nm with an isosbestic point at ca.685 nm. This clearly indicates that the transition moment for the 650 nm absorption stands nearly perpendicular to the film plane, i.e., OTiPc molecules in the multilayer film are preferentially arranged with such an orientation. An X-ray diffraction pattern for the 40-layer film of OTiPc [Fig. S(a)] showed a weak but distinct diffraction peak at 28=6.7" 20I 1 "0 20 40 60 80 100 120 area per moIecule/A* Fig. 6 Surface pressure-area isotherm (20"C) for OTiPc monolayer 146 J. Muter. Chem., 1996,6(2), 143-148 0.4 0.2 E n+? 400 600 800 1000 v v)ncd 0.4 0.2 400 600 800 1000 Alnm Fig.7 Polarized visible and near IR absorption spectra for (a) as-deposited and (b) CH,Cl,-treated OTiPc multilayers (40 layers) at various incident angles of p-polarized light beam.-, 0"; ..., 30"; ___ 45". ___ 600. _._._ ,70".39 >> 10 20 30 40 2O/degrees Fig. 8 XRD patterns for (a)as-deposited and (b)CH,Cl,-treated OTiPc multilayers (40 layers) (d= 13.2 A),but no other peaks in the higher 28 region except the halo from the substrate. These observations suggest that the OTiPc multilayer film may be amorphous but has a layered strycture in the molecular arrangement with a layer spacing of 13 A. This means that the interactions between neighbouring molecules are relatively weak to allow no severe regulation in their relative orientations, though OTiPc molecules adopt the preferential edge-on orientation with respect to the film plane.In other words, the preferential molecular orientation is afforded rather by the method of the film preparation, than by any self-organizing character of the molecule, in the present case. (In most LB film studies, ordering of molecules is considered to be afforded by both.) The edge-on arrangement of OTiPc is consistent with the limiting area in the surface pressure-area isotherm. On exposing the LB film to dichloromethane vapour for 2 h, a drastic change in the VIS-NIR absorption spectra occurred with a considerable bathochromic shift of the longest absorption maximum to 840 nm [Fig. 7(b)].This change is due to a phase transformation of amorphous OTiPc to an a-OTiPc ~rystal.~,~,~~ As seen in Fig. 7(b),the p-polarized absorp- tion spectra of the vapour-treated film again showed a clear dependence on the incident angles: the absorbance at ca.640nm increases with a decrease of that at 840nm as the incident angle increases. An X-ray diffraction pattern of the vapoFr-treated film [Fig. 8(b)]showed a peak at 28=7.5" (d= 11.8 A), but no other distinct peaks besides the halo from the plate. The observed peak is attributable to the diffraction from the (010) plane of a-OTiPc. Since other peaks reported for a-OTiPc were not detected, it can be inferred that the a-form crystals in the treated film are preferentially oriented with the (010) plane nearly paqllel to theofilm plane. The decrease in d-spacings from 13.2A to 11.8 A upon the DCM vapour treatment may imply that the OTiPc molecules in the treated film has a more 'inclined' molecular orientation than in the as-deposited film.This model of molecular orientation is illustrated in Fig. 9, based on the crystallographic data of Hiller et Note that the phase transformation by the vapour treatment occurs while the preferential orientation of the molecule is retained. Conclusions The spectroscopic investigation into the chemical behaviour of OTiPc in TFAA-DCM solvents demonstrated that the protonation of OTiPc occurs in equilibrium at room tempera- ture and that a major pathway for the decomposition of OTiPc in this solvent involves the one-electron oxidation of OTiPc followed by hydrolysis of the OTiPc radical cation. It was confirmed that the final products are mainly 3-iminoisoindolin- 1-one and phthalimide, whereas H,Pc is a minor product. On the other hand, TCAA (0.1mol dm-3)-DCM solvent gave a very stable solution with negligible decomposition of OTiPc.A monolayer of OTiPc on the water surface was successfully prepared from this solution. A deposited multilayer of OTiPc had a preferential orientation of the molecules with respect to C Fig. 9 Molecular orientation model for CH,Cl,-treated OTiPc multilayers the substrate plane. The molecular ordering was retained even after the crystal phase transition caused by the vapour treatment. 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