Preparation, solution behaviour and electrical properties of octasubstituted phthalocyaninato and 2,3-naphthalocyaninato oxotitanium(IV ) complexes Wing-Fong Law,a K. M. Luib and Dennis K. P. Ng*a† aDepartment of Chemistry, T he Chinese University of Hong Kong, Shatin, N.T ., Hong Kong bMaterials T echnology Research Centre, T he Chinese University of Hong Kong, Shatin, N.T ., Hong Kong The octasubstituted oxo(phthalocyaninato)titanium(IV) complexes TiO[Pc¾(CH2OC5H11)8] (Pc¾=2,3,9,10,16,17,23,24- octasubstituted phthalocyaninate) and TiO[Pc(OC4H9)8] (Pc=1,4,8,11,15,18,22,25-octasubstituted phthalocyaninate) and the first 2,3,-naphthalocyaninato titanium complex TiO[Nc¾(C6H13)8] (Nc¾=2,5,11,14,20,23,29,32-octasubstituted naphthalocyaninate) have been prepared by treating the corresponding dicyano-benzenes or -naphthalene with titanium(IV) ethoxide and urea in n-pentanol.These compounds and their analogues TiO[Pc¾(R)8] (R=C7H15, OC5H11) tend to form molecular aggregates in solutions and the eVects of solvent and concentration on their aggregation behaviour were investigated by UV–VIS and 1H NMR spectroscopy. The electrochemistry and electrical properties of these compounds were also studied.Oxo(phthalocyaninato)titanium(IV) (TiOPc) is a well known, method or treating the corresponding dinitriles with TiCl4 followed by hydrolysis.11 All of these macrocycles could be near-IR-active photoconductive dye used practically as xerographic photoreceptors in copiers and GaAs laser printers.1 purified by column chromatography or simply washing with appropriate solvents.The use of this material in optical disk information recording2 and as a p-type semiconductor in photovoltaic cells3 has also been documented. Owing to its poor solubility in usual organic solvents, the purification of TiOPc usually requires tedious procedures such as train sublimation4 and acid pasting,5 and most of the studies have been concentrated on its physical properties in the solid state such as single crystals6,7 and thin solid films.3,7,8 The solution properties of this compound, however, remain relatively unexplored.9 Substitution on the periphery of phthalocyanine will not only enhance its solubility that may facilitate the fabrication of homogeneous thin films, but will also provide entries to tailor the properties of this material. Surprisingly, substituted TiOPc complexes are still scarce.10,11 Here, we describe the preparation of a series of octasubstituted phthalocyaninato and 2,3-naphthalocyaninato oxotitanium(IV) complexes along with their spectroscopic and electrochemical behaviour in solutions.The electrical properties of thin films of TiO[Pc¾(C7H15)8] and TiO[Nc¾(C6H13)8] are also discussed.Results and Discussion Treatment of substituted dinitriles 1, 5 and 6 with titanium(IV) ethoxide and urea in n-pentanol led to the formation of the corresponding metallophthalocyanines 7 and 10, and 2,3- naphthalocyanine 11, respectively. The method was similar to that reported by Pac and co-workers in the synthesis of unsubstituted TiOPc,12 but the yields were considerably lower (12–25%) for substituted analogues.It is noteworthy that compound 11, to our knowledge, represents the first titanium The UV–VIS spectra of these complexes displayed a typical complex of 2,3-naphthalocyanine reported. Reaction of dinitrile Q band and a Soret band which are attributed to the p–p* 2 under similar conditions aVorded a dark green powder.transitions of the macrocycles. As shown in Table 1, the Q Although its UV–VIS spectrum in CHCl3 showed characteristic B and Q bands at 351 and 700 nm, respectively, other spectroscopic methods [1H NMR, IR and liquid secondary ion (LSI) Table 1 UV–VIS data of the oxotitanium complexes 7–11 in CHCl3 MS] and elemental analyses did not support the formation of TiO[Pc¾(CH2OPh)8]. This unknown species seemed to be compound lmax/nm (log e/dm3 mol-1 cm-1) highly aggregated in solutions which hampered the purification 7 352 (4.90), 631 (4.55), 665 (4.60), 700 (5.28) and characterisation processes. The heptyl (8) and pentyloxy 8 347 (4.80), 639 (4.48), 680 (4.42), 710 (5.27) (9) analogues could however be prepared by either using this 9 348 (4.68), 439 (4.37), 631 (4.28), 669 (4.28), 702 (5.12) 10 340 (4.63), 479 (3.99), 704 (4.46), 791 (5.10) 11 337 (4.85), 451 (4.23), 734 (4.56), 782 (4.58), 826 (5.31) † E-mail: dkpn@cuhk.edu.hk J.Mater. Chem., 1997, 7(10), 2063–2067 2063mined for some other substituted phthalocyanines in nonpolar solvents.13a,d The increased aggregation of these complexes in hexanes can be rationalised by the fact that hexanes, having a lower relative permittivity (er=1.89) than CHCl3 (er= 4.81),15 has a weaker screening eVect to disrupt the Pc–Pc interactions. For compounds 9 and 10, such spectral analysis could not be performed because of their poor solubility in hexanes.The naphthalocyaninato complex 11, as expected, is even more highly aggregated in hexanes solution. No absorption spectrum assignable to the monomeric species could be obtained.However, by gradual addition of CHCl3 into dilute hexanes solution of 11, the initial broad band around 770 nm became sharpened giving a characteristic spectrum of metallonaphthalocyanines (Fig. 2). As revealed by UV–VIS spectroscopy, the aggregation of compounds 7–11 in CHCl3 appears to be insignificant. However, at suYciently high concentration, these complexes Fig. 1 Concentration dependence of UV–VIS spectra of 8 in hexanes still have a tendency to form aggregates as shown by 1H NMR spectroscopy. We recorded the 1H NMR spectra of 7–10 in band absorption of 1,4-substituted phthalocyanine 10 shows a CDCl3 over the concentration range 1×10-3–2×10-2 M. bathochromic shift (lmax=791 nm) in comparison with those Fig. 3 shows the concentration dependence of chemical shift of 2,3-substituted analogues 7–9 (lmax=700–710 nm).The Q of the aromatic ring protons for 7–10. The resonances for 2,3- band absorption of naphthalocyanine 11 is even more red- substituted phthalocyanines 7–9 shift upfield by 0.3–0.4 ppm shifted (lmax=826 nm) owing to the more extended p as the concentration approaches 2×10-2 M. It is likely that conjugation.the cone of aromaticity generated by the ring current of one It is well known that phthalocyanines, even in dilute solution, phthalocyanine macrocycle causes upfield shifts of its aggretend to form molecular aggregates such as dimers, trimers and oligomers. These aggregated species may have very diVerent characteristics from the corresponding monomer and the degree of association is largely dependent on the polarity of the solvent and the concentration of the solution.13 We measured the absorption spectra of compounds 7–11 in CHCl3 over the concentration range of 10-6–10-5 M and found no significant spectral change with concentration.The species exhibited spectra typical of monomeric phthalocyanines. Thus, it is apparent that aggregation is not important for these complexes in CHCl3 in these concentrations.In contrast, the absorption spectra of 7 and 8 in hexanes were concentration dependent. Fig. 1 shows the variation of the UV–VIS spectrum of 8 with concentration ranging from 7.34×10-7 to 2.20×10-5 M. The Q band absorption maximum at 696 nm is unshifted but broadened as the concentration increases.Since further spectral change was not observed at concentrations lower than 7.34×10-7 M, the spectrum observed in this concentration can be attributed to the purely monomeric 8. Fig. 2 UV–VIS spectral change of 11 in hexanes upon addition of Compound 7 behaved similarly and the absorption spectrum CHCl3; (a) hexanes only, gradual addition of CHCl3 from (b) to (d) due to monomeric species was obtained at concentrations below 3.25×10-6 M.We assumed that a one-step equilibrium between phthalocyanine monomer (Pc) and aggregated phthalocyanine (Pcn) exists [eqn. (1)], where K is the aggregation constant and n is the aggregation number. nPc ,b) K Pcn (1) By following the treatment described by Mataga,14 eqn. (2) could be derived in which Ct is the total concentration of phthalocyanine, e and em are the observed molar absorptivity and the corresponding value for pure monomer, respectively.log[Ct(1-e/em)]=log(nK)+nlog[Ct(e/em)] (2) Plots of log[Ct(1-e/em)] vs. log[Ct(e/em)] for 7 and 8 gave straight lines from which the values of n and K were determined to be 2.07 and 6.84×104 (for 7) and 2.34 and 1.75×106 (for 8), respectively.The aggregation numbers for both complexes are close to two suggesting that dimer of these compounds may be the dominant species in hexanes solution. The higher value of K for 8 indicates that this compound has a higher aggregation tendency than compound 7 in hexanes and the Fig. 3 Chemical shift of aromatic protons of 7–10 in CDCl3 as a function of log (concentration); (&) 7, (2) 8, (+) 9, ($) 10 value is comparable with the dimerisation constants deter- 2064 J.Mater. Chem., 1997, 7(10), 2063–2067gated partners.16 However, for the 1,4-substituted phthalocyan- oxidation couples, which resembles the energy gap between the highest occupied and the lowest unoccupied molecular ine 10, the chemical shift of ring protons is essentially independent of concentration.This may be attributed to the orbitals (HOMO and LUMO), for 2,3-substituted TiO[Pc¾(R)8] 7–9 was found to be 1.54–1.57 V. These values fact that the ring protons are farther away from the core of phthalocyanine or a weaker aggregation occurs for this substi- are in the range (1.5–1.7 V) reported for the HOMO–LUMO separation of phthalocyanines.17 The corresponding diVerence tution pattern.The 1H NMR spectrum of naphthalocyanine 11 in CDCl3 showed only broad bands for the hexyl side for 1,4-substituted analogue 10 was however smaller (1.29 V) which is in accord with the bathochromic shift of Q band in chains; the aromatic protons’ signals were not observed. However, by adding a few drops of [2H5]pyridine, a broad its UV–VIS spectrum.According to the electrochemical data, the narrowing of the HOMO–LUMO gap in 10 was due to band at d 9.42 and a relatively sharp singlet at d 7.36 appeared which could be ascribed to H1 and H3, respectively. Thus an increase of the HOMO level. The data for 7–9 were also consistent with the electron donating ability of the substituents pyridine is able to disrupt the interactions among these macrocycles to some extent.which follows the order OC5H11>C7H15>CH2OC5H11. The solubility of the substituted macrocyclic compounds The electrochemistry of compounds 7–11 was examined by cyclic voltammetry in CH2Cl2. The voltammograms for8 7–11 in organic solvents renders these compounds suitable for deposition with technique such as spin-coating. Fig. 5 shows (Fig. 4) and 9 revealed two reversible one-electron reduction couples, one quasi-reversible one-electron oxidation couple, the absorption spectra of spin-coated films of 8 and 11 (thickness: ca. 500 A ° ). In comparison with the solution spectra, together with one irreversible oxidation which may be associated with decomposition. Compound 7 gave a similar voltam- the Q bands are significantly broadened, in particular for compound 11.In addition, the lmax for 11 (ca. 775 nm) shows mogram except that an extra quasi-reversible reduction wave was observed, while for compound 10, the second oxidation a hypsochromic shift while that for 8 (ca. 715 nm) remains relatively unchanged. These observations may also indicate also appeared to be quasi-reversible. The voltammogram for naphthalocyanine 11 also displayed two reversible reductions, that the naphthalocyanine 11 has a higher columnar stacking tendency.18 but the oxidation waves were poorly defined. The reversibility of the reduction waves of 7–11 was judged by the separations The electrical properties of the films of 8 and 11 were also briefly examined. The dark conductivity increased with tem- between the anodic and cathodic potentials (62–84 mV) which were almost invariable with scan rates of 50–200 mV s-1, the perature and followed the Arrhenius equation of temperature dependence of conductivity [s=s1 exp(-E1/kT )+s2 cathodic to anodic peak current ratios (ipc/ipa) which approached unity and the linear plots of peak current vs.exp(-E2/kT )]. The Arrhenius plots for both compounds (Fig. 6) show two linear regions of diVerent activation energies square root of the scan rate. A summary of the electrochemical data for these compounds is given in Table 2. All of these (Table 3) with no indication of sharp transition. Since the samples were not intentionally doped, the low activation redox couples are attributed to the macrocyclic ligand as TiIVNO can be considered as a redox inactive moiety.17 energies (E1 ca. 0.04 and 0.24 eV for 8 and 11, respectively) indicate that extrinsic conduction, which may arise from The potential diVerence between the first reduction and structural defects, dominates in the low-temperature regime. Both complexes exhibited semiconducting properties with room-temperature conductivity in the range 10-8 –10-9 V-1 cm-1 which seem to be higher than the values reported for related phthalocyanines.10c As all the conductivity measurements were carried out in complete darkness, we exclude the possibility of extra charge carrier generations due to photo-absorption.On the other hand, it is well known that Fig. 4 Cyclic voltammogram of 8 in CH2Cl2 containing 0.1 M Fig. 5 UV–VIS spectra of spin-coated films of 8 (—) and 11 (A) with a thickness of ca. 500 A ° [NBu4][PF6] at a scan rate of 50 mV s-1 Table 2 Redox potentials for the oxotitanium complexes 7–11a compound Epa b(ox.2) E1/2 c(ox.1) E1/2(red.1) E1/2(red.2) E1/2(red.3) 7 1.54 0.96 -0.58 -0.93 -1.39 8 1.54 0.86 -0.71 -1.06 — 9 1.40 0.80 -0.77 -1.11 — 10 1.00 0.59 -0.70 -1.06 — 11 — — -0.65 -0.97 — aRecorded with [NBu4][PF6] as electrolyte in CH2Cl2 (0.1 M) at ambient temperature. Scan rate=50 mV s-1.Expressed in volts relative to SCE. bAnodic peak potential. cHalf-wave potential. J. Mater. Chem., 1997, 7(10), 2063–2067 2065trations were in the range 10-4 M. All potentials were referenced to the ferrocenium–ferrocene couple (internal standard) at +0.45 V relative to the saturated calomel electrode (SCE).Thin films of 8 and 11 were prepared by dissolving the compounds in tetrahydrofuran and CHCl3, respectively, then spin-coating the solutions on Corning 7059 glass substrates with a spin-coater (Cost EVective Equipment, Model 100 CB) at a spinning rate of 6000 rev min-1 for 30 s. The films were baked at 70 °C for 30 min prior to subsequent measurements. The thicknesses of the films were determined by a film thickness profiler (Tencor Instrument, a-step 500).For conductivity measurements, silver electrodes which defined a gap width of ca. 2 mm were first evaporated onto the samples, then the resistances of the samples were measured in a dynamic vacuum (ca. 10-6 Torr) with an electrometer (Keithley, Model 617) in the V–I mode. The measuring processes started at 200 °C with a ramping down rate of 0.5 °C min-1 to 30 °C.TiO[Pc¾(CH2OC5H11)8] 7 A mixture of 1,2-dicyano-4,5-bis(pentyloxymethyl)benzene 1 (200 mg, 0.61 mmol) and urea (19 mg, 0.31 mmol) was dis- Fig. 6 Plots of log (conductivity) vs. 103 T -1 for spin-coated films of solved in n-pentanol (0.5 ml) to which titanium(IV) ethoxide 8 (#) and 11 ($) (0.04 ml, 0.18 mmol) was added via a micropipette.The mixture was refluxed under nitrogen for 20 h, then mixed with methanol physical properties of thin film materials depend strongly on (15 ml ). After refluxing for a further 15 min, the mixture was their film structures, which may exhibit vast diVerences as a filtered and the residue was washed with methanol (10 ml ), result of diVerent preparation methods and conditions, and of water (10 ml ), and methanol (5 ml) again.The resulting dark any subsequent thermal treatments.19 A direct comparison of blue solid was chromatographed with ethyl acetate–hexanes these data should thus be made with great care. (151) as eluent giving a greenish blue band which was collected and rotary evaporated. The greenish blue solid was dried in vacuo. Yield 53 mg (25%). 1H NMR (CDCl3, 1.6×10-2 M): d Experimental 9.49 (s, 8 H, Pc¾H), 5.14–5.26 (m, 16 H, Pc¾CH2), 3.84 (t, J General 6.7 Hz, 16 H, OCH2), 1.83–1.95 (m, 16 H, CH2), 1.42–1.65 (m, 32 H, CH2), 1.02 (t, J 7.0 Hz, 24 H, CH3); 13C{1H} NMR n-Pentanol and hexanes were distilled from sodium and anhy- (CDCl3): d 151.6, 140.4, 136.3, 123.4, 71.3 (two overlapping drous calcium chloride, respectively.Chromatographic purifi- signals), 29.7, 28.6, 22.7, 14.2. IR: n=978 cm-1 (TiNO); MS cations were performed on silica gel columns (Merck, 70–230 (LSI): an isotopic envelope peaking at m/z 1376.82 (100%) mesh). Dichloromethane used for electrochemical studies was {calc. for M+ based on 48Ti, 1376.81}; Anal. Calc. for freshly distilled from calcium hydride and the electrolyte [NBu4][PF6] was recrystallised from tetrahydrofuran.All C80H112N8O9Ti: C, 69.74; H, 8.19; N, 8.13. Found: C, 67.17; H, other reagents and solvents were of reagent grade and used 8.16; N, 7.68%. without prior purification. The compounds 1,2-dicyano-4,5- bis(pentyloxymethyl)benzene 1,20 3,6-bis(butyloxy)-1,2-dicyanobenzene 5,21 and 2,3-dicyano-5,8-dihexylnaphthalene 622 TiO[Pc(OC4H9)8] 10 were prepared according to the literature procedures.To a mixture of 3,6-bis(butyloxy)-1,2-dicyanobenzene 5 NMR spectra were recorded on a Bruker WM 250 spec- (200 mg, 0.73 mmol) and urea (22 mg, 0.37 mmol) in n-penta- trometer (250 MHz for 1H and 62.9 MHz for 13C) with nol (1 ml) was added titanium(IV) ethoxide (0.05 ml, Si(CH3)4 as an internal standard (d=0).UV–VIS spectra were 0.23 mmol). The mixture was refluxed under nitrogen for 48 h, measured on a Hitachi U-3300 spectrophotometer. IR spectra then mixed with methanol (20 ml ). After refluxing for a further were recorded on a Perkin Elmer 1600 or a Nicolet Magna 30 min, the mixture was cooled to room temperature then 550 spectrometer as KBr pellets. LSI mass spectra were taken filtered, and the solid was washed with diethyl ether (3×10 ml).on a Bruker APEX 47e Fourier transform ion cyclotron The resulting fine dark brown microcrystals were collected resonance spectrometer with 3-nitrobenzyl alcohol as matrix. and dried in vacuo. Yield 26 mg (12%). 1H NMR (CDCl3, Elemental analyses were performed by the Shanghai Institute 2.2×10-2 M): d 7.70 (s, 8 H, PcH), 4.80–5.05 (m, 16 H, OCH2), of Organic Chemistry, Chinese Academy of Sciences. 2.16–2.31 (m, 16 H, CH2), 1.60–1.78 (m, 16 H, CH2), 1.10 (t, Electrochemical measurements were carried out with a BAS J 7.4 Hz, 24 H, CH3); 13C{1H} NMR (CDCl3): d 151.6, 151.2, CV-50W potentiostat using a conventional three-electrode cell 127.0, 118.4, 71.7, 31.5, 19.4, 14.1. IR: n=966 cm-1 (TiNO); equipped with a glassy carbon disc working electrode (3 mm MS (LSI): an isotopic envelope peaking at m/z 1153.59 (100%) diameter), a platinum wire counter electrode and a silver wire {calc.for MH+ based on 48Ti, 1153.56}; Anal. Calc. for pseudo-reference electrode. Typically, experiments were per- C64H80N8O9Ti: C, 66.65; H, 6.99; N, 9.72. Found: C, 65.12; H, formed in CH2Cl2 containing 0.1 M [NBu4][PF6] at ambient temperature under a dry nitrogen atmosphere.The concen- 6.97; N, 9.48%. Table 3 Electrical properties of thin films of compounds 8 and 11 compound temp. range E1/eV temp. range E2/eV s303/V-1 cm-1 8 303–413 K 0.04 413–473 K 0.47 2.5×10-8 11 303–353 K 0.24 353–473 K 0.41 2.2×10-9 2066 J. Mater. Chem., 1997, 7(10), 2063–20678 T. J. Klofta, J.Danziger, P. Lee, J. Pankow, K. W. Nebesny and TiO[Nc¾(C6H13)8] 11 N. R. Armstrong, J. Phys. Chem., 1987, 91, 5646; H. Yanagi, A mixture of 2,3-dicyano-5,8-dihexylnaphthalene 6 (200 mg, S. Chen, P. A. Lee, K. W. Nebesny, N. R. Armstrong and A. Fujishima, J. Phys. Chem., 1996, 100, 5447. 0.58 mmol) and urea (18 mg, 0.30 mmol) was dissolved in n- 9 T. Harazono and I. Takagishi, Bull.Chem. Soc. Jpn., 1993, 66, 1016; pentanol (0.5 ml). Then titanium(IV) ethoxide (0.04 ml, K. Ogawa, J. Yao, H. Yonehara and C. Pac, J.Mater. Chem., 1996, 0.18 mmol) was introduced via a micropipette. The mixture 6, 143; J. Zhou, Y. Wang, J. Qiu, L. Cai, D. Ren and Z. Di, Chem. was refluxed under nitrogen for 20 h, then methanol (20 ml ) Commun., 1996, 2555. was added. After refluxing for a further 30 min, the mixture 10 (a) T.Kashima, Jpn. Kokai T okkyo Koho, JP 63 149 188, 1988 was cooled to room temperature and decanted. The black (Chem. Abstr., 1989, 110, P105174c); (b) JP 63 149 189, 1988 (Chem. Abstr., 1989, 110, P105175d); (c) P. Haisch, G. Winter, M. Hanack, residue was dissolved in hexanes (20 ml ) and precipitated with L. Lu� er, H.-J. Egelhaaf and D.Oelkrug, Adv. Mater., 1997, 9, 316. methanol (10 ml ). The solid was chromatographed with CHCl3 11 W.-F. Law, R. C. W. Liu, J. Jiang and D. K. P. Ng, Inorg. Chim. as eluent. The dark green band developed was collected and Acta, 1997, 256, 147. rotary evaporated to give a dark green solid which was dried 12 J. Yao, H. Yonehara and C. Pac, Bull. Chem. Soc. Jpn., 1995, 68, in vacuo. Yield 50 mg (24%). 1H NMR {CDCl3+ca. 0.6 M 1001. [2H5]pyridine, 4.0×10-3 M): d 9.42 (br s, 8 H, Nc¾H), 7.36 (s, 13 (a) A. R. Monahan, J. A. Brado and A. F. DeLuca, J. Phys. Chem., 1972, 76, 446; (b) A. W. Snow and N. L. Jarvis, J. Am. Chem. Soc., 8 H, Nc¾H), 3.29 (br s, 16 H, Nc¾CH2), 2.00 (br s, 16 H, CH2), 1984, 106, 4706. (c) Ot E. Sielcken, M. M. van Tilborg, M. F. M. 1.66 (br s, 16 H, CH2), 1.47 (br s, 32 H, CH2), 0.95 (t, J 6.9 Hz, Roks, R.Hendriks, W. Drenth and R. J. M. Nolte, J. Am. Chem. 24 H, CH3). IR: n=970 cm-1 (TiNO); MS (LSI): an isotopic Soc., 1987, 109, 4261; (d) W. J. Schutte, M. Sluyters-Rehbach and envelope peaking at m/z 1449.94 (100%) {calc. for M+ based J. H. Sluyters, J. Phys. Chem., 1993, 97, 6069. on 48Ti, 1449.91}; Anal. Calc.for C96H120N8OTi: C, 79.52; H, 14 N. Mataga, Bull. Chem. Soc. Jpn., 1957, 30, 375; S. Tai and 8.34; N, 7.73. Found: C, 77.74; H, 8.27; N, 7.31%. N. Hayashi, J. Chem. Soc., Perkin T rans. 2, 1991, 1275. 15 J. A. Dean, L ange’s Handbook of Chemistry, 14th edn., McGraw- Hill, New York, 1992. We thank The Chinese University of Hong Kong for support 16 D. S. Terekhov, K. J. M. Nolan, C.R. McArthur and C. C. LeznoV, (Direct Grant 94/95). J. Org. Chem., 1996, 61, 3034; C. C. LeznoV and D. M. Drew, Can. J. Chem., 1996, 74, 307. 17 A. B. P. Lever, E. R. Milaeva and G. Speier, in Phthalocyanines: References Properties and Applications, ed. C. C. LeznoV and A. B. P. Lever, VCH, New York, 1993, vol. 3, pp. 1–69. 1 K.-Y. Law, Chem. Rev., 1993, 93, 449. 18 M. J. Cook, J.Mater.Chem., 1996, 6, 677 and references therein. 2 T. N. Gerasimova and V. V. Shelkovnikov, Russ. Chem. Rev., 1992, 19 S. Hasegawa, M. Arai and Y. Kurata, J. Appl. Phys., 1992, 71, 1462; 61, 55. Y. Wu and A. Stesmans, J. Non-Cryst. Solids, 1987, 90, 151; 3 T. Tsuzuki, N. Hirota, N. Noma and Y. Shirota, T hin Solid Films, K. P. Chik and S. H. Fung, J. Non-Cryst. Solids, 1977, 24, 431; 1996, 273, 177; H. Yonehara and C. Pac, T hin Solid Films, 1996, B. A. Orlowski, W. E. Spicer and A. D. Baer, T hin Solid Films, 278, 108. 1976, 34, 31; A. J. Mountvala and G. Abowitz, Vacuum, 1965, 4 J. Mizuguchi, Cryst. Res. T echnol., 1981, 16, 695; H. J. Wagner, 15, 359. R. O. Loutfy and C. K. Hsiao, J.Mater. Sci., 1982, 17, 2781. 20 M. Hanack, A. Beck and H. Lehmann, Synthesis, 1987, 703; 5 G. A. Page, E. G. Tokoli, R. T. Cosgrove and J. W. Spiewak, US M. Hanack, P. Haisch, H. Lehmann and L. R. Subramanian, Pat. 4 557 868, 1985 (Chem. Abstr., 1986, 104, P131453p); Synthesis, 1993, 387. G. Liebermann, A. M. Hor and A. E. J. Toth, Eur. Pat. Appl., EP 21 M. J. Cook, A. J. Dunn, S. D. Howe, A. J. Thomson and 280 520, 1988 (Chem. Abstr., 1989, 110, P97171g). K. J. Harrison, J. Chem. Soc., Perkin T rans. 1, 1988, 2453. 6 W. Hiller, J. Stra�hle, W. Kobel and M. Hanack, Z. Kristallogr., 22 Y.-O. Yeung, R. C. W. Liu, W.-F. Law, P.-L. La. Jiang and 1982, 159, 173; O. Okada and M. L. Klein, J. Chem. Soc., Faraday D. K. P. Ng, T etrahedron, 1997, 53, 9087. T rans., 1996, 92, 2463. 7 J. Mizuguchi, G. Rihs and H. R. Karfunkel, J. Phys. Chem., 1995, 99, 16 217. Paper 7/02637D; Received 17th April, 1997 J. Mater. Chem., 1997, 7(10), 2063–2067 2067