Langmuir–Blodgett films of a tetrathiafulvalene derivative substituted with an azobenzene group Leonid M. Goldenberg,a,b† Martin R. Bryce,*‡a Stefan Wegener,a,b Michael C. Petty,*b John P. Cresswell,b Igor K. Lednev,c Ronald E. Hester*c and John N. Moorec aDepartment of Chemistry and Centre forMolecular Electronics, University of Durham, Durham, UK DH1 3L E bSchool of Engineering and Centre forMolecular Electronics, University of Durham, Durham, UK DH1 3L E cDepartment of Chemistry, University of York, Heslington, York, UK YO1 5DD The new tetrathiafulvalene derivative 1, functionalised with an azobenzene substituent, has been synthesised.Cyclic voltammetric and spectroelectrochemical studies in solution demonstrate the reversible formation of the radical cation of 1. UV–VIS spectroscopy suggests that there is a weak interaction between the TTF and azobenzene moieties in compound 1, and demonstrates that trans–cis isomerisation occurs upon photolysis of the azobenzene substituent.Semi-conducting LB films of 1 have been assembled without the need for added fatty acid: room temperature conductivity values of the films before and after doping with iodine vapour were srt=10-3–10-5 S cm-1 and 2×10-2–10-3 S cm-1, respectively.No change in the conductivity of the LB films was observed under irradiation. Tetrathiafulvalene (TTF) derivatives have attracted consider- layer at the air–water interface unless at least 50% mole ratio of a fatty acid was added,14 and a bis(EDT-TTF) derivative, able interest in recent years as a number of their crystalline cation-radical salts are molecular metals and superconduc- which did not require the addition of any fatty acid.15 The azobenzene group was incorporated into compound 1 tors.1–5 In order to achieve high conductivity in thin films,6 several amphiphilic analogues of TTF have been prepared and because azo derivatives are known to undergo photochemical cis–trans isomerisation in LB films and this reaction can their Langmuir–Blodgett (LB) films built up.These generally exhibit in-plane direct current (dc) room temperature conduc- be monitored electrochemically.16–19 Moreover, for pyridinium–TCNQ LB films, where the pyridinium moiety tivity values in the range srt=10-3–10-1 S cm-1 after formation of a mixed valence state by doping with iodine vapour,7,8 was substituted with an azobenzene derivative, Matsumoto et al.18 reported a 30% conductivity change upon photochem- although higher conductivities, srt=ca. 1 S cm-1, have been achieved with a few derivatives.9,10 ical isomerisation of the azo group in the LB film. A polypyrrole copolymer film, where polypyrrole was substituted with an In the present work we report on the properties of compound 1 in solution and in LB films.This compound is a novel non- azobenzene group, is reported to change its conductivity by up to 50% under illumination; however, the authors of this amphiphilic derivative of TTF. work concluded that the conductivity change was not triggered by isomerisation of the azo group.20 Experimental Synthesis Compound 1 was synthesised as follows.Equimolar quantities of 4-phenylazophenol (Aldrich) and triethylamine were dis- Based upon experimental and theoretical data for related solved in dichloromethane. After 10 min, a solution of tetrathi- TTF–C(O)R derivatives (R=OBu and NMe2)11 the electron- afulvalenecarbonyl chloride [TTF–C(O)Cl] (prepared from withdrawing ester group attached to the TTF ring in com- tetrathiafulvalene carboxylic acid,21 by a modification of the pound 1 should increase the polar nature of the TTF ring literature procedure)22 in dichloromethane was syringed into system, which presumably serves as the hydrophilic portion of the solution, and the mixture was stirred at room temp.for the molecule, while the aromatic rings replace the ‘traditional’ 12 h. The mixture was then acidified with 2 M aqueous HCl; hydrophobic alkyl chain(s).This present study is timely in the the organic layer was separated, washed with water and dried light of current interest in the formation of LB films of charge- (MgSO4). The solvent was evaporated in vacuo and the residue transfer materials which do not possess long alkyl chains. In was chromatographed on a neutral alumina column (eluent this context, we have recently reported the formation of dichloromethane) to aVord compound 1 as a purple solid in conductive LB films of three derivatives of ethylenedithio-TTF 51% yield, mp 148 °C (Found: C, 52.9; H, 3.0; N, 6.2.(EDT-TTF) bearing aromatic substituents (phenyl, pyridyl C19H12N2O2S4 requires: C, 53.2; H, 2.8; N, 6.5%); m/z (DCI) and pyridinium); for these compounds 25 mol% of fatty acid 429 (M++1); dH (200 MHz, CDCl3) 7.99 (2H, dd), 7.92 (2H, was needed to stabilise monolayer formation.12,13 Other dd), 7.63 (1H, s), 7.52 (3H, m), 7.33 (2H, dd) and 6.37 (2H, s).examples of non-amphiphilic TTF materials which form LB films are (PhCH2S)4TTF, which did not form a stable mono- Characterisation Cyclic voltammetry (CV) was performed using an EG&G † Visiting scientist from the Institute of Chemical Physics in PARC model 273 potentiostat with an Advanced Bryans XY Chernogolovka, Russian Academy of Sciences, Chernogolovka 142432, recorder. Pt mesh served as the counter electrode, a saturated Moscow Region, Russia. ‡ E-mail M.R.Bryce@durham.ac.uk calomel electrode (SCE) served as the reference electrode in J.Mater. Chem., 1997, 7(10), 2033–2037 2033HClO4 solution, and Ag wire as the quasi-reference electrode (Aldrich) in acetonitrile (ca. 10-5 M) (Aldrich, spectrometric grade) were contained in 10 mm cells and studied at 18 °C. A in acetonitrile solution. Potentials for solution CV were corrected to Ag/AgCl with the ferrocene/ferrocenium couple as xenon lamp (1 kW) with a glass bandpass filter centred at 350 nm was used for photolysis of the LB films.the internal reference (+0.35 V vs. Ag/AgCl). CV in solution was performed in 0.2 M Bu4NPF6–acetonitrile on a Pt disk working electrode (1.6 mm diameter, Bioanalytical System Inc.) Results and Discussion employing IR compensation. Bu4NPF6 (Fluka, electrochemical grade), HClO4 (Aldrich, ACS reagent), acetonitrile (Aldrich, Solution studies HPLC) and ultrapure water were used for preparation of the The cyclic voltammetry of compound 1 in acetonitrile solution electrolyte solutions.revealed two, reversible, one-electron waves, which are typical Spectroelectrochemistry was undertaken using a Perkinof TTF esters,11 at E1/2=+0.40 and +0.75 V, vs. Ag/AgCl. Elmer Lamda 19 spectrometer with a Ministat (Thomson These redox potentials are raised slightly relative to TTF Electrochem.Ltd, Newcastle upon Tyne, UK) using a 0.2 M under the same conditions (E1/2=0.34 and 0.71 V) by conju- Bu4NPF6–acetonitrile solution of compound 1. The spectrogation of the TTF system and the electron-withdrawing carelectrochemical cell was based on a 1 cm thick cuvette; Pt wire bonyl group.11 No reduction peak was observed between 0 was used as the counter electrode, while Ag wire (with a and -1.8 V in the cyclic voltammetric measurements where potential approximately equal to that of Ag/AgCl in this reduction of an azo group would be expected;16,17 this is solution) served as a quasi-reference.Thin layer electrodes consistent with a trans-azo group, which usually gives a very were constructed from indium tin oxide (ITO, sheet resistance broad, ill-defined reduction wave, whereas the cis-isomer gives 30 V per square, from Balzers) and glass with a ca. 100 mm a clearly observable peak.16 We have measured the spectroelecthick PTFE spacer. trochemistry of compound 1 in acetonitrile solution (Fig. 1). The Durham LB troughs were housed in a class 10 000 The appearance of a new absorption peak at lmax 430 nm microelectronics clean room and have been described prealong with a shoulder at lmax 580 nm, when the spectrum was viously.23 Compound 1 was spread on the surface of ultrapure obtained at +1.2 V, are consistent with the generation of the water (obtained by reverse osmosis, deionisation and ultraviolet cation radical of compound 1.27 (We note that the potentials sterilisation) from CH2Cl2 solutions (0.1 g l-1).The surface are shifted considerably with respect to the CV data reported pressure versus molecular area isotherm was measured at above, due to uncompensated resistance in the thin-layer 20±2 °C, pH=5.8±0.2 and a compression rate ca. spectroelectrochemical cell.) These spectroscopic changes were 4×10-3 nm2 molecule-1 s-1.The optimal dipping pressure reversible and the spectrum of compound 1 reverted to that was found to be 35 mN m-1. LB films were deposited onto of the neutral species measured initially when the potential glass slides, quartz, conducting ITO glass slides (sheet resistwas returned to 0 V. ance 300 V per square, from Balzers) and Au- and Ag-coated The optical absorption spectrum of compound 1 in acetoglass slides by the conventional vertical dipping technique. nitrile [Fig. 2(a)] is similar but not identical to a linear Unless specified otherwise, a dipping speed of 10 mm min-1 combination of the absorption spectra of TTF28 and transwas employed and the first monolayer was dipped on the azobenzene Fig. 2(d) (inset).This suggests that there is some upstroke when the slide was immersed in the subphase before interaction between the TTF and azobenzene groups, but the compression of the monolayer. To improve the hydrophilic small diVerences in comparison with those between azobenzene properties of ITO, the slides were pretreated with saturated and its disubstituted donor–acceptor derivatives29 indicates Na2Cr2O7–concentrated H2SO4 solution for approximately that this interaction is relatively weak.The diVerences between 10 s and carefully washed with ultrapure water.24 Substrates the spectra of compound 1 shown in Fig. 1 (0 V) and Fig. 2(a) with areas between 20 and 30 cm2 were used for LB film are due to absorption by the ITO thin layer electrode in the transfer. After LB film deposition, the slides were cut carefully former case.with a diamond tipped stylus to form several electrodes with Photolysis of 1 in acetonitrile resulted in changes in the contact areas between 0.1 and 0.5 cm2. Au electrodes with a absorption spectrum [Fig. 2(b,c)] which were similar to those gap of ca. 30 mm and interdigitated Au electrodes with a gap observed on photolysis of trans-azobenzene [Fig. 2(e,f,g)]. of 20 mm were used for electrochemical doping of the LB films. Photolysis of trans-azobenzene is known to result in trans–cis These electrodes were produced as previously described.25 photoisomerisation,30 and the changes in the spectra are Electrochemical doping during LB film transfer was achieved characteristic of this process. These changes were observed to on a KI subphase (0.1 M) with a current of 6 mA, using a persist for at least 1 h in the dark, indicating that cis-1, like similar procedure to that described earlier.25 Chemical doping of the LB films was carried out by exposure to iodine vapour for a given time in a sealed vessel.The dc conductivity data were obtained in air by a standard two-contact method using silver paste contacts.By varying the distance between the electrodes, it was established that the contact resistance was negligible. The conductivity values were calculated using a monolayer thickness of 1.5 nm (estimated from molecular modelling). The capacitance was measured by a Boonton Electronics model 72BD capacitance meter. The conductivity normal to the film surface was measured by using evaporated Au top contact dots (diameter 0.1 cm, slowly evaporated at rate of about 0.5–1.0 nm min-1) for films deposited on Au-coated glass slides. The Pockels electro-optic eVect was measured for monolayers deposited on Ag-coated glass using the technique of surface plasmon resonance at a wavelength of 633 nm.26 Optical absorption spectra of solutions were obtained using a Hitachi U-3000 spectrometer, and of LB films using a Perkin-Elmer Lamda 19 spectrophoto- Fig. 1 Optical absorption spectra of compound 1 in 0.2 M meter. A mercury lamp with a 314 nm interference filter was Bu4NPF6–acetonitrile solution using a thin layer electrode: measurements at (a) 0 and (b) +1.2 V vs. Ag wire used for photolysis in solution. Solutions of 1 or azobenzene 2034 J.Mater. Chem., 1997, 7(10), 2033–2037Fig. 2 Optical absorption spectra of compound 1 and azobenzene (inset) in acetonitrile solution. Spectra of 1 obtained: (a) before photolysis, (b) 3 and (c) 8 min after photolysis at 314 nm. Spectra of trans-azobenzene obtained: (d) before photolysis, (e) 1.5, ( f ) 4 and (g) 10 min after photolysis at 314 nm. Fig. 4 A model of an LB monolayer of compound 1; molecular geometry optimisation obtained using Chem3D for Macintosh cis-azobenzene,30 is relatively stable to thermal cis–trans iso- LB film characterisation merisation.Prolonged photolysis (>10 min) of the azobenzene The in-plane dc conductivities for as-deposited LB films of solution resulted in no further changes in the spectrum, indicatcompound 1 were in the range srt=10-3–10-5 S cm-1.After ing that Fig. 2(g) is the spectrum of the photostationary state doping with iodine vapour, the room temperature conductivity mixture. In contrast, prolonged photolysis of 1 resulted in value of each sample rose by ca. one order of magnitude to further changes in the spectrum for which the earlier isosbestic values of srt=2×10-2–10-3 S cm-1.For LB films with a top points were not maintained, indicating that additional photo- Au contact, ohmic current–voltage characteristics were chemical pathways are available to 1. observed. Normal to the film surface, conductivity values of srt=10-6 and 5×10-5 S cm-1 were measured for a 25-layer Monolayer behaviour of 1 on the air–water interface and LB LB film, before and after iodine doping, respectively.The film transfer capacitance of the same device was found to be 550±27 and The condensed pressure vs. area isotherm for compound 1 is 627±56 pF, for the as-deposited and doped films, respectively, shown in Fig. 3. This was reproducible and stable at the which correspond to permittivity values, er=2.9 and 3.3 (using deposition pressure and was not aVected by the time that the a film thickness of 1.5 nm per monolayer obtained from monolayer remained on the subphase before compression.31 molecular modelling studies).There was no evidence of collapse during compression of the It is well known that azo compounds undergo trans–cismonolayer up to the highest pressure measured (40 mN m-1). isomerisation under illumination, even as thin films, and that The extrapolated limiting area (to zero pressure) is 0.21 nm2 this can lead to a change in conductivity.16–19 However, molecule-1, which is slightly less than that expected for the illumination of a 15 layer LB film of 1 using either visible cross-sectional area of the molecule obtained from molecular light, a wide-range UV source, a sodium lamp or monochromodelling studies with geometry optimisation using Chem3D matic light at 320 nm, did not result in any detectable change for Macintosh.We have observed this previously in isotherms in the value of the lateral conductivity. This result suggests of TTF derivatives,32 and we suspect that this is simply due that for LB films of 1 either: (a) trans–cis-isomerisation does to a very slight solubility of the compound in the subphase.A not proceed due to steric hindrance in a compact LB film schematic representation of a possible close-packing arrangestructure, or (b) the isomerisation proceeds but this structural ment of molecules of 1 in the LB film structure is shown in change does not influence the conductivity of the film. Fig. 4 Fig. 4. LB films of compound 1 were built up by predominantly shows a geometry optimisation of an LB monolayer of 1 which Z-type deposition with a transfer ratio on the upstroke of indicates that the molecules are bent to aVord the experimen- 0.9±0.1.tally observed molecular area of 0.21 nm2. This tightly packed structure could imply that the first explanation is more plausible. We note that the LB films studied by Matsumoto et al.18 were assembled at a lower surface pressure than we used in the present work, and by using the horizontal touching technique.Both these experimental conditions may result in a less dense film structure which would allow isomerisation to proceed. However, the optical absorption spectra of LB films of compound 1 (Fig. 5) suggest that some trans–cis-isomerisation does occur upon photolysis: after 10 min irradiation there was a slight decrease in the intensity of the absorption peaks at 320 and 235 nm, and a slight increase in the absorption in the range 420–550 nm.These data are qualitatively similar to those of the solution spectra shown in Fig. 2, and, therefore, we favour explanation (b) above. We also attempted to increase the conductivity values of LB films of compound 1 by electrochemical oxidation either during or after LB film deposition.33 However, neither of these methods aVected the conductivity of the films, and a low Fig. 3 Pressure vs. area isotherm for compound 1 transfer ratio was observed in the former experiments. This J. Mater. Chem., 1997, 7(10), 2033–2037 2035Conclusions We have synthesised the new TTF derivative 1 and demonstrated that trans–cis isomerisation of the azobenzene substituent occurs upon photolysis.Semi-conducting LB films of 1 have been assembled without the need for added fatty acid: presumably, the TTF group (the polarity of which is increased by conjugation with the carbonyl substituent) is hydrophilic, and the azobenzene unit serves as the hydrophobic portion of the molecule, instead of the traditional alkyl chain(s).No change in the conductivity of the LB films was observed under irradiation. Further studies on LB films of non-amphiphilic TTF systems will be reported in due course. We are grateful to the EPSRC for an Advanced Fellowship to J.N.M. and for a research grant (S.W. and J.P.C.). L.M.G. Fig. 5 Optical absorption spectra for a six-layer LB film of compound thanks the Royal Society, the ERSPC, the Russian Foundation 1 on quartz: (a) before photolysis and (b) after photolysis for 10 min for Fundamental Research (project 97-03-32268a) and the University of Durham for financial support.References 1 J. R. Ferraro and J. M. Williams, Introduction to Synthetic Electrical Conductors, Academic Press, London, 1987. 2 M. R. Bryce, Chem. Soc. Rev., 1991, 20, 355. 3 A. E. Underhill, J.Mater. Chem., 1992, 2, 1. 4 J.M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geiser, H. H. Wang, A. M. Kini and M-H. Whangbo, Organic Superconductors (including Fullerenes) Prentice Hall, New Jersey, 1992. 5 J. Mater. Chem., Special Issue on Molecular Conductors, 1995, 5, 1469. 6 For a review of electronically conductive LB films see: M.R. Bryce and M. C. Petty, Nature 1995, 374, 771. 7 J. Richard, M. Vandevyver, A. Barraud, J. P. Morand, R. Lapouyade, P. Delhaes, J. F. Jacquinot and M. Roulliay, J. Chem. Soc., Chem. Commun., 1988, 754. Fig. 6 Cyclic voltammogram for a five-layer LB film of compound 1 8 L. M. Goldenberg, R. Andreu, M. Saviro� n, A. J. Moore, J.Garý�n, deposited on an ITO electrode, 0.2 M HClO4, scan rate 50 mV s-1 M. R. Bryce and M. C. Petty, J.Mater. Chem., 1995, 5, 1593. 9 A. S. Dhindsa, Y. P. Song, J. P. Badyal, M. R. Bryce, Y. M. Lvov, M. C. Petty and J. Yarwood, Chem. Mater., 1992, 4, 724. Table 1 Pockels eVect measurements 10 L. M. Goldenberg, V. Yu. Khodorkovsky, J. Y. Becker, P. J. Lukes, M. R. Bryce, M. C.Petty and J. Yarwood, Chem. Mater., 1994, compound thickness/nm x(2) (-v;v,0)/pm V-1 r/pm V-1 6, 1426. 11 A. S. Batsanov, M. R. Bryce, J. N. Heaton, A. J. Moore, 1 1.2 5.0 1.6 P. J. Skabara, J. A. K. Howard, E. Ortý�, P. M. Viruela and R. Viruela, J.Mater. Chem., 1995, 5, 1689. 12 L. M. Goldenberg, J. Y. Becker, O. Paz-Tal Levi, V. Yu. Khodorkovsky, M. R. Bryce and M. C. Petty, J.Chem. Soc., Chem. Commun., 1995, 475. may be explained by two factors: (a) hindered anion diVusion 13 L. M. Goldenberg, J. Y. Becker, O. Paz-Tal Levi, V. Yu. within the multilayer assembly, which is quite compact as Khodorkovsky, L. M. Shapiro, M. R. Bryce, J. R. Cresswell and judged by the molecular area obtained from the isotherm; (b) M. C. Petty, J. Mater. Chem., 1997, 7, 901.instability of the films upon application of an electrochemical 14 Y. Xiao, Z. Yao and D. Jin, L angmuir, 1994, 10, 1848. potential. The cyclic voltammetric response of LB films of 1 15 R. P. Parg, J. D. Kilburn, M. C. Petty, C. Pearson and T. G. Ryan, J.Mater. Chem., 1995, 5, 1609. was measured and the best response was obtained for a five- 16 Z. F. Liu, B. H. Loo, K. Hashimoto and A.Fujishima, layer film (Fig. 6). However, the electroactivity disappeared J. Electroanal. Chem., 1991, 297, 133. after a few cycles, which is consistent with film desorption. We 17 Z. F. Liu, K. Hashimoto and A. Fujishima, Faraday Discuss., 1992, consider, therefore, that (a) explains the results of attempted 94, 221. post-deposition electrochemical oxidation, and (b) aVects the 18 H.Tochibana, T. Nakamura, M. Matsumoto, H. Komizu, E. electrochemical doping during film deposition. Mauda, H. Niino, A. Yabe and Y. Kawabata, J. Am. Chem. Soc., 1989, 111, 3080. The Pockels electro-optic eVect was measured for mono- 19 For a review of organic switches based on azobenzene derivatives, layers of compound 1. The surface plasmon resonance method see: F. Vo� gtle, Supramolecular Chemistry, Wiley, Chichester, 1991, also allowed an estimate of the film thickness to be made by ch. 7. assuming a value of the permittivity (er=2.5 was used in this 20 S-A. Chen and C. S. Liao, Synth.Met., 1993, 57, 4950. case). The results are given in Table 1. In each case the 21 J. Garý�n, J. Orduna, S. Uriel, A. J. Moore, M. R. Bryce, S.Wegener, nonlinear optical r coeYcient is relatively small.This is likely D. S. Yufit and J. A. K. Howard, Synthesis, 1994, 489. For an eYcient synthesis of TTF see: A. J. Moore and M. R. Bryce, to be due to poor alignment of the molecules of 1, as the Synthesis, 1997, 407. chromophores themselves should have large values of hyperpo- 22 C. A. Panetta, J. Baghdadchi and R. Metzger, Mol. Cryst. L iq. larisability.The thickness obtained for a monolayer of 1 Cryst., 1984, 107, 103. (1.2 nm) is consistent with the value for the length of the 23 C. A. Jones, M. C. Petty, G. G. Roberts, G. Davies, J. Yarwood, molecule obtained from molecular modelling studies (1.5 nm, N. M. RatcliVe and J. W. Barton, T hin Solid Films, 1987, 155, 187. see above) and suggests that the film is one molecule in 24 Y.Fu, J. Ouyang and A. B. P. Lever, J. Phys. Chem., 1993, 97, 13753. thickness. 2036 J. Mater. Chem., 1997, 7(10), 2033–203725 L. M. Goldenberg, C. Pearson, M. R. Bryce and M. C. Petty, 31 It has been noted that the structure of multilayer films of amphiphilic metal(dmit)2 charge-transfer salts (metal=Ni, Pd, Pt) J.Mater. Chem., 1996, 6, 699. 26 G. H. Cross, I. R. Girling, I. R. Peterson and N. A. Cade, depends upon the time that the floating film is left on the subphase surface before compression. S. K. Gupta, D. M. Taylor, Electrooptics L ett., 1986, 22, 1111. 27 J. B. Torrance, B. A. Scott, B. Welber, F. B. Kaufman and P. E. P. Dynarowicz, E. Barlow, C. E. A. Wainwright and A. E. Underhill, L angmuir, 1992, 8, 3057; C. Pearson, A. S. Dhindsa, Seiden, Phys. Rev. B, 1979, 19, 730. 28 R. Dieing, V. Morisson, A. J. Moore, L. M. Goldenberg, M. R. L. M. Goldenberg, R. A. Singh, R. Dieing, A. J. Moore, M. R. Bryce and M. C. Petty, J. Mater. Chem., 1995, 5, 1601. However, Bryce, J-M. Raoul, M. C. Petty, J. Garý�n,M. Saviro�n, I. K. Lednev, R. E. Hester and J. N. Moore, J. Chem. Soc., Perkin T rans. 2, this is not usually observed with TTF derivatives. 32 A. S. Dhindsa, J. P.Badyal, M. R. Bryce, M. C. Petty, A. J. Moore 1996, 1587. 29 J. GriYths, Colour and Constitution of Organic Molecules, and Y. M. Lvov, J. Chem. Soc., Chem. Commun., 1990, 970. 33 B. Tieke, Adv. Mater., 1991, 2, 222. Academic Press, London, 1976. 30 H. Rau, in Photochromism.Molecules and Systems, ed. H. Durr and H. Bouas-Laurent, Elsevier, Amsterdam, 1990, ch. 4, p.165. Paper 7/02797D; Received 24tJ. Mater. Chem., 1997, 7(10), 2033–2037 2037