J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 321-325 32 1 Electrochemical Synthesis of Soluble Poly(9-hexylfluorene) and Poly(1-hexylindene) Junzo Matsuda, Kunitsugu Aramaki and Hiroshi Nishihara" Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan Electrochemical oxidation of 9-hexylfluorene and 1-hexylindene in 0.1 mol dm-, Bu,NBF,-nitromethane affords n-conjugated polymers, PHF and PHI, respectively, which are fusible and soluble in common organic solvents. Electrode materials affect the formation of the polymers significantly; the molecular weight of PHF increases in the order, glassy carbon (GC) < indium-tin oxide (ITO) < SnO,, and that for PHI, SnO, % GC < ITO. 'H NMR and IR spectra of the polymers indicate that PHF and PHI comprise mainly linkage of 1,4-fluorenylene and 2,4-indenylene units, respectively.PHF undergoes a reversible oxidation reaction in acetonitrile, whereas electro- chemical oxidation of PHI is irreversible. The electrical conductivities of PHF and PHI doped with SO, are and S cm-', respectively, at room temperature. Recent advances in the processing of linearly n-conjugated conducting polymers have enlarged their application to areas such as composite materials, devices, sensors (for some recent reviews, see ref. 1). One method to improve the solubility of these polymers in common organic solvents is by the intro- duction of long alkyl chains to the polymer backbones., We have recently reported an electrochemical method to prepare soluble p~ly(n-alkylphenylene)s,~and their utilization as n-acid ligands for the synthesis of n-conjugated organometallic polymer^.^.' Our greatest interest in such n-conjugated organometallic polymers centres around the modification of the physical and chemical properties of the n-conjugated systems by electronic interactions with d elec- trons and/or vacant d orbitals of the transition metal centres.The present study deals with soluble polymers of 9-hexylfluorene and 1-hexylindene, PHF and PHI, respectively, which could act as either 9'-or q6-n-coordinating ligands,6 and might also be of use in homogeneous synthetic reactions. Poly(9-alkylfluorene) and poly(9,9-dialkylfluorene) have been prepared by a chemical method using FeC1,,7 and their properties such as electroluminescence has been studied by Yoshino and co-workers.8 We here employ an electrochemical method for synthesis of PHF and PHI, whose structures are then determined based on FTIR and 'H NMR spectra.The redox and electrical properties of the polymers are also reported. Experimental Chemicals and Equipment Anhydrous solvents were obtained from Kanto Chemicals Co., Inc. Other reagents were guaranteed reagent-grade chemicals and used as received. IT0 and Sn0,-coated glasses with resistance <30 R were purchased from Nippon Sheet Glass Co. Ltd. Tokai Carbon GC-20 glassy carbon plates were used as electrodes. Bulk electrolysis was carried out with a Toho Technical Research 2001 potentiostat/ galvanostat and a 3320 coulometer.Infrared, UV-VIS and 'H NMR spectra were recorded with Shimadzu FTIR8100M, MPS-2000 and JEOL GX400 spectrometers, respectively. The molecular weights of the polymers were determined by gel permeation chromatography (GPC) with a Shimadzu LC-5A system based on polystyrene standards. Spin coating was carried out with a Kyowa K-359s-1 spinner, and the film thickness was measured with a Tokyo Seimitsu Surfcom 900A surface profilometer. Cyclic voltammetry was carried out in a standard one-compartment cell equipped with a Pt wire counter-electrode and an Ag/Ag+ [lo mmol dm-, AgClO, in 0.1 mol dm-, Bu,NBF,-MeCN, Eo (ferrocenium/ ferrocene) = 0.197 V us. Ag/Ag+] reference electrode with a Toho Technical Research PS-07 polarization unit and a Riken Denshi F-35 X-Y plotter.Spectroelectrochemical mea- surements were carried out in the same manner as has been reported previo~sly.~.~ The electrical conductivity of the polymer films was measured by the two-probe method under vacuum using a Toho Technical Research 2020 potentlostat/ galvanos tat. Preparation of 9-Hexylfluorene and 1-Hexylindene Introduction of hexyl groups to fluorene and indene was carried out by deprotonation with Bu"Li in THF followed by reaction with hexyl bromide according to the method reported by Cedheim and Eberson," and the product was purified by distillation under reduced pressure. 9-Hexylfluorene, v,,,/cm -: 3065s, 304Om, 301 7m, 2928s, 2857s, 1913w, 1908w, 1802w, 1607w, 1582w, 1478m, 1449s, 1377w, 1321w, 1296w, 1102w, 1030w, 936w, 739s, 662m, 621m, 426w and 415w (neat); 6, (CD,Cl,) 7.7-7.1 [8 H, m, H(1)-(8) of ring], 3.89 [l H, t, JHH 5.9 Hz, H(9) of ring], 1.96 (2 H, m, ring-CH,), 1.16 (8 H, m, CH,C,H,CH3), 0.76 (3 H, t, JHH = 5.7 Hz, CH3).1-Hexylindene, v,,Jcm- ': 3065m, 2928s, 2857s, 1609w, 1460s, 1449s, 1377m, 1364m, 1069w, 1019m, 936m, 774s, 741m, 712m, 436w and 405w (neat); 6, (CDCl,) 7.2-6.9 [4 H, m, H(4)-(7) of ring], 6-61 [1 H, dd, HH(I)H(3) Hz, JH(2)H(3) 5.7 Hz, H(3) of ring], 6.36 [1 H, dd, JH(,)H(,) 1.8 Hz, H(2) of ring], 3.27 (2 H, m, ring-CH,), 1.12 (8 H, m, CH2C4H8CH3), 0.69 (3 H, t, JHH = 5.8 Hz, CH3). Preparation of Soluble PHF and PHI A typical procedure for the synthesis of PHF is as follows.Controlled potential electrolysis of 2.7 mmol 9-hexylfluorene was carried out at an Sn0,-coated glass electrode (electrode area: 5.60 cm') in 0.1 mol dmP3 Bu,NBF4-nitromethane (150 cm3) at 2.5 V us. Ag/Ag+ for 30 h (1378 C of electricity were passed) in a standard H-type two-compartment cell, fol- lowed by electrochemical dedoping at -1.0 V until the cathodic current decreased to the background level. The elec- trode surface was covered with some fragile film, but most of the products were dissolved in solution. The contents of the cell were collected using dichloromethane and the solution thus obtained was concentrated into a volume of 10 cm3. Addition of a hydrazine-MeOH mixture to this solution yielded dark-brown precipitates, which were collected by fil- tration and washed with distilled water and MeOH.After drying, the solid products were extracted with dichloro-methane for 2 days. Solvents were evaporated from the extract to give a dark-brown solid, PHF, in a yield of 0.81 g (23% based on the current passed). Elemental analysis: (Found: C, 90.75; H, 7.64%. C19,,HZOn requires C, 91.95: H, 8.05%). As for PHI, similar procedures were used to obtain soluble products. Elemental analysis of PHI prepared at ITO: (Found: C, 87.09; H, 8.85, N, 1.52%. ClSnHIBnrequires C, 90.92; H, 9.08%). Results and Discussion Electrochemical Synthesis of PHI and PHF We have employed Bu,NBF,-nitromethane as the electrolyte solution for the polymerization of 9-hexylfluorene and 1-hexylindene, since solvents with low donor numbers have been known to be effective for the electro-oxidative poly- merization of benzene and its derivative^.^.' Cyclic voltam- mograms of hexylfluorene at GC, SnO, and ITO, fluorene at IT0 and hexylindene at IT0 are displayed in Fig.1. In the first cycle, the anodic current for the oxidation of hexylfluo- rene increases from ca. 1.2 V us. Ag/Ag+ at every electrode, and its magnitude at GC is larger than that at SnO, or ITO. During the backward scan, a cathodic wave appears at 0.8 V, and the corresponding anodic wave is seen at 1.2 V in the second cycle. This suggests the formation of an electroactive polymer," whereas growth of these waves with the number of cyclic scans is considerably less significant than that for unsubstituted fluorene [cf.Fig. l(c) and (41. This is due to the higher solubility of the hexyl-substituted polyfluorene than the unsubstituted one. It has been observed visually that the dark-brown products formed from hexylfluorene at the electrode surface diffuse into the solution. As shown in Fig. l(e), oxidation of hexylindene takes place at a potential cu. 1.5 V us. Ag/Ag+ more positive than that of hexylfluorene. Coloration of the solution around the elec- trode is also observed, as in the case of hexylfluorene, but no redox waves appear in the cyclic voltammograms in the fol- lowing potential scans. This indicates that the electrochemi- cally formed polymer, PHI, in the doped (oxidized) form, is electrochemically inactive in the potential range, 0-2.5 V.Controlled-potential electrolysis of hexylfluorene and hexy- lindene was carried out for the bulk syntheses of PHF and PHI, respectively. The concentration of the monomers was 0.2 mol dm-3 and the applied potential was 2.5 V us. Ag/Ag+. Soluble components in the products were dedoped electrochemically and then chemically with hydrazine, and extracted with dichloromethane as described in the Experi- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1 I I I I I 0 0.5 1.0 1.5 2.0 2.5 EN vs. Ag/Ag + Fig. 1 Cyclic voltammograms of 9-hexylfluorene at GC (a), SnO, (b) and IT0 (c), fluorene at IT0 (6)and l-hexylindene at IT0 (e) in 0.1 mol dm-3 Bu,NBF,-nitromethane at 0.1 V s-'.S = 5, 2, 2, 5 and 1 mA for (a), (b), (c), (d) and (e), respectively. Numbers in the figure refer to those of the cyclic scans. Electrode area, 1.4 cm2. mental. Both the PHF and PHI thus obtained are soluble in organic solvents of low polarity such as chloroform, benzene or tetrahydrofuran, but insoluble in polar solvents such as acetonitrile or propylene carbonate. Results of the synthesis for the three kinds of electrode materials are given in Table 1. In the electrolysis of hexylfluorene, the initial current was high but the current decay with time was rapid at GC, resulting in a smaller total amount of electricity passed during the electrolysis compared with that for IT0 and SnO, . The electrode material also influences the physical properties of the soluble products: the average molecular weight estimated based on polystyrene standards and the Table 1 Dependence of yield and physical properties of PHF and PHI on the electrochemical conditions" form at polymer electrode electricity/(= yield (%)b room temperature Mw PHF GC 1210 14 solid 1250 PHF SnO, 1330 13 solid 2730 PHF IT0 1380 23 solid 2540 PHI GC 1220 13 solid 2680 PHI PHI IROSnO, 1340 1440 20 3 solidliquid 4560 < lo00 ~~ Electrolysis was carried out at a 5.6 cmz area plate electrode in 0.1 mol dmV3 Bu,NBF,-nitromethane monomer at 2.5 V us.Ag/Ag+ for 30 h. Based on electricity passed during electrolysis. mp/"C E,,IeV E,zIeV 130 3.31 3.26 160 2.97 2.85 160 3.06 2.83 120 3.06 - - - - 160 2.99 - ~~ (150 cm-3) containing 2.7 mmol of the J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 melting point are higher for the products at SnO, and IT0 than those at GC. The effects of the electrode materials on the preparation and properties of soluble PHI are different from those for PHF. The soluble product obtained at SnO, is oily and of low molecular weight, whereas the product at IT0 is a film- processable solid with a molecular weight of 4560. The glassy carbon electrode affords a solid product with a lower molecu- lar weight and lower melting point than that obtained at ITO. Such significant effects of the electrode materials on the electrosynthesis of n-conjugated polymers have been reported for several cases, but the reasons have not yet been wholly clarified.12*13 One likely rationale might be that the regiosel- ectivity of the polymerization (coupling) reaction depends on the electrode material.Even a slight formation of non-conjugated products in the coupling reaction would decrease the conductivity of the product seriously, resulting in termi- nation of the polymerization. We speculate that the differ- ences in the effects of electrode materials for PHF and PHI are related to the discrepancies in their structures, since both the effect of electrode material and the structure for PHF are similar to those for poly(hexylpheny1ene) but different from those for PHI (vide infr~).~ Characterization of PHF and PHI The chemical structures of PHF and PHI were characterized by their infrared and 'H NMR spectra, which are shown in Fig. 2 and 3.Absorption peaks due to the out-of-plane CH deformation appear at 818, 768 and 737 cm-I in the infrared spectrum of PHF. This indicates the existence of benzene rings with four adjacent and two adjacent free hydrogen atom^,'^ and thus connection of the fluorene rings at the 1,4- positions in the polymerization, as shown in Scheme 1, can be elucidated. This structure is inconsistent with that of poly(9-alkylfluorene) reported by Fukuda et aE., where fluor- ene moieties are connected at the 2,7-positions.' This might be attributed to the different oxidation method used for the alkylfluorene. The chemical oxidation with FeCl, employed by Fukuda et al.may proceed by way of charge-transfer com- I I I I I I0 2000 1500 1000 50( wavenumber/cm-I Fig. 2 Infrared spectra of PHI (a)and PHF (b) 323 87654321 I I I I I I I 1 87654321 6 Fig. 3 'H NMR spectra of PHF (a) and PHI (b)in CD,CI, plexes with the aromatic rings, leading to a different type of monomer coupling from that of the electrochemical oxidation carried out in this study. The 'H NMR spectrum of PHF in Fig. 3(a) shows a peak due to the proton in the five-membered ring at S = 3.9-4.2 ppm, but gives insufficient information on the linkage structure of the fluorenylene moi- eties because of the broadness of the peaks. The infrared spectrum of PHI shows a broad and strong band around 740 cm- ' due to the out-of-plane CH deforma- tion, indicative of benzene rings with three or four adjacent free hydrogen atoms.The former case denotes that the coup- ling of the indene proceeds at either the 4-or 7-position in the six-membered ring and a position in the five-membered ring. The latter indicates coupling at two positions in the five- membered ring. In the 'H NMR spectrum of PHI, peaks due to the proton at the 3-position around 6.8 ppm exist but those due to the proton at the 2-position around 6.5 ppm disappear.' This indicates that the coupling occurs at the 2-position, and thus the possible positions for polymerization are limited to the 2,4- or 2,7-positions. Between these two possibilities, only poly(2,4-indenylene) is n-conjugated.As the polymer formed is semiconducting as described below, it is concluded that poly(2,4-indenylene) is the most likely struc- ture from the spectroscopic data. + 2.5 V W. AglA# c Bu4NBF4-nitromethane PHF Bu4NBF4-n#ranethane+ 2.5 V vs. Ag/Ag+ -6 C6H13 C6H13 PHI Scheme 1 324 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1.01 240 300 400 500 600 700 wavelength/nm Fig. 4 UV-VIS absorption spectra of the polymers prepared at ITO: (a) PHF in CH,Cl, (9.8 mg drn-,), (b) PHF coated on quartz, (c) PHI in CH,Cl, (6.1 mg drn-,) and (d)PHI coated on quartz Physical Properties The UV-VIS spectra of PHF and PHI prepared at ITO, dis- solved in dichloromethane and in the film form, are displayed in Fig.4. The peak positions of the films are similar to those of the solutions. The band gap values were estimated from the absorption edge using an equation for inorganic semicon- ductors,16 and the results for the polymers prepared at three electrodes are listed as E,, in Table 1. Note that the higher molecular weight polymer has lower band gap, probably owing to the increase in the conjugation length. The values obtained, 2.9-3.3 eV, are similar to those reported for poly(fluorene) derivatives by Fukuda et aL7 Cyclic voltammograms of PHF spin-coated on ITO-coated glass electrodes are shown in Fig. 5(a) where oxidation re- I I I I 0 0.5 1 .o 1.6 E/V vs. AglAg+ Fig. 5 Cyclic voltammograms of PHF (a) and PHI (b) spin-coatedon IT0 in 0.1 mol dm-3 Bu4NBF4-MeCN.The film thickness was 0.5 pm for PHF and 0.1 pm for PHI. Numbers indicate the scan rates in V. waveleng th/nm Fig. 6 Visible absorption spectra of PHF/ITO (film thickness 0.5 pm) at the potentials: (a) 0.2, (b) 0.8, (c) 1.0, (d)1.1, (e) 1.2 and (f) 1.4 V vs. Ag/Ag+ reduction waves appear around 1.0 V us. Ag/Ag+. Fig. 6 dis-plays changes in the visible spectrum of PHF with potential shift in the positive direction from 0.2 V us. Ag/Ag+. An increase in absorbance above 460 nm and a decrease at 400 nm owing to the formation of the polaronic state are seen at potentials more positive than 1.0 V, and the change saturates at ca. 1.2 V. The band gap energy was estimated from the isosbestic point appearing in the UV-VIS spectra with poten- 71 ** m O (b) m 0 m a 0 (d) 0 00 0 no O 0 no 0 0 0 I n 00 0 0 103KIT Fig.7 Arrhenius plots of electrical conductivity for undoped PHF (a),SO,-doped PHF(b), undoped PHI(c)SO,-doped PHI(d) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 tial shift and the results are listed as Eg2 in Table 1. The discrepancy between E,, and E,,, is within 0.23 eV, and is most likely derived from the considerable distribution in con- jugation length and structural disorder in PHF. The cyclic voltammogram of PHI in acetonitrile shows only an irreversible oxidation wave at 1.3 V us. Ag/Ag+ as displayed in Fig.5(b). In the course of this cyclic voltam- metry, the PHI film was not dissolved in acetonitrile nor detached from the electrode, and thus this electrochemical irreversibility should be caused by chemical changes in PHI. The irreversibility is in accordance with the cyclic voltam- mogram for the formation of PHI given in Fig. 1; i.e. the oxidized form is not electroactive in the potential region 0-1.6 V us. Ag/Agf. This difference between the electro- chemical behaviour of PHI and that of PHF or poly(he~y1phenylene)~might be related to the dissimilarity in the structure as described above. Note that the undoped PHI was obtained by electrochemical and chemical reduction of PHI in the doped form generated under electro-polymerization conditions. Since the electrochemical dedop- ing is almost ineffective as described above, reduction with hydrazine is the important process for making undoped PHI.The usefulness of the hydrazine treatment to give a perfectly undoped form of polyaniline has also been reported.17 Arrhenius plots of the electrical conductivity of PHF and PHI undoped and doped with SO, measured by the two- probe method under vacuum are shown in Fig. 7. The con- ductivity of the undoped form is S cm-' at 60°C, and increases greatly upon SO,-doping for both polymers. This indicates that PHI can be doped chemically, and therefore PHI should have a n-conjugated structure as noted above. Doping mehods for obtaining higher conductivities for PHI and PHF are currently under investigation.Conclusion Electrochemical oxidation of 9-hexylfluorene in Bu,NBF,-nitromethane gives poly(9-hexyl-1,4-fluorenylene), which is electroactive and soluble in common organic sol- vents. Electrochemical oxidation of 1-hexylindene affords soluble poly( l-hexyl-2,4-indenylene),of which the electro-chemical oxidation is irreversible. The physical properties of the polymers such as the molecular weight or the melting point depend markedly on the electrode materials used for the preparation. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan (no. 056406059), the Ogasawara Foundation for the Promotion of Science and Engineering, and the General Sekiyu Research and Development Encouragement and Assistance Foundation.References 1 Handbook of Conducting Polymers, ed. T. A. Skotheim, Marcel Dekker, New York, 1986; A. 0.Patil, A. J. Heeger and F. Wudl, Chem. Rev., 1988, 88, 183; J. L. Bredas and G. B. Street, Acc. Chem. Res., 1985, 18, 309; H. Kumany, M. Mehring and S. Roth, Electronic Properties of Conjugated Polymers, Springer, Heidelberg, 3rd edn, 1989; S. Roth, Synth. Met., 1989, 34, 617; A. J. Heeger, Faraday Discuss. Chem. SOC., 1989, 88, 203; S. Roth, H. Bleier and W. Pukacki, Faraday Discuss. Chem. SOC., 1989, 88, 223; A. G. MacDiarmid and A. Epstein, Faraday Discuss. Chem. SOC., 1989, 88, 317; A. J. Heeger, Synth. Met., 1993,57,347 1. 2 M. Sato, S. Tanaka and K. Kaeriyama, J.Chem. SOC., Chem. Commun., 1986, 873; R. L. Elsenbaumer, K. U. Jen and R. Oboode, Synth. Met., 1986, 15, 169; R. Sugimoto, S. Takeda, H. B. Gu and K. Yoshino, Chem. Express, 1986, 1, 635; M. R. Bryce, A. Chissel, P. Kathirgamanathan, D. Parker and N. R. M. Smith, J. Chem. SOC., Chem. Commun., 1987,466. 3 T. Shimura, H. Funaki, H. Nishihara, K. Aramaki, T. Ohsawa and K. Yoshino, Chem. Lett., 1992,457. 4 H. Hunaki, K. Aramaki and H. Nishihara, Chem. Lett., 1992, 2065. 5 H. Nishihara, H. Funaki, T. Shimura and K. Aramaki, Synth. Met., 1993,55,942. 6 W. E. Watts, in Comprehensive Organometallic Chemistry, ed. G. Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon, Oxford, 1982, vol. 8, p. 1013. 7 M. Fukuda, K. Sawada and K. Yoshino, Jpn. J. Appl. Phys., 1989,28, L1433. 8 Y. Ohmori, M. Uchida, K. Muro and K. Yoshino, Jpn. J. Appl. Phys., 1991, 30,L1941; M. Uchida, Y. Phmori, C. Morishima and K. Yoshino, Synth. Met., 1993,57,4168. 9 H. Nishihara, M. Noguchi and K. Aramaki, Inorg. Chem., 1987, 26,2862. 10 L. Cedheim and L. Eberson, Synthesis, 1973, 159. 11 T. Ohsawa, H. Nishihara, K. Aramaki, S. Takeda and K. Yoshino, Polym. Commun., 1987,28, 628. 12 H. Nishihara, H. Harada, K. Ohashi and K. Aramaki, J. Chem. SOC.,Faraday Trans., 1991,87, 1187. 13 H. Nishihara, M. Tateishi, K. Aramaki, T. Ohsawa and 0. Kimura, Chem. Lett., 1987, 1064. 14 L. J. Bellamy, The Infrared Spectra of Complex Molecules, Chapman and Hall, London, 1975. 15 E. Pretsch, J. Seibl, W. Simon and T. Clerc, Tabellen zur Strukturaujklarung Organischer Verbindungen mit Spektroskopis- chen Methoden, Springer, Berlin, 1981. 16 I. Kudmar and T. Seidel, J. Appl. Phys., 1962,33, 771. 17 T. Ohsawa, H. Nishihara, K. Aramaki and K. Yoshino, Chem. Lett., 1991, 1707. Paper 3/04614A; Received 2nd August, 1993