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Synthesis, characterization and electrochemical properties of an iron cyclopentadienyl complex of poly(n-hexylphenylene)
ELSEVIER
Synthetic Metals 74 (1995) 59-64
Synthesis, characterization and electrochemical properties of an iron cyclopentadienyl complex of poly ( n-hexylphenylene) Hiroshi Funaki, Kunitsugu Aramaki, Hiroshi Nishihara * PRESTO, JRDC and Department
of Chemistry, Faculty of Science and Technology, Keio University, 3-14-I Hiyoshi, Kohoku-ku, Yokohama 223, Japan Received
16 February
1995; revised 28
March 1995;accepted 10 April 1995
Abstract An iron cyclopentadienyl complex of a conducting polymer, poly (n-hexylphenylene) (abbreviated as PHP) , was prepared by a reaction of PHP with ferrocene, Al and AlCl, followed by an anion exchange using NH,PF,. Cyclic voltammetry of the complex thus formed with a formula, [ (C,HsC,H,,) ,,6[ ( $-C,H3C,H,3)Fe( $-C,H,) ]PF,), (abbreviated as PHP-[ FeCp] PF,, Cp = $-cyclopentadienyl) , has indicated that it is reduced with a moderate chemical reversibility in Bu,NBF,-THF at E?‘= - 1.7 V versus Ag/Ag+. Spectroelectrochemical measurements have suggested that the chemical reaction of the reduced form is a coupling reaction of phenylene rings coordinated to iron, resulting in a formation of insoluble thin network polymer film on the electrode surface. Photoluminescence of PHP is modified in intensity and maximum emission wavelength by coordinating the FeCp moiety. Keywords: Polyfluorene;
Iron complex; Electrochemistry;
Photoluminescence;
1. Introduction Our recent interest has focused on the modification of Tconjugated organic polymers [ 1] by electronic interaction with d-transition metals. We have previously reported the synthesis, characterization and physical properties of transition metal complexes of poly (alkylphenylene) s, poly (9-hexylfluorene) and poly( I-hexylindene) [ 2-51. For example, redox potential and electric conductivity of poly( n-hexylphenylene) (PHP) are changed by ligating to the molybdenum tricarbonyl units, whereas the bandgap energy of the polymer is scarcely altered [ 21. We report here the second example of PHP complexes, involving [ ($-arene)FeCp] + units (Cp = $-cyclopentadienyl) . Monomeric complexes of [ ( $-arene)FeCp] + are generally air-stable and reducible to form a neutral radical in aprotic solvents [6]. Theoretical and electrochemical studies of the [ (polyarene)FeCp] system have shown that an increase in the number of fused rings of a polyaromatic such as pyrene causes the shift in the singly occupied molecular orbital (SOMO) charge distribution from the Fe metal to the arenic ligand [ 71. These facts indicate that physical properties of 1 ($-arene)FeCp] + depend on the structure of arene and, instead, physical properties of PHP would be changed by coordinating to FeCp units. The main purpose of this study * Corresponding
author.
0379-6779/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI0379-6779(95)03345-K
Synthesis
is to understand the degree of difference in physical properties of the PHP- [ FeCp] + compared to free PHP or the corresponding monomeric complex, [ ($-n-hexylbenzene)FeCp] +. We describe synthesis, characterization and electrochemical properties of PHP- [ FeCp] +, together with photoluminescence of PHP and its MO and Fe complexes.
2. Experimental 2.1. Chemicals and equipment PHP in the undoped form [ 81 and PHP-Mo( CO), [ 21 were prepared electrochemically as reported previously. Tetrahydrofuran (THF) was refluxed over Na and benzophenone and distilled under nitrogen. Other anhydrous solvents were obtained from Kanto Chemicals Co., Inc. Other reagents were guaranteed reagent grade chemicals and used as received. ITO-coated glass with resistance less than 30 fi per square was purchased from Nippon Sheet Glass Co. Ltd. The glassy carbon (GC) rod and sheet used as the electrodes were Tokai Carbon GC-20. IR, UV-Vis, ‘H NMR, ESR and photoluminescence spectra were recorded with Shimadzu IT-IR 8 lOOM, Shimadzu MPS-2000, JEOL GX400, JES-TE 300 and Hitachi F-4.500 spectrometers, respectively.
60
H. Funaki et al. /Synthetic
Metals 74 (1995) 59-64
2.2. Preparation of PHP-[FeCpIPF,
A solution of PHP (480 mg), ferrocene (5.58 g), AlCl, (1.35 g) and aluminum powder ( 1.35 g) in cyclohexane (100 cm3) was refluxed with vigorous mechanical stirring for 18 h under nitrogen. After cooling the solution to room temperature, ice-cold water (20 cm3) was added slowly with stirring. Precipitates thus formed were filtered, washed thoroughly with water and cyclohexane, and extracted with methanol. After the solvent was evaporated under vacuum, the residue was dissolved again in 30 cm3 CH$&. The anion exchange from AlCL- to PF6- was carried out by mixing the solution with three portions of 50 cm3 saturated aqueous solution of NH4PF6 in a separating funnel. The CHICIZ layer separated and dried with Na2S04 was filtered, evaporated and dried under vacuum to give PHP- [ FeCp] PF, in the yield of 262 mg (34%). Anal. Calc. for (&Hr6) ,,6(C,7H2,F6FeP) : C, 63.70; H, 6.83. Found: C, 64.10; H, 6.32%. 2.3. Preparation of [(n-hexylbenzene)FeCp]PF, Fig. 1. IR spectra of PHP (a), [ ( $-hexylbenzene)FeCp]
To a stirred solution of ferrocene (5.58 g) , A1C13(20 g) and aluminum powder ( 1.35 g) in cyclohexane ( 100 dm3) was added slowly 4.86 g of n-hexylbenzene, which was then refluxed with mechanical stirring for 18 h under nitrogen. After deaerated ice-cold water was added to the mixture cooled on an ice bath, the solution was filtered. The aqueous layer of the filtrate was separated and washed three times with cyclohexane (50 cm3). Addition of NH.,PFs to the solution gave a yellow precipitate, which was filtered, washed with water and recrystallized from acetonitrile-ethyl ether to give [ (hexylbenzene)FeCp] PF, in the yield of 5.07 g (40%). ‘H NMR S: 6.17 (m, 5H, Ph), 4.96 (s, 5H, Cp), 2.72 (t, 2H, Ph-CH,-, J= 7.8 Hz), 1.62 (tt, 2H, Ph-CH,-CH,), 1.35 (m, 6H, -CH,-C,H,-CH,), 0.91 (t, 3H, -CH,). Anal. Calc. for C,,H,,F$eP: C, 47.70; H, 5.37. Found: C, 47.57; H, 5.37%.
PF, (b) and PHP-
[FeCplPF,(c). 3. Results aud discussion 3.1. Synthesis and characterization of PHP-[FeCp]PF,
The molecular weight of PHP samples employed in this study was 1700 (the polymerization degree is 11) based on the polystyrene standard. PHP- [ FeCp] PF, was prepared by a method similar to that for monomeric complexes of [ ($arene) FeCp] PF6 [ 91; that is, the treatment of PHP with ferrocene, A1C13and Al. PHP-[FeCp] PF, thus formed was soluble in dichloromethane, THF or methanol, but insoluble in hexane or cyclohexane. 1) Cp,Fe, Al, AU, cyclohexane.
2.4. Electrochemical measurements A 5 mm o.d. GC rod was embedded in a Pyrex glass and the cross section was used as a disk electrode. Cyclic voltammetry was carried out in a standard one-compartment cell equipped with a Pt wire counter electrode and an Ag/Ag+ (10 mm01 dmv3 AgClO, in 0.1 mol dm-3 Bu,NClO,MeCN, I?( ferrocenium/ferrocene) = 0.197 V versus Ag/ Ag+) reference electrode with a Toho Technical Research PS-07 polarization unit and a Riken Denshi F-35 X-Y plotter. Spectroelectrochemicalmeasurements were carried out in the same manner as reported previously [ 21. Deposition of a film from PHP- [ FeCp] PF, was carried out by consecutive potential scans between 0 and -2.2 V versus Ag/Ag+ at a GC sheet in 0.1 mol dm - 3Bu,NClO,-THF at 20 mV s - ’for 300 scans.
reflux,
18h
w
2) NH,PF,
R = n-C,H,, PHP
(1) n
PHP-[FeCp]PFb
IR spectra of free PHP, PHP-[FeCp]PF, and [ (n-hexylbenzene)FeCp] PF, are displayed in Fig. 1. The spectrum of the polymer complex shown in Fig. 1(c) gives peaks at 2954,
H. Funaki et al. /Synthetic Metals 74 (1995) 59-44
61
to the higher heterogeneity in the structure of the polymer complex. However, this slight change indicates that the bandgap energy is not significantly altered by the complexation, as has been observed for the polyphenylene-Mo( CO) 3 complex, PHP-Mo(CO), [2]. The Eg value roughly estimated from the hv versus (u+r~)~ plot is 2.72 eV, similar to the value of free PHP and PHP-Mo( CO) 3 [ 21. 3.2. Electrochemical properties ----__ , 3
300
500
700
9
Wavelength / nm
Fig. 2. UV-Vis spectraofPHP (a). [ ($-hexylbenzene)FeCp]PF, PHP-[FeCp]PF, (c, d).
(b) and
2928 and 2857 cm-’ due to v(C-H), at 1458 and 1420cm-’ due to v( C=C ) of aromatic rings, at 477 cm- ’due to v (FeC),and at 842 and 558 cm- ’ due to PF,-.This spectral pattern is quite similar to that of the hexylbenzene complex given in Fig. 1(b), indicating that the complex sites in the polymer are in the form of [ ( $-arene)FeCp] PF,. This structure was supported from the electrochemical properties (vide infra) . The ratio in absorbance of the peak at 2928 cm-’ due to v (C-H), to the one at 477 cm- 1due to v (Fe-C) indicates that the molar ratio of FeCp units to benzene rings is 1:2.6, based on the peak ratio for [ (n-hexylbenzene) FeCp] PF,. This estimation is in accord with the elemental analysis data. However, we could not obtain any ‘H NMR spectrum expected for the [ ( $-arene) FeCp] + structure; the spectrum showed no detectable sharp peaks in spite of careful purification and treatment of the sample. A sharp signal was observed at g = 2.003 with A EPP= 7.2 G in the ESR spectrum of the polymer complex powders. It is unlikely that the [ ( $arene)FeCp] + structure which is an l&electron system is paramagnetic even if the arene is highly r-conjugated, and thus the spectral results imply a contamination of paramagnetic species in the polymer chain. Although we have not specified the structure of the paramagnetic species, we tentatively conjecture that a [bis( $-dienyl)iron] ‘-type cationic StrUCtUre, such as [ ( q5-C6H,C6Hi3)FeCp] +, is a likely candidate, since it is analogous to the ferrocenium ion with relatively high stability to heat and oxygen. Very little contamination of such species could be effective in changing drastically the magnetic properties of r-conjugated polymers E51. UV-Vis spectra of free PHP, PHP- [FeCp]PF, and [ (nhexylbenzene)FeCp] PF6 are displayed in Fig. 2. The chargetransfer bands, which are seen at 400 and 450 nm for the hexylbenzene complex, are not clearly observed for the polymer complex because of the overlapping of a strong W-rr* transition band due to the polyphenylene ligand. The absorption edge in the long wavelength region is duller for the polymer complex compared to free PHP, corresponding
Cyclic voltammograms for reduction of PHP- [ FeCp] PF, and [ (n-hexylbenzene) FeCp] PF, at GC in Bu,NClO,-THF are shown in Fig. 3. It has been reported that [ (q6arene)FeCp] + undergoes one-electron reduction in aprotic media affording a neutral radical [ 61. This radical dimerizes in the solid state or in nonpolar solvents to give dicyclohexadienyls via intramolecular electron transfer from the metal to the arene ring when arene is not highly substituted [ 10,111. Both the voltammograms of the monomeric and polymeric complexes given in Fig. 3 exactly show the oneelectron reduction process at ,??‘I= ( Ep,=+ E,,J /2 = - 1.75 and - 1.7 V versus Ag/Ag+, respectively. The peak-to-peak separation, A EP = E,,= - EP,C,is 370 and 560 mV at 20 mV s-i for the monomeric and polymeric complexes, respectively, and increases with increasing scan rate, indicating that the heterogeneous electron transfer is rather slow. The broadness of the peak for the polymer might originate from the involvement of heterogeneous complex sites with different environments and/or a formation of mixed valence states giving complex waves due to an electronic interaction of complex sites via r-conjugated bonds [ 121. This electrochemical behavior indicates that the LUMO charge distributes mainly on the complex sites, and this orbital locates between valence and conduction bands of T-conjugated chains.
1
-2.5
I
I
I
-2.0
-1.5
-1.0
I
-0.5
E 1 V YS.Ag/Ag+
Fig. 3. Cyclic voltammograms of [ ( ~6-hexylbenzene)FeCplPF, (a) and PHP- [ FeCp] PF, (b) at a GC electrode in 0.1 mol dm - ’ Bu,NClO.,-THF. The numbers given in tbe figure denote scan rates in mV s-‘.
H. Funuki et al. /Synthetic Metals 74 (1995) 59-44
62
-O.lI
chains, forming a network polymer, that may cause formation of an insoluble film on the electrode surface. In fact, a thin film was deposited at a GC sheet electrode in Bu,NClO,THF by repeated potential scans through the reduction potential of the polymer complex. An IR spectrum of the deposited film is given in Fig. 5. The peaks appearing at 484,749 and 2926 cm-’ are assignable to v(FeC) , n-( CH) and V( CH) , respectively, and no peaks were observed due to anions such as PF6- or ClO,- . This implies that the complex sites are in the neutral form, supporting an occurrence of the coupling reaction given in Eq. (2)) while the spectrum could not be a definitive evidence of the $-hexadienyl complex form.
I
0.5
7do
500
300
900
1, (b)
\I
__---
-0.J 300
I
1
I
500 Wavelength
700 / nm
900
Fig. 4. Vis spectra for electrochemical reduction of [ ($‘-hexylbenzene)FeCp] PF, (a) and PHP- [FeCp] PF, (b) at an IT0 electrode in 0.1 mol dmw3 Bu,NC104-THF at given potentials. The numbers in the figure denote electrode potentials in V vs. Ag/Ag+.
Spectroelectrochemical measurements of the monomeric and polymeric complexes at IT0 in BudNC1O,-THF were carried out and the results are shown in Fig. 4. The spectroscopic measurements at given potentials were made after holding the potential until the current decreased to the background level (taking about 10 min) . The monomeric complex gives an increase of peaks at 420 and 450 nm. This can be assigned not to the neutral form of the monomer (A,, = 700 nm) [ 111 but to its dimerization product, the color of which was reported to be orange [ 131. The neutral radical form could not be detected in these measurements, taking a longer time than those of cyclic voltammetry. As shown in Fig. 4(b), the polymer complex also shows a similar change, probably due to the dimerization reaction, where the A_ value is 540 nm, longer than that of the hexylbenzene complex. The bands in both spectra are assignable to d-d transition since the dimeric form is a ferrocene-like structure [ 141. The dimerization reaction noted above would act to bridge the polymer
1
8OF
I
I
I
I
4000
2000
1500
1000
Wavenumbers
/cm-
I 400
1
Fig. 5. IR spectrum of a film deposited at a GC sheet electrode by electroreduction of PHP-[FeCp] PF, in 0.1 mol drnm3 Bu4NC10.,-THF (KBr disk).
Fe
.~_____
Ra____(2) Fe
3.3. Photoluminescence Photoluminescence spectra of PHP and its MO and Fe complexes in solution are shown in Figs. 6 and 7. The MO complex involves Mo( CO), units binding to about one-fifth of the phenylene rings on average [ 21. Photoluminescence is an important fundamental property of r-conjugated polymers [ 15 1, and polyphenylene derivatives have been reported to be effective electroluminescent materials [ 161. Figs. 6 and 7 denote that PHP and its complexes show photoluminescence in the visible region, and that both maximum excitation and fluorescence wavelength of PHP are modified by coordination to metals. This wavelength shift and also the fluorescence intensity decrease are more significant for PHF- [ FeCp] PF, than PHF-Mo( CO) 3. This is reasonable because it is generally known that transition metal r-cornplexes show little or no photoluminescence [ 171, and the concentration of complex units, which would act as quenching sites of photoluminescence, is higher for the Fe complex than the MO complex. As has been noted above, the energy level of the LUMO locating on complex sites is between the valence and conduction bands of the conjugated chain and, consequently, electrons generated by photo-irradiation can easily be transferred to the metal d-characterized orbital before recombining holes, giving a path of non-radiative
H. Funaki et al. /Synthetic
Metals 74 (1995) 59-64
63
decay. We have observed similar quenching effects of metal complex sites for polyfluorene complexes [ 51.
4. Conclusions We have described electrochemical properties and photoluminescence behaviors of PHP- [ FeCp] + which are different from those of free PHP. It can be concluded that these differences come from an engagement of the d-orbital of the metals in the LUMO, which locates between the energy levels of valence (or conduction) bands of rr-conjugated chains. This shows that the r-coordination to transition metals is a unique method to modify the electronic properties of g-conjugated organic polymers.
Acknowledgements
This work was partly supported by a Grant-in-Aid for Scientific Research No. 06226274 from the Ministry of Education, Science and Culture of Japan, and an Iketani Research Grant.
References [I] T.A. Skotheim (ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986; A.O. Patil, A.J. Heeger and F. Wudl, Chem. Rev., 88 (1988) 183; J.L. Bredas and G.B. Street,Acc. Chem. Res., 28 (1985) 309; H. Kuzmany, M. Mehring and S. Roth, Electronic Properties of Conjugated Polymers, Springer, Berlin, 3rd edn, 1989; S. Roth, Synth. Met., 34 (1989) 617; A.J. Heeger, Faraday Discuss. Chem. Sot.. 88 (1989) 203; S. Roth, H. Bleier and W. Pukacki, Faraday Discuss. Chem. Sot., 88 (1989) 223; A.G. MacDiarmid and A. Epstein, Faraday Discuss. Chem. Sot., 88 ( 1989) 317; A.J. Heeger, Synth. Met., 55-57 (1993) 3471. [2] H. Funaki, K. Aramaki and H. Nishihara, Chem. Lett., (1992) 2065. [ 31 H. Nishibara, H. Funaki, T. Shimura and K. Aramaki, Synch. Met., 5%
Fig. 6. Excitation/emission spectraof photoluminescence for PHP (a), PHPMo(CO), (b) and PHP-[FeCp]PF, (c) in CH,CI,.
57 (1993) 942. [4] H. Nishihara, J. Matsuda, T. Hirao and K. Aramaki, Synth. Met., 69 (1995) 551. [5] J. Matsuda, K. Aramaki and H. Nishihara, J. Chem. Sot.. Faraday Trans., 91 (1995) 1477. [6] W.E. Silverthom, Inorg. Chem., 18 (1979) 1835; A.N. Nesmeyanov, L.I. Denisovich, S.P. Gubin, N.A. Volkenau, E.I. Sirotkina and I.N. Bolesova, J. Organomet. Chem., 210 (1981) 103; D. Astruc and R. Dabard, Bull. Sot. Chim. Fr., (1976) 228; P. Michaud, D. Astruc and J.H. Ammeter, J. Am. Chem. Sot., 104 (1982) 3755. [7] M. Lacoste, H. Rabaa, D. Astruc, A. Le Beuse, J.-Y. Saillard, G. Preeigoux, C. Courseille, N. Ardoin and W. Bowyer, Organometallics, 8 (1989) 2233. [8] T. Shimura, H. Funaki, H. Nishihara, K. Aramaki, T. Ohsawa and K.
Wavelength
/ nm
Fig. 7. Photoluminescence spectra of PHP (a), PHP-Mo(CO), (b) and PHP- [ FeCp] PF6 (c) in CH& at excitation wavelengths of 400 (a), 340 (b) and 520 nm (c).
Yoshino, Chem. L&t., (1992) 457. [9] R.B. King, Organometallic Syntheses, Vol. 1, Academic Press, New York, 1965, p.-138. [lo A.N. Nesmeyanov, S.P. Solodovnikov, N.A. Volkenau, L.S. Kotova and N.N. Sinitsyna, J. Organomet. Chem., 148 ( 1978) C5; D. Astruc, J.-R. Hamon, G. Althoff, E. Roman, P. Batail, P. Michaud, J.-P. Mariot, F. Varret and D. Cozak, J: Am. Chem. Sot., 101 (1979) 544.5. [ll J.R. Hamon, D. Astruc and P. Michaud. J. Am. Chem. Sot., IO3 ( 198 1) 758.
64
H. Funaki et al. /Synthetic Metals 74 (1995) 59-64
[ 121 K. Aoki and J. Chen, J. Electroanal. Chem., 380 (1995) 35. [ 131 W.H. Morrison, Jr., E.Y. Ho and D.N. Hendrickson, Inorg. C/rem., 14 (1975) 500. [ 141 Y.S. Sohn, D.N. Hendrickson and H.B. Gray, J. Am. Chem. Sot., 93 (1971) 3063. [15] A.B. Holmes, D.D.C. Bradley, A.R. Brown, P.L. Bum, J.H. Burroughes, R.H. Friend, N.C. Greenham, R.W. Gymer, D.A.
Hall&y, R.W. Jackson, A. Kraft, J.H.F. Martens, K. Pichler and I.D.W. Samuel, Synth. Met., 55-57 (1993) 4031 and Refs. therein. [ 161 G. Grem,G.Leditzky,B.UllrichandG.Leising,Au’v. Mater.,4 (1992) 3621. [17] D. Scott and R.S. Becker, J. Chem. Phys., 35 (1961) 516; J. Smith and B. Meyer, J. Chem. Phys., 48 (1968) 5436.
Synthetic Metals 74 (1995) 59-64
Synthesis, characterization and electrochemical properties of an iron cyclopentadienyl complex of poly ( n-hexylphenylene) Hiroshi Funaki, Kunitsugu Aramaki, Hiroshi Nishihara * PRESTO, JRDC and Department
of Chemistry, Faculty of Science and Technology, Keio University, 3-14-I Hiyoshi, Kohoku-ku, Yokohama 223, Japan Received
16 February
1995; revised 28
March 1995;accepted 10 April 1995
Abstract An iron cyclopentadienyl complex of a conducting polymer, poly (n-hexylphenylene) (abbreviated as PHP) , was prepared by a reaction of PHP with ferrocene, Al and AlCl, followed by an anion exchange using NH,PF,. Cyclic voltammetry of the complex thus formed with a formula, [ (C,HsC,H,,) ,,6[ ( $-C,H3C,H,3)Fe( $-C,H,) ]PF,), (abbreviated as PHP-[ FeCp] PF,, Cp = $-cyclopentadienyl) , has indicated that it is reduced with a moderate chemical reversibility in Bu,NBF,-THF at E?‘= - 1.7 V versus Ag/Ag+. Spectroelectrochemical measurements have suggested that the chemical reaction of the reduced form is a coupling reaction of phenylene rings coordinated to iron, resulting in a formation of insoluble thin network polymer film on the electrode surface. Photoluminescence of PHP is modified in intensity and maximum emission wavelength by coordinating the FeCp moiety. Keywords: Polyfluorene;
Iron complex; Electrochemistry;
Photoluminescence;
1. Introduction Our recent interest has focused on the modification of Tconjugated organic polymers [ 1] by electronic interaction with d-transition metals. We have previously reported the synthesis, characterization and physical properties of transition metal complexes of poly (alkylphenylene) s, poly (9-hexylfluorene) and poly( I-hexylindene) [ 2-51. For example, redox potential and electric conductivity of poly( n-hexylphenylene) (PHP) are changed by ligating to the molybdenum tricarbonyl units, whereas the bandgap energy of the polymer is scarcely altered [ 21. We report here the second example of PHP complexes, involving [ ($-arene)FeCp] + units (Cp = $-cyclopentadienyl) . Monomeric complexes of [ ( $-arene)FeCp] + are generally air-stable and reducible to form a neutral radical in aprotic solvents [6]. Theoretical and electrochemical studies of the [ (polyarene)FeCp] system have shown that an increase in the number of fused rings of a polyaromatic such as pyrene causes the shift in the singly occupied molecular orbital (SOMO) charge distribution from the Fe metal to the arenic ligand [ 71. These facts indicate that physical properties of 1 ($-arene)FeCp] + depend on the structure of arene and, instead, physical properties of PHP would be changed by coordinating to FeCp units. The main purpose of this study * Corresponding
author.
0379-6779/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI0379-6779(95)03345-K
Synthesis
is to understand the degree of difference in physical properties of the PHP- [ FeCp] + compared to free PHP or the corresponding monomeric complex, [ ($-n-hexylbenzene)FeCp] +. We describe synthesis, characterization and electrochemical properties of PHP- [ FeCp] +, together with photoluminescence of PHP and its MO and Fe complexes.
2. Experimental 2.1. Chemicals and equipment PHP in the undoped form [ 81 and PHP-Mo( CO), [ 21 were prepared electrochemically as reported previously. Tetrahydrofuran (THF) was refluxed over Na and benzophenone and distilled under nitrogen. Other anhydrous solvents were obtained from Kanto Chemicals Co., Inc. Other reagents were guaranteed reagent grade chemicals and used as received. ITO-coated glass with resistance less than 30 fi per square was purchased from Nippon Sheet Glass Co. Ltd. The glassy carbon (GC) rod and sheet used as the electrodes were Tokai Carbon GC-20. IR, UV-Vis, ‘H NMR, ESR and photoluminescence spectra were recorded with Shimadzu IT-IR 8 lOOM, Shimadzu MPS-2000, JEOL GX400, JES-TE 300 and Hitachi F-4.500 spectrometers, respectively.
60
H. Funaki et al. /Synthetic
Metals 74 (1995) 59-64
2.2. Preparation of PHP-[FeCpIPF,
A solution of PHP (480 mg), ferrocene (5.58 g), AlCl, (1.35 g) and aluminum powder ( 1.35 g) in cyclohexane (100 cm3) was refluxed with vigorous mechanical stirring for 18 h under nitrogen. After cooling the solution to room temperature, ice-cold water (20 cm3) was added slowly with stirring. Precipitates thus formed were filtered, washed thoroughly with water and cyclohexane, and extracted with methanol. After the solvent was evaporated under vacuum, the residue was dissolved again in 30 cm3 CH$&. The anion exchange from AlCL- to PF6- was carried out by mixing the solution with three portions of 50 cm3 saturated aqueous solution of NH4PF6 in a separating funnel. The CHICIZ layer separated and dried with Na2S04 was filtered, evaporated and dried under vacuum to give PHP- [ FeCp] PF, in the yield of 262 mg (34%). Anal. Calc. for (&Hr6) ,,6(C,7H2,F6FeP) : C, 63.70; H, 6.83. Found: C, 64.10; H, 6.32%. 2.3. Preparation of [(n-hexylbenzene)FeCp]PF, Fig. 1. IR spectra of PHP (a), [ ( $-hexylbenzene)FeCp]
To a stirred solution of ferrocene (5.58 g) , A1C13(20 g) and aluminum powder ( 1.35 g) in cyclohexane ( 100 dm3) was added slowly 4.86 g of n-hexylbenzene, which was then refluxed with mechanical stirring for 18 h under nitrogen. After deaerated ice-cold water was added to the mixture cooled on an ice bath, the solution was filtered. The aqueous layer of the filtrate was separated and washed three times with cyclohexane (50 cm3). Addition of NH.,PFs to the solution gave a yellow precipitate, which was filtered, washed with water and recrystallized from acetonitrile-ethyl ether to give [ (hexylbenzene)FeCp] PF, in the yield of 5.07 g (40%). ‘H NMR S: 6.17 (m, 5H, Ph), 4.96 (s, 5H, Cp), 2.72 (t, 2H, Ph-CH,-, J= 7.8 Hz), 1.62 (tt, 2H, Ph-CH,-CH,), 1.35 (m, 6H, -CH,-C,H,-CH,), 0.91 (t, 3H, -CH,). Anal. Calc. for C,,H,,F$eP: C, 47.70; H, 5.37. Found: C, 47.57; H, 5.37%.
PF, (b) and PHP-
[FeCplPF,(c). 3. Results aud discussion 3.1. Synthesis and characterization of PHP-[FeCp]PF,
The molecular weight of PHP samples employed in this study was 1700 (the polymerization degree is 11) based on the polystyrene standard. PHP- [ FeCp] PF, was prepared by a method similar to that for monomeric complexes of [ ($arene) FeCp] PF6 [ 91; that is, the treatment of PHP with ferrocene, A1C13and Al. PHP-[FeCp] PF, thus formed was soluble in dichloromethane, THF or methanol, but insoluble in hexane or cyclohexane. 1) Cp,Fe, Al, AU, cyclohexane.
2.4. Electrochemical measurements A 5 mm o.d. GC rod was embedded in a Pyrex glass and the cross section was used as a disk electrode. Cyclic voltammetry was carried out in a standard one-compartment cell equipped with a Pt wire counter electrode and an Ag/Ag+ (10 mm01 dmv3 AgClO, in 0.1 mol dm-3 Bu,NClO,MeCN, I?( ferrocenium/ferrocene) = 0.197 V versus Ag/ Ag+) reference electrode with a Toho Technical Research PS-07 polarization unit and a Riken Denshi F-35 X-Y plotter. Spectroelectrochemicalmeasurements were carried out in the same manner as reported previously [ 21. Deposition of a film from PHP- [ FeCp] PF, was carried out by consecutive potential scans between 0 and -2.2 V versus Ag/Ag+ at a GC sheet in 0.1 mol dm - 3Bu,NClO,-THF at 20 mV s - ’for 300 scans.
reflux,
18h
w
2) NH,PF,
R = n-C,H,, PHP
(1) n
PHP-[FeCp]PFb
IR spectra of free PHP, PHP-[FeCp]PF, and [ (n-hexylbenzene)FeCp] PF, are displayed in Fig. 1. The spectrum of the polymer complex shown in Fig. 1(c) gives peaks at 2954,
H. Funaki et al. /Synthetic Metals 74 (1995) 59-44
61
to the higher heterogeneity in the structure of the polymer complex. However, this slight change indicates that the bandgap energy is not significantly altered by the complexation, as has been observed for the polyphenylene-Mo( CO) 3 complex, PHP-Mo(CO), [2]. The Eg value roughly estimated from the hv versus (u+r~)~ plot is 2.72 eV, similar to the value of free PHP and PHP-Mo( CO) 3 [ 21. 3.2. Electrochemical properties ----__ , 3
300
500
700
9
Wavelength / nm
Fig. 2. UV-Vis spectraofPHP (a). [ ($-hexylbenzene)FeCp]PF, PHP-[FeCp]PF, (c, d).
(b) and
2928 and 2857 cm-’ due to v(C-H), at 1458 and 1420cm-’ due to v( C=C ) of aromatic rings, at 477 cm- ’due to v (FeC),and at 842 and 558 cm- ’ due to PF,-.This spectral pattern is quite similar to that of the hexylbenzene complex given in Fig. 1(b), indicating that the complex sites in the polymer are in the form of [ ( $-arene)FeCp] PF,. This structure was supported from the electrochemical properties (vide infra) . The ratio in absorbance of the peak at 2928 cm-’ due to v (C-H), to the one at 477 cm- 1due to v (Fe-C) indicates that the molar ratio of FeCp units to benzene rings is 1:2.6, based on the peak ratio for [ (n-hexylbenzene) FeCp] PF,. This estimation is in accord with the elemental analysis data. However, we could not obtain any ‘H NMR spectrum expected for the [ ( $-arene) FeCp] + structure; the spectrum showed no detectable sharp peaks in spite of careful purification and treatment of the sample. A sharp signal was observed at g = 2.003 with A EPP= 7.2 G in the ESR spectrum of the polymer complex powders. It is unlikely that the [ ( $arene)FeCp] + structure which is an l&electron system is paramagnetic even if the arene is highly r-conjugated, and thus the spectral results imply a contamination of paramagnetic species in the polymer chain. Although we have not specified the structure of the paramagnetic species, we tentatively conjecture that a [bis( $-dienyl)iron] ‘-type cationic StrUCtUre, such as [ ( q5-C6H,C6Hi3)FeCp] +, is a likely candidate, since it is analogous to the ferrocenium ion with relatively high stability to heat and oxygen. Very little contamination of such species could be effective in changing drastically the magnetic properties of r-conjugated polymers E51. UV-Vis spectra of free PHP, PHP- [FeCp]PF, and [ (nhexylbenzene)FeCp] PF6 are displayed in Fig. 2. The chargetransfer bands, which are seen at 400 and 450 nm for the hexylbenzene complex, are not clearly observed for the polymer complex because of the overlapping of a strong W-rr* transition band due to the polyphenylene ligand. The absorption edge in the long wavelength region is duller for the polymer complex compared to free PHP, corresponding
Cyclic voltammograms for reduction of PHP- [ FeCp] PF, and [ (n-hexylbenzene) FeCp] PF, at GC in Bu,NClO,-THF are shown in Fig. 3. It has been reported that [ (q6arene)FeCp] + undergoes one-electron reduction in aprotic media affording a neutral radical [ 61. This radical dimerizes in the solid state or in nonpolar solvents to give dicyclohexadienyls via intramolecular electron transfer from the metal to the arene ring when arene is not highly substituted [ 10,111. Both the voltammograms of the monomeric and polymeric complexes given in Fig. 3 exactly show the oneelectron reduction process at ,??‘I= ( Ep,=+ E,,J /2 = - 1.75 and - 1.7 V versus Ag/Ag+, respectively. The peak-to-peak separation, A EP = E,,= - EP,C,is 370 and 560 mV at 20 mV s-i for the monomeric and polymeric complexes, respectively, and increases with increasing scan rate, indicating that the heterogeneous electron transfer is rather slow. The broadness of the peak for the polymer might originate from the involvement of heterogeneous complex sites with different environments and/or a formation of mixed valence states giving complex waves due to an electronic interaction of complex sites via r-conjugated bonds [ 121. This electrochemical behavior indicates that the LUMO charge distributes mainly on the complex sites, and this orbital locates between valence and conduction bands of T-conjugated chains.
1
-2.5
I
I
I
-2.0
-1.5
-1.0
I
-0.5
E 1 V YS.Ag/Ag+
Fig. 3. Cyclic voltammograms of [ ( ~6-hexylbenzene)FeCplPF, (a) and PHP- [ FeCp] PF, (b) at a GC electrode in 0.1 mol dm - ’ Bu,NClO.,-THF. The numbers given in tbe figure denote scan rates in mV s-‘.
H. Funuki et al. /Synthetic Metals 74 (1995) 59-44
62
-O.lI
chains, forming a network polymer, that may cause formation of an insoluble film on the electrode surface. In fact, a thin film was deposited at a GC sheet electrode in Bu,NClO,THF by repeated potential scans through the reduction potential of the polymer complex. An IR spectrum of the deposited film is given in Fig. 5. The peaks appearing at 484,749 and 2926 cm-’ are assignable to v(FeC) , n-( CH) and V( CH) , respectively, and no peaks were observed due to anions such as PF6- or ClO,- . This implies that the complex sites are in the neutral form, supporting an occurrence of the coupling reaction given in Eq. (2)) while the spectrum could not be a definitive evidence of the $-hexadienyl complex form.
I
0.5
7do
500
300
900
1, (b)
\I
__---
-0.J 300
I
1
I
500 Wavelength
700 / nm
900
Fig. 4. Vis spectra for electrochemical reduction of [ ($‘-hexylbenzene)FeCp] PF, (a) and PHP- [FeCp] PF, (b) at an IT0 electrode in 0.1 mol dmw3 Bu,NC104-THF at given potentials. The numbers in the figure denote electrode potentials in V vs. Ag/Ag+.
Spectroelectrochemical measurements of the monomeric and polymeric complexes at IT0 in BudNC1O,-THF were carried out and the results are shown in Fig. 4. The spectroscopic measurements at given potentials were made after holding the potential until the current decreased to the background level (taking about 10 min) . The monomeric complex gives an increase of peaks at 420 and 450 nm. This can be assigned not to the neutral form of the monomer (A,, = 700 nm) [ 111 but to its dimerization product, the color of which was reported to be orange [ 131. The neutral radical form could not be detected in these measurements, taking a longer time than those of cyclic voltammetry. As shown in Fig. 4(b), the polymer complex also shows a similar change, probably due to the dimerization reaction, where the A_ value is 540 nm, longer than that of the hexylbenzene complex. The bands in both spectra are assignable to d-d transition since the dimeric form is a ferrocene-like structure [ 141. The dimerization reaction noted above would act to bridge the polymer
1
8OF
I
I
I
I
4000
2000
1500
1000
Wavenumbers
/cm-
I 400
1
Fig. 5. IR spectrum of a film deposited at a GC sheet electrode by electroreduction of PHP-[FeCp] PF, in 0.1 mol drnm3 Bu4NC10.,-THF (KBr disk).
Fe
.~_____
Ra____(2) Fe
3.3. Photoluminescence Photoluminescence spectra of PHP and its MO and Fe complexes in solution are shown in Figs. 6 and 7. The MO complex involves Mo( CO), units binding to about one-fifth of the phenylene rings on average [ 21. Photoluminescence is an important fundamental property of r-conjugated polymers [ 15 1, and polyphenylene derivatives have been reported to be effective electroluminescent materials [ 161. Figs. 6 and 7 denote that PHP and its complexes show photoluminescence in the visible region, and that both maximum excitation and fluorescence wavelength of PHP are modified by coordination to metals. This wavelength shift and also the fluorescence intensity decrease are more significant for PHF- [ FeCp] PF, than PHF-Mo( CO) 3. This is reasonable because it is generally known that transition metal r-cornplexes show little or no photoluminescence [ 171, and the concentration of complex units, which would act as quenching sites of photoluminescence, is higher for the Fe complex than the MO complex. As has been noted above, the energy level of the LUMO locating on complex sites is between the valence and conduction bands of the conjugated chain and, consequently, electrons generated by photo-irradiation can easily be transferred to the metal d-characterized orbital before recombining holes, giving a path of non-radiative
H. Funaki et al. /Synthetic
Metals 74 (1995) 59-64
63
decay. We have observed similar quenching effects of metal complex sites for polyfluorene complexes [ 51.
4. Conclusions We have described electrochemical properties and photoluminescence behaviors of PHP- [ FeCp] + which are different from those of free PHP. It can be concluded that these differences come from an engagement of the d-orbital of the metals in the LUMO, which locates between the energy levels of valence (or conduction) bands of rr-conjugated chains. This shows that the r-coordination to transition metals is a unique method to modify the electronic properties of g-conjugated organic polymers.
Acknowledgements
This work was partly supported by a Grant-in-Aid for Scientific Research No. 06226274 from the Ministry of Education, Science and Culture of Japan, and an Iketani Research Grant.
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Fig. 6. Excitation/emission spectraof photoluminescence for PHP (a), PHPMo(CO), (b) and PHP-[FeCp]PF, (c) in CH,CI,.
57 (1993) 942. [4] H. Nishihara, J. Matsuda, T. Hirao and K. Aramaki, Synth. Met., 69 (1995) 551. [5] J. Matsuda, K. Aramaki and H. Nishihara, J. Chem. Sot.. Faraday Trans., 91 (1995) 1477. [6] W.E. Silverthom, Inorg. Chem., 18 (1979) 1835; A.N. Nesmeyanov, L.I. Denisovich, S.P. Gubin, N.A. Volkenau, E.I. Sirotkina and I.N. Bolesova, J. Organomet. Chem., 210 (1981) 103; D. Astruc and R. Dabard, Bull. Sot. Chim. Fr., (1976) 228; P. Michaud, D. Astruc and J.H. Ammeter, J. Am. Chem. Sot., 104 (1982) 3755. [7] M. Lacoste, H. Rabaa, D. Astruc, A. Le Beuse, J.-Y. Saillard, G. Preeigoux, C. Courseille, N. Ardoin and W. Bowyer, Organometallics, 8 (1989) 2233. [8] T. Shimura, H. Funaki, H. Nishihara, K. Aramaki, T. Ohsawa and K.
Wavelength
/ nm
Fig. 7. Photoluminescence spectra of PHP (a), PHP-Mo(CO), (b) and PHP- [ FeCp] PF6 (c) in CH& at excitation wavelengths of 400 (a), 340 (b) and 520 nm (c).
Yoshino, Chem. L&t., (1992) 457. [9] R.B. King, Organometallic Syntheses, Vol. 1, Academic Press, New York, 1965, p.-138. [lo A.N. Nesmeyanov, S.P. Solodovnikov, N.A. Volkenau, L.S. Kotova and N.N. Sinitsyna, J. Organomet. Chem., 148 ( 1978) C5; D. Astruc, J.-R. Hamon, G. Althoff, E. Roman, P. Batail, P. Michaud, J.-P. Mariot, F. Varret and D. Cozak, J: Am. Chem. Sot., 101 (1979) 544.5. [ll J.R. Hamon, D. Astruc and P. Michaud. J. Am. Chem. Sot., IO3 ( 198 1) 758.
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[ 121 K. Aoki and J. Chen, J. Electroanal. Chem., 380 (1995) 35. [ 131 W.H. Morrison, Jr., E.Y. Ho and D.N. Hendrickson, Inorg. C/rem., 14 (1975) 500. [ 141 Y.S. Sohn, D.N. Hendrickson and H.B. Gray, J. Am. Chem. Sot., 93 (1971) 3063. [15] A.B. Holmes, D.D.C. Bradley, A.R. Brown, P.L. Bum, J.H. Burroughes, R.H. Friend, N.C. Greenham, R.W. Gymer, D.A.
Hall&y, R.W. Jackson, A. Kraft, J.H.F. Martens, K. Pichler and I.D.W. Samuel, Synth. Met., 55-57 (1993) 4031 and Refs. therein. [ 161 G. Grem,G.Leditzky,B.UllrichandG.Leising,Au’v. Mater.,4 (1992) 3621. [17] D. Scott and R.S. Becker, J. Chem. Phys., 35 (1961) 516; J. Smith and B. Meyer, J. Chem. Phys., 48 (1968) 5436.