Dual photoluminescence of polythiophene thin films

SVflTH|TIC |TRLS ELSEVIER

Synthetic Metals 95 (1998) 107-112

Dual photoluminescence of polythiophene thin films Tadatake Sato, Mamoru Fujitsuka 1, Hiroshi Segawa 2, Takeo Shimidzu, Kazuyoshi Tanaka * Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan Received 29 September 1997; received in revised form 2 March 1998; accepted 5 March 1998

Abstract

Transient photoluminescence of electrochemically prepared thin films of polybithiophene and polyterthiophene was analyzed. It was observed for the first time that time-integrated spectra exhibited dual photoluminescence. In the time interval right after the excitation, a shortlifetime emission corresponding to the bandgap energy was observed, whose lifetime was estimated to be several tens of ps. Moreover, another emission of smaller energy than the bandgap was observed in the following time interval. This emission had relatively longer lifetime: several hundred ps. These results indicated that at least two types of excitons were formed in the films. Both of these could be the self-trapped excitons generated at the recombination sites with different local environments. Polyterthiophene contains many microcrystalline regions in comparison with polybithiophene. Enhancement of the longer-lifetime photoluminescence (PL) component in polyterthiophene suggests that the exciton with longer lifetime is generated in the microcrystalline region. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Polythiophene; Photophysics; Photoluminescence

1. Introduction

Photophysical processes in conjugated polymers have been widely studied both experimentally and theoretically in the last decade. The overall theoretical view was described for conjugated polymers with both the degenerated and the nondegenerated ground states [ 1 ]. Although such theoretical view has explicitly rationalized many experimental results, there still remains some open question in the interpretation of the experimental results possibly because of the structural ambiguity characteristic to the actual polymeric system. Moreover, some unexpected phenomena not predicted by the conventional theoretical model have been observed as pointed out by Kanner et al. [2]. Polythiophene has been adopted as the representative of the conjugated polymers with non-degenerated ground state, since it has a simple structure and remarkable stability in the air [3]. In polythiophene, photoluminescence (PL) was observed at the absorption-edge energy upon photoexcitation [4,5]. PL has also been observed in other conjugated polymers such as polyacetylene [6], polydiacetylene [2,7], poly(p-phenylene) [8], and poly(p-pbenylenevinylene) * Corresponding author. Tel.: +81 75 753 5923; fax: +81 75 771 0172. Present address: Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan. 2 Present address: Department of Chemistry, College of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-0041, Japan. 0379-6779/98/$19.000 1998 Elsevier Science S.A. All rights reserved. PIIS0379-6779 ( 9 8 ) 0 0 0 4 0 - X

(PPV) [9-11 ]. In all cases, the PL bands have been observed at the absorption-edge energy; the self-trapped exciton formed in a single chain has been assigned to the origin of the PL [ 12,13]. Among the polymers listed above, the exciton in PPV and its derivatives has extensively been studied. This exciton can be formed from two oppositely charged polarons injected from two electrodes placed at the both sides of the film in conjugated-polymer electroluminescent devices [ 14,15]. The relaxation mechanism of the exciton has been studied in association with modification of its efficiency. Measurement of PL decay with the time-integrated spectra provides an opportunity to probe their decay mechanism. For polythiophene electrochemically prepared, it has been reported that the lifetime of its PL was very short. For instance, Wong et al. [7] have reported that the lifetime of PL was shorter than 9 ps. In a recent study [2], the dominant photoexcited species in a short time, e.g., time interval up to 3 ns, was assigned to the self-trapped exciton, which decayed via radiative and nonradiative recombinations, and partially separated into a charged polaron pair. The PL has been observed to decay within several hundred ps and the observed PL decay dynamics was non-exponential. In general, the PL decay curves obtained for conjugated polymers are non-exponential. The origin of this non-exponential decay is still controversial. We have already pointed out that the morphological factor affects the relaxation process of the photoluminescent exci-

108

T. Sato et al. / Synthetic Metals 95 (1998) 107-112

tons in polybithiophene films [ 16]. In the present study, the transient PL spectra were investigated on the thin films of polybithiophene and polyterthiophene electrochemically prepared. It is noted that homogeneous thin film of electrochemically polymerized thiophene could not be obtained under similar potentiostatic conditions applied for bithiophene and terthiophene and, hence, thiophene was not employed for the starting material. In both polybithiophene and polyterthiophene, different time-integrated PL spectra were observed in two distinct time intervals. Such dual PL has not been observed hitherto. These results indicate that at least two types of excitons were generated, which should be the excitons formed at the recombination sites with different local environment.

2. Experimental

2.1. Chemicals 2,2'-Bithiophene (BT) was purchased from Aldrich and purified by recrystallization from hot methanol. 2,2':5',2"Terthiophene (TT) (Aldrich) was used as purchased. LiC104 and CH3CN were of the best commercial grade available and used without further purification.

from ITO glass. ITO glass shows no notable absorption peaks in the present wavenumber region. X-ray diffraction (XRD) patterns were recorded with a Rigaku Geigerflex diffractometer using Cu Kct radiation (35 mW-20 mA). In these measurements, thick film prepared under the same conditions as that for the films for PL measurements was employed. In order to estimate accurately their crystallinity, these films were peeled off from the ITO glass and then fixed on a CaF2 single-crystal plate by Pt mesh.

2.3. Optical measurements Steady-state PL spectra were recorded under vacuum by pumping with the 488 nm line of a cw argon ion laser (American Laser Corp. model 60C, 10.6 W c m - 2 ) . Collected emission was detected by a multi-channel photodiode array system (Otsuka Electronics Co., Ltd., MCPD110A). Transient PL was measured with a picosecond two-dimensional singlephoton counting laser flash photolysis system (C4334 streakscope, Hamamatsu Photonics). The sample films were excited by the second-harmonic pulse ( h = 4 0 0 nm, fwhm = 200 ps) of a mode-locked Ti:sapphire laser (SpectraPhysics, Tsunami) pumped by an argon-ion laser (SpectraPhysics, BeamLok). All the measurements were carried out under ambient conditions.

2.2. Preparation and characterization of sample films The sample films were prepared by the electrochemical polymerization of BT or TT. The polymerization was carried out in CH3CN solution containing 0.1 M of LiC104 and 10 mM of BT or TT. The indium-tin oxide (ITO) glass electrode was adopted for the working electrode; this was made to rotate at 1000 rpm during the polymerization so as to obtain fiat and homogeneous thin film [ 17]. The Pt wire and the saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The electrolyte solutions were degassed by N2 bubbling before the polymerization. The polymerization was carried out under potentiostatic conditions at room temperature. In order to suppress any side reactions such as crosslinking, the potentials for polymerization were set to 1.1 and 0.87 V (versus SCE) for BT and TT, respectively. Each of these potentials was the same as that for the onset of the anodic current for their oxidation. Thickness of the films was controlled by the electric charge supplied during the polymerization; films of about 1000 thickness were used in the present study. All the sample films were completely reduced by the successive electrochemical treatment in the CH3CN solution containing only LiCIO4 (0.1 M), and then applied to the optical measurements. Thickness of the films was measured by a Sloan Dektak 3 surface profiler. The Fourier transform infrared (FT-IR) attenuated total reflection (ATR) spectra were measured with a BIO-RAD FTS-30 spectrometer with a horizontal ATR accessory using a ZnSe crystal. The prepared polythiophene films were applied to the FT-IR ATR measurement without removing

3. Results

3.1. Characterization of polybithiophene and polyterthiophene thin films The FT-IR ATR spectra of thin films of polybithiophene (PBT) and polyterthiophene (PTT) used in PL measurements are shown in Fig. 1. The broad band around 1100 c m - J and sharp intense band at 780 cm-~ were assigned to the skeletal vibration of ~r-conjugated systems and the C - H out(a)

d

E

1500

I

I

I

1250

1000

750

500

W a v e n u m b e r / cm 1 Fig. 1. FT-IR ATR spectra of the films of (a) PBT and (b) PT'I'.

7". Sato et al. / Synthetic Metals 95 (1998) 107-112

of-plane bending vibration of a--a' coupled thiophene [ 18 ], respectively. Thus it is confirmed that thiophene rings in PBT are coupled at their a-positions. On the contrary, some additional bands were observed on the FT-IR ATR spectra of PTT film. Among them, the sharp bands observed at 670-690 c m - ~ were assigned to the C-H out-of-plane bending vibration of 2-substituted thiophene [ 19 ]. Since 2-substituted thiophene should be positioned at the ends of a polymer chain, PTT should contain the polymer chains with lower polymerization degree. Indeed, Roncali et al. [20] have reported that the polymer prepared by electrochemical polymerization of TT contained unreacted TT together with important quantities of dimerization product, sexithiophene. They have also claimed that the number of et-[3' couplings increased by employing TT for polymerization. Although the C - H out-ofplane bending vibrations of 2,4-thienylene have been observed at 730 and 820 c m - J [ 18], these bands were not clearly observed in the present FT-IR ATR spectra of PTT. It was thus confirmed that PTT film obtained under the present conditions contained oligomers with short conjugation length and few defects such as e~-13' coupling. XRD measurements were carried out for thick films prepared under identical conditions with those applied to the PL measurements because the XRD patterns of the same films that applied to the PL measurements could not be obtained. Diffraction peaks in Fig. 2 observed for the thick film of P T r indicated that PTT contained the microcrystalline region; its crystallinity was estimated to be 17%. On the other hand, no diffraction peak was observed in PBT. The XRD pattern obtained for PTT was similar to the reported patterns of electrochemically prepared PTT [ 21] and chemically prepared polythiophene [ 22]. In the absorption spectra of PBT films not shown here, a distinct peak assigned to the 7r-~r* transition was observed at 460 nm, its onset being at around 620 nm. On the contrary, such a distinct peak could not be observed in the spectra of PTT films, although the onset of absorption was also observed at around 620 nm. It has been reported that the absorption maximum of electrochemically prepared PTT was observed at 370 nm [20]. Although the peak position in the absorption band of PTT was not obvious in the present case probably

i ....

10

i ....

15

i ....

20

i ....

2s

i ....

30

i . ,

3s

2 9 / degree F i g . 2. X R D

patterns of thick films of (a) PBT and (b) PTF.

109

.m

c

(b)

1.6

1.8

2.0

2.2

Photon energy / eV F i g . 3. P L s p e c t r a o f ( a )

PBT

and (b)

PTT observed

under steady-state

irradiation

because of the strong absorption by the ITO glass substrate, the absorption maximum of the present PTT ought to be positioned around 370 nm because of the coexistence of oligomer. On the other hand, the PL spectra of PBT and PTT thin films observed under steady-state irradiation were positioned at almost the same energy as seen in Fig. 3. The PL band was observed at the absorption-edge energy. These spectra were similar to those of polythiophene chemically [ 5 ] and electrochemically [ 12 ] prepared from thiophene. The PL maximum of PTI" slightly shifted to higher energy compared with that of PBT, implying that the effective conjugated length of the segment responsible for PL was slightly shorter in PTT. However, the exciton formed in the segment of shorter conjugation length, e.g. sexithiophene, should immediately migrate to the segment with longer conjugation length as observed in the heterogeneous oligothiophene dimer [ 23 ]. Thus, almost the same PL spectra were obtained for PBT and PTT in spite of their different absorption spectra. Therefore, it is pointed out that the coexistence of oligomers in PTT has little effect on their PL spectra. 3.2. Transient P L measurement

The observed PL decay curve of PBT is shown in Fig. 4(a). The non-exponential PL decay dynamics was observed as described above. This decay curve can be fitted with two linear lines for the distinct time intervals; the time interval of 8-16 ps (interval A) and of 223-730 ps (interval B) after the excitation. The time-integrated PL spectra obtained in those time intervals are shown in Fig. 5(a). A similar spectrum to the steady-state PL one in Fig. 3 was observed for interval A; the PL band maximum was observed at 2.0 eV. On the other hand, the broad band was observed around 1.85 eV for interval B. The provisional lifetimes of those excitons were estimated as the reciprocal of the slopes of these fitting lines. The estimated lifetimes were 21 ps at interval A and 250 ps at interval B. At interval A, the ratio of the longer-lifetime PL compo-

7". Sato et al. / Synthetic Metals 95 (1998) 107-112

110

A

,,-t

._1

(a) -100

0

100 200 300 400 500 600 700

t,,,-. IV ¢-

B

x 25

Time / ps A (a)

1.7

1.8

1.9

2

2.1

2.2

Photon energy / eV

J

_J

(b) -100

0

100 200 300 4013 500 600 700

Time/ps Fig. 4. PL decay curves of (a) PBT and (b) PTT. The carve can be fitted with linear lines in the two distinct time intervals labeled A and B.

nent was smaller than 10% of the total PL. On the other hand, the contribution of shorter-lifetime PL component was negligible at interval B, because that component completely decayed at this time interval. Thus, it can be said that the spectra obtained at two time intervals were derived from distinct excitons. Similar dual PL was observed in the PTT thin films; the PL decay curve and time-integrated spectra are shown in Figs. 4(b) and 5(b), respectively• Energies of the PL maximum and their provisional lifetimes were slightly different from those of PBT. The maximum of the short-lifetime PL component was observed at 2.10 eV and that of the longer-lifetime one at 1.95 eV. Moreover, their provisional lifetimes were estimated as 36 and 370 ps, respectively• Note that the ratio of longer-lifetime PL component rather increased in the PL decay curves of PTT compared with the case of PBT. Thus, the time-integrated PL spectrum of the longer-lifetime component could be more clearly observed and the PL band was found to consist of a broad and structureless one.

4. Discussion Dual luminescence has been observed in several cases [24]: (i) the complex was formed in the excited state; (ii) the molecular structure changed in the excited state; (iii) emission from the upper level such as, e.g., the $2 state, was observed; and (iv) several emission centers with different local environments existed. In all the cases, the origin of dual luminescence came from the existence of two different excited species. Thus, the observed dual PL clearly indicated

._.g, t/3

(b)

1.7

1.8

1.9

2

2.1

2.2

Photon energy / eV Fig. 5. Time-integrated PL spectra of (a) PBT and (b) P I T observed at time intervals A and B.

the existence of two distinct types of excitons. The two types of excitons generated in polythiophene have been proposed by Kobayashi et al. [ 25 ] as ascribing to free and self-trapped excitons. The former exciton is a bound electron-hole pair directly generated by photoexcitation and the latter the electron-hole pair confined by the lattice relaxation. The free exciton relaxes to the self-trapped one within a few hundred fs, this being a characteristic lattice relaxation time. Kanner et al. [2] have also confirmed that the formation time of selftrapped excitons was 310 fs by the transient photomodulation spectrum. In the present study, however, the two types of excitons were observed in the time interval from a few ps to a few hundred ps and, hence, free excitons were unobservable in this time interval. Therefore, both of them were assigned to the self-trapped species. FT-IR ATR spectra of PBT and P'I'T films showed that the polymer chains in these films had almost identical chemical structure and that the latter film contained oligomers with shorter conjugation length. In both films, the PL bands in the

T. Sato et al. / Synthetic Metals 95 (1998) 107-112

time interval A were observed at the energy equal to the bandgap. These results indicated that the excitons formed in the segment with shorter conjugation length completely migrated to that with a longer one within a few ps, as described above. Thus, we can only observe the excitons in the segment with long enough conjugation length and with almost identical chemical structure. Therefore, it can be concluded that the difference in transient PL of PBT and PTT is based on the arrangement of polymer chains rather than the chemical structure of them; the differences in their lifetime and spectra would be based on the local environment of the recombination sites varied with the arrangement of polymer chains. The XRD pattern of PTT films showed the existence of a microcrystalline region in the film indicating inhomogeneous arrangement of polymer chains; there were microcrystalline and amorphous regions. XRD results indicated that the thin film of PTI" also contained many microcrystalline regions but that of PBT only few. The longer-lifetime PL component was enhanced in PTT film, which suggested that the exciton with longer lifetime is formed in the microcrystalline region. This longer-lifetime PL component showed the broad structureless band at lower energy than the bandgap. It has been reported that increase of the hydrostatic pressure brought about a red shift of the PL band and a decrease of its intensity [26,27]. Jenekhe and Osaheni [28] have reported that emission from the excimer state was seen in the conjugated polymer film. These results implied that the tight packing of polymer chains caused the PL band at lower energy. In chemically prepared polythiophene showing similar XRD patterns, crystal structure with tightly packed straight polymer chains has been suggested [22]. Therefore, it is highly possible that the exciton with longer lifetime showing the PL band at lower energy appeared in the microcrystalline region where the polymer chains were tightly packed. The mechanism causing the non-exponential decay of PL is controversial. In the present study, it should be taken into account that the observed dual PL evidences the existence of at least two different types of excitons. Various kinetics models have been proposed to explain the non-exponential behaviors, in which the double exponential [29], the stretched exponential [ 2,11 ], and the bimolecular recombination [ 13 ] models are typical. Among these, only the double exponential model represented by Eq. ( 1) deals with the existence of two kinds of recombination sites:

I=A exp(-t/~-i ) + B e x p ( - t/~-2)

( 1)

The results of nonlinear least-squares fitting by Eq. (1) are shown in Fig. 6 (broken line) and the best parameters used for this fitting are listed in Table 1. However, the double exponential model could not successfully describe the longlifetime PL component of decay curves as seen in Fig. 6. There can be two reasons to explain this failure: (i) since the excitons diffuse to the quenching center such as the photoinduced polaron, the dynamics must be described as the combination of the stretched exponential model; (ii) since there

111

(a) A

..J

|

0

I

200

400

600

time / ps (b) A e-

_=

11 0

i 200

i 400

i 600

time / ps

Fig. 6. PL decaycurvesof (a) PBTand (b) PTT.Bestfittingresultsof Eqs. ( 1) and (2) (see text) are shownby dashedand solidlines, respectively. Table 1 The best parameters used for the fitting by Eqs. ( 1) and (2) Sample

PBT

PTT

(1)

A B ~'i (ps) rz (ps)

6.41×104 1.52X 103 19 142

4.46X 104 6.62X 103 30 200

(2)

A B rl (ps) r~in-

6.23X 104 6.33 X 103 19 1.31 × 10 -~l

3.61Xl04 2.61 × 104 26 2.49X 10 -I1

is a number of recombination sites with different environmental conditions, the dynamics must be described as the multi-exponential model. Eq. ( 1) was modified on the basis of the following two reasons: (I) migration of the exciton has been observed in several aromatic crystals [30,31]; (II) the monomolecular decay of PL described by the single exponential was observed for the solutions of soluble conjugated polymers [ 10,32]. Therefore, the second term of Eq. ( 1 ) describing the longlifetime PL component is replaced by the following:

I=A exp(-t/Tl)+B exp(--t/'rdiff) I/3

exp(--t/%)

(2)

where 3

3

--1

and e x p ( - t/~'aiff)1/3 represents the survival probability of excitons diffusing in one-dimensional (ID) randomly dis-

112

T. Sato et al. / Synthetic Metals 95 (1998) 107-112

tributed quenching centers, c and D denote the I D concentration of quenching sites and diffusion coefficient, respectively [ 2]. ~'ois the intrinsic lifetime ofmonomolecular decay of polythiophene; To= 500 ps was applied in this case, which is the lifetime of poly(3-octylthiophene) in solution [32]. The results of fitting by Eq. (2) are shown in Fig. 6 and Table 1. This equation successfully describes the whole decay curves. Using D = 2 . 4 X 10 - 4 c m 2 / s , the diffusion coefficient of excitons estimated for polythiophene electrochemically prepared [33], 1/c was estimated to be 46 and 63 A for PBT and PTT, respectively. Meanwhile, using D = 10 2 cm2/s employed by Kanner et al. [2], 1/c became 3.0 × 102 and 4.1 × 102 A for PBT and PTT, respectively. The non-exponential PL decay observed for a single crystal of dihexylsexithiophene [34] could be described by only the second term of Eq. (2) giving comparable Zd~rfwith those obtained for PBT and PTT. Thus, it is also probable that the exciton with longer lifetime is generated in the microcrystalline region. On the other hand, the exciton with shorter lifetime is generated in the amorphous region in the film; twisting of the polymer chain existing in the amorphous region, which increases the probability of the intersystem crossing [35], should contribute to the non-radiative decay process of the exciton.

5. Conclusions Transient PL of electrochemically prepared thin films of PBT and PTT were investigated in detail. There are four major findings in the present study: 1. In both the polybithiophene and polyterthiophene, dual PL was clearly observed. These results indicate that there are at least two distinct excitons in the film. 2. Arrangement of polymer chains in the film is inhomogeneous. XRD results indicate that there are both amorphous and crystalline regions in the film and that PTT contains many microcrystalline regions but PBT does not. 3. The enhancement of the longer-lifetime PL component in PTT suggests that the exciton with longer lifetime is generated in the microcrystalline region. 4. The non-exponential decay curves were successfully described by the double exponential model considering the exciton diffusion.

Acknowledgements This work is a part of the project of the Institute for Fundamental Chemistry, supported by Japan Society for the Promotion of Science - - Research for the Future Program (JSPS-RFrF96P00206).

References [ 11 A.J. Heeger, S. Kivelson, J.R. Schrieffer, W.-P. Su, Rev. Mod. Phys. 60 (1988) 781. [21 G.S. Kanner, X. Wei, B.C. Hess, L.R. Chen, Z.V. Vardeny, Phys. Rev. Lett. 69 (1992) 538. [3] See, for instance: J. Roncali, Chem. Rev. 92 (1992) 711 and Refs. therein. [4] S. Hayashi, K. Kaneto, K. Yoshino, Solid State Commun. 61 (1987) 249. [5] Z. Vardeny, E. Ehrenfreund, J. Shinar, F. Wudl, Phys. Rev. B 35 (1987) 2498. 16] T.A. Skotheim (Ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986. [71 K.S. Wong, W. Hayes, T. Hattori, R.A. Taylor, J.F. Ryan, K. Kaneto, K. Yoshino, D. Bloor, J. Phys. C: Solid State Phys. 18 (1985) L843. [8] J. Stampfl, S. Tasch, G. Leising, U. Scherf, Synth. Met. 69-71 (1995) 2125. 19] M. Furukawa, K. Mizuno, A. Matui, S.D.D.V. Rughooputh, W.C. Walker, J. Phys. Soc. Jpn. 58 (1989) 2976. [ 10] L. Smilowitz, A. Hays, A.J. Heeger, G. Wang, J.E. Bowers, J. Chem. Phys. 98 (1993) 6504. [ 11 ] L.J. Rothberg, M. Yan, F. Papadimitrakopoulos, M.E. Galvin, E.W. Kwock, T.M. Miller, Synth. Met. 80 (1996) 41. [ 12 ] K. Kaneto, F. Uesugi, K. Yoshino, J. Phys. Soc. Jpn. 56 (1987) 3703. [ 13] R.H. Friend, D.D.C. Bradley, P.D. Townsend, J. Phys. D: Appl. Phys. 20 (1987) 1367. [14] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [ 15] D.C.C. Bradley, Synth. Met. 54 (1993) 401~ [ 161 T. Sato, M. Fujitsuka, H. Segawa, T. Shimidzu, Synth. Met. 69-71 (1995) 335. [ 171 M. Fujitsuka, R. Nakahara, T. Iyoda, T. Shimidzu, H. Tsuchiya, J. Appl. Phys. 74 (1993) 1283. [ 18] T. Yamamoto, K. Sanechika, A. Yamamoto, Bull. Chem. Soc. Jpn. 56 (1983) 1497. [ 19] Y. Furukawa, M. Akimoto, I. Harada, Synth. Met. 17-19 (1987) 151. [20] J. Roncali, M. Lemaire, R. Garreau, F. Gamier, Synth. Met. 17-19 (1987) 139. [21 ] Y. Yumoto, S. Yoshimura, Synth Met. 13 (1986) 185. [221 Z. Mo, K.-B. Lee, Y.B. Moon, M. Kobayashi, A.J. Heeger, F. Wudl, Macromolecules 18 (1985) 1972. [23] T. Sato, H. Ino, M. Fujitsuka, K. Tanaka, Tetrahedron Lett. 38 (1997) 6039. [24] J.B. Birks, Photophysics of Aromatic Molecules, Wiley-lnterscience, New York, 1970. [25] T. Kobayashi, M. Yoshizawa, U. Stamm, M. Tajii, M. Hasegawa, J. Opt. Soc. Am. B 7 (1990) 1558. [26] B.C. Hess, G.S. Kanner, Z. Vardeny, Phys. Rev. B 47 (1993) 1407. [27] K. Yoshino, K. Nakao, M. Onoda, R. Sugimoto, J. Phys.: Condens. Matter 1 (1989) 1009. [28] S.A. Jenekhe, J.A. Osaheni, Science 265 (1994) 765. [29] I.D.W. Samuel, B. Crystall, G. Rumbles, P.L. Burn, A.B. Holmes, R.H. Friend, Synth. Met. 54 (1993) 281. [30] O. Simpson, Proc. R. Soc. London, Ser. A 238 (1956) 402. [31] W. Klrpffer, J. Chem. Phys. 50 (1969) 2337. [32] B. Kraabel, D. Moses, A.J. Heeger, J. Chem. Phys. 103 (1995) 5102. [33] K. Kaneto, S. Hayashi, K. Yoshino, J. Phys. Soc. Jpn. 57 (1988) 1119. [34] T. Sato, M. Fujitsuka, K. Tanaka, M. Shiro, Synth. Met. 95 (1998) 143. [35] N.D. Crsare, M. Bellet~te, F. Raymond, M. Leclerc, G. Durocher, J. Phys. Chem. A 101 (1997) 776.