Photoinduced electron transfer between semiconducting p-conjugated polymers and fullerenes has been demostrated to be an efficient process. It has been detected with several tecniques, in blends of C60 with polymers (polythiophenes and poly(p-phenylene-vinylenes)) as well as with various oligomers. The charge separated state is metastable; although the forward charge transfer occurs in the sub-picosecond time scale, the back charge transfer rate is many order of magnitude slower. Therefore steady state photoinduced absorption (PIA) spectroscopy provides an useful tool to study the transfer features.
In this contribution we report a photophysical study of a series of thiophene-based copolymers and of their blends with C60, by steady state PIA spectroscopy.
The conjugated polymers have been obtained by the random copolymerization of 3-butylthiophene and 3,4-dibutylthiophene monomers in different ratio which is an easy route to obtain materials with tunable optical properties [1]. The steric hindrance of 3,4-dibutylthiophene units induces a twisting of the conjugated backbone. By increasing the fraction f of disubstituted monomer in the copolymer, the degree of conformational disorder is increasing and a reduction of the conjugation length and a consequent blue shift of the absorption-emission spectra are obtained. The energy position of the HOMO and LUMO levels, as well as the compatibility with C60, are thus easly controlled by varying f. For these reasons these materials provide an interesting system to study the photoinduced electron transfer mechanisms between conjugated polymers and fullerenes. The signatures of long lived triplets and charged excitations appears in the PIA spectra of the pure copolymers. Their spectral weight and their energy position vary with the copolymer composition. For the copolymers with a quite distorted backbone, f > 0.5, the contribution of the charged excitations is drastically reduced with respect to triplet states.
The visible and infrared absorption spectra of the blends rule out doping effects in the ground state. The whole copolymer series exhibit a strong photoluminescence quenching upon adding C60. This is suggesting that fast photoinduced electron transfer from the polymer to fullerene competes with singlet radiative decay. The PIA spectra are affected by the presence of fullerene: the C60 anion transition at 1.15 eV, appears in the spectra and the PIA activity of the popolymer charged excitations is increasing. This evidences the enhancement of quantum efficiency for charged carriers photogeneration due to charge separation in the excited state. Even for the copolymers with a high content of disubstituted monomers, where the photogeneration of charged states is quite unlikely without fullerene, the photoproduction of polymer charged states and of C60 anions is observed in the PIA spectra. This indicates that the photoinduced electron transfer to C60 helps in stabilizing the charged defects in polythiophenes with a twisted backbone conformation.
In the copolymer series it is possible to photoexcite the fullerene without exciting the polymer as, for f>0.75, the onset of the polymer absorption band is much higher than the first transition of C60. The selective photoexcitation of C60 brings to the same charge transfer spectral features observed by exciting the conjugated polymer.
In this study the spectroscopical evidences of the photoinduced charge transfer between a series of polythiophenes and C60 are reported. The tunability of the optical properties of the conjugated polymer gives the possibility to obtain the following improvements with respect to previous results: i) a better disentanglement of the PIA transitions of the polymer and of the C60 charged species; ii) the detection of the spectral signatures of the charge transfer arising either from the electron transfer from the excited polymer to C60 or to the hole transfer from the excited C60 to the polymer.

[1]	M. Catellani, S. Luzzati, R. Mendichi, A. Giacometti Schieroni, Polymer 37 (1996) 1059; G. Bongiovanni, M.A. Loi, A. Mura, A. Piaggi, S. Luzzati, M. Catellani, Chem. Phys. Lett. 288 (1998) 749; S. Luzzati, M. Panigoni, M. Catellani, Synth. Met., in press, ICSM '98 proceedings.

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