Photoinduced electron transfer processes in oligothiophene/C60 composite films

We present near steady-state photoinduced absorption (PIA) spectroscopy and steady-state light-induced electron spin resonance (LESR) studies on photoinduced electron transfer reactions in composite films of well defined α-oligothiophenes (Tn, n=6, 7, 9, and 11) as electron donor with buckminsterfullerene (C60) and a methanofullerene derivative (1-(3-cholestanoxycarbonyl)propyl-1-phenyl-[6.6]C61) as electron acceptors. Dispersion on a molecular level of these fullerenes in oligothiophene films causes quenching of both the photoluminescence and intersystem crossing after photoexcitation across the π–π* energy gap as a result of a fast electron transfer reaction. The PIA spectra exhibit bands due to Tn+• radical cations, fullerene radical anions and electroabsorption oscillations of the π–π* band edge resulting from strong local electric fields set up by the photoinduced charges. LESR spectra give additional unambiguous evidence of the photoinduced electron transfer reaction. Using PIA and LESR, we do not observe an effect of the conjugation length of the photoinduced electron transfer reaction or on the metastability of the charge-separated state.