Photoinduced electron transfer from oligothiophenes/polythiophene to fullerenes (C60/C70) in solution:: Comprehensive study by nanosecond laser flash photolysis method

Photoinduced electron-transfer processes from oligothiophenes (nT)/polythiophene (poly-T) to fullerenes (C-60/C-70) have been studied by the nanosecond laser flash photolysis method, observing the transient absorption spectra in the visible and near-IR regions. When fullerene was selectively photoexeited in polar solvents, electron transfer from nT to the excited triplet state of fullerene was confirmed. The electron-transfer rate constants increased with the number of repeating unit (n) of nT. On the other hand, the efficiency of electron transfer showed a maximal value at It = 4; for n > 4, electron-transfer efficiency of nT decreased, indicating contribution of other processes such as energy transfer. By the photoexcitation of nT in polar solvent, both electron and energy transfer processes were observed for 4T and 6T. in the case of 3T, energy transfer occurred predominantly even in polar solvent. In nonpolar solvent, energy transfer was a predominant deactivation process. Electron-transfer efficiencies among these oligothiophenes and polythiophene were explained on the basis of free-energy changes for the electron transfers and triplet energy levels of nT.