Electron transfer from oligothiophenes in the higher triplet excited states

In the present paper, we have investigated the inter- and intramolecular electron transfer processes from the higher triplet excited state (T(n)) of oligothiophenes (3T and 4T). In the case of the intermolecular systems, two-color two-laser flash photolysis using nanosecond lasers was applied to the solution including benzophenone, oligothiophene, and halogenated benzene as a photosensitizer, an electron donor, and an electron acceptor, respectively. The first laser light irradiation generated the lowest triplet excited state (T(1)) of oligothiophene via energy transfer from benzophenone. Upon the second laser light irradiation, the absorption band of the radical cation of oligothiophene appeared with the simultaneous bleaching of the absorption band of the T(1) state, indicating the electron transfer from the T(2) state of the oligothiophene to the electron acceptor. The observed electron transfer rate dependent on the free energy change was explained on the basis of the Marcus theory. The intramolecular electron transfer in the dyad molecule of oligothiophene and acceptor was investigated using the two-color two-laser flash photolysis employing femtosecond laser. Upon the second laser light irradiation, which generates the T(n) state, the kinetic trace of the absorption band of T(1) state showed the bleaching and recovery, the rate of which depends on the driving force for the charge separation from the T(2) state of the oligothiophene. This observation suggests the existence of charge separation process from the T(2) state, and the observation of the charge-separated state was difficult probably due to the low charge separation yield and fast charge recombination.