Photochemical generation of radical cations of dithienothiophenes, condensed thiophene trimers, studied by laser flash photolysis

Photoinduced electron-transfer reactions between dithienothiophene (DTT) and the electron acceptors, p-dinitrobenzene (DNB) and CCl₄, have been investigated using nanosecond laser flash photolysis and time-resolved fluoresence spectroscopy. Generation of the radical cations of DTTs and radical anion of DNB in acetonitrile solution was confirmed by transient absorption spectra in the visible and near-IR regions. Observed transient absorption bands corresponded to those of radical ions generated by γ irradiation in a frozen matrix. Electron transfer occurred both from the singlet and triplet excited states of DTT to DNB at the diffusion-controlled rate, ∼10¹⁰ M^-1 s^-1. When CCl₄ was used as an acceptor, electron transfer from the singlet excited state occurred at the diffusion-controlled rate, while the reaction from the triplet excited state occurred at <10⁹ M^-1 s^-1 due to small ΔG₀ for this reaction. For both acceptors, the singlet route tended to dominate the triplet route when concentration of the acceptor was increased. The radical ions decayed with second-order kinetics by back-electron transfer at a rate closed to the diffusion-controlled limit. In cyclohexane, generation of radical ions was completely suppressed and the triplet excited state of DTT was deactivated by energy transfer to DNB.