Photoinduced electron-transfer reactions between dithienothiophene (DTT) and the electron accepters, 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 gamma 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, similar to 10(10) 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 < 10(9) M(-1) s(-1) due to small Delta G(0) for this reaction. For both accepters, 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.