Laser flash photolysis study on the photoinduced reactions of 3,3′-bridged bithiophenes

Photophysical properties and photoinduced reactions of 3,3'-bridged-2,2'-bithiophenes {dithieno[3,2-b :2',3'-d]-thiophene, 4H-cyclopenta[2,1-b : 3,4-b']dithiophene, and 4H-dithieno[3,2-b;2',3'-d]pyrrole (DTP)} and 2,2'-bithiophene (BT) were investigated by observing the transient absorption spectra in the visible and near-IR regions using nanosecond laser flash photolysis. Fluorescence quantum yields for the bridged bithiophenes were low compared with that for BT. An especially low quantum yield for DTP in acetonitrile was attributed to an addition reaction with the solvent. The triplet energy of BT was the lowest amongst the examined bithiophenes, indicating some conformational change in the triplet state. Triplet-energy-transfer reactions at diffusion limited rates were confirmed between bithiophenes and triplet-energy donors or accepters. Photochemical generation of the radical cations of the bithiophenes was confirmed by the transient absorption spectra, which show good correspondence with those observed in gamma-ray radiolysis. It was found that the triplet quenching rates of the electron-transfer reactions were small when the Gibbs energy change for the reaction was greater than or equal to -25 kJ mol(-1). The observed tendency agreed with the semi-empirical equation of Rehm-Weller. The generated radical cations decay according to a second-order function, indicating deactivation by a back electron-transfer reaction.