Terthiophene Photophysics Revisited: Independence of the Triplet Quantum Yield on the Excitation Wavelength

A significant part of our understanding of the excited-state properties of polythiophenes comes from studies of smaller oligothiophenes, which have a better-defined structure. Among them, terthiophene (3T) was reported to have an excitation-wavelength dependent triplet quantum yield, ΦT. This was explained by the opening of a second intersystem crossing (ISC) pathway upon the high-energy excitation of distorted molecules. Here, we reinvestigate the excited-state dynamics of 3T in solvents of various viscosity and in a polymer film. Our results reveal that, although different subpopulations are excited upon high- and low-energy irradiation, the ISC dynamics, and hence ΦT, are the same. We show that the distorted molecules excited at short wavelength undergo rapid planarization independent of the viscosity of the environment before significant ISC takes place. The apparent increase of ΦT with increasing excitation wavelength reported earlier can be explained by the neglect of the early relaxation dynamics.