Time-Resolved Photoelectron Studies of Thiophene and 2,5-Dimethylthiophene

The photoinduced dynamics of thiophene and 2,5-dimethylthiophene (2,5-DMT) were investigated upon excitation at 200 and 255 nm (2,5-DMT only) using time-resolved photoelectron spectroscopy and compared with results from ab initio coupled cluster calculations. For thiophene, depopulation of the initially excited B₂(π₃π₄*) state to the lower-lying A₁(π₂π₄*) state occurs within 25 ± 20 fs, with a subsequent bifurcation into a ring-puckering channel and a ring-opening channel with lifetimes of 80 ± 20 and 450 ± 50 fs, respectively. For 2,5-DMT, the dynamics following excitation at 200 nm is described by a monoexponential decay with a time constant of 120 ± 20 fs, while that following excitation at 255 nm is best fit by a biexponential decay with time constants of 115 ± 20 fs and 15 ± 3 ps, respectively. The fast signal observed after excitation of 2,5-DMT is assigned to the ring-opening channel, which is favored with respect to thiophene due to a lower excited-state barrier along the ring-opening coordinate and an increased inertia toward the ring-puckering channel. Coupled cluster calculations have been undertaken to compare the relaxation dynamics of thiophene to thiazole and isothiazole. For the latter two molecules, we find a strong gradient along the ring-opening coordinate in the Franck-Condon region of the initially populated ππ* state and predict that ring-opening is the dominating relaxation channel after photoexcitation. We use the extracted information for a comparison of the thiophene dynamics with the light-induced processes observed in other five-membered heterocyclic molecules.