Enhanced intersystem crossing in a thiohelicene

The photophysics of a helicene derivative in which two benzene units are replaced by thiophene units (thiohelicene, 6H) was studied by steady state and transient absorption and emission spectroscopies covering time ranges from femtoseconds to minutes. Efficient intersystem crossing (ISC) to the triplet state was observed, by far exceeding that of the parent helicene and the corresponding oxo-helicene. Quantum chemical calculations indicate that the helical distortion and the heavy atom effect of sulfur cooperate in promoting spin-orbit coupling, and that the most efficient decay channel involved the T2 or even the T3 state. These insights can help in the design of more efficient triplet sensitizes for may applications.