Rapid intersystem crossings in benzothieno[3,2-b][1]benzothiophene and its derivatives

Transient absorption and UV-vis spectroscopy were used to investigate the optical properties of single benzothieno[3,2-b][1]benzothiophene (BTBT) molecules and substituted derivatives in dilute toluene solution. The time-resolved spectra point to excited state dynamics with two transient absorption signals, detected at different time-scales related to the non-radiative and radiative deactivation channels. The latter is takes place after an intersystem crossing occurring as fast as ∼30 ps for alkoxy substituted BTBT (C8-O-BTBT) due to a spin-conversion mechanism populating a triplet excited state manifold. Bromine and iodine substituents were also studied, and contrary to the possibility of increased spin-orbit coupling, the transition was less rapid as that for the alkyloxy substituent, requiring ~500 ps. It is suggesed that out-of-plane vibrational modes are the key factor enabling intersystem crossing, which is in consistent with that for alkyl substututed BTBT (C8-BTBT) and similar small molecues. It is suggested that, for bromine and iodine substitutions, their strong electronegativity suppresses certain out-of-plane vibrational modes associated with the spin-orbit-coupling matrix elements by disrupting the equilibrium geometry of the molecule, hence resulting in slower intersystem crossing.