Spin–Vibronic Coupling Enhanced Intersystem Crossing beyond El-Sayed Restrictions

Abstract

Intersystem crossing is a crucial photochemical process driven by spin–orbit coupling. With intersystem crossing at the forefront of photochemical advancements, there is significant motivation to design molecular systems that can dynamically modulate spin–orbit coupling strength. This work seeks to understand the unusually rapid intersystem crossing rates that occur in select organic molecular systems that do not contain heavy atoms to induce spin–orbit coupling. This work presents a case where intersystem crossing is enabled by symmetry-perturbation rather than through the lifting of El-Sayed restrictions. Computational results indicate that the intersystem crossing between the lowest-lying S(nπ*) and T(ππ*) excited states of dibutylaniline thiosquaraine can only occur through a symmetry-perturbing spin–vibronic mechanism. Our analysis helps provide new insight into how synthetic chemists, materials scientists, and chemical engineers might incorporate symmetry considerations in designing new heavy-atom-free molecular systems with a propensity for efficient intersystem crossing.