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

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 <i>S</i>(<i>n</i>π*) and <i>T</i>(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.