Machine learning photodynamics reveal competing inversion paths of methylated cyclooctatetrathiophene

We used state-of-the-art machine-learning nonadiabatic molecular dynamics to investigate the stereochemical inversion reaction of a methylated thiophene-fused cyclooctatetraene derivative, MeCOTh. Minimum energy path calculations suggest that the pseudo-dominant pathway of MeCOTh is towards a non-productive fluorescence decay pathway. Our machine learning photodynamics calculations revealed that relative stereochemical inversion occurs mainly on the S1 surface (74% of trajectories), and we identified two competing inversion pathways. The first and main mechanistic pathway, seen in 62% of trajectories, showcases a “crown” structure with unidirectional sulfurs resulting from S-S closed-shell repulsions. The second pathway is the previously proposed inversion mechanism, which passes through a planar geometry of MeCOTh, and appeared in only 8% of trajectories. Our photodynamic simulations indicate that while excited-state Baird aromaticity contributes to the relative stereochemical inversion mechanism of MeCOTh, it is not the primary electronic effect. Instead, closed-shell repulsions generally drive the inversion mechanism.