Singlet 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 steepest descent path on the S₁ surface 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 proceeds through a planar geometry of MeCOTh, and appeared in only 8% of trajectories. Our photodynamic simulations show that although excited-state Baird aromaticity contributes to the relative stereochemical inversion mechanism of MeCOTh, it is not the only electronic effect. Instead, the overall inversion mechanism is primarily governed by the interplay between Baird aromaticity and the S-S closed-shell repulsions.