Competing Isomerization Channels and Rotor Selectivity in a Third-Generation Molecular Motor: Insights from...

Third-generation light-driven molecular motors, characterized by a unique double-rotor architecture, represent a pivotal advancement in nanotechnology. Recently, a novel third-generation molecular motor based on bridged-isoindigo has been proposed. However, the microscopic mechanism behind its ultrafast photoisomerization remains to be explored. In this work, nonadiabatic molecular dynamics simulations at the semiempirical OM2/MRCI level were employed to investigate the excited-state dynamics of a third-generation molecular motor, starting from the E _S Z _S configuration. The simulation reveals five competitive isomerization channels, identifies new ways to directly reach the stable configuration, and confirms the mechanical coupling between the central stator and the rotor. We demonstrate that the isomerization is kinetically biased toward the lower rotor due to the greater energetic accessibility of its associated conical intersections. Furthermore, our results attribute the modest overall quantum yield (16.5%) to a dominant "geometric preservation" decay channel and confirm the existence of an intermediate dark state preceding the nonadiabatic transition.