Tracking Chirality during Molecular Motor Photoisomerization via Simulated Time-Resolved Circular Dichroism

Ultrafast spectroscopic techniques are widely used to investigate photoinduced processes, yet they remain largely blind to molecular chirality. Here we introduce a framework for simulating time-resolved electronic circular dichroism (TRCD) along an ensemble of nonadiabatic molecular dynamics and apply it to the photoisomerization of a second-generation molecular motor (Koumura, N.; et al.; Nature1999, 401, 152.). While transient absorption captures the overall excited-state dynamics, it cannot distinguish the two photoproduct pathways. The TRCD response, by contrast, resolves the stereochemical branching: trajectories returning to the stable P isomer (right-handed helix) retain a distinct chiroptical band in the visible region, whereas those forming the M isomer (left-handed helix) become chiroptically dark as they twist through the conical intersection. This asymmetry constitutes a potentially measurable signature of the stereochemical branching, offering a real-time probe of the formation of molecular chirality and concrete predictions for future TRCD experiments.