Chiral Photochemistry of Achiral Molecules: The Emblematic Case of Stilbene and Stiff-Stilbene

Chirality is a fundamental molecular property governed by the topography of the potential energy surface (PES). The barrier height between two enantiomers relative to the thermal energy dictates the timescale for their interconversion (and thus whether this can be observed). Thermally achiral molecules interconvert rapidly when the interconversion barrier is comparable to or lower than the thermal energy, in contrast to thermally stable chiral configurations. In principle, a change in the PES topography on the excited electronic state may diminish interconversion, leading to electronically prochiral molecules that can be converted from achiral to chiral by electronic excitation. We demonstrate that this is the case for two prototypical examples – cis-stilbene and cis-stiff stilbene. Both systems exhibit unidirectional photoisomerization for each enantiomer as a result of their electronic prochirality. We propose and simulate an experiment to demonstrate this effect in cis-stilbene based on its interaction with circularly polarized light. Our results highlight the drastic change in chiral behavior upon electronic excitation, opening up the possibility for asymmetric photochemistry from an effectively nonchiral starting point.