Light-Driven Out-of-Equilibrium Helicity Control in a Diarylethene-Helicene Photosystem.

Molecular photoswitches are key tools to control responsive functions and to drive systems away from equilibrium by enabling kinetic asymmetry and energy ratchet mechanisms. Incorporating dynamic chirality control within such non-equilibrium frameworks, however, remains challenging. Here, the responsive behavior of a diarylethene-[5]helicene photoswitch comprising multiple interconverting diastereomers is investigated. Distinct energy profiles govern the helicity dynamics in the open and closed forms of the switch. While helix inversion in the open state is kinetically hindered by a high energy barrier, the closed state allows rapid equilibration toward the thermodynamically favored diastereomer. Upon photochemical ring-opening, the helicity acquired in the closed state is transferred and kinetically trapped in the open form, generating a persistent out-of-equilibrium configuration. This sequence constitutes a rare energy ratchet mechanism operating within a single covalently linked molecule, enabling directional control over helicity. These findings provide a concept for light-driven helicity control in adaptive chiral systems.