Orchestrating photoswitch thermal backconversion using hydrogen bonds to reshape energy landscapes.

Thermodynamic control of molecular photoswitches remains a fundamental challenge, centered on the stabilization of high-energy metastable states. Inspired by concepts of selective kinetic regulation found in Maxwell's demon-like systems, we explore hydrogen bonding as a molecular-level selector for modulating the thermal back-conversion of quadricyclane (QC) to norbornadiene (NBD). By strategically incorporating hydrogen-bond donors and acceptors, we show that these interactions build a water "bridge" that restricts rotational degrees of freedom. This bridge significantly elevates the kinetic barrier and redirects the isomerization along an alternative reaction coordinate. In this manner, the hydrogen-bonding system selectively regulates the thermal back-conversion dynamics and enables directional control over the isomerization kinetics, achieving up to a 5000-fold extension of the thermal half-life. This work illustrates how hydrogen bonds can facilitate precise control over back-conversion dynamics and molecular thermal stability in molecular photoswitches.