Conformational Control Governs Photoswitching Efficiency in Diazocines

Diazocines are emerging as superior alternatives to azobenzenes in photoswitching applications due to their favorable photophysical properties. Here, we show that the metastable trans isomer of diazocine derivatives can adopt two distinct conformations, a twist (E-t) and a chair (E-c) form, with markedly different switching efficiencies, based on the photochemistry of selected diazocine derivatives (parent diazocine 1, 2,9-diiododiazocine 2 and S-diazocine 3) under matrix isolation conditions in an inert solid argon and nitrogen at cryogenic conditions (4 K). The experimental identification of dominant conformation among them in solution phase is a strenuous task due to the possible conformational dynamics and thermal relaxations of the trans isomers. Despite this, observations at cryogenic temperatures do provide important and useful design parameters also at room/physiological temperature. The combined experimental and computational explorations demonstrate the robustness in the bidirectional photoswitching even in condensed state. While the twist conformation exhibits excellent photoswitching behavior and dominates the equilibrium, the chair conformation leads to significantly reduced performance, as exemplified by S-diazocine (3) where both conformations are similarly stable. These findings reveal a key structure-function relationship and provide a design principle for diazocine-based photoswitches, namely to avoid stabilization of the chair conformation in the trans state.