Spectral Tuning and Photoisomerization Efficiency in Push-Pull Azobenzenes: Designing Principles

This work demonstrates how push-pull substitution can induce spectral tuning toward the visible range and improve the photoisomerization efficiency of azobenzene-based photoswitches, making them good candidates for technological and biological applications. The red-shifted bright pi pi* state (S-2) behaves like the lower and more productive dark n pi* (S-1) state because less potential energy along the planar bending mode is available to reach higher energy unproductive ne/S-0 crossing regions, which are responsible for the lower quantum yield of the parent compound. The stabilization of the bright pi pi* state and the consequent increase in isomerization efficiency may be regulated via the strength of push-pull substituents. Finally, the torsional mechanism is recognized here as the unique productive route because structures with bending values attributable to the inversion mechanism were never detected, out of the 280 pi pi* time-dependent density functional theory (RASPT2-validated) dynamics simulations.