The Photoisomerization Pathway(s) of Push-Pull Phenylazoheteroarenes

Azoheteroarenes are the most recent derivatives targeted to further improve the properties of azo-based photoswitches. Their light-induced mechanism for trans-cis isomerization is assumed to be very similar to that of the parent azobenzene. As such, they inherited from the controversy about the dominant isomerization pathway (rotation vs. inversion) depending on the excited state (nπ* vs. ππ*). While the controversy seems settled in azobenzene, the extent to which the same conclusions apply to the more structurally-diverse family of azoheteroarenes is unclear. Here, we unravel by means of non-adiabatic molecular dynamics, the photoisomerization mechanism of three prototypical phenyl-azoheteroarenes with an increasing push-pull character. The evolution of the rotational and inversion conical intersection energies, the preferred pathway, and the associated kinetics upon both nπ* and ππ* excitations can be linked directly with the push-pull substitution effects. Overall, we clarify the working conditions of this family of azo-dyes and identify a possibility to exploit push-pull substituents to tune their photoisomerization quantum yield.