Nonadiabatic simulation study of photoisomerization of azobenzene: Detailed mechanism and load-resisting capacity

TL;DR

This study uses nonadiabatic simulations to explore azobenzene photoisomerization mechanisms and load-resisting capacities, revealing out-of-plane rotation as the dominant process and identifying force thresholds for mechanical isomerization.

Contribution

It provides a detailed nonadiabatic simulation analysis of azobenzene's photoisomerization and mechanical switching, challenging previous in-plane inversion hypotheses and quantifying load thresholds.

Findings

Photoisomerization occurs mainly via out-of-plane rotation.

External forces of 90-200 pN can inhibit photoisomerization.

Mechanical isomerization requires forces of 1250-1650 pN.

Abstract

Nonadiabatic dynamical simulations were carried out to study cis-to-trans isomerization of azobenzene under laser irradiation and/or external mechanical loads. We used a semiclassical electron-radiation-ion dynamics method that is able to describe the coevolution of the structural dynamics and the underlying electronic dynamics in a real-time manner. It is found that azobenzene photoisomerization occurs predominantly by an out-of-plane rotation mechanism even under a nontrivial resisting force of several tens of piconewtons. We have repeated the simulations systematically for a broad range of parameters for laser pulses, but could not find any photoisomerization event by a previously suggested in-plane inversion mechanism. The simulations found that the photoisomerization process can be held back by an external resisting force of 90 - 200 pN depending on the frequency and intensity of the lasers. This study also found that a pure mechanical isomerization is possible from the cis state if the azobenzene molecule is stretched by an external force of 1250 -1650 pN. Remarkably, the mechanical isomerization first proceeds through a mechanically activated inversion, and then is diverted to an ultrafast downhill rotation that accomplishes the isomerization. Implications of these findings to azobenzene-based nanomechanical devices are discussed.