Nonadiabatic Dynamics Simulation of the Wavelength-Dependent Photochemistry of Azobenzene Excited to the nπ* and ππ* Excited States

Azobenzene
is one of the most ubiquitous photoswitches in photochemistry and
a prototypical model for photoisomerizing systems. Despite this, its
wavelength-dependent photochemistry has puzzled researchers for decades.
Upon excitation to the higher energy ππ* excited state
instead of the dipole-forbidden nπ* state, the quantum yield
of isomerization from trans- to cis-azobenzene is halved. The difficulties associated with unambiguously
resolving this effect both experimentally and theoretically have contributed
to lasting controversies regarding the photochemistry of azobenzene.
Here, we systematically characterize the dynamic photoreaction pathways
of azobenzene by performing first-principles simulations of the nonadiabatic
dynamics following excitation to both the ππ* and the
nπ* states. We demonstrate that ground-state recovery is mediated
by two distinct S1 decay pathways: a reactive twisting
pathway and an unreactive planar pathway. Increased preference for
the unreactive pathway upon ππ* excitation largely accounts
for the wavelength-dependent behavior observed in azobenzene.