Surface Hopping Dynamics for Azobenzene Photoisomerization: Effects of Packing Density on Surfaces, Fluorination, and Excitation Wavelength

Azobenzenes easily photoswitch in solution, while their photoisomerization at surfaces is often hindered. In recent work, it was demonstrated by nonadiabatic molecular dynamics with trajectory surface hopping [Titov et al., J. Phys. Chem. Lett.2016,7, 3591–3596] that the experimentally observed suppression of trans → cis isomerization yields in azobenzenes in a densely packed SAM (self-assembled monolayer) [Gahl et al., J. Am. Chem. Soc.2010, 132, 1831–1838] is dominated by steric hindrance. In the present work, we systematically study by ground-state Langevin and nonadiabatic surface hopping dynamics, the effects of decreasing packing density on (i) UV/vis absorption spectra, (ii) trans → cis isomerization yields, and (iii) excited-state lifetimes of photoexcited azobenzene. Within the quantum mechanics/molecular mechanics models adopted here, we find that above a packing density of ∼3 molecules/nm2, switching yields are strongly reduced, while at smaller packing densities, the “monomer limit” is quickly approached. The UV/vis absorption spectra, on the other hand, depend on packing density over a larger range (down to at least ∼1 molecule/nm2). Trends for excited-state lifetimes are less obvious, but it is found that lifetimes of ππ* excited states decay monotonically with decreasing coverage. Effects of fluorination of the switches are also discussed for single, free molecules. Fluorination leads to comparatively large trans → cis yields, in combination with long ππ* lifetimes. Furthermore, for selected systems, also the effects of nπ* excitation at longer excitation wavelengths have been studied, which is found to enhance trans → cis yields for free molecules but can lead to an opposite behavior in densely packed SAMs.