The photo-orientation of azobenzene in viscous solutions, simulated by a stochastic model.

We report a computational study of the photo-orientation kinetics in a viscous solution of azobenzene in ethylene glycol, under irradiation with linearly polarized light. The development of anisotropy and its interplay with photoisomerization are simulated by a stochastic model. A distinctive feature of the model is that it takes into account the photo-orientation angular distributions, specific for each isomer, obtained by nonadiabatic dynamics simulations at the molecular level. We find that the anisotropy, as measured by optical absorption dichroism, does not necessarily increase monotonously with time. As expected, the photo-orientation turns out to be strongly coupled with photoisomerization, but the latter is not a mandatory ingredient of this phenomenon: we predict that any chromophore undergoing large amplitude geometry relaxation during its excited state dynamics can develop anisotropy under suitable conditions.