Exciton Localization and Transfer in Azobenzene Aggregates: A Surface Hopping Molecular Dynamics Study

Molecular photoswitches are a promising class of molecules for the development of new functional, light-controlled materials. In complex systems composed of multiple photoswitchable units, photophysical and photochemical properties may be altered as compared to isolated chromophores. And phenomena such as excitation energy transfer may arise in the aggregated state. In the present work, using nonadiabatic molecular dynamics simulations in conjunction with transition density matrix analysis, we study exciton dynamics in H-type tetramers of azobenzene, a prototypical molecular switch. We consider “free” and “constrained” (embedded in an environment of additional azobenzene molecules) models with different intermolecular distances (3.5 and 5.5 Å). Our simulations reveal ultrafast exciton localization upon ππ* excitation, occurring on a sub-100 fs timescale and proceeding faster for the longer separation distance than for the shorter one. We also find that exciton transfer takes place during excited-state dynamics in the ππ* manifold, but it is strongly inhibited in the nπ* manifold. Moreover, we find that the ππ* trans → cis isomerization quantum yields are lower by a factor of about three for free/not strongly constrained tetramers than for the monomer, and no switching is observed for the most tightly packed model.