Dynamics of exciton-polaron transition in molecular assemblies: the variational approach

Dynamics of excitonic polaron formation in molecular systems coupled to an overdamped bath are investigated using the Dirac-Frenkel variational principle and Davydov D1 Ansatz. Using a two-site model system we show that a few qualitatively distinct relaxation regimes of an optically created exciton are possible, depending on the timescale of bath fluctuations. A slow bath always leads to adiabatic polaron formation. Non-adiabatic exciton self-trapping occurs when the system is strongly coupled to a fast bath. Weak coupling to such bath does not perturb the excitonic picture. The complex system-bath dynamics can then be mapped to an effective model where the resonant coupling between sites is quenched during relaxation. The timescale of the polaron formation can be defined by the timescale of resonant coupling quenching, and is found to directly correlate with the bath relaxation time.