Potential surfaces and delocalization of excitons in dimers

In the present work we will demonstrate that the nuclear dynamics have a strong influence on the delocalization of an exciton in a dimer, even if they do not effect the excitonic interaction. It will be shown that the internal nuclear conformation of the molecules forming the dimer depends critically on the delocalization of the exciton state in the dimer and vice versa. The resulting closed loop enforces a localization of the lower excitonic state, but, contrary to the commonly accepted view, a delocalization of the upper one. Qualitatively different time-evolution of the delocalization length for the lower and upper excitonic state will be shown. Besides, it will turn out that the nuclear motions inhibit a complete delocalization of the excitonic state in any case. To accomplish nuclear and exciton dynamics, the nonadiabatic coupling between the two excitonic states will be deduced. This causes a relaxation from the upper to the lower excitonic state, which limits the maximum reachable exciton delocalization.