Exciton-vibrational coupling in molecular aggregates: Electronic versus vibronic dimer

The influence of exciton-vibrational coupling on the energy level structure, oscillator strength, and relaxation dynamics is investigated for two different excitonic dimer models. As compared with a purely electronic dimer, the inclusion of local vibrational modes within a vibronic dimer gives rise to a complex energy level structure including avoided crossings and changes of the nature of the exciton states from electronic to vibrational character. Besides these static properties, the dissipative dynamics of the two models is systematically investigated using Redfield relaxation theory. In case of the vibronic dimer this allows to treat selected vibrational degrees of freedom beyond the limits of perturbation theory and Markov approximation. It is demonstrated that the vibronic dimer gives rise to transient vibrational population trapping in the one-exciton manifold. (C) 2011 Elsevier B.V. All rights reserved.