Signatures of exciton-phonon coupling in linear absorption spectra of molecular aggregates: A polaron transformation approach

Silvis, M. H.

Master's thesis. University of Groningen, The Netherlands (2012).



Motivated by suggestions that exciton–phonon coupling plays an important role in determining the electronic and optical properties of molecular aggregates, in the current research project we have aimed at modeling such dynamical vibrational effects. Taking the Holstein Hamiltonian as a starting point, a new method that utilizes a variational polaron transformation was devised to calculate the linear optical response of molecular aggregates in which vibrations play a role. Focusing on small model systems containing a few to a few of tens of molecules, it was shown that this method is valid over a large range of exciton–phonon and electronic coupling strengths, as verified by comparing to a two-particle basis set approach. Not only does the current method go beyond the usual perturbative approaches by accurately probing the intermediate regime of couplings, it also provides insightful analytic expressions for linear absorption spectra at zero and finite temperatures. The computational costs of the calculation of such zero-temperature spectra were seen to be much smaller than that for two-particle method calculations. Examining absorption spectra at increasing temperature two interesting effects were observed, the first of which is a broadening of the vibrational band associated with exciton–phonon coupling, and the other being thermal destruction of exciton coherence.