Exciton-vibrational coupling in pinwheel aggregates of π-conjugated molecules

Absorption spectra of para-distyrylbenzene (DSB) aggregates are calculated based on a Hamiltonian which includes excitonic coupling between molecules, each containing a single electronic transition coupled to an intramolecular vibration (with frequency omega(0) and Huang-Rhys factor lambda(2)). Aggregates are based on the four-molecule pinwheel unit which maximizes the favorable edge-face interactions. Pinwheel aggregates of DSB lie within the intermediate coupling regime, J approximate to lambda(2)(h) over bar omega(0), where thr excitonic bandwidth J is of the order of the nuclear relaxation energy. A square lattice of pinwheel units lies primarily within the strong coupling regime J >> lambda(2)(h) over bar omega(0). Theoretical spectra compare favorably with experiment. (C) 1999 Elsevier Science B.V. All rights reserved.