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 ω0 and Huang–Rhys factor λ2). 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≈λ2ℏω0, where the 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≫λ2ℏω0. Theoretical spectra compare favorably with experiment.