Elucidating the Microscopic Origin of the Unique Optical Properties of Polypyrene

Abstract

A combination of Time-Dependent Density Functional Theory (TD-DFT) and approximate coupled cluster theory (CC2) is used to elucidate the microscopic origin of the experimental observation that the absorption and fluorescence spectra of 1,3-polypyrene display a much smaller shift with chain length than other conjugated polymers. The optical absorption and fluorescence spectra of a large range of oligomers are calculated using TD-DFT and CC2 and successfully compared with available experimental data. The calculations show that the lowest singlet excitation is excitonic in nature and that this exciton becomes strongly localized upon excited state relaxation. This strong localization explains the negligible shift in fluorescence energy between the dimer/trimer and polymer, observed in experiment.