Elucidating the Microscopic
Origin of the Unique Optical
Properties of Polypyrene
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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