Vibronic Structure of Frenkel and Charge-Transfer Excitons in PTCDA

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Abstract
Perylenetetracarboxylic dianhydride (PTCDA) is ideal for applying molecular exciton theory to the evolution of molecular to solid-state excitations. The spectra of crystalline films and superlattices differ from solution due to the formation of uniform stacks. PTCDA shows mixing of nearly degenerate Frenkel and charge-transfer (CT) excited states and strong coupling to an out-of-phase C-C, C=C intramolecular stretch that corresponds to the effective conjugation coordinate of polymers. Its structural, electronic and vibrational simplicity can be modeled by a diatomic stack rather than a three-dimensional molecular lattice. Uniform stacks are realizations of Holstein models and provide the first example of vibronic coupling to mixed Frenkel-CT excitons. CT participation is analogous to polaron pairs on adjacent polymer chains, while Frenkel excitons are related to polyene and intrachain excitations. Calculated molecular parameters are consistent with spectroscopic values. Absorption processes show little dispersion and increased vibronic width; fluorescence is phonon-assisted and shifts to lower energy with increasing slack length; substituted perylenes form stacks with different overlaps and spectra. Localized and extended PTCDA states illustrate both interchain interactions in polymer films where single-crystal resolution is lacking and organic molecular crystals with more extensive and complicated solid-state contributions.