Vibronic structure of Frenkel and charge-transfer excitons in PTCDA

Perylenetetracarboxylic dianhydride (PTCDA) is ideal for applying molecular exciton theory to the evolution of molecular to solid-state excitations. T he 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 effect ive conjugation coordinate of polymers. Its structural, electronic and vibr ational simplicity can be modeled by a diatomic stack rather than a three-d imensional molecular lattice. Uniform slacks are realizations of Holstein m odels and provide the first example of vibronic coupling to mixed Frenkel-C T excitons. CT participation is analogous to polaron pairs on adjacent poly mer chains, while Frenkel excitons are related to polyene and intrachain ex citations. Calculated molecular parameters are consistent with spectroscopi c values. Absorption processes show little dispersion and increased vibroni c width; fluorescence is phonon-assisted and shifts to lower energy with in creasing stack length; substituted perylenes form stacks with different ove rlaps and spectra. Localized and extended PTCDA states illustrate both inte rchain interactions in polymer films where single-crystal resolution is lac king and organic molecular crystals with more extensive and complicated sol id-state contributions.