The Spectral Signatures of Frenkel Polarons in H- and J-Aggregates

摘要

Electronic excitations in small aggregates, thin films, and crystals of conju-ngated organic molecules play a fundamental role in the operation of a widenarray of organic-based devices including solar cells, transistors, and light-nemitting diodes. Such excitations, or excitons, are generally spread out overnseveral molecules: a balance between the delocalizing influence of resonantnintermolecular coupling and the localizing influence of static and dynamic dis-norder determines the coherence range of the exciton. Because of the “soft”nnature of organic materials, significant nuclear relaxation in the participatingnmolecules also accompanies the electronic excitations. To properly understand energy or charge transport, one must treatnintermolecular (excitonic) coupling, electron-vibrational coupling, and disorder on equal footing. In this Account, we reviewnthe key elements of a theoretical approach based on a multiparticle representation that describes electronic excitations innorganic materials as vibronic excitations surrounded by a field of vibrational excitations. Such composite excitations are appro-npriately called Frenkel excitonic polarons.nFor many conjugated molecules, the bulk of the nuclear reorganization energy following electronic excitation arises fromnthe elongation of a symmetric vinyl stretching mode with energy ∼1400 cm 1n. To appreciate the impact of aggregation,nwe study how the vibronic progression of this mode, which dominates the isolated (solvated) molecule absorption and emis-nsion spectra, is distorted when molecules are close enough to interact with each other. As we demonstrate in this Account,nthe nature of the distortion provides a wealth of information about how the molecules are packed, the strength of the exci-ntonic interactions between molecules, the number of molecules that are coherently coupled, and the nature of the disorder.nWe show that the aggregation-induced deviations from the Poissonian distribution of vibronic peak intensities take on twonextremes identified with ideal H- and J-aggregates.nThe sign of the nearest neighbor electronic coupling, positive for H and negative for J, distinguishes the two basic aggre-ngate forms. For several decades, researchers have known that H-aggregates exhibit blue-shifted absorption spectra and arensubradiant while J-aggregates exhibit the opposite behavior (red-shifted absorption and superradiance). However, the exactninclusion of exciton vibrational coupling reveals several more distinguishing traits between the two aggregate types: innH(J)-aggregates the ratio of the first two vibronic peak intensities in the absorption spectrum decreases (increases) withnincreasing excitonic coupling, while the ratio of the 0 0to0 1 emission intensities increases (decreases) with disordernand increases (decreases) with increasing temperature. These two extreme behaviors provide the framework for under-nstanding absorption and emission in more complex morphologies, such as herringbone packing in oligo(phenylene vinylene)s,noligothiophenes and polyacene crystals, as well as the polymorphic packing arrangements observed in carotenoids.