Energy Transfer Mechanisms and the Molecular Exciton Model for Molecular Aggregates

The molecular exciton model is defined as the resonance interaction between the excited states of loosely bound molecular aggregates (molecular crystals, nonconjugated polymers, molecular laminae, molecular dimers and trimers). The Simpson and Peterson vibrational-electronic coupling cases are applied to spectral characteristics of molecular aggregates. The quasi-classical electrostatic vector model is described for simple molecular aggregates as an approach to understanding the formation of the exciton-state formation. Selection rules and exciton band structures are described for several molecular aggregate structures in the strongcoupling exciton model. In strong-coupling exciton cases, pronounced spectral effects of molecular aggregates compared with individual molecules are predicted, with very fast resonance transfer rates (≲,1015 sec-1) depending on the inverse cube of intermolecular distance. In weak-coupling exciton cases, no conspicuous spectral changes are predicted, but the transfer rates are still high (∼10 12 to 1013 sec-1) and preserve the inverse-cube intermolecular distance dependence. Fö rster's vibrational-relaxation resonance transfer mechanism is emphasized as a contrasting mechanism based on a different theoretical origin (interaction of continua model), with inverse sixth-power dependence on intermolecular distance and much slower transfer rates (106 to 1011 sec-1). Emphasis is placed on the need for new discriminative experimental work. © 2012 by Radiation Research Society. All rights of reproduction in any form reserved.