Non-Kasha Behavior in Quadrupolar Dye Aggregates: The Red-Shifted H-Aggregate

Aggregates of quadrupolar DAD-type chromophores are investigated theoretically using the essential states model (ESM) enhanced to include vibronic coupling. On the basis of vibronic signatures in the absorption and photoluminescence spectra, important connections are made to the Frenkel exciton model, which is the basis for defining conventional H- and J-aggregates. In contrast to the exciton theory, the ESM allows for an unusual type of the red-shifted H-aggregate, driven by the large quadrupole-quadrupole interaction. Except for the red shift of the main absorption peak, such aggregates display all of the characteristics of conventional H-aggregates, including suppressed radiative decay rates and all of the vibronic signatures in the absorption and photoluminescence spectra consistent with Frenkel exciton models. Red-shifted H-dimers occur for a range of slip angles, which separate conventional H- and J-aggregate behavior, with the slip range diminishing with increasing intermolecular separation. Davydov splitting and vibronic signatures for dimers with nonparallel transition dipole moments are also investigated. The theoretical insight provided herein may be exploited for optimizing optoelectronic device applications for growing families of quadrupolar molecules.