Unusual Non-Kasha Photophysical Behavior of Aggregates of Push–Pull Donor–Acceptor Chromophores

Unlike the Frenkel exciton model, the essential state model (ESM) for aggregates of donor–acceptor (DA) chromophores accounts self-consistently for the effect of the reaction field on the ground and excited states of the individual chromophores, an effect that can lead to divergences from Kasha’s H- and J-aggregate classification schemes for highly polarizable molecules. In this work, two slip-stack dimer geometries of DA chromophores are considered, one with parallel permanent dipole moments (Geometry I) and the other with antiparallel dipole moments (Geometry II). Based on the ESM, it is shown that in both GI and GII dimers the aggregation-induced spectral shifts characteristic of H- and J-aggregates agree exactly with the Kasha/Frenkel model, but only in the cyanine limit, where both the ground and excited states of an isolated chromophore consist of equal admixtures of the neutral and charge-separated diabatic states D0A0 and D+A–. The agreement is also limited to the perturbation regime where intermo...