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 D(0)A(0) and D(+)A(-). The agreement is also limited to the perturbation regime where intermolecular interactions are relatively weak. However, in the polyene limit, where the ground state of the monomer is dominated by the D(0)A(0) component, the agreement with the Kasha/ Frenkel model breaks down. In this limit, GI dimers exhibit exaggerated spectral shifts, as much as twice as large as the Kasha shifts, while GII aggregates exhibit shifts which can be much smaller than the Kasha shifts. Hence, in the latter case, one can obtain practically unshifted H- and J-aggregates. When vibronic coupling is incorporated into the ESM via a Holstein-like Hamiltonian, it is further revealed that the vibronic signatures derived from the Frenkel exciton theory carry over to the ESM in the weak coupling regime, providing an unambiguous means for determining H- and J-aggregation. In the strong coupling limit, the divergences from the Kasha model are even more pronounced with the emergence of strongly blue-shifted J-aggregates with enhanced vibronic coupling. Moreover, when the intermolecular coupling V becomes equal to -2 eta, where eta is the monomer's diabatic excitation energy (with eta > 0), the dimer behaves as if no vibronic coupling were present at all. In other words, the strong vibronic progressions characteristic of a monomer in the polyene limit vanish when V = -2 eta. Applications are made to a merocyanine dimer, which abides by the GII configuration.