Intermolecular Charge Transfer in H- and J-Aggregates of Donor-Acceptor-Donor Chromophores: The Curious Case of Bithiophene-DPP

A vibronic exciton model is developed to describe low-energy electronic excitations in slip-stack aggregates of donor-acceptor-donor (DAD) chromophores with substantial overlap between the neighboring donor and acceptor fragments. In such stacks, J- and H-aggregate behavior is driven by intermolecular charge transfer (ICT) and not Coulomb coupling as is assumed in the Kasha model. In-phase (out-of-phase) intermolecular charge transfer integrals result in J-aggregate (H-aggregate) behavior, as unambiguously determined by the vibronic spectral signatures. Interestingly, both J- and H-aggregates are red-shifted, in contrast to the predictions of the Kasha model. Simulated spectral line shapes agree well with recent experiments on two derivatives of the bithiophene diketopyrrolopyrrole chromophore, 2T-DPP-2T, with different terminal groups.