Absorption and Photoluminescence in π‑Stacks of Donor−Acceptor−Donor Chromophores: A Frenkel-Holstein approac

A vibronic exciton model is introduced to describe the excited state band structure and associated absorption and photoluminescence (PL) spectra of low bandgap donor– acceptor π-conjugated polymers¹. The Hamiltonian is represented in a diabatic basis, comprising Frenkel-like donor and acceptor fragment excitations, along with charge-transfer (CT) excitations between neighboring fragments. The J-aggregate behavior in the crystal is enhanced by intermolecular charge-transfer (ICT) integrals. In-phase ICT integrals lead to J-aggregate behavior, while out-of-phase ICT integrals result in H-aggregate behavior. Notably, both J- and H-aggregates exhibit red shifts, contrary to the predictions of the Kasha model. Later, a two-level system with vibronic coupling based on an effective Frenkel-Holstein (EFH) Hamiltonian is proposed to simulate the absorption and PL spectra with recent experiments on bithiophene-DPP (2T-DPP-2T) thin films and nanoparticles². Finally, triplet pair states are incorporated into the Frenkel-Holstein model to describe singlet fission (SF) in aggregated perylene diimide (PDI) chromophores. The CT states play vital roles in the optical signature of SF state. We finally compare the simulated absorption spectra with experimental measurements, capturing insights into the nature of singlet fission in EP-PDI chromophores.