Absorption and Photoluminescence in π-Stacks of Donor-Acceptor-Donor Chromophores: Effective Frenkel-Holstein Hamiltonian Approach

Aggregates and crystals of donor-acceptor-donor (DAD) chromophores are of significant current interest due to their ability to express both intra- and intermolecular charge transfer, making them ideal candidates for optoelectronic applications, such as field effect transistors. The current work focuses on DAD slipped pi-stacks in which the donor of one molecule substantially overlaps the acceptor of an adjacent molecule, leading to efficient inter molecular charge transfer (ICT). A model is introduced that treats each DAD chromophore as a two-level system with vibronic coupling based on an effective Frenkel-Holstein (EFH) Hamiltonian. The approach allows one to accurately capture the details of the lowest-energy optical absorption band of DAD assemblies with much less computational cost than a more robust model used in previous works, which treats each DAD chromophore as three interacting donor/acceptor fragments. Absorption and photoluminescence spectral simulations are in very good agreement with recent experiments on bithiophene-DPP (2T-DPP-2T) thin films and nanoparticles.