Excitons in a hexagonal graphene-like donor–acceptor lattice

Spectra of excitonic excitations in a high-symmetry 2D molecular lattice with two different entities—donors and acceptors have been studied. We use the geometry of two hexagonal sub-lattices in graphene created by donors and acceptors, respectively, and investigate both Frenkel excitons (FEs) and charge-transfer excitons (CTEs) along with their coupling and manifestation in the linear absorption spectra. Vibronic spectra that consist of excitons and one-mode intramolecular vibrations of the donor or acceptor molecules are explored both analytically and numerically. Our calculations model the excitonic density of state and the linear absorption spectra in excitonic and vibronic regimes for two polarization of light, notably in the lattice plane and perpendicular to it. Lineshapes of the absorption depend upon the FE–CTEs–phonon coupling and exhibit bound and unbound exciton–phonon states in one-phonon and two-phonon vibronic spectra.