Excitonic spectra of dual hexagonal-trigonal 2D donor-acceptor lattices

Theoretical model of the excitonic spectra in hexagonal 2D structures of two components (donors (D) and acceptors (A) respectively) has been explored. The A molecules occupy the nodes of a hexagonal lattice and D molecules are located in the centers of hexagons and create a trigonal lattice. 2D compounds of similar structure DA2 have been realized in MXenes — transition metal carbides, nitrides, or carbonitrides. Two types of molecular excitons and their coupling have been treated: (a) Charge Transfer Excitons (CTEs) of ionized DA pair of neighbor molecules (six branches of CTEs); (b) Frenkel excitons with the origin of electronic excitations of neutral excited D or A molecule. The symmetry analysis and Green functions methods are the basis of calculations of the excitonic spectra and of the levels of linear absorption in polarizations perpendicular and parallel to the molecular layer.