Time Dependent Density Functional Theory calculations of the optical properties of charge-transfer complexes

Abstract: Charge transfer complexes are materials with a wide range of interesting optical and electronic properties. They have seen a great deal of research over the past decade, both in device development as well as research to elucidate the underlying mechanisms of charge transfer and transport. Here we present time dependent density-functional theory (TD-DFT) calculations of the energy levels of the donor-acceptor complexes perylene-TCNQ (7,7,8,8-tetracyanoquinodimethane) and hexamethylbenzene-chloranil. The calculations show a good match to the experimentally measured optical absorption spectra, and indicate that low energy CT absorption bands appear at the dimer/tetramer stage of cocrystal formation. We also report the degree of charge transfer for these complexes in various stoichiometries and compare them.