  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.
