Molecular Charge Transfer Effects on Perylene Diimide Acceptor and Dinaphthothienothiophene Donor Systems

The interactions between organic donor and acceptor molecules and the related charge transfer (CT) effects are of great interest in organic optoelectronics. Here, we present a comprehensive investigation of cocrystal formation and charge transfer effects in weakly interacting organic semiconductor mixtures. As a model system, we choose dinaphthothienothiophene (DNTT) as a donor molecule and two different perylene diimide derivatives (PTCDI-C-8-CN2 and PDIF-CN2) as acceptors, which differ in the fluorination of the side chains in the imide position. Experimentally, both systems show a small ground-state CT governed by hybridized HOMO-1 and LUMO+1 levels. In contrast, the respective HOMO and LUMO levels of the complex are localized on the acceptor and donor molecule. This leads to the observation of a nearly pure charge transfer excitation from the acceptor to the donor in the absorption spectra. We discuss the general impact of localized HOMO and LUMO levels on the optoelectronic properties in CT complexes dependent on comparison with first-principles calculations based on density functional theory and many-body perturbation theory.