Influence of Alkyl Chain Variation on Cocrystal Formation and Molecular Charge Transfer in DIP:Perylene Diimide Binary Systems

We present a comprehensive investigation of cocrystal formation and charge transfer effects in weakly interacting organic semiconductor mixtures. As a model system, we choose diindenoperylene (DIP) as a donor molecule and several perylene diimide derivatives (PDI) as acceptor molecules that differ in the n-alkyl side chain in the imide position and in the cyano (CN) group in the bay position. We identified the optimized side groups for the acceptors in thin films with the donor DIP concerning the mixing behavior and molecular charge transfer (CT) effects. The two systems, which form a well-defined cocrystal and show excited-state charge transfer effects, are DIP:PTCDI-C3 with an n-propyl side chain and DIP:PTCDI-C8-CN2 with incorporated cyano groups. Important for the mixing behavior and the charge transfer effects with DIP are the intermolecular interactions of the pure perylene diimide derivatives and the orientation of these molecules on the substrate (SiO2). For the DIP:PTCDI-C3 system, a sharp CT peak in absorption with a well-defined CT energy is observed. In contrast, the DIP:PTCDI-C8-CN2 mixed films show a broad CT band in absorption and two different CT energies. The mixing behavior and charge transfer effects with DIP are strongly influenced by the structure of the acceptors, which are easily chemically tunable in the desired way.