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

We present a comprehensive investigation of cocrystalformationand charge transfer effects in weakly interacting organic semiconductormixtures. As a model system, we choose diindenoperylene (DIP) as adonor molecule and several perylene diimide derivatives (PDI) as acceptormolecules that differ in the n-alkyl side chain inthe imide position and in the cyano (CN) group in the bay position.We identified the optimized side groups for the acceptors in thinfilms with the donor DIP concerning the mixing behavior and molecularcharge transfer (CT) effects. The two systems, which form a well-definedcocrystal and show excited-state charge transfer effects, are DIP:PTCDI-C-3 with an n-propyl side chain and DIP:PTCDI-C-8-CN2 with incorporated cyano groups. Importantfor the mixing behavior and the charge transfer effects with DIP arethe intermolecular interactions of the pure perylene diimide derivativesand the orientation of these molecules on the substrate (SiO2). For the DIP:PTCDI-C-3 system, a sharp CT peak in absorptionwith a well-defined CT energy is observed. In contrast, the DIP:PTCDI-C-8-CN2 mixed films show a broad CT band in absorptionand two different CT energies. The mixing behavior and charge transfereffects with DIP are strongly influenced by the structure of the acceptors,which are easily chemically tunable in the desired way.