Polymorphic Phase-Dependent Optical and Electrical Properties of a Diketopyrrolopyrrole-Based Small Molecule

Four different polymorphic conformations of diethyl 5,5′-[5,5′-[2,5-bis­(2-ethylhexyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo­[3,4-<i>c</i>]­pyrrole-1,4-diyl]­bis­(thiophene-5,2-diyl)]­difuran-2-carboxylate (DPP-(CF)<sub>2</sub>), namely, DPP-(CF)<sub>2</sub>-α, DPP-(CF)<sub>2</sub>-β, DPP-(CF)<sub>2</sub>-γ, and DPP-(CF)<sub>2</sub>-ω, were identified from X-ray diffraction analysis conducted on their thin films and single crystals. Highly crystalline and well-textured thin films of these four polymorphs were successfully prepared via postgrowth solvent vapor and thermal annealing treatments to investigate the polymorphic phase-dependent optical and electrical properties of DPP-(CF)<sub>2</sub>. Interestingly, during the phase transition from DPP-(CF)<sub>2</sub>-α to DPP-(CF)<sub>2</sub>-ω, the optical band gap decreases from 1.75 to 1.5 eV because of the enhanced π–π interaction between the neighboring molecules. Except for DPP-(CF)<sub>2</sub>-γ, the other three phases show ambipolar charge transport. Although DPP-(CF)<sub>2</sub>-β and DPP-(CF)<sub>2</sub>-γ exhibit a similar way of packing, a small increment in the π–π-stacking distance (0.006 Å) and twist conformation of the grafted electron-donating moieties of DPP-(CF)<sub>2</sub>-γ are found to reduce its hole mobility.