New ambipolar organic semiconductors. 1. Synthesis, single-crystal structures, redox properties, and photophysics of phenoxazine-based donor-acceptor molecules

We report the synthesis and structural, photophysical, and electrochemical properties of a series of eight new phenoxazine-based donor-acceptor molecules utilizing phenoxazine as a common donor and quinoline, quinoxaline, benzoylquinoxaline, and benzoquinoxaline as various electron acceptor moieties. The phenoxazine-phenylquinoline molecules crystallized in the monoclinic system with the space groups C2/c and P2(1)/c, respectively, revealing a 169 degrees dihedral angle in the phenoxazine unit. The new materials had high glass transition temperatures (128-193 degrees C) and ambipolar redox properties with reversible electrochemical oxidation and reduction. Their HOMO levels were at 5.1-5.2 eV, whereas their LUMO levels (2.3-3.0 eV) increased with the electron acceptor strength. The materials showed intramolecular charge transfer fluorescence in dilute solutions with emission colors varying from blue to red and a quantum yield ranging from 86% in the phenoxazine-biphenyl molecule to as low as 16% in the benzoquinoxaline-containing molecule. Density functional theory calculations provided insight into the molecular geometry and electronic structures of the donor-acceptor molecules and were in good agreement with the experimental results. These results demonstrate that the new phenoxazine-based donor-acceptor molecules represent a new class of promising ambipolar semiconductors for organic electronics.