(678f) Phenoxazine-Based Organic Semiconductors for Light-Emitting Diodes and Thin Film Transistors

Organic and conjugated polymer semiconductors are currently of great interest for applications in organic electronics, such as thin film transistors, photovoltaic cells, and light emitting devices. Development of new building blocks for the design of polymer and organic semiconductors are essential to improving the performance of organic electronic devices. We recently synthesized and characterized a new class of conjugated polymers and organic semiconductors based on the tricyclic phenoxazine ring as the building block, and successfully used them as the semiconductors in organic field effect transistors (OFETs) and as the highly fluorescent ambipolar materials in organic light emitting diodes (OLEDs). The phenoxazine-based polymers, synthesized by Suzuki or Yamamoto coupling polymerization, have high glass transition temperatures (112-230^oC), high decomposition temperatures (393-414^oC), and optical band gaps of 2.43-2.65 eV. These polymers showed highly reversible electrochemical oxidation with low ionization potentials (4.8-4.9 eV), which facilitate ready injection of holes from gold and other high work function electrodes. Compared to the widely used tricyclic carbazole ring, the ionization potential of phenoxazine is 0.7 eV lower in energy and thus hole injection and transport should be more facile in phenoxazine-based semiconductors. Organic field-effect transistors based on the phenoxazine copolymers had a field effect hole mobility of up to 6 ×10^-4cm²/(Vs) with an on/off ratio exceeding 10⁴. New donor-acceptor (D-A) molecules exhibiting intramolecular charge transfer fluorescence were designed with phenoxazine as the donor and phenylquinoline or quinoxaline as the acceptor. The resulting D-A materials had high glass transition temperatures and possessed reversible electrochemical oxidation and reduction that facilitated efficient hole/electron injection, transport, and recombination. Simple OLEDs made from phenoxazine-quinoline molecules showed high brightness (9510 cd/m²) and efficient (3.42 cd/A at 6580 cd/m²) green electroluminescence at low voltages in ambient air. Other electroluminescence colors were achieved by incorporating different electron acceptors into the phenoxazine-containing molecules. A field effect hole mobility of 7×10^-4cm²/Vs was found in the phenoxazine-quinoline molecules, suggesting that efficient light-emitting transistors could be achieved if phenoxazine-based semiconductors are incorporated into appropriate device structures. These results demonstrate that phenoxazine is a very promising building block for the design of new charge transport and emissive materials for field effect transistors, OLEDs, and other organic electronics.