New ambipolar organic semiconductors. 2. Effects of electron acceptor strength on intramolecular charge transfer photophysics, highly efficient electroluminescence, and field-effect charge transport of phenoxazine-based donor-acceptor materials

Substantial variations (up to factors of 5) were observed in the intramolecular charge-transfer (ICT) fluorescence quantum yields and electroluminescence efficiencies among a series of emissive bipolar donor-acceptor (D-A) materials based on a phenoxazine donor and different acceptors with a varying electron acceptor strength, including quinoline, quinoxaline, benzoquinoxaline, and benzoylquinoxaline. High-efficiency organic light-emitting diodes (OLEDs) with colors spanning, the visible spectrum were achieved from the new emissive ambipolar materials. The performance of the OLEDs based on the D-A molecules decreased with increasing electron acceptor strength, largely owing to the reduction in fluorescence efficiencies. Green OLEDs (CIE = 0.27, 0.61) from a phenoxazine-quinoline molecule gave the best performance (36190 cd/m(2), 10.9 cd/A at 5115 cd/m(2)). Red OLEDs (CIE = 0.63, 0.37) with moderate performance (9580 cd/m(2), 2.3 cd/A at 230 cd/m(2)) were obtained from the phenoxazinebenzoylquinoxaline molecule. These results show that the electron accepting strength of the acceptor moiety in a D-A molecule is a convenient method for varying the HOMO/LUMO energy levels and the resultant electroluminescence emission colors. Field-effect hole mobilities of up to 7 x 10-4 cm(2)/Vs were obtained in the phenoxazine-quinoline D-A molecules, demonstrating the potential of phenoxazine as a building block for developing new emissive and charge-transport materials for OLEDs.