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

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Abstract

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/m2, 10.9 cd/A at 5115 cd/m2). Red OLEDs (CIE = 0.63, 0.37) with moderate performance (9580 cd/m2, 2.3 cd/A at 230 cd/m2) were obtained from the phenoxazine-benzoylquinoxaline 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 × 10−4 cm2/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.

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