Phenothiazine-based bipolar green-emitters containing benzimidazole units: synthesis, photophysical and electroluminescence properties

Abstract

A series of bipolar green emitters bearing hole-transporting phenothiazine and electron-transporting N-phenylbenzimidazole are synthesized and characterized by their photophysical, electrochemical and thermal properties. All the dyes exhibited solvent dependent emission characteristics indicative of a more polarized excited state probably arising from the enhanced intramolecular charge transfer (ICT) character in the photo-excited state. Dyes possessing two N-phenylbenzimidazole units and pyridine showed a larger degree of ICT in the excited state than the remaining dyes as evidenced from the huge solvatochromic shifts in emission spectra. The dyes exhibited facile oxidation attributable to the removal of an electron predominantly from the phenothiazine donor. However, the oxidation is positively shifted due to the electronic perturbations arising from the electron-withdrawing N-phenylbenzimidazole and pyridine units. The compounds exhibited marked thermal stability with high decomposition temperatures in the range, 412–454 °C. Solution processed organic light-emitting diodes fabricated using the dyes as neat hole-transporting emitting layer or dopant emitter in CBP host showed promising device parameters. A dye possessing three phenothiazine units exhibited bright green electroluminescence with maximum luminance of 1365 cd m⁻², CIE coordinates (0.314, 0.593) resembling the prescribed NTSC 1987 standards with a current efficiency of 1.9 cd A⁻¹ at 100 cd m⁻².