Unusual dual-emissive heteroleptic iridium complexes incorporating TADF cyclometalating ligands

Abstract

Five new neutral heteroleptic iridium(III) complexes IrL₂(pic) (2–6) based on the archetypical blue emitter FIrpic have been synthesised. The cyclometallating ligands L are derived from 2-(2,6-F₂-3-pyridyl)-4-mesitylpyridine (7), 2-(3-cyano-2,6-F₂-phenyl)-4-mesitylpyridine (8), 2-(2,6-F₂-phenyl)-4-[2,7-(HexO)₂-9H-carbazol-9-yl]pyridine (9), 2-(2,6-F₂-3-pyridyl)-4-[2,7-(HexO)₂-9H-carbazol-9-yl]pyridine (10) and 2-(3-cyano-2,6-F₂-phenyl)-4-[2,7-(HexO)₂-9H-carbazol-9-yl]pyridine (11) for complexes 2, 3, 4, 5 and 6, respectively. The carbazole-functionalised ligands 9–11 show weak thermally activated delayed fluorescence (TADF) in solution. Complexes 5 and 6 reveal dual emission in polar solvents. A broad charge transfer (CT) band appears and increases in intensity relative to the higher energy emission band as solvent polarity is increased. The dual emission occurs when the energy of the ligand ³CT state is comparable to that of the ³MLCT state of the complex, resulting in fast interconversion between the two. Assignment of the ligand TADF and dual emission properties is supported by hybrid density functional theory (DFT) and time dependent DFT (TD-DFT) calculations. Phosphorescent organic light emitting devices (PhOLEDs) have been fabricated using these complexes as sky-blue emitters, and their performance is compared to devices using FIrpic and the previously reported complex IrL₂(pic) 1 (L from the 2-(2,6-F₂-phenyl)-4-mesitylpyridine ligand). For identical device structures, the device containing the carbazole complex 4 performs best out of the seven complexes. The dual emission observed in solution for complexes 5 and 6 is not observed in their devices.