Deep blue phosphorescent iridium(III) cyclometalates with o-carboranes

Abstract

Heteroleptic iridium(III) cyclometalates, [5-(2-MeCB)dfppy]2Ir(LX) (2-MeCB = 2-methyl-o-carboran-1-yl; dfppy = 2-(4,6-difluorophenyl)pyridinato-C2,N; LX = picolinate (5a) or tetrakis(1-pyrazolyl)borate (5b)), with o-carborane at the 5-position of the phenyl ring of the dfppy ligand were prepared to investigate the impact of the o-carborane cage on the blue phosphorescence of complexes. X-ray diffraction analysis of 5a revealed a distorted octahedral coordination geometry around the iridium center with the peripheral carborane substitution of the dfppy ligand. The absorption and emission bands of 5a and 5b were hypsochromically shifted relative to those of the sky blue (dfppy)2Ir(pic) (FIrpic) complex, resulting in deep blue phosphorescence (λPL = 459 nm for 5a and 452 nm for 5b vs. 468 nm for FIrpic). The complexes showed high to moderate quantum efficiencies in toluene (ΦPL = 0.89 for 5a; 0.47 for 5b) and in PMMA film (ΦPL = 0.45 for 5a; 0.34 for 5b), despite a large decrease in quantum efficiency in THF. A slight decrease in quantum efficiency was observed with increasing doping concentrations of the complexes in PMMA films, indicating suppression of the emission quenching owing to the steric bulkiness of the 2-Me-o-carborane cage. Anodic shifts in the oxidation potentials of 5a and 5b compared with that of FIrpic indicated that the stabilization of the HOMO level by the inductive electron-withdrawing effect of the o-carborane is responsible for the deep blue emission of 5a and 5b.