Methoxy-substituted bis-tridentate iridium(iii) phosphors and fabrication of blue organic light emitting diodes

Abstract

Both monoanionic dicarbene pincer chelate and dianionic azole-pyridine-carbazole cyclometalate were successfully employed in the preparation of respective bis-tridentate Ir(III) metal complexes (Cz6–9) in moderate yields. Tuning of emission to blue was achieved by the addition of dual methoxy substituents at the carbazole cyclometalate, as well as the introduction of either methoxy or dimethylamino group at the central pyridinyl fragment of the azole-pyridine-carbazole cyclometalate. Single-crystal X-ray diffraction study was conducted on complex Cz6 in an attempt to provide an unambiguous structural proof. Photophysical properties were next measured in degassed CH₂Cl₂ solution at RT, giving structureless emission with peak maximum spanning 460–508 nm and photoluminescence quantum yield of 22–87%. A TD-DFT calculation confirmed the dominance of azole-pyridine-carbazole cyclometalate in controlling the emission energy gap, while the carbene pincer was less influential on the detected emission peak wavelength, i.e. acted as an ancillary. Importantly, the experimentally detected radiative lifetime (e.g. 2.78–17.18 μs) showed a clear correlation to the calculated metal-to-ligand charge transfer percentage of the electronic transitions (10.8–7.5%). Organic light-emitting diode devices gave turn-on voltages of 4.3 and 4.3 V, maximum EQE = 16.3 and 12.2%, power efficiencies of 17.6 and 10.4 lm W⁻¹ and with CIEx,y coordinates of (0.16, 0.26) and (0.16, 0.20) for Cz6 and 7 at 15 wt%, confirming their potential as blue dopants for phosphorescent organic light-emitting diodes.