Impact of the number of o-carboranyl ligands on the photophysical and electroluminescent properties of iridium(III) cyclometalates

A series of iridium(III) cyclometalates with mono-, bis-, and tris-o-carboranyl ligands, fac-[m-(2-MeCB)ppy]nIr(ppy)3−n (n = 1–3, m = 5 (3a–5a); m = 4 (3b–5b); 2-MeCB = 2-methyl-o-carboran-1-yl; ppy = 2-phenylpyridinato-C2,N), was prepared to investigate the impact of the number of o-carboranyl ligands on the photophysical and electroluminescent properties. X-ray diffraction analysis of 3b–5b revealed that the carboranyl C–C bond distances remained nearly unchanged with different numbers of carboranyl ligands. While the phosphorescence wavelengths of 3a–5a and 3b–5b were blue-shifted or red-shifted with respect to that of fac-Ir(ppy)3 (6), respectively, both series underwent gradual hypsochromic shifts as the number of carboranyl ligands was increased. All of the complexes exhibited moderate to high phosphorescence quantum efficiencies in toluene with a slightly higher efficiency for the 5-substituted 3a–5a (ΦPL = 0.43–0.98) than for the 4-substituted 3b–5b (ΦPL = 0.42–0.92). In particular, the quantum efficiency increased as the number of carboranyl ligands increased. Solution-processed green phosphorescent organic light-emitting diodes (PhOLEDs) incorporating the 5-substituted complexes (3a–5a) as emitters exhibited a similar level of high performance, of which the best performance was observed for devices based on 4a. Furthermore, vacuum-deposited devices were successfully fabricated with heteroleptic mono- and bis-carboranyl complexes, 3a and 4a. The resulting devices showed excellent performance, which was improved by ca. 40% compared to those of the solution-processed devices. The devices based on 4a also displayed a performance that was superior to those of the devices based on 3a (max. CE = 45.8 cd A−1; PE = 35.4 lm W−1; EQE = 15.1% for 4a at 7 wt% Ir).