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] n Ir(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-C 2 ,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 ( Φ PL = 0.43–0.98) than for the 4-substituted 3b–5b ( Φ PL = 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).