The substituent effect of 2-R-o-carborane on the photophysical properties of iridium(iii) cyclometalates

A family of heteroleptic iridium(iii) cyclometalates, 4-(2-RCB)ppy2Ir(acac) (3c-3g; CB = o-carboran-1-yl; ppy = 2-phenylpyridinato-C(2),N; acac = acetylacetonate; R = (i)Pr (3c), (i)Bu (3d), Ph (3e), CF3C6H4 (3f), C6F5 (3g)) with various 2-R substituted o-carboranes at the 4-position in the phenyl ring of the ppy ligand, were prepared. The X-ray crystal structure of all complexes, including 3a (R = H) and 3b (R = Me), showed that while the carboranyl C-C bond distance increases with the increasing steric effect of the 2-R substituent (3a-3d), the bond distance is more likely to be influenced by the electronic effect of the substituent for the 2-aryl substituted complexes (3e-3g). The absorption and emission bands of all complexes were red-shifted with respect to those of the parent (ppy)2Ir(acac) (4). While 3a-3d exhibited intense green phosphorescence with good quantum efficiency in toluene (ΦPL = 0.17-0.47), the complexes were poorly emissive in THF (ΦPL < 0.004). In a PMMA film, however, 3a-3d became substantially emissive (ΦPL = 0.11-0.14, 8 wt% Ir), albeit less emissive than 4 (ΦPL = 0.15). In contrast, the 2-aryl substituted 3e-3g were almost non-emissive in both the solution and solid states. All complexes underwent facile carborane-centered, electrochemical reduction (Eonset = -2.01 to -1.22 V). The potential values of these reductions increased with increases in the electron accepting ability of the 2-R group (H < alkyl < aryl) and were largely shifted anodically with respect to those of 4 and 5-carborane substituted complexes. This reduction behavior of 3a-3g implies that the LUMO contribution of 2-R-o-carborane units increases through the substitution at the 4-position of the ppy ligand and is influenced by the nature of the 2-R group, which may be responsible for facile carboranyl C-C bond variation leading to efficient quenching of the emissive excited states.