Phosphorescence Color Tuning of Cyclometalated Iridium Complexes by o-Carborane Substitution

Heteroleptic ((CN)-N-boolean AND)(2)Ir(acac) ((CN)-N-boolean AND = 4-CBppy (1); 5-CBppy (2), 4-fppy (4) CB = ortho-methylcarborane; ppy = 2-phenylpyridinato-C-2,N, 4-fppy = 2-(4-fluorophenyl)-pyridinato-C-2,N, acac = acetylacetonate) complexes were prepared and characterized. While 1 exhibits a phosphorescence band centered at 531 nm, which is red-shifted compared to that of unsubstituted (ppy)(2)Ir(acac) (3) (lambda(em) = 516 nm), the emission spectrum of 2 shows a blue-shifted band at 503 nm. Comparison with the emission band for the 4-fluoro-substituted (lambda(em) = 493 nm) indicates a substantial bathochromic shift in 1. Electrochemical and theoretical studies suggest that while carborane substitution on the 4-position of the phenyl ring lowers the (MLCT)-M-3 energy by a large contribution to lowest unoccupied molecular orbital (LUMO) delocalization, which in turn assigns the lowest triplet state of 1 as [d(pi)(Ir)-->pi*((CN)-N-boolean AND)] (MLCT)-M-3 in character, the substitution on the 5-position raises the (MLCT)-M-3 energy by the effective stabilization of the highest occupied molecular orbital (HOMO) level because of the strong inductive effect of carborane. An electroluminescent device incorporating 1 as an emitter displayed overall good performance in terms of external quantum efficiency (6.6%) and power efficiency (10.7 lm/W) with green phosphorescence.