Phosphorescence color tuning of cyclometalated iridium complexes by o-carborane substitution

摘要

Heteroleptic (C~(a?)N)_2Ir(acac) (C~(a?)N = 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)_2Ir(acac) (3) (λ_(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 4 (λ_(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 ~3MLCT energy by a large contribution to lowest unoccupied molecular orbital (LUMO) delocalization, which in turn assigns the lowest triplet state of 1 as [d_π(Ir)→π*(C~(a?)N)] ~3MLCT in character, the substitution on the 5-position raises the ~3MLCT 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.