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

Heteroleptic (CN)2Ir(acac) (CN = 4-CBppy (I); 5-CBppy (2), 4-fppy (4) CB = ortho-methylcarborane; ppy = 2-phenyIpyridinato-Cz,N, 4-fppy = 2-(4-fluorophenyl )-pyridinato-C²,N, acac = acetylacetonate) complexes were prepared and characterized. While 1 exhibits a phosphor-escence band centered at 531 nm, which is red-shifted compared to that of unsubstituted (ppy)2Ir(acac) (3) (λem = 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 (λem = 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) derealization, which in turn assigns the lowest triplet state of 1 as [dπ(ir)--π*(CAT)]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 die 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. [ABSTRACT FROM AUTHOR]