Synthesis and Photophysical Properties of Substituted Tris(phenylbenzimidazolinato) IrIII Carbene Complexes as a Blue Phosphorescent Material

Substitution effects on the photoluminescence and electrochemical properties and the photochemical stability of tris(phenylbenzimidazolinato)lr(III) complexes were investigated. Facial and meridional isomers of a series of complexes having the general structure of Ir(CC)(3), where (CC) is (4-R-phenyl)benzimidazolinato (R = H, CF3, CN, OCH3), were prepared. They are abbreviated to Ir(pmb)(3) (1), Ir(CF(3)pmb)(3) (2), Ir(CNpmb)(3) (3), and Ir(Opmb)(3) (4), respectively. Electron-donatirig or -withdrawing groups on the phenyl ring lead to both higher emission quantum yields and longer emission lifetimes compared to those of 1 in each mer and fac series, Particularly, the emission quantum yields were high in the cases of 2a, 3a, 3b, and 4a. No photochemical isomerization was observed in both fac-(a) and mer (b) isomers of Ir(pmb)(3) Ir(CF3pmb)(3), and Ir(CNpmb)(3) on irradiation with a 313-nm wavelength light from a 400 W mercury lamp for more than 20 h. All complexes excluding fac-Ir(pmb)(3) (1a) show excellent photochemical stability in degassed anhydrous thf solution. The locations of the HOMO and LUMO and the lowest excitation energy of these complexes were investigated by DFT and TD-DFT calculations on fac- and mer-Ir(Opmb)(3) (4a and 4b). The HOMO is mainly localized over the Ir metal center and the phenyl moiety, and the LUMO mainly localized over the benzimidazole moiety. The calculated lowest excitation energies agree with the experimental values. The X-ray single crystal structures obtained for fac-Ir(CF(3)PMb)(3) (2a) and mer-Ir(Opmb)(3) (4b) are also discussed.