Highly phosphorescent iridium complexes containing both tridentate bis(benzimidazolyl)-benzene or -pyridine and bidentate phenylpyridine: Synthesis, photophysical properties, and theoretical study of Ir-bis(benzimidazolyl) benzene complex

Novel mixed-ligand Ir( III) complexes, [ Ir( L)( N boolean AND C) X](n+) ( L) N boolean AND C boolean AND N or N boolean AND N boolean AND N; X) Cl, Br, I, CN, CH3CN, or - CCPh; n) 0 or 1), were synthesized, where N boolean AND C boolean AND N) bis( N-methylbenzimidazolyl) benzene ( Mebib) and bis( N-phenylbenzimidazolyl) benzene ( Phbib), N boolean AND N boolean AND N) bis( N-methylbenzimidazolyl) pyridine ( Mebip), and N boolean AND C) phenylpyridine ( ppy) derivatives. The X-ray crystal structures of [ Ir( Phbib)( ppy) Cl] and [ Ir( Mebib)( mppy) Cl] [ mppy) 5-methyl-2-( 2'-pyridyl) phenyl] indicate that the nitrogen atom of the ppy ligand is located trans to the coordinating carbon atom in Me- or Phbib, while the coordinating carbon atom in ppy occupies the trans position of Cl. [ Ir( Mebip)( ppy) Cl](+) showed a quasireversible Ir( III/IV) oxidation wave at + 1.05 V, while the Ir complexes, [ Ir( Mebib)( ppy) Cl], were oxidized at + 0.42 V versus Fc/Fc(+). The introduction of an Ir - C bond in [ Ir( Mebib)( ppy) Cl] induces a large potential shift of 0.63 V in a negative direction. Further, the oxidation potential of [ Ir( Mebib)( Rppy) X] was altered by the substitution of R, R', and X groups. Compared to the oxidation potential, the first reduction potential revealed an almost constant value at - 2.36 to - 2.46 V for [ Ir( L)( ppy) Cl] ( L) Mebib and Phbib) and - 1.52 V for [ Ir( Mebip)( ppy) Cl. The UV-vis spectra of [ Ir( Mebib)( R-ppy) X] show a clear singlet metal-to-ligand charge-transfer transition around 407 similar to 425 nm and a triplet metal-to-ligand charge-transfer transition at 498 similar to 523 nm. [ Ir( Mebip)( ppy) Cl] + emits at 610 nm with a luminescent quantum yield of Phi = 0.16 at room temperature. The phosphorescence of [ Ir( Mebib)( ppy) X] was observed at 526 nm for X) CN and 555 nm for X) Cl with the high luminescent quantum yields, Phi = 0.77 similar to 0.86, at room temperature. [ Ir( Phbib)( ppy) Cl] shows the emission at 559 nm with a luminescent quantum yield of) 0.95, which is an unprecedentedly high value compared to those of other emissive metal complexes. Compared to the luminescent quantum yields of the Ir( ppy) 2( L) derivatives and [ Ir( Mebip)( ppy) Cl](+), the neutral Ir complexes, [ Ir( L)( R-ppy) X] ( L) Me- or Phbib), reveal very high quantum yields and large radiative rate constants ( k(r)) ranging from 3.4 x 10(5) to 5.5 x 10(5) s(-1). The density functional theory calculation suggests that these Ir complexes possess dominantly metal-to-ligand charge-transfer and halide-to-ligand charge-transfer excited states. The mechanism for a high phosphorescence yield in [ Ir( bib)( ppy) X] is discussed herein from the perspective of the theoretical consideration of radiative rate constants using perturbation theory and a one-center spin - orbit coupling approximation.