ABSTRACT
Novel mixed-ligand Ir(III) complexes, [Ir(L)(NwedgeC)X]n+ (L = N/\C/\N or N/\N/\N; X = Cl, Br, I, CN, CH3CN, or -CCPh; n = 0 or 1), were synthesized, where N/\CwedgeN = bis(N-methylbenzimidazolyl)benzene (Mebib) and bis(N-phenylbenzimidazolyl)benzene (Phbib), N/\N/\N = bis(N-methylbenzimidazolyl)pyridine (Mebip), and N/\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 approximately 425 nm and a triplet metal-to-ligand charge-transfer transition at 498 approximately 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 approximately 0.86, at room temperature. [Ir(Phbib)(ppy)Cl] shows the emission at 559 nm with a luminescent quantum yield of Phi = 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 (kr) 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.
ABSTRACT
A series of novel emissive Ir(III) complexes having the coordination environments of [Ir(N--N--N)2]3+, [Ir(N--N--N)(N--N)Cl]2+, and [Ir(N--N--N)(N--C--N)]2+ with 2,6-bis(1-methyl-benzimidazol-2-yl)pyridine (L1, N--N--N), 1,3-bis(1-methyl-benzimidazol-2-yl)benzene (L2H, N--C--N), 4'-(4-methylphenyl)-2,2':6',2' '-terpyridine (ttpy, N--N--N), and 2,2'-bipyridine (bpy, N--N) have been synthesized and their photophysical and electrochemical properties studied. The Ir(III) complexes exhibited phosphorescent emissions in the 500-600 nm region, with lifetimes ranging from approximately 1-10 micros at 295 K. Analysis of the 0-0 energies and the redox potentials indicated that the lowest excited state of [Ir(L1)(L2)]2+ possessed the highest contribution of 3MLCT (MLCT = metal-to-ligand charge transfer) among the Ir(III) complexes, reflecting the sigma-donating ability of the tridentate ligand, ttpy < L1 < L2. The emission quantum yields (phi) of the Ir(III) complexes ranged from 0.037 to 0.19, and the highest phi value (0.19) was obtained for [Ir(L1)(bpy)Cl]2+. Radiative rate constants (k(r)) were 1.2 x 10(4) s(-1) for [Ir(ttpy)2]3+, 3.7 x 10(4) s(-1) for [Ir(L1)(bpy)Cl]2+, 3.8 x 10(4) s(-1) for [Ir(ttpy)(bpy)Cl]2+, 3.9 x 10(4) s(-1) for [Ir(L1)2]3+, and 6.6 x 10(4) s(-1) for [Ir(L1)(L2)]2+. The highest radiative rate for [Ir(L1)(L2)]2+ with the highest contribution of 3MLCT could be explained in terms of the singlet-triplet mixing induced by spin-orbit coupling of 5d electrons in the MLCT electronic configurations.
