Excited-State Switching Between Ligand-Centered And Charge Transfer Modulated By Metal-Carbon Bonds In Cyclopentadienyl Iridium Complexes

Three series of pentamethylcyclopentadienyl (Cp*) Ir(III) complexes with different bidentate ligands were synthesized and structurally characterized, [Cp*Ir(tpy)L](n+) (tpy = 2-tolylpyridinato; n = 0 or 1), [Cp*Ir(piq)L](n+) (piq = 1-phenylisoquinolinato; n = 0 or 1), and [Cp*Ir(bpy)L](m+) (bpy = 2,2'-bipyridine; m = 1 or 2), featuring a range of monodentate carbon -donor ligands within each series [L = 2,6-dimethylphe-nylisocyanide; 3,5-dimethylimidazol-2-ylidene (NHC); methyl)]. The spectroscopic and photophysical properties of these molecules and those of the photocatalyst [Cp*Ir(bpy)H](+) were examined to establish electronic structure-photophysical property relationships that engender productive photochemical reactivity of this hydride and its methyl analogue. The Ir(III) chromophores containing ancillary CNAr ligands exhibited features anticipated for predominantly ligand-centered (LC) excited states, and analogues bearing the NHC ancillary exhibited properties consistent with LC excited states containing a small admixture of metal-to-ligand charge-transfer (MLCT) character. However, the molecules featuring anionic and strongly sigma-donating methyl or hydride ligands exhibited photophysical properties consistent with a high degree of CT character. Density functional theory calculations suggest that the lowest energy triplet states in these complexes are composed of a mixture of MLCT and ligand-to-ligand CT originating from both the Cps* and methyl or hydride ancillary ligands. The high degree of CT character in the triplet excited states of methyliridium complexes bearing C<^>N-cyclometalated ligands offer a striking contrast to the photophysical properties of pseudo-octahedral structures fac-Ir(C<^>N)(3) or Ir(C<^>N)(2)(acac) that have lowest-energy triplet excited states characterized as primarily LC character with a more moderate MLCT admixture.