Emissive Osmium(II) Complexes Supported by N-Heterocyclic Carbene-based C∧C∧C-Pincer Ligands and Aromatic Diimines

Osmium(II) complexes containing N-heterocyclic carbene (NHC)-based pincer ligand 1,3-bis(1-methylimidazolin-2-ylidene)phenyl anion ((CCC1)-C-1 boolean AND-C-boolean AND) or 1,3-bis(3-methylbenzimidazolin-2-ylidene)phenyl anion ((CCC2)-C-2 boolean AND-C-boolean AND) and aromatic diimine (2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), or 4,4'-diphenyl-2,2'-bipyridine (Ph(2)bpy)) in the form of [Os((CCC)-C-boolean AND-C-boolean AND)((NN)-N-boolean AND)(CO)](+) have been prepared. Crystal structures for these complexes show that the Os-C-NHC bonds are essentially single (Os-C-NHC distances = 2.079(5)-2.103(7) angstrom). Spectroscopic comparisons and time-dependent density functional theory (TD-DFT) calculations suggest that the lowest-energy electronic transition associated with these complexes (lambda(max) = 493-536 nm, epsilon(max) = (5-10) x 10(3) dm(3) mol(-1) cm(-1), solvent = CH3CN) originate from a d(pi)(Os-II) -> pi*((NN)-N-boolean AND) metal-to-ligand charge transfer transition, where the d pi(Os-II) and pi*((NN)-N-boolean AND) levels contain significant contribution from the (CCC)-C-boolean AND-C-boolean AND ligands. All these complexes are emissive in the red-spectral region (674-731 nm) with quantum yields of 10(-4)-10(-2) and emission lifetimes of around 1-6 mu s. Transient absorption spectroscopy and spectroelectrochemical measurements have also been used to probe the nature of the emissive excited-states. Overall, this joint experimental and theoretical investigation reveals that the (CCC)-C-boolean AND-C-boolean AND ligands can be used to modulate the photophysical properties of a [Os((NN)-N-boolean AND)] core via the formation of the hybrid [Os + (CCC)-C-boolean AND-C-boolean AND] frontier orbitals.