Photophysics and Dynamics of Tautomerization in Two-Coordinate Carbene-Metal-Amide Complexes

A series of Au(I) carbene-metal-amide (cMa) complexes using N-heterocyclic carbenes and N-benzo[d]benzo[4,5]imidazo[1,2-a]imidazolyl (bim) amide ligands have been synthesized as models to investigate coordination tautomerization in cMa compounds. Methyl substituents on the bim ligand were used to increase steric hindrance and break the degeneracy of tautomers. The presence of tautomers was identified using X-ray crystallography and ¹H NMR spectroscopy, and the kinetics of tautomerization were measured. The dimethylbim (DMbim) ligand in cMa complexes thermodynamically favors a specific tautomer and suppresses the rate of tautomerization. The steric bulk of the NHC carbene and amide ligands and metal-carbene bond strength were found to alter the tautomerization rate. The photophysical properties of DMbim-based cMa are comparable to their bim-based analogs. The DMbim-based cMa complexes exhibit short emission lifetimes (τ = 200 ns) and fast radiative rates (k_r = 4.9 × 10⁶ s^–1) with high photoluminescent quantum yields (Φ_PL = 98%). Analysis of the temperature-dependent photophysical properties indicates fast lifetimes for the singlet state (τ = 14 ns) with small exchange energies (ΔE_ST ∼ 40 meV). The compounds provide general insight for rational molecular designs to suppress tautomerization.