Observation of long-lived phosphorescence in Au(i) complexes bearing chromophoric (N-heterocyclic carbene) ligands

Synthesis and optical characterization of two new gold(i) complexes bearing chromophoric N-heterocyclic carbene (NHC) ligands are described. Ligands with electron-releasing (diphenylamino) and electron-withdrawing (benzothiazolyl) substituents are described, and metalation with gold(i) bromide is demonstrated. Crystallographic characterization reveals both complexes to have linear, two-coordinate gold and an absence of aurophilic interactions in the solid state. The absorption spectra of both gold(i) complexes are similar to the free ligand absorption spectra indicating ligand localized pi-pi* absorption. The new gold complexes show moderate fluorescence and observable phosphorescence; binding of a single gold atom suffices to populate triplet excited states. The fluorescence of the benzothiazolyl derivative is pi-pi* in nature, while the fluorescence of the diphenylamino derivative is charge-transfer in nature. The phosphorescence of both complexes is vibronically structured, indicative of metal-perturbed pi-pi* character. Phosphorescence at 298 K persists for hundreds of microseconds. Intersystem crossing is the dominant excited state kinetic pathway with quantum yields of intersystem crossing greater than 50% for both gold complexes. Density-functional theory calculations indicate that the lowest excited singlet and triplet states are dominated by the carbene-fluorenyl-substituent conjugate. We believe the incorporation of gold as a heavy atom through a strongly sigma-donating and moderately pi-accepting NHC-linker provides a general strategy for achieving NHC complexes with phosphorescence.