Water-soluble luminescent cyclometalated gold(III) complexes with cis-chelating bis(N-heterocyclic carbene) ligands: Synthesis and photophysical properties

A new class of cyclometalated Au^III complexes containing various bidentate C-deprotonated C^N and cis-chelating bis(N-heterocyclic carbene) (bis-NHC) ligands has been synthesized and characterized. These are the first examples of Au^III complexes supported by cis-chelating bis-NHC ligands. [Au(C^N)(bis-NHC)] complexes display emission in solutions under degassed condition at room temperature with emission maxima (λ _max) at 498-633 nm and emission quantum yields of up to 10.1 %. The emissions are assigned to triplet intraligand (IL) π→π* transitions of C^N ligands. The Au^III complex containing a C^N (C-deprotonated naphthalene-substituted quinoline) ligand with extended π-conjugation exhibits prompt fluorescence and phosphorescence of comparable intensity with λ_max at 454 and 611 nm respectively. With sulfonate-functionalized bis-NHC ligand, four water-soluble luminescent Au ^III complexes, including those displaying both fluorescence and phosphorescence, were prepared. They show similar photophysical properties in water when compared with their counterparts in acetonitrile. The long phosphorescence lifetime of the water-soluble AuIII complex with C-deprotonated naphthalene-substituted quinoline ligand renders it to function as ratiometric sensor for oxygen. Inhibitory activity of one of these water-soluble Au ^III complexes towards deubiquitinase (DUB) UCHL3 has been investigated; this complex also displayed a significant inhibitory activity with IC₅₀ value of 0.15 μM.