Synthesis and crystallization of a carboxylate functionalized -heterocyclic carbene-based Au cluster with strong photo-luminescence.

Here we report the synthesis and crystallization of a -COOH-capped -heterocyclic carbene (NHC)-protected Au cluster. The single-crystal structure of the -COOH-capped NHC-Au cluster reveals a classic icosahedral core with one Au atom in its center. The icosahedral core is surrounded by five NHC ligands with pseudo C5 symmetry and exposed carboxyls in a pentagonal antiprism fashion. The detailed formula of the Au cluster was identified as Au(bi-NHC carboxyl)Cl (hereafter abbreviated as Au-c). The density functional theory (DFT) calculations confirm that Au-c is an electronically stable eight-electron super-atom cluster and elucidate its optical transitions in the UV-Vis range. The Au-c cluster exhibits excellent thermal and chemical stability under bio-relevant conditions. Additionally, this cluster shows a strong red emission in DMF and HO with an excellent quantum yield (QY) of 40% and 12.6%, respectively. The high QY of Au-c enables its use in cell imaging on both cancer and noncancerous cells.