Water-Soluble Mixed-Phosphine-Protected Gold Clusters: Can a Single Axially Chiral Ligand Lead to Large Chiroptical Responses?

Abstract

We present isolation and exploration of chiroptical activity of water-soluble achiral/chiral mixed-phosphine-protected Au9 and Au11 clusters. The mixed-phosphine we use is triphenylphosphine monosulfonate (TPPS)/tetrasulfonated-(R)-BINAP in a molar equivalent ratio of 3/1 with respect to the P atom, where BINAP represents 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl. In both Au9 and Au11 magic-number clusters, (R)-BINAP protection is partial (single-molecule ligation in each cluster), and thus the obtained chiroptical responses are rather small. Quantum-chemical calculations for model nanogold (Au9) cluster compounds are performed to evaluate the optical and chiroptical responses. On this basis, we find that (i) single coordination of axially chiral diphosphine ligand onto the Au9 cluster surface can give small CD responses only in the high-energy region (≥ ∼3 eV) and (ii) if the axially chiral diphosphine ligand contains π-electron systems, then it can additionally enhance the CD intensity in the low-ener...