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

摘要

We present isolation and exploration of chiroptical activity of water-soluble achiral/chiral mixed-phosphine-protected Au-9 and Au-11 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 Au-9 and Au-11 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 (Au-9) 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 Au-9 cluster surface can give small CD responses only in the high-energy region (= similar to 3 eV) and (ii) if the axially chiral diphosphine ligand contains p-electron systems, then it can additionally enhance the CD intensity in the low-energy region (similar to 2.0 to 2.5 eV). The effect of the number of chiral diphosphine ligands in Au-9 clusters is also examined, and an effective modulation in the CD intensity is observed as a function of its number.