Highly Luminescent NHC-Stabilized Au13 Clusters as Efficient Excited-State Electron Donors

Atomically precise N-heterocyclic carbene (NHC)-stabilized gold nanoclusters (Au NCs) have attracted great attention among a variety of gold nanoclusters. Herein, a series of three NHC-stabilized Au13 NCs have been synthesized featuring ligands with different steric hindrances. Their electrochemical and photophysical properties were investigated. The bidentate NHC ligands with pendant pyrenes rendered the most sterically hindered Au13 NC, [Au13(bis-NHCPyr)5Cl2]3+, highly luminescent (quantum yield, 56%) and long-lived (3.4 mu s, dichloromethane) among the three Au13 NCs. The excellent photoluminescence properties were attributed to the suppression of excited-state vibrational relaxation arising from the cluster structural rigidity. Further transient spectroscopic investigation of the Au NC excited state also showed favorable electron transfer to methyl viologen with the highest quantum yield from [Au13(bis-NHCPyr)5Cl2]3+ (phi e-T, 12%). Understanding the relationship between the excited-state behavior and the sterically hindered structure of NHC-ligated Au NCs will assist in better designing NHC-stabilized Au NCs as photosensitizers for applications in organic photoredox catalysis, sensors, and solar energy conversion.