Ligand Engineering of an N-Heterocyclic Carbene Au13 Nanocluster as a Strong Photoreductant for Sensitized Photocatalytic CO2 Reduction

Developing photosensitizers with enhanced photoredox potential while preserving superior visible-light absorption and sufficient excited-state lifetimes poses a formidable challenge in photocatalytic redox reactions. In this study, we synthesized a series of bis-N-heterocyclic carbene Au13 nanoclusters (bis-NHC Au13 NCs) functionalized with various electron-donating/withdrawing groups. While the as-developed bis-NHC Au13 NCs all retain excellent optical absorption in the visible-wavelength region (exhibiting a molar absorption coefficient exceeding 2.0 × 104 M-1 cm-1 at 426 nm and 1.3 × 104 M-1 cm-1 at 502 nm) and long-living excited states (with lifetimes greater than 2 μs), the reductive power of bis-NHC Au13 NCs was successfully tuned from -1.42 V to -1.91 V vs SCE, rendering potent photoreduction capabilities. As a result, the sensitization of Re(bpy)(CO)3Cl catalysts by the photoexcited bis-NHC Au13 NCs led to 1 order-of-magnitude enhancement in the photocatalytic CO2 reduction yields. An increase in the electron transfer rate from excited bis-NHC Au13 NCs to Re(bpy)(CO)3Cl correlates with improved photostability, which underscores the significant implications of the bis-NHC Au13 nanocluster system for enhancing the efficiency and durability of photocatalytic redox reactions.