The solely motif-doped Au36-xAgx(SPh-tBu)24 (x=1-8) nanoclusters: X-ray crystal structure and optical properties

We report the observation of new doping behavior in Au36-xAgx(SR)(24) nanoclusters (NCs) with x = 1 to 8. The atomic arrangements of Au and Ag atoms are determined by X-ray crystallography. The new gold-silver bimetallic NCs share the same framework as that of the homogold counterpart, i.e. possessing an fcc-type Au28 kernel, four dimeric AuAg(SR)(3) staple motifs and twelve simple bridging SR ligands. Interestingly, all the Ag dopants in the Au36-xAgx(SR)(24) NCs are selectively incorporated into the surface motifs, which is in contrast to the previously reported Au-Ag alloy structures with the Ag dopants preferentially displacing the core gold atoms. This distinct doping behavior implies that the previous assignments of an fcc Au-28 core with four dimers and 12 bridging thiolates for Au-36(SR)(24) are more justified than other assignments of core vs. surface motifs. The UV-Vis adsorption spectrum of Au36-xAgx(SR)(24) is almost the same as that of Au-36(SR)(24), indicating that the Ag dopants in the motifs do not change the optical properties. The similar UV-Vis spectra are further confirmed by TD-DFT calculations. DFT also reveals that the energies of the HOMO and LUMO of the motif-doped AuAg alloy NC are comparable to those of the homogold Au-36 NC, indicating that the electronic structure is not disturbed by the motif Ag dopants. Overall, this study reveals a new silver-doping mode in alloy NCs.