Crystal Structure of Au36-xAgx(SPh- tBu)24 Nanoalloy and the Role of Ag Doping in Excited State Coupling

We report the X-ray crystal structure of Au36-xAgx(SPh-tBu)24 alloy nanomolecules, optical properties, and the substitutional disorder refinement protocol to obtain a reliable structural model. Single crystal X-ray crystallography (SC-XRD) revealed a composition of Au33.17Ag2.83(SPh-tBu)24 with 2 Ag doped on the 28-atom face centered cubic core surface and 0.83 Ag distributed over metal atoms on dimeric staple motifs. Electrospray ionization mass spectrometry revealed a composition of Au32.5Ag3.5(SPh-tBu)24 complementing the SC-XRD data. Optical properties were investigated by steady-state and transient absorption spectroscopies and computational studies, showing faster excited-state decay for Ag-doped clusters due to enhanced electronic coupling. A previously published SC-XRD based Au36-xAgx(SPh-tBu)24 structure used positional disorder refinement and concluded that the structure is "solely motif-doped". But the structure has unusually large and small thermal ellipsoids indicating potential problems with the atom assignment. Here, we have modeled our SC-XRD data using both positional disorder and substitutional disorder. Subtitutional disorder modeling gave better R1 and other refinement indicators, and similarly sized thermal ellipsoids. The resulting substitutional disorder model structure has Ag atoms not as "solely motif-doped" but is found both in the staple motifs and in the core. The substitutional disorder refinement for alloy nanomolecules must be performed at each metal site with independent free variables to determine the partial occupancy of hetero atoms. The positional disorder refinement should be performed for atoms or groups disordered over different positions typically found in disordered tBu group ligands.