Influence of Silver Doping on the Photoluminescence of Protected AgnAu25–n Nanoclusters: A Time-Dependent Density Functional Theory Investigation

The effect of silver doping on the electronic properties and photoluminescence of a class of structurally similar AgnAu25–n2+ nanoclusters (0 ≤ n ≤ 13) has been investigated here by means of time-dependent density functional calculations. As very recently reported in the literature, a mixture of these clusters showed an unexpected 200-fold fluorescence quantum yield boost with respect to Au252+, but no mechanism has been proposed to date to explain this phenomenon. The results presented here suggest that the origin of this boost lies in the nature of the first excited state (S1), which is affected differently by the increasing presence of Ag atoms into the network of Au atoms. In fact, doping the cluster with silver atoms has the effect of shifting the lowest-energy “dark” excited states to higher energy, leaving a very “bright” highest occupied molecular orbital → lowest unoccupied molecular orbital (HOMO → LUMO) transition as the lowest-energy excitation. We propose that when fluorescence occurs from “bright” S1 states, it receives a boost in the quantum yield because of the high oscillator strength of these HOMO → LUMO transitions.