Re-examining the electronic structure of fluorescent tetra-silver clusters in zeolites

Abstract

In the present study, we have examined the electronic structures related to the fluorescence properties of small Ag₄²⁺ complexes encapsulated in zeolites. We find that interaction between Ag₄²⁺ and coordinated water molecules, which was previously proposed to be the origin of fluorescence, may not be a sufficient condition by itself. Refinement of the previously used all-silicon-cage model to include framework Al atoms leads to an asymmetric environment, and this alters the electronic structure in favor of fluorescence. We have further examined the substitution of the H₂O ligands by NH₃, H₂S, PH₃, CO and CS. Among these systems, Ag₄²⁺ binds most strongly to NH₃ but the energetics for the H₂S and PH₃ complexes is also reasonable. The energy of the fluorescent light is related to the energy of the lowest-energy triplet state, and these energies for the H₂O, NH₃, H₂S and PH₃ systems span the range of ∼2–3 eV, i.e., roughly the visible range. Thus, the use of different ligands appears to be an attractive means for tailoring the luminescence properties.