Manipulation and Theoretical Modeling of the Luminescence Performance of Silver Clusters in LTA Zeolites by Adjusting Extra-framework Cations

Luminescent silver clusters that are confined inside porous zeolites have attracted great research interest due to the combined advantages of high quantum yield, tunable emission, excellent chemical stability, and desirable adoption ability of this micronano-composite. In this research, a series of Ag-exchanged R-LTA/Ag (R = Li, Na, K) zeolites were synthesized, in which the silver clusters showed blue-shifted excitation in the UV region, red-shifted emission in the visible region, and decreased emission intensity when taking Li⁺, Na⁺, and K⁺ as the extra-framework cations, respectively. The [Ag₄(H₂O)_xR₄-sod]^6- (x = 2, 4; R = Li, Na, K) structures were constructed, and the TD-DFT method was further carried out to study the influence of extra-framework cations on the luminescence performance of [Ag₄]²⁺ clusters, in which the substitution of Al for Si and the reasonable hydrate levels were both taken into consideration. It is estimated that the S₁ and S₃ levels for the [Ag₄(H₂O)₄Li₄-sod]^6- cluster, the S₄ level for the [Ag₄(H₂O)₄Na₄-sod]^6- cluster, and the S₅ level for the [Ag₄(H₂O)₂K₄-sod]^6- cluster can be effectively populated through direct excitation, and the S₁ → S₀ transition is assumed to be responsible for the radiative emission in the visible region for the [Ag₄]²⁺ pseudo-tetrahedrons. This calculated energy level diagram and simulated absorption spectra of [Ag₄(H₂O)_xR₄-sod]^6- (x = 2, 4; R = Li, Na, K) clusters explained the modified luminescence property of silver clusters in R-LTA/Ag (R = Li, Na, K) zeolites studied in this research and reported literature.