Temperature dependent molecular fluorescence of [Agm]n+ quantum clusters stabilized by phosphate glass networks

Abstract

Molecule like silver quantum clusters ([Ag_m]ⁿ⁺ QCs) exhibit an ultrasmall size confinement resulting in efficient broadband fluorescence. However, free [Ag_m]ⁿ⁺ QCs are also chemically active, so their stabilization is required for practical applications. We report in this work a phosphate oxyfluoride glass network enabled stabilization strategy of [Ag_m]ⁿ⁺ QCs. A series of silver-doped P₂O₅–ZnF₂–xAg glasses were prepared by a conventional melt-and-quench method. The NMR and XPS results reveal that two types of [P(O,F)₄] tetrahedrons (Q¹, Q²) form chain structures and Zn(IV) connects [P(O,F)₄] chains into a 3-dimension network in the glasses. The frameworks with limited void spaces were designed to restrict the polymerization degree, m, of [Ag_m]ⁿ⁺ QCs; the negatively charged tetrahedrons were designed to restrict the charge, n, of [Ag_m]ⁿ⁺ QCs. Through optical and mass spectroscopy studies, silver quantum clusters, [Ag₂]²⁺ and [Ag₄]²⁺, were identified to be charge compensated by [ZnO₄] tetrahedrons and surrounded with [P(O,F)₄] complex anions. The fluorescence thus gives high quantum efficiencies of 55.2% and 83.4%, for P₂O₅–ZnF₂–xAg glass stabilized [Ag₂]²⁺ and [Ag₄]²⁺ QCs, respectively. This further reveals that the peak fixed fluorescence of [Ag₂]²⁺ and [Ag₄]²⁺ can be described by molecular fluorescence mechanisms. These are parity-allowed singlet–singlet transitions (S₁ → S₀), parity-forbidden triplet–singlet transitions (T₁ → S₀) and intersystem crossings between singlets (S₁) and triplets (T₁). The phonon coupled intersystem crossing between singlets (S₁) and triplets (T₁) determines the phosphate stabilized [Ag₄]²⁺ QCs to exhibit a series of temperature dependent fluorescence behaviors. These include fluorescence intensity (at 50–200 K), intensity ratio (FIR) (at 50–200 K), peak shift (at 100–300 K) and lifetime (at 300–450 K) with maximum sensitivities of 1.27% K⁻¹, 0.94% K⁻¹, 0.29% K⁻¹ and 0.41% K⁻¹, respectively. Therefore, phosphate stabilized [Ag₄]²⁺ QCs can be applied as temperature sensing probes, especially at low temperatures (10–300 K) and for color-based visualized temperature sensors.