Structural and Photophysical Characterization of Ag Clusters in LTA Zeolites

Ag clusters (AgCLs) confined within Na-exchanged Linde type A zeolites are studied by X-ray absorption and steady-state and time-resolved photoluminescence spectroscopies in a coordinated effort to elucidate the photophysical properties and link them to the precise cluster structure. The hydrated sodalite cage contains mostly tetrahedral [Ag-4(H2O)(4)] clusters located at the center of the sodalite cages, whereas octahedral Ag-6 clusters coordinated with the zeolite framework oxygen (O-F) [Ag-6(O-F)(14)] are formed upon dehydration. The time-dependent density functional theory and electron spin resonance reports suggest that both Ag-4 and Ag-6 clusters have formally a double positive charge of 2+. Time-resolved spectroscopy shows that at room temperature, the emission of the hydrated sample decays with 3.4 ns from a state with the same multiplicity as the ground state. Upon dehydration, the entire excited-state dynamics speeds up to 1.2 ps. The microsecond-scale lifetimes observed at 77 K suggest the occurrence of two main decay processes for the initially populated singlet state: intersystem crossing and charge transfer. We show that the intersystem crossing yields the formation of a long-lived (409 mu s) triplet state P-3 from the P-1 state located at lower energy from which luminescence occurs. There is evidence that electron transfer is followed by electron-hole recombination or back electron transfer, yielding a relaxed singlet excited state. Upon removal of water ligands, the electrostatic field of the framework is enhanced, which leads to an increase in the rate constant of charge transfer due to stronger electronic coupling between Ag-6(2+) and trap levels. The dependence of luminescence lifetime on the water content indicates a possible way to speed or delay the electron-hole recombination in a controlled way.