Silver (Ag) clusters confined in matrices possess remarkable luminescence properties, but little is known about their structural and electronic properties. We characterized the bright green luminescence of Ag clusters confined in partially exchanged Ag-Linde Type A (LTA) zeolites by means of a combination of x-ray excited optical luminescence-extended x-ray absorption fine structure, time-dependent-density functional theory calculations, and time-resolved spectroscopy. A mixture of tetrahedral Ag(HO) ( = 2 and = 4) clusters occupies the center of a fraction of the sodalite cages. Their optical properties originate from a confined two-electron superatom quantum system with hybridized Ag and water O orbitals delocalized over the cluster. Upon excitation, one electron of the s-type highest occupied molecular orbital is promoted to the p-type lowest unoccupied molecular orbitals and relaxes through enhanced intersystem crossing into long-lived triplet states.

Luminescent silver clusters (AgCLs) stabilized inside partially Ag exchanged Na LTA zeolites show a remarkable reversible on-off switching of their green-yellowish luminescence that is easily tuned by a hydration and dehydration cycle, making them very promising materials for sensing applications. We have used a unique combination of photoluminescence (PL), UV-visible-NIR Diffuse Reflectance (DRS), X-ray absorption fine structure (XAFS), Fourier Transform-Infrared (FTIR) and electron spin resonance (ESR) spectroscopies to unravel the atomic-scale structural changes responsible for the reversible optical behavior of the confined AgCLs in LTA zeolites. Water coordinated, diamagnetic, tetrahedral AgCLs [Ag4(H2O)4]2+ with Ag atoms positioned along the axis of the sodalite six-membered rings are at the origin of the broad and intense green-yellowish luminescence in the hydrated sample. Upon dehydration, luminescent [Ag4(H2O)4]2+ clusters are transformed into non-luminescent (dark), diamagnetic, octahedral AgCLs [Ag6(OF)14]2+ with Ag atoms interacting strongly with zeolite framework oxygen (OF) of the sodalite four-membered rings. This highly responsive on-off switching reveals that besides quantum confinement and molecular-size, coordinated water and framework oxygen ligands strongly affect the organization of AgCLs valence electrons and play a crucial role in the opto-structural properties of AgCLs.

The appealing luminescent properties of Ag-zeolites have been shown to be dependent on the local environment of the confined silver clusters. Herein, we shed light on the properties of Ag clusters inside hydrated Linde-type A (LTA) zeolites and relate them to the nature of the host framework when expanded and compressed by the incorporation of Li cations and the Ag loading. Within this scenario, we measure a strong emission color shift in these materials, which we directly correlate with the fine structure details derived by optical luminescence-detected X-ray absorption in combination with deep UV-Raman spectroscopy and X-ray diffraction. Strong guest-host-guest interactions are revealed to underpin the variations in the optical properties; a modification in the zeolite lattice parameter results in changing bond lengths of the silver cluster. This interplay between the host zeolite and its confined guests can thus be harnessed to easily tune the Ag-zeolites' emission properties.

Metal clusters confined inside zeolite frameworks display unique electronic, catalytic, and optical properties. However, so far only confined silver clusters have shown peculiar luminescent properties, displaying high photoluminescent quantum efficiencies reaching almost unity. In this study, we demonstrate the self-assembly and confinement of highly luminescent lead (Pb) clusters into the molecular-sized cavities of Linde Type A (LTA) zeolites. These Pb-LTA samples display an intense deep-blue emission with external quantum efficiencies up to 69% in their partially dehydrated state. A tetrahedral lead cluster (Pb 4 ) with unusually short Pb−Pb distances and hydroxyl ligands was identified as responsible for the luminescence as determined by X-ray absorption fine structure (XAFS) analysis. The in-depth characterization of the Pb-zeolites, reported here, sets the stage for elucidating the structure-to-luminescent relationship of other zeolite-embedded clusters.

Ag clusters (AgCLs) confined within Na exchanged-LTA zeolites are studied by X-ray absorption, and steadystate 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 (H 2 O) 4 ] clusters located at the center of the sodalite cages, whereas upon dehydration octahedral Ag 6 clusters coordinated with the zeolite framework oxygen (O F) [Ag 6 (O F) 14 ] are formed. TD-DFT and ESR reports suggest that both the 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 77K suggest the occurrence of two main decay processes for the initially populated singlet state: intersystem crossing and charge transfer. We show that intersystem-crossing yields the formation of a long-lived (409 µs) triplet state 3 P from the 1 P state located at lower energy from which luminescence occurs. There is evidence that when electron transfer takes place, it 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. ASSOCIATED CONTENT Supporting Information. Detailed discussions, structural properties, steady-state spectra, fs fluorescence upconversion traces, TC-SPC fluorescence decay traces, luminescence decay traces. This material is available free of charge via the Internet at http://pubs.acs.org.

The applicability of Ag-exchanged zeolites as efficient phosphors for the development of near ultra-violet primary LEDs is described.

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