**3.2 State of the art**

The long pathway laid and impressive work of scientists toward structural characterization of metal clusters provided a momentum wisely used by the recent scientists who combined time-resolved spectroscopy, X-ray excited optical luminescence (XEOL), electron spin resonance (ESR), X-ray crystallographic, Ag K-edge Extended X-ray Absorption Fine Structure (EXAFS) and density functional theory (DFT), time-resolved density functional theory (TD-DFT), X-ray photoelectron spectrometry (XPS), and used all the tools at hand to reveal with astonishing details the shape of these "molecules" and tag their luminescence properties [2–5, 11, 48–50].

A general and versatile synthesis protocol used for encapsulation of metal clusters within zeolite micropores enabled selective incorporation of a broad range of metals (Pt, Pd, Ir, Rh, and Ag) within NaA, a zeolite with channel windows too small for post synthesis encapsulation of metal precursors [51]. Encapsulation provides protection for clusters against sintering and contact with toxic impurities within environments. Better conditions of stabilization of Ag and Cu clusters in zeolite voids with formation of a self-assembled clusters system were found by Gurin *et al*. [52] As expected, zeolite voids could incorporate and stabilize clusters of smaller size than a void size. A few geometric structures have been proposed fitting erionite and mordenite voids (0.63–0.70 nm). The structures belong to the following point groups: Oh (cube), Td (tetrahedron), D4h (rectangular parallelepiped), and C2v (3D polyhedron). A stable geometry (rhombus) is found for Ag4 + in accordance with calculations presented earlier.

Restricted by the chapter size, we will further focus on the case of Ag clusters as these appear to be representative for these class of luminescent systems [49]. In this field, notable studies investigating the geometry and energetic properties of Agn clusters 3 ≤ n ≤ 6 have been carried out using density functional theory

(DFT) calculations. FAU topologies predominantly accommodate Ag2, Ag3, Ag4 and Ag8 clusters while LTA frameworks prefer Ag3 and Ag6 clusters [3–5, 9, 33, 39, 53]. A number of geometries have been optimized inside a ZSM-5 zeolite whose ten-membered ring contains different numbers of Al atoms substituted for Si atoms of the SiO2 framework. For Ag5 + , a ditrigonal orthogonal geometry appeared as the most energetically stable configuration while a triangle Ag3 cluster while for Ag4 both a square and a tetragonal clusters have been considered. Ag6 clusters inside ZSM-5(Al1) and ZSM-5(Al2) cavities preserve two stable configurations: a tetragon pair with a shared bond and edge and a pair with shared bonds [54]. DFT calculations also show that the cluster has two 5 s electrons populating the totally symmetric frontier orbitals, which leads to a stabilization of the cluster structure. The totally symmetric 5 s-based orbital corresponds to a super atom S-orbital. The optical transition modeled through time-dependent DFT calculations attributed the absorption peaks to an electronic transitions based on 5 s-type orbitals from the totally symmetric occupied orbital (S-orbital) to an unoccupied orbital with one node (P-orbital) [54].

**The curious case of Ag3 cluster**. Experimental crystallographic data showed that Ag3 + and Ag3 2+ clusters form a linear configuration (with the later slightly bent) along the 3-fold axes through double six-rings of dehydrated zeolite X [55]. A weakly attractive interaction between Ag<sup>+</sup> -Ag<sup>+</sup> could be concluded. Later, the assumption of a linear clusters has been confirmed by Zhao *et al* in a comprehensive series of DFT calculations [56]. The optimized geometries and binding energies of the most stable Ag<sup>n</sup> , Ag<sup>n</sup> -, Ag<sup>n</sup> +, Ag<sup>n</sup> H, Ag<sup>n</sup> H- and Ag<sup>n</sup> H+ with 2 ≤ n ≤ 7 showed remarkable odd-even alternation behaviors. Silver behaves like an alkalimetal atom in the interaction between H and silver clusters. Surprisingly, in a different and fascinating DFT modeling study, the geometry of Ag3 inside the void of ZSM-5 was demonstrated as a triangle and that the Ag-Ag orbital interaction as well as Ag-O electrostatic interactions determine such a different structure [57]. The structure is the same in both lower and high spin states, however, the high spin state leads to two types of triangles significantly distorted from the D3h configuration. The authors also established that the structural and electronic features are governed by the number of Al atoms trough Ag-Ag and Ag-O interactions. The modeling calculations support both a linear and a triangular structure [58].

**Ag4, a clear tetrahedron.** Using already classical X-ray absorption measurements, the geometry of Ag4 cluster has been carefully investigated in the FAU or LTA topologies [3–5, 48, 49, 59]. About 67% of Ag atoms constitute the oligomeric Ag4 clusters and found located inside the sodalite cage [4]. Each Ag atom is bonded in to 2.2 water molecules and surrounded mostly by ca. 33% of Ag isolated cations located at the center of the S6R rings of the sodalite cage (18%) and in the center of the hexagonal prisms connecting the sodalite cages (15%). The fully Ag-exchanged sample FAUY contains a similar Ag4 geometry, with the difference that the cation distribution is slightly changed with of Ag4 cluster surrounded by ca. 25% of silver ions located close to the center of the S6Rs and ca. 13% of silver atoms located in the center of the hexagonal prisms. This excellent study is particularly interesting for its extensive characterization which indicates a strong and direct influence of silver loading and host environment on the cluster ionization potential. This new finding is also correlated to the cluster's optical and structural properties. By fine-tuning of the zeolite environment the researches achieved clusters with photoluminescence quantum yield approaching unity. Another Ag4 cluster (dehydrated Ag1Li11-LTA zeolite) was found to feature a remarkable EQE of 83% with an emission maxima around 545 nm when excited between 300 and 400 nm [48]. The presence of Li<sup>+</sup> clearly changes the luminescence properties of this cluster in two ways: by shielding the interaction between cluster and zeolite oxygen framework and by contraction of the lattice parameter leading to a tightly confined cluster inside the sodalite cage of LTA zeolite.

**The octahedron cluster, Ag6**. Using a different, Ag clusters in LTA and FAU zeolites have been characterized via luminescence properties which were shown before to depend on the nature of the co-cation, the amount of exchanged silver, and the host topology. A broad pallet of emissive species Agn,Na-X and Agn,Na-Y, 1 ≤ n ≤ 12 were observed with spectral properties ranging from 380 to 700 nm and further used as "fingerprint" in cluster type identification [3]. The luminescence decays on a time scale ranging from ns to μs and the transitions were attributed to excited state processes involving spin forbidden transitions and intersystem crossing. A singlet-triplet transition in the case of Ag6 2+ cluster or a doublet-quadruplet transition in the case of Ag6 + have been suggested. Similar long luminescence decay times were also found in other types of silver-zeolite systems. The exact structure of each luminescent species and the nature of the luminescent electronic state still remain a subject of investigations. The structure of hexasilver molecule Ag6 0 has been further resolved by the group of Seff *et al* via single crystal diffraction experiments and showed that these crystals contains octahedral subunits [60]. Ag6 0 had formed in varying amounts in up to 60% in the sodalite cavities of the crystals studied. The hexasilver was shown to be surrounded by eight Ag+ ions distributed in a cub, one in each face of the octahedron. In addition, the presence of even larger clusters is Ag8 + or Ag14 8+ was detected.
