Nanoarchitectonics of Glass Coatings for Near-Infrared Shielding: From Solid-State Cluster-Based Niobium Chlorides to the Shaping of Nanocomposite Films

The high potential of [{Nb₆Clⁱ₁₂}­L^a₆] cluster-based building blocks as near-infrared radiation blockers for energy saving applications is exposed in the present paper (i = inner edge-bridging ligand, a = apical ligand of the Nb₆; L = H₂O and/or Cl). To do so, a combined experimental and theoretical investigation of edge-bridged [{Nb₆Clⁱ₁₂}­Cl^a_6–x(H₂O)_x]^m+/0/n− cluster unit series (x = 0, 4, 6; m = 2, 3, 4; n = 2, 3, 4) has been carried out. By using the K₄[{Nb₆Clⁱ₁₂}­Cl^a₆] starting solid-state precursor, we explored the behavior of the [{Nb₆Clⁱ₁₂}­Cl^a₆]^4– cluster unit during the different steps of its integration as a building block into a polyvinylpyrrolidone (PVP) matrix to form a glass coating composite denoted {Nb₆Clⁱ₁₂}^m+@PVP (m = 2 or 3). The optical, vibrational and redox properties [{Nb₆Clⁱ₁₂}­Cl^a_6–x(H₂O)_x]^m+/0/n− building blocks have been interpreted with the support of electronic structure calculations and simulation of properties. The chemical modifications and oxidation properties have been identified and studied thanks to various techniques in solution. Combining Raman and ultraviolet–visible spectroscopies, electrochemistry, and quantum chemical simulations, we bring new knowledge to the understanding of the evolution of the properties of the [{Nb₆Clⁱ₁₂}­Cl^a_6–x(H₂O)_x]^m+/0/n− cluster units as a function of the number of valence electron per cluster (VEC) and the nature of terminal ligands (x = 0, n = 4; x = 4, charge = 0; x = 6, m = 4). The fine understanding of the physical properties and vibrational fingerprints depending on the VEC and chemical modifications in solution are mandatory to master the processing of cluster-based building blocks for the controlled design and shaping of glass coating nanocomposites. On the basis of this acquired knowledge, [{Nb₆Clⁱ₁₂}­Cl^a_6–x(H₂O)_x]^m+/0/n− building blocks were embedded in a PVP matrix. The resulting {Nb₆Clⁱ₁₂}²⁺@PVP nanocomposite film shows excellent ultraviolet (UV, 280–380 nm) and near-infrared (NIR, 780–1080 nm) blocking ability (>90%) and a highly visible light transmittance thanks to the controlled integration of the {Nb₆Clⁱ₁₂}²⁺ cluster core. The figures of merit (FOM) value of T_vis/T_sol (T_vis = visible transmittance and T_sol = solar transmittance) as well as the haze, clarity, and the NIR shielding values (S_NIR) were measured. After optimization of the integration process, a {Nb₆Clⁱ₁₂}²⁺@PVP nanocomposite on glass substrate has been obtained with a high FOM equal to 1.29. This high value places the transparent green olive {Nb₆Clⁱ₁₂}²⁺@PVP nanocomposites at the top system in the benchmark in the field of glass coating composites for energy-saving applications.