Thermochromic luminescence of sol-gel films based on copper iodide clusters

The incorporation of copper iodide clusters in sol-gel silica has been investigated to prepare materials with original luminescent properties. The synthesis, structural characterizations, and optical properties of sol-gel films containing [CU₄I₄L₄] clusters, L = phosphine-based ligands, are reported. Clusters studied are [Cu ₄I₄(PPh₂(CH₂)₂Si(OCH ₂CH₃)₃)₄] (C1), able to copolymerize with the silica matrix, and [Cu₄I₄-(PPh ₂(CH₂)₂CH₃)₄] (C2) used as a reference for the characterizations. The luminescent films exhibit the optical properties of these clusters in accordance with XPS and NMR studies demonstrating their integrity in the gel matrix. The temperature dependence of light emission properties of clusters and films shows, for the first time for phosphine-based [Cu₄X₄L₄] clusters, thermochromic luminescence with bright yellow luminescence at room temperature and purple emission at 77 K. As a result of weak Cu-Cu interactions, the two emissive states appear as highly coupled with a low energy barrier (2 kJ·mol-1), leading to a controlled thermochromism in a large temperature range.