Unveiling the Water-Responsive Solid-State Photoluminescence Properties of a Series of Li 6 TeW 6 O 24 · n H 2 O ( n = 10, 7, 0) Polyoxometalates

The first series of water-responsive photoluminescent materials incorporating the lanthanide-free TeW6O246- polyoxometalate (POM), namely Li6TeW6O24·10H2O (LiTeW6·10H2O), Li6TeW6O24·7H2O (LiTeW6·7H2O), and Li6TeW6O24 (LiTeW6), is reported. The structures of the hydrates have been determined by single crystal (for LiTeW6·10H2O) and ab initio powder (for LiTeW6·7H2O) X-ray diffraction (XRD). Phase relationships between the three compounds upon exposure to variable temperatures and relative humidity (RH) have been investigated by a combination of thermal analyses, optical thermomicroscopy, powder XRD, Raman spectroscopy, and dynamic vapor sorption. Under high humidity levels (>75% RH) at room temperature, LiTeW6 is converted over many days into LiTeW6·10H2O. In marked contrast, LiTeW6·7H2O is converted into LiTeW6·10H2O at 75% RH in a couple of hours. The photophysical characterizations of the compounds have revealed, for the first time, the inherent greenish-white emission of TeW6O246-. The photoluminescence efficiency of the salts drastically increases with decreasing the number of water molecules in direct hydrogen bonding interactions with the POM. Upon exposure to humidity at room temperature, the emission of LiTeW6 and LiTeW6·7H2O is gradually quenched with variable rates and photocontrasts, due to their conversion into LiTeW6·10H2O which is nonluminescent. The two PL-active phases can then be restored by controlled thermal dehydration.