Moisture-responsive films consisting of luminescent polyoxometalates and agarose

Abstract

Novel switchable and tunable luminescent composite films consisting of Na₉[EuW₁₀O₃₆]·32H₂O (EuW₁₀) and agarose have been prepared. EuW₁₀ can be converted into a non-luminescent species by photoreduction and the recovery is facilitated by oxidation under moisture air. The luminescence recovery of the UV-reduced films showed dependence on the relative humidity (RH) ranging from 43 to 78% after a certain exposure time. X-ray photoelectron energy spectroscopy (XPS) measurements confirm the generation of the reduced W⁵⁺ species in EuW₁₀ polyoxoanions. Further analysis of EuW₁₀ indicates that the photo-generated d¹ electron hopping is prohibited in the hybrid films. The absence of recombination of the photogenerated d¹ electrons and holes, and the consumption of holes to form ˙OH radicals are responsible for the luminescence quenching rather than luminescence resonance energy transfer (LRET). The moisture-responsive behavior of the EuW₁₀–agarose films is attributed to the kinetically controlled back reaction of W⁵⁺O₆ with O₂. The present study provides new insights into lanthanide-containing POMs as tunable luminescent components in UV and moisture dual-responsive materials.