Origin of the Anomalous Temperature Dependence of the Photochromic Reaction of Cu-Doped ZnS Nanocrystals.

The temperature dependence of the color fading process of thermally reversible photochromic reactions is one of the most important challenges for their industrial applications. Generally, photochromic reactions of organic molecules have a strong temperature dependence due to the occurrence of large conformational changes during the reactions. In contrast, we recently reported that the photochromic reaction of Cu-doped ZnS nanocrystals (NCs) exhibits a very small temperature dependence around room temperature. However, the mechanism underlying this phenomenon has not been clarified yet. Here, we reveal that the anomalous temperature dependence of Cu-doped ZnS NCs originates from the balance between the temperature dependence of the charge recombination and that of the adsorption/desorption of water molecules on the surface of the NCs, which act as hole acceptors. Exploring temperature-insensitive photochromic reactions is important not only for gaining fundamental insight into nanomaterials but also for developing novel photochromic materials for outdoor applications.