Photochromism in the periodic nanospace of zeolite LTA by the transfer of photoexcited electrons of adsorbed Na atoms

Cationic sodium (Na) clusters incorporated into a dehydrated Na-form LTA (Na-LTA) zeolite by the adsorption of Na atoms exhibit diamagnetism regardless of their adsorption amount. These clusters preferentially form in β-cages under the condition of dilute adsorption. In this study, photochromism was observed on the dilute Na-adsorbed Na-LTA, which showed a color change from yellow-green to dark blue-green at room temperature. This phenomenon resulted in dissociation of the diamagnetic Na clusters in β-cages upon light irradiation and thereby in the formation of two metastable Na₄³⁺ clusters with <i>C</i>_3<i>v</i> symmetry in α-cages from the photoabsorption and electron spin resonance spectra. We also observed photochromism in a sample of insufficiently dehydrated Na-LTA with dilute Na adsorption-<i>i.e.</i>, the initial white color changed to blue, then blue-green and finally dark moss-green upon irradiation with UV light. This phenomenon consists of two steps. First, products in the β-cages, formed by the reaction of residual water molecules with adsorbed Na atoms, release electrons upon irradiation. The electrons transfer to the α-cages, forming the metastable Na₄³⁺ clusters. Next, the diamagnetic Na clusters are formed in vacant β-cages by the thermal transfer of electrons from the Na₄³⁺ clusters in the α-cages. Na clusters including multiple electrons were also formed in the α-cages by long exposure to UV light irradiation, which had a similar effect to increasing the adsorption amount of Na atoms onto the dehydrated Na-LTA. After termination of irradiation, the sample required a week to change its color to light yellow.