Spiropyran-Embedded Metal–Organic Frameworks with Thermoresponsiveness for Tunable Gas Adsorption

An ideal adsorbent is supposed to possess a strong adsorbate–adsorbent interaction during adsorption and a weak adsorbate–adsorbent interaction during desorption. However, conventional adsorbents have fixed properties and are less likely to meet the demand for adsorption and desorption simultaneously. In this paper, we report spiropyran (SP)-based smart adsorbents with dynamically tunable properties for gas adsorption. Nitro-substituted SP molecules were embedded in a typical metal–organic framework (MOF), MIL-101(Cr), leading to the generation of SP@M. Upon thermal stimulation, the embedded SP isomerizes to the merocyanine (MC) form and can revert back to the initial ring-closed state through spontaneous thermal relaxation. The adsorption capacity of the adsorbent on CO is tunable upon isomerization, and the CO adsorption amount is increased by 37% when SP is isomerized to MC. Density functional theory (DFT) calculations suggest that MC is more inclined to capture the CO molecule than SP due to stronger binding energy. We also demonstrate that the tunable adsorption capacity upon isomerization can be maintained during 2.5 cycles without degradation.