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.