Achieving Enhanced Photochromism of Spiropyran in Pretreated Nanoporous Lanthanide Metal–Organic Frameworks for Information Storage Applications

Spiropyran and its derivatives are an important class of photochromic molecules that have a wide range of applications in information storage and anti-counterfeiting. However, the easy tight accumulation of molecules under the solid state of spiropyran affects its discoloration efficiency, which is still a challenge to improve the solid photochromic properties. In this paper, a method of loading spiropyran by pretreatment of a lanthanide metal–organic framework (MOF) to improve its photo-response ability is reported. The lanthanide MOF is pretreated by methanol activation and 330 °C calcination to remove the unwanted ligands and some unreacted impurities in the framework. Compared to the pore size of the untreated framework (2.06 nm), the pore sizes of the framework activated by methanol (2.5 nm) and sintering at 330 °C (4.23 nm) were increased. Then, the spiropyran was loaded into a lanthanide MOF by solvent adsorption. After pretreatment of the Tb-MOF, the loading content of spiropyran increased from the initial 4.3 to 9.08 and 12.13%. After 365 nm UV illumination, the difference value in UV absorbance of framework pretreated composites gradually increased from 0.24 to 0.36, accompanied by color transition from yellow to violet. Due to the fluorescence resonance energy transfer effect between the donor lanthanide ion and the acceptor spiropyran, the fluorescence emission transition of the composite material was realized within 10 min. Among them, the fluorescence emission of the untreated composite of the frame can only change from pale yellow to orange, and the maximum fluorescence intensity is 4.4 × 105 CPS, while the fluorescence emission can change from yellow to red after frameworks' pretreatment, and the fluorescence intensity is significantly improved, which is 6.4 × 105 and 7.5 × 105 CPS, respectively. By applying a composite material to a filter paper, a pattern with high contrast can be made. The pattern can be irradiated with ultraviolet light to realize the physical and fluorescence changes and can be restored to the initial state after being irradiated with green light with a wavelength of 530 nm for 10 min. This not only improves the solid photochromic properties of spiropyran but also provides the application possibility for information storage.