https://doi.org/10.1016/j.mtsust.2022.100149
“Until now, most efforts have been devoted to the development of azobenzene based photoswitchable MOFs; it is expected that more other photochromic units, such as DET and SP, would be employed in MOFs. This will endow MOFs with better switching properties. For azobenzene, large geometrical alteration and an increase of dipole moment are associated with isomerization, which can be transferred into MOFs materials and then contribute to switching effects of MOFs. Compared to azobenzene, spiropyran show even larger dipole moment changes, and thus have more potential to lead to higher switching effects in MOFs. Although DET displays small geometrical transitions, large electronic transitions occur upon isomerization. Thus, DET-based MOFs could also be utilized to switch the electronic and optical properties of the materials and then applied in memory, information processing areas.
Considering the different strategy of imposing switching features on MOFs, the influence of incorporating photoswitchable molecules on the structure of MOFs also need to be discussed when designing the photoswitching MOF materials. For loading the photoswitchable molecules as guest into MOF structure, the photoswitchable guests are not covalently incorporated into the framework and just occupy specific pore volume in MOF pore channels. Thus, the property of original MOF structure will not be affected. Compared to loading as a guest, attaching photoswitchable units as a side group of MOF linkages is a more synthetically simple method. The photoswitching units are covalently attached to the MOF framework like pendants. Thus after photoswitching functionalization, the thermal stability and chemical stability will be affected by the property of photoswitching units. For example, The thermal stability of U-mazo and U-pazo slightly decreased compared to CMOF-2 and IRMOF-3 because of the incorporation of photoswitchable azobenzene groups as pendants [104]. Apart from that, the pore volume and BET surface area will also decrease owing to the loss of specific pore space that photoswitching molecules have occupied [63]. For incorporating photoswitchable units as the backbone of MOFs, the photoswitchable units are directly served as linkages to form the MOF structure. Owing to the unstable and photoswitching property of this moiety, the stability of MOF may be affected [49]. However, this also endows MOF with higher photoswitching efficiency. In addition, incorporating as backbones could help prolonged the monomer units and obtain high Brunauer–Emmett–Teller (BET) surface area MOFs [31]. Modification subsequent to synthesis method requires the reaction between existing MOF materials and photoswitchable units. Such reaction process may affect the chemical stability and decrease the surface area of original MOF materials. Thus high chemical stability and accessible pore space are strictly required for MOFs to be synthetically modified.
Although some detailed understanding of the switching process and mechanism in MOFs have been reported, the change of isomer content during the switching process is still not sufficiently characterized. Only a few reported switchable MOFs powders or films were quantified between the switching process through UV–Vis [63], Fourier transform infrared (FT-IR) spectroscopy [78] or NMR spectra [12], which is the fundamental information for understanding the switching process and performance enhancement. Furthermore, the switching efficiency of trans to cis isomerization of MOFs is another key challenge and needs to be improved. For example, the isomerization of many azobenzene pendants in MOFs can be sterically resisted by dense packing of MOF solids, causing a low switching efficiency. Moreover, in some cases, switching efficiency and reversibility of isomerization cannot be ensured at the same time. Some efforts have been made to solve these problems. For instance, multistimuli switchable MOF system was proposed to enhance the switching efficiency up to 99% [65]. Further high switching efficiency of MOFs materials with reversibility is still expected in the future.”