Introduction to photo-switching MOFs

“Photo switching ability is a kind of photoresponsive property that after exposure to the light, molecules or moieties can undergo a transformation between different molecular states, which is typically known as reversible isomerization, and also commonly involves photo-induced electron/energy transfer, chemical bond break, as well as conformational rearrangements [10]. After absorbing sufficient photon energy, photochromic molecules can convert to a different conformer. Such photo-induced conversion is often reversible, with back transition initiated by irradiation under a specific light of different wavelength, simple heating, or long time placement under darkness [11]. These two transitions allow the photochromic units to switch between trans and cis isomers or bond breaking and forming states, which lead to molecular property changes such as polarity, light absorption, charge density [12,13], and then finally contribute to visualized photoresponse or macroscopic property changes.

Photo-switching MOFs, as a new sort of MOF material, has attracted significant research attention. These materials are commonly fabricated by incorporating photoswitchable units into MOF materials since they can reversibly change their molecular structure and electronic state through light exposure, such as azobenzene (AB) which can switch between trans(E)-cis(Z) configuration, and diarylethene (DAE) with light trigger structural changed caused by ring-opening/-closing reactions [11,[14][15][16]], as well as spiropyran (SP) which undergoes photo-induced isomerization to zwitterionic merocyanine (MC) counterpart [[17][18][19]]. Among them, azobenzene based MOFs are the most widely researched, followed by diarylethene, while only a few spiropyran based MOFs were studied.

Although the isomerization of these photoswitchable units can be discovered in terms of free molecules in the gas phase or solution [20], they are often sterically resisted by intermolecular interactions in dense packing and thus cannot display photoswitchable ability in the solid state. Therefore, it is challenging to construct photoswitchable solid materials where the structure change upon light exposure in a single molecular is capable of being accumulated to produce macroscopic property changes. Apart from that, the deprivation of crystallinity also makes it difficult to design a rational procedure to integrate photochromic unites into solid materials. Furthermore, the comparatively low density of photoswitchable units in the materials also restricts their optical performance.”

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