Introduction of azobenzene based MOFs

Azobenzene (AB) based photochemistry is the most commonly researched cis(Z)-trans(E) isomerization in photochromic moieties contained switching MOFs, owing to the composition of an azo group bonded by two side aryl groups. The pristine trans isomer of AB based MOFs is capable of switching to cis under the irradiation of UV light [28] or only a few cases in visible light [29], accompanied by the decreasing n→π∗ transitions and increasing π→π∗ transitions. In addition, the distance between para carbon atoms what in trans isomer of azobenzene is 9 Å drastically drop to 5.5 Å in cis isomer, with an induced dipole moment change from 0 to 3 Da, respectively. Therefore, trans isomer of AB based MOFs is more thermodynamically stable than cis due to the nearly perfect planner structure and longer distance between the aryl termini.

The cis isomer of AB based MOFs are also able to switch back to trans isomer under the condition of exposing to visible light, thermally heating, or even long-time darkness placement, which indicates a reversible isomerization performance. In addition, multiple recycles or even dynamic light triggered isomerization of AB based MOFs can also be accomplished. For instance, bisazobenzene-terphenyl linker based core-shell-MOF II managed to undergo 9 cycles of alternating between green (λirr = 565 nm) or blue (λirr = 420 nm) light with no fatigue observed, which indicated the flexibility and stability of AB isomerization in the pores of MOFs [30]. After digesting MOF particles, approximately 20 min irradiation with green light led to up to 93% containing Z-isomers, whereas only subsequently 8 min irradiation with blue light gave the composition of 78% of E-isomers. This remarkable result shows the photo-induced isomer ratios in the solid MOF samples are practically as high as the linkage in a solution.

Besides, the wavelength of irradiation light may affect the photo-isomerization of AB based MOFs. Zn(AzDC)(4,4′-BPE)0.5 reported by Bradley et al. [31] was found to dynamically oscillate between trans and cis isomeric conformations under continuous exposition to either 200–500 nm (cis-AzDC) or 460 nm light (trans AzDC), which was then applied to the dynamic low energy uptake and release of CO2. Moreover, photo-isomerization of both AzDC and ligands is promoted by filtering 200–500 nm light to 365 nm to enhance the efficiency of CO2 release. These results imply that light intensity is not a limiting factor of photoswitching while the filtration of the light range is preferable [31].

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