Isomerization of DASA Molecules in the Nanopores of Metal–Organic Frameworks: What Determines Its Reversibility?**

Graphical Isomerization reversibility of visible-light responsive donor–acceptor Stenhouse adducts in nanopores of metal–organic frameworks is strongly dependent on the polarity of their pore environment. In low polar MIL-53(Al), proton conductivity as high as 0.013 S cm −1 and at least 30 switching cycles are achieved under visible light at 80 °C and 98 % RH. In recent years, light-responsive molecules have been incorporated in metal–organic frameworks (MOFs) to fabricate light-responsive intelligent devices, where reversible isomerization of the guest molecules in the nanopores is crucial. However, how to design a porous environment of MOFs to achieve a reversible isomerization remains unknown until now. In this work, donor–acceptor Stenhouse adducts (DASAs), a new kind of visible light responsive compound, were confined in the nanopores of different MOFs to study their isomerization upon visible-light irradiation/mild heating. We found that the polarity of the pore environment is the key to control the reversibility of isomerization of such guest molecules. Under the guidance of this principle, MIL-53(Al) was screened to investigate the proton conductivity and switching performance of the DASA-confined MOF. The proton conductance was up to 0.013 S cm −1 at 80 °C and 98 % RH, and at least 30 switching cycles were achieved thanks to the Grotthuss-type mechanism and the low polarity of MIL-53(Al) pore environment.