Solution-Like Behavior of Photoswitchable Spiropyrans Embedded in Metal–Organic Frameworks

1,3,3-Trimethylindolino-6′-nitrobenzopyrylospiran (SP-1) as an example of a photoswitchable spiropyran was loaded into the pores of different prototypical metal–organic frameworks, namely MOF-5, MIL-68­(In), and MIL-68­(Ga), by a vapor-phase process. The successful incorporation in the pores of the MOF was proven by X-ray powder diffraction, and the amount of the embedded photoswitchable guest was determined by X-ray photoelectron spectroscopy and elemental analysis. In contrast to the sterically hindered crystalline state, SP-1 embedded in solid MOF hosts shows photoswitching under irradiation with UV light from the spiropyran to its merocyanine form with a nearly complete photoisomerization. Switching can be reversed by heat treatment. These switching properties were confirmed by means of UV/vis and IR spectroscopy. Remarkably, the embedded guest molecules show photoswitching and absorption properties similar to those in the dissolved state, so that MOFs might be considered as “solid solvents” for photoswitchable spiropyrans. In contrast to that, embedment of SP-1 in the smaller pores of MIL-53­(Al) was not successful. SP-1 is mainly adsorbed on the surfaces of the MIL-53­(Al) particles, which also leads to photoswitching properties.