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

Abstract

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.

Metal‐Organic Frameworks as Hosts for Fluorinated Azobenzenes: A Path towards Quantitative Photoswitching with Visible Light

“…These residues of the guest can be removed by further heating in vacuum. Even more, for spiropyran, which was attempted to be embedded in MIL‐53(Al), we showed that unchanged intensities after potential gas‐phase loading are a clear indicator that (almost) no embedment of the guest within the pores of the MOF has taken place . In Figure S21 (Supporting Information), XRPD patterns of MOF‐5 loaded with different starting ratios of pristine azobenzene are shown.…”

“…A few years ago we started to establish MOFs as suitable hosts for the embedment of photochromic guests like azobenzenes and spiropyrans . To embed the photochromic guests we developed a gas phase procedure (see Experimental Section) .…”

“…In our previous work we showed that IR spectroscopy is well‐suited to characterize the photochromic behaviour of azobenzene and spiropyran embedded in different MOF hosts, as the IR bands of the guest hardly overlap with those of the MOF host (Figure ). Therefore, it allows for a direct quantitative analysis of the switching process through the determination of the changes of the relevant IR bands.…”

“…Additionally, we found that the photochromic properties are affected by the size/shape of the MOF′s pore, by the degree of guest loading, that is, the packing of the guest molecules within the MOF′s pores, and/or by the chemical environment given by the MOF pores. For spiropyrans embedded in MOFs we used the solvatochromic behaviour of the excited merocyanine form to investigate the influence of the MOF′s polarity on the photostationary state . But, as the excited Z form of azobenzene is not solvatochromic, this approach cannot be used here and any further interpretation remains speculative at the moment.…”

Novel Photoactive Spirooxazine Based Switch@MOF Composite Materials

“…While most applications focus on gas adsorption and storage, combining MOFs and photoswitches has only been given increasing attention in the recent past . Different photochromic dyes were embedded into MOFs: stilbenes, 2‐(phenylazo)pyridine, azobenzenes and (fluorinated) derivatives, diarylethenes, or spiropyrans . Surprisingly, so far spirooxazines as non‐covalently attached switchable guest molecules or as part of the framework itself have not been considered at all, even though they feature promising photophysical properties.…”