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

Abstract

Fifteen new photochromic hybrid materials were synthesized by gas phase loading of fluorinated azobenzenes, namely ortho‐tetrafluoroazobenzene (tF‐AZB), 4H,4H′‐octafluoroazobenzene (oF‐AZB), and perfluoroazobenzene (pF‐AZB), into the pores of the well‐known metal‐organic frameworks MOF‐5, MIL‐53(Al), MIL‐53(Ga), MIL‐68(Ga), and MIL‐68(In). Their composition was analysed by elemental (CHNS) and DSC/TGA. For pF‐AZB0.34@MIL‐53(Al), a structural model based on high‐resolution synchrotron powder diffraction data was developed and the host‐guest and guest‐guest interactions were elucidated from this model. These interactions of O−H⋅⋅⋅F and π⋅⋅⋅π type were confirmed by significant shifts of the O−H frequencies in loaded and unloaded MOFs of the MIL‐53 and MIL‐68 series. Most remarkably, all of the synthesized F‐AZB@MOF systems can be switched with visible light, and some of them show almost quantitative (>95 %) photo‐isomerization between its E and Z forms with no significant fatigue after repeated switching cycles.

Novel Photoactive Spirooxazine Based Switch@MOF Composite Materials

“…The synthesis and design of functional hybrid switch@MOF materials has become a central point of research in recent years. [1,2] These smart composites are obtained by introducing photoswitchable dye molecules such as azobenzenes and (fluorinated) derivatives, [3][4][5][6][7][8] diarylethenes [9] or spiropyrans [10][11][12] as non-covalently attached guests into the crystalline porous MOF (metal-organic framework) host scaffold.…”

“…While most applications focus on gas adsorption and storage, combining MOFs and photoswitches has only been given increasing attention in the recent past. [1,2] Different photochromic dyes were embedded into MOFs: stilbenes, [28] 2-(phenylazo)pyridine, [29,30] azobenzenes and (fluorinated) derivatives, [3][4][5][6][7][8] diarylethenes, [9] or spiropyrans. [10][11][12] 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.…”

Atomic/Molecular Layer Deposited Iron–Azobenzene Framework Thin Films for Stimuli‐Induced Gas Molecule Capture/Release

“…[9,10] Introduction of azobenzene moieties in porous MOF-like materials has already been challenged in recent studies. [17,18] Thef irst alternative would be truly exciting but difficult to realize;t he challenges arise from the fact that azobenzene is an integral part of ar igid crystal structure in which the trans-cis transformation either does not occur owing to the lack of free space,o ro ccurs but destroys the crystal lattice in an irreversible manner. [17,18] Thef irst alternative would be truly exciting but difficult to realize;t he challenges arise from the fact that azobenzene is an integral part of ar igid crystal structure in which the trans-cis transformation either does not occur owing to the lack of free space,o ro ccurs but destroys the crystal lattice in an irreversible manner.…”

“…[11,12] These efforts can be divided into three groups depending on whether the azobenzene moiety is accommodated as 1) the actual framework linker, [13,14] 2) as ide-group pendant to the linker, [15,16] or 3) ag uest species in the pores incorporated either during the synthesis or through ap ostsynthesis treatment. [17,18] Thef irst alternative would be truly exciting but difficult to realize;t he challenges arise from the fact that azobenzene is an integral part of ar igid crystal structure in which the trans-cis transformation either does not occur owing to the lack of free space,o ro ccurs but destroys the crystal lattice in an irreversible manner. [19][20][21][22] Apossible solution could be to fabricate the azobenzenebased metal-organic material in ap olycrystalline form (preferably consisting of small crystallites) on top of as upporting substrate to allow more flexibility for the bond rearrangements during the isomerization.…”

Atomic/Molecular Layer Deposited Iron–Azobenzene Framework Thin Films for Stimuli‐Induced Gas Molecule Capture/Release

“…Introduction of azobenzene moieties in porous MOF‐like materials has already been challenged in recent studies . These efforts can be divided into three groups depending on whether the azobenzene moiety is accommodated as 1) the actual framework linker, 2) a side‐group pendant to the linker, or 3) a guest species in the pores incorporated either during the synthesis or through a post‐synthesis treatment . The first alternative would be truly exciting but difficult to realize; the challenges arise from the fact that azobenzene is an integral part of a rigid crystal structure in which the trans – cis transformation either does not occur owing to the lack of free space, or occurs but destroys the crystal lattice in an irreversible manner …”