Metal-organic frameworks as hosts for photochromic guest molecules

Area of Science:

• Materials Science
• Photochemistry
• Supramolecular Chemistry

Background:

• Azobenzene (AZB) is a photo-switchable molecule with potential applications in various fields.
• Metal-organic frameworks (MOFs) are porous materials that can host guest molecules.
• Controlling the photoisomerization of guest molecules within MOFs is crucial for developing advanced materials.

Purpose of the Study:

• To investigate the effect of different MOF host materials on the light-induced trans/cis isomerization of azobenzene.
• To understand the relationship between MOF structure and the switching behavior of embedded azobenzene molecules.
• To determine optimal MOF structures for enhanced azobenzene photoisomerization.

Main Methods:

• Loading of azobenzene (AZB) into MOF-5, MIL-68(Ga), MIL-68(In), and MIL-53(Al) host materials.
• Confirmation of guest loading using X-ray powder diffraction (XRPD) and elemental analysis.
• Investigation of light-induced trans/cis isomerization using IR spectroscopy and high-resolution synchrotron powder diffraction.

Main Results:

• IR spectroscopy confirmed improved light-induced trans/cis isomerization of AZB within MOF hosts compared to solid AZB.
• AZB0.66@MIL-68(In) achieved up to 30% population of the excited cis state.
• No light-induced isomerization was observed for AZB0.5@MIL-53(Al) due to dense packing of AZB molecules within its channels.
• Structural analysis revealed that the channel size of MOFs, such as MIL-68(Ga) with larger channels, influences AZB isomerization.

Conclusions:

• The choice of MOF host material significantly impacts the photoisomerization efficiency of embedded azobenzene.
• MIL-68(In) and MIL-68(Ga) show promise for enhanced azobenzene photo-switching due to their suitable pore structures.
• MIL-53(Al) is unsuitable for azobenzene photoisomerization due to steric hindrance within its narrow channels.