Stimuli‐Responsive Metal‐Organic Frameworks with Photoswitchable Azobenzene Side Groups

Abstract

Metal‐organic frameworks (MOFs) are nanoporous, crystalline hybrid materials, which enable various functionalities by incorporating functional organic molecules. By using organic linker molecules that possess photoswitchable azobenzene side groups, the remote control over certain properties was introduced to MOFs. Different MOF materials in the form of powders and thin films have been used to demonstrate the photoswitching. The applications of these stimuli‐responsive nanoporous solids range from switching the adsorption capacity of various gases over remote‐controlled release of guest molecules to continuously tunable membrane separation of molecular mixtures. A particular focus of this review is the effect of the azobenzene photoswitching on the host‐guest interaction, enabling smart applications of the material. Steric hindrance, which may suppress the photoswitching in some MOF structures, is also discussed.

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

“… The photochromic entity is part of the linker backbone of the respective MOF . However, it is a severe drawback of this approach that the large structural changes upon irradiation often lead to a degradation of the framework. The photochromic functionality is added as a substituent to the linker . This leads to the necessary steric freedom for successful photo‐isomerization, but like for 1) typically elaborate synthetic efforts are needed to obtain these linker molecules. The most simple way is to directly embed the photochromic molecules into the empty pores of a MOF by gas‐phase loading, or by solution‐based processes .…”

“…2) The photochromicf unctionality is addeda sasubstituent to the linker. [28][29][30][31][32][33][34][35][36][37] This leads to the necessary steric freedom for successful photo-isomerization, but like for 1) typically elaborate synthetic efforts are needed to obtain these linker molecules.…”

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

“…Theb ackswitching is driven by visible light illumination or thermal relaxation. [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. [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.…”

Synthesis of Functionalized Azobiphenyl‐ and Azoterphenyl‐ Ditopic Linkers: Modular Building Blocks for Photoresponsive Smart Materials

“…Embedded into materials such as polymers, [3] liquid crystals, [4] porous frameworks, [5] and molecular machines, [6] stimuli-responsive units undergo reversible rearrangements in their geometry, polarity and electronic states by external stimuli (e. g. photons, electrons, chemicals, or mechanical forces). [7] Among the stimuli-responsive molecular switches are photoswitchable units À such as spiropyrans (SP), [8] diarylethenes (DAE), [9] azobenzenes (ABs) and stilbenes, [10] (Figure 1)-that have attracted enormous interest, both of experimental and theoretical nature, for their potential applications in materials and life sciences. [11] Prominent among such photoswitchable units are azobenzenes which, when embedded into proteins and peptides, allow conformational control, [12] optical cell signaling, [13] cancer chemotherapy [14] antibiotic treatment, [15] and in vivo neuronal activity control.…”