Abstract
Metal organic frameworks (MOFs) are porous hybrid crystalline materials that consist of organic linkers coordinated to metal centres. The trans–cis isomerisation kinetics of the azobenzene-4,4′-dicarboxylic acid (AZB(COOH)2) precursor, as well as the Al3+ (Al-AZB)- and Zr4+ (Zr-AZB)-based MOFs with azobenzene-4,4′-dicarboxylate linkers, are presented. The photo-isomerization in the MOFs originates from singly bound azobenzene moieties on the surface of the MOF. The type of solvent used had a slight effect on the rate of isomerization and half-life, while the band gap energies were not significantly affected by the solvents. Photo-responsive MOFs can be classified as smart materials with possible applications in sensing, drug delivery, magnetism, and molecular recognition. In this study, the MOFs were applied in the dye adsorption of congo red (CR) in contaminated water. For both MOFs, the UV-irradiated cis isomer exhibited a slightly higher CR uptake than the ambient-light exposed trans isomer. Al-AZB displayed a dye adsorption capacity of over 95% for both the UV-irradiated and ambient light samples. The ambient light exposed Zr-AZB, and the UV irradiated Zr-AZB had 39.1% and 44.6% dye removal, respectively.
Highlights
IntroductionMetal organic frameworks (MOFs) are 3D crystalline porous materials obtained by the self-assembly of metal ions with appropriate bridging organic ligands
Academic Editors: Girolamo CasellaMetal organic frameworks (MOFs) are 3D crystalline porous materials obtained by the self-assembly of metal ions with appropriate bridging organic ligands
The rate of the trans–cis isomerization is of importance; this study reports the trans–cis isomerisation kinetics of azobenzene-4,40 -dicarboxylic acid and azobenzene dicarboxylate MOFs with Al3+ and Zr4+ metal centres
Summary
Metal organic frameworks (MOFs) are 3D crystalline porous materials obtained by the self-assembly of metal ions with appropriate bridging organic ligands. Electron-withdrawing groups typically have a retarding effect on the rate of azobenzene isomerisation. The electron-withdrawing groups retard the rate of isomerisation, they extend the half-lives of azobenzene derivatives [2]. Enantioselective adsorption of chiral molecules [6] In all these applications, the rate of the trans–cis isomerization is of importance; this study reports the trans–cis isomerisation kinetics of azobenzene-4,40 -dicarboxylic acid and azobenzene dicarboxylate MOFs with Al3+ and Zr4+ metal centres. Surface-mounted Cu MOFs (SURMOFs) with pillar linkers containing azobenzene side groups between the Cu atoms show photo-switching, without reporting the kinetics thereof [6,10,11]. A theoretical study on Zr-AZB MOF-containing azobenzene linkers and singly bound azobenzenes on the surface focused only on the adsorption of gases [12].
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