Structural Effects in Visible‐Light‐Responsive Metal–Organic Frameworks Incorporating ortho‐Fluoroazobenzenes

Abstract

The ability to control the interplay of materials with low-energy photons is important as visible light offers several appealing features compared to ultraviolet radiation (less damaging, more selective, predominant in the solar spectrum, possibility to increase the penetration depth). Two different metal-organic frameworks (MOFs) were synthesized from the same linker bearing all-visible ortho-fluoroazobenzene photoswitches as pendant groups. The MOFs exhibit different architectures that strongly influence the ability of the azobenzenes to isomerize inside the voids. The framework built with Al-based nodes has congested 1D channels that preclude efficient isomerization. As a result, local light-heat conversion can be used to alter the CO2 adsorption capacity of the material on exposure to green light. The second framework, built with Zr nodes, provides enough room for the photoswitches to isomerize, which leads to a unique bistable photochromic MOF that readily responds to blue and green light. The superiority of green over UV irradiation was additionally demonstrated by reflectance spectroscopy and analysis of digested samples. This material offers promising perspectives for liquid-phase applications such as light-controlled catalysis and adsorptive separation.

Tunable molecular separation by nanoporous membranes

“…Also, for MOFs in the form of powders or thin films, photoswitchable moieties222324 were used to demonstrate the remote-control of the MOF properties like colour25, release262728 and adsorption2930313233 of the guest molecules, in particular of CO 2 (refs 32, 33). While in most cases illumination with ultraviolet-light was needed, linker molecules that can be switched with visible light were used for the MOF preparation recently34.…”

Photoresponsive spiropyran-functionalised MOF-808: postsynthetic incorporation and light dependent gas adsorption properties

“…6 Metal-organic frameworks (MOFs) are a class of multidimensional porous networks composed of organic struts and inorganic nodes, and can be systematically tuned in terms of chemical composition and precise arrangement. [10][11][12] These studies have focused extensively on azobenzene frameworks, [13][14][15][16][17][18][19] as well as on diarylethenes 20,21 and others. [10][11][12] These studies have focused extensively on azobenzene frameworks, [13][14][15][16][17][18][19] as well as on diarylethenes 20,21 and others.…”

Orchestrating Molecular Motion with Light – From Single (macro)Molecules to Materials

“…A second MOF incorporating the same ligand and based on the aluminum MIL‐53 framework displays interesting photomodulation of CO 2 adsorption capacities upon green light irradiation . Remarkably, in that case the photoinduced effect is not caused by E/Z isomerization, but by the suppression of the photoswitching ability due to steric congestion inside the 1D channels of the framework.…”

“…In order to create visible‐light‐responsive materials, ortho ‐fluoroazobenzenes (see Figure ) have been introduced as pendant groups in the walls of carefully selected architectures . The terephthalate linker used for that purpose (see L , Figure A) bears two ortho ‐fluorines on the same phenyl ring and readily isomerizes in solution upon exposure to green‐ and blue‐light, producing PSSs comprising 86% of Z‐ and 88% of E ‐isomers, respectively.…”