Switching the Proton Conduction in Nanoporous, Crystalline Materials by Light

Abstract

Proton conducting nanoporous materials attract substantial attention with respect to applications in fuel cells, supercapacitors, chemical sensors, and information processing devices inspired by biological systems. Here, a crystalline, nanoporous material which offers dynamic remote-control over the proton conduction is presented. This is realized by using surface-mounted metal-organic frameworks (SURMOFs) with azobenzene side groups that can undergo light-induced reversible isomerization between the stable trans and cis states. The trans-cis photoisomerization results in the modulation of the interaction between MOF and guest molecules, 1,4-butanediol and 1,2,3-triazole; enabling the switching between the states with significantly increased (trans) and reduced (cis) conductivity. Quantum chemical calculations show that the trans-to-cis isomerization results in the formation of stronger hydrogen bridges of the guest molecules with the azo groups, causing stronger bonding of the guest molecules and, as a result, smaller proton conductivity. It is foreseen that photoswitchable proton-conducting materials may find its application in advanced, remote-controllable chemical sensors, and a variety of devices based on the conductivity of protons or other charged molecules, which can be interfaced with biological systems.

Surface‐Mounted Metal–Organic Frameworks: Crystalline and Porous Molecular Assemblies for Fundamental Insights and Advanced Applications

“…A particular interesting goal is the dynamic remote-control over the proton conduction properties. [56] The dynamic remote-control of the electronic conductance was recently demonstrated using photoresponsive spiropyran embedded in passive MOF films. The trans-cis photoisomerization results in the modulation of the interaction between MOF and guest molecules, 1,4-butanediol and 1,2,3-triazole; enabling the switching between the states with significantly increased (trans) and reduced (cis) conductivity, Figure 7.…”

“…Following UV irradiation, the rate constants of the thermal relaxation, i.e., the decay rate of cis azobenzene and the increase rate of trans azobenzene, in the dark were determined by analyzing the intensity of vibrational bands characteristic for the trans-or the cis-state. [56] Copyright 2018, Wiley-VCH. a) Sketch of the photoswitchable SURMOF film (green) on the interdigitated gold electrodes (yellow) on quartz (light blue) substrate.…”

Thermal cis-to-trans Isomerization of Azobenzene Side Groups in Metal-Organic Frameworks investigated by Localized Surface Plasmon Resonance Spectroscopy

“…Using fluorinated azobenzene side groups enables the switching of the MOF material properties with visible light only, avoiding UV light [27]. As a result, the proton conduction properties of the guest molecules can be remotecontrolled [28]. The experimental data of the well-defined, crystalline model system allowed us to compare the results with theoretical data in detail.…”

Metal–Organic Frameworks in Modern Physics: Highlights and Perspectives

“…Magnetic phase transitions between low spin and high spin states have also been observed at 20 K with coercive fields of 240 Oe. [98][99][100][101][102][103] The proton semiconductive hysteresis observed in the 3D MOF, Cu 2 (F 2 AzoBDC) 2 (dabco) (F 2 AzoBDC = (E)-2-((2,6-difluorophenyl)diazenyl)terephthalic acid; dabco = 1,4-Diazabicyclo[2.2.2]octane), is an indicator of such structural changes. This high bipolar reversible magnetization switching is different from the irreversible rotation of ferro/ferrimagnetic domains through the use of external magnetic fields.…”

“…[96] The 3D niccolite MOF mentioned earlier, [(CH 3 CH 2 ) 2 NH 2 ] [Fe III Fe II (HCOO) 6 ], also appeared to demonstrate magnetic hysteresis regulated by a thermal and magnetic state at low temperatures, [80] a phenomenon which is the response of the magnetization of Fe II and Fe III sublattices to external stimuli. [98] In general, this photoisomerisation process leads to conformation changes of the ligand, and modulates the electronic structure of MOFs and, therefore, their chemical, [98][99][100][101][102] optical, [103] and electrical [104] properties. [80] Mechanical hysteresis has been reported for some examples of 3D MOFs (e.g., rigid layers of Co/1,3,5-benzenetricarboxylate which are connected to each other with flexible and spherical dipyridyl linkers).…”