cis-to-trans isomerization of azobenzene investigated by using thin films of metal–organic frameworks

Abstract

The activation barrier for cis-to-trans isomerization is a key parameter for governing the properties of photoswitchable molecules. This quantity can be computed by using theoretical methods, but experimental determination is not straightforward. Photoswitchable molecules typically do not change their conformation in the pure crystalline state. When the molecules are in solution, the switching is affected by the viscosity and polarity of the solvent and when embedded in polymers, the conformational change is affected by the polymer matrix. Here, we describe a novel approach where the photoswitchable group is integrated in a highly crystalline, porous molecular framework. Sufficiently large pore sizes in such metal-organic frameworks, MOFs, allow unhindered switching and the strictly periodic structure of the lattice eliminates virtually all contributions from inhomogeneities. Using IR spectroscopy to probe the conformational state of azobenzene, the energy barrier separating the cis and the trans state could be determined by an Arrhenius analysis of the data accumulated in a temperature regime between 314 K and 385 K. The result, 1.09 ± 0.09 eV, is in very good agreement with the activation energy reported for the thermal cis-to-trans isomerization of free azobenzene as computed by DFT calculations.

Tunable molecular separation by nanoporous membranes

“…By analysing the intensity of the infrared vibrational band at about 720 cm −1 , which can be assigned to trans azobenzene42, the percentage of trans azobenzene and therefore of cis azobenzene in the MOF structure was determined. This band may be assigned to the γ(CH) and τ(ring) vibration of the trans azobenzene side group, which, because of the bonding to the framework, is red shifted in comparison to the vibration band of the isolated azobenzene4344.…”

“…3c. It should be noted that cis azobenzene can undergo thermal relaxation back to trans azobenzene with a thermal relaxation time of about 20 days at room temperature42. Therefore, the isomerization state of the photoswitchable SURMOF can be considered as stable in the dark, this means, without light irradiation, for the duration of the experiment (approximately a few hours).…”

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

“…[63] Copyright 2015, Royal Chemical Society. a) AzoBiPyB linker molecules are incorporated in Cu 2 (DMTPDC) 2 (AzoBiPyB) SURMOF, resulting in essentially isolated azobenzene pendant groups.…”

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

“…Please note that the azobenzene side groups in Cu 2 (BDC) 2 (AzoBiPyB) are not fully isolated and may interact with each other. For comparison, in Cu 2 (DMTPDC) 2 (AzoBiPyB) with large pores, where the azobenzene moieties are fully isolated, an activation energy for the thermal cis-to-trans isomerization of 1.09 ± 0.09 eV was previously determined [30]. Unfortunately, the preparation of large pore Cu 2 (DMTPDC) 2 (AzoBiPyB) SURMOF on the LSPR substrates did not result in crystalline thin films, possibly due to the substrate roughness or functionalization.…”

“…There, steric hindrance of the azobenzene side groups was identified in certain MOF structures [29]. Recently, we used thin MOF films which possess virtually isolated azobenzene pendant groups to investigate the thermal cis-to-trans isomerization [30]. Since the experiments were performed in ultrahigh vacuum, solvent effects were avoided and it serves as a valuable model system for the isomerization of isolated azobenzene molecules.…”