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

The energy barrier for cis-to-trans isomerization is among the key
parameters for photoswitchable molecules such as azobenzene. Recently,
we introduced a well-defined model system based on thin films of crystalline,
nanoporous metal-organic frameworks, MOFs. The system enables the precise
investigation of the thermal cis-to-trans relaxation of virtually isolated azobenzene
pendant groups by means of infrared spectroscopy in vacuum. Here, this
approach is extended by using localized surface plasmon resonance spectroscopy.
This simple and relatively inexpensive setup enables the investigation of
the thermal cis-to-trans isomerization in different environments, here in argon
gas or in liquid butanediol. The energy barrier for the cis-to-trans-relaxation in
argon, 1.17 ± 0.20 eV, is identical to the barrier in vacuum, while the energy barrier
in liquid butanediol is slightly larger, 1.26 ± 0.15 eV.