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±0.20eV, is identical to the barrier in vacuum, while the energy barrier in liquid butanediol is slightly larger, 1.26±0.15eV.