Back Isomerization Kinetics of Molecular Photoswitches: Complementary Insights from Liquid Chromatography and Ion Mobility Measurements

Storing solar energy in chemical bonds through the reversible isomerization of UV−vis absorbing molecules offers a promising approach to energy storage. These molecules form high-energy photoisomers, which can store energy if kinetically protected by a significant activation barrier against spontaneous thermal back-isomerization. In this study, we compare the back-isomerization kinetic parameters (ΔH^‡ and ΔS^‡) of model azobenzene-based photoswitches in solution with those obtained in the gas phase using an original tandem ion mobility mass spectrometer. Our findings show that the activation enthalpy is well-reproduced from the solution phase to the gas phase, whereas the activation entropy is significantly affected by the absence of solvent, revealing further different relaxation mechanisms.