Kinetic study of azobenzene E/Z isomerization using ion mobility-mass spectrometry and liquid chromatography-UV detection

Z and E azobenzene isomers are molecular switches which can interconvert both photochemically and thermally. Presently, we studied a ketal-substituted bridged azobenzene in which two stable diastereomeric conformers (Z1 and Z2) photochemically interconvert through transient E isomer. UV-VIS absorption spectroscopy is commonly used to study the relaxation kinetics of azobenzenes, but it does not allow direct quantitation of the process in this case. In the present paper, liquid chromatography coupled to UV detection (LC-UV) and ion mobility-mass spectrometry (IMS-MS) were combined to study the thermal back relaxation kinetics of the E isomer. LC separation of the three isomers was achieved in less than 10 minutes, allowing the characterization of the relatively slow thermal back relaxation kinetics at low temperature through UV detection. In addition, the faster E→Z thermal back relaxation at higher temperature was studied using IMS-MS, which allows shorter time scale separation than LC. Baseline separation of the two Z isomers was achieved in IMS-MS for [Z+Ag]⁺ ions, and their gas-phase conformations were determined by IRMPD experiments. Both IMS-MS and LC-UV methodologies succeeded to study the E→Z thermal back relaxation kinetics, and appeared to be complementary techniques. We show that the combination of the two techniques allows the characterization of the isomerization processes over a broad temperature range, and the determination of the associated thermodynamic observables.