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

Azobenzene molecular switches can interconvert both photochemically and thermally. Due to the great interest focused toraised by azobenzene’s photochromic properties, their fast identification has become a challenge. Usually identified using UV-VIS spectrophotometry, we propose different coupling to MS such as ion mobility spectrometry (IMS) and liquid chromatography (LC) to study the Z to E isomerisation. This process, known as thermal back relaxation, spontaneously occurs in the dark, where the metastable E isomer relaxes towards the thermodynamically more stable Z form. In the specific case of cyclic azobenzene, three isomers were isolated: two stable diastereomers with Z stereochemistry and an unstable E conformer .Analyses of isomers by tandem mass spectrometry were carried out on azobenzene sample using different metallic salts and generating ternary complexes without allowing differentiation of isomers. An upstream separation dimension was then necessary to characterize each isomer. For these compounds, IMS-MS and LC-(UV)-MS were evaluated in parallel to study the kinetics of thermal Z to E isomerisation. On one hand, baseline separation of Z isomers was achieved in IMS-MS from [M+Ag]+ ions, while the E conformer was differentiated from the dimers [2M+Ag]+. On the other hand, LC separation of the three isomers was achieved in less than 10 minutes. These methodologies can be applied to different systems in order to determine the thermal back relaxation kinetic of unstable isomers. However, IMS-MS method will be preferred for fast systems as it allowed isomers separation in the millisecond time scale.