A cautionary tale of basic azo photoswitching in dichloromethane finally explained

Abstract

Many studies of azobenzene photoswitches are carried out in polar aprotic solvents as a first principles characterization of thermal isomerization. Among the most convenient polar aprotic solvents are chlorinated hydrocarbons, such as DCM. However, studies of azobenzene thermal isomerization in such solvents have led to spurious, inconclusive, and irreproducible results, even when scrupulously cleaned and dried, a phenomenon not well understood. We present the results of a comprehensive investigation into the root cause of this problem. We explain how irradiation of an azopyridine photoswitch with UV in DCM acts not just as a trigger for photoisomerization, but protonation of the pyridine moiety through photodecomposition of the solvent. Protonation markedly accelerates the thermal isomerization rate, and DFT calculations demonstrate that the singlet-triplet rotation mechanism assumed for many azo photoswitches is surprisingly abolished. This study implies exploitative advantages of photolytically-generated protons and finally explains the warning against using chlorinated solvent with UV irradiation in isomerization experiments. Azobenzenes undergo reversible light-induced photoisomerization, resulting in marked spectroscopic, electronic, and mechanical changes, but their sensitivity towards solvents is not fully understood. Here, the authors report how irradiation of an azopyridine photoswitch with UV light in dichloromethane triggers protonation of the pyridine moiety through photodecomposition of the solvent, consequently accelerating thermal back isomerization and abolishing singlet-triplet rotation mechanisms.