Modulating the Thermal Stability of cis Isomers of Azobenzene Photoswitches via n→π* Interactions

Azobenzenes are a class of important molecular photoswitches with wide-ranging applications in energy storage, supramolecular assembly, biomedicine and beyond.However, it remains a significant challenge to effectively tune the thermal stability of their cis-isomers without compromising the key photoswitching properties such as absorption wavelength and isomerization efficiency.In this research, a unique n→π* interaction based noncovalent strategy is deccribed for tuning the cis-isomer thermal stability of azobenzene photoswitches, for which a series of azo molecules were designed and synthesized by introducing a methoxy group (providing n electron) and different carbonyl units (providing π* orbital) at the meta-positions of azo double bond, respectively.The UV-Vis and 1 H NMR experiments revealed that these photoswitches could undergo reversible E⇄Z photoisomerization upon 365 nm UV and 430 nm visible light irradiation.The FT-IR spectra verified the existence of n→π* interactions in the cis-isomers, which could be enhanced by reducing the electron-withdrawing nature of the carbonyl units or increasing the solvent polarity, thereby improving the thermal stability of the photoswitches.These findings provide new insights for the design of new molecular photoswitches with tunable thermal stability.