Azobisheteroarene photoswitches based on isoxazoles and pyrazoles: tunable photostationary states, thermal relaxation and sensitization under confinement

Abstract

Replacing one or both phenyl moieties of azobenzene with aromatic heterocycles is a versatile strategy to expand molecular diversity and to customize photophysical properties specifically for the intended application. Numerous mono-heteroaryl azo structures have been explored and characterized, with especially arylazopyrazoles (AAPs) and arylazoisoxazoles (AIZ) finding widespread application in various fields such as material science, photopharmacology and supramolecular chemistry. However, bis-heteroaryl motifs have been sparingly explored, with only a handful of examples known to date. Here, we introduce two previously unexplored classes of bisheteroaromatic structures: azobisisoxazole and isoxazoleazopyrazoles. Both classes readily undergo photoisomerization upon irradiation with UV (E–Z) and green light (Z–E), with half-life times of the Z isomer ranging from a few minutes to multiple months. Notably, some of the compounds combine very long half-life times of the Z isomer with near-quantitative photoisomerization in both directions. Furthermore, sensitized isomerization of these photoswitches under confinement enables rapid E–Z isomerization with selectable lower-energy photons, enabling high conversion even for derivatives performing poorly upon direct excitation. In general, these azobisheteroarenes represent new and easily accessible platforms for the design of light responsive molecules with favorable photophysical properties in photopharmacology and beyond.