Chalcogen-Guided Control of Azoarene Photoswitching: Tuning Excited-State Energies Through Electronic Property Modulation.

In recent years, chalcogen bonding has emerged as a promising alternative to classical supramolecular interactions such as hydrogen or halogen bonds. While its behavior in the electronic ground state has been extensively studied, its role in the excited state is gaining increasing attention. We recently demonstrated that the lack of photoswitchability of ortho-tellurated azobenzenes is due to an excitation-induced conversion of the classical chalcogen bond into a more pronounced, electron-rich three-electron σ bond. This transformation significantly strengthens the interaction between the chalcogen and the Lewis base center, effectively preventing isomerization. Based on these findings, we have now investigated the photoswitching behavior of ortho-tellurium-substituted azoarenes by modulation of the electronic properties of the aryl substituent and the oxidation state of the tellurium center. Our results show that electron-donating groups destabilize the excited-state geometry associated with the formation of a three-electron σ bond, thereby restoring photoisomerizability. Furthermore, oxidation to the Te(IV) species disrupts this bonding interaction, leading to significantly enhanced photoswitching properties. Together, these findings provide valuable design principles for the development of multiresponsive molecular switches based on chalcogen bonding and excited-state control.