Triphenylene Diazocines: Butterfly-Type Rigid Photoswitches with Annulated Aromatic Ring Systems and Increased Switching Amplitude

Abstract

Azobenzenes are arguably the most frequently used photoswitches, but systems in which the benzene units are replaced by larger π systems are rare. Azonaphthalene is known, but the next higher homologue azoanthracene already undergoes irreversible intramolecular cycloaddition during photoisomerization. Such side reactions are not possible with rigid diazocines. In the present work, we have succeeded in integrating triphenylene groups into diazocines, making them the azo switches with the largest aromatic, annulated ring systems published to date. Both the unsubstituted system and several derivatives are easy to access. The extension of the π systems results in a larger molecular switching amplitude compared to parent diazocine, which should lead to better force transmission to the environment in material applications. The switching wavelengths are shifted bathochromically into the visible range, although the photostationary equilibria are decreased. Potential applications include switchable liquid crystals, mechanophores, photoactuators, and many other responsive materials. Particularly noteworthy is the 4-fold substitution of the system, which allows incorporation into larger systems, e.g. as multifunctional cross-linkers in polymers or as building blocks in COFs and MOFs.