Visible-Light Switchable Rings and Chains in Dynamic Covalent Imine Chemistry

The self-assembliy of red-light switchable, functionalised ortho-difluoroazobenzene isomers E-/Z-A with aliphatic diamines exhibits an alternating match/mismatch behaviour depending on the diamine's chain length. While even-numbered diamines exclusively form imine macrocycles with the Z-configured azobenzene isomer, odd-numbered diamines form photoswitchable, defined macrocycles. The formation of Z,Z-A²X²-type macrocycles was demonstrated with butane-1,4- (B) and hexane-1,6-diamine (H), while additionally E,E-A²X²-type macrocycles were obtained with propane-1,3- (Pr), pentane-1,5-diamine (Pe), and diethylene glycol bisamine (O), validated by single-crystal X-ray structures of E,E-A²Pr² and E,E-A²O². The observed reactivity differences arise from the preferred conformations of the flexible diamines in solution, which alternate predictably with odd and even numbers of methylene groups, investigated by ¹⁹F-DOSY NMR experiments, MALDI-MS measurements, and UV/Vis spectroscopy. These findings provide detailed insight into photoresponsive self-assembled systems and highlight the potential of azobenzenes in dynamic covalent chemistry, offering new opportunities for controlling structure and function in adaptive materials.