Mechanistic Insights into the Self-Assembly of an Acid-Sensitive Photoresponsive Supramolecular Polymer.

The supramolecular polymerization of an acid-sensitive pyridyl-based ligand (L ) bearing a photoresponsive azobenzene moiety was elucidated by mechanistic studies. Addition of trifluoroacetic acid (TFA) led to the transformation of the antiparallel H-bonded fibers of L in methylcyclohexane into superhelical braid-like fibers stabilized by H-bonding of parallel-stacked monomer units. Interestingly, L dimers held together by unconventional pyridine-TFA N⋅⋅⋅H⋅⋅⋅O bridges represent the main structural elements of the assembly. UV-light irradiation caused a strain-driven disassembly and subsequent aggregate reconstruction, which ultimately led to short fibers. The results allowed to understand the mechanism of mutual influence of acid and light stimuli on supramolecular polymerization processes, thus opening up new possibilities to design advanced stimuli-triggered supramolecular systems.