Metastable Azobenzene Nanotoroids as Kinetic Reservoirs for Seeded and Photoinduced Gelation

Abstract

Cyclic molecules and supramolecular assemblies play important roles in both covalent and supramolecular polymerization because their distinctive topology eliminates chain ends and thereby suppresses one-dimensional elongation pathways. Here we show that larger, mesoscopic cyclic supramolecular assemblies function as off-pathway kinetic traps that inhibit supramolecular polymerization by virtue of their high kinetic stability. We previously reported that an azobenzene dyad bearing amide hydrogen-bonding sites forms exceptionally stable cyclic assemblies (nanotoroids). In the present work, relocating the amide groups renders the nanotoroids metastable: after an induction period they ultimately convert into supramolecular fibers. At high concentrations the nanotoroids remain remarkably stable and resist transformation; however, addition of fiber seeds triggers fiber growth and gelation. Moreover, UV-induced trans→cis isomerization of the azobenzene units collapses the nanotoroids and promotes fiber growth through the formation of short fibrous species, leading to gel formation. This system therefore constitutes a rare case in which generation of the assembly-inactive cis-isomer induces gelation, offering a new design principle for off-pathway trapping strategies that leverage cyclic mesoscopic architectures.