Molecular-level insights into the supramolecular gelation mechanism of urea derivative.

Despite being a promising soft material embodied by molecular self-assembly, the formation mechanism of supramolecular gels remains challenging to fully understand. Here we provide molecular to nanoscopic insights into the formation mechanism of gel-forming fibers from a urea derivative. High-speed atomic force microscopy of the urea derivative revealed the presence of a lag phase prior to the formation of supramolecular fibers, suggesting a nucleation process. The fiber growth kinetics differ at both termini of the fiber, indicating a directional hydrogen-bonding motif by the urea units, which is supported by single-crystal X-ray crystallography of a reference compound. Moreover, we observed an intermittent growth pattern of the fibers with repeated elongation and pause phases. This unique behavior can be simulated by a theoretical block-stacking model. A statistical analysis of the concentration-dependent lag time on macroscopic observation of the gelation suggests the presence of a tetrameric or octameric nucleus of the urea molecules.