Viologen-based supramolecular crystal gels: gelation kinetics and sensitivity to temperature

Abstract

Supramolecular crystal gels, a subset of molecular gels, are formed through the self-assembly of low molecular weight gelators into interconnecting crystalline fibers, creating a three-dimensional soft solid network. This study focuses on the formation and properties of viologen-based supramolecular crystalline gels. It aims to answer key questions about the tunability of network properties and the origin of these properties through in-depth analyses of the gelation kinetics triggered by thermal quenching. Experimental investigations, including UV-Vis absorption spectroscopy, rheology, microscopy and scattering measurements, contribute to a comprehensive and self-consistent understanding of the system kinetics. We confirm that viologen-based gelators crystallize by forming nanometer radius hollow tubes that assemble into micro to millimetric spherulites. We then show that crystallization follows the Avrami theory and is based on pre-existing nuclei. We also establish that the growth is interface-controlled, leading the hollow tubes to branch into spherulites with fractal structures. Finally, we demonstrate that the gel properties can be tuned depending on the quenching temperature. Lowering the temperature results in the formation of denser and smaller spherulites. In contrast, the gel's elasticity is not significantly affected by the quench temperature, leading us to hypothesize that the densification of spherulites occurs at the expense of connectivity between spherulites.