Supramolecular gelling agents: can they be designed?

The last two decades have witnessed an upsurge of research activities in the area of supramolecular gelators, especially low molecular mass organic gelators (LMOGs), not only for academic interests but also for their potential applications in materials science. However, most of the gelators are serendipitously obtained; their rational design and synthesis is still a major challenge. Wide structural diversities of the molecules known to act as LMOGs and a dearth of molecular level understanding of gelation mechanisms make it difficult to pin-point a particular strategy to achieve rational design of gelators. Nevertheless, some efforts are being made to achieve this goal. Once a gelling agent is serendipitously obtained, new gelling agents with novel properties may be prepared by modifying the parent gelator molecule following a molecular engineering rationale; however, such approach is limited to the same class of gelling agent generated from the parent gelating scaffold. A crystal engineering approach wherein the single-crystal structure of a molecule is correlated with its gelling/nongelling behaviour (structure-property correlation) allows molecular level understandings of the self-assembly of the gelator and nongelator molecules and therefore, provides new insights into the design aspects of supramolecular gelling agents. This tutorial review aims at highlighting some of the developments covering both molecular and crystal engineering approaches in designing LMOGs.