Supramolecular gels in crystal engineering

What is a Supramolecular Gel and why the link to CrystEngComm? This question brings this editorial to the main purpose of this themed issue, which is to highlight the close link between crystal engineering and supramolecular gels. A supramolecular gel is a colloidal soft material showing viscoelastic properties which has a discontinuous dispersed liquid phase and a solid-like continuous network phase. What makes gels so interesting is that by weight and volume they are mostly liquid, yet they behave like a solid. The solid-like gel network is built up of small molecules interacting together through supramolecular interactions and Coulombic forces, hence supramolecular gels. Similarly, a crystal can be thought of as a network of molecules connected through these same sorts of interactions. This link between crystal engineering and gels was, in most likelihood, first clearly introduced through the work of Prof. Dastidar¹ and has blossomed into a means to not only predict gelation occasionally but fully understand the structure landscape of the possible self-assembled materials. Supramolecular gels and crystals are both often controlled by nucleation processes (cooperative self-assembly of the supramolecular polymerisation, although examples do exist of isodesmic supramolecular polymerisation in supramolecular gelation). Supramolecular gels are increasingly finding uses in industry with applications in drug delivery, fracking, cell growth and food products, with more likely to come on to the market soon.² We hope that the readership of CrystEngComm find this themed issue informative and we highlight the contributions below from our diverse set of authors from all over the world.

We are happy to have two Highlights. These present the nucleation theory behind supramolecular gelation (Liu and co-workers, DOI: 10.1039/C5CE00854A) and the link between some crystalline metal–organic frameworks and gels (Marrero-Tellado and Díaz, DOI: 10.1039/c5ce01032b). A number of papers are presented highlighting the use of crystallisation in gel media. As a fairly well-recognised technique for reducing convection and sedimentation during crystallisation it has had a recent renewed resurgence of interest. As a crystallisation technique it has importance in biomineralisation and, through better control and understanding of the gel properties and their effects on crystal growth, new applications have been envisioned. Gavira, de Cienfuegos and co-workers present the importance of protein crystallisation (DOI: 10.1039/c5ce00850f). They clearly show that supramolecular gels have a future in aiding structural biology. This is complemented by the paper presented by Sugiyama et al. on protein crystallisation in high-strength hydrogels (DOI: 10.1039/c5ce00844a). Li et al. present crystallisation of inorganic materials with gel matrices showing how diverse the technique is (DOI: 10.1039/c5ce01085c). Sánchez and co-workers present pharmaceutical crystallisation studies utilising supramolecular gelators (DOI: 10.1039/C5CE01293G). They highlight how sensitive supramolecular gels are to the presence of other additives in solution. With co-assembly and self-sorting possibilities abounding, synergistic and antagonistic effects must always be considered during supramolecular assembly.

Important links between biology and supramolecular gels are clearly represented by a number of interesting amino acid and peptide gelator based manuscripts in the issue. Adams et al. present work on FMOC amino acid gelators where they have managed to crystallise two of the gelators and show through use of the exciting technique of fibre diffraction how the molecules self-assemble within the fibres (DOI: 10.1039/c5ce00801h). Fibre diffraction is without a doubt an underutilised technique to understand the molecular packing structure of supramolecular gelators. Yam and co-workers present a very interesting set of gelators synthesised to combine fluorescent gold π–π stacking groups and biologically relevant amino acids and peptides (DOI: 10.1039/C5CE01136A). Gazit and co-workers round up the amino acid gelators presented in the themed issue (DOI: 10.1039/c5ce01051a). Their paper reports on the mixed assembly of two gelators. This type of multi-component gelator has become an important research subject highlighting the possibilities of co-assembly or self-sorting.

Metal–organic or metallogels are represented in the issue as well besides the gold gels mentioned above. Damodaran, Steed and co-workers present a tour de force of selective gelation by N-(4-pyridyl)nicotinamide with copper(II) salts (DOI: 10.1039/c5ce00901d). Burrows, Raithby, Wilson and et al. show that metallogels are increasingly diverse with a manuscript presenting what could easily be the first description of a lead gelator (DOI: 10.1039/c5ce01689d). Zhang, Liu and co-workers highlight that alkali metals within gel media can have a profound effect on the supramolecular aggregation and can and will be incorporated into the gel fibres, altering the properties of the gel materials (DOI: 10.1039/c5ce00826c). Vittal et al. present an elegant paper on a magnesium-containing fluorescent gelator (DOI: 10.1039/c5ce00662g).

Another type of multi-component gelator that is possible is when the gel material is made of more than one type of molecule. In the crystal engineering community these are known as cocrystals or salts. Pal et al. show this in a gel built from citrazinic acid and melamine (DOI: 10.1039/c5ce01001b). The materials produced are of interest beyond just their gelation. They show some interesting iodine sorption and delivery in the gel matrix. Although it is often mistakenly claimed that applications for supramolecular gels are thin on the ground. Nandi and co-workers show how there is a growing interest in the development of applications such as conducting nanostructures formed during gelation (DOI: 10.1039/c5ce00837a). They utilise a multi-component gel system constructed of a supramolecular gelator, a naphthalene tetracarboxylic dianhydride derivative and a complementary polymer polyaniline.

Díaz et al. present a remarkable paper describing the use of a salt gelator to show the importance of the kinetic nature of supramolecular gels (DOI: 10.1039/c5ce00397k). The temporal nature of the gels described highlights the close link between crystal growth and gelation, with Ostwald's rule of stages clearly highlighted in the article. Classical gel studies are also presented in the issue. Yi, Ma and co-workers present gelation by a compound that contains what must be one of the most common functionalities found in studied gelators, specifically cholesterols and related groups (DOI: 10.1039/c5ce00636h). They present some phase-selective gelation, which Díaz also highlights. Weiss presents work on a set of classical gelators based on fatty acids and shows the use of fast infrared spectroscopy methods to follow the changes of the gel assemblies as they set (DOI: 10.1039/c5ce00733j). Rogers and Lan present a very detailed but no less important description of the gelation of 12-hydroxystearic acid (DOI: 10.1039/c5ce00652j). 12-Hydroxystearic acid is without a doubt a model gelator and the manuscript describes studies of its supramolecular aggregation in a number of diol solvents from nano to micro and macro levels. Their profound conclusion, that the drastic differences found between each of the solvents do not correlate with general solubility parameters, highlights how important interfacial interactions of the fibres with solvents and inclusion of solvent within fibres is still not fully understood. Hisaki, Miyata and co-workers pose a question that nearly every reader and author of this issue asks, “Gelation or crystallisation?” (DOI: 10.1039/c5ce00778j). Their study focuses on discotic-like octadehydrodibenzo[12]annulene molecules highlighting the close relationship between gelation and crystallisation.

Taken together, the papers in this issue illustrate the clear, but often understated, relationship between crystal engineering and supramolecular gelation. We hope the readership of CrystEngComm will find this themed issue stimulating, informative and insightful. Finally, we should like to thank all the contributors, reviewers, and the editorial team for their high quality and professional work.

References

1. P. Dastidar, Chem. Soc. Rev., 2008, 37, 2699.
2. A. R. Hirst, B. Escuder, J. F. Miravet and D. K. Smith, Angew. Chem., Int. Ed., 2008, 47, 8002.

---