Supramolecular gels

Molecular gels - nanostructured soft materials , in Organic Nanostructures, Atwood, J. L. and Steed, J. W. (eds), Wiley-VCH Weinheim, 2008. 

Gels are a colloidal state of matter that are commonplace in everyday life, e.g. jelly (or jell-o), toothpaste, contact lenses, hair gel, meat jelly etc. Fundamentally a gel is characterised by the following properties.  

On the macroscale gels do not flow and hence a simple test for a gel is to invert its container - if it lands on the floor with a splash it is probably not a gel On the microscale the solid-like component generally comprises hbrillar bundles of high aspect ratio and in the case of chiral gelators can adopt 

Despite their solid appearance, within the gel the liquid component is mobile and is only held by capillary forces. The solid network can be either a covalent polymer or a supramolecular assembly of small molecules. The latter class of compound, termed low molecular weight gelators (LMWG) of which 14.11-14.15 are examples, is perhaps of most interest to supramolecular chemists. Perhaps the most well known gels are metal oxide based polymeric materials produced by the sol-gel process. The sol-gel process involves the hydrolysis and polycondensation of monomeric metal salts such as early transition 

The monomers were composed of a hydrocarbon linkage that was functionalized on both ends by a hydrophilic, hydroxyl-terminated dendron. TEM revealed the presence of rodlike aggregates with diameters of 34-36 A and variable lengths in the range of 2000 A. Such rodlike assemblies are considered to be ideally suited for the immobilization of large quantities of solvent (Terech and Weiss 1997). 

This conformational preference in conjunction with the preference of the terminal amides to exist in an i-trans conformation constrains the system such that the terminal anthranilamide substituents are positioned above and below the plane of the molecule, resulting in a local helical structure. The helical antipodes experience a 

No conformational preference between Aland Pat first dendron generation 

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Weng W, Beck JB, Jamieson AM, Rowan SJ. Understanding the mechanism of gelation and stimuli-iesponsive nature of a class of metallo-supramolecular gels. J Am Chem Soc 2006 128 11663-11672. 

Such control over the mechanical strength of the supramolecular gel network is of particular interest for applications in cell culture, particularly in stem cell research because the elastic modulus of cell substrates has been found to dictate the differentiation pathways chosen by mesenchymal stem cells [57]. 

The behaviour and magnitude of the storage and loss moduli and yield stress as a function of applied stress or oscillatory frequency and concentration can be modelled mathematically and leads to conclusions about the structure of the material.3 For supramolecular gels, for example, their structure is not simple and may be described as cellular solids, fractal/colloidal systems or soft glassy materials. In order to be considered as gels (which are solid-like) the elastic modulus (O ) should be invariant with frequency up to a particular yield point, and should exceed G" by at least an order of magnitude (Figure 14.2). 

Figure 14.2 Frequency sweep rheometry data for a supramolecular gel. G is invariant with frequency up to the yield point and ca. one order of magnitude greater than G".

A great variety of self-assembled supramolecular structures can be found in supramolecular gels . Association of low molecular weight compounds in a topologically defined way yields long fibres with often well defined diameters [420], An amazing variety of very complex superstructures has been described, i.e. rods and tubes formed from concentric or would up lamellae, helices, or braid-like structures. In all cases we can assume a hierarchy of superstructures which ultimately link the molecular structure to the nanoscopic assembly. However, this has only been investigated in detail in a few cases so far. 

Most frequently, the formation of such supramolecular gels is observed upon cooling aqueous solutions [419—423]. In Figure 37a self-assembled thin fibres of a carbohydrate substituted porphyrin are depicted, while Figure 37b shows a multilayered fibre of a octylmannonamide [424]. 

Figure 22. Guest entrapment experiments using supramolecular gel 46. Below guest release from material 46 can be followed by absorption spectroscopy. The release rates are dependent on temperature, solvent polarity, and other factors.

Fig. 3.37 Possibility of the tris(urea) 49 related to the transformation between dimeric capsules and supramolecular gels by sonication. Reproduced from Ref. [90] by permission of The Royal Society of Chemistry...

F. 27 Proposed mechanism for the liquefaction of phosphine-Rh or phosphine-Ir supramolecular gels uprm ultrasonication [91]... 

Makarevic. J. Jokic, M. Peric, B. Tomisic. V. Kojic-Prodic. B. Zinic, M. Bisfamino acid) oxalyl amides as ambidextrous gelators of water and organie solvents, supramolecular gels with temperature dependent assem-bly/dissolution equilibrium. Chem. Eur. J. 2001, 7, 3328-3341. 

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