Division of Organic Chemistry and Biochemistry, Laboratory for Supramolecular and Nucleoside Chemistry, Ruder Bosković Institute Bijenicka 54, Zagreb, Croatia.
Chem Commun (Camb). 2010 Jan 28;46(4):522-37. doi: 10.1039/b920353m. Epub 2009 Dec 3.
Bis(amino acid)- and bis(amino alcohol)oxalamide gelators represent the class of versatile gelators whose gelation ability is a consequence of strong and directional intermolecular hydrogen bonding provided by oxalamide units and lack of molecular symmetry due to the presence of two chiral centres. Bis(amino acid)oxalamides exhibit ambidextrous gelation properties, being capable to form gels with apolar and also highly polar solvent systems and tend to organise into bilayers or inverse bilayers in hydrogel or organic solvent gel assemblies, respectively. (1)H NMR and FTIR studies of gels revealed the importance of the equilibrium between the assembled network and smaller dissolved gelator assemblies. The organisation in gel assemblies deduced from spectroscopic structural studies are in certain cases closely related to organisations found in the crystal structures of selected gelators, confirming similar organisations in gel assemblies and in the solid state. The pure enantiomer/racemate gelation controversy is addressed and the evidence provided that rac-16 forms a stable toluene gel due to resolution into enantiomeric bilayers, which then interact giving gel fibres and a network of different morphology compared to its (S,S)-enantiomer gel. The TEM investigation of both gels confirmed distinctly different gel morphologies, which allowed the relationship between the stereochemical form of the gelator, the fibre and the network morphology and the network solvent immobilisation capacity to be proposed. Mixing of the constitutionally different bis(amino acid) and bis(amino alcohol)oxalamide gelators resulted in some cases in highly improved gelation efficiency denoted as synergic gelation effect (SGE), being highly dependent also on the stereochemistry of the component gelators. Examples of photo-induced gelation based on closely related bis(amino acid)-maleic acid amide and -fumaramide and stilbene derived oxalamides where gels form by irradiation of the solution of a non-gelling isomer and its photo-isomerisation into gelling isomer are provided, as well as examples of luminescent gels, gel-based fluoride sensors, LC-gels and nanoparticle-hydrogel composites.
双（氨基酸）-和双（氨基醇）草酰胺凝胶剂代表了一类多功能凝胶剂，其凝胶能力是由于草酰胺单元提供的强和定向分子间氢键以及由于存在两个手性中心而缺乏分子对称性的结果。双（氨基酸）草酰胺表现出两面手性的凝胶化性质，能够与非极性和高极性溶剂系统形成凝胶，并分别倾向于在水凝胶或有机溶剂凝胶组装体中组织成双层或反向双层。（1）H NMR 和 FTIR 对凝胶的研究表明了组装网络和较小的溶解凝胶剂组装体之间平衡的重要性。从光谱结构研究中推断出的凝胶组装体的组织在某些情况下与所选凝胶剂晶体结构中发现的组织密切相关，证实了凝胶组装体和固态中的相似组织。纯对映体/外消旋凝胶化争议得到了解决，并提供了证据表明 rac-16 由于解析成对映体双层而形成稳定的甲苯凝胶，然后相互作用形成不同形态的凝胶纤维和网络与它的（S，S）-对映体凝胶相比。两种凝胶的 TEM 研究都证实了明显不同的凝胶形态，这使得能够提出凝胶剂的立体化学形式、纤维和网络形态以及网络溶剂固定能力之间的关系。结构上不同的双（氨基酸）和双（氨基醇）草酰胺凝胶剂的混合在某些情况下导致了高度改善的凝胶化效率，称为协同凝胶化效应（SGE），并且还高度依赖于组分凝胶剂的立体化学。基于密切相关的双（氨基酸）-马来酸酰胺和-富马酰胺和芪衍生草酰胺的光诱导凝胶化的例子，其中凝胶通过照射非凝胶化异构体的溶液并将其光异构化为凝胶化异构体形成，以及发光凝胶、基于凝胶的氟化物传感器、LC-凝胶和纳米粒子-水凝胶复合材料的例子。
