Urea derivatives as low-molecular-weight gelators
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REVIEW ARTICLE
Urea derivatives as low-molecular-weight gelators Masamichi Yamanaka
Received: 31 January 2013 / Accepted: 16 February 2013 Ó Springer Science+Business Media Dordrecht 2013
Abstract This paper reports an overview of low-molecular-weight gelators (LMWGs) that have a ureide moiety as a hydrogen-bonding site. Various mono-, bis-, tris-, and tetrakis-urea compounds can form supramolecular gels with organic solvents. The author developed a C3-symmetrical tris-urea molecule that can form a ubiquitous framework of LMWGs. The supramolecular organogel of the tris-urea molecule exhibited a chemical-stimuliresponsive reversible gel–sol phase transition. Supramolecular hydrogels are constructed from self-assemblies of amphiphilic urea derivatives. Sugar-connected amphiphilic tris-urea was found to form a gel with water, and the hydrogels showed chemical-stimuli-responsive gel–sol phase transitions. The potential of supramolecular hydrogels as matrices of electrophoresis has been demonstrated through the supramolecular gel electrophoresis (SUGE) of protein samples using our developed amphiphilic tris-urea LMWG. Keywords Gel Hydrogen bond Molecular recognition Self-assembly Urea
Introduction Gels are familiar materials in our modern life. Wide ranges of applications of gels in, e.g., agriculture, biological
This is a paper selected for the ‘‘HGCS Japan Award of Excellence 2012’’. M. Yamanaka (&) Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan e-mail: [email protected]
research, cosmetics, and medical instruments, have been realized [1]. Gels are classified into two groups: chemical gels, which have covalent cross-linking, and physical gels, which have noncovalent cross-linking. Supramolecular gels constructed from the self-assemblies of small molecules called low-molecular-weight gelators (LMWGs) are a type of physical gel. The general processes for the formation of supramolecular gels are as follows. An LMWG is self-assembled in one dimension to produce supramolecular polymers [2, 3]. The resulting pseudo-polymers are bundled to form a fibrous aggregate that is tens to hundreds of nanometers thick and has a high aspect ratio. Several such fibrous aggregates are then intertwined three-dimensionally, which immobilizes liquids and physically confines them in the small spaces created. In recent decades, various artificial LMWGs have been developed [4–14]. One of the major advantages of supramolecular gels over polymer gels is their responsive nature toward external stimuli such as light, pH, and chemical. Supramolecular gels have potential applications in smart materials, but the rational design of LMWGs remains a challenging problem. A fundamental strategy for the synthesis of LMWGs involves the introduction of appropriate functional groups to induce one-dimensional assembly through interactions such as van der Waals interactions, p–p interactions, and hydrogen bonding. The ureide group is a typical hydrogen-bonding functional gro
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