Supramolecular self-assembly of fibres Dario Pasini a, * , Arno Kraft b a Dipartimento di Chimica Organica, Universit a di Pavia, Viale Taramelli 10, 27100 Pavia, Italy b School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK Received 20 June 2003; accepted 30 January 2004 Abstract This paper describes recent advances in the fields of unnatural supramolecular materials that are able to show polymer and/or gel properties by association through multiple weak, noncovalent bonds. The field is reviewed with a particular eye on materials science aspects, and on substrates which seem to hold promise for future applications. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Supramolecular; Self-assembly; Hydrogen bonds; Fibres; Gelators 1. Introduction Supramolecular chemistry makes extensive use of the directionality of hydrogen bonds, the electrostatic at- traction of oppositely charged ions, as well as other favourable attractive (mainly p–p stacking, hydropho- bic/solvophobic) interactions between molecules or functional groups. Nature, too, uses such supramolec- ular (also called noncovalent) interactions extensively, for example, in bringing proteins together to form en- zyme superclusters, or sometimes to form structurally less defined fibrous aggregates. Supramolecular chem- istry tries to mimic this approach by Nature, although chemists generally choose to simplify the structural requirements. Whereas proteins are complex macro- molecules, synthetic compounds designed to have a supramolecular component have to be made in the lab, usually in a short time, at a reasonably large scale, and as cheaply as possible, and they therefore tend to be much simpler in structure and complexity. A hydrogen bond is worth between about 4 and 120 kJmol 1 , which is a fraction of the strength of a cova- lent bond (400 kJmol 1 for a C–C bond). Despite this, if two molecules are held together by many hydrogen bonds (as is the case for protein clusters), the supra- molecular attraction can become quite strong. This pa- per provides an outline of some of the recent developments where supramolecular interactions have been used to make polymeric materials and gel-like aggregates in which hydrogen bonding and other non- covalent interactions are crucial. The discussion of the self-assembly of supramolecular polymers and fibres will be limited to papers published in the last two to three years and the general trends in this field. 2. Supramolecular polymers The term ‘supramolecular polymer’ has become popular in recent years as the result of the synergism between the areas of supramolecular chemistry and polymer science. A supramolecular polymer can be broadly defined as a macromolecular system in which the monomers are linked by noncovalent interactions instead of covalent bonds [1]. In traditional polymeric systems, inter- and intra-chain supramolecular interac- tions play a very important role in determining three- dimensional architectures (a typical example would be nylon), and even some simple organic crystals can be considered as a succession of monomers stabilised in the solid state by supramolecular interactions; however, we will focus here on a novel class of polymeric materials in which the covalent bonds that hold together the monomeric units in a macromolecule are totally re- placed by highly directional noncovalent interactions * Corresponding author. Tel.: +39-0382-507312; fax: +39-0382- 507323. E-mail addresses: dario.pasini@unipv.it (D. Pasini), a.kraft@hw. ac.uk (A. Kraft). 1359-0286/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2004.01.009 Current Opinion in Solid State and Materials Science 8 (2004) 157–163