Rigidity in synkinetic molecular monolayers of functional lipids containing Angstrom gaps

Bolaamphiphiles ("bola") are compounds with a hydrophobic core and hydrophilic head groups at both ends. The head groups as well as the hydrophobic core may be functional in nonsymmetric arrangements. One end group may, for example, be bound to a solid subphase, the other may dissolve and react with solutes in the bulk water phase. The hydrophobic core may contain reactive C=C double bonds. If a monolayer membrane is formed from the bolaamphiphile, only the outer head groups and the double bonds at the edges of the membrane can react with polar reagents in the bulk water phase, provided the membrane is rigid enough. Rigidity may be achieved either by rigid hydrophobic skeletons (carotenes, steroids, porphyrins) or by two hydrogen bond chains between secondary amide groups within an oligomethylene chain. Rigid monolayers have so far been used to prepare water-free, dry vesicle membranes, to protect solid surfaces from aggressive reagents by an ultrathin lipid layer and, most important, to construct functionalized gaps in membranes on solid surfaces. The latter are thought to contain water molecules, which do not exchange with the bulk water ("hydrophobic water") and may bind watersoluble molecules. INTRODUCTION Bolas with two different head groups at the ends of oligomethylene chains or oligomethylene macrocycles provide membranes with a specific type of reactivity and stereochemistry. It has been possible in several cases to dissolve them in water under given conditions and to convert them afterwards quantitatively and reversibly to polymeric molecular assemblies. Well-known examples are the one-sided precipitation of bolas to give vesicles by a change of pH (ref. 1) or ammonium-counterions (Br-+ClOi; ref. 2) and the self-assembly of a-mercapto-o-carboxylates on gold surfaces (ref. 3). One obtains, in general, spherical or planar monolayers with a totally unsymmetrical arrangement of the head groups. In vesicles, all large head groups may be on the larger outer surface (refs. 1 and 4), and all small head groups on the smaller inner surface. In self-assembled monolayers, one may position electron-accepting quinones on the gold surface or inside the membrane and corresponding donors on the outside. Many such asymmetric donor-acceptor systems have been produced, both in bulk, vesicular solution and on solid surfaces, but attempted lightinduced charge separation has not become useful. Charge recombination in such fluid membranes is invariably at least as fast as charge separation (ref. 5) . Furthermore, the flexibility of alkyl chains connecting the head groups not only causes fluidity in vesicle membranes, but also leads to a massive disordering of the edges of monolayer domains on solid surfaces (Fig.1; ref. 6): It thus becomes difficult to produce stable molecular gaps of Angstrom size in molecular monolayers by a stepwise deposition of flat-lying molecules and up-standing bolas. To regulate distances between reactive clusters and produce membrane gaps of defined size, one has to replace the fluid membrane structures by more rigid ones. An immediate consequence is that the nondirectional repulsive hydration forces between the head group acting in spherical micelles and vesicles must be replaced by directional binding forces. Instead of "self-organization" (ref. 7), non-covalent synthesis or synkinesis (ref. 8) must be applied. Each molecule of the molecular assembly is considered now as a fitting synkinon, which binds in all parts to its neighbors in the polymeric molecular assembly. Synkinesis must, however, not lead to three-dimensional crystals. Crystallisation can be prevented either by curvature of the molecular assemblies, which leads either to undulating vesicle or molecular fibers. +Lecture presented at the 1st International Conference on Supramolecular Science and Technology, Zakopane, Poland, 27 September-3 October 1998. Other presentations are published in this issue, pp. 2337-2408.