Design and construction of chemically controlled molecular muscles

The development of supramolecular chemistry in the field of molecular recognition systems has led to a plethora of (supra)molecular architectures that would otherwise require a major effort to construct if conventional synthetic methods were used. Molecular-level architectures, based on these molecular recognition systems, have also led to the construction of functional molecules, such as molecular switches, in which the relative positions of the component parts can be controlled either chemically, photochemically or electrochemically, at will. Introduction of a new molecular synthon---a self-complementary molecular recognition motif---that is made by covalently linking two complementary recognition motifs, has enabled us to build superstructures such as cyclic daisy-chain dimers (Chapter 1). The functionalization of a cyclic daisy-chain dimer leads to a molecule whose function is reminiscent of a molecular muscle (Chapter 2). The molecular muscle is constructed via dimerization of monomers, followed by stoppering with a bulky group, where the monomer consists of a pi-electron rich dibenzo[24]crown-8 ring and two electron-poor sites such as an ammonium center and a bipyridinium unit. The position of the crown ethers was determined to be on the ammonium center when it is protonated, and on the bipyridinium unit, when the ammonium center is deprotonated, following the addition of a base. The switching process is reversible several times upon the addition of acid base. The molecular muscle, based on a cyclic daisy chain, is hermaphroditic since the two monomers come together to form a molecule, while the four-station [3]rotaxane ( Chapter 3) is palindromic, since it reads the same from left to right as from right to left. This palindromic rotaxane is composed of the same molecular recognition motifs as the hermaphroditic molecular muscle. 2D 1H-NMR Spectroscopic studies have shown that the position of the crown ether rings can be monitored through the NOEs between the protons on the crown ether ring and the guest. The different binding affinities of the ring toward the two electron poor sites---the ammonium center and the bipyridinium unit---have led to an investigation (Chapter 4) of the binding affinities of similar-sized crown ethers, such as dipyrido[24]crown-8 and benzo- meta-phenylene[25]crown-8 rings, in addition to dibenzo[24]crown-8, toward the bipyridinium-containing guests. This study has confirmed much stronger binding of crown ethers toward the ammonium center than to the bipyridinium unit. However, although the binding is weak and the cavity size of the crown ethers are tight, construction of symmetric [2]pseudorotaxanes and [2]rotaxanes have been achieved, upon bringing the crown ether rings with the bipyridinium-containing guests together, and, in the latter case, modifying covalently the complex.