No abstract

Traffic control: By exploiting the interplay of kinetic and thermodynamic effects, the direction of threading/dethreading in a nonsymmetric calixarene wheel can be selected by an appropriate choice of the head group incorporated in the molecular axle (see figure).The possibility of obtaining full control on the direction of axle threading in calix[6]arene wheel 1 either from its upper or lower rim was evaluated in solution. To this aim, we prepared nonsymmetric axles characterised by a 4,4'-bipyridinium recognition unit with two alkyl side chains, one of which terminates with a stopper, and the other with either ammonium (2), hydroxy (3) or methyl (4 and 5) head groups. When the axles were mixed with 1 in apolar solvents at room temperature, the formation of oriented pseudorotaxanes derived from the threading of the axles from the upper rim was observed. The stability constants of such complexes are in the order of 10(7) m(-1) and are almost independent of the type of axle. A detailed thermodynamic and kinetic study revealed that stability constants and activation parameters for complex formation between 1 and axles 2 and 3 are of the same order of magnitude, suggesting a common threading process. However, upon heating a solution of 1 and 2 in benzene at 340 K, the formation of another supramolecular complex was observed, the structure of which is consistent with an oriented pseudorotaxane derived from the threading of axle 2 from the lower rim of the calixarene wheel. By carrying out the threading-stoppering reaction sequence between 1 and 2 in the presence of an excess of diphenylacetyl chloride, the orientational rotaxane isomers R1 and R2, derived from lower- and upper-rim threading, respectively, were collected in about a ratio of 7:3 as the unique chromatographic fraction. Our results suggest that at room temperature the threading process is under kinetic control for all axles. On increasing the temperature only the threading behaviour of axle 2 is substantially modified, most likely because the process becomes thermodynamically controlled owing to the peculiar recognition properties of the ammonium head of this axle.

We have investigated the self-assembly of pseudorotaxanes composed of viologen-type axle and calix[6]arene wheel components. The distinctive feature of this system is that both components are structurally nonsymmetric; hence, their self-assembly can follow four distinct pathways and eventually give rise to two different orientational pseudorotaxane isomers. We found that the alkyl side chains of the viologen recognition site on the molecular axle act as strict kinetic control elements in the self-assembly, thereby dictating which side of the axle pierces the calixarene cavity. Specifically, nonsymmetric axles with alkyl side chains of different length thread the wheel with the shorter chain. Such a selectivity, in combination with the face-selective threading of viologen-type axles afforded by tris(N-phenylureido)calix[6]arenes, enables a strict directional control of the self-assembly process for both the face of the wheel and the side of the axle. This kinetic selectivity allows both intramolecular self-sorting between two different side chains in a nonsymmetric axle and intermolecolar self-sorting among symmetric axles with alkyl substituents of different length.

We describe the synthesis and characterization of a series of desymmetrized donor-acceptor [2]catenanes where different donor and acceptor units are assembled within a confined catenated geometry. Remarkable translational selectivity is maintained in all cases, including two fully desymmetrized [2]catenanes where both donors and acceptors are different, as revealed by X-ray crystallography in the solid state, and by (1)H NMR spectroscopy and electrochemistry in solution. In all desymmetrized [2]catenanes the co-conformation is dominated by the strongest donor and acceptor pairs, whose charge-transfer interactions also determine the visible absorption properties. Voltammetric and spectroelectrochemical experiments show that the catenanes can be reversibly switched among as many as seven states, characterized by distinct electronic and optical properties, by electrochemical stimulation in a relatively narrow and easily accessible potential window. Moreover in some of these compounds the oxidation of the electron donor units or the reduction of the electron acceptor ones causes the circumrotation of one molecular ring with respect to the other. These features make these compounds appealing for the development of molecular electronic devices and mechanical machines.