The interaction of appropriate metal ions (Pb(II), Zn(II)) with helical ligand strands, obtained by hydrazone polycondensation, generates polymetallic supramolecular architectures of rack and grid types, by uncoiling of the ligand. The interconversion between the helical free ligand and the linearly extended ligand in the complexes produces reversible ion-induced, nanomechanical molecular motions of large amplitude. It has been integrated in an acid-base neutralisation fuelled process, which links the extension/contraction of the ligand strands to alternating changes in pH.

Dedicated to Professor Jack D. Dunitz on the occasion of his 80th birthday Control over the folding of molecular strands may be achieved by appropriate choice of the constituting subunits, in particular for chains of specific heterocycles such as sequences of directly connected pyridine (py) and pyrimidine (pym) rings, which are known to fold into extended helical structures. Since the hydrazone (hyz) group represents an isomorphic analogue of a py site, the condensation of hydrazine and carboxaldehyde derivatives of pym offers a very efficient approach to strands incorporating hyz instead of py units and constituted by sequences of alternating hyz and pym groups. A series of such strands of different lengths, up to ten hyz units, i.e., 1 ± 7, were synthesized. Their spectral properties indicate that they fold indeed into helical shapes. Extensive characterization was performed in solution by 1 H-NMR spectroscopy and in the solid state by determination of the crystal structures of eight such strands. They all display the expected helical geometry with up to 3 1 / 3 turns and direct stacking contacts. The efficiency and flexibility of the synthetic approach as well as its wide potential for generation of diversity through lateral decoration make the (hyzÀpym) subunit a particularly attractive helicity codon.Introduction. ± The design of molecular strands capable of taking up defined and predictable shapes is of much interest in view of its relation to biological folding processes in proteins as well as of the access it provides to the generation of welldefined geometries for functional devices and materials. Intense activity has recently been displayed in this area, implementing various types of supramolecular, noncovalent interactions (H-bonding, stacking and electrostatic interactions, metal-ion coordination) as well as medium effects, to induce the self-organization of a molecular strand into specific architectures [1]. A range of geometries from entirely helical to fully linear may thus be targetted.In our laboratories, we have made use of different interaction patterns and structuration subunits to generate single helices [2 ± 8]

General design principles have been developed for the control of the structural features of polyheterocyclic strands and their effector-modulated shape changes. Induced defined molecular motions permit designed enforcement of helical as well as linear molecular shapes. The ability of such molecular strands to bind metal cations allows the generation of coiling/uncoiling processes between helically folded and extended linear states. Large molecular motions are produced on coordination of metal ions, which may be made reversible by competition with an ancillary complexing agent and fueled by sequential acid/base neutralization energy. The introduction of hydrazone units into the strands confers upon them constitutional dynamics, whereby interconversion between different strand compositions is achieved through component exchange. These features have relevance for nanomechanical devices. We present a morphological and functional analysis of such systems developed in our laboratories.