Dynamic chemical devices: modulation of contraction/extension molecular motion by coupled-ion binding/pH change-induced structural switching.

Dynamic chemical devices involve morphological or constitutional modifications in molecular or supramolecular systems, induced by internal or external physical or chemical triggers. Reversible changes in shape result in molecular motions and define motional dynamic devices presenting mechanical-like actions of various types. Suitably designed polyheterocyclic strands such as compounds 1-5 wrap into helical conformations. The binding of lead(II) ions to the coordination subunits contained in the strand leads to complete uncoiling and yields a polymetallic complex presenting a fully extended shape. The addition of a cryptand complexing agent that strongly binds lead(II) ions and releases them under protonation allows a reversible pH-modulation of lead(II) levels in the medium, which in turn induces coiling/uncoiling of the molecular strand. This system thus represents a motional dynamic device which performs a mechano-chemical process, realizing alternating extension/contraction motions triggered by ion binding. It achieves a linear motor-type of action of very large stroke amplitude fueled by ionic processes.