Stimuli-Responsive Europium-Containing Metallo- Supramolecular Polymers

摘要

The ability to utilize supramolecular interactions towards the design of new stimuli-responsive materials has gained a great amount of interest in recent years. For example, main-chain supramolecular polymers, whose polymer backbone consists of both covalent bonds and non-covalent interactions, can result in large (e.g. mechanical) responses to small changes in the environment. Reversible metalligand coordination is one class of non-covalent interaction that has been widely utilized to access main-chain supramolecular polymers. Most of these metallo-supramolecular polymers rely on ligand coordination with transition metals ions. Such systems have been targeted, at least in part, on account of the strength of the metal-ligand interaction and the fact that 2:1 ligand to metal complexes (required for linear chain extension) can be easily accessed with either terdentate ligands bound to hexacoordinate metal ions or bidentate ligands bound to tetracoordinate metal ions. A series of metal ions that have seen much less attention in this field are the lanthanide metals, which can be of special interest as their complexes have been shown to have many potential applications. Combining the distinctive properties of lanthanide complexes with the mechanical properties of polymers presents an interesting method of creating stimuli-responsive metalc o n t a i n i n g m a t e r i a l s . We h a v e s h o w n t h a t 2 , 6 - b i s - ( 1 ' - methylbenzimidazolyl)pyridine (Mebip) ligands can be placed at the ends of ditopic macromonomers, which consist of either conjugated or polyether cores, and produce polymeric materials upon addition of an appropriate transition metal ion (e.g. Fe~(2+), Co~(2+), or Zn~(2+)) in a 1:1 ratio of metal to ditopic macromonomer. Piguet and coworkers have taken an in-depth look at the interaction between lanthanides and terdentate aza-ligands in both the solid-state and in solution. With their preference for high coordination numbers (8-10) , lanthanides, such as Eu~(3+), can coordinate Mebip in a 3:1 ligand to metal ratio. Thus with the goal of developing s t i m u l i - responsive thermoplastic elastomers we report herein the effects of replacing a percentage of the Zn~(2+) ion complexes with the more responsive Eu~(3+) complexes in these metallo-supramolecular polymers