Metallo-Supramolecular Gels Based on a Multitopic Cyclam Bis-Terpyridine Platform

Abstract

The domain of soluble metallopolymers (i.e., polymers spontaneously obtained from the self-assembly of metal ions and polytopic bridging ligands) is promising for the construction of novel materials with perspectives in magnetic, redox, optical, electrochromic, or mechanical properties. We report the preparation and characterization of metallo-supramolecular gels based on a multitopic cyclam bis-terpyridine platform (CHTT) that are representative of a second generation of molecular gels exhibiting "intelligent" properties. The systems were characterized by UV-visible spectroscopy, cyclic voltammetry, viscosimetry, rheology, and small-angle neutron scattering (SANS). A basic analysis of the viscosity taking into account the interplay of hydrodynamic versus Brownian motions of 1D species is proposed. SANS data demonstrate the formation of rod-like species whose radius R and aspect ratio f = L/R were extracted. Rheological features are typical of weak gels having storage moduli of more than 1 order of magnitude weaker than for ordinary molecular organogels. These new molecular materials exhibit interesting properties: (i) chemosensitivity, with the gelation being dependent on the type of metal, stoichiometry M/CHTT, solvent, and counterion; (ii) electro-sensitivity: the Co(II)/CHTT system can be electrochemically and reversibly commuted between the gel (red) and liquid (green) states; and (iii) mechanical sensitivity, with the system being able to cycle from weakly to highly viscous states upon fast application/suppression of a shearing stress.