Redox-Triggered Folding of Self-Assembled Coordination Polymers incorporating Viologen Units.

We report the study of stimuli-responsive Zn and Fe coordination polymers (MC3 or MC2 with M=Fe or Zn ). These soluble metallopolymers were formed spontaneously by reaction of an organic ligand (C3 or C2 ) with one molar equivalent of metal ions. The C3 and C2 ligands incorporate two chelating terpyridine groups bridged by a redox responsive hinge featuring two viologen units (viologen=N,N'-dialkyl-4,4'-bipyridinium) linked either with propyl (C3 ) or ethyl (C2 ) chains. The viologen units in the polymer chains were reduced (1 e per viologen group) either by bulk electrolysis or by visible-light irradiation carried out in the presence of a photosensitizer. The 1 e reduction of the viologen units in the MC2 polymers induced a slight decrease in the viscosity of the solutions due to a modification of the overall charge carried by the metallopolymers. In strong contrast, reduction of coordination polymers involving propyl linkers (MC3 ) led to a remarkable increase (≈+400 %) in observed viscosity. This reversible effect was attributed to a folding of the polymer chains triggered by π-dimerization of the photo-generated viologen cation radicals.