A photo-responsive polymeric azopyridine ligand with metal-complexation sensitivity: application to coordination equilibrium studies on the polymer complexes of a cobalt(ii) Schiff base

Abstract

In a solution of the copolymer, P(EHMA-APMA), consisting of ethylhexyl methacrylate (EHMA) and the photo-responsive azopyridine methacrylate (APMA), the E form of APMA showed photo-reversible isomerization to the Z form upon UV irradiation. After addition of the cobalt(II) Schiff base (CoS), the copolymer complex, P(EHMA-(E)-APMA)-CoS, was produced, and the (E)-APMA moieties coordinated to CoS no longer underwent photo-induced isomerization. The complexation-sensitive isomerization of (E)-APMA had a large extinction coefficient and the apparent equilibrium constant (K_app) for the coordination reaction of (E)-APMA with CoS in a dilute solution was calculated to be 4.4 × 10⁴ M⁻¹ ([APMA moiety] = 0.05 mM) and 1.0 × 10⁵ M⁻¹ ([APMA moiety] = 0.003 mM) at 25 °C. The resulting concentration-dependent large K_app values were due mainly to a micro-heterogeneous solution environment, consisting of a micro-suspension with a large solvent phase and a small polymer-domain phase caused by the extreme dilution, in addition to the thermodynamic stabilization of the five-coordinate CoS via the local concentration effect of the coordinating polymeric APMA ligands located only in the polymer domain.