Trivial positional isomerism in ligands triggering different properties in Fe(ii)-metallopolymers; design, synthesis, and characterization

Abstract

The tunable molecular scaffold of organic moieties in metallopolymers generates variation in their properties, but what could be the minimal change that can produce variation in the properties of these macromolecules is still untouched. This research has meticulously explored the trivial change in the molecular scaffold of the ligand capable of making a mammoth difference in the nonvolatile memory and coordination pattern in two metallopolymers. The significance of this research lies in the fact that it demonstrates how a slight change in the organic building block can significantly alter the memristive and fluorescence properties of iron(II) metallopolymers, opening up new possibilities for their design and synthesis. Two novel positional isomeric ligands and their corresponding iron(II)-polymers were synthesized and thoroughly characterized using NMR, XRD, ATR-IR, FESEM, AFM and other techniques. Bright orange solid and solution state fluorescence was observed both in the solid and solution states for ligand L₂ (3,3′-bis((E)-(pyridin-3-ylimino)methyl)-[1,1′-biphenyl]-4,4′-diol), while ligand L₁ (3,3′-bis((E)-(pyridin-2-ylimino)methyl)-[1,1′-biphenyl]-4,4′-diol) showed blue fluorescence in the solution state only. A robust memristive property for Fe(II)-L₁-poly with a high current ON/OFF ratio of 10⁴, remarkable random access behaviour, and a long retention time greater than 35 000 seconds was observed while its counterpart was entirely silent. Both polymers showed solution-state electrochromism. These synthesised metallopolymers also showed good specific capacitance in the range of 50–60 F g⁻¹ with a remarkable retention of 98% of the initial value even after 5000 charge–discharge cycles. The AFM and FESEM micrographs revealed the formation of long polymer nano-rods, which correlates with the NMR, ATR-IR, and XRD results. The difference in the properties of polymers generated by such a slight change in the organic building block forces different coordination patterns of these two ligands around the same central metal ion, and this is also evident in all the characterization methods.