Relationship Between Structure and Electrochemical Properties of Polymers Based on Transition Metal Complexes with Schiff Bases

Grant Number: 13-03-00843

Project Description

This project is devoted to the study of metal-containing conducting polymers based on transition metal complexes with Schiff bases. These substances are considered for use in various sensing, catalytic, and optical devices; however, literature lacks a recognized model linking structure, electrochemical, and optical properties.

Goal:
To address this, the project applies a combined methodology:

• Experimental electrochemical studies
• Molecular dynamics simulations
• Quantum chemical calculations of electronic structure and spectral characteristics

Special attention is given to correlating computational results with observed electronic properties of monomers and polymers, enabling targeted synthesis of materials with desired spectral and electrochemical features.

Project Report

Comprehensive investigation of salen-type complexes, varying ligand structure and central metal atom, revealed:

• Polymerization yields relatively short chains, crosslinked via covalent C–C bonds, forming supramolecular structures on electrode surfaces, held together by stacking interactions of d–d and d–π types.
• Ligand substituents affect order and packing of these supramolecular structures, altering polymer density and morphology.
• Substituents in the diamine bridge have the greatest influence—molecular dynamics shows they determine molecule orientation.
• Polymers demonstrate two redox processes:
  1. Oxidation of the organic ligand forms a delocalized cation radical
  2. Oxidation of the metal center yields a localized metal charge

The ratio of charge flow in these processes is defined by ligand structure and metal atom nature. By selecting appropriate substituents, one can separate ligand- and metal-centered redox processes, increasing the number of electrons transferred and improving electrode capacity.

Experimental data and quantum chemical calculations confirmed that forming metal-centered oxidized polymer requires coordination of axial ligands in the square-planar complex. In acetonitrile, the solvent itself can serve as ligand and is reversibly removed during polymer reduction. However, impurities (e.g., water or halide ions) in the electrolyte, coordinating and oxidizable at the relevant potentials, can transfer electrons to the metal atom, generating reactive oxidation products that degrade the ligand and gradually destroy film electroactivity.

Steric hindrance—via substituents in the diamine bridge—retards impurity coordination by blocking access between polymer layers. Thus, capacity retention during long-term cycling is best achieved in polymers with steric substituents in the diamine bridge, with careful control of electrolyte purity.