Reversible Redox Processes in Polymer of Unmetalated Salen-Type Ligand: Combined Electrochemical in Situ Studies and Direct Comparison with Corresponding Nickel Metallopolymer

Abstract

Most non-metalized Salen-type ligands form passivation thin films on electrode surfaces upon electrochemical oxidation. In contrast, the H₂(3-MeOSalen) forms electroactive polymer films similarly to the corresponding nickel complex. There are no details of electrochemistry, doping mechanism and charge transfer pathways in the polymers of pristine Salen-type ligands. We studied a previously uncharacterized electrochemically active polymer of a Salen-type ligand H₂(3-MeOSalen) by a combination of cyclic voltammetry, in situ ultraviolet-visible (UV-VIS) spectroelectrochemistry, in situ electrochemical quartz crystal microbalance and Fourier Transform infrared spectroscopy (FTIR) spectroscopy. By directly comparing it with the polymer of a Salen-type nickel complex poly-Ni(3-MeOSalen) we elucidate the effect of the central metal atom on the structure and charge transport properties of the electrochemically doped polymer films. We have shown that the mechanism of charge transfer in the polymeric ligand poly-H₂(3-MeOSalen) are markedly different from the corresponding polymeric nickel complex. Due to deviation from planarity of N₂O₂ sphere for the ligand H₂(3-MeOSalen), the main pathway of electron transfer in the polymer film poly-H₂(3-MeOSalen) is between π-stacked structures (the π-electronic systems of phenyl rings are packed face-to-face) and C-C bonded phenyl rings. The main way of electron transfer in the polymer film poly-Ni(3-MeOSalen) is along the polymer chain, while redox processes are ligand-based.

Keywords: Schiff bases; UV–vis–NIR spectroelectrochemistry; conducting polymer; electrochemical polymerization; salen-type ligand; π-stacked structures.

Scheme 1

Structures of the ligand H₂(3-MeOSalen) (a) and corresponding nickel(II) complex Ni(3-MeOSalen) (b).

Figure 1

Polymerization of 0.001 mol dm⁻³ Ni(3-MeOSalen) (a) and 0.001 mol dm⁻³ H₂(3-MeOSalen) (b) in 0.1 mol dm⁻³ Et₄NBF₄ acetonitrile solution, v = 0.15 V s⁻¹. The first polymerization cycle is marked with red color. The arrows show the growth of the film oxidation/reduction currents from cycle to cycle.

Scheme 2

Mechanism of H₂(3-MeOSalen) polymerization.

Figure 2

IR-spectra of the monomeric nickel complex Ni(3-MeOSalen) and the monomeric ligand H₂(3-MeOSalen) in the frequency range of O-H stretching.

Figure 3

IR spectra of monomeric and polymeric nickel complexes poly-Ni(3-MeOSalen) (a) and monomeric and polymeric ligands poly-H₂(3-MeOSalen) (b).

Figure 4

Cyclic voltammograms of the polymeric films poly-Ni(3-MeOSalen) (a) and poly-H₂(3-MeOSalen) (b) in 0.1 mol dm⁻³ Et₄NBF₄/AN, 0–1.30 V, v = 0.05 V s⁻¹.

Figure 5

The change in the polymer film mass as a function of potential (blue line), cyclic voltammograms (black line) and corresponding Δm(q) curves for poly-Ni(3-MeOSalen) (a,b) and poly-H₂(3-MeOSalen) (c,d).

Figure 6

Evolution of in situ UV–VIS spectra during electrochemical oxidation of poly-H₂(3-MeOSalen) in 0.1 mol dm⁻³ Et₄NBF₄/AN electrolyte, each UV–VIS spectrum was collected relative to that of the neutral (non-charged) polymer at the initial potential (a), and potential dependence of absorbance (b).

Scheme 3

Schematic drawing of π-dimerization in a general case. Perturbation diagrams showing mixing of SOMO and LUMO to form new orbitals of π-mer (blue) and π-dimer (green). Black arrows indicate the spin of an electron in the corresponding orbital. The delta value characterizes the difference in energy between HOMO and LUMO.

Figure 7

Scanning electron micrographs of polymer films poly-H₂(3-MeOSalen) (a,c) and poly-Ni(3-MeOSalen) (b,d) deposited electrochemically on a platinum electrode by cycling from 0.0 to 1.0 V during 3 cycles (a,b) and 20 cycles (c,d).

Scheme 4

Schematic representation of polymer film poly-Ni(3-MeOSalen) at different doping levels. Depending on the doping level such charge carriers as cation radicals (pink), dications (red) and π-dimers (green) are formed.

Scheme 5

Schematic representation of oxidation of poly-Ni(3-MeOSalen).

Scheme 6

Schematic representation of the polymer film poly-H₂(3-MeOSalen) at different doping levels. At a low doping level the main charge carriers in the film are cation radicals (pink) and π-mer structures (blue). With an increase in the doping level, the conversion of π-mers (blue) to π-dimers (green) occurs. Defects in the polymer structure such as “trapped” charge (purple) and neutral units (baby blue) are not involved in redox processes. The deviation of monomeric units from planarity was ignored for clarity.

Scheme 7

Schematic representation of oxidation of the poly-H₂(3-MeOSalen).