Spectroelectrochemical Approaches to Mechanistic Aspects of Charge Transport in meso-Nickel(II) Schiff Base Electrochromic Polymer

A new redox conducting polymer, viz. poly­[meso-<i>N</i>,<i>N</i>′-bis­(salicylidene)-2,3-butane­diaminonickel­(II)], poly­[meso-Ni­(II)-SaldMe], belonging to the Schiff base polymer family, was electrochemically synthesized. The charge transfer and polymerization mechanism were unraveled by simultaneous cyclic voltammetry (CV) and in situ UV–vis, FTIR-ATR, and ex situ low-temperature ESR spectroscopy. With the latter, a short-living paramagnetic transient form of electro-oxidized poly­[meso-Ni­(II)-SaldMe] was detected. This form was identified as the bisphenolic radical cation. In situ UV–vis and FTIR-ATR spectroelectrochemistry measurements revealed that the charge transfer of the polymer involved bisphenolic radical cation formation at the potential lower than 0.80 V vs Ag/Ag<sup>+</sup> and then dication formation at the potential exceeding 0.80 V. The proposed mechanism of electropolymerization of meso-<i>N</i>,<i>N</i>′-bis­(salicylidene)-2,3-butane­diaminonickel­(II), meso-Ni­(II)-SaldMe, involves two steps. First, electro-oxidation of the monomer results in bisphenolic radical cation generation, and then mutual binding of these radicals at the <i>para</i> positions of aromatic rings is activated by electron-donating phenol moieties. In this electropolymerization, the Ni­(II) metal center played the role of a template providing planarity to the monomer molecule. Structures responsible for the charge transfer in the polymer and formed during electropolymerization were modeled with quantum chemistry calculations using the DFT method at the PBE level. The resulting polymer film was highly conducting and stable with respect to potential multicycling under cyclic voltammetry conditions, from 0 to 1.3 V vs Ag/Ag<sup>+</sup>. Under these conditions, it changes color from yellow through orange to russet for its neutral, bisphenolic radical cation, and bisphenolic dication form, respectively. High electrochemical stability and a wide potential range of electroactivity (0.40–1.30 V vs Ag/Ag<sup>+</sup>) of the polymer are very promising for its application as a new electrochromic electrode material for supercapacitors. That is, an anode composed of poly­[meso-Ni­(II)-SaldMe] can serve as an internal charging–discharging indicator in these supercapacitors.