Abstract
We revealed a general guidance on preparation of electrochemically stable meso forms of nickel-salen based polymers suitable as new anode materials for electrochemical devices. Electrochemical performance of nickel-salen complexes, used as monomers of nickel-salen based polymers, was directly related to the salen ligand structure. We have considered it in specially designed and synthesized herein a new meso form of the nickel-salen polymer, vis., poly[meso-N,N′-bis-(3-methylosalicylidene)-2,3-butanediaminonickel(II)]. Then, electrochemical conditions for providing the environment appropriate for electropolymerization and charge transfer were optimized. In dependence upon the purpose of the use of the poly(meso-Ni-salen) films coated electrodes, it was possible to increase the amount of the charge stored in these conjugated polymer systems by proper tuning of both the chemical structure of the polymer and electrochemical conditions. Apparently, the charge transfer through nickel-salen polymers strongly depended upon steric hindrance between polymer layers, thus ensuring efficient counter ion doping. Therefore, this hindrance was carefully selected. We have defined steric requirements with respect to the type and position of moderately electrodonating substituents on imine bridges and phenolic rings of the nickel-salen polymers that must be met to achieve high conductivity and stability/durability during potential multi-cycling of the polymer films prepared.
Highlights
Nickel-salen based polymers are Schiff base semiconducting macromolecular compounds composed of multiple monomer units of the same salen metal complex bound in para positions of their aromatic rings
Preparation of new poly(meso-Ni-SaldMe-3dMe) improved meso form of the nickelsalen based polymer with methyl substituents in the 3-positions of each salicylaldehyde moiety was inspired by results of the research focused on electrochromic behavior of nickel-salen polymers behavior, which was different in the presence and absence of two dimethyl substituents on each carbon atom of the imine bridge [27]
The charge decrease in each consecutive cycle (Figure 4c and 4d) for the poly(NiSaltMe) under cyclic voltammetry (CV) conditions confirms that even small chemical structure changes lead to the decrease of electrochemical stability of the nickel-salen based polymer while meso forms of nickel-salen polymers, namely, poly(meso-Ni-SaldMe-3dMe) (Figure 4a and 4d) and poly(meso-Ni-SaldMe) (Figure 4b and 4d) containing two moderately electrodonating methyl substituents located on the imine bridge in the one plane of monomer unit, reveal high electrochemical stability under these conditions
Summary
Nickel-salen based polymers are Schiff base semiconducting macromolecular compounds composed of multiple monomer units of the same salen metal complex bound in para positions of their aromatic rings. The encouraging results of our former studies focused on unsubstituted in phenyl rings poly(meso-Ni-SaldMe) stability and charge transfer mechanism [2, 23, 24] prompted us to find the first structure-reactivity requirement whit respect to nickel-salen based polymers ensuring its electrochemical stability.
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