Spectroelectrochemical characterisation of copper salen-based polymer-modified electrodes

Harmonizing Energies: The Interplay Between a Nonplanar SalEn-Type Molecule and a TEMPO Moiety in a New Hybrid Energy-Storing Redox-Conducting Polymer

Redox-conducting polymers based on SalEn-type complexes have attracted considerable attention due to their potential applications in electrochemical devices. However, their charge transfer mechanisms, physical and electrochemical properties remain unclear, hindering their rational design and optimization. This study aims to establish the influence of monomer geometry on the polymer's properties by investigating the properties of novel nonplanar SalEn-type complexes, poly[N,N'-bis(salicylidene)propylene-2-(hydroxy)diaminonickel(II)], and its analog with 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO)-substituted bridge (MTS). To elucidate the charge transfer mechanism, operando UV-Vis spectroelectrochemical analysis, electrochemical impedance spectroscopy, and electron paramagnetic resonance are employed. Introducing TEMPO into the bridge moiety enhanced the specific capacity of the poly(MTS) material to 95 mA h g-1, attributed to TEMPO's and conductive backbone's charge storage capabilities. Replacement of the ethylenediimino-bridge with a 1,3-propylenediimino- bridge induced significant changes in the complex geometry and material's morphology, electrochemical, and spectral properties. At nearly the same potential, polaron and bipolaron particles emerged, suggesting intriguing features at the overlap point of the electroactivity potentials ranges of polaron-bipolaron and TEMPO, such as a disruption in the connection between TEMPO and the conjugation chain or intramolecular charge transfer. These results offer valuable insights for optimizing strategies to create organic materials with tailored properties for use in catalysis and battery applications.

A Comparative XPS, UV PES, NEXAFS, and DFT Study of the Electronic Structure of the Salen Ligand in the H2(Salen) Molecule and the [Ni(Salen)] Complex

A comparative study of the electronic structure of the salen ligand in the H₂(Salen) molecule and the [Ni(Salen)] complex was performed using the experimental methods of XPS, UV PES, and NEXAFS spectroscopy along with DFT calculations. Significant chemical shifts of +1.0 eV (carbon), +1.9 eV (nitrogen), and -0.4 eV (oxygen) were observed in the 1s PE spectra of the salen ligand atoms when passing from a molecule to a complex, unambiguously indicating a substantial redistribution of the valence electron density between these atoms. It is proposed that the electron density transfer to the O atoms in [Ni(Salen)] occurred not only from the Ni atom, but also from the N and C atoms. This process seemed to be realized through the delocalized conjugated π-system of the phenol C 2p electronic states of the ligand molecule. The DFT calculations (total and partial DOS) for the valence band H₂(Salen) and [Ni(Salen)] described well the spectral shape of the UV PE spectra of both compounds and confirmed their experimental identification. An analysis of the N and O 1s NEXAFS spectra clearly indicated that the atomic structure of the ethylenediamine and phenol fragments was retained upon passing from the free salen ligand to the nickel complex.

Asymmetric Monomer Design Enables Structural Control of M(Salen)-Type Polymers

Conductive and electrochemically active polymers consisting of Salen-type metal complexes as building blocks are of interest for energy storage and conversion applications. Asymmetric monomer design is a powerful tool for fine-tuning the practical properties of conductive electrochemically active polymers but has never been employed for polymers of M(Salen)]. In this work, we synthesize a series of novel conducting polymers composed of a nonsymmetrical electropolymerizable copper Salen-type complex (Cu(3-MeOSal-Sal)en). We show that asymmetrical monomer design provides easy control of the coupling site via polymerization potential control. With in-situ electrochemical methods such as UV-vis-NIR (ultraviolet-visible-near infrared) spectroscopy, EQCM (electrochemical quartz crystal microbalance), and electrochemical conductivity measurements, we elucidate how the properties of these polymers are defined by chain length, order, and cross-linking. We found that the highest conductivity in the series has a polymer with the shortest chain length, which emphasizes the importance of intermolecular iterations in polymers of [M(Salen)].

A Novel Cobalt Metallopolymer with Redox-Matched Conjugated Organic Backbone via Electropolymerization of a Readily Available N4 Cobalt Complex

Fast and reversible cobalt-centered redox reactions in metallopolymers are the key to using these materials in energy storage, electrocatalytic, and sensing applications. Metal-centered electrochemical activity can be enhanced via redox matching of the conjugated organic backbone and cobalt centers. In this study, we present a novel approach to redox matching via modification of the cobalt coordination site: a conductive electrochemically active polymer was electro-synthesized from [Co(Amben)] complex (Amben = N,N′-bis(o-aminobenzylidene)ethylenediamine) for the first time. The poly-[Co(Amben)] films were investigated by cyclic voltammetry, electrochemical quartz crystal microbalance (EQCM), in situ UV-vis-NIR spectroelectrochemistry, and in situ conductance measurements between −0.9 and 1.3 V vs. Ag/Ag⁺. The polymer displayed multistep redox processes involving reversible transfer of the total of 1.25 electrons per repeat unit. The findings indicate consecutive formation of three redox states during reversible electrochemical oxidation of the polymer film, which were identified as benzidine radical cations, Co(III) ions, and benzidine di-cations. The Co(II)/Co(III) redox switching is retained in the thick polymer films because it occurs at potentials of high polymer conductivity due to the optimum redox matching of the Co(II)/Co(III) redox pair with the organic conjugated backbone. It makes poly-[Co(Amben)] suitable for various practical applications based on cobalt-mediated redox reactions.

Electronic structure of the [Ni(Salen)] complex studied by core-level spectroscopies

The nature and structure of occupied and empty valence electronic states (molecular orbitals, MOs) of the [Ni(Salen)] molecular complex (NiO2N2C16H14) have been studied by X-ray photoemission and absorption spectroscopy combined with density functional theory (DFT) calculations. As a result, the composition of the high-lying occupied and low-lying unoccupied electronic states has been identified. In particular, the highest occupied molecular orbital (HOMO) of the complex is found to be predominantly located on the phenyl rings of the salen ligand, while the states associated with the occupied Ni 3d-derived molecular orbitals (MOs) are at higher binding energies. The lowest unoccupied molecular orbital (LUMO) is also located on the salen ligand and is formed by the 2pπ orbitals of carbon atoms in phenyl groups of the salen macrocycle. The unoccupied MOs above the LUMO reflect σ- and π-bonding between Ni and its nearest neighbours. All valence states have highly mixed character. The specific nature of the unoccupied Ni 3d-derived σ-MO is a consequence of donor-acceptor chemical bonding in [Ni(Salen)].

Redox-conducting polymers based on metal-salen complexes for energy storage applications

Metal-salen polymers are electrochemically active metallopolymers functionalized with multiple redox centers, with a potential for high performance in various fields such as heterogeneous catalysis, chemical sensors, energy conversion, saving, and storage. In light of the growing world demand for the development of superior energy storage systems, the prospects of employing these polymers for advancing the performance of supercapacitors and lithium-ion batteries are particularly interesting. This article provides a general overview of the results of investigating key structure-property relationships of metal-salen polymers and using them to design polymer-modified electrodes with improved energy storage characteristics. The results of independent and collaborative studies conducted by the members of two research groups currently affiliated to the Saint–Petersburg State University and the Ioffe Institute, respectively, along with the related data from other studies are presented in this review.

Water-stable [Ni(salen)]-type electrode material based on phenylazosubstituted salicylic aldehyde imine ligand

We have synthesized a new water-stable [Ni(salen)]-type electrode material, which was found to be a stacked polymer due to the presence of substituents, preventing oxidative coupling of the phenyl rings.

High-Performance Electrochromic Devices Based on Poly[Ni(salen)]-Type Polymer Films

We report the application of two poly[Ni(salen)]-type electroactive polymer films as new electrochromic materials. The two films, poly[Ni(3-Mesalen)] (poly[1]) and poly[Ni(3-MesaltMe)] (poly[2]), were successfully electrodeposited onto ITO/PET flexible substrates, and their voltammetric characterization revealed that poly[1] showed similar redox profiles in LiClO4/CH3CN and LiClO4/propylene carbonate (PC), while poly[2] showed solvent-dependent electrochemical responses. Both films showed multielectrochromic behavior, exhibiting yellow, green, and russet colors according to their oxidation state, and promising electrochromic properties with high electrochemical stability in LiClO4/PC supporting electrolyte. In particular, poly[1] exhibited a very good electrochemical stability, changing color between yellow and green (λ = 750 nm) during 9000 redox cycles, with a charge loss of 34.3%, an optical contrast of ΔT = 26.2%, and an optical density of ΔOD = 0.49, with a coloration efficiency of η = 75.55 cm(2) C(-1). On the other hand, poly[2] showed good optical contrast for the color change from green to russet (ΔT = 58.5%), although with moderate electrochemical stability. Finally, poly[1] was used to fabricate a solid-state electrochromic device using lateral configuration with two figures of merit: a simple shape (typology 1) and a butterfly shape (typology 2); typology 1 showed the best performance with optical contrast ΔT = 88.7% (at λ = 750 nm), coloration efficiency η = 130.4 cm(2) C(-1), and charge loss of 37.0% upon 3000 redox cycles.

Electropolymerization and electrochemical behavior of nickel Schiff base complexes with different groups between imine linkages

Nickel Schiff base complexes Ni(salen), Ni(salphen) and Ni(saldmp) are synthesized and electropolymerized on multiwalled carbon nanotube electrodes.

Novel layer-by-layer interfacial [Ni(salen)]-polyelectrolyte hybrid films

A novel multilayer film containing a cationic phosphonium-derivatized Ni(salen)-type complex and poly(sodium-4-styrenesulfonate (NaPSS) was assembled onto quartz, mica, and metal surfaces using the layer-by-layer (LbL) technique. Spectroscopic (UV-vis) and gravimetric (QCM) responses for the multilayer films show regular stepwise growth and the signature of strong electrostatic interactions between the component layers. The gravimetric responses indicate the presence of substantial additional (net neutral) material in the PSS layers, which XPS shows is not polyelectrolyte or salt, so charge compensation is intrinsic; we deduce the presence of space-filling solvent. Direct electrostatic interaction of the two-component layers is enhanced by a secondary noncovalent interaction between the delocalized pi-systems of the two components. Permeability of the film to the redox probe [Fe(CN)(6)](3-/4-) was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Qualitatively similar results were obtained in the absence and presence of a precursor PSS/PAH multilayer, but with a general shift in kinetic and diffusional processes to longer time scales (lower frequencies) in the presence of the precursor layer and with increasing numbers of PSS/[Ni(salen)] bilayers. Quantitatively, the EIS data were interpreted using a capillary membrane model (CMM) to yield values of coverage, apparent charge transfer resistance, double-layer capacitance, pore size, and diffusion coefficient. The coverage values were consistent with a model in which there are no preferential growth sites and the surface charge density is independent of the number of bilayers.