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Analyte concentration effects on the first reduction process of methyl viologens and diquat redox flow battery electrolytes were examined by cyclic voltammetry in aqueous media. A simple one-electron transfer mechanism to form radical cations was detected for diquat, 4,4′-dimethyl diquat, and bis(3trimethylammonio)-propyl viologen compounds. The radical cations attach to the electrode surface when the source of their electrogeneration is methyl viologen molecules bearing PF 6 − ions as a counterpart. However, this inner sphere reduction mechanism was not observed in methyl viologen having an I − counterion. For the latter compound, as well as for 5,5′-dimethyl diquat and 1,1′bis(3-sulfonatopropyl)-4,4′-bipyridinium, a piece of experimental evidence for unexpected, fast, and reversible dimerization interactions between their electrogenerated radical cations is presented. To get information on these bimolecular interactions, a screening methodology (using different levels of theory) was employed in finding suitable dimeric structures and their related interaction energies. By using diquat as a reference system, a relationship between calculated interaction energies and the corresponding experimental dimerization constants was obtained. The examination of redox-active molecules using this experimental and theoretical approach will allow a better selection of redox flow battery electrolytes.

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Molecular information in SMILES code was used as input in Open Babel 2.4.0 \cite{O'Boyle2011} to find the minimum energy conformer using the MMFF94 force field. The generated geometries provided the cartesian coordinates that were used to generate a Gaussian input file. In some cases, it was necessary to apply a preliminary minimization using pseudopotentials (PM7) before performing the actual DFT calculation. Molecular energies of the 156 2,2'-bipirydines were obtained using the Gaussian 16 suite at DFT, with B3LYP hybrid functional and 6-31G(d) basis. Analysis of vibrational frequencies was performed to verify the existence of an energy minimum. All geometries were optimized using both the polarizable continuum model (PCM) \cite{Miertus1981,Scalmani2010} and the solvation model based on density (SMD) \cite{Marenich2009} with water as solvent; the results presented below correspond to SMD as it is better correlated with experimental values

Calculations of structure, redox potential and spin transition energies were performed using DFT approximations for a series of [Fe(bztpen)OR] 2 + / + type Fe III /Fe II systems (R=Me, Et, nPr, nBu), which have a temperature dependent spin crossover behavior. These compounds exhibit changes in redox and magnetic properties, related to structural variations quite important for their possible applications among which signal generator materials stand out. Functionals B3LYP, wB97XÀD and TPSS along with PCM solvation model were evaluated for redox potential, whereas for spin crossover the TPSSh functional was also included. The multireference character of these compounds was tested as well. Calculations were compared to experimental measurements, and wB97XÀD proved able to accurately describe the geometries observed in solid state for the low spin (LS) and high spin (HS) states; moreover, it had the best correlation between calculated and experimental redox potential values. However, in the description of the spin transition energies the TPSS functional is needed to correctly describe the LS state as the observed ground state in the complexes at low temperature, which allows to calculate proper spin transition curves as a function of temperature. From these results, we obtained suitable approximations for an accurate description of redox potential and magnetic properties for the Fe III coordination compounds, which can be extended to model similar systems.

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