The one electron electrochemical reduction of vanadium(V) substituted-Keggin type polyoxometalate anions [XV^VM₁₁O₄₀]⁴⁻ (XV^VM₁₁) where X = P, As; M = Mo or W has been investigated in CH₃CN as a function of acid concentration. EPR studies confirm that the product is the highly basic and hence readily protonated V(IV), (XV^IVM₁₁), species. In the absence of acid or in the presence of a large concentration excess, the V(V /IV) redox couple gives one well defined chemically and electrochemically reversible process under conditions of cyclic voltammetry. In contrast, three well resolved V(V /IV) processes are observed in the presence of a moderate concentration of acid in the case of XV^VMo₁₁ but only two with XV^VW₁₁. NMR and EPR spectra have been obtained as a function of acid concentration when the polyoxometalate anions are in the V(V) and V(IV) oxidation levels respectively. All voltammetric and spectroscopic data indicate that protonation reactions are coupled to the V(V) and V(IV) redox chemistry but that the reduced V(IV) state is much more basic than the V(V) one and that (XV^IVMo₁₁ ) is more basic than (XV^IVW₁₁). Digital simulations of voltammograms for reduction of XV^VMo₁₁ and XV^VW₁₁ have been undertaken in CH₃CN as a function of acid concentration in order to define the thermodynamics and kinetics associated with the V(V /IV) process and the equilibrium constants that accompany the coupled acid–base chemistry. EPR spectra allow an estimation of the relative concentrations of protonated species present in frozen glasses derived from one-electron bulk electrolysis and also allow inferences to be drawn on their nature. This study provides a far more detailed analysis of the coupling of proton and electron transfer than previously available.

Obtaining highly efficient photoluminescence with Mn⁴⁺-activated phosphors, which have been extensively studied in diverse lighting devices, requires the precise control of the manganese valence states. However, this control is difficult to achieve because manganese ions can have various valence states ranging from divalent to heptavalent. Additionally, the concentrations of Mn ions in each valence state, especially the effective Mn⁴⁺ concentration, have never been quantitatively determined in a phosphor crystal lattice. The relationship between the effective Mn⁴⁺ concentration and the luminescence properties of Mn⁴⁺-activated phosphors is of current interest for improving the phosphor properties. In the present study, the effective Mn⁴⁺ concentration in Li₂TiO₃:Mn⁴⁺ (LTO:Mn) phosphors prepared by the sol–gel method with heating at various temperatures was quantitatively analyzed by X-ray absorption near-edge spectroscopy. Moreover, the effect of the existence of Mn²⁺, Mn³⁺, and Mn⁴⁺ ions on the photoluminescence efficiency was investigated. The effective Mn⁴⁺ concentration was found to be over 60% in all phosphor samples. The quantum efficiencies (QEs) of all LTO:Mn phosphors strongly depend on the effective Mn⁴⁺ concentration. In particular, the LTO:Mn phosphor prepared by heating at 800 °C (LTO:[email protected]) contained the highest effective Mn⁴⁺ concentration of 98.1% and exhibited the highest internal QE of 31.6%. The results of this work provide new and important insights for the development of Mn⁴⁺-activated phosphors with high efficiency.

Structures of the n-tetrabutylammonium salts of [SVM₁₁O₄₀]³⁻ (M = Mo, W) have been determined by X-ray crystallography and exhibit 3D networks with the V atom disordered over several sites. The cyclic voltammetric behavior of SVM₁₁ in neutral and acidified acetonitrile solutions also has been investigated with respect to the V^V/V^IV couple. Results have been interpreted in conjunction with data provided by ⁵¹V NMR spectroscopy on the oxidized V^V form and by EPR spectroscopy on the reduced V^IV form. Based on mechanistic details inferred from these studies, simulations of the cyclic voltammograms have been undertaken and results compared with experimental data in acidic media (two protonated forms) in order to provide estimates of equilibrium and kinetic parameters. For the V^V/V^IV couple in the series [XVM₁₁O₄₀]ⁿ⁻ (X = Si,Ge,P,As,S; M = Mo,W), the reversible potentials in neutral acetonitrile linearly depend on the total charge of the vanadium-substituted polyoxometalates, similar to the dependence previously reported for the non-substituted parent Keggin polyoxometalates [XM₁₂O₄₀]^m−.