Redox behaviour, electrochromic properties and photoluminescence of potassium lanthano phosphomolybdate sandwich-type compounds

Abstract

Sandwich-type phosphomolybdates with general formula K_n[Ln^III(PMo₁₁O₃₉)₂], where Ln^III = Sm, Eu, Gd, Tb and Dy, were prepared and characterized by several techniques. The crystal structure of Sm(PMo₁₁)₂ and Gd(PMo₁₁)₂ were studied and showed that they crystallise in a P2₁/c space group. All Ln(PMo₁₁)₂ revealed four Mo-based electrochemical reduction processes with very similar E_1/2 values (≈0.5, ≈0.3, ≈0.02 and ≈−0.1 V) for all the Ln^III atoms. The two more positive reduction processes correspond to pH independent one-electron reduction processes, whereas the two more negative processes correspond to pH dependent two-electron reduction processes. Electrolysis at two different potentials, 0.1 V – related to the two one-electron reduction processes – and −0.3 V – related to the two more negative two-electron reduction processes, confirmed the electrochromic properties of the Ln(PMo₁₁)₂ species: their original yellow coloured solutions turned blue, corresponding to the appearance of four new electronic bands in the near UV-Vis-near IR region. These electronic bands were tentatively assigned based on their molar absorption coefficients (ε) and absorbance (Abs) variation as a function of electrolysis time: bands A (λ_max ≈ 855–870 nm) and B (λ_max ≈ 670–695 nm) were assigned to Mo^V → Mo^VI intervalence charge transfer transitions, band C (λ_max ≈ 525 nm) to d–d transition due to d¹ configuration of the reduced addenda atom (Mo^V) and band D (λ ≈ 310–315 nm) to an O → Mo^V charge transfer transition (CTT). The emission features of the Eu(PMo₁₁)₂ and Tb(PMo₁₁)₂ samples reveal broad emission in the UV/vis spectral region resulting from d–d transition transitions. The Eu(PMo₁₁)₂ also display the typical Eu^{3+ 5}D₀ → ⁷F_0–4 transitions, when excited through the O → Eu^III and O → Mo^VI CCTs.