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

Abstract

Sandwich-type phosphomolybdates with general formula Kn[LnIII(PMo11O39)2], where LnIII = 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 E1/2 values (≈0.5, ≈0.3, ≈0.02 and ≈−0.1 V) for all the LnIII 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 (e) and absorbance (Abs) variation as a function of electrolysis time: bands A (λmax ≈ 855–870 nm) and B (λmax ≈ 670–695 nm) were assigned to MoV → MoVI intervalence charge transfer transitions, band C (λmax ≈ 525 nm) to d–d transition due to d1 configuration of the reduced addenda atom (MoV) and band D (λ ≈ 310–315 nm) to an O → MoV 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 Eu3+ 5D0 → 7F0–4 transitions, when excited through the O → EuIII and O → MoVI CCTs.