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 P2(1)/c space group. All Ln(PMo11) 2 revealed four Mo-based electrochemical reduction processes with very similar E-1/2 values (approximate to 0.5, approximate to 0.3, approximate to 0.02 and approximate to-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 (epsilon) and absorbance (Abs) variation as a function of electrolysis time: bands A (lambda(max) approximate to 855-870 nm) and B (lambda(max) approximate to 670-695 nm) were assigned to Mo-V -> Mo-VI intervalence charge transfer transitions, band C (lambda(max) approximate to 525 nm) to d-d transition due to d(1) configuration of the reduced addenda atom (Mo-V) and band D (lambda approximate to 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 Eu3+ D-5(0) -> F-7(0-4) transitions, when excited through the O -> Eu-III and O -> Mo-VI CCTs.