Tetra-Ln3+-Implanted Tellurotungstates Covalently Modified by dl-Malic Acid: Proton Conduction and Photochromic Properties

Three unique dl-malic acid covalently modified tetra-Ln³⁺-implanted tellurotungstates [H₂(CH₃)₂]₉NaH₉[Ln₄(H₂O)₁₄W₆O₁₃(OH)₅(Mal)₂(B-α-TeW₉O₃₃)₄]·48H₂O [Ln = La³⁺ (1), Ce³⁺ (2), Pr³⁺ (3); H₃Mal = dl-malic acid] were fabricated by reacting Na₂TeO₃, Na₂WO₄·2H₂O, Mal, and LnCl₃·6H₂O with dimethylamine hydrochloride in an aqueous solution. The most prominent architectural feature of these compounds is the covalent connection mode of an organic ligand and a polyoxometallate backbone, which is relatively rare in the realm of polyoxotungstates. The tetrameric polyanion can be deemed as four [TeW₉O₃₃]^8- fragments fused together via an intriguing hexanuclearity [W₆O₁₃(OH)₅(Mal)₂Ln₄(H₂O)₁₄]¹³⁺ cluster. Impedance measurements manifest that all three complexes display splendid proton conduction properties, with an exceptional conductivity for 2 up to 2.48 × 10^-2 S·cm^-1 under 85 °C and 95% relative humidity. Moreover, compounds 1 and 3 exhibited fast reversible photochromic properties with allochroic half-life periods t_1/2 of 1.046 and 0.544 min, respectively.