Sensitization of Lanthanoid Luminescence by Organic and Inorganic Ligands in Lanthanoid-Organic-Polyoxometalates

The reaction of terbium and europium salts with the lacunary polyxometalate (POM) [As₂W₁₉O₆₇(H₂O)]^14– and 2-picolinic acid (picH) affords the ternary lanthanoid-organic-polyoxometalate (Ln-org-POM) complexes [Tb₂(pic)­(H₂O)₂(B-β-AsW₈O₃₀)₂(WO₂(pic))₃]^10– (<b>1</b>), [Tb₈(pic)₆(H₂O)₂₂(B-β-AsW₈O₃₀)₄(WO₂(pic))₆]^12– (<b>2</b>), and [Eu₈(pic)₆(H₂O)₂₂(B-β-AsW₈O₃₀)₄(WO₂(pic))₆]^12– (<b>3</b>). A detailed synthetic investigation has established the conditions required to isolate pure bulk samples of the three complexes as the mixed salts H_0.5K_8.5Na­[<b>1</b>]·30H₂O, K₄Li₄H₄[<b>2</b>]·58H₂O, and Eu_1.66K₇[<b>3</b>]<b>·</b>54H₂O, each of which has been characterized by single crystal X-ray diffraction. Complexes <b>2</b> and <b>3</b> are isostructural and can be considered to be composed of two molecules of <b>1</b> linked through an inversion center with four additional picolinate-chelated lanthanoid centers. When irradiated with a laboratory UV lamp at room temperature, compounds K₄Li₄H₄[<b>2</b>]<b>·</b>58H₂O and Eu_1.66K₇[<b>3</b>]<b>·</b>54H₂O visibly luminesce green and red, respectively, while compound H_0.5K_8.5Na­[<b>1</b>]·30H₂O is not luminescent. A variable temperature photophysical investigation of the three compounds has revealed that both the organic picolinate ligands and the inorganic POM ligands sensitize the lanthanoid­(III) luminescence, following excitation with UV light. However, considerably different temperature dependencies are observed for Tb­(III) versus Eu­(III) through the two distinct sensitization pathways