Sensitization of lanthanoid luminescence by organic and inorganic ligands in lanthanoid-organic-polyoxometalates

Abstract

The reaction of terbium and europium salts with the lacunary polyxometalate (POM) [As 2W 19O 67(H 2O)] 14- and 2-picolinic acid (picH) affords the ternary lanthanoid-organic-polyoxometalate (Ln-org-POM) complexes [Tb 2(pic)(H 2O) 2(B-β-AsW 8O 30) 2(WO 2(pic)) 3] 10- (1), [Tb 8(pic) 6(H 2O) 22(B-β-AsW 8O 30) 4(WO 2(pic)) 6] 12- (2), and [Eu 8(pic) 6(H 2O) 22(B-β-AsW 8O 30) 4(WO 2(pic)) 6] 12- (3). 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[1] ·30H 2O, K 4Li 4H 4[2] ·58H 2O, and Eu 1.66K 7[3] ·54H 2O, each of which has been characterized by single crystal X-ray diffraction. Complexes 2 and 3 are isostructural and can be considered to be composed of two molecules of 1 linked through an inversion center with four additional picolinate-chelated lanthanoid centers. When irradiated with a laboratory UV lamp at room temperature, compounds K 4Li 4H 4[2]·58H 2O and Eu 1.66K 7[3] ·54H 2O visibly luminesce green and red, respectively, while compound H 0.5K 8.5Na[1] ·30H 2O 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. © 2011 American Chemical Society.