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– (1), [Tb8(pic)6(H2O)22(B-β-AsW8O30)4(WO2(pic))6]12– (2), and [Eu8(pic)6(H2O)22(B-β-AsW8O30)4(WO2(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 H0.5K8.5Na­[1]·30H2O, K4Li4H4[2]·58H2O, and Eu1.66K7[3]·54H2O, 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 K4Li4H4[2]·58H2O and Eu1.66K7[3]·54H2O visibly luminesce green
and red, respectively, while compound H0.5K8.5Na­[1]·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.