New lanthanide complexes of 3-hydroxypicolinic acid (HpicOH) were prepared: [Ln(H 2 O)-(picOH) 2 (µ-HpicO)]‚3H 2 O (Ln ) Eu, Tb, Er). The complexes were characterized using photoluminescence, infrared, Raman, and 1 H NMR spectroscopy, and elemental analysis. The crystal structure of [Eu(H 2 O)(picOH) 2 (µ-HpicO)]‚3H 2 O 1 was determined by X-ray diffraction. Compound 1 crystallizes in a monoclinic system with space group P2 1 /c and cell parameters a ) 9.105(13) Å, b ) 18.796(25) Å, and c ) 13.531(17) Å, and ) 104.86(1) deg. The 3-hydroxypicolinate ligands coordinate through both N,O-or O,O-chelation to the lanthanide ions, as shown by X-ray and spectroscopic results. Photoluminescence measurements were performed for the Eu(III) and Tb(III) complexes; the Eu(III) complex was investigated in more detail. The Eu(III) compound is highly luminescent and acts as a photoactive center in nanocomposite materials whose host matrixes are silica nanoparticles.

The hydrothermal reaction between lanthanide nitrates and 2,3-pyrazinedicarboxylic acid led to a new series of two-dimensional (2D) lanthanide-organic frameworks: [Ln(2)(2,3-pzdc)(2)(ox)(H(2)O)(2)](n) [where 2,3-pzdc(2-) = 2,3-pyrazinedicarboxylate, ox(2-) = oxalate, and Ln(III) = Ce, Nd, Sm, Eu, Gd, Tb, or Er]. The structural details of these materials were determined by single-crystal X-ray diffraction (for Ce(3+) and Nd(3+)) that revealed the formation of a layered structure. Cationic monolayers of {(infinity)(2)[Ln(2,3-pzdc)(H(2)O)](+)} are interconnected via the ox(2-) ligand leading to the formation of neutral (infinity)(2)[Ln(2)(2,3-pzdc)(2)(ox)(H(2)O)(2)] bilayer networks; structural cohesion of the crystalline packing is reinforced by the presence of highly directional O-H...O hydrogen bonds between adjacent bilayers. Under the employed hydrothermal conditions 2,3-pyrazinedicarboxylic acid can be decomposed into ox(2-) and 2-pyrazinecarboxylate (2-pzc(-)), as unequivocally proved by the isolation of the discrete complex [Tb(2)(2-pzc)(4)(ox)(H(2)O)(6)].10H(2)O. Single-crystal X-ray diffraction of this latter complex revealed its co-crystallization with an unprecedented (H(2)O)(16) water cluster. Photoluminescence measurements were performed for the Nd(3+), Sm(3+), Eu(3+), and Tb(3+) compounds which show, under UV excitation at room temperature, the Ln(3+) characteristic intra-4f(N) emission peaks. The energy level of the triplet states of 2,3-pyrazinedicarboxylic acid (18939 cm(-1)) and oxalic acid (24570 cm(-1)) was determined from the 12 K emission spectrum of the Gd(3+) compound. The (5)D(0) and (5)D(4) lifetime values (0.333 +/- 0.006 and 0.577 +/- 0.017 ms) and the absolute emission quantum yields (0.13 +/- 0.01 and 0.05 +/- 0.01) were determined for the Eu(3+) and Tb(3+) compounds, respectively. For the Eu(3+) compound the energy transfer efficiency arising from the ligands' excited states was estimated (0.93 +/- 0.01).

Novel three-dimensional lanthanide-organic frameworks with 2,5-pyridinedicarboxylic (2,5-H 2 pdc) and 1,4phenylenediacetic acids (1,4-H 2 pda) were synthesized by hydrothermal synthesis, and characterized structurally using single-crystal and powder X-ray diffraction, elemental and thermogravimetric analyses, FT-IR and photoluminescence spectroscopies, and diffuse reflectance. The structural details of [Ln 2 (2,5-pdc) 2 (1,4-pda)(H 2 O) 2 ] [where Ln 3+ ) Eu 3+ , Tb 3+ , (Eu 0.2 Tb 0.8 ) 3+ and (Eu 0.1 Tb 0.9 ) 3+ ] and the dehydrated [Eu 2 (2,5-pdc) 2 (1,4-pda)] phase were unveiled by single-crystal X-ray diffraction. Photoluminescence measurements performed for the Eu 3+ and Tb 3+ compounds show emission at room temperature. Energy transfer from Tb 3+ to Eu 3+ has been observed for the Eu 3+ /Tb 3+ mixed materials.

Photoluminescent lanthanopolyoxotungstate core/shell nanoparticles are prepared by the encapsulation of lanthanide-containing polyoxometalates (POMs) with amorphous silica shells. The preparation of morphological well-defined core/shell nanoparticles is achieved by the hydrolysis of tetraethoxysilane in the presence of POMs using a reverse microemulsion method. The POMs used are decatungstolanthanoates of [Ln(W 5 9À type (Ln(III) ¼ Eu, Gd and Tb). Photoluminescence studies show that there is efficient emission from the POM located inside the SiO 2 shells, through excitation paths that involve O / Eu/Tb and O / W ligand-to-metal charge transfer. It is also shown that the excitation of the POM containing europium(III) may be tuned towards longer wavelengths via an antenna effect, by coordination of an organic ligand such as 3-hydroxypicolinate. The POM/SiO 2 nanoparticles form stable suspensions in aqueous solution having the advantage of POM stabilization inside the core and the possibility of further surface grafting of chemical moieties via well known derivatization procedures for silica surfaces. These features together with the possibility of tuning the excitation wavelength by modifying the coordination sphere in the lanthanopolyoxometalate, make this strategy promising to develop a new class of optical bio-tags composed of silica nanobeads with multi-wavelength photoluminescent lanthanopolyoxometalate cores.