Lanthanide-mediated supramolecular cages and host-guest interactions.

The structure and thermodynamic properties of lanthanide complexes with a new tripodal ligand L2 have been elucidated using different physicochemical methods. At stoichiometric ratios, the tetrahedral three-dimensional complexes with lanthanide cations are formed in acetonitrile with good stabilities. Despite minor structural changes comparing to previously investigated tripodal ligands, the resulting assembly exhibits different features revealed with the crystal structure of [Eu(4)L2(4)](OH)(ClO(4))(11) (orthorhombic, Pbcn). Interestingly, the highly charged edifice contains an inner cage encapsulating a perchlorate anion. Such lanthanide mediated cage-like assemblies are rare, and may be of interest for different sensing applications. Indeed, the anionic guest can be exchanged with different anions. The related host-guest equilibria were investigated with NMR techniques. Various aspects of these reactions are qualitatively discussed.

Effect of external pressure on the excitation energy transfer from [Cr(ox)3]3- to [Cr(bpy)3]3+ in [Rh(1-x)Cr(x)(bpy)3][NaM(1-y)Cr(y)(ox)3]ClO4.

Resonant excitation energy transfer from [Cr(ox)(3)](3-) to [Cr(bpy)(3)](3+) in the doped 3D oxalate networks [Rh(1-x)Cr(x)(bpy)(3)][NaM(III) (1-y)Cr(y)(ox)(3)]ClO(4) (ox=C(2)O(4) (-), bpy=2,2'-bipyridine, M=Al, Rh) is due to two types of interaction, namely super exchange coupling and electric dipole-dipole interaction. The energy transfer probability for both mechanisms is proportional to the spectral overlap of the (2)E→(4)A(2) emission of the [Cr(ox)(3)](3-) donor and the (4)A(2)→(2)T(1) absorption of the [Cr(bpy)(3)](3+) acceptor. The spin-flip transitions of (pseudo-)octahedral Cr(3+) are known to shift to lower energy with increasing pressure. Because the shift rates of the two transitions in question differ, the spectral overlap between the donor emission and the acceptor absorption is a function of applied pressure. For [Rh(1-x)Cr(x)(bpy)(3)][NaM(1-y)Cr(y)(ox)(3)]ClO(4) the spectral overlap is thus substantially reduced on increasing pressure from 0 to 2.5 GPa. As a result, the energy transfer probability decreases with increasing pressure as evidenced by a decrease in the relative emission intensity from the [Cr(bpy)(3)](3+) acceptor.

Structural study of mixed crystals of [Zn(1-x)Ru(x)(bpy)(3)][NaCr(ox)(3)] probed by high-resolution absorption spectroscopy and high-pressure experiments.

In the mixed crystal series of the cubic three-dimensional networks of composition [Zn(1-x)Ru(x)(bpy)(3)][NaCr(ox)(3)] (0 (2)E transition (R-lines) reveals the creation of five specific spectroscopic sites for the [Cr(ox)(3)](3-) complex. The concentration of these spectroscopic sites follows a binomial distribution of [Zn(bpy)(3)](2+) and [Ru(bpy)(3)](2+) among the four nearest neighbors of a given [Cr(ox)(3)](3-) complex within the network. The tris-bipyridine complexes occupying those positions have an optimal pi-pi interaction with the oxalate ligands of the tris-oxalate chromophore. The energy of each spectroscopic [Cr(ox)(3)](3-) site depends on the total concentration of [Ru(bpy)(3)](2+) in the mixed crystal and on its specific distribution among the four nearest neighbors. Single crystal X-ray diffraction indicates a reduction of the unit cell volume when [Zn(bpy)(3)](2+) (a = 15.6365(18) A) is substituted by [Ru(bpy)(3)](2+) (a = 15.5098(6) A). This alone would lead to a red-shift of the R lines in analogy to the red-shift of 25.2 cm(-1)/GPa due to the decrease of the metal ligand Cr-O bond length as observed in high-pressure luminescence experiments. However, specific pi-pi interactions with the nearest neighbors have the opposite effect and shift the transition in discrete jumps to higher energies with increasing [Ru(bpy)(3)](2+) mole fraction.