Structural Study of Mixed Crystals of [Zn1−xRux(bpy)3][NaCr(ox)3] Probed by High-Resolution Absorption Spectroscopy and High-Pressure Experiments

Abstract

In the mixed crystal series of the cubic three-dimensional networks of composition [Zn(1-x)Ru(x)(bpy)(3)][NaCr(ox)(3)] (0 <or= x <or=1, ox = C(2)O(4)(2-), bpy = 2,2'-bipyridine), high-resolution absorption spectroscopy in the region of the (4)A(2)-->(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.