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 <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) complex. The concentration of these spectroscopic sites follows a binomial distribution of Zn(bpy)(3) and Ru(bpy)(3) among the four nearest neighbors of a given Cr(ox)(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) site depends on the total concentration of Ru(bpy)(3) 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) (a = 15.6365(18) A) is substituted by Ru(bpy)(3) (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) mole fraction.