Structural Study of Mixed Crystals of [Zn_1−<i>x</i>Ru_<i>x</i>(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 [Zn1−xRux(bpy)3][NaCr(ox)3] (0 ≤ x ≤1, ox = C2O42−, bpy = 2,2′-bipyridine), high-resolution absorption spectroscopy in the region of the 4A2→2E 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 π−π 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) Å) is substituted by [Ru(bpy)3]2+ (a = 15.5098(6) Å). 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 π−π 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.