Resonant excitation energy transfer from [Cr(ox)3]^3- to [Cr(bpy)3]³⁺ in the doped 3D oxalate networks [Rh_1-xCr_x(bpy)₃][NaM^III_1-yCr_y(ox)₃]ClO₄ (ox=C₂O₄^-, 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 ²E→⁴A₂ emission of the [Cr(ox)₃]^3- donor and the ⁴A₂→²T₁ absorption of the [Cr(bpy)₃]³⁺ acceptor.The spin-flip transitions of (pseudo-)octahedral Cr³⁺ 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-xCr_x(bpy)₃][Na-M_1-yCr_y(ox)₃]ClO₄ 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.

In the mixed crystal series of the cubic three-dimensional networks of composition [Zn_1−xRu_x(bpy)₃][NaCr(ox)₃] (0 ≤ x ≤1, ox = C₂O₄^2−, 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) Ã…) is substituted by [Ru(bpy)₃]²⁺ (a = 15.5098(6) Ã…). 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.