Excitation energy transfer processes play an important role in many areas of physics, chemistry and biology. The three-dimensional oxalate networks of composition [MIII(bpy)3][MIMIII(ox)3]ClO4 (bpy=2,2-bipyridine, ox=oxalate, MI=alkali ion) allow for a variety of combinations of different transition metal ions. The combination with chromium(III) on both the tris-bipyridine as well as the tris-oxalate site constitutes a model system in which it is possible to differentiate unambiguously between energy transfer from [Cr(ox)3]3– to [Cr(bpy)3]3+ due to dipole-dipole interaction on the one hand and exchange interaction on the other hand. Furthermore it is possible to just as unambiguously differentiate between the common temperature dependent phonon-assisted energy migration within the 2E state of [Cr(ox)3]3–, and a unique resonant process.

Efficient resonant energy transfer occurs within the R₁ line of the ⁴A₂ → ²E transition of the [Cr(ox)₃]^3- chromophore in mixed crystal [Rh(bpy)₃][NaAl_1-xCr_x(ox)₃]ClO₄ (x = 0.05−0.9, ox = oxalate, bpy = 2,2‘-bipyridine). This manifests itself in the form of multiline patterns in resonant fluorescence line narrowing (FLN) experiments at 1.5 K. The conditions for such a resonant process to occur are that the inhomogeneous line width of the R₁ line is larger than the zero-field splitting of the ground state, which, in turn, is larger than the homogeneous line width of the transition. The number of lines and their relative intensities depend critically upon the [Cr(ox)₃]^3- concentration and the excitation wavelength within the inhomogeneous distribution. The basic model for resonant energy transfer as presented by von Arx et al. (Phys. Rev B 1996, 54, 15800) is extended to include the effects of diluting the chromophores in an inert host lattice and of nonresonant R₂ excitation. In addition, Monte Carlo simulations serve to explain the temporal evolution of the multiline pattern following pulsed excitation.

Electronic energy transfer from [Cr(ox)₃]^3- (ox = oxalate) in three-dimensional (3D) anionic oxalate networks to encapsulated [Cr(bpy)₃]³⁺ (bpy = 2,2‘-bipyridine) cations at 1.5 K was investigated by time-resolved luminescence spectroscopy. Two series of mixed crystals of nominal compositions [NaAl_1-xCr_x(ox)₃][Rh_0.99Cr_0.01(bpy)₃]ClO₄ (x = 0, 0.01, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, and 1) and [NaAl_0.99Cr_0.01(ox)₃][Rh_1-yCr_y(bpy)₃]ClO₄ (y = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) were utilized. Energy transfer from [Cr(ox)₃]^3- to [Cr(bpy)₃]³⁺ occurs by two mechanisms. Rapid, short-range transfer (k_et > 10⁶ s^-1) is attributed to superexchange coupling between the Cr³⁺ ions via Ï€ overlap of the oxalate and bipyridine ligands. In addition, at low [Cr(ox)₃]^3- concentrations (nominally x = 0.01) a very much slower process with a maximum k_et ≈ 200 s^-1 is identified in the time-resolved spectra and attributed to a dipole−dipole mechanism. Furthermore, the resonant [Cr(ox)₃]^3- to [Cr(ox)₃]^3- energy migration previously reported by von Arx et al. (Phys. Rev. (1996), B54, 15800) assists [Cr(ox)₃]^3- to [Cr(bpy)₃]³⁺ transfer as the [Cr(ox)₃]^3- concentration increases.