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.

The [Ru(bpy)₃][LiCr(ox)₃] system (bpy = 2,2‘-bipyridine, ox = oxalate) has two crystallographically non-equivalent [Cr(ox)₃]^3- sites. In steady-state resonant and nonresonant fluorescence line narrowing (FLN) experiments on the R₁ lines of the two non-equivalent [Cr(ox)₃]^3- chromophores, multiline spectra are observed at 1.6 K. Such multiline spectra are clear evidence for resonant energy transfer processes within the inhomogeneously broadened R₁ lines. In addition, time-resolved experiments show that also site-to-site energy transfer occurs, which turns out to be resonant, too, however with a non-negligible phonon-assisted contribution even at 1.5 K.

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.

Luminescence and energy transfer in [Zn_1-xRu_x(bpy)₃][NaAl_1-yCr_y(ox)₃] (x ≈ 0.01, y = 0.006 − 0.22; bpy = 2,2‘-bipyridine, ox = C₂O₄^2-) and [Zn_1-x-yRu_xOs_y(bpy)₃][NaAl(ox)₃] (x ≈ 0.01, y = 0.012) are presented and discussed. Surprisingly, the luminescence of the isolated luminophores [Ru(bpy)₃]²⁺ and [Os(bpy)₃]²⁺ in [Zn(bpy)₃][NaAl(ox)₃] is hardly quenched at room temperature. Steady-state luminescence spectra and decay curves show that energy transfer occurs between [Ru(bpy)₃]²⁺ and [Cr(ox)₃]^3- and between [Ru(bpy)₃]²⁺ and [Os(bpy)₃]²⁺ in [Zn_1-xRu_x(bpy)₃][NaAl_1-yCr_y(ox)₃] and [Zn_1-x-yRu_xOs_y(bpy)₃] [NaAl(ox)₃], respectively. For a quantitative investigation of the energy transfer, a shell type model is developed, using a Monte Carlo procedure and the structural parameters of the systems. A good description of the experimental data is obtained assuming electric dipole−electric dipole interaction between donors and acceptors, with a critical distance R_c for [Ru(bpy)₃]²⁺ to [Cr(ox)₃]^3- energy transfer of 15 Ã… and for [Ru(bpy)₃]²⁺ to [Os(bpy)₃]²⁺ energy transfer of 33 Ã…. These values are in good agreement with those derived using the Förster−Dexter theory.

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.

Based on a synthetic strategy, extended anionic, homo and bimetallic oxalato-bridged transition-metal compounds with two (2D) and three-dimensional (3D) connectivities can be synthesized and crystallized. Thereby, the choice of the templating counterions will determine the crystal chemistry. Since the oxalato bridge is a mediator for both antiferro and ferromagnetic interactions between similar and dissimilar metal ions, long-range magnetic ordering will occur. Examples of the determination of magnetic structures in 2D and 3D compounds by means of elastic neutron scattering methods will be discussed. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems.

In resonant fluorescence line narrowing (FLN) experiments in the R₁ transition of the [Cr(ox)₃]^3- chromophore in [Ru(bpy)₃][NaAl:Cr(1%)(ox)₃] and [Rh(bpy)₃][NaCr(ox)₃]ClO₄ multiline spectra are observed at 1.8 K, (ox=oxalate, bpy=2,2’-bipyridine). For [Rh(bpy)₃][NaCr(ox)₃]ClO₄ the number of lines and their relative intensities depend critically upon the excitation wavelength within the inhomogeneous distribution, and in time-resolved FLN experiments additionally upon the delay. This behavior is clear evidence for a resonant energy-transfer process. At 4.2 K the more common phonon-assisted process becomes dominant, manifesting itself as spectral diffusion.

Chemical variation and combination of metal ions of different valencies in the oxalate backbone as well as in the tris-bpy cation of the three-dimensional network structures of the type [M^II₂(ox)₃][M^II(bpy)₃] (bpy = 2,2'-bipyridine, ox = C₂O₄^2-), [M^IM^III(ox)₃][M^II(bpy)3] and [M^IM^III(ox)3][M^III(bpy)3]ClO₄ offer unique opportunities for studying a large variety of photophysical processes. Depending upon the relative energies of the excited states of the chromophores, excitation energy transfer either from the tris-bipyridine cation to the oxalate backbone or vice versa is observed, as for instance from [Ru(bpy)₃]²⁺ as photo-sensitiser to [Cr(ox)₃]^3- as energy acceptor in the combination [NaCr(ox)₃][Ru(bpy)₃], or from [Cr(ox)₃]^3- to [Cr(bpy)₃]³⁺ in [NaCr(ox)₃][Cr(bpy)₃]ClO₄. In addition efficient energy migration within the oxalate backbone is observed. Furthermore, depending upon the excited state redox potentials, light-induced electron transfer processes may be envisaged.