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.

Polymeric two- and three-dimensional, homo- and heterometallic oxalatebridged coordination compounds offer exciting opportunities, mainly in the fields of molecular magnetism and photophysics. Given that a large variety of magnetic phenomena have been reported so far from these molecular magnets, very limited experience is gained from elastic neutron scattering experiments. Therefore, with two examples, we will address the topic of the elucidation of magnetic structures by means of the neutron scattering technique. 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.

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.

Resonant fluorescence line narrowing of the R₁ line of the [Cr(ox)₃]^3− chromophore in [Rh(bpy)₃][NaCr(ox)₃]ClO₄ at 1.6 K neither gives rise to the usual three-line pattern nor to spectral diffusion. Instead multi-line spectra with spacings equal to the zero-field splitting of the ground state are observed. This phenomenon is attributed to efficient non-radiative resonant energy transfer within the R₁ line.

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.

In analogy to the [M^II(bpy)₃]²⁺ cations, where M^II is a divalent transition-metal and bpy is 2,2‘-bipyridine, the tris-chelated [M^III(bpy)₃]³⁺ cations, where M^III is Cr^III or Co^III, induce the crystallization of chiral, anionic three-dimensional (3D) coordination polymers of oxalate-bridged (μ-ox) metal complexes with stoichiometries [M^II₂(ox)₃]_n^2n- or [M^IM^III(ox)₃]_n^2n-. The tripositive charge is partially compensated by inclusion of additional complex anions like ClO₄^-, BF₄^-, or PF₆^- which are encapsulated in cubic shaped cavities formed by the bipyridine ligands of the cations. Thus, an elaborate structure of cationic and anionic species within a polymeric anionic network is realized. The compounds isolated and structurally characterized include [Cr^III(bpy)₃][ClO₄] [NaCr^III(ox)₃] (1), [Cr^III(bpy)₃][ClO₄][Mn^II₂(ox)₃] (2), [Cr^III(bpy)₃][BF₄] [Mn^II₂(ox)₃] (3), [Co^III(bpy)₃][PF₆][NaCr^III(ox)₃] (4). Crystal data:  1, cubic, P2₁3, a = 15.523(4) Ã…, Z = 4; 2, cubic, P4₁32, a = 15.564(3) Ã…, Z = 4; 3, cubic, P4₁32, a = 15.553(3) Ã…, Z = 4; 4, cubic, P2₁3, a= 15.515(3) Ã…, Z = 4. Furthermore, it seemed likely that 1,2-dithiooxalate (dto) could act as an alternative to the oxalate bridging ligand, and as a result the compound [Ni^II(phen)₃][NaCo^III(dto)₃]·C₃H₆O (5) has successfully been isolated and structurally characterized. Crystal data:  5, orthorhombic, P2₁2₁2₁, a = 16.238(4) Ã…, b = 16.225(4) Ã…, c = 18.371(5) Ã…, Z = 4. In addition, the photophysical properties of compound 1 have been investigated in detail. In single crystal absorption spectra of [Cr^III(bpy)₃][ClO₄][NaCr^III(ox)₃] (1), the spin−flip transitions of both the [Cr(bpy)₃]³⁺ and the [Cr(ox)₃]^3- chromophores are observed and can be clearly distinguished. Irradiating into the spin-allowed ⁴A₂ → ⁴T₂absorption band of [Cr(ox)₃]^3- results in intense luminescence from the ²E state of [Cr(bpy)₃]³⁺as a result of rapid energy transfer processes.