Excitation energy migration is an important phenomenon at high concentration of luminescent chromophores. In crystalline solids it results in a quenching of the intrinsic luminescence of the chromophore as the excitation energy migrates to impurity centres or other forms of trap sites. As concluded from the extensively studied systems where Cr³⁺ is doped as the active chromophore into inert host lattices, energy migration in crystalline solids is usually a phonon-assisted process, in which the simultaneous creation or annihilation of phonons helps to bridge the energy miss-match in the energy levels of two neighbouring chromophores within a inhomogeneously broadened absorption band. However, in the three-dimensional network systems [Ru(bpy)₃][NaCr(ox)₃] and [Rh(bpy)₃][NaCr(ox)₃]ClO₄, it proved possible to unambiguously identify three different mechanisms for energy migration within the R₁ line of the ⁴A₂ → ²E transition of Cr³⁺. In addition to the common temperature dependant phonon-assisted process, a resonant process between the zero-field split components of the ⁴A₂ ground state leading to a multi-line pattern in a fluorescence line narrowing spectrum and a quasi-resonant process within the same component leading to fast spectral diffusion can be identified at very low temperature. The parameters governing these processes are discussed and the behaviour of the model systems is compared to more conventional doped oxides and related systems.

The optical properties of a thin film of the [Ru(bpy)₃][NaCr(ox)₃] network structure obtained by pulsed laser deposition are described. The luminescence shows the characteristic doublet of R lines at 14 400 cm^−1 of the spin-forbidden ligand field transition ^2E(t2g3)→^4A₂(t2g3) of the [Cr(ox)₃]^3− chromophore. The resonant energy migration within the R₁ line shows that the three-dimensional crystallographic structure is preserved during the coating process. The observation of the R lines of [Cr(bpy)₃]³⁺ at 13 710 cm^−1 indicates that a small fraction of Cr³⁺ ions migrate from the oxalate network to the tris-bipyridine cation site in the cavities of the network.