The role of ligand-field states for the photophysical properties of d⁶ systems has been discussed in a large number of publications over the past decades. Since the seminal paper by Houten and Watts, for instance, the quenching of the ³MLCT luminescence in ruthenium(II) polypyridyl complexes is attributed to the presence of the first excited ligand-field state, namely a component of the ³T₁(t_2g⁵e_g¹) state, at similar energies. If this state lies above the ³MLCT state, the luminescence is quenched via thermal population at elevated temperatures only. If it lies well below, then the luminescence is quenched down to cryogenic temperatures. In this contribution we present transient absorption spectra on non-luminescent ruthenium polypyridyl complexes such as [Ru(m-bpy)₃]²⁺, m-bpy = 6-methyl-2,2’-bipyridine, in acetonitrile at room temperature, which reveal an ultra-rapid depopulation of the ³MLCT state but a much slower ground state recovery. We propose that in this and related complexes the methyl groups force longer metal-ligand bond lengths, thus resulting in a lowering of the ligand-field strength such that the ³dd state drops to below the ³MLCT state, and that furthermore the population of this state from the ³MLCT state occurs faster than its decay to the ground state. In addition we demonstrate that in this complex the luminescence can be switched on by external pressure, which we attribute to a destabilisation of the ligand-field state by the pressure due to its larger molecular volume compared to the ground state as well as the ³MLCT state.

The ground-state electronic structures of K₃V(ox)₃·3H₂O, Na₃V(ox)₃·5H₂O, and NaMgAl_1–xV_x(ox)₃·9H₂O (0 < x <= 1, ox = C₂O₄^2–) have been studied by Fourier–transform electronic absorption and inelastic neutron scattering spectroscopies. High-resolution absorption spectra of the ³Î“(t_2g²) → ¹Î“(t_2g²) spin-forbidden electronic origins and inelastic neutron scattering measurements of the pseudo-octahedral [V(ox)₃]^3– complex anion below 30 K exhibit both axial and rhombic components to the zero-field-splittings (ZFSs). Analysis of the ground-state ZFS using the conventional S = 1 spin Hamiltonian reveals that the axial ZFS component changes sign from positive values for K₃V(ox)₃·3H₂O (D ≈ +5.3 cm^–1) and Na₃V(ox)₃·5H₂O (D ≈ +7.2 cm^–1) to negative values for NaMgAl_1–xV_x(ox)₃·9H₂O (D ≈ –9.8 cm^–1 for x = 0.013, and D ≈ –12.7 cm^–1 for x = 1) with an additional rhombic component, |E|, that varies between 0.8 and 2 cm^–1. On the basis of existing crystallographic data, this phenomenon can be identified as due to variations in the axial and rhombic ligand fields resulting from outer-sphere H-bonding between crystalline water molecules and the oxalate ligands. Spectroscopic evidence of a crystallographic phase change is also observed for K₃V(ox)₃·3Y₂O (Y = H or D) with three distinct lattice sites below 30 K, each with a unique ground-state electronic structure.

The spin transition of the [Co(terpy)₂]²⁺ complex (terpy = 2,2′:6′,2″-terpyridine) is analysed based on experimental data from optical spectroscopy and magnetic susceptibility measurements. The single crystal absorption spectrum of [Co(terpy)₂](ClO₄)₂ shows an asymmetric absorption band at 14 400 cm^−1 with an intensity typical for a spin-allowed d–d transition and a temperature behaviour typical for a thermal spin transition. The single crystal absorption spectra of suggest that in this compound, the complex is essentially in the high-spin state at all temperatures. However, the increase in intensity observed in the region of the low-spin MLCT transition with increasing temperature implies an unusual partial thermal population of the low-spin state of up to about 10% at room temperature. Finally, high-spin → low-spin relaxation curves following pulsed laser excitation for [Co(terpy)₂](ClO₄)₂ dispersed in KBr discs, and as a comparison for the closely related [Co(4-terpyridone)₂](ClO₄)₂ spin-crossover compound are given.

Whereas there are hundreds of known iron(II) spin-crossover compounds, only a handful of cobalt(II) spin-crossover compounds have been discovered to date, and hardly an in depth study on any of them exists. This review begins with an introduction into the theoretical aspects to be considered when discussing spin-crossover compounds in general and cobalt(II) systems in particular. It is followed by case studies on [Co(bpy)₃]²⁺ and [Co(terpy)₂]²⁺ (bpy = 2,2′-bipyridine, terpy = 2,2′:6′,2″-terpyridine) presenting and discussing results from magnetic susceptibility measurements, X-ray crystallography, optical spectroscopy, and EPR spectroscopy.

The high-spin → low-spin relaxation in spin-crossover compounds can be described as non-adiabatic multi-phonon process in the strong coupling limit, in which the low-temperature tunnelling rate increases exponentially with the zero-point energy difference between the two states. Based on the hypothesis that the experimental bond length difference between the high-spin and the low-spin state of ~0.2 Å is also valid for low-spin iron(II) complexes, extrapolation of the single configurational coordinate model allows an estimate of the zero-point energy difference for low-spin complexes from kinetic data. DFT calculations on low-spin [Fe(bpy)₃]²⁺ support the structural assumption. However, for low-spin [Fe(terpy)₂]²⁺ the relaxation rate constant shows an anomalous behaviour in so far as it is more in line with spin-crossover systems. This is attributed to very anisotropic bond length changes associated with the spin state change, and the subsequent breakdown of the single mode model.

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.

The physical and photophysical properties of three classic transition metal complexes, namely [Fe(bpy)₃]²⁺, [Ru(bpy)₃]²⁺, and [Co(bpy)₃]²⁺, can be tuned by doping them into a variety of inert crystalline host lattices. The underlying guest-host interactions are discussed in terms of a chemical pressure.