This Perspective reviews the impact of exciton coupling on the spectroscopic properties of coordination compounds. Exciton coupling features arise in electronic absorption and circular dichroism spectra when chromophores are brought into close spatial proximity, for example by coordination to a metal centre. The analysis of these features can reveal much information such as the geometry of a complex and its absolute configuration. The extension of the exciton coupling model to polynuclear metallosupramolecular arrays is discussed.

Ground-state structures and vibrational frequencies are calculated for complexes of the nitrate anion with one and two water molecules at the ab initio Hartree–Fock level with a basis set including diffuse and polarization functions. Two local minimum geometries are found for each complex. Calculations of the electronically excited states at the CIS level are then used to find the forces on each of the atoms upon vertical excitation to the two lowest-lying (near-degenerate) strongly allowed electronic transitions. These forces are converted to gradients of the excited-state potential surfaces along the ground-state normal modes and compared with the parameters obtained previously from empirical simulations of the experimental resonance Raman intensities of NO3− in dilute aqueous solution. The calculations on two-water clusters agree well with the experimental excited-state geometry changes along the totally symmetric N–O stretch. The calculations underestimate the frequency splitting of the antisymmetric stretching vibration (degenerate in the isolated D3h ion) and the resonance Raman intensity in this mode, suggesting that bulk solvent polarization enhances the asymmetry of the local environment for NO3− in water. Comparison of the ground-state vibrational frequency splitting of the antisymmetric stretch with the corresponding values for the nitrate ion in salts having known crystal structures suggests that the rms difference among the three N–O bond lengths for nitrate anion in water probably exceeds 0.01 Å.

Resonance Raman spectra are presented for the nitrate anion, ${\mathrm{NO}}_{\mathrm{3}}^{\mathrm{-}},$ in water, ethylene glycol, methanol, and acetonitrile solution at six excitation wavelengths from 246 to 204 nm, on resonance with the lowest ${\mathrm{\pi \to \pi }}^{*}$ excitation. Absolute Raman cross sections for the CH stretches of ethylene glycol and methanol at these wavelengths are also reported. The nitrate spectra in all four solvents are dominated by fundamentals, overtones, and combination bands of the totally symmetric NO stretch ${\mathrm{(\nu }}_1)$ near 1043 cm⁻¹ and the out-of-phase NO stretches ${\mathrm{(\nu }}_3)$ at 1340–1400 cm⁻¹, consistent with substantial changes in NO bond length upon π-electron excitation. The intensity in ${\nu }_3$ and the ≈60 cm⁻¹ splitting of this nominally degenerate vibration are indicative of pronounced breaking of the isolated molecules $D_{\text{3h}}$ symmetry by the local solvent environment. Intensity in the overtone of the out-of-plane mode ${\mathrm{(\nu }}_2)$ near 830 cm⁻¹ suggests a change in the equilibrium geometry from planar to pyramidal upon electronic excitation. The absorption spectra and absolute Raman cross sections are simulated with a model that considers resonance with two orthogonally polarized electronic states whose degeneracy is broken by the locally asymmetric environment. Both solvent reorganization and geometry changes along the nitrate molecular vibrations make major contributions to the breadth of the absorption band. No differences between resonant and nonresonant linewidths are observed for the ${\nu }_1$ band.

The synthesis, characterisation, and TiO₂ binding studies of a series of chromophoric complexes of 5-(4-carboxyphenyl)-4,6-dipyrrin (L^b) are presented. The synthesis of [Ru(bipy)(L^b)₂] (bipy = 2,2′-bipyridine), [Rh(L^b)₃], and [Pd(L^b)₂] was achieved by initial coordination of 5-(4-methoxycarbonylphenyl)-4,6-dipyrrin (L^a) followed by hydrolysis of the ester group. The carboxyl groups that are located on the peripheries of these complexes are able to engage in intermolecular hydrogen bonding interactions in the solid state, as revealed by X-ray crystallography. These groups also allow the complexes to anchor to the surface of TiO₂ nanoparticles, as evidenced by colouration of the TiO₂ and FT-IR spectroscopy. The ability of these complexes to capture a significant fraction of sunlight and to adhere to TiO₂ surfaces renders them viable dyes for photochemical devices such as dye sensitised solar cells.