Manipulating the properties of MLCT excited states

Systematic variation of the ligand environment has allowed design of the absorbance characteristics of polypyridyl complexes of ruthenium(II) to produce "black absorbers" which absorb throughout the visible region. The presence of acceptor ligands with low-lying pi* levels red shift the energies of the lowest energy MLCT bands, while MLCT and pi --> pi* bands originating on other ligands can be used to fill in the higher-energy regions of the spectrum. Incorporation of anionic ligands or other electron-donating ligands causes a red shift in MLCT band energies compared to bpy by manipulation of dpi energy levels. Attention to these design principles has led to the synthesis of complexes which absorb appreciably in the near IR, and are free from complications caused by thermally accessible dd states. Although their emission energies (and energy gaps) are at low energy in the near IR, the use of lowest lying, delocalised acceptor ligands provides lifetime enhancements (compared to bpy) that can be dramatic.