Joseph A. Treadway

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 π* levels red shift the energies of the lowest energy MLCT bands, while MLCT and π → π* 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 dπ 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.

Step-scan Fourier transform infrared absorption difference time-resolved spectroscopy studies of excited state decay kinetics and electronic structure of low-spin d6 transition metal polypyridine complexes with 10 nanosecond time resolution

Step-scan Fourier transform absorption difference time-resolved spectroscopy (S2FTIR ∆A TRS) has been used to collect mid-IR time-resolved infrared spectra of the transient electronic excited states of polypyridine transition metal complexes with 10 ns time resolution. The time-resolved data can be used for kinetic analysis or to generate “snapshots” of the lowest lying excited state. Shifts of vibrational bands in the excited state relative to the ground state can be used to infer significant details of the electronic structure of the excited state. The multiplex advantage of the FTIR technique allows a wide variety of vibrational bands to be analyzed for this purpose. In the example illustrated, the shift of the ester ν(CO) band in {Ru(bpy)[4,4′-(COOEt)2bpy]2}2