Probing the excited state nature of coordination complexes with blended organic and inorganic chromophores using vibrational spectroscopy

The use of transient vibrational spectroscopy in the analysis of rhenium(I) and ruthenium(II) complexes is discussed. Particular focus is given to the use of resonance Raman spectroscopy to probe initial photoexcitation and transient resonance Raman and infrared spectroscopy to observe subsequent relaxation processes. Several types of excited states are accessible for these systems, which can be probed on a range of timescales by these techniques. These include ligand-centered π,π, intraligand chargetransfer, and metal-to-ligand charge transfer; a number of these states may be overlapping and show mixing. As such, these techniques are described in some detail and their utility is given by discussion of examples in which the electronic complexity of the system increases from systems which are metal-to-ligand charge-transfer through to systems which have complex interplay between intraligand and metal-to-ligand charge transfer states. Particular attention is paid to complexes containing [Re(CO)₃(L)(N^N)] metal centers and dipyrido[3,2-a:2′,3′-c]phenazine ligands, as subtle structural changes on these often manifest in significant changes in the photophysical properties and they are therefore well-suited to the investigation of excited states. The use of these complexes in areas such as solar energy conversion and the probe of biological systems are also discussed.