Transient resonance Raman investigation of excited states of [Ru(phen)2dppz]2+ and deuterated analogues in aqueous and non-aqueous environments

Transient resonance Raman (TR2) spectroscopy was used to investigate the complex [Ru(phen)2dppz]2+ (phen = 1,10‐phenanthroline; dppz = dipyridophenazine) following nanosecond pulsed laser excitation in three types of media: aqueous solution, DNA and acetonitrile. Studies were carried out at several excitation wavelengths spanning the absorption range of the metal–ligand charge‐transfer (MLCT) excited state populated by the pulsed laser. In addition to the complex with the natural abundance dppz ligand, studies were also conducted with the complex containing the dppz ligand isotopically substituted at the phenanthroline hydrogens (i.e. d6‐dppz). The intensity variations observed for some of the prominent bands of the dppz−· radical anion in the TR2 spectra as a function of excitation wavelength show that several distinct transitions contribute to the excited‐state absorption. While many of the bands observed can most readily be attributed to vibrations which involve the entire dppz ligand skeleton, the behaviour of a few features upon isotopic substitution suggests the involvement also of more confined vibrational modes. The results of the transient resonance Raman studies are discussed in the context of recent proposals concerning the nature of the MLCT excited states possibly involved in the well‐known use of the [Ru(phen)2dppz]2+ complex as a ‘light‐switch reporter’ for DNA. Copyright © 2000 John Wiley & Sons, Ltd.