The absorption spectroscopy of [Ru(phen) 2 dppz] 2+ and [Ru(tap) 2 dppz] 2+ (phen=1,10-phenanthroline, tap=1,4,5,8-tetraazaphenanthrene; dppz=dipyridophenazine) complexes used as molecular light switches by intercalation in DNA has been analysed by means of Time-Dependent Density Functional Theory (TD-DFT). The electronic ground state structures have been optimized at the DFT (B3LYP) level of theory. The absorption spectra are characterized by a high density of excited states between 500nm and 250nm. The absorption spectroscopy of [Ru (phen) 2 dppz] 2+ in vacuum is characterized by metal-to-ligand-charge-transfer (MLCT) transitions corresponding to charge transfer from Ru(II) either to the phen ligands or to the dppz ligand with a strong MLCT (dRu→πdppz*) absorption at 411nm. In contrast, the main feature of the lowest part of the vacuum theoretical spectrum of [Ru(tap) 2 dppz] 2+ between 522nm and 400nm is the presence of various excited states such as MLCT (dRu→πTAP*), ligand-to-ligand-charge-transfer LLCT (πdppz→πTAP*) or intra-ligand IL (πdppz→πdppz*) states. When taking into account solvent corrections within the polarizable continuum model (PCM) approach (H 2 O, CH 3 CN) the absorption spectrum of [Ru(tap) 2 dppz] 2+ is dominated by a strong absorption at 388nm (CH 3 CN) or 390nm (H 2 O) assigned to a 1 IL (πdppz→πdppz*) corresponding to a charge transfer from the outside end of the dppz ligand to the site of coordination to Ru(II). These differences in the absorption spectra of the two Ru(II) complexes have dramatic effects on the mechanism of deactivation of these molecules after irradiation at about 400nm. In particular, the electronic deficiency at the outside end of the dppz ligand created by absorption to the 1 IL state will favour electron transfer from the guanine to the Ru(II) complex when it is intercalated in DNA.