A series of mixed-metal bimetallic complexes [(TL)2M(dpp)RhCl2(TL)]3 (M = Ru and Os, terminal ligands (TL) = phen, Ph2phen, Me2phen and bpy, terminal ligands (TL) = phen, bpy and Me2bpy ), which couple one Ru or Os polyazine light absorber (LA) to a cis-RhIIICl2 center through a dpp bridging ligand (BL), were synthesized using a building block method. These are related to previously studied trimetallic systems [{(TL)2M(dpp)2RhCl2]5+, but the bimetallics are synthetically more complex to prepare due to the tendency of RhIII halide starting materials to react with diimine ligands to form cis-[Rh(NN)2Cl2]+ motifs. The bimetallic complexes, [(phen)2Ru(dpp)RhCl2(bpy)]3+, [(phen)2Ru(dpp)RhCl2(phen)]3+, [(Ph2phen)2Ru(dpp)RhCl2(phen)]3+, [(Me2phen)2Ru(dpp)RhCl2(phen)]3+, [(bpy)2Ru(dpp)RhCl2(bpy)]3+, [(bpy)2Ru(dpp)RhCl2(Me2bpy)]3+ and [(bpy)2Os(dpp)RhCl2(phen)]3+, were characterized and studied by electrochemistry, electronic absorption spectroscopy, ESI-mass spectrometry, steady-state and time-resolved emission spectroscopy. Ï¿" ï¿" The electrochemical properties of bimetallic complexes with polyazine ligands exhibit a reversible one-electron metal-based oxidation, a quasi-reversible RhIII/IICl2 overlapped with a small amount of RhII/ICl and an irreversible RhII/ICl2 �reductions prior to the reversible bridging ligand dpp0/- �reduction. ï¿" ï¿" The title bimetallic complexes are efficient light absorbers due to the [(TL)2MII(dpp)] light absorber subunit. The bimetallics display ligand-based ï¿"'ï¿"* transitions in the UV region and metal-to-ligand charge transfer (MLCT) transitions in the visible region of the spectrum with approximately half the absorption extinction coefficient values relative to the trimetallics in the spectrum. The Os,Rh bimetallic complex, [(bpy)2Os(dpp)RhCl2(phen)]3+, displays Os(dï¿")'dpp(ï¿"*) CT transition at 521 nm and a low energy absorption band at 750 nm in the near-infrared region representing direct 1GS'3MLCT excitation due to the high degree of spin orbital coupling in Os complexes. The bimetallic complexes [(phen)2Ru(dpp)RhCl2(bpy)]3+, [(phen)2Ru(dpp)RhCl2(phen)]3+, [(Ph2phen)2Ru(dpp)RhCl2(phen)]3+, [(Me2phen)2Ru(dpp)RhCl2(phen)]3+, [(bpy)2Ru(dpp)RhCl2(bpy)]3+ and [(bpy)2Ru(dpp)RhCl2(Me2bpy)]3+ display Ru(dï¿")'dpp(ï¿"*) MLCT transitions centered at 505, 508, 515, 516, 510 and 506 nm, respectively. The bimetallic complex [(Ph2phen)2Ru(dpp)RhCl2(phen)]3+ displays enhanced absorption. Ï¿" ï¿" The photophysical properties of Ru,Rh bimetallic complexes are close to those of trimetallic analogues. In room temperature acetonitrile, both bimetallic and trimetallic complexes display a weak and short-lived emission from the Ru(dï¿")'dpp(ï¿"*) 3MLCT excited state. For example, the bimetallic complex [(phen)2Ru(dpp)RhCl2(bpy)]3+ emits at 766 nm and the trimetallic complex [{(phen)2Ru(dpp)}2RhCl2]5+ emits at 760 nm. At 77 K in 4:1 ethanol/methanol glass, the bimetallics, as well as trimetallics, exhibit a more intense blue-shifted emission with a longer lifetime, which is from the same 3MLCT excited state. At 77 K, the low temperature emission from the same 3MLCT state of [{(phen)2Ru(dpp)}2RhCl2]5+ blue-shifts to 706 nm with the emission lifetime of 1.8 ms and the bimetallic [(phen)2Ru(dpp)RhCl2(bpy)]3+ emits at 706 nm (t = 1.8 ms). The Ru,Rh complexes 3MLCT excited states can populate Ru(dï¿")'Rh(ds*) triplet metal-to-metal charge transfer (3MMCT) excited states through intramolecular electron transfer at room temperature, which is impeded in the rigid matrice at 77 K due to the large reorganizational energy and restricted molecular motion. The emission of Os,Rh bimetallic complex [(bpy)2Os(dpp)RhCl2(phen)]3+ could not be detected by our instruments likely due to its expected red-shifted emission which lies outside our detector window. ï¿" ï¿" �The Ru,Rh bimetallics display interesting and efficient photo-reactivity with DNA activated by visible light. The DNA gel shift assay, selective precipitation, ESI-mass spectrometry and polymerase chain reaction (PCR) studies suggest that Ru,Rh bimetallic complexes photobind to DNA following visible light excitation. This reactivity is not observed for analogous Ru,Rh,Ru trimetallics due to the steric protection of the Rh site in that motif. The bimetallic [(TL)2Ru(dpp)RhCl2(TL)]3+ systems can photobind and photocleave DNA through low-lying 3MMCT excited states when excited by the low energy visible light, with or without molecular oxygen. This is unusual but desirable reactivity for photodynamic therapy (PDT) drug development. The Os,Rh bimetallic complex [(bpy)2Os(dpp)RhCl2(phen)]3+ photobinds and photocleaves DNA under red therapeutic light excitation without molecular oxygen, an unprecedented result. Polymerase chain reaction experiments were used to evaluate the impact on DNA amplification of the DNA photo-modification and photo-damage induced by [(bpy)2Os(dpp)RhCl2(phen)]3+ under red light irradiation. Either photobinding or photocleavage induced by red light excitation of [(bpy)2Os(dpp)RhCl2(phen)]3+ on DNA inhibits amplification via PCR methods, a model for in vivo replication. Moreover, significant thermal stability of DNA photo-modification over 90 "C is required for PCR. A red light-activated drug that acts in an oxygen-independent mechanism to impede DNA amplification is unique in this field and desirable for study as a new class of PDT drugs. / Ph. D.