Excited state localization and internuclear interactions in asymmetric Ruthenium (II) and Osmium (II) bpy/tpy based dinuclear compounds

The synthesis of two asymmetric dinuclear complexes with the formula [M(bpy)2(bpt)Ru(tpy)Cl]2+, where M = Ru (1a), Os(2a); bpy = 2,2’-bipyridyl; Hbpt = 3,5-bis(pyridin-2-yl)1,2,4-triazole and tpy = 2,2’,6’,2”-terpyridine, is reported. The compounds obtained are characterized by mass spectrometry, 1H-NMR, UV/vis/NIR absorption, luminescence and resonance Raman spectroscopy. Deuterium isotope labeling facilitates assignment of the 1H-NMR and resonance Raman spectra. The interaction between the two metal centers, mediated by the bridging 1,2,4-triazolato moiety in the mixed valent state, is assigned as type II based on the observation of metal to metal charge transfer absorption bands at 7090 and 5990 cm-1 for 1a and 2a, respectively. The extent of localization of the emissive excited state was determined by transient resonance Raman and emission spectroscopy. Both 1a and 2a show phosphorescence at the same wavelengths; however, whereas for compound 1a the emission is based on the Ru(tpy)Cl- center, for 2a the emissive state is localized on the Os(bpy)2- unit. This indicates that also in the excited state there is efficient interaction between the two metal centers.