Absorption Spectra, Photophysical Properties, and Redox Behavior of Stereochemically Pure Dendritic Ruthenium(II) Tetramers and Related Dinuclear and Mononuclear Complexes

Publisher Website
Google Scholar
Abstract
The absorption spectra, luminescence properties, and redox behavior of stereochemically pure, dendritic Ru(II) tetramers have been studied. Furthermore, the investigation has also been performed on stereochemically resolved dinuclear complexes of the same family and on racemic forms of their mononuclear precursors and models. The complexes studied are the racemic species [(phen)2Ru(1,10-phenanthroline-5,6-dione)](PF6)2 (A, phen = 1,10-phenanthroline), [(phen)2Ru(1,10-phenanthroline-5,6-diamine)](PF6)2 (B), [Ru(1,10-phenanthroline-5,6-dione)3](PF6)2 (C), [(phen)2Ru(tpphz)](PF6)2 (1, tpphz = tetrapyrido[3,2-a:2‘,3‘-c:3‘ ‘,2‘ ‘-h:2‘ ‘,3‘ ‘-j]phenazine), [(phen)2Ru(μ-tpphz)Ru(phen)2](PF6)4 (2), and [{(phen)2Ru(μ-tpphz)}3Ru](PF6)8 (4), the stereochemically pure dinuclear species ΔΔ-[(phen)2Ru(μ-tpphz)Ru(phen)2](PF6)4 (ΔΔ-2), ΛΛ-[(phen)2Ru(μ-tpphz)Ru(phen)2](PF6)4 (ΛΛ-2), and ΔΛ-[(phen)2Ru(μ-tpphz)Ru(phen)2](PF6)4 (ΔΛ-2), and the stereochemically pure dendritic tetranuclear complexes [(Δ-(phen)2Ru(μ-tpphz))3-Δ-Ru](PF6)8 (Δ3Δ-4), [(Δ-(phen)2Ru(μ-tpphz))3-Λ-Ru](PF6)8 (Δ3Λ-4), and [(Λ-(phen)2Ru(μ-tpphz))3-Λ-Ru](PF6)8 (Λ3Λ-4). All the complexes exhibit reversible metal-centered oxidation processes: the mononuclear complexes undergo a one-electron oxidation within the potential range +1.30 to +1.70 V vs SCE, whereas the dinuclear complexes undergo a two-electron oxidation at about +1.35 V and the tetranuclear compounds undergo a three-electron process at about +1.35 V followed by a one-electron process at +1.46 V. On reduction, each compound undergoes several reversible or quasireversible ligand-centered reductions within the potential window investigated (+2.00/−1.80 V vs SCE). The absorption spectra of the complexes exhibit intense ligand-centered (LC) bands in the UV region (ε up to 106 M-1 cm-1) and moderately intense metal-to-ligand charge-transfer (MLCT) bands in the visible region (ε in the range 104−105 M-1 cm-1). All the complexes are luminescent both in fluid acetonitrile solution at room temperature (λmax in the range 600−720 nm) and in MeOH/EtOH 4:1 (v/v) rigid matrix at 77 K (λmax in the range 560−620 nm), except C which is luminescent only at 77 K. In all the cases, luminescence decays are monoexponential with lifetimes in the range 10-5−10-8 s. Energy transfer occurs in the dendritic tetranuclear complexes from the central chromophore to the peripheral ones. For the oligonuclear tpphz-containing complexes, luminescence at room temperature and at 77 K originates from different MLCT states. When the experimental uncertainties are taken into account, the absorption spectra, luminescence properties, and redox behavior of the various stereoisomers studied here are practically undistinguishable one another. Comparison of our results with the photophysical results reported for other stereochemically pure luminescent multimetallic arrays is attempted.