Synthesis and study of a mixed-ligand ruthenium(II) complex in its ground and excited states: bis(2,2′-bipyridine)(dipyrido[3,2-a : 2′,3′-c]phenazine-N4N5)ruthenium(II)

Abstract

The ruthenium (II) complex [Ru(bipy)₂(dppz)]²⁺(bipy = 2,2′-bipyridine, dppz = dipyrido[3,2-a : 2′,3′-c]phenazine) was synthesized, characterized, and studied. Its oxidation and first reduction potentials are respectively 1.24 and –1.02 V (vs. saturated calomel electrode). The maxima of the metal-to-ligand charge-transfer absorption and emission occur respectively at 448 and 610 nm. These data suggest that [Ru(bipy)₂(dppz)]²⁺ is made up of two electronically independent units, one behaving as a [Ru(bipy)₃]²⁺-like chromophore, the other as a phenazine-like electron acceptor. Excited-state absorption spectra were obtained for [Ru(bipy)₂(dppz)]²⁺ and its parent complex [Ru(bipy)₃]²⁺. Above 500 nm the latter shows only one maximum (at 510 nm), and the former shows two maxima (around 526 and 557 nm), whereas radical anions of both bipyridine and phenazine or dipyridophenazine show two maxima (at 512 and 552 nm for phenazine). In the case of [Ru(bipy)₂(dppz)]²⁺ these results can be interpreted in terms of a light-induced directed charge transfer from the ruthenium to the phenazine part of the dipyridophenazine ligand, and its localization on this ligand moiety. Photochemical properties of [Ru(bipy)₂(dppz)]²⁺ were studied in ethanol. The excited state of the complex is quenched both by an electron acceptor (methylviologen, k_q= 1.37 × 10⁹ dm³ mol^–1 s^–1) and an electron donor (triethanolamine, k_q= 4.40 × 10⁷ dm³ mol^–1s^–1).