Preparation and spectroscopic, electrochemical and photophysical properties of mono-, di-nuclear and mixed-valence species derived from the photosensitizing group (2,2′-bipyridine)(2,2′:6′,2″-terpyridine)ruthenium(II)

Abstract

New mono-, di-nuclear, and mixed-valence complexes derived from the photosensitizing moiety Ru(terpy)(bipy)²⁺(bipy = 2,2′-bipyridine, terpy = 2,2′:6′,2″-terpyridine) have been prepared and characterized by spectroscopic, electrochemical and luminescence techniques. The mononuclear species [Ru(terpy)(bipy)(4,4′-bipy)]²⁺(4,4′-bipy = 4,4′-bipyridine) exhibits λ_max at 466 and 420 nm in MeCN, E_½(Ru^III–Ru^II)= 1.23 V (vs. saturated calomel electrode, SCE) in MeCN and λ_em= 625 nm in an EtOH–MeOH (4:1 v/v) glass at 77 K. The dinuclear complex [(terpy)(bipy)Ru^II(4,4′-bipy)Ru^II(NH₃)₅]⁴⁺ has a new absorption maximum at 538 nm and a new voltammetric wave at E_½= 0.39 V in MeCN ascribable to a metal-to-ligand charge transfer and a Ru^III–Ru^II redox process respectively involving the capping ammineruthenium group. On oxidation of this species with bromine vapour the mixed-valence complex [(terpy)(bipy)Ru^II(4,4′-bipy)Ru^III(NH₃)₅]⁵⁺ is obtained, which seems to exhibit a metal-to-metal charge-transfer band, at λ_max≈640 nm. Both dinuclear species emit at λ_em≈620 nm at 77 K, but with reduced intensity with respect to the parent mononuclear complex, thus pointing to the operation of reductive and oxidative quenching processes for the II,II and the II,III complexes, respectively. The weak metal–metal interaction detected in the mixed-valence ion implies the possibility of using these systems as ‘molecular switches’ and/or models for the obtention of charge-separated states.