Photoinduced Electron Transfer in Linear Triarylamine-Photosensitizer-Anthraquinone Triads with Ruthenium(II), Osmium(II), and Iridium(III)

A rigid rod-like organic molecular ensemble comprised of a triarylamine electron donor, a 2,2'-bipyridine (bpy) ligand, and a 9,10-anthraquinone acceptor was synthesized and reacted with suitable metal precursors to yield triads with Ru(bpy)(3)(2+), Os(bpy)(3)(2+), and [Ir(2-(p-tolyl)pyridine)(2)(bpy)](+) photosensitizers. Photoexcitation of these triads leads to long-lived charge-separated states (tau = 80-1300 ns) containing a triarylamine cation and an anthraquinone anion, as observed by transient absorption spectroscopy. From a combined electrochemical and optical spectroscopic study, the thermodynamics and kinetics for the individual photoinduced charge-separation and thermal charge-recombination events were determined; in some cases, measurements on suitable donor-sensitizer or sensitizer-acceptor dyads were necessary. In the case of the ruthenium and iridium triads, the fully charge-separated state is formed in nearly quantitative yield.