Electron Donor−Acceptor Dyads and Triads Based on Tris(bipyridine)ruthenium(II) and Benzoquinone: Synthesis, Characterization, and Photoinduced Electron Transfer Reactions

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Abstract
Two electron donor−acceptor triads based on a benzoquinone acceptor linked to a light absorbing [Ru(bpy)3]2+ complex have been synthesized. In triad 6 (denoted RuII−BQ−CoIII), a [Co(bpy)3]3+ complex, a potential secondary acceptor, was linked to the quinone. In the other triad, 8 (denoted PTZ−RuII−BQ), a phenothiazine donor was linked to the ruthenium moiety. The corresponding dyads RuII−BQ (4) and PTZ−RuII (9) were prepared for comparison. Upon light excitation in the visible band of the ruthenium moiety, electron transfer to the quinone occurred with a rate constant kf = 5 × 109 s-1 (τf = 200 ps) in all the quinone containing complexes. Recombination to the ground state followed, with a rate constant kb ∼ 4.5 × 108 s-1 (τb ∼ 2.2 ns), for both RuII−BQ and RuII−BQ−CoIII with no indication of a charge shift to generate the reduced CoII moiety. In the PTZ−RuII−BQ triad, however, the initial charge separation was followed by a rapid (k > 5 × 109 s-1) electron transfer from the phenothiazine moiety to give the fairly long-lived PTZ•+−RuII−BQ•- state (τ = 80 ns) in unusually high yield for a [Ru(bpy)3]2+-based triad (> 90%), that lies at ΔG° = 1.32 eV relative to the ground state. Unfortunately, this triad turned out to be rather photolabile. Interestingly, coupling between the oxidized PTZ•+ and the BQ•- moieties seemed to occur. This discouraged further extension to incorporate more redox active units. Finally, in the dyad PTZ−RuII a reversible, near isoergonic electron transfer was observed on excitation. Thus, a quasiequilibrium was established with an observed time constant of 7 ns, with ca. 82% of the population in the PTZ−*RuII state and 18% in the PTZ•+−RuII(bpy•-) state. These states decayed in parallel with an observed lifetime of 90 ns. The initial electron transfer to form the PTZ•+−RuII(bpy•-) state was thus faster than what would have been inferred from the *RuII emission decay (τ = 90 ns). This result suggests that reports for related PTZ−RuII and PTZ−RuII−acceptor complexes in the literature might need to be reconsidered.

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