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 Ru II-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.+-Ru II(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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry