Synthesis and Structural Characterization of Porphyrinic Enediynes: Geometric and Electronic Effects on Thermal and Photochemical Reactivity

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 Ru^II^-BQ^-Co^III), 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 Ru^II-BQ (4) and PTZ^-Ru^II (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 (f = 200 ps) in all the quinone containing complexes. Recombination to the ground state followed, with a rate constant kb ~ 4.5 × 108 s^-1 (b ~ 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^-Ru^II-BQ triad, however, the initial charge separation was followed by a rapid (k (greater than) 5 × 10^9 s^-1) electron transfer from the phenothiazine moiety to give the fairly long-lived PTZ^+-Ru^II-BQ^- state ((tau)= 80 ns) in unusually high yield for a [Ru(bpy)3]^2+-based triad ((greater than) 90%), that lies at (Delta)G = 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^-Ru^II 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^-*Ru^II 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^+-Ru^II(bpy^-) state was thus faster than what would have been inferred from the *Ru^II emission decay ((tau)= 90 ns). This result suggests that reports for related PTZ-RuII and PTZ-Ru^II-acceptor complexes in the literature might need to be reconsidered.