Phenothiazine Radical Cation Excited States as Super-oxidants for Energy-Demanding Reactions

We demonstrate that the 10-phenyl-10H-phenothiazine radical cation (PTZ^+•) has a manifold of excited doublet states accessible using visible and near-infrared light that can serve as super-photooxidants with excited-state potentials is excess of +2.1 V vs SCE to power energy demanding oxidation reactions. Photoexcitation of PTZ^+• in CH₃CN with a 517 nm laser pulse populates a D_n electronically excited doublet state that decays first to the unrelaxed lowest electronic excited state, D₁′ (τ < 0.3 ps), followed by relaxation to D₁ (τ = 10.9 ± 0.4 ps), which finally decays to D₀ (τ = 32.3 ± 0.8 ps). D₁′ can also be populated directly using a lower energy 900 nm laser pulse, which results in a longer D₁′→D₁ relaxation time (τ = 19 ± 2 ps). To probe the oxidative power of PTZ^+• photoexcited doublet states, PTZ^+• was covalently linked to each of three hole acceptors, perylene (Per), 9,10-diphenylanthracene (DPA), and 10-phenyl-9-anthracenecarbonitrile (ACN), which have oxidation potentials of 1.04, 1.27, and 1.6 V vs SCE, respectively. In all three cases, photoexcitation wavelength dependent ultrafast hole transfer occurs from D_n, D₁′, or D₁ of PTZ^+• to Per, DPA, and ACN. The ability to take advantage of the additional oxidative power provided by the upper excited doublet states of PTZ^+• will enable applications using this chromophore as a super-oxidant for energy-demanding reactions.