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

We demonstrate that the 10-phenyl-10 H-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 CHCN with a 517 nm laser pulse populates a D electronically excited doublet state that decays first to the unrelaxed lowest electronic excited state, D' (τ < 0.3 ps), followed by relaxation to D (τ = 10.9 ± 0.4 ps), which finally decays to D (τ = 32.3 ± 0.8 ps). D' can also be populated directly using a lower energy 900 nm laser pulse, which results in a longer D'→D 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, D', or D 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.