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

We demonstrate that the 10-phenyl-10H-phenothiazine radical cation (PTZ(+center dot)) 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(+center dot) 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, D-1' (tau < 0.3 ps), followed by relaxation to D-1 (tau = 10.9 +/- 0.4 ps), which finally decays to D-0 (tau = 32.3 +/- 0.8 ps). D-1' can also be populated directly using a lower energy 900 nm laser pulse, which results in a longer D-1'-> D-1 relaxation time (tau = 19 +/- 2 ps). To probe the oxidative power of PTZ(+center dot) photoexcited doublet states, PTZ(+center dot) 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, D-1', or D-1 of PTZ(+center dot) to Per, DPA, and ACN. The ability to take advantage of the additional oxidative power provided by the upper excited doublet states of PTZ(+center dot) will enable applications using this chromophore as a super-oxidant for energy-demanding reactions.