Designing High-Triplet-Yield Phenothiazine Donor-Acceptor Complexes for Photoredox Catalysis

Phenothiazine, owing to its ease of oxidation and modularity with respect to facile functionalization, is an attractive central chemical unit from which to construct highly reducing organic photoredox catalysts. While design improvements have been made in the community, the yield of intersystem crossing (Phi(ISC)), which determines access to the long-lived triplet excited state, has yet to be systematically optimized. Herein, we explore the impacts of N-aryl substituent variation on excited-state dynamics using picosecond to millisecond transient absorption and emission spectroscopies. Design principles are uncovered that center on controlling the energy of an intermediate charge transfer (CT) state within the singlet excited-state manifold, which, in turn, dictates the yield of CT-state formation and the rate constants for its depletion. Ultimately, we find Phi(ISC) to be highly sensitive to the electron-withdrawing character of the N-aryl electron acceptor in the aforementioned CT state, with Phi(ISC) ranging from similar to 0 to 0.96.