Parasitic Singlet Equilibrium in Phenoxazine Photocatalysts

The N-aryl phenoxazine architecture shows great promise as a modular synthetic platform for robust visible-light-absorbing organic photoredox catalysts (PCs). When properly constructed, such systems can exploit different directional charge-transfer states in their relaxation dynamics, ultimately leading to high yields of long-lived triplet excited states that are potent reductants. However, desirable characteristics can also be inadvertently diminished in the process of making structural modifications due to the energetic proximity of the first two singlet excited states: S1 and S2. Previously, we observed a case where alteration of N-naphthyl connectivity introduced an S2-S1 equilibrium, hindering the PC's ability to form triplets. Herein, we report on another phenoxazine PC called N1N, where parasitic excited-state equilibrium is also observed, this time arising from modification of the core substituents. The PC N1N and its N-phenyl analog NP are characterized through a combination of steady-state and time-resolved spectroscopies, electrochemical experiments, density functional theory calculations, and kinetic analyses. NP and N1N, like other PCs of their family, are strong visible-light absorbers and powerful excited-state reductants and possess triplet lifetimes exceeding 3 ms. NP is a bright emitter (Φem = 0.77, ΦISC = 0.085) exhibiting simple photophysics. On the other hand, N1N is an equilibrium-bearing phenoxazine, which negatively impacts its yield of ISC (Φem = 0.13, ΦISC = 0.61). A kinetic model is proposed that links Φem of N1N to the S2-S1 equilibrium constant and allows for its determination (Keq = 5.1). We conclude that unlike in the previously studied system, this equilibrium arises from the presence of trifluoromethyl groups on the core substituents, which destabilize S1 due to their electron-withdrawing nature. These results highlight the care needed in evolving PCs of this class.