Tuning Electron‐Withdrawing Strength on Phenothiazine Derivatives: Achieving 100 % Photoluminescence Quantum Yield by NO2 Substitution

Abstract The weak fluorescence (quantum yield <1 % in cyclohexane) of phenothiazine ( PTZ ) impedes its further application. In addition, the nitro group (NO 2 ) is a well‐known fluorescence quencher. Interestingly, we obtained a highly fluorescent chromophore by combining these two moieties, forming 3‐nitrophenothiazine ( PTZ‐NO 2 ). For comparison, a series of PTZ derivatives bearing electron‐withdrawing groups (EWGs; CN and CHO) or electron‐donating groups (EDGs; OMe) at the 3‐position have been designed and synthesized. The phenothiazines bearing EWGs exhibited enhanced emission compared with the parent PTZ or EDG derivatives. Computational approaches unveiled that for PTZ and PTZ‐OMe , the transitions are from HOMOs dominated by π orbitals to LUMOs of mixed sulfur nonbonding–π* orbitals, and hence are partially forbidden. In contrast, the EWGs lower the energy level of the lone‐pair electrons on the sulfur atom, thereby suppressing the mixing of the nonbonding orbital with the π* orbital in the LUMO, such that the allowed ππ* transition becomes dominant. This work thus demonstrates a judicious chemical design to fine‐tune the transition character in PTZ analogues, with PTZ‐NO 2 attaining 100 % emission quantum yields in nonpolar solvent.