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

Weak fluorescence (quantum yields < 1% in cyclohexane) of phenothiazine ( PTZ ) impedes its further utilization. Besides, 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 with electron-withdrawing groups (EWG) (cyano -CN and formyl -CHO) and electron-donating group (EDG) (methoxy -OMe) group at C3-position were designed and synthesized. The EWGs exhibit intensified emission compared with parent PTZ or EDG derivatives. Computational approaches unveil that for PTZ and PTZ-OMe , the transitions are from HOMOs dominated by pi orbitals to LUMOs of sulphur nonbonding-mixed pi* orbitals, and hence are partially forbidden. On the contrary, the EWGs lower the energy level of lone pair electrons on the sulphur atom, suppressing the mixing of nonbonding orbital with pi* in LUMO such that the allowed pipi* transition becomes dominant. This work thus demonstrates a judicious chemical design to fine-tune the transition character in PTZ analogues, making PTZ-NO 2 attains unity emission quantum yield in nonpolar solvents.