The weak fluorescence( quantum yield < 1% in cyclohexane)o fp henothiazine (PTZ)i mpedes its further application. In addition, the nitro group (NO 2 )i sawell-known fluorescence quencher.I nterestingly,w eo btained ah ighly 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;C Na nd CHO) or electron-donating groups (EDGs;O Me) at the 3-position have been designed and synthesized. The phenothiazines bearing EWGs exhibited enhanced emission compared with the parent PTZ or EDGd e-rivatives. Computational approaches unveiled thatf or PTZ and PTZ-OMe,t he transitions are from HOMOs dominated by p orbitals to LUMOso fm ixed sulfur nonbonding-p*o rbitals, 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 mixingo ft he nonbondingo rbital with the p*o rbital in the LUMO, such that the allowed pp*t ransition becomes dominant. This work thus demonstrates aj udiciousc hemical design to fine-tune the transition character in PTZ analogues,w ith PTZ-NO 2 attaining 100 %e mission quantumy ields in nonpolar solvent.
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