Resonance‐Enhanced Emission Effects toward Dual‐State Emissive Bright Red and Near‐Infrared Emitters

Abstract Resonance‐enhanced emission (REE) effect was discovered and lead to a novel dye family of hydrostyryl pyridinium derivatives in our recent work. Herein, the REE effect was employed to design a red and near‐infrared dual‐state emissive fluorophore family of SW−OH−NO 2 derivatives which were easily synthesized by coupling an electron‐withdrawing group (W) onto nitro(hydroxyl)styryl (S−OH−NO 2 ) through a C=C double bond as π‐bridge. The deprotonation of a phenolic hydroxyl group promoted by a nitro group and the electron‐withdrawing group (W) on the other side of the π‐bridge triggered resonance, resulting in significantly red‐shifted emission. All the resultant SW−OH−NO 2 compounds showed excellent dual‐state emission behavior. Remarkably, hydrostyryl quinolinium (SQ−OH−NO 2 ) is one of the smallest NIR emitter molecular skeleton ( λ em =725 nm, MW<400) and showed dual‐state emission characteristics and obvious viscosity‐depended fluorescent behaviors. In addition to constructing electron donor‐acceptor structures and prolonging π‐bridges, the REE effect promises a reliable strategy toward novel fluorophores with small size, long emissive wavelength, and dual‐emission characteristics, and importantly, feasible industrial manufactures and applications due to their easy and low‐cost synthesis strategy.