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

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-NO2 derivatives which were easily synthesized by coupling an electron-withdrawing group (W) onto nitro(hydroxyl)styryl (S-OH-NO2 ) 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-NO2 compounds showed excellent dual-state emission behavior. Remarkably, hydrostyryl quinolinium (SQ-OH-NO2 ) is one of the smallest NIR emitter molecular skeleton (λem =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.