Electron‐Withdrawing Substituents Allow Boosted NIR‐II Fluorescence in J‐Type Aggregates for Bioimaging and Information Encryption

Developing molecular fluorophores with enhanced fluorescence in aggregate state for the second near‐infrared (NIR‐II) imaging is highly desirable but remains a tremendous challenge due to the lack of reliable design guidelines. Herein, we report an aromatic substituent strategy to construct highly bright NIR‐II J‐aggregates. Introduction of electron‐withdrawing substituents at 3,5‐aryl and meso positions of classic boron dipyrromethene (BODIPY) skeleton can promote slip‐stacked J‐type arrangement and further boost NIR‐II fluorescence of J‐aggregates via increased electrostatic repulsion and intermolecular hydrogen bond interaction. Notably, NOBDP‐NO2 with three nitro groups (−NO2) shows intense NIR‐II fluorescence at 1065 nm and high absolute quantum yield of 3.21 % in solid state, which can be successfully applied in bioimaging, high‐level encoding encryption, and information storage. Moreover, guided by this electron‐withdrawing substituent strategy, other skeletons (thieno‐fused BODIPY, aza‐BODIPY, and heptamethine cyanine) modified with −NO2 are converted into J‐type aggregates with enhanced NIR‐II fluorescence, showing great potential to convert aggregation caused emission quenching (ACQ) dyes into brilliant J‐aggregates. This study provides a universal method for construction of strong NIR‐II emissive J‐aggregates by rationally manipulating molecular packing and establishing relationships among molecular structures, intermolecular interactions, and fluorescence properties. An electron‐withdrawing substituent (−NO2) strategy is reported to construct J‐type aggregates with boosted NIR‐II fluorescence, which can promote slip‐stacked J‐type movement via increased electrostatic repulsion as well as facilitate intermolecular tight stacking by hydrogen bond. This method can be used to manipulate different kinds of organic fluorophores to obtain enhanced fluorescence in aggregate state.