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.

Highly Twisted Conformation Thiopyrylium Photosensitizers for In Vivo Near Infrared-II Imaging and Rapid Inactivation of Coronavirus

Despite significant effort, a majority of heavy-atom-free photosensitizers have short excitation wavelengths, thereby hampering their biomedical applications. Here, we present a facile approach for developing efficient near-infrared (NIR) heavy-atom-free photosensitizers. Based on a series of thiopyrylium-based NIR-II (1000-1700 nm) dyads, we found that the star dyad HD with a sterically bulky and electron-rich moiety exhibited configuration torsion and significantly enhanced intersystem crossing (ISC) compared to the parent dyad. The electron excitation characteristics of HD changed from local excitation (LE) to charge transfer (CT)-domain, contributing to a ≈6-fold reduction in energy gap (ΔE_ST ), a ≈10-fold accelerated ISC process, and a ≈31.49-fold elevated reactive oxygen species (ROS) quantum yield. The optimized SP@HD-PEG_2K lung-targeting dots enabled real-time NIR-II lung imaging, which precisely guided rapid pulmonary coronavirus inactivation.

NIRII-HDs: A Versatile Platform for Developing Activatable NIR-II Fluorogenic Probes for Reliable In Vivo Analyte Sensing

Small-molecule-based second near-infrared (NIR-II) activatable fluorescent probes can potentially provide a high target-to-background ratio and deep tissue penetration. However, most of the reported NIR-II activatable small-molecule probes exhibit poor versatility owing to the lack of a general and stable optically tunable group. In this study, we designed NIRII-HDs, a novel dye scaffold optimized for NIR-II probe development. In particular, dye NIRII-HD5 showed the best optical properties such as proper pK_a value, excellent stability, and high NIR-II brightness, which can be beneficial for in vivo imaging with high contrast. To demonstrate the applicability of the NIRII-HD5 dye, we designed three target-activatable NIR-II probes for ROS, thiols, and enzymes. Using these novel probes, we not only realized reliable NIR-II imaging of different diseases in mouse models but also evaluated the redox potential of liver tissue during a liver injury in vivo with high fidelity.

Facial Control Intramolecular Charge Transfer of Quinoid Conjugated Polymers for Efficient in Vivo NIR-II Imaging

Low-band gap conjugated polymers with donor-acceptor (D-A) structures have emerged as  second near-infrared (NIR-II) fluorescence probes for biological imaging. However, how to control the intramolecular charge transfer (ICT) to maintain the low band gap and improve the NIR-II fluorescence intensity is an urgent issue. Here, the quinoid polymers have been developed to effectively regulate the ICT for brighter NIR-II fluorescence signals. Thiophene repeat chain units of different lengths (T, 2T, and 3T) were utilized to link with electron-withdrawing ester-substituted thieno[3,4- b]thiophene (TT) to alter the density of the electron-withdrawing side groups for controlling the ICT. By increasing the thiophene chain length from TT-T to TT-3T, the density of the electron-withdrawing groups decreased and the ICT was weakened. In the case of NIR absorption and NIR-II emission, weakened ICT leads to brighter NIR-II fluorescence. After the preparation of the water-soluble quinoid polymer probes (CPs), TT-3T CPs with weak ICT exhibited the brightest NIR-II fluorescent signals among the three quinoid polymer probes. Several NIR-II biomedical imaging applications, including in vivo cell tracking, blood vascular system images, and lymphatic drainage mapping, show that the TT-3T CP-based nanoprobe had excellent characteristics of long-term stability and high NIR-II spatial resolutions in vivo.