Fig. 1. Representative design strategies for fluorescent probes with long emission wavelengths. (a) Extending the conjugation chain^[32]; (b) exchanging of heteroatoms; (c) donor modifications^[34]; (d) heterocycle substitute^[33]; (e) constructing J-aggregates^[71]

Fig. 2. Representative design strategies to improve fluorescent brightness. (a) Introducing steric hindrance^[33]; (b) self-assembling with protein^[33]; (c) improving the molecular rigidity^[37]

Fig. 3. Representative design strategies to improve water solubility. (a) Nanoprecipitation^[35]; (b) embedding hydrophilic groups^[38]

Fig. 4. Physical properties and biological applications of CH1055^[39]. (a) Chemical structure of CH1055; (b) absorbance and fluorescent emission spectra of CH1055-PEG; (c) temporal profiles of CH1055-PEG and SWCNTs invivo (1200 nm long-pass filter, exposure time of 100 ms); (d) fluorescent signal intensity of both the liver and bladder regions for CH1055-PEG; (e) NIR-II imaging with CH10555-PEG and NIR-I imaging with ICG on lymphatic vessels; (f) fluorescence intensity profiles at cross-section indicated by arrows shown in Fig. 4(e)

Fig. 5. Organic small molecules with BBTD as the core

Fig. 6. Chemical structures of acceptor

Fig. 7. Representative NIR-II fluorescent probes with different acceptors. (a) Chemical structures of BTB and BBT^[48]; (b) absorption and emission spectra of BTB NPs and BBT NPs^[48]; (c) quantum yields of BTB and BBT in toluene, BTB NPs in water;^[48] (d) schematic illustration of the design of self-assembled L6-PEG_nk^[49]

Fig. 8. Comparison of different acceptor NIR-II dyes^[90]. (a) Chemical structures of BBTD, TQ and TQT; (b) HPLC chromatograms of TQT and BBTD under various acid-base conditions, and bright-field images of TQT and BBT in MeOH (5% DMF, 1 mL) under various acid-base conditions; (c) in vivo NIR-II imaging for the vascular network of brain and tumor with FT-TQT; (d) real-time monitoring of the tumor vascular disruption after treatment with combretastatin A4 phosphate (CA4P)

Fig. 9. Strategies to increase QY. (a) Schematic illustration of the design of S-D-A-D-S type NIR-II fluorophores^[44]; (b) suppressing the twisted intramolecular charge transfer of fluorophore^[50]; (c) chemical structures of CH-4T, and fluorescence photos of CH-PEG or CH-4T in DI water, FBS, and PBS buffer^[40]; (d) schematic of constructing CQL, and fluorescence photo of CQ-4T in water, HSA, and HSA-HT; (e) scheme of constructing nanoparticle p-FE and chemical structures of FE and the PS-g-PEG polymer^[60]; (f) absorption and emission spectra (excited by an 808 nm laser) of FE in toluene, and absorption and emission spectra (excited by an 808 nm laser) of p-FE in PBS buffer^[60]

Fig. 10. Chemical structures of NIR-II AIEgens

Fig. 11. Representative small molecules with AIE properties. (a) Schematic illustration for AIE molecular design^[53]; (b) chemical structures and optimized ground state (S₀) geometries of TT1-oCB, TT2-oCB, and TT3-oCB^[53]; (c) chemical structures and optimized ground state (S₀) geometries of 2TT-oC6B, 2TT-oC26B, and 2TT-oC610B; (d) NIR-IIb fluorescence imaging of vasculature in living mice^[54]

Fig. 12. The first polymer was applied to NIR-II in vivo imaging^[61]. (a) Chemical structures of pDA-1, pDA-2, pDA-3 and pDA-4; (b) a schematic of the pDA-PEG nanoparticle showing a hydrophobic polymer core and a hydrophilic PEG shell; (c) a typical AFM image of pDA-PEG nanoparticles deposited on a silicon substrate; (d) absorption and emission spectra of pDA-PEG; (e) ultrafast NIR-II imaging of arterial blood flow

Fig. 13. Chemical structures of organic conjugated polymers

Fig. 14. Fluorination strategy to improve QY^[63]. (a) Schematic illustration of nanoscale fluorous effect to maintain hydrophobic interior and minimize structure distortion of the Pdots (the fluorination redshifts the optical spectra and enhances the fluorescence QY); (b) in vivo NIR-II whole-body fluorescence imaging of C57BL/6 mice in prone and supine positions after tail-vein injection of 100 mL m-PBTQ4F Pdots (200 µg/mL); (c) in vivo NIR-II fluorescence imaging of cerebral vasculature of C57BL/6 mice injected with 100 mL ICG or m-PBTQ4F Pdots (200 µg/mL) at certain time intervals from 1 to 120 min (70 mW/cm², 808 nm laser, 1319 nm LP filter)

Fig. 15. Representative polymers with AIE properties. (a) Fluorescence images of IR26 and Pdots in different states (left), illustration of the formation of Pdots (right)^[68]; (b) molecular design of three semiconducting polymers^[70];^{(c) schematic illustration of the molecular design philosophy of highly bright SPN[69]; (d) chemical structures of pNIR-1, pNIR-2, pNIR-3 and pNIR-4 [69]; (e) NIR-II fluorescent imaging of blood vessels in the cerebral cortex and hindlimb under different LP filters (left), comparison of NIR-IIa fluorescent imaging quality between pNIR-4 and pNIR-3 nanoparticles (right) [69]}

Fig. 16. Representative polymers for the integration of diagnosis and treatment. (a) Schematic illustration of L1057 NPs as a theranostic agent^[67]; (b) chemical structures of PTQ and DSPE-PEG₂₀₀₀ and a schematic illustration of the preparation of L1057 NPs^[67]; (c) normalized absorption and emission spectra of L1057 NPs in water^[67]; (d) schematic illustration of the effect of the repeating unit number on the phototheranostic performance of semiconducting polymer probe^[64]; (e) synthetic route to PBQx^[64]