Preparation Of NIR-Ⅱ Probe Based On Cyclopentadithiophene And Its Application In Tumor Fluorescence Imagin

Organic small molecule fluorescent probes are powerful support tools for fluorescence imaging to achieve visual medical diagnosis and treatment.Compared with the short-wavelength（400-900 nm）fluorescence used in traditional techniques,the near-infrared second-region（1000-1700 nm）fluorescence can significantly reduce the scattering and autofluorescence interference of biological tissues,and greatly innovate imaging resolution and imaging depth.However,the low quantum yield,poor biocompatibility and complex imaging system severely limit the clinical transformation of NIR-Ⅱ fluorescence imaging.Finding suitable fluorophores is the key to the clinical transformation of NIR-Ⅱ optical imaging.At present,carbon nanotubes,nanoclusters,rare earth doped nanoparticles,quantum dots and organic fluorophores can realize NIR-Ⅱ emission.Although many inorganic and carbon-based nanomaterials,such as quantum dots and rare earth nanoparticles,have been developed as NIR-Ⅱ fluorophores,the difficulties in excretion and self-toxicity of these fluorophores after imaging severely limit their development.The selection of small organic molecules as the main body of the fluorescent probe material not only improves the safety of the fluorescence probe in vivo imaging,but also facilitates the later modification of the fluorescent probe.For example,small organic molecules can combine the complex physiological and pathological characteristics and microenvironment of the organism to tune the main structure.In terms of fluorescence performance,fluorescent molecules in the NIR-Ⅱ band can greatly reduce the interaction between photons and tissues and provide higher resolution.Therefore,the preparation of NIR-Ⅱ organic small molecule fluorophores with high quantum yield and biocompatibility can significantly improve the drawbacks of NIR-Ⅱ imaging and promote clinical transformation.In this paper,the planar structure cyclopentadithiophene（CPDT）with excellent optical properties was used as the donor core,and the following work was carried out to improve the quantum yield and biocompatibility of NIR-Ⅱ organic small molecule fluorophores:（1）The strong acceptor diketopyrrolopyrrole（DPP）was selected as the acceptor to increase the electron delocalization.Cyclopentadithiophene（CPDT）was used as the donor,and isooctane was introduced as the shielding unit on CDPT to weaken the intermolecular force and prevent the interaction between organic fluorescent molecules with larger conjugated skeletons.We introduce 3,4-ethylenedioxythiophene（EDOT）as aπ-bridge between the acceptor and the donor to connect CPDT and DPP,which not only prolongs the conjugatedstructure and increases the charge transfer,but also distorts the spatial structure of the fluorophore.As a result,the excited state of the molecule is partially limited by the acceptor DPP,and the excited state center is protected,which is more conducive to the T1 reverse intersystem crossing back to S1 and improves the quantum yield.Finally,nanoparticles（CED NPs）were prepared by nano-precipitation method,and CED NPs showed high quantum yield（1.15%）and good biocompatibility.With fluorescence imaging,bright CED NPs can clearly present the tumor contour of subcutaneous tumor-bearing nude mice,and high optical penetration can mark the swollen lymph nodes in the metastatic tumor nude mouse model.（2）There is a strongπ-πinteraction betweenπ-conjugated molecules,resulting in aggregation fluorescence quenching（ACQ）of fluorescent molecules,which seriously weakens the quantum yield of fluorophores.Although the distorted structure can weaken the ACQ effect,it also affects the quantum yield.Through a series of characterizations,it is found that CPDT molecules with planar structure（π-conjugated molecules）can provide excellent optical properties in D-A-D organic fluorophores,and AIEgens can provide a certain AIE effect.Therefore,we incorporated the planar structure（CPDT）into the twisted structural skeleton（D-A-D type）,and then introduced the strong acceptor TTQ molecule and donor triphenylamine（AIEgens）to achieve aggregation-induced luminescence and NIR-Ⅱ emission,thereby avoiding aggregation fluorescence quenching and improving the fluorescence intensity in the aggregation state.Through a series of reactions,we designed and synthesized a NIR-Ⅱ organic small molecule fluorophore（TCD）.In addition,it was found that TDA NPs with certain AIE characteristics could achieve tumor fluorescence imaging in subcutaneous tumor-bearing nude mice at a lower concentration(100μg·mL^-1).