空间位阻对D-A型近红外二区共轭聚合物荧光调控作用及成像性能研究

Fluorescence imaging within the second near-infrared window (NIR-II, 1000-1700 nm) has the advantages of high spatial resolution and deep tissue penetration depths. However, the defects of the existing NIR-II fluorophores, relatively low fluorescence quantum yield and unfavourably short emission wavelength, are urgent to be improved. This proposal will focus on enhancing fluorescence quantum yield. Donor-acceptor (D-A) type conjugated polymer will be studied as the research object, strong donors and acceptors will be employed to construct D-A conjugated polymers with NIR-II absorption and emission. The introduced side chains with strong steric hindrance will twist the backbone of polymers, reduce the π-π interaction in aggregated state, thus high fluorescence quantum efficiency can be obtained in the fluorescent probes. A nanoprecipitation method will be utilized to prepared good biocompatible fluorescent probes. It is expected to achieve a NIR-II fluorescence quantum efficiency over 10% in water phase, and the relationship of D-A type conjugated polymers skeleton torsion degree and the fluorescence quantum efficiency will be elucidated. The innovation of this project is skeleton torsion will be employed to reduce the polymeric fluorescence quenching, and by investigating the quantum yield of polymers with different skeleton torsions, the relationship between fluorescence quantum efficiency and polymeric structure will be more clear. Meanwhile, it could be a universal applicable method to improve the fluorescence quantum efficiency of D-A type conjugated polymers, which can promote the application of D-A type conjugated polymers in the field of NIR-II fluorescence imaging.

近红外二区（NIR-II, 1000-1700 nm）荧光成像具有高探测深度、高空间分辨率等优点，然而现有NIR-II荧光探针存在荧光波长短、荧光量子效率低等缺陷亟待改善。本项目围绕荧光量子效率低的问题，以D-A型共轭聚合物（DAP）为研究对象，拟采用强给体与强受体单元构筑DAP，使其吸收和发射红移至近红外二区。通过在骨架中引入较大位阻的侧链，适度扭转共轭骨架，减弱其在聚集态时的π-π相互作用，缓解聚合物荧光的聚集淬灭，以达到提高荧光量子效率的目的。采用纳米沉淀法制备NIR-II荧光探针。预期实现水相中NIR-II荧光量子效率10%以上的目标，阐明DAP骨架扭转程度与荧光量子效率之间的关系。本项目将创新性地通过骨架扭转来减弱聚合物荧光聚集淬灭，这一方法有助于探索DAP结构与荧光性能之间的构效关系，并可能成为构建高荧光量子效率聚合物探针的普适方法，促进DAP在NIR-II荧光成像领域的应用。

相比于近红外一区（NIR-I, 750-900 nm）荧光成像，近红外二区（NIR-II, 1000-1700 nm）荧光发射波长更长，可显著降低光在穿透生物组织时的散射及自荧光的影响，成像时探测深度更深、空间分辨率更高。现有NIR-II荧光探针如碳纳米管、硫化银量子点等无机纳米材料存在荧光波长短、荧光量子效率低等缺陷亟待改善。我们采用强给体单元与强受体单元构筑D-A型共轭聚合物，使其吸收和发射落入近红外二区，选用了带有sp3杂化结构的环戊二烯并二噻吩与受体单元TT和DPP共聚，获得聚合物P-TT和P-DPP，近红外二区荧光量子效率分别为0.5%和1.5%，研究成果发表于J. Mater. Chem. B，并被杂志评为年度热点论文；我们通过增大受体单元上取代基位阻来调控聚合物的堆积，合成了聚合物PDTSDTBT，大位阻侧链有利于减弱荧光的聚集淬灭，其荧光量子效率为0.5%，研究成果已发表于J. Innov. Opt. Health Sci.；在前期工作的基础上，将环戊二烯并二噻吩中桥碳原子替换为尺寸更大的硅原子，进一步放大sp3杂化结构的优势，同时在受体单元上引入氟原子，通过F…H作用力加强分子内电荷传输，其荧光量子效率进一步提升至3.4%-4.4%，研究成果已发表于RSC Adv.。.本项目开发了一系列摩尔吸光系数高、光稳定性好、荧光量子产率高、吸收和发射均在NIR-II的D-A型共轭聚合物，并通过结构优化，使之荧光信号得到增强，这些聚合物还具有光热和光声性能。本项目通过引入sp3杂化结构、调整受体单元侧链、引入氟原子通过F…H作用力加强分子内电荷传输等手段成功避免了聚合物荧光的聚集淬灭，获得了较强的近红外二区荧光信号。这些设计理念对未来如何通过优化分子结构来获得性能优异的近红外二区荧光成像材料具有重要参考意义。