【JACS】中科院车延科等|荧光量子产率飙升5倍、双光子截面增强4倍，动态分子晶体，41℃温和热刺激触发分子内扭转！
文章标题： Dynamic Molecular Crystal Triggered by Near-Ambient Intramolecular Twisting for Switchable Luminescence and Two-Photon Absorption
通讯作者： Ling Zang, Yanke Che
文章概要
引言
动态分子晶体在智能光学响应材料和生物医学成像等领域具有巨大的应用潜力。然而，传统荧光晶体内部往往存在强烈的分子间相互作用，将分子牢牢锁定在刚性晶格中，导致其多晶型转变通常需要高于80°C或低于0°C的极端热驱动，这极大限制了它们在温和温区（如人体或生物环境）的实际应用。如何在近室温温和条件下，通过大尺寸共轭基团的协同扭转来直接构筑高效、可逆的光电调控系统，一直是该领域面临的重大挑战。
Figure 1. (a) Molecular structure of 1 and schematic diagram of the D–A group torsion. (b) Normalized absorption (black) and fluorescence spectra (orange) of molecule 1 in toluene (1 μM).
主要实验及结论
研究团队精心设计并合成了一种由苯并硒二唑受体和咔唑供体构成的D-A-D型活性有机分子，并通过溶液自组装成功制备出了高质量、高长径比的棒状动态分子晶体。单晶X射线衍射分析表明，在常温初始状态下，晶体内部独特的分子间硒···（chalcogen bonding）相互作用充当了“构象锁”，将供受体之间的二面角锁定在较大的77.5°，限制了分子内的共轭程度，此时晶体的荧光量子产率仅为6%。令人兴奋的是，当将晶体微热至41 °C时，温和的热能便足以破坏这种较弱的硒···作用，瞬间释放构象限制，使分子协同弛豫至更加平整的构象，供受体二面角大幅减小至26.2°–34.5°。这种空间平面化显著增强了分子内共轭，使晶体发射光谱红移至633 nm，且由于受体间距增大解除了聚集猝灭，荧光量子产率骤增5倍至约30%。
Figure 2. (a) Fluorescence-mode optical microscopic image and (b) SEM image of rod-shaped crystals of 1. (c) Normalized absorption (black) and fluorescence spectra (blue) of rod-shaped crystals of 1. (d) Molecular packing of 1 within a rod-shaped crystal, illustrating distinct intermolecular interactions along different directions. Selected carbazole groups have been omitted for clarity to highlight the chalcogen bonding (right).
Figure 3. (a) Time-dependent fluorescence images illustrating the rapid propagation of thermofluorochromism from the ends of the crystal. (b) Changes in the fluorescence spectra of rod-shaped crystals of 1 measured at 20 °C (blue) and 41 °C (red). (c) Molecular packing of the ordered component (55%) of 1 in the rod-shaped crystals of polymorph β, highlighting different intermolecular interactions. Selected carbazole groups are omitted for clarity to reveal the absence of chalcogen bonding (right).
这种由热驱动的多晶型转变不仅响应迅速，还展现出极其优异的可逆性、构象恢复完整度以及耐疲劳性能。当晶体冷却至20 °C时，分子间硒···键会重新建立并锁定原有构象，使晶体结构在20秒内完全恢复，且在经历20次加热-冷却循环后仍未出现任何的光学性能衰减；同时，微观构象的协同平面化还成功转化为宏观机械运动，使晶体产生6.3%的轴向可逆伸长。更重要的是，得益于高温相下更强、更平整的分子内共轭网络，研究团队利用810 nm飞秒脉冲激光激发晶体时，发现其在41 °C下的双光子吸收截面最大可达约6700 GM，相比常温初始状态实现了4倍以上的剧烈增强。
Figure 4. (a) Time-dependent fluorescence images illustrating the thermofluorochromism propagating from the crystal end across the entire structure. (b) Changes in the fluorescence spectra of rod-shaped crystals of 1 measured at 41 °C (red) and 20 °C (blue). (c) Heating–cooling cycles induce pronounced, yet reversible, changes in the fluorescence intensity of the rod-shaped crystals. (d) Molecular packing of 1 in the α lattice obtained after cooling, highlighting different intermolecular interactions. (e) Under repeated heating–cooling cycles, the fluorescence emission maximum of the rod-shaped crystals changes in a reversible manner. (f) Powder XRD patterns of rod-shaped crystals of 2 were compared with those of the rod-shaped crystals of 1 in polymorphs α, β, and the α obtained upon cooling from β. (g) DSC heating–cooling cycles of rod-shaped crystals of 1. Three consecutive thermal cycles are presented. The DSC curves were recorded at a heating–cooling rate of 5 °C/min. Endothermic peaks are plotted upward.
Figure 5. (a) Two-photon fluorescence microscopic images and (b) two-photon fluorescence spectra of the crystals of polymorphs α (blue) and β (red) excited by a femtosecond pulsed laser at 810 nm. (c) Fluorescence intensity of the crystals of polymorphs α (blue) and β (red) as a function of femtosecond pulsed laser power (810 nm, 80 MHz, 140 fs pulsed duration). (d) TPA cross sections of the crystals of polymorphs α (blue) and β (red) at different excitation wavelengths of a femtosecond pulsed laser (80 MHz, 140 fs pulsed duration).
总结及展望
该研究成功阐明了一种基于弱相互作用控制的“触发-弛豫”分子内扭转调控机制。通过巧妙平衡分子固有的空间柔韧性与大尺寸功能基团的共轭特性，实现了在近室温温和刺激下对晶体发光效率、发射波长以及非线性光学响应的高效、快速且完全可逆的调控。这一研究成果不仅为开发下一代智能自适应光学材料提供了全新的设计范式，也为其在高分辨率双光子生物成像、微纳光驱执行器以及先进光电器件中的应用铺平了道路。