研究背景与科学问题#

在宏观世界中，人类通过将一维纤维交叉编织成二维网络或三维布料，能够显著改变材料的机械强度与物理特性。将这种编织概念引入到分子或纳米尺度，构筑具有长程有序结构的分子纺织品，是化学与材料科学领域长期追求的目标。然而，要在无机或有机高分子体系中精确驱动单条分子链发生自发纠缠与定角编织，依然面临着巨大挑战。

目前大多数共价有机框架或聚合物晶体依赖于氢键、π-π堆积或金属配位等作用力。这些作用力往往缺乏足够的强定向性与多功能协同调控能力。为了解决分子链间缺少有效“分子摩擦力”以及缺乏定角编织驱动力的问题，研究团队提出引入给体-受体（D-A）交替序列。利用电子富集与电子匮乏单元之间的强相互作用作为分子摩擦点，配合分子链连接处特定扭转角的设计，成功诱导了分子链的有序编织。

Fig.1 (a) Schematic illustration of the similarity between crystal and weave structures. (b) HOMO and LUMO energy level diagrams for BACT, p-TCDI, m-TCDI, and p-MeTCDI. (c) Schematic representation of a single chain with alternating D–A units.

核心实验过程与发现#

研究团队首先基于B←N配位键构建了交替包含给体与受体单元的单链结构。给体部分选用富电子的BACT单体，受体部分则选用包含不同扭转接头的萘二酰亚胺（NDI）衍生物p-TCDI、m-TCDI与p-MeTCDI。理论计算表明，这种组合在分子链上形成了明显的HOMO和LUMO空间分离，建立了良好的电子传递基础，如图1所示。随后，研究人员采用溶剂热法成功制备出了具有厘米级尺寸的高质量单晶。

晶体结构分析揭示了扭转角对分子编织拓扑的决定性作用。如图2所示，在p-BNF晶体中，两条呈之字形构象的聚合物链以87.9°的交叉角相互交织，分别充当经线和纬线，借助链间D-A-D三元夹心相互作用，构建出双轴三维编织拓扑结构。相比之下，m-BNF和p-MeBNF则表现为二维片层堆积。m-BNF内部的儿茶酚给体单元上下交错并与相邻链的受体结合，形成厚度为10.64 Å的二维单层。而p-MeBNF因亚甲基接头产生了98.49°的平面外扭转，促使分子链形成“榫卯”状互锁结构。

Fig.2 Crystal structure of p-BNF viewed along the c-axis (a), corresponding simplified diagram (b), and structure along the a-axis (c). 2D monolayer of m-BNF shown down the b-axis (d), corresponding simplified diagram (e), and inter-layer packing mode (f). Single-layer chain–chain mortise-tenon structure of p-MeBNF (g), corresponding simplified diagram (h), and 2D layer stacking (i). Color code: B pink, C cyan, N blue, O red, H white. Adjacent chains or layers are color-coded for clarity. In the simplified diagrams, acceptor and donor units are drawn as blue rectangles and red circles, respectively, B←N bonds are marked by orange dots, and the remaining backbones are lines (dashed for those in the lower layer). The D–A–D sandwich structure is highlighted by a purple box.

为了直观验证这种分子级别的编织模式，研究人员利用低剂量集成微分相衬扫描透射电子显微镜（iDPC-STEM）进行了实空间观测。如图3所示，图像清晰地展现了高结晶度且无缺陷的超晶格阵列。p-BNF呈现出由经纬链交织构成的菱形网格，而m-BNF与p-MeBNF则展现出平行的之字形亮带，这与单晶衍射模拟的电势图高度吻合。控制实验进一步证实，若移除受体单元或替换为非D-A对照组，体系仅能得到无定形产物，这充分证明了D-A相互作用是驱动分子链交织的核心动力。

Fig.3 Low-dose iDPC-STEM images of (a) p-BNF along [101] direction, (b) m-BNF along [110] direction and (c) p-MeBNF along [001] direction (insets: raw images). Enlarged false-colored images of (d) p-BNF, (f) m-BNF, and (h) p-MeBNF, respectively. Simulated projected potentials: (e) p-BNF, (g) m-BNF, and (i) p-MeBNF superlattices (insets: ideal superlattice structural models from SCXRD). (j) Schematic illustration of experiments involving acceptor-unit replacement and acceptor-unit removal. (k) PXRD patterns of p-BNF crystals after post-synthetic exchange competition experiments. Fig.4 (a) Simplified diagram of D–A sandwich: red, donor; blue, acceptor; grey, COP backbone. Schematic D–A sandwiches in (b) p-BNF, (c) m-BNF, and (d) p-MeBNF, respectively. IGMH scatter plots for (e) p-BNF, (f) m-BNF, and (g) p-MeBNF (insets: corresponding IGMH isosurfaces). Hirshfeld 2D fingerprint plots of the NDI unit in (h) p-BNF, (i) m-BNF, and (j) p-MeBNF, with D–A interaction regions density-colored. (k) UV–vis absorption spectra, (l) band structure and (m) PDOS for p-BNF, m-BNF, and p-MeBNF (insets: photographs of COPs powders).

光电与机械性能测试表明，不同的编织拓扑对材料特性产生了深远影响。如图4所示，D-A单元在三维空间中的重叠程度直接决定了电荷转移能力。p-MeBNF由于具有最短的3.34 Å层间堆积距离以及1.36 eV的低光学带隙，展现出最高的电导率。在机械性能方面，纳米压痕实验与密度泛函理论计算表明，采用三维交叉编织拓扑的p-BNF拥有高达1.38 GPa的杨氏模量。正如如图5所示，这种经纬交织的结构能够在受到外力挤压时实现高效的应力分散，表现出优异的抗变形能力。

Fig.5 (a) 3D excitation-emission matrix for p-BNF, m-BNF, and p-MeBNF. (b) Comparison of the kinetics for p-BNF, m-BNF, and p-MeBNF probed at 606 nm. fs-TAS of (c) p-BNF, (d) m-BNF, and (e) p-MeBNF. NAMD simulations of electron and hole decay kinetics of (f) p-BNF, (g) m-BNF, and (h) p-MeBNF.

Fig.6 2D representation of the SPM images of the residual indent impressions for (a) p-BNF, (b) m-BNF, and (c) p-MeBNF. (d) The corresponding height profiles along the line shown in the SPM images. (e) P–h curves for p-BNF, m-BNF, and p-MeBNF at a fixed load (5 mN). (f) Continuous hardness evolution of p-BNF, m-BNF, and p-MeBNF during PUM tests. (g-i) In situ nanoindentation-Raman spectra of p-BNF. Calculated spatially dependent linear compressibility for (j) p-BNF, (k) m-BNF, and (l) p-MeBNF in the xz plane. Positive and negative linear compressibility are represented by green and red, respectively.

总结与展望#

该工作成功开发了一套基于D-A配对与扭转角调控的模块化分子编织策略，从实验和理论层面阐明了分子编织拓扑与宏观物理性质之间的构效关系。研究不仅实现了共价有机聚合物晶体的高高效构筑，更为后续设计高强度机械材料、柔性电子器件以及光电功能材料提供了全新的分子层面的设计思路。未来，基于这一拓扑调控概念，研究人员有望将更多功能性发光基团或催化活性位点引入编织网络中，进一步拓展分子纺织品在能源转化与智能响应材料领域的应用场景。