【JACS】27%晶格巨幅伸长！飞秒激光无标记成像新宠：扭曲弹性驱动的分子马达级智能晶体#

Figure 1. Mechanistic classification of thermosalient molecular crystals. Crystals (1–29) are grouped by a dominant structural transformation pathway. The reported crystal elongation (Δ_L/L_0 × 100, %) is summarized alongside the corresponding molecular structures. References are provided in the Introduction.#

Figure 2. Outline of the chemical structure, polymorphs, and phase transition of αDDDCS. (a) Chemical structure of αDDDCS, consisting of a twist-elastic π-conjugated backbone and adaptive alkoxy side chains. Here, α denotes cyano substitution at the olefinic carbon adjacent to the inner phenyl ring. (b) Schematic conformations and packing motifs of the three enantiotropic polymorphs: the yellow-emissive phase (Y-phase) adopts a nearly planar backbone with a π-stacked packing, the blue-emissive phase (B-phase) features a twisted backbone arranged in a μ-herringbone (μ-HB) motif, and the cyan-emissive phase (C-phase) consists of a slightly planarized twisted backbone in a μ-HB motif. (c) Fluorescence images of Y-, B-, and C-phase crystals and a schematic phase map summarizing their interconversion: Upon heating, the Y- and B-phases transform into the high-temperature C-phase with large (+27%) and small (−1.2%) thermoelastic strain, respectively. Notably, mechanical input can trigger a B/C → Y conversion.#

Figure 3. Thermally driven phase transitions among the three polymorphs of αDDDCS. (a) DSC traces of Y-phase crystalline powders showing a first-order transition upon heating (TY → C: 94.8 °C) and cooling (TC → B: 66.7 °C). (b) VT-FM images capturing pronounced deformation during the Y → C transition. (c) DSC traces of the B-phase crystalline powders showing two first-order transitions upon heating (TB → C: 71.5 °C, TY → C: 87.5 °C). (d) VT-FM images capturing minimal deformation and transient C–Y coexistence during the B → C transition. (e,f) Variable-temperature powder X-ray diffraction (VT-PXRD) patterns of the B- and Y-phase crystalline powders, respectively.#

为了厘清产生这种超大形变差异的结构本质，研究团队利用同步辐射单晶X射线衍射（SCXRD）对室温结构进行了高精度解析。如图4所示，B相中的分子呈现高度扭曲的分子构象，这种畸变排斥了高效的面对面$\pi$-重叠，转而稳定了具有横向偶极网络相互作用的$\mu$-鱼骨状堆积。与此形成鲜明对比的是，Y相中的分子骨架几近完全平面化，从而能够形成紧密的、滑移式的高效$\pi$-堆积通道，其核心层间距缩短至3.37-3.40 Å。结合光学图谱分析（如图5所示），这种从强扭曲到高平面化的构象跃迁不仅完美解释了高达0.39 eV的超大荧光色移，也证实了Y相到C相的相变伴随着从$\pi$-堆积到$\mu$-鱼骨状堆积的彻底转换，这正是产生27%巨幅伸长的结构根源。

Figure 4. Molecular and packing structures of B- and Y-phase crystals. (a) Twisted geometry in the B-phase (θi = −152.4(6)°, θo = −145.7(6)°) and nearly planar geometry in the Y-phase (θo,1 = −179.5(5)°, θi,1 = 174.0(5)°, θi,2 = −175.7(5)°, θo,2 = 178.9(5)°). (b–d) μ-Herringbone (μ-HB) packing motif of the B-phase: (b) vertically oriented μ-HB arrangement, (c) edge-to-face contacts (C···H, 2.93 and 2.96 Å) in the x-slip (Δ_x_ = 0.56 Å) view, and (d) herringbone angles in the y-slip (Δ_y_ = 3.37 Å) view (central ring: 55.2°, terminal ring: 70.0°) with C–N···H–C interactions (2.53, 2.67 Å). (e–g) π-Stack packing motif of the Y-phase: (e) vertically oriented π-stack arrangement, (f) short π-stacking distance (3.37–3.40 Å) with minimal x-slip (Δ_x_ = 0.05 Å), and (g) C–N···H–C interactions along [−210] with a y-slip (Δ_y_ = 2.07 Å). Intermolecular packing parameters of the B- and Y-phases (e.g., x- and y-slips) are illustrated and summarized in detail in Figure S12 and Table 1#

Figure 5. UV–vis absorption (dotted lines) and photoluminescence spectra (solid lines) of the B-, Y-, and C-phases (λex = 365 nm (for B and C), 420 nm (for Y)).#

为了揭示该相变在原子尺度的动态演变过程，研究人员通过变温选区电子衍射（SAED）与分子动力学（MD）模拟进行了深度剖析。如图6和图7所示，在加热触发下，Y相密堆积晶格首先发生预相变弹性软化。当热摆动累积超过临界点时，骨架发生扭曲解锁。分子并非随机乱动，而是通过C–N$\cdots$H–C网络诱导，在层内向同一方向协同扭曲，而在相邻层间反向旋转，最终锁定在高度有序的$\mu$-鱼骨状拓扑 registry 中，其长烷基侧链则自适应转变为全反式构象以消除位阻。此外，这种由扭曲弹性主导的模式转换在变温机械探针测试中展现出独特的双重力致变色与机械跳跃特性。如图8所示，在特定温度窗口内实施机械剪切，能够突破动力学能垒，成功激活反向相变，促使扭曲晶格自主传播重组为平面化的Y相，首次实现了扭曲弹性的力致跳跃自维持传播。

Figure 6. Thermoelastic transitions and deformation behavior of αDDDCS crystals. (a) Lattice orientation of the B-phase from SAED; the inset shows the corresponding transmission electron microscopy (TEM) image. (b) Fluorescence micrographs of B- and C-phase crystals. (c) Molecular packing structure viewed along the a-axis, with habit planes and lattice directions assigned from SAED and SCXRD. (d) Lattice orientation of the Y-phase from SAED; the inset shows the corresponding TEM image. (e) Fluorescence micrographs of the Y → C → B transition. (f,g), Molecular packing structures of the Y- and B-phases viewed along the c- and a-axes, respectively. Habit planes and lattice vectors are assigned based on SAED and SCXRD.#

Figure 7. MD simulation of the Y → C transition mechanism. (a) Structural snapshots capturing a stable π-stack at NVT (0 ps), followed by stability breakdown at the onset of NPT (30 ps) and formation of a layer-correlated μ-herringbone arrangement (130 ps). (b) Changes in π-stacking and secondary stacking distances. (c) Mean and standard deviation of θo,top. (d) Distributions of θo,top for 64 molecules. (e) Individual θo,top traces for 64 molecules. (f) Dihedral angle distributions of 128 side chains. (g) Model structure of steric clash caused by a gauche side chain in the μ-herringbone motif (dotted circle). (h) Side-chain conformations illustrating how gauche side chains avoid steric clash.#

Figure 8. Mechanically induced transitions of αDDDCS crystals. (a) Schematic Gibbs free energy diagram of the αDDDCS polymorphs, constructed from DSC data and computational analysis. The gray region denotes the temperature range where B → Y or C → Y transitions are expected to be mechanically accessible. (b) Mechanical probing test on αDDDCS crystals under varying temperatures. Mechanically induced transitions were observed in the B-phase at 60–70 °C and in the C-phase at 75–85 °C. Regions where the Y-phase locally formed at 60 and 85 °C are marked with dotted circles. (c) At 70 °C, the B → Y transition is initiated by mechanical probing-induced nucleation along the [010] direction and autonomously propagates along the [001] direction. The observed deformation, calculated as Δ_L_/_L_0 × 100, is −22.7%, smaller than the +29% value for the Y → B transition, due to the difference in the reference length _L_0: in this case, taken from the B-phase rather than the Y-phase. (d) At 80 °C, the C → Y transition displays a ferroelasticity-like response, wherein Y-phase formation occurs only within regions subjected to mechanical shear.#