驱动非共价作用与超分子螺旋#

在超分子化学领域，构筑具有外部刺激响应性的螺旋结构一直是一项巨大挑战。大多数天然与人工螺旋体系依赖弱非共价作用力维持稳定性，但在外力刺激下容易发生结构坍塌。为了解决这一难题，研究人员将目光投向了硫属键（Ch chalcogen bonding）。与传统的氢键或卤键不同，二价硫属原子具有两个互相垂直的极性缺陷区，能够提供更为丰富的几何取向与结合位点。将光响应的偶氮苯单元引入到氢键芳香酰胺折叠体中，结合硫属键的各向异性作用，能够实现对螺旋结构的精细调控。

Scheme.1 The structures of compounds 1–3.

核心实验过程与发现#

研究团队首先设计并合成了较短骨架的模型化合物1。如图1a与图1b所示，单晶X射线衍射分析表明化合物1在固态下呈现稳定的全反式（E）构象。分子内部形成了四对对称的三中心氢键，极大地增强了芳香环之间的共平面性。如图1d与图1e所示，在365 nm紫外光照射下，化合物1发生E到Z的光异构化，异构化产物在溶液中自发卷曲形成螺旋构象。通过核磁共振氢谱定量分析，照射15分钟后E与Z构象的比例达到38.5<61>.5的光稳态。而在避光条件下，Z构象又可缓慢回复为能量更低的E构象。理论计算表明，E与Z构象之间的吉布斯自由能差值为15.8 kcal mol⁻¹，验证了E构象更高的热力学稳定性。

Fig.1 (a) Crystal structure of the trans-configuration of compound 1, with the hydrogen bond length data indicated. (b) Side view of the crystal structure of the E configuration of compound 1. (c) Z configuration of compound 1 simulated by a computer. (d) The photoresponsive interconversion between the E and Z configurations of compound 1 (irradiation with 365 nm UV light and subsequent dark storage), wherein the Z conformation adopts a helical conformation. (e) The 1H NMR spectrum of compound 1 exhibit distinct changes in response to light exposure and dark storage duration, reflecting the interconversion between the E and Z configurations. The relative ratio of these two configurations can be quantitatively determined based on 1H NMR (in CDCl3, 3.0 mM). (f) ΔG values of the E and Z configurations of compound 1. (The geometric structures of compound 1 were optimized at M06-2X-D3(0)/def-TZVP level of theory with Gaussian 16.)

为了进一步扩展骨架的空间伸展能力，研究团队合成了骨架更长的化合物2。如图2所示，化合物2表现出强烈的溶剂依赖性与光响应行为。在弱极性的氘代二氯甲烷中，分子内三中心氢键保持完好，紫外光照射后Z构象比例高达90.1%，形成稳定的螺旋结构。有趣的是，在极性强的氘代二甲亚砜中，溶剂竞争破坏了分子内氢键，使分子转变为线性（E）或U形（Z）构象。二维核磁共振谱图分析证实，通过改变溶剂极性与光照条件，化合物2可以在S形、螺旋形、线性以及U形四种构象之间灵活切换。此外，使用600 nm红光照射可以在15分钟内促使Z构象完全恢复为E构象，展现出极高的异构化效率。

Fig.2 (a) Molecular structure and schematic diagram of compound 2 in the E configuration in CD2Cl2, featuring intramolecular three-center hydrogen bonding (all the hydrogen atoms in the molecule were systematically numbered and highlighted). (b) The molecular structure and schematic diagram of compound 2 in the Z configuration in CD2Cl2 (5.0 mM), featuring intramolecular three-center hydrogen bonding. (c) Molecular structure and schematic diagram of compound 2 in the E configuration in DMSO-d6. (d) Molecular structure and schematic diagram of compound 2 in the Z configuration in DMSO-d6 (5.0 mM).

Fig.3 The 1H NMR spectrum of compound 2 shows distinct changes in response to light exposure and duration of dark storage, reflecting the interconversion between the E and Z configurations. The relative ratio of these two configurations can be quantitatively determined based on 1H NMR in CD2Cl2 (a) (5.0 mM) and DMSO-d6 (b) (5.0 mM).

Fig.4 2D ROE spectra of the E (a) and Z (b) configurations of compound 2 in DMSO-d6 (5.0 mM). (The signals corresponding to the same group of hydrogen atoms are indicated by circles of identical color.)

在化合物3的设计中，末端被替换为具有双重电子供受体特性的苯并硒二唑基团。如文章后续分析所示，末端硒原子与氮原子之间形成了强烈的分子间Se⋯N硫属键。这种协同非共价作用力驱动分子在溶液中高选择性地自组装成长程有序的超分子螺旋结构。UV-vis光谱与光化学动力学测试表明，化合物3在光照下的量子产率与异构化速率均受到硫属键作用的显著调控。分子动力学模拟与密度泛函理论计算进一步证实，双重Se⋯N交互作用为螺旋超分子的构象锁定提供了关键的能量补偿。

Fig.5 (a) Schematic potential energy profile for the E → Z photoisomerization in the ground (S0) and excited (S1) states. (b) Calculated free energies (G) and electronic energies (energy) of E/Z conformations in DCM. (The geometric structure of compound 2 was optimized at B3LYP-D3(BJ)/6-311G** level of theory using IEFPCM solvation model in dichloromethane. The vertical electronic excitation energies of the E, Z conformations and transition states were calculated using TD-DFT, and five singlet excited states were considered at B3LYP-D3(BJ)/6-311G** level of theory with IEFPCM solvation model in dichloromethane.52 ) Fig.6 UV-vis absorption spectra of compound 2 under various irradiation durations at 365 nm and 600 nm. (a) The spectra when dichloromethane was used as the solvent. (b) The spectra when DMSO was used as the solvent. (The regions at 400 nm and 500 nm have been specifically magnified for enhanced clarity. The sample concentrations of compound 2 in DCM and DMSO were all 1.1 × 10−6 mol L−1.) Fig.7 The 1H NMR spectrum of compound 3 shows distinct changes in response to light exposure and duration of dark storage, reflecting the interconversion between the E and Z configurations. The relative ratio of these two configurations can be quantitatively determined based on 1H NMR in CDCl3 (5.0 mM) (a) and a mixed solvent of CDCl3/DMSO-d6 at a volume ratio of 3/1 (b) (5.0 mM). Fig.8 (a) Crystal structure of compound 3. The bond length data of intramolecular three-center hydrogen bonds. (b) The Se⋯N interaction mechanism between molecules and the corresponding bond lengths (H atoms were omitted for clarity for (b)). Fig.9 (a) Photoresponsive interconversion between the E and Z configurations of compound 3, along with a schematic representation of the helical structure in the Z configuration. (b) Planar curved self-assembled structure in the crystal structure induced by intermolecular Se⋯N interactions in the E configuration. (c) Computer-simulated optimized supramolecular helix induced by intermolecular Se⋯N interactions in the Z configuration. (d) Self-assembly behavior guided by Se⋯N noncovalent interactions between E configuration in chloroform through molecular dynamics (MD) simulations. (e) and (f) The electrostatic potential and Gibbs free energy of the E and Z configurations of compound 3, as well as the bond energy of the Se⋯N ChB. (The obtained intermediates were fully optimized at the DFT level using the M062X-D3 and B3LYP functional.) (g) Self-assembly behavior guided by Se⋯N noncovalent interactions between the Z configuration in chloroform through molecular dynamics (MD) simulations. (h) ΔG values of the E and Z configurations of compound 3, as well as Se⋯N bond energy. (H atoms are omitted for clarity for (b) and (c). The intermolecular Se⋯N interactions are indicated within the red circle for (d) and (g). The geometric structures of compound 3 were optimized at M06-2X-D3(0)/def-TZVP level of theory with Gaussian 16. MD simulations: the molecules were optimized at the B3LYP level using the 6-31+G(d,p) basis set with Gaussian 16 program, where the solvent effects were corrected by applying the SDM method. The RESP charges were further optimized alongside the wave function using Multiwfn.) Fig.10 1H-DOSY NMR spectra of compound 3 at varying concentrations in CDCl3 (a) and in a mixed solvent of CDCl3/DMSO-d6 at a volume ratio of 3/1 (b).

意义与展望#

本研究展示了一种利用硫属键与氢键正交协同作用构筑光响应超分子螺旋的创新策略。该体系不仅实现了对超分子螺旋解螺旋与重卷曲过程的精准光调控，还揭示了非共价作用力在动态装配中的互补机制。这一发现为设计智能响应型材料、靶向分子识别载体以及动态人工生物模拟体系奠定了重要的化学基础。