Construction and Photo-Responsiveness of Cyanostilbene-Based Supramolecular Polymers
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摘要: 通过在氰基二苯乙烯基元上引入氢键识别位点，驱使单体分子之间产生氢键与π-π堆积作用力的高效加合，构建了具有“成核-链增长”机制的超分子聚合物。通过获取组装热力学参数，揭示了酰胺及脲等不同氢键结构对于超分子聚合行为的影响规律。进一步利用超分子聚合态对于氰基二苯乙烯基元的限域效应，光照条件下特异性获得[2+2]环加成产物。与之相比，单体态时氰基二苯乙烯基元倾向于发生Z/E光异构化反应。这一研究结果可为多路径光化学反应的高效调控提供新的思路。Abstract: The nucleation-elongation cooperative mechanism, as widely adopted by natural systems such as microtubules and actin filaments, exhibits superior properties for supramolecular systems in comparison to the isodesmic mechanism by avoiding the formation of short-ranged oligomers. In this study, a novel type of artificial supramolecular polymers with the nucleation-elongation mechanism have been constructed, by incorporating hydrogen bonding recognition motifs adjacent to the cyanostilbene core. It facilitates the combination of hydrogen bonds and π-π stacking interactions between the neighboring monomers in a synergistic manner. The different hydrogen bonding units (urea versus amide) exert crucial impacts on the supramolecular polymerization behaviors, which can be qualified by the thermodynamic parameters of the self-assembly processes. Furthermore, supramolecular polymerization imparts confinement effect to the cyanostilbene unit, giving rise to the specific formation of [2+2] cycloaddition products under 430 nm photo-irradiation. It is in stark contrast to that in the monomeric state, which undergoes Z/E photoisomerization reactions under the same conditions. Overall, the current study provides new insights to modulate multi-path photochemical reactions in a precise manner.
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图 2 单体1、2的超分子聚合及光化学行为示意图
Figure 2. Supramolecular polymerization of monomers 1, 2, together with the distinct photo-chemical behaviors
图 3 单体1在氯仿（黑线）和甲基环己烷（红线）中的（a）紫外-可见光谱,（b）荧光发射光谱，（c）CD光谱（图3（b）中的插图显示单体1在365 nm紫外灯下的荧光发光颜色：氯仿溶液（左）和甲基环己烷溶液（右））
Figure 3. (a) UV-Vis, (b) fluorescent emission, and (c) CD spectra of monomer 1 in CHCl3 (black lines) and MCH (red lines) (Inset of Fig.3(b): the fluorescent luminescence colors of monomer 1 under a 365 nm UV lamp in CHCl3 (left) and MCH (right))
图 4 单体1在甲基环己烷中随温度变化的（a）UV-Vis光谱和（b）CD光谱；（c）单体1和2的熔融曲线图（通过监控353 nm处CD信号强度随温度（降温速率为60 ℃/h）的变化并进行归一化处理）;（d）单体1和2的理论聚合度（图4（b）中的插图显示单体1在293 K时的超分子凝胶化行为）
Figure 4. Temperature-dependent (a) UV-Vis and (b) CD spectra of 1 in MCH; (c) Melting curves of monomer 1 and 2 (By monitoring temperature (cooling rate: 60 ℃/h)-dependent CD signal change at 353 nm); (d) <Nn> of monomers 1 and 2 (Inset of Fig.4(b): photograph of supramolecular gel derived from monomer 1 at 293 K)
图 5 （a）氯仿和（b）甲基环己烷中单体1在430 nm LED光照下（c=0.02 mmol/L，293 K）的紫外-可见光谱；（c）单体1和2在甲基环己烷中发生[2+2]环加成反应的归一化光照（λ=270 nm，c=0.02 mmol/L，293 K）动力学曲线；（d）单体1及其光照产物的核磁共振氢谱：（上）光照前的单体1、（中）氯仿溶液中的光照（t=10 min，c=2.00 mmol/L，293 K）产物和（下）甲基环己烷溶液中的光照（ t=60 min，c =1.00 mmol/L，293 K）产物
Figure 5. UV-Vis spectra of monomer 1 (c=0.02 mmol/L, 293 K) upon 430 nm light irradiation in (a) CHCl3 and (b) MCH; (c) Normalized absorption changes of monomer 1 and 2 upon light irradiation in MCH (λ=270 nm, c = 0.02 mmol/L，293 K); (d) 1H-NMR spectra of (top) monomer 1, (middle) irradiation sample of monomer 1 in CHCl3 (irradiating condition: t=10 min, c = 2.00 mmol/L, 293 K) and (bottom) monomer 1 in MCH (irradiating condition: t=60 min, c = 1.00 mmol/L, 293 K)
表 1 单体1和2在甲基环己烷中的超分子聚合参数
Table 1. Thermodynamic parameters for supramolecular polymerization of monomers 1 and 2 in methylcyclohexane
Monomer Te /K △He /(kJ·mol−1) △Se /(J·mol−1·K−1) △Ge /(kJ·mol−1) (T=293 K) <Nn> (T=293 K) 1 315 −74 −147 −30.9 627 2 344 −139 −316 −46.4 1372 
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