动态共价键是一种受到热力学平衡调控，在外界刺激下能够发生断裂、生成或重组等反应的特殊化学键。相比于非共价相互作用和传统的共价键，动态共价键兼具良好的稳定性和响应性，因此已被广泛使用在超分子体系的构筑及动态材料的制备等领域。表界面是材料和环境相互作用最直接的场所，具有动态特征的表界面体系在自清洁、信息存储、微流控和生物传感等领域有重要价值。在过去的五年中，一系列的含硒动态共价键被陆续报道，它们包括二硒键、硒硫键、硒碲键以及硒氮键等等。其中二硒键在可见光辐照下即可发生交换反应，受制于热力学平衡该反应的转化率为50%。然而，我们对二硒动态化学的认识远远不足，对其在不同领域的应用也刚刚起步。本论文从单分子的尺度上研究为何二硒键具有温和的响应性，并将其独特的可见光响应性应用到表界面化学的领域，主要取得了以下两个方面的重要成果：一、从单分子水平揭示了二硒动态共价键具有响应性的原因，成功实现了二硒键动态平衡的调控。采用基于原子力显微镜的单分子力谱技术，测量了二硒键、硒硫键、二硫键的断裂力值分别为~1100 pN、1320 pN、1450 pN。从单分子的尺度上证明二硒键有力响应性，并解释了为何二硒键相比于二硫键能在更温和的条件下发生动态交换。进一步的，我们利用含有二硒键的两亲性高分子在水中组装形成囊泡结构，通过向水中加入电解质在囊泡内外形成渗透压作用力打断二硒键，实现了含二硒键囊泡的控制释放。将亲水及疏水二硒分子分别溶于水相及油相中，在可见光照射下利用油水界面张力的下降证明了二硒交换反应可以在界面处发生。亲疏水二硒交换生成两亲分子，可以自组装形成稳定的胶束结构，进一步的分离提纯即可实现对二硒动态平衡的调控。二、探索了二硒键在表界面化学领域的可能应用，包括二维材料的共价修饰以及温和、快速、可逆的表面修饰。利用二硒键的可见光及氧化还原响应性，实现对氧化石墨烯二维材料的温和共价修饰，所得的含硒氧化石墨烯材料具有在体外调控活性氧物种浓度的功能。同时，二硒动态化学可以发展成一种温和、快速、可逆的表面修饰手段。通过将二硒键修饰在石英等基底表面，利用可见光辐照下二硒分子之间的交换反应，赋予表面不同的化学特征。基于该方法可以实现浸润性调控、表面图案化、可见光驱动的液体输运过程以及生物偶联反应。
Dynamic covalent bond (DCB) is a type of chemical bond that undergoes cleavage, formation and metathesis under external stimuli modulated by thermodynamic equilibrium. DCB is both stable and stimuli responsive compared with non-covalent interactions and classic covalent bonds and has thus been extensively used in research fields like supramolecular chemistry and dynamic materials. Surface/interface is the frontier between material and external environment. Surfaces/interfaces with dynamic features have found important applications in fields like self-cleaning, information storage, microfluid and biosensing. In the past five years a series of selenium-containing DCBs have been reported, including diselenide bond, selenide-sulfide bond, selenide-telluride bond and selenide-nitrogen bond, etc. Among them diselenide bond is unique since it can undergo exchange reaction simply by visible light irradiation without any catalysts. The metathesis reaction reaches a 50% exchange ratio due to the thermodynamic equilibrium. However, our understanding of dynamic diselenide chemistry is limited, so is its applications in different fields. In this thesis, the reason of diselenide bond’s mild responsive behavior was studied in the single molecule level, and its unique properties were applied in surface/interface chemistry. This work mainly consists of two parts as listed below:(1) Revealed the reason behind the responsiveness of diselenide dynamic covalent bond at the single molecule level and successfully modulated the dynamic equilibrium of the diselenide bond. By employing Atomic Force Spectroscope based Single Molecule Force Spectroscopy, the rupture forces of SeSe, SS and SS bonds were measured to be ~ 1100 pN, 1320 pN and 1450 pN, respectively. This result indicated that SeSe is force responsive and implied the reason why SeSe bond undergoes exchange in a milder condition compared with SS bond. Additionally, we synthesized a diselenide bond containing amphiphilic polymer that can self-assemble to form vesicles in aqueous solution. Adding electrolytes in water could apply osmotic force between two sides of the vesicles’ shell, which could break the diselenide bond and led to a controlled release manner. Hydrophilic and hydrophobic diselenide molecules were dissolved in water and oil phase respectively. When giving visible light irradiation, the decline of the oil/water interface tension illustrated that diselenide metathesis could occur at the interface. The obtained exchange product was amphiphilic and could self-assemble into stable micelles in water. Further isolation and purification of the system could lead to the modulation of the dynamic equilibrium.(2) Investigated the potential applications of diselenide bond in surface/interface chemistry, including covalent functionalization of two dimensional (2D) materials and a mild, fast, reversible surface modification. The mild covalent modification of 2D material graphene oxide (GO) was achieved by the visible light and redox response of diselenide bond. The obtained selenium-containing GO could modulate the concentration of reactive oxygen species in vitro. Additionally, we developed the diselenide dynamic chemistry into a mild, fast and reversible surface modification method. After modifying diselenide bond on substrates like quartz, we could endow surface with different functions by exchanging with different diselenide molecules. This method could achieve wettability adjustment, surface patterning, light driven liquid motion and bioconjugate reaction.
