氧化还原对Lindqvist型多金属氧簇复合物自组装的动态调控

Redox-Regulated Dynamic Self-Assembly of a Lindqvist-Type Polyoxometalate Complex

自组装的动态调控对材料科学的发展至关重要,它在智能材料和器件的开发中具有巨大的潜力。多金属氧簇是一类纳米尺度的无机功能簇,尤其是当它被有机组分共价修饰或者非共价修饰之后,已经发展成为超分子自组装中优异的构筑基元之一。多金属氧簇是典型的刺激响应功能团簇,光化学或电化学方法可将其还原为混合价态,即使经历了逐步的多电子氧化还原后多金属氧簇的结构仍保持不变。多金属氧簇独特的光致变色使其表现出不同的光物理性质,这激励着我们去开发基于多金属氧簇复合物的动态自组装。Lindqvist型六钼酸盐簇[Mo6O19]2-是负电荷数目最少的多金属氧簇之一,是构筑新颖组装结构的理想构筑基元。本文在乙腈溶剂中用十八烷基三甲基铵替换Lindqvist型多金属氧簇复合物(TBA)2[Mo6O19]的抗衡离子,通过简单的替换方法制备了一种单链表面活性剂包覆的多金属氧簇复合物(ODTA)2[Mo6O19]。通过核磁共振氢谱、红外光谱、热重分析和元素分析表征了多金属氧簇复合物的结构。复合物(ODTA)2[Mo6O19]在乙腈和异丙醇(体积比是4比1)中的溶液在经过交替的紫外光照和空气氧化后表现出了可逆的光致变色性质。复合物的溶液经紫外光照后由浅黄色快速变成蓝色。紫外可见吸收光谱在751 nm处出现的宽吸收带归属于MoV→MoVI的价间电荷转移跃迁,这表明还原态多金属氧簇的形成。经过空气氧化之后蓝色褪去。这种可逆的光致变色可以多次循环实现。扫描电镜、透射电镜和X射线衍射的测试证实了复合物在乙腈和异丙醇中形成了螺旋形貌的组装体。更有意义的是,复合物在光致变色的同时发生了从螺旋条带到球形组装体的形貌转变。光致变色过程中的形貌演变是从短螺旋条带经过海胆型聚集体变为球形组装体。最重要的是,经过空气氧化后螺旋组装体可以再次恢复,表明这是由氧化还原刺激驱动的可逆形貌转变。X射线光电子能谱和核磁共振氢谱的测试表明,氧化还原调控的可逆自组装是由无机簇与有机阳离子之间静电吸引力和无机簇之间静电排斥力的变化所驱动的。此研究结果将有助于更好地理解动态自组装的机理,并有助于推动先进智能材料的精准制备。

Dynamic regulation of self-assembly is of vital importance in chemistry,biology and material science thanks to its great potential for development of smart materials and devices.Polyoxometalates(POMs)are a class of functional inorganic nanoclusters,which has become one of the excellent building blocks for supramolecular self-assemblies,especially when covalently or non-covalently modified by organic species.As typical stimuli-responsive functional clusters,the POMs could be photochemically or electrochemically reduced to mixed-valence states,of which the structural integrity remains even after encountering stepwise multi-electron redox process.The intriguing photochromism of the POMs in different states exhibits distinct photophysicalproperties,which motivates us to exploit the dynamic self-assemblies of POM-based complexes.The divalent Lindqvisttype hexamolybdate cluster[Mo6O19]2-is one of the least negative-charged POMs,which is the ideal building blocks to construct novel assembly structures.Based on this motivation,herein,a single chain surfactant-encapsulated polyoxometalate(POM)complex(ODTA)2[Mo6O19]was prepared by simple counterion replacement of Lindqvist-type(TBA)2[Mo6O19]with octadecyltrimethylammonium(ODTA)in acetonitrile solution.The structure of the POM complex was confirmed by 1 H nuclear magnetic resonance(NMR),Fourier transform infrared(FT-IR)spectroscopy,thermogravimetric analysis(TGA)and elemental analysis.The solution of complex(ODTA)2[Mo6O19]in the mixed solvents of acetonitrile and isopropanol with the volume ration of 4 to 1 exhibited reversible photochromism upon alternate UV light irradiation and air exposure.Upon UV light irradiation,the light yellow transparent solution of(ODTA)2[Mo6O19]turned into blue quickly.The new broad absorption band appearing at ca.751 nm assigned to the MoV→MoVI intervalence charge-transfer(IVCT)transition,indicated the formation of reduced POM,as revealed by UV-Vis absorption spectra.After exposed to air,the blue solution was bleached.The alternate photochromism could be conducted for multiple cycles.Helical self-assembled morphology of(ODTA)2[Mo6O19]was formed in acetonitrile/isopropanol,characterized by scanning electron microscope(SEM),transmission electron microscopy(TEM)and X-ray diffraction(XRD)methods.More interestingly,morphology transformation of the complex from helical strips to spherical assemblies occurred accompanied by photochromism occurrence.The morphology evolution during the photochromism process experienced from shortened helical strips through sea urchin-like aggregates to spherical assemblies.Most significantly,the helical assemblies could be recovered again after air oxidation,implying the reversible morphology transformation driven by redox stimulus.The redox-modulated reversible self-assembly is driven by the variation of electrostatic attraction between organic cations and inorganic anions as well as the electrostatic repulsion between inorganic ionic clusters,proved by X-ray photoelectron spectroscopy(XPS)and 1 H NMR spectra.The results will contribute to better understanding the mechanism of dynamic assemblies and inspire the precise fabrication of advanced smart materials.