杂化大阴离子簇自组装及光致组装结构转变

Self-assembly and Photo-triggered Structural Transformation of Hybrid Polyanionic Clusters

通过将具有光响应性的香豆素基团共价接枝到盘形多金属氧簇两侧,构筑了一类可光控聚合的杂化大阴离子簇单体.利用X-射线单晶衍射、核磁共振氢谱、红外光谱、元素分析和电喷雾质谱等手段对所获得的杂化物和组装体中的光照聚合过程进行了表征.通过离子替换,将抗衡离子由四丁基铵阳离子替换为具有更强自组装能力的双十八烷基二甲基铵阳离子.利用香豆素基团在不同紫外光照射条件下的光二聚反应,以及外围表面活性剂提供的疏水性,实现了超分子组装体中的聚合.通过研究亦发现,在光控二聚过程中,聚合物不仅展现新颖的自组装行为,而且表现出聚合诱导的荧光增强.

To realize the in situ modulation of self-assembled structure by photo coupling reactions, nanosized clusters were prepared through a covalent grafting of photoactive coumarin and methylcoumarin connecting with triol group onto both sides of a disk-like [MnMo6019]3- in one-pot reaction, where tetrabutylammonium （TBA） or dioctadecyldimethylammonium （DODA） served as the counterions. The chemical composition and photo polymerization of the obtained hybrids were characterized by elemental analysis, 1H-NMR, FTIR, electrospray ionization-mass spectra （ESI-MS） and single-crystal X-ray diffraction analysis. Because of the synergistic interaction of organic and inorganic components, the hybrid monomers displayed spherical assembly when TBA was acting as the counterion, and flake assembly was observed when DODA served as the counterion of the inorganic cluster. The detailed characterization for the self-assembled structure in the case of DODA indicated the layered sub-structure in organic solution： That is, the organic surfactant component self-assembled into a fully interdigited bilayer with the cationic head locating toward outside of the bilayer while the coumarin grafted inorganic cluster polyanions located between two bilayers. The geometric structure of grafted cluster and spectral measurement revealed a close packing of the photoactive groups. Interestingly, the favorable packing structure made the in situ photoreaction become convenient. Due to the ring coupling occurred in adjacent coumarin groups and the space blocking of polyanionic clusters, the preferred ring addition conformation tended to the linear growth and finally formed polymers in the inner layer of the assembly. Though the present study could not give a strict molecular weight, the coupling degrees were estimated to be in 60% - 68%, depending on the organic counterions used. Because of the photo reaction occurring in inner layer, the layered structure was still maintained yet the phase separation between polymerized and non-polymerized domains was observed by transmission electron microscope （TEM）. Accompanied by the in situ polymerization, a fluorescence enhancement, ascribed to the coupled coumarin lying in limited space, was found. The present study demonstrated an effective method to construct polyoxometalate based supramolecular polymers in self-assembled systems.