Multi-responsed Azobenzene Based Polyoxometalate Complex Self-assembly

The term “self-assembly” refers to the spontaneous formation of ordered structures from basic structural units(molecules and nano-/larger-scale materials)that is mostly governed by non-covalent interactions and affords stable and regular geometric structures(e.g.,vesicles,micelles,tubes,and helices)widely used as components of molecular devices,highly efficient catalysts,and medicinal/biomaterials.However,the manipulation of building blocks to construct the desired assemblies(e.g.,for building smart materials)is still faced with significant challenges,even though a number of host-guest interaction–based techniques for precise building block assembly/disassembly regulation have been established.Cyclodextrins(CDs)are cyclic oligosaccharides extensively employed as host molecules in supramolecular chemistry that can interact with and recognize guest molecules such as adamantane,azobenzenes,and ferrocene by engaging in hydrogen bonding,hydrophobic,and van der Waals interactions.Importantly,host-guest interactions endow supramolecular assemblies with the capacity of self-healing and dynamic reversibility,which can be applied in the construction of handily functional materials.For example,Zhang et al.connected guest molecules with organic molecules to prepare p H-and photosensitive materials,while Tian et al.reported the construction of polymeric host-guest materials.Most of the known host-guest complexes are based on organic and polymer systems,while the integration of inorganic clusters is still underexplored.Hence,we intend to select inorganic clusters with uniform sizes and definite structure to combine with guest molecules for researching the effect of host-guest interactions and stimuli on the assembly/disassembly of the above compound in aqueous solutions.Polyoxometalates(POMs),inorganic nanoclusters featuring a broad range of compositions and multifarious topological architectures,can be applied in the fields of redox reactions,catalysis,and medicinal/biological material fabrication,and have therefore attracted much attention over the past two decades.Functionalization of POMs with organic moieties not only results in solubility changes,but also increases the stability of the resulting hybrid complexes,which exhibit a wealth of architectures and feature distinctive properties determined by the nature of organic components.Importantly,the synergy between organic and inorganic components significantly broadens the scope of POM applications in biomedicine and engineering.Proust et al.first used guest group grafting to the POM,but this approach ignores the dynamic assembly behavior of the organic-inorganic hybrid complex.Hence,we designed and synthesized a novel hybrid POM complex(POM-azo)by grafting an azobenzene moiety onto a lacunary Keggin-type POM followed by ion exchange on a silica gel column to increase the solubility of the amphiphilic hybrid complex in aqueous media and formed vesicle-structured assemblies.The addition of ?-or ?-CDs destroyed the amphiphilicity of POM-azo,resulting in the formation of assemblies with two hydrophilic ends and thus inducing a vesicle ? monodispersed state transition.Moreover,whereas the trans-azobenzene moiety strongly interacted with ?-CD,this interaction was weak or non-existent in the case of the cis-isomer,and the abovementioned host-guest interaction could therefore be controlled by light-induced trans-cis isomerization of azobenzene.Specifically,UV light irradiation promoted the monodispersed state ? vesicle transition,whereas the reverse process was observed upon visible light irradiation,with similar behavior observed when ?-CD was substituted for ?-CD.Disaggregation/aggregation control was also realized via the addition of adamantanecarboxylic acid(Ad-COOH)as a competitive guest to the POM-azo/?-CD system,which led to the restoration of the primary vesicle structure of POM-azo.Thus,host-guest interaction control allowed us to precisely regulate the assembly/disassembly of building blocks,and the developed technique based on multiple stimuli responsiveness was shown to be potentially applicable in the fields of drug loading,sensing,and functional nanodevice fabrication.