Photo-responsive Performance Of Azobenzene-based Ionic Liquids In Solution

As a new type of green media and functional materials,ionic liquids?ILs?have been developed with an exponential explosion of interest in the past two decades.The most attractive properties of ILs are their tuneable physical and chemical properties with a judicious design of the structures of cations and anions.Thus,introducing stimuli-responsive groups into ILs would generate more attractive functional ILs for particular applications.By applying external stimuli,the structure,property and performance of the"smart"ILs may have a significant change,they are expected to have potential applications in micro-fluid devices,phase transfer catalysis,etc.As an important type of external stimulus,light trigger has attracted great attention of the scientists,because light can be remotely controlled and rapidly changed in a clean and non-invasive manner.At present,researches on photo-responsive ILs are still in its infancy.We have not yet had a deep understanding of the photo-responsive performance,interactions between ILs and solution,and the solvation states of cations and anions.In this work,therefore,a serious of photo-responsive ILs have been designed and synthesized by introducing high medium sensitive and reversible azobenzene structure into cations of ILs.The key factors affacting photo-regulated aggregation behavior,photo-induced conductivity enhancement,and photo-triggered reversible phase transfer were investigated in detail,and stable photo-responsive Pickering emulsions consisting of azobenzene-based ILs was constructed and used for highly efficient catalytic hydrogenation.The results are expected to provide new insights into the relationships of molecular structure,salvation state and light responsive performance of the azobenzene-based ILs.The main contents are as follows:?1?4 kinds of azobenzene-based photo-responsive ILs[ChoC₁₀Azo]Br,[ChoC₆Azo]Br,[ChoC₄Azo]Br and[ChoC₂Azo]Br were designed,synthesized and characterized.By using UV-vis spectroscopy,conductivity,small-angle X-ray scattering and dynamic light scattering techniques,photo-isomerization performance and light modulation of aggregation behavior of the ILs in aqueous solution were investigated,and their possible mechanism was elucidated.It is shown that these ILs had rapid photo-responsive property,and their photo-isomerization efficiency was greater than 83%after 5 s UV irradiation,head group of the ILs is an electron withdrawing group,which could reduce the isomerization energy barrier and accelerate the isomerization process of azobenzene.Among the ILs studied in this chapter,only[ChoC₁₀Azo]Br could form spherical micelle in water,and UV/vis light irradiation only changed the size of its aggregates,but could not change its structure.The ILs with azobenzene at the end of the alkyl chain was not beneficial to the formation of aggregates of other structures?such as vesicles,worm-like micelle?due to the hindering of azobenzene group for the hydrophobic interaction of the alkyl chains.These findings are useful for the design of photo-responsive IL aggregates.?2?5 kinds of azobenzene-based photo-responsive ILs,[C₄AzoC₂MIM]Br,[C₄AzoC₂TMA]Br,[C₄AzoC₂DMEA]Br,[C₄AzoC₄DMEA]Br and[C₄AzoC₆DMEA]Br were synthesized and characterized.The conductivity modulation by UV/vis light was investigated in aqueous solutions.It was found that conductivity of the ILs in water exhibited significant increase upon UV irradiation,and the maximum reach to 2.5 times of the initial state.Moreover,solution conductivity was restored to the initial value by further irradiation of visible light.Thus IL solution conductivity can be modulated by alternatively irradiation of UV and visible light.The mechanism study showed that upon light irradiation,significant changes in the solvation and aggregation behavior of the ILs were the main reasons for the conductivity modulation.To achieve a highly efficient conductivity modulation,it is necessary to design azobenzene-based ILs with short alkyl spacer in the cation,which are able to aggregate in water before UV irradiation,but the aggregate could be destroyed by UV irradiation.These findings provide new insights for the understanding of photo-responsive conductivity modulation from solvation and photo-regulation aggregation of the ILs,and are expected to be useful for the design of microscale photo-control devices and fluid sensors.?3?10 kinds of azobenzene-based photo-responsive ILs,[C₄AzoC₂DMEA]Br,[C₄AzoC₄DMEA]Br,[C₄AzoC₆DMEA]Br,[C₄AzoC₂TMA]Br,[C₄AzoC₂Py]Br,[C₄AzoC₂MIM]Br,[AzoC₂PySH]Br,[AzoC₂PySH][Asc],[AzoC₂PySH][Lac]and[AzoC₂PySH][Cit]were design and synthesized.The reversible photo-induced phase transfer of them between organic phase and water phase were investigated for the first time.It was found that the photo-responsive ILs could transfer from organic phase to aqueous phase by UV irradiation,and they would move back to organic phase from water by subsequent irradiation of visible light.By alternatively irradiation of UV and visible light,ILs exhibited reversible phase transfer between organic phase and aqueous phase.In addition to the photo-isomerization property,the regulation of solvation states of ILs in solutios was the key factor to affect the reversible phase transfer.Under optimal conditions,transfer efficiency greater than 89%was achieved between n-octanol/n-hexane?2:1,v/v?and water.It is suggested that water has weak solvation ability for cis-isomer,which is not benefit for the transfer of the ILs from organic phase to aqueous phase.The results obtained here provide a new pertential way for IL phase transfer catalysis as well as recovery and recycling of catalyst from products and solvents.?4?The functionalized silica microspheres with surface-loaded metal Pd?Pd@SiO₂?were synthesized and characterized.The Pd@SiO₂ and photo-responsive IL?[C₄AzoC₂DMEA]Br,[C₄AzoC₄DMEA]Br,[C₄AzoC₆DMEA]Br or[C₄AzoC₈DMEA]Br?were used as composite emulsifiers to form Pickering emulsions with water and n-octane.It is shown that the Pickering emulsions were destoryed upon UV irradiation,and they could reform stable Pickering emulsions by alternative irradiation of visible light.Thus phase behavior could be switched by alternatively irradiation of UV and visible light.Upon UV irradiation,the solvation state of the ILs was changed and their surface activity was reduced,leading to phase separation of the system.Based on this unique photo-responsive phase behavior,the Pickering emulsions were used for the highly efficient catalytic hydrogenation of some unsaturated hydrocarbons at room temperature and 101 kPa.In this process,the coupling of product separation,IL and catalyst recycling were successfully achieved,which opens a new pathway for the development of sustainable chemical processes.