Preparations And Property Investigations Of Azobenzene Surfactants With Bulky Head Groups

Azobenzenes are an important part of photoswitches,which has a unique N=N molecular structure of light-induced reversible structural transition,can undergo reversible trans?cis photoisomerizations with large differences of the isomers in molecular property,and they have received widespread interest in developing photoresponsive phase change materials(PPCMs).However,materials composed solely of small azobenzene molecules lack sufficient free space for isomerization,and it is difficult for photoinduced solid-liquid state changes to occur at room temperature,which limits the application of such materials in the field of energy storage.Increasing the steric hindrance of azobenzene molecules can effectively solve the problem of photoisomerization and material phase transition,but it will reduce the energy storage density of azobenzene materials.In view of this,we designed two azobenzene surfactant molecules from the molecular level for the purpose of developing azobenzene photophase change materials with high energy storage density.In the solvent-free state,the two surfactant molecules can achieve the storage of light energy and low temperature heat energy through photoisomerization and its induced phase transition.The main research work is as follows:(1)In the design of the two molecules,the problem of insufficient space for the photoisomerization of azobenzene molecules was solved by introducing a quaternary ammonium salt head structure with large steric resistance,so that the two azobenzene surfactants could be reversible isomerized by light at room temperature,and at the same time cause incomplete reversible phase change of the materials.Then,by introducing flexible alkyl long chain structure,the intermolecular force between molecules is increased to improve the overall energy storage density of the material.The energy materials composed of these surfactants can achieve the highest energy storage density of 131.18 J/g by exposing the solid to ultraviolet light.Moreover,the molar isomerization enthalpy of azobenzene was increased by2.4 times due to the strengthening of intermolecular forces.The energy stored in the material can be quickly released in the form of heat when exposed to visible light,causing the ambient temperature to rise.(2)At room temperature,the problem that the two surfactant molecules cannot achieve complete phototransformation can be solved by doping some nonazobenzene surfactant molecules.The doped non-azobenzene surfactant can effectively weaken the π-π force between azobenzene groups,reduce the packing density between azobenzene surfactant molecules,and provide sufficient free space for the photoisomerization of azobenzene.Through this strategy,we developed two kinds of ionic materials that can achieve complete phototransformation at room temperature,and summarized the relationship between the doping ratio of nonazobenzene surfactants and the ability of phase transformation,packing density of the two kinds of materials.This study proposes a new molecular design strategy for azobenzene PCM and provides a new direction for the development of azobenzene PCM with high energy storage density.