Synthesis And Luminescence Properties Of Tetraphenylethylene Derivatives

Compared with traditional fluorescent molecules, aggregation-induced emission（AIE） compounds have great advantage-when they aggregated, their fluorescencewas enhanced rather than being quenched. This character makes them desirablecandidates for chemical and biological sensors. Besides silole, many organicmolecules with AIE nature have been gradually found. Among these AIE molecules,tetraphenylethylene （TPE） and its derivatives have the advantages of facile synthesis,high efficient emission, and high thermal stability. People are trying to expand itsapplication, hoping to achieve functional reversible change upon external stimuli.In this work, a series of water-soluble TPE derivatives were designed andsynthesized via introducing hydrophilic oligo（ethylene oxide） segments into TPE.These TPE derivatives not only show the intrinsic temperature-responsive propertyof a nonionic amphiphile, but also maintains the AIE behavior of TPE moiety. Theymight work as a potential fluorescence probe to detect the subtle temperature changethat occurs in chemical and biological reaction.Firstly, three TPE derivatives containing oligo（ethylene oxide） were synthesized,which were noted as M（EO）₃-TPE, M（EO）₄-TPE and M（EO）₅-TPE. Because theoligo（ethylene oxide） group is hydrophilic, the TPE derivatives can dissolve inwater.Secondly, we investigated thermoresponsive property of the TPE derivatives.Within the concentration range of0.5to4mmol/L, the phase-transition temperatureof M（EO）_n-TPE decreased with increasing solution concentration, which indicatedthat the TPE derivatives exhibited lower critical solution temperature （LCST）phenomenon. In addition, the phase transition temperature of TPE derivativesincreases with the increase in length of oligo（ethylene oxide） chains.Thirdly, we investigated the luminescence properties of the M（EO）_n-TPE inwater-THF mixture. The fluorescence became stronger when large amounts of waterwas added into the THF solution of M（EO）_n-TPE. This proves that the TPE derivatives maintain the AIE nature of the TPE moiety.Finally, the fluorescence of M（EO）_n-TPE is temperature-dependent. Whenbeing heated above its phase-transition temperature, the M（EO）_n-TPE moleculesstarted to aggregate, which resulted in the restricted intramolecular rotation of TPEmoiety. Consequently, the aqueous solution of M（EO）_n-TPE showed a strongerfluorescence. When the aqueous solution of M（EO）_n-TPE was cooled, a roughlyreversible fluorescence-temperature curve was obtained. Additionally, when theconcentration of M（EO）_n-TPE was decreased, the starting temperature for thefluorescence enhancement was increased.