Design,Synthesis And Luminescence Properties Of Tetraphenylethylene Modified Curcumin Derivatives Fluorescent Dyes

Organic fluorescent dyes have great applications in fluorescent probes,bioimaging,sensors,and optoelectronic devices,the demand for organic fluorescent dyes with excellent performance is increasing with scientific and technological progress and social development.As a highly bioactiveβ-diketone natural product extracted from plants,curcumin has outstanding chromogenic properties,such as visible light excitation,large molar extinction coefficient,high fluorescence quantum yield,and long wavelength emission.In addition,theβ-diketone structure of curcumin is also an excellent ligand for cations,therefore it has promising applications in the direction of fluorescent probes.However,the photophysical properties of curcumin are strongly affected by solvents,especially its fluorescence will be quenched by water due to the aggregation-caused quenching（ACQ）phenomenon,and its molecular structure is easily decomposed by photobleaching.These defects limit the application of curcumin and its derivatives.Fortunately,the aggregation-induced emission（AIE）strategy developed by Ben-Zhong Tang’s group from 2001 can effectively resolve the ACQ problem,and a variety of AIE molecules have been discovered and applied in different areas after 20 years of advancement.Tetraphenylethylene（TPE）as a typical AIE moiety has highly twisted molecular structure which will limit the nonradiative energy transfer by hampering the intermolecularπ–πstacking in aggregated states.In order to resolve the ACQ problem of curcumin,we have introduced the TPE moiety into curcumin and synthesized a series of TPE-modified curcumin derivatives and their boron difluoride complexes by changing their para-substituents,and their photophysical properties have been studied in-depth.The main achievements are concluded in the following:1.Four TPE-modified curcumin fluorescent dyes,namely L1,L2,L3,and L4,were designed and synthesized with different para-substituents,and their molecular structures were characterized by¹H NMR,¹³C NMR,and high resolution mass spectrometry.The fluorescence experiments carried out in mixed solvent systems（water/tetrahydrofuran）with different ratios demonstrated that L1-3 had significant aggregation-induced enhanced emission（AIEE）properties.Especially,L1 showed a significant fluorescence response to Cu²⁺,and a simple Cu²⁺fluorescent indicator paper prepared based on L1 particles displayed effective and rapid detection for Cu²⁺in aqueous solution.2.Furthermore,five boron difluoride complexes based on TPE-modified curcumin derivative,namely L₁-BF₂,L₂-BF₂,L₃-BF₂,L₄-BF₂,and L₅-BF₂,were synthesized and investigated their photophysical properties.The results showed that all complexes exhibited significant AIE/AIEE properties in hexane/tetrahydrofuran mixed solvent systems and mechanochromic properties after grinding them.Especially,the electron-donating and electron-withdrawing abilities of para-substituents could affect the fluorescence of grinded complexes except for L₅-BF₂.For example,the complex（L₁-BF₂）with electron-donating group showed a blue-shifted emission spectrum after grinding,and the electron-withdrawing group made the emission spectrum of complex（L₃-BF₂）red-shift.Meanwhile,the substituent groups also could affect the fluorescence lifetime of grinded complexes.Further research should pay more attention to investigate the structure-property relationship by replacing more substituent groups and explore the mechanism based on the theoretical calculation and more characterizations.