Push-Pull Chromophores Of Isoindole:Synthesis,Properties And Bioimaging Application

Fluorescent dyes with donating and accepting groups（D-A）have been widely studied in the fields of nonlinear optical materials,organic light-emitting diodes,and environmentally sensitive probes,because of excellent properties such as large Stokes shift,high molar extinction coefficient,and excellent photostability.However,D-A systems with isoindole as theπbridge is quite rare,especially for those isoindole-based fluorescent probes with aggregation-induced emission（AIE）.In this study,D-A organic fluorescent molecules were designed and constructed by using isoindole as the skeleton,thiophene,p-hydroxybenzene and aromatic amine as donor,aldehyde group,dicyanogen methyl,4-（dicyanogen methyl）pyran（DCM）and quinoline malononitrile（QM）as acceptor.1)Eight D-A fluorescent probes with different electron-donating groups were synthesized by Suzuki coupling reaction,starting from two kinds of bromoisindolaldehydes and phenylboric acids containing thiophene,p-hydroxy benzene,p-dimethylaminobenzene and triphenylamine.We found arylamine dyes show high sensitivity to polarity.Besides,N,N-dimethyl substituted probe showed great correlation coefficient between the emission wavelength and polarity reached 0.99,and100 nm redshift from toluene to methanol.In addition,we further studied the localization of liposomes and used it to map the polarity distribution of zebrafish by ratio imaging and real-time monitoring the changes of polarity in cellular environment.2)Four D-A isoindole-based fluorescence probes were,respectively,synthesized by Knoevenagel condensation reaction of isoindolaldehydes and malononitrile.They show100 nm redshift by replacing the aldehyde to cyanide,and represent the properties of AIE and two-photon absorption.More importantly,the emission wavelength reaches the near-infrared region after aggregation.Besides,it was found that only those isoindole-based probes with the alkyl chain gave great AIE feature,which might result from hindering the intermolecularπ-πaccumulation of the alkyl chain.3)Five kinds of D-A near-infrared AIE-active isoindole dyes with largeπconjugation were,respectively,synthesized by Knoevenagel condensation from electron-donating isoindole aldehydes with DCM,QM and pyridine salt.All the obtained dyes exhibited near-infrared emission wavelength ranging from 654 nm to 731 nm in methanol.Besides,they all exhibited AIE properties,for example,one obtained isoindole-based probe with pyridine salt emit at 765 nm in water.Furthermore,the dye with pyridine salt showed good mitochondrial localization ability.4)A series of bis（difluoroboron）isoindole-based fluorescent dyes were synthesized by condensation of five isoindole-based aldehydes with pyridinhydrazine and then coordinated with BF₃·OEt₂ by one-pot method.Their structures were characterized by NMR(¹H、¹³C、¹⁹F、¹¹B),HRMS,X-Ray single-crystal,UV-Vis and emission spectra.Compared with unsubstituted BOPPY reported before,the absorption and emission maximum wavelength of these isoindole-based BOPPY fluorescent probes redshift at least 100 nm.Besides,they also showed relatively large Stokes shift and high fluorescence quantum yield up to 0.92.These isoindole-based BOPPY probes with p-methylbenzene sulfonic acid display highly specific localization of endoplasmic reticulum.