Synthesis,Self-Assembly and Aggregation-Induced Emission of Fluorescent Polypeptoids
Tetraphenyl ethylene(TPE)-capped block copolypeptoid poly(N-butylglycine)-b-poly(N-ethylglycine)(TPE-PNBG-b-PNEG)was synthesized through the 1-(4-aminophenyl)-1,2,2-triphenylethene(TPE-NH2)-initiated ring opening polymerization of N-butylglycine N-thiocarboxyanhydride(NBG-NTA)and N-ethylglycine N-thiocarboxyanhydride(NEG-NTA).Three copolypeptoids with different mass ratios of hydrophilic PNEG block(wPNEG),TPE-PNBG10-b-PNEG41(wPNEG=70% ),TPE-PNBG38-b-PNEG44(wPNEG=45% )and TPE-PNBG53-b-PNEG36(wPNEG=33% ),were prepared by changing the feed molar ratios of monomer to initiator,whose compositions were confirmed by the proton nuclear magnetic resonance(1H-NMR).Size exclusion chromatography(SEC)traces of the three copolypeptoids were all unimodal with narrow molecular mass distributions(Đ<1.30).The ultraviolet-visible(UV-Vis)absorption spectra exhibited that TPE-PNBG-b-PNEG had a strong absorption at 316 nm,which was corresponding to the characteristic absorption of TPE group.TPE-PNBG-b-PNEG showed distinct aggregation-induced emission(AIE)property by adding water to its N,N-dimethylformamide(DMF)solution,which was caused by the assembly of hydrophobic TPE groups and PNBG segments in DMF-H2O system.When the initial water content(volume fraction)was 33%,TPE-PNBG53-b-PNEG36 assembled to spindle-like aggregates with an average length of 550 nm and width of 160 nm.When the initial water content was increased to 67%,TPE-PNBG53-b-PNEG36 assembled to worm-like aggregates with an average length of 650 nm and width of 90 nm.The higher initial water content would lead to the more tightly packed assemblies.With the increasing of wPNEG,TPE-PNBG38-b-PNEG44 assembled to pine leaf-like aggregates with an average length of 300 nm,while TPE-PNBG10-b-PNEG41 assembled to spherical micelles with a diameter around 50 nm.Under the excitation of 335 nm UV light,the assembly solution of the three copolypeptoids could emit strong blue light,with a maximum emission wavelength of 485 nm.The fluorescence intensity and fluorescence quantum yield of the assembly solution increased with the decreasing of wPNEG.As wPNEG gradually decreased,the packing parameters of TPE-PNBG-b-PNEG gradually increased,which led to the more compacted packing of the hydrophobic segments and subsequently enhanced the fluorescence intensity and fluorescence quantum yield due to the stronger AIE effect.
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