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 (w_PNEG), TPE-PNBG₁₀-b-PNEG₄₁ (w_PNEG = 70%), TPE-PNBG₃₈-b-PNEG₄₄ (w_PNEG = 45%) and TPE-PNBG₅₃-b-PNEG₃₆ (w_PNEG = 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 w_PNEG, 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 w_PNEG. As w_PNEG 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.