Studies On Synthesis And Assembly Behavior Of Novel Conjugated Organic Molecules—Oligopeptides Block Oligomers

Oligothiophene and its derivatives as a good organic semiconductor have been attracted great attention during the past decade, due to their excellent optical-electrical properties in organic electronics. One of the challenges in oligothiophene and its derivatives systems with semiconducting function is the achievement of molecular orientation over molecular-level to improve its photoelectric conversion efficiency. Self-assembly has been regarded as one of most promising approaches to prepare nanostructure materials by chemists and material scientists. Oligomeric polypeptide GAGA (G-Glycine, A-Alanine) a repeating unit found in nature silk protein can be assembled by hydrogen bonds to form anti-parallel β-sheet microcrystalline structure. These microcrystalline structure with amorphous region of silk protein alternating resulting in silk with superior mechanical strength and toughness, therefore become bionics research hotspot.An artificial peptide, N3-GVGV-OMe (G=Glycine, V=Valine), which inspired the repeating GAGA (A=Alanine) sequence in silk protein, was synthesized via solution phase synthesis. Compared with N3-GAGA-OMe sequence, N3-GVGV-OMe showed high solubility in common organic solvents (such as CHCl3, THF, CH2Cl2), and easily formed organogels simply by adding poor solvents (such as toluene or ether) to the peptide solution at room temperature certified the GVGV component had strong ability to assemble. The hierarchical nanostructure of N3-GVGV-OMe organogel was dependent on the nature of the poor solvents, although in all cases, β-sheets were formed exclusively. Gels formed in ether showed higher level hierarchical assembly, as evidenced by AFM and CD studies. Solution state FT-IR analysis showed that the pre-organization of the peptides in solution was not significant, and well-defined antiparallel β-sheets were formed after the addition of the poor solvent. The high solubility and strong tendency for self-assembly of N3-GVGV-OMe, together with its terminal azide group, might facilitate the modification of functional organic molecules even macromolecules for better nanostructure control. Based on N3-GVGV-OMe segment, we further synthesized N3-GVGV-N3tetrapeptide which both terminal was functionalized by azide group. Quaterthiophenes (T) decorated by silk-inspired tetrapeptide segment (Gly-Val-Gly-Val, GVGV) were synthesized as peptide-thiophene-peptide (PTP) and thiophene-peptide-thiophene (TPT) to investigate their assembly behavior. FTIR shows, both TPT and PTP formed organogels and left-handed helical twisted nanostructures were obtained, however, anti-parallel β-sheets and parallel (3-sheets were formed for PTP and TPT respectively, although both oligothiophenes adopted hydrogen bonds stacking. Supramolecular chirality was derived not only from the β-sheets but also from the oligothiphene parts; achiral solvent induced supramolecular chirality inversion was obtained for both oligothiophenes, as evidenced by CD studies. The optical property of the block copolymer was characterized through UV spectra and fluorescence spectra. So we can infer that the main driving force of the assemblies is hydrogen bonding for PTP, while the hydrogen bonding and π-π stacking might contribute to the self-assembly equally for TPT. X-ray analysis provided data support for the establishment of molecular models. Based on all the test results, a self-assembly model of PTP and TPT block copolymers was established, this would provide a theoretical model for the application of polypeptide-thiophene block copolymer.