Self-assembly And Optoelectronic Properties Of Oligopeptides Modified Perylene Diimide Derivative

Large π-conjugated molecules have remarkable photoelectric properties based on their long-range ordered self-assembly driven by efficient π-orbital stacking,However,designing molecular structures to accurately construct π-stacked supramolecular materials with high photoelectric properties remains a major challenge.Herein,perylene diimide(PDI)as a classical n-type semiconductor material was selected as a large π conjugated group model molecule.Oligopeptides with different compositions were modified asymmetrically PDI to construct amphiphilic structure,which induced large π-conjugated PDI molecules to construct long range ordered assemblies by taking advantage of directivity and selectivity of multiple hydrogen bonds and electrostatic interactions between the peptide chains.The relationship between the supramolecular structure and photoelectrical properties was then investigated.The details are as follows:(1)An amphiphilic peptide-perylene diimide derivative(EEAβAUPDI,EAUP)was designed and synthesized.One imide position in PDI was substituted by an oligopeptide composed of β-alanine(βA,linking group),L-alanine(A),L-glutamic acid(E)and L-glutamic acid(E),and the other side was introduced a long alkyl chain(U).The self-assembly behavior and the structure of EAUP were systematically studied by adjusting the external environmental factors such as concentration,temperature,pH and solvent polarity.The photoelectric properties of EAUP assemblies constructed under different conditions were investigated to reveal the relationship between the structure and photoelectric properties of EAUP assemblies.It is found that EAUP can self-assemble into right-handed π-stacked aggregates at extremely low concentration.Moreover,the supramolecular chirality of the assembly was reversed to form left-handed helical nanofibers by lowering pH.Solvent polarity was used to regulate the balance between hydrogen bonds of peptide chains and π-π stacking of PDI rings,so that EAUP changed from assembly state to monomer state.EAUP films with different assembly structures show different photoelectric responsiveness.The photocurrent responsiveness of the fabricated nanomaterials formed in mixture of THF/NaOH(aq)with fTHF=0.7 was the most significant,which was 1.4 times that of the assembled materials constructed under acidic conditions.(2)Compared to EAUP,L-alanine(A)was replaced by L-histidine(H),and D-A molecular structure(EEHβAUPDI,EHUP)was constructed as imidazole(D)with electronrich characteristics in histidine residues and electron-deficient PDI ring(A).The effect of amino acid residue structure on the self-assembly behavior of the derivatives was investigated.The results showed that EHUP mainly existed in the form of assembly at the lowest concentration investigated.However,unlike EAUP,left-handed π-stacked EHUP selfassemblies were observed.Changing pH is capability of regulating the supramolecular chirality of the assembly,but could not induce the reversal of supramolecular chirality.The transformation between EHUP assembly and monomer can also be achieved by adjusting solvent polarity.EHUP supramolecular assemblies constructed under different conditions have remarkably different photoelectrical properties.The photoelectric responsiveness of the EHUP assembly fabricated under neutral condition was the lowest at only 0.48 μA cm-2,and the response to triethylamine was 2.5 times higher.In contrast,the assemblies constructed under fTHF=0.7 exhibited the highest photocurrent values of 1.20 μA cm-2 and their response to triethylamine increased by 3.2 times.(3)Based on EHUP molecule,one more L-alanine(A)was added between histidine(H)and hydrophilic amino acid(E)to construct a novel amphiphilic oligopeptides perylene diimide derivative(EEAHβAUPDI,EAHUP),and the effect of peptide chain length on the self-assembly behavior of the derivative was investigated.The results indicated that EAHUP mainly formed H-type aggregates with left-handed helical characteristics,and exhibited the greatest preference to self-assembly among these three oligopeptide-PDI derivatives.The supramolecular chirality of self-assembles gradually diminished with the decrease of pH.Unlike EAUP and EHUP,two single-electron redox peaks of EAHUP assemblies with different supramolecular structures cannot be distinguished.The EAHUP assembly without supramolecular chirality exhibited the lowest photo-stimulus responsiveness with a photocurrent of 0.11 μA cm-2,and the addition of triethylamine produced a 5.4 times enhancement.In contrast,the EAHUP assembly fabricated at fTHF=0.7 had the highest photoelectric responsiveness of 1.17 μA cm-2,and its stimulation response to triethylamine increased 2.5 times,which was the weakest among these three oligopeptide-PDI derivatives.