This project aims to create unique optoelectronic materials based on the self-assembly of helical β-peptides.
Charge carrying π-conjugated materials are the focus of intense research with applications in biomedicine ranging from neural electrode coatings for brain implants, to biosensors in cell and tissue engineering, and artificial muscles. Photoinduced electron transfer (PET) and excited energy transfer (EET) reactions are essential processes for mimicking natural photosynthesis and are integral to various applications such as optoelectronics, photovoltaic and solar cells. Rigid polymers that are intrinsically π-conjugated are the most extensively used organic semiconductors for the development of optoelectronic devices. However, recent work on hydrogen-bonded assemblies containing π-conjugated systems derived from biologically relevant groups such as nucleotides, steroids, carbohydrates and amino acids has resulted in the intermolecular π – π interactions by the formation of hierarchical structures. In particular, the propensity of oligopeptides to self-assemble in polar or aqueous media into fibrillary aggregates induces hydrophobic effects, by isolating the hydrophobic interface on the interior of the fibril, that enhances the intermolecular π – π interactions necessary for efficient charge-transport. Thus the bottom-up approach of the self-assembly process allows the fabrication of well-ordered, solution-processable organic semiconductors from molecular components. The extended arrays of ordered π-conjugated self-assemblies that has the potential to facilitate long-range charge migration are achieved through inter-digitation of the chromophores mostly driven by β-sheet formation along the axis of the nanostructure. However, utilizing long oligopeptide assembling subunits can restrict the charge-transporting ability because of the “insulating” properties of peptide moieties, thus making it challenging to build electronic devices. This limitation can be potentially overcome by decreasing the volume fraction of the peptide moiety compared to the π-conjugated system by either using truncated peptide sequence, such as the short β-tripeptides with densely populated π-conjugated chromophores. The unique self-assembly of the N-acetylated β-peptides results in a near perfect pitch of three residues per turn of the helix. A direct consequence of this precise assembly is the lateral alignment of the side chain residues of each peptide monomer, resulting in a high degree of symmetry along the periphery of the helical β-peptide backbone should result in ordered arrangement of pendant groups along the peptide chain conferring directionality and degree of organisation in the π-conjugated molecular inserts leading to the formation of columnar stacks. This would allow the chromophore moieties, such as diverse aryl units, ethynylene linkers or oligothiophenes, to form highly ordered configurations, a prerequisite for effective charge-transport in materials. As a consequence of the self-assembly, π-electron systems restrained within the peptide assemblies should display optoelectronic properties that differ from their molecularly dissolved counterparts (monomers). Therefore, UV−vis absorption and fluorescence spectroscopy will be used to provide evidence for extended arrays of ordered π-conjugated self-assemblies. The ordered assembly between the chromophores should lead to unique spatial arrangements of their transition dipoles that have apparent consequences in the optical response of the material resulting in blue or red shifted absorption and photoluminescence profiles. Circular dichroism spectroscopy, in conjunction with FT-IR spectroscopy of the amide bond region, will also be employed to examine the specific secondary structures of the polymeric peptide assemblies and the collective orientations of the π-electron transition dipoles within the peptide assemblies. The interpretation of the spectral signatures can then be used to “fine-tune” the π-interactions between the chromophores, which can directly influence the optoelectronic behaviour of these materials. The peptide assemblies will then be incorporated as the active semiconducting layer in an organic field-effect transistor to evaluate the charge mobility within the peptide-chromophore assemblies. This proposal involves basic and applied interdisciplinary research that includes: a) Design and synthesis of orthogonally-protected β-amino acids b) Synthesis of functional N-acetyl-β-tripeptide monomers and evaluation of their self-assemblies c) Analysis of the photophysical outcome of the π-conjugated units within the peptide and self-assembled structures