Peptide-Directed Supramolecular Self-Assembly of N-Substituted Perylene Imides
Synthetic peptides offer enormous potential to encode the assembly of molecular electronic components, provided that the complex range of interactions is distilled into simple design rules. Herein is reported a spectroscopic investigation of aggregation in an extensive series of peptide-perylene imide conjugates designed to interrogate the effect of structural variations. Throughout the course of this study, the self-assembly and photophysical properties of the compounds are explored to better understand the behavior and application of these materials. Three principal avenues of inquiry are applied: (1) the evaluation of structure-property relationships from a thermodynamic perspective, (2) the examination of peptide chiral effects upon properties and self-assembly, and (3) an application of the understanding gained from rationally designed systems to effectively utilize naturally optimized peptides in bio-organic electronics. By fitting different contributions to temperature-dependent optical absorption spectra, this study quantifies both the thermodynamics and the nature of aggregation for peptides with incrementally varying hydrophobicity, charge density, length, amphiphilic substitution with a hexyl chain, and stereocenter inversion. Coarse effects like hydrophobicity and hexyl substitution are seen to have the greatest impact on binding thermodynamics, which are evaluated separately as enthalpic and entropic contributions. Moreover, significant peptide packing effects are resolved via stereocenter inversion studies, particularly when examining the nature of aggregates formed and the coupling between π-electronic orbitals. Peptide chirality overall is seen to influence the self-assembly of the perylene imide cores into chiral nanofibers, and peptide stereogenic positions, stereochemical configurations, amphiphilic substitution, and perylene core modification are evaluated with respect to chiral assembly. Stereocenters in peptide residue positions proximal to the perylene core (1-5 units) are seen to impart helical chirality to the perylene core, while stereocenters in more distal residue positions do not exert a chiral influence. Diastereomers involving stereocenter inversions within the proximal residues consequently manifest spectroscopically as pseudo-enantiomers. Increased side-chain steric demand in the proximal positions gives a similar chiral influence but exhibits diminished Cotton Effect intensity with additional longer wavelength features attributed to interchain excimers. Amphiphilic substitution of a peptide with an alkyl chain disrupts chiral self-assembly, while an amphiphilic structure achieved through the modification of the perylene imide core with a bisester moiety prompts strongly exciton-coupled, chiral, solvent-responsive self-assembly into long nanofilaments. Informed by rationally designed sequences, and capitalizing upon the optimization seen in many natural systems, specific peptide sequences designed by inspection of protein-protein interfaces have been identified and used as tectons in hybrid functional materials. An 8-mer peptide derived from an interface of the peroxiredoxin family of self-assembling proteins is exploited to encode the assembly of perylene imide-based organic semiconductor building blocks. By augmenting the peptide with additional functionality to trigger aggregation and manipulate the directionality of peptide-semiconductor coupling, a series of hybrid materials based on the natural peptide interface is presented. Using spectroscopic probes, the mode of self-assembly and the electronic coupling between neighboring perylene units is shown to be strongly affected by the number of peptides attached, and by their backbone directionality. The disubstituted material with peptides extending in the N-C direction away from the perylene core exhibits strong coupling and long-range order, which are both attractive properties for electronic device applications. A bio-organic field-effect transistor is fabricated using this material, highlighting the possibilities of exploiting natural peptide tectons to encode self-assembly in other functional materials and devices. These results advance the development of a quantitative framework for establishing structure-function relationships that will underpin the design of self-assembling peptide electronic materials. The results further advance a model for adapting natural peptide sequences resident in β-continuous interfaces as tectons for bio-organic electronics.