Robust Assembly of Cross-Linked Protein Nanofibrils into Hierarchically Structured Microfibers

Natural, high-performance fibers generally have hierarchically organized nanosized building blocks. Inspired by this, whey protein nanofibrils (PNFs) are assembled into microfibers, using flow-focusing. By adding genipin as a nontoxic cross-linker to the PNF suspension before spinning, significantly improved mechanical properties of the final fiber are obtained. For curved PNFs, with a low content of cross-linker (2%) the fiber is almost 3 times stronger and 4 times stiffer than the fiber without a cross-linker. At higher content of genipin (10%), the elongation at break increases by a factor of 2 and the energy at break increases by a factor of 5. The cross-linking also enables the spinning of microfibers from long straight PNFs, which has not been achieved before. These microfibers have higher stiffness and strength but lower ductility and toughness than those made from curved PNFs. The fibers spun from the two classes of nanofibrils show clear morphological differences. The study demonstrates the production of protein-based microfibers with mechanical properties similar to natural protein-based fibers and provides insights about the role of the nanostructure in the assembly process

Semiaromatic polyamides with enhanced charge carrier mobility

The control of local order in polymer semiconductors using non-covalent interactions may be used to engineer materials with interesting combinations of mechanical and optoelectronic properties. To investigate the possibility of preparing n-type polymer semiconductors in which hydrogen bonding plays an important role in structural order and stability, we have used solution-phase polycondensation to incorporate dicyanoperylene bisimide repeat units into an aliphatic polyamide chain backbone. The morphology and thermomechanical characteristics of the resulting polyamides, in which the aliphatic spacer length was varied systematically, were comparable with those of existing semiaromatic engineering polyamides. At the same time, the charge carrier mobility as determined by pulse-radiolysis time-resolved microwave conductivity measurements was found to be about 10-2 cm2 V-1 s-1, which is similar to that reported for low molecular weight perylene bisimides. Our results hence demonstrate that it is possible to use hydrogen bonding interactions as a means to introduce promising optoelectronic properties into high-performance engineering polymers.Peer ReviewedPostprint (author's final draft

A nanomesh scaffold for supramolecular nanowire optoelectronic devices

Supramolecular organic nanowires are ideal nanostructures for optoelectronics because they exhibit both efficient exciton generation as a result of their high absorption coefficient and remarkable light sensitivity due to the low number of grain boundaries and high surface-to-volume ratio. To harvest photocurrent directly from supramolecular nanowires it is necessary to wire them up with nanoelectrodes that possess different work functions. However, devising strategies that can connect multiple nanowires at the same time has been challenging. Here, we report a general approach to simultaneously integrate hundreds of supramolecular nanowires of N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) in a hexagonal nanomesh scaffold with asymmetric nanoelectrodes. Optimized PTCDI-C8 nanowire photovoltaic devices exhibit a signal-to-noise ratio approaching 107, a photoresponse time as fast as 10 ns and an external quantum efficiency >55%. This nanomesh scaffold can also be used to investigate the fundamental mechanism of photoelectrical conversion in other low-dimensional semiconducting nanostructures

Hierarchical Conjugated Polymer Systems Prepared by Controlled Chain-Growth Polymerization

π-Conjugated polymer materials may have a significant economic impact on society by providing means for designing affordable, flexible, and portable organic electronic devices. Their successful commercialization will depend on major scientific advancements, which will challenge human society to seek out ever more detailed and fundamental processes to command. Controlled polymerization affords such a power; allowing for the design of meticulous and precisely defined systems granting detailed insight into structure-property relationships in the polymer materials and bettering understanding of novel physical phenomena. This dissertation primarily focuses on development and preparation of well-defined hierarchically organized macromolecular systems. The novel chain-growth polymerization methodologies described herein depended on gaining new fundamental insights into transition metal catalyzed controlled polymerization reactions, including many intriguing aspects of their catalytic and self-assembly processes. For example, a general approach towards deeper mechanistic understanding and improving the controlled polymerization reactions for preparing conjugated polymer is presented; subsequent applying this knowledge resulted in an approach for a general synthesis of various classes of conjugated polymers precisely incorporating specific structural units. Preparation of such materials led to the observation of novel and unusual photophysical phenomena that exclusively appear in these unique polymer systems. In addition, the photophysical properties and energy transfer processes occurring in nano- and mesoscale conjugated polymer donor-acceptor materials were found to depend on the non-equilibrium conformations of supramolecular assemblies forming in kinetically controlled catalyst-transfer polymerization

Tailoring the excited-state energy landscape in supramolecular nanostructures

Nature's photosynthetic machinery uses precisely arranged pigment-protein complexes, often representing superstructures, for efficient light-harvesting and transport of excitation energy (excitons) during the initial steps of photosynthesis. This function is achieved by defined electronic Coulomb interactions between the conjugated molecules resulting in tailored excited-state energy landscapes. While such complex natural structures are synthetically difficult to achieve, supramolecular chemistry is now on its advent to realize defined artificial supramolecular nanostructures with tailored functionalities via controlled self-assembly processes of small molecules. In this review, we focus on recent work reporting photophysical studies on self-assembled and hierarchical nanostructures as well as complex superstructures. We discuss how the resulting excited-state energy landscapes influence energy transport. Progress in the field of supramolecular chemistry allows for the realization of distinct kinds of H- or J-aggregates with well-defined morphologies on the mesoscale. Advances in the field of optical spectroscopy and microscopy have permitted to resolve the incoherent/coherent dynamics of exciton transport in such systems down to the level of single nanostructures. Although outstanding diffusion lengths of up to several mu m were found in selected nanostructures, a full understanding of the underlying principles is still missing. In particular, the unavoidable structural and electronic disorder in these systems influences the excited-state energy landscapes and thus the transport characteristics, which can be exploited to refine the molecular design criteria of supramolecular nanostructures and complex superstructures. Despite the rapid progress in the field of functional supramolecular nanostructures, we believe that revealing the full potential of such systems is far from complete. In particular, criteria for tailored and optimized (hierarchical) supramolecular nanostructures in view of applications are not yet established. Finally, we outline current challenges and future perspectives for optical and optoelectronic applications of supramolecular nanostructures

Chiroptical Properties in Thin Films of π-Conjugated Systems

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (gabs and glum) and on the application of ECD and CPL to derive structural information on aggregated states

Composition, thermodynamics, and morphology: A multi-scale computational approach for the design of self-assembling peptides

Peptide self-assembly has generated significant interest as a means for the bottom-up fabrication of highly tunable biocompatible nanoaggregates. Individual peptides can be synthesized to include non-natural π-conjugated subunits, endowing assembled aggregates with a range of optical and electronic properties that render them useful in applications as biocompatible organic electronics. The immense number of possible peptides, however, causes the exhaustive traversal of sequence space to be intractable. This massive composition space lends itself toward the use of computer simulation and data science tools to understand molecular aggregation and guide experimental synthesis and design. In this dissertation, I present work employing a hierarchy of molecular modeling techniques to identify self-assembling peptides with specific photophysical properties by probing thermodynamic and structural characteristics of peptide aggregation. We employ classical molecular dynamics simulation to probe the key molecular forces governing the morphology and free energy of oligomerization, time dependent density functional theory to predict photophysical properties as a function of aggregate morphology, and data-driven quantitative structure property models to perform high-throughput virtual screening of chemical space to identify promising peptide chemistries. This work establishes a multi-scale framework for the principled computational design of self-assembling π-conjugated peptides with engineered photophysical properties

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

MOLECULAR ENGINEERING STRATEGIES FOR THE DEVELOPMENT OF ENERGY-TRANSPORTING CONJUGATED SYSTEMS TOWARDS BIOELECTRONICS

Peptidic nanostructures appended with π-electron units present a powerful class of biomaterials that merges the chemical versatility of self-assembling peptides and the optoelectronic function of organic π-electron units. This dissertation spans research efforts from understanding some rational design concepts to the bioelectronic utility of π-conjugated peptides. Chapter 1 describes the progress in the field of peptidic nanomaterials bearing π-electron systems and how this relates to advances in bioelectronics. In Chapters 2 and 3, key properties and aspects of molecular design that can be utilized to rationally tune optoelectronic and mechanical properties of oligothiophene-peptide hybrid hydrogelators are discussed. Chapter 2 focuses on the effect of varied amino acid size and hydrophobicity on material properties at different length scales. Chapter 3 investigates the photophysical effects of confining individual π-units within nanostructures and how the coassembly behavior is affected by local fields imparted by the peptide moieties. The next two chapters introduce a multicomponent strategy to create and characterize the nanostructure of different functional materials, either with the co-incorporation of bioactive epitopes within peptide nanostructures or showing energy and/or electron transfer events among π-electron systems that are spatially engineered within peptidic constructs. Chapter 4 presents energy-transporting nanomaterials comprised of donor-acceptor peptide pairs existing in either solution or hydrogel phase. Chapter 5 aims to shed light on the local structure formed upon supramolecular coassembly in both solution and hydrogel phases using solid state NMR and small-angle neutron scattering techniques. Finally, Chapter 6 presents the developmental efforts towards creating biological scaffolds out of these peptidic nanostructures with tunable physicochemical properties that can potentially facilitate nanostructural energy transport upon electrical or light stimulation. These peptide nanomaterials offer a platform for scaffolds that can mimic the natural environment of electrosensitive tissues such as nerves. Much of the progress accomplished towards this application is to establish a stable system against material degradation during long periods under cell culture conditions. We show that aligned constructs that are pre-assembled by an external trigger followed by covalent crosslinking were successful in imparting topographical guidance to human neural stem cells or dorsal root ganglion neuron explants. The latter part of this Chapter reports the extension of these efforts towards developing known electroactive organic polymers that can be processed as aligned soft materials for neural stem cells and neonatal rat ventricular cardiomyocytes. This dissertation aims to bridge the understanding of chemical design and self-assembly principles with the biomaterial applications of peptide-based optoelectronic assemblies and related conjugated polymers