Effect of Core Oligomer Length on the Phase Behavior and Assembly of π-Conjugated Peptides.

Biohybrid molecules are a versatile class of materials for controlling the assembly behavior and functional properties of electronically active organics. In this work, we study the effect of the size of the π-conjugated core on the assembly and phase behavior for a series of π-conjugated peptides consisting of oligothiophene cores of defined lengths flanked by sequence-defined peptides (OTX, where X = 4, 5, 6 is the number of thiophene core units). Interestingly, we find that π-conjugated peptides with relatively short OT4 cores assemble into ordered, high aspect ratio, one-dimensional (1D) structures, whereas π-conjugated peptides with longer OT5 and OT6 cores assemble into disordered structures or lower aspect ratio 1D structures depending on assembly conditions. Phase diagrams for assembled materials are experimentally determined as a function of ionic strength, pH, temperature, and peptide concentration, revealing the impact of molecular sequence and π-conjugated core length on assembled morphologies. Molecular dynamics (MD) simulations are further used to probe the origins of microscale differences in assembly that arise from subtle changes in molecular identity. Broadly, our work elucidates the mechanisms governing the assembly of π-conjugated peptides, which will aid in efficient materials processing for soft electronic applications. Overall, these results highlight the complex phase behavior of biohybrid materials, including the impact of molecular sequence on assembly behavior and morphology.

Torsional Impacts on Quaterthiophene Segments Confined within Peptidic Nanostructures.

The co-assembly behavior of peptide-π-peptide and peptide-alkyl-peptide triblock molecules that form one-dimensional (1D) nanostructures under acidic, aqueous environments is dependent on the peptide sequence and the torsional constraints imposed within the nanomaterial volume. Although a hydrophilic tripeptide sequence (Asp-Asp-Asp, DDD-) previously promoted isolation/dilution of minority π-electron components in the matrix of aliphatic peptides, a ß-sheet promoting sequence (Asp-Val-Val, DVV-) led to blocks of the two components distributed within larger 1D self-assembled nanostructures. Furthermore, torsional restrictions exerted on the oligoaromatic π-electron unit by the self-assembly process can lead to changes in its conformation (for example, planarity), which has ramifications on its functionality within the peptide matrix. Here, we study this impact on thiophene-based π-electron units with inherently different geometries, viz., relatively planar 2,2':5',2″:5″,2‴-quaterthiophene and 3″,4'-dimethyl-2,2':5',2″:5″,2‴-quaterthiophene, which is twisted at the core bithiophene unit due to the presence of two methyl groups. These peptides were co-assembled at 5 and 20 mol % with peptide- n-decyl-peptide triblock molecules, and the resultant assemblies were studied using UV-vis absorption, photoluminescence, and circular dichroism spectroscopies. We found that torsional restriction in dimethylated quaterthiophene units can impact the stacking behavior of these 1D peptide nanoassemblies and have consequences on their photophysical properties. Additionally, these insights help in the understanding of the dependence of the optoelectronic properties of these materials on both the intrinsic conformation of π-units and the geometric constraints imposed by their immediate local environment under aqueous conditions.

Synthesis and Evaluation of Self-Assembled Nanostructures of Peptide-π Chromophore Conjugates.

Peptides provide a biomolecular scaffold for solubilizing and assembling π-conjugated molecules in aqueous media. The properties of such peptide-π chromophore conjugates can be manipulated by varying the constituent amino acid residues in the peptide backbone as well as the chromophore moieties. Such a precise handle on molecular structure leads to molecules with diverse macromolecular and material properties. We developed a versatile synthetic protocol that leads to a wide range of peptide-π chromophore conjugates in which the chromophores are covalently attached to the peptide backbone such that the chromophore is flanked by two peptide chains. This "trimer" structure lends interesting self-assembly properties to these materials which may be useful for a plethora of biological applications.

Highly Contrasting Static Charging and Bias Stress Effects in Pentacene Transistors with Polystyrene Heterostructures Incorporating Oxidizable N,N'-Bis(4-methoxyphenyl)aniline Side Chains as Gate Dielectrics.

Charge storage and trapping properties of polymer dielectrics govern the charge densities of adjacent semiconductors and greatly influence the on-off switching voltage (threshold voltage, V t h ) of organic field-effect transistors (OFETs) when the polymers are used as gate insulators. Intentional charging of polymer dielectrics in OFETs can change V t h and affect the bias stress. We describe a chemical design and fabrication protocol to construct multilayer-stack dielectrics for pentacene-based OFETs using different polystyrene (PS)-based polymers in each layer, with oxidizable N,N-bis(4-methoxyphenyl)anilino (TPAOMe)-substituted styrene copolymers in arbitrary vertical positions in the stacks. Thermal, byproduct-free cross-linking of benzocyclobutene subunits provides integrity to the multilayer structure by preventing dissolution of the previous deposited layer. Neutron reflectivity data verified the multilayer morphology. We compared the V t h shift before and after charging the stacks by application of ±100 V across 0.5-1 µm total film thicknesses. Bias stress was the dominant effect in bilayer devices with a TPAOMe layer in contact with the pentacene, indicated by the direction of V t h shift associated with either polarity of external electric field. In structures with no TPAOMe subunit in contact with the pentacene, when charging with -100 V on top of the source and drain electrodes, electron injection from pentacene to dielectric was the major charging mechanism, again consistent with the bias stress direction. When charging with +100 V, bilayer devices without TPAOMe showed little change in V t h , suggesting there was no bias stress effect or charge injection in these devices for this charging polarity. For the bilayer devices with the TPAOMe layer in the bottom, and the trilayer devices with TPOMe in the middle, when +100 V was applied, the V t h shifts were opposite those expected from bias stress. Dipole formation or partial ionization of chargeable groups at the interface between the dielectric layers are likely polarization mechanisms in these cases. A simple analytical model levelports the plausibility of these mechanisms. This work provides examples of both stabilization and shifting of V t h , and therefore controlling charge carrier density, in semiconductors overlying the dielectric multilayers.

Nonresonant and Local Field Effects in Peptidic Nanostructures Bearing Oligo(p-phenylenevinylene) Units.

Peptide nanostructures with built-in electronic functions offer a new platform for biomaterial science. In this report, we interrogate the influences of the immediate peptide environment around oligo(p-phenylenevinylene) (OPV3) electronic units embedded within one-dimensional peptide nanostructures on the resulting photophysics as assessed by UV-vis, photoluminescence (PL), and circular dichroism spectroscopies. To do so, we studied peptide-core-peptide molecules where the core was either OPV3 or an aliphatic n-decyl chain. Coassemblies of these molecules wherein the π-core was diluted as a minority component within a majority aliphatic matrix allowed for the variation of interchromophore exciton coupling commonly found in homoassemblies of peptide-OPV3-peptide monomers. Upon coassembly of the peptides, a hydrophilic tripeptide sequence (Asp-Asp-Asp-, DDD-) promoted the dilution/isolation of the peptide-π-peptide molecules in the majority peptide-decyl-peptide matrix whereas a hydrophobic tripeptide sequence (Asp-Val-Val-, DVV-) promoted the formation of self-associated stacks within the nanostructures. We also performed temperature variation studies to induce the reorganization of π-electron units in the spatially constrained n-decyl environment. This study elucidates the nonresonant (e.g., conformational) and local peptide field effects enforced within the internal environment of peptide nanomaterials and how they lead to varied photophysical properties of the embedded π-electron cores. It offers new insights on tuning the optoelectronic properties of these types of materials on the basis of the local electronic and steric environment available within the nanostructures.

Solid-Phase Synthesis of Self-Assembling Multivalent π-Conjugated Peptides.

We present a completely solid-phase synthetic strategy to create three- and four-fold peptide-appended π-electron molecules, where the multivalent oligopeptide presentation is dictated by the symmetries of reactive handles placed on discotic π-conjugated cores. Carboxylic acid and anhydride groups were viable amidation and imidation partners, respectively, and oligomeric π-electron discotic cores were prepared through Pd-catalyzed cross-couplings. Due to intermolecular hydrogen bonding between the three or four peptide axes, these π-peptide hybrids self-assemble into robust one-dimensional nanostructures with high aspect ratios in aqueous solution. The preparation of these systems via solid-phase methods will be detailed along with their self-assembly properties, as revealed by steady-state spectroscopy and transmission electron microscopy and electrical characterization using field-effect transistor measurements.

Low band gap thiophene-perylene diimide systems with tunable charge transport properties.

Perylenediimide-pentathiophene systems with varied architecture of thiophene units were synthesized. The photophysical, electrochemical, and charge transport behavior of the synthesized compounds were studied. Both molecules showed a low band gap of ∼1.4 eV. Surprisingly, the molecule with pentathiophene attached via ß-position to the PDI unit upon annealing showed a predominant hole mobility of 1 × 10(-4) cm(2) V(-1) s(-1) whereas the compound with branched pentathiophene attached via ß-position showed an electron mobility of 9.8 × 10(-7) cm(2) V(-1) s(-1). This suggests that charge transport properties can be tuned by simply varying the architecture of pentathiophene units.

Supramolecular assemblies of amphiphilic homopolymers.

Amphiphilic molecules self-assemble in solvents because of the differential solvation of the hydrophilic and lipophilic functionalities. Small-molecule surfactants have long been known to form micelles in water that can solubilize lipophilic guest molecules in their water-excluded interior. Polymeric surfactants based on block copolymers are also known to form several types of aggregates in water owing either to the mutual incompatibility of the blocks or better solvation of one of the blocks by the solvent. Incorporating amphiphilicity at smaller length scales in polymers would provide an avenue to capture the interesting properties of macromolecules and fine tune their supramolecular assemblies. To address this issue, we designed and synthesized amphiphilic homopolymers containing hydrophilic and lipophilic functionalities in the monomer. Such a polymer can be imagined to be a string of small-molecule surfactants tethered together such that the hydrophilic and lipophilic functionalities are located on opposite faces, rendering the assemblies facially amphiphilic. This feature article describes the self-assembly of our amphiphilic homopolymers in polar and apolar solvents. These homopolymers not only form micelles in water but also form inverse micelles in organic solvents. Subtle changes to the molecular structure have been demonstrated to yield vesicles in water and inverted micelles in organic solvents. The characterization of these assemblies and their applications in separations, catalysis, and sensing are described here.