Blended Conjugated Host and Unconjugated Dopant Polymers Towards N-type All-Polymer Conductors and High-ZT Thermoelectrics.

N-Type thermoelectrics typically consist of small molecule dopant+polymer host. Only a few polymer dopant+polymer host systems have been reported, and these have lower thermoelectric parameters. N-type polymers with high crystallinity and order are generally used for high-conductivity ( σ ${\sigma }$ ) organic conductors. Few n-type polymers with only short-range lamellar stacking for high-conductivity materials have been reported. Here, we describe an n-type short-range lamellar-stacked all-polymer thermoelectric system with highest σ ${\sigma }$ of 78 S-1 , power factor (PF) of 163 µW m-1 K-2 , and maximum Figure of merit (ZT) of 0.53 at room temperature with a dopant/host ratio of 75 wt%. The minor effect of polymer dopant on the molecular arrangement of conjugated polymer PDPIN at high ratios, high doping capability, high Seebeck coefficient (S) absolute values relative to σ ${\sigma }$ , and atypical decreased thermal conductivity ( κ ${\kappa }$ ) with increased doping ratio contribute to the promising performance.

In Vivo Formation and Tracking of π-Peptide Nanostructures.

The photophysics associated with the self-assembly of π-peptide molecules into 1-D nanostructures has been well-established, thus revealing the creation of nanoscale electronic conduits in aqueous media. Such materials have therapeutic potential in many biomedical applications. In this work, we report the in vivo deployment of these π-peptide nanostructures in brain tissue using photothrombotic stroke as a model application. A test peptide was used for brain injections, and the nanostructures formed were visualized with electron microscopy. A new peptide bearing a low-energy fluorescence dye was prepared to facilitate direct visualization of π-peptide localization in the brain cavity by way of fluorescence microscopy. This work demonstrates feasibility for in vivo application of π-peptide nanostructures toward pressing biomedical challenges.

Evolution of π-Peptide Self-Assembly: From Understanding to Prediction and Control.

Supramolecular materials derived from the self-assembly of engineered molecules continue to garner tremendous scientific and technological interest. Recent innovations include the realization of nano- and mesoscale particles (0D), rods and fibrils (1D), sheets (2D), and even extended lattices (3D). Our research groups have focused attention over the past 15 years on one particular class of supramolecular materials derived from oligopeptides with embedded π-electron units, where the oligopeptides can be viewed as substituents or side chains to direct the assembly of the central π-electron cores. Upon assembly, the π-systems are driven into close cofacial architectures that facilitate a variety of energy migration processes within the nanomaterial volume, including exciton transport, voltage transmission, and photoinduced electron transfer. Like many practitioners of supramolecular materials science, many of our initial molecular designs were designed with substantial inspiration from biologically occurring self-assembly coupled with input from chemical intuition and molecular modeling and simulation. In this feature article, we summarize our current understanding of the π-peptide self-assembly process as documented through our body of publications in this area. We address fundamental spectroscopic and computational tools used to extract information regarding the internal structures and energetics of the π-peptide assemblies, and we address the current state of the art in terms of recent applications of data science tools in conjunction with high-throughput computational screening and experimental assays to guide the efficient traversal of the π-peptide molecular design space. The abstract image details our integrated program of chemical synthesis, spectroscopic and functional characterization, multiscale simulation, and machine learning which has advanced the understanding and control of the assembly of synthetic π-conjugated peptides into supramolecular nanostructures with energy and biomedical applications.

A New Polystyrene-Poly(vinylpyridinium) Ionic Copolymer Dopant for n-Type All-Polymer Thermoelectrics with High and Stable Conductivity Relative to the Seebeck Coefficient giving High Power Factor.

A novel n-type copolymer dopant polystyrene-poly(4-vinyl-N-hexylpyridinium fluoride) (PSpF) with fluoride anions is designed and synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. This is thought to be the first polymeric fluoride dopant. Electrical conductivity of 4.2 S cm-1 and high power factor of 67 µW m-1 K-2 are achieved for PSpF-doped polymer films, with a corresponding decrease in thermal conductivity as the PSpF concentration is increased, giving the highest ZT of 0.1. An especially high electrical conductivity of 58 S cm-1 at 88 °C and outstanding thermal stability are recorded. Further, organic transistors of PSpF-doped thin films exhibit high electron mobility and Hall mobility of 0.86 and 1.70 cm2 V-1 s-1 , respectively. The results suggest that polystyrene-poly(vinylpyridinium) salt copolymers with fluoride anions are promising for high-performance n-type all-polymer thermoelectrics. This work provides a new way to realize organic thermoelectrics with high conductivity relative to the Seebeck coefficient, high power factor, thermal stability, and broad processing window.

Splitting the Ring: Impact of Ortho and Meta Pi Conjugation Pathways through Disjointed [8]Cycloparaphenylene Electronic Materials.

In this report, we describe the synthesis and electronic properties of small-molecule and polymeric [8]cycloparaphenylenes ([8]CPPs) with disjointed pi-conjugated substituents. Arylene-ethynylene linkers were installed on opposite sides of the [8]CPP nanohoop as separated by three phenyl units on either side such that the monomer systems have syn (C2 symmetry) and anti (C1 symmetry) conformers with a small energy gap (0.1-0.6 kcal/mol). This disjoined substitution pattern necessarily forces delocalization through and around the CPP radial structure. We demonstrate new electronic states from this radial/linear mixing in both the small molecules and the pi extended polymers. Quantum chemical calculations reveal that these electronic processes arise from multiple operative radial/linear conjugation pathways, as the disjoint pattern results in both ortho and meta connections to the CPP ring. These results affirm the unique nature of hybrid radial and linear pi electron delocalization operative in these new conjugation pathways.

Computationally Guided Tuning of Peptide-Conjugated Perylene Diimide Self-Assembly.

Peptide-π-conjugated materials are important for biointerfacing charge-transporting applications due to their aqueous compatibility and formation of long-range π-electron networks. Perylene diimides (PDIs), well-established charge-transporting π systems, can self-assemble in aqueous solutions when conjugated with amino acids. In this work, we leveraged computational guidance from our previous work to access two different self-assembled architectures from PDI-amino acid conjugates. Furthermore, we expanded the design rule to other sequences to learn that the closest amino acids to the π core have a significant effect on the photophysical properties of the resulting assemblies. By simply altering glycine to alanine at the closest residue position, we observed significantly different electronic properties as revealed through UV-vis, photoluminescence, and circular dichroism spectroscopies. Accompanying molecular dynamics simulations revealed two distinct types of self-assembled architectures: cofacial structures when the smaller glycine residue is at the closest residue position to the π core versus rotationally shifted structures when glycine is substituted for the larger alanine. This study illustrates the use of tandem computations and experiments to unearth and understand new design rules for supramolecular materials and exposes a modest amino acid substitution as a means to predictably modulate the supramolecular organization and engineer the photophysical properties of π-conjugated peptidic materials.

Carbonyl-Directed Aliphatic Fluorination: A Special Type of Hydrogen Atom Transfer Beats Out Norrish II.

Recently, our group reported that enone and ketone functional groups, upon photoexcitation, can direct site-selective sp3 C-H fluorination in terpenoid derivatives. How this transformation actually occurred remained mysterious, as a significant number of mechanistic possibilities came to mind. Herein, we report a comprehensive study describing the reaction mechanism through kinetic studies, isotope-labeling experiments, 19F NMR, electrochemical studies, synthetic probes, and computational experiments. To our surprise, the mechanism suggests intermolecular hydrogen atom transfer (HAT) chemistry is at play, rather than classical Norrish hydrogen atom abstraction as initially conceived. What is more, we discovered a unique role for photopromoters such as benzil and related compounds that necessitates their chemical transformation through fluorination in order to be effective. Our findings provide documentation of an unusual form of directed HAT and are of crucial importance for defining the necessary parameters for the development of future methods.

Computationally Guided Tuning of Amino Acid Configuration Influences the Chiroptical Properties of Supramolecular Peptide-π-Peptide Nanostructures.

Self-assembled supramolecular materials derived from peptidic macromolecules with π-conjugated building blocks are of enormous interest because of their aqueous solubility and biocompatibility. The design rules to achieve tailored optoelectronic properties from these types of materials can be guided by computation and virtual screening rather than intuition-based experimental trial-and-error. Using machine learning, we reported previously that the supramolecular chirality in self-assembled aggregates from VEVAG-π-GAVEV type peptidic materials was most strongly influenced by hydrogen bonding and hydrophobic packing of the alanine and valine residues. Herein, we build upon this idea to demonstrate through molecular-level experimental characterization and all-atom molecular modeling that varying the stereogenic centers of these residues has a profound impact on the optoelectronic properties of the supramolecular aggregates, whereas the variation of stereogenic centers of other residues has only nominal influence on these properties. This study highlights the synergy between computational and experimental insight relevant to the control of chiroptical or other electronic properties associated with supramolecular materials.

Quinonoid vs. aromatic structures of heteroconjugated polymers from oligomer calculations.

Conjugated polymers with quinonoid ground states can display low optical band gaps. The design of novel conjugated polymers with quinonoid ground states offers insights into the relative stabilities of aromatic vs. quinonoid structures. In this work, we present parameters such as the quinonoid (Q)/aromatic (A) energy difference, the band gap, and the C-C distances between the repeat units. This study reveals eight new polymers which exist in quinonoid ground state among twenty-nine polymers of varying structural composition that were subject to analysis. We expect that copolymerizing such quinonoid ground state monomers with aromatic ground state monomers will modulate the bandgap of the resulting polymers.

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.

Discovery of Self-Assembling π-Conjugated Peptides by Active Learning-Directed Coarse-Grained Molecular Simulation.

Electronically active organic molecules have demonstrated great promise as novel soft materials for energy harvesting and transport. Self-assembled nanoaggregates formed from π-conjugated oligopeptides composed of an aromatic core flanked by oligopeptide wings offer emergent optoelectronic properties within a water-soluble and biocompatible substrate. Nanoaggregate properties can be controlled by tuning core chemistry and peptide composition, but the sequence-structure-function relations remain poorly characterized. In this work, we employ coarse-grained molecular dynamics simulations within an active learning protocol employing deep representational learning and Bayesian optimization to efficiently identify molecules capable of assembling pseudo-1D nanoaggregates with good stacking of the electronically active π-cores. We consider the DXXX-OPV3-XXXD oligopeptide family, where D is an Asp residue and OPV3 is an oligophenylenevinylene oligomer (1,4-distyrylbenzene), to identify the top performing XXX tripeptides within all 203 = 8000 possible sequences. By direct simulation of only 2.3% of this space, we identify molecules predicted to exhibit superior assembly relative to those reported in prior work. Spectral clustering of the top candidates reveals new design rules governing assembly. This work establishes new understanding of DXXX-OPV3-XXXD assembly, identifies promising new candidates for experimental testing, and presents a computational design platform that can be generically extended to other peptide-based and peptide-like systems.

Unusually Conductive Organic-Inorganic Hybrid Nanostructures Derived from Bio-Inspired Mineralization of Peptide/Pi-Electron Assemblies.

Supramolecular materials derived from pi-conjugated peptidic macromolecules are well-established to self-assemble into 1D nanostructures. In the presence of KOH, which was used to more fully dissolve the peptide macromolecules prior to triggering the self-assembly by way of exposure to HCl vapor, we report here an unexpected mineralization of KCl as templated presumably by the glutamic acid residues that were present along the backbone of the peptide macromolecules. In order to decouple the peptidic side chains from the central pi-electron unit, three-carbon spacers were added between them on both sides. The assembled structures that resulted from the collective formation of ß-sheets, π-orbital overlaps, and mineralization resulted in highly interconnected dendritic structures under suitable KOH concentrations. Electrical measurements indicated that when well-interconnected, these dendritic structures maintained conductivities comparable to those of metals at around 1800 S/cm. About 50 mA current was measured for 0.5 V/37.5 µm. Varying the gate voltage in a transistor configuration had no effect on the current levels, indicating a conductive instead of a semiconducting pathway. Control experiments without the peptide, measurements of conductivity over time, and conductivity quenching by ammonia suggested the conductivity of these dendritic networks was derived from proton doping of the central π-electron units in a strong acid environment and was facilitated by closely spaced chromophores, as suggested in the literature, leading to facile π-electron transfer along the interconnected dendritic pathways. Our findings suggest that mineralization templated by appropriate amino acids combined with peptide/π-electron self-assembly can lead to highly conductive dendritic macrostructures as well as control of nanowire growth in specific directions.

Core structure dependence of cycloreversion dynamics in diarylethene analogs.

Diarylperfluorocyclopentenes are a well-characterized class of molecular photoswitches that undergo reversible photocyclization. The efficiency of cycloreversion (<∼30%), in particular, is known to be limited by a competition with excited-state deactivation by internal conversion that is strongly impacted by the electron-withdrawing/donating character of pendant aryl groups. Here we present a first study to determine how varied structural motifs for the core bridge group impact excited-state dynamics that control cycloreversion quantum yields. Specifically, we compare photophysical behaviors of 3,3'-(perfluorocyclopent-1-ene-1,2-diyl)bis(2-methylbenzo[b]thiophene) with diarylethene derivatives possessing the same benzo[b]thiophene pendant group but with a rigid 1-methyl-1H-pyrrole-2,5-dione and a rigid/aromatic thieno[3,4-b]thiophene bridge (TT) core bridge group. We find that the flexible perfluorocyclopentene core undergoes cycloreversion 3-4× slower than the rigid core photoswitches (9 vs. 2-3 ps in acetonitrile, 25 vs. 5-6 ps in cyclohexane) despite comparable cycloreversion quantum yields. To distinguish effects induced by bridge vs. pendant groups, we also studied a series of photoswitches with the same thieno[3,4-b]thiophene bridging group, but with varied pendant groups including 2,5-dimethylthiophene and 2-(3,5-bis(trifluoromethyl)phenyl)-5-methylthiophene. Analysis of temperature-dependent excited-state lifetimes and cycloreversion quantum yields reveals that both the rates of nonreactive internal conversion and reactive cycloreversion increase with greater structural rigidity of the core. This difference is attributed to smaller energy barriers on the excited-state potential energy surface for both reactive and non-reactive deactivation from the 21A electronic state relative to the flexible perfluorocyclopentene switch, implying that a rigid core results in a net shallower excited-state potential energy surface.

Linear and Radial Conjugation in Extended π-Electron Systems.

We describe the synthesis and electronic properties of new π-conjugated small molecules and polymers that combine the linear intramolecular conjugation pathways commonly associated with organic electronic materials with the emerging properties of radial conjugation found in cycloparaphenylenes (CPPs) and other curved π-surfaces. Using arylene ethynylenes as prototypical linear segments and [6]/[8]CPP as the radial segments, we demonstrate the formation of new electronic states that are not simply additive responses from the individual components. Quantum chemical calculations of model oligomeric structures reveal these electronic processes to arise from the hybrid nature of wave function delocalization over the linear and radial contributors in the photophysically relevant electronic states.

Correction: Solid-state electrical applications of protein and peptide based nanomaterials.

Correction for 'Solid-state electrical applications of protein and peptide based nanomaterials' by Sayak Subhra Panda et al., Chem. Soc. Rev., 2018, 47, 3640-3658.

Two-electron transfer stabilized by excited-state aromatization.

The scientific significance of excited-state aromaticity concerns with the elucidation of processes and properties in the excited states. Here, we focus on TMTQ, an oligomer composed of a central 1,6-methano[10]annulene and 5-dicyanomethyl-thiophene peripheries (acceptor-donor-acceptor system), and investigate a two-electron transfer process dominantly stabilized by an aromatization in the low-energy lying excited state. Our spectroscopic measurements quantitatively observe the shift of two π-electrons between donor and acceptors. It is revealed that this two-electron transfer process accompanies the excited-state aromatization, producing a Baird aromatic 8π core annulene in TMTQ. Biradical character on each terminal dicyanomethylene group of TMTQ allows a pseudo triplet-like configuration on the 8π core annulene with multiexcitonic nature, which stabilizes the energetically unfavorable two-charge separated state by the formation of Baird aromatic core annulene. This finding provides a comprehensive understanding of the role of excited-state aromaticity and insight to designing functional photoactive materials.

Revealing the Sequence-Structure-Electronic Property Relation of Self-Assembling π-Conjugated Oligopeptides by Molecular and Quantum Mechanical Modeling.

Self-assembled nanoaggregates of π-conjugated synthetic peptides present a biocompatible and highly tunable alternative to silicon-based optical and electronic materials. Understanding the relationship between structural morphology and electronic properties of these assemblies is critical for understanding and controlling their mechanical, optical, and electronic responses. In this work, we combine all-atom classical molecular simulations with quantum mechanical electronic structure calculations to ascertain the sequence-structure-electronic property relationship within a family of Asp-X-X-quaterthiophene-X-X-Asp (DXX-OT4-XXD) oligopeptides in which X is one of the five amino acids {Ala, Phe, Gly, Ile, Val} ({A, F, G, I, V}). Molecular dynamics simulations reveal that smaller amino acid substituents (A, G) favor linear stacking within a peptide dimer, whereas larger groups (F, I, V) induce larger twist angles between the peptides. Density functional theory calculations on the dimer show the absorption spectrum to be dominated by transitions between carbon and sulfur p orbitals. Although the absorption spectrum is largely insensitive to the relative twist angle, the highest occupied molecular orbital strongly localizes onto one molecule within the dimer at large twist angles, impeding the efficiency of transport between molecules. Our results provide a fundamental understanding of the relation between peptide orientation and electronic structure and offer design precepts for rational engineering of these systems.

Controlling Supramolecular Chirality in Peptide-π-Peptide Networks by Variation of the Alkyl Spacer Length.

Self-assembled supramolecular organic materials with π-functionalities are of great interest because of their applications as biocompatible nanoelectronics. A detailed understanding of molecular parameters to modulate the formation of hierarchical structures can inform design principles for materials with engineered optical and electronic properties. In this work, we combine molecular-level characterization techniques with all-atom molecular simulations to investigate the subtle relationship between the chemical structure of peptide-π-peptide molecules and the emergent supramolecular chirality of their spontaneously self-assembled nanoaggregates. We demonstrate through circular dichroism measurements that we can modulate the chirality by incorporating alkyl spacers of various lengths in between the peptides and thienylene-phenylene π-system chromophores: even numbers of alkyl carbons in the spacer units (0, 2) induce M-type helical character whereas odd numbers (1, 3) induce P-type. Corroborating molecular dynamics simulations and explicating machine learning analysis techniques identify hydrogen bonding and hydrophobic packing to be the principal discriminants of the observed chirality switches. Our results present a molecular-level design rule to engineer chirality into optically and electronically active nanoaggregates of these peptidic building blocks by exploiting systematic variations in the alkyl spacer length.

Quantum Interference Enhanced Chemical Responsivity in Single-Molecule Dithienoborepin Junctions.

Providing a chemical control over charge transport through molecular junctions is vital to developing sensing applications at the single-molecule scale. Quantum-interference effects that affect the charge transport through molecules offer a unique chance to enhance the chemical control. Here, we investigate how interference effects can be harnessed to optimize the response of single molecule dithienoborepin (DTB) junctions to the specific coordination of a fluoride ion in solution. The single-molecule conductance of two DTB isomers is measured using scanning tunneling microscopy break-junction (STM-BJ) before and after fluoride ion exposure. We find a significant change of conductance before and after the capture of a fluoride ion, the magnitude of which depends on the position of the boron atom in the molecular structure. This single-molecule sensor exhibits switching ratios of up to four orders of magnitudes, suggesting that the boron-fluoride coordination can lead to quantum-interference effects. This is confirmed by a quantum chemical characterization, pointing toward a cross-conjugated path through the molecular structure as the origin of the effect.

Energy- and conformer-dependent excited-state relaxation of an E/Z photoswitchable thienyl-ethene.

Bis(bithienyl)-1,2-dicyanoethene (4TCE) is a photoswitch that operates via reversible E/Z photoisomerization following absorption of visible light. cis-to-trans photoisomerization of 4TCE requires excitation below 470 nm, is relatively inefficient (quantum yield < 5%) and occurs via the lowest-lying triplet. We present excitation-wavelength dependent (565-420 nm) transient absorption (TA) studies to probe the photophysics of cis-to-trans isomerization to identify sources of switching inefficiency. TA data reveals contributions from more than one switch conformer and relaxation cascades between multiple states. Fast (∼4 ps) and slow (∼40 ps) components of spectral dynamics observed at low excitation energies (>470 nm) are readily attributed to deactivation of two conformers; this assignment is supported by computed thermal populations and absorption strengths of two molecular geometries (PA and PB) characterized by roughly parallel dipoles for the thiophenes on opposite sides of the ethene bond. Only the PB conformer is found to contribute to triplet population and the switching of cis-4TCE: high-energy excitation (<470 nm) of PB involves direct excitation to S2, relaxation from which prepares an ISC-active S1 geometry (ISC QY 0.4-0.67, kISC∼ 1.6-2.6 × 10-9 s-1) that is the gateway to triplet population and isomerization. We ascribe low cis-to-trans isomerization yield to excitation of the nonreactive PA conformer (75-85% loss) as well as loses along the PB S2→ S1→ T1 cascade (10-20% loss). In contrast, electrocyclization is inhibited by the electronic character of the excited states, as well as a non-existent thermal population of a reactive "antiparallel" ring conformation.