Molecular Electrets Research Articles

(Invited) Bioinspired Molecular Electrets: “Misbehavior” of Dipoles, Solvation, and Interfaces

Charge transfer (CT) and charge transport are of fundamental importance for sustaining life on Earth and for making our modern ways of living possible (Phys. Chem. Chem. Phys. 2020, 22, 21583-21629). Concurrently, the ubiquitous nature of electric dipoles warrants deep understanding of how they affect CT. Possessing order electric dipoles, electrets are the electrostatic analogues of magnets, and they present outstanding paradigms for exploring dipole effects on CT (Can. J. Chem. 2018, 96, 843-858). Polypeptide α-helices are some of the best-known molecular electrets. These natural structures, however, do not efficiently mediate CT at distances exceeding 2 nm, which limits their utility. Therefore, we undertake the task for developing bioinspired molecular electrets based on synthetic amino acids that provide sites for charge-hopping and allow CT beyond the 2-nm limit. Even a single electret amino-acid residue can rectify CT (Angew. Chem. Int. Ed. 2018, 57, 12365-12369; J. Am. Chem. Soc. 2014, 136, 12966-12973). The molecular macrodipoles and the CT properties of these bioinspired molecular electrets strongly depend on their structure and conformational flexibility. Sinergy between molecular-dynamics (MD) simulations and quantum-mechanical (QM) calculations provides insights into the structural properties of these macromolecules. The structures of the molecular electrets do not fluctuate much. Nevertheless, the macrodipoles of these conjugates show picosecond fluctuations with amplitudes amounting to a factor of two or three from the average values. The structural variations of the electret oligomers cannot account for the dipole fluctuations. The analysis, however, reveals that the solvent dynamics, inducing fast-oscillating Onsager reaction fields in the solvated cavities, is principally responsible for the huge dipole fluctuations. The impact of this discovery extends way beyond the realm of electrets and our explorations show that solvent-induced picosecond dipole fluctuations are a norm, rather than an exception, for solvated species and interfaces. Averaging these dipole fluctuations over 10-ps or 20-ps intervals drastically decreases their amplitudes. It indicates that they would have a minimum effect on nanosecond and sub-nanosecond CT. Nevertheless, these dipole fluctuations may have decisive effects on picosecond and sub-picosecond processes, such as ultrafast CT. These revelations about the dipole dynamics present unexplored paradigms for condensed-phase interfaces and electronic materials.

How Permanent Are the Permanent Macrodipoles of Anthranilamide Bioinspired Molecular Electrets?

Dipoles are ubiquitous, and their impacts on materials and interfaces affect many aspects of daily life. Despite their importance, dipoles remain underutilized, often because of insufficient knowledge about the structures producing them. As electrostatic analogues of magnets, electrets possess ordered electric dipoles. Here, we characterize the structural dynamics of bioinspired electret oligomers based on anthranilamide motifs. We report dynamics simulations, employing a force field that allows dynamic polarization, in a variety of solvents. The results show a linear increase in macrodipoles with oligomer length that strongly depends on solvent polarity and hydrogen-bonding (HB) propensity, as well as on the anthranilamide side chains. An increase in solvent polarity increases the dipole moments of the electret structures while decreasing the dipole effects on the moieties outside the solvation cavities. The former is due to enhancement of the Onsager reaction field and the latter to screening of the dipole-generated fields. Solvent dynamics hugely contributes to the fluctuations and magnitude of the electret dipoles. HB with the solvent weakens electret macrodipoles without breaking the intramolecular HB that maintains their extended conformation. This study provides design principles for developing a new class of organic materials with controllable electronic properties. An animated version of the TOC graphic showing a sequence of the MD trajectories of short and long molecular electrets in three solvents with different polarities is available in the HTML version of this paper.

What defines biomimetic and bioinspired science and engineering?

Abstract Biomimicry, biomimesis and bioinspiration define distinctly different approaches for deepening the understanding of how living systems work and employing this knowledge to meet pressing demands in engineering. Biomimicry involves shear imitation of biological structures that most often do not reproduce the functionality that they have while in the living organisms. Biomimesis aims at reproduction of biological structure-function relationships and advances our knowledge of how different components of complex living systems work. Bioinspiration employs this knowledge in abiotic manners that are optimal for targeted applications. This article introduces and reviews these concepts in a global historic perspective. Representative examples from charge-transfer science and solar-energy engineering illustrate the evolution from biomimetic to bioinspired approaches and show their importance. Bioinspired molecular electrets, aiming at exploration of dipole effects on charge transfer, demonstrate the pintail impacts of biological inspiration that reach beyond its high utilitarian values. The abiotic character of bioinspiration opens doors for the emergence of unprecedented properties and phenomena, beyond what nature can offer.

On the Search of a Silver Bullet for the Preparation of Bioinspired Molecular Electrets with Propensity to Transfer Holes at High Potentials.

Biological structure-function relationships offer incomparable paradigms for charge-transfer (CT) science and its implementation in solar-energy engineering, organic electronics, and photonics. Electrets are systems with co-directionally oriented electric dopes with immense importance for CT science, and bioinspired molecular electrets are polyamides of anthranilic-acid derivatives with designs originating from natural biomolecular motifs. This publication focuses on the synthesis of molecular electrets with ether substituents. As important as ether electret residues are for transferring holes under relatively high potentials, the synthesis of their precursors presents formidable challenges. Each residue in the molecular electrets is introduced as its 2-nitrobenzoic acid (NBA) derivative. Hence, robust and scalable synthesis of ether derivatives of NBA is essential for making such hole-transfer molecular electrets. Purdie-Irvine alkylation, using silver oxide, produces with 90% yield the esters of the NBA building block for iso-butyl ether electrets. It warrants additional ester hydrolysis for obtaining the desired NBA precursor. Conversely, Williamson etherification selectively produces the same free-acid ether derivative in one-pot reaction, but a 40% yield. The high yields of Purdie-Irvine alkylation and the selectivity of the Williamson etherification provide important guidelines for synthesizing building blocks for bioinspired molecular electrets and a wide range of other complex ether conjugates.

Molecular electrets – Why do dipoles matter for charge transfer and excited-state dynamics?

For decades, the natural systems, and in particular photosynthesis, have provided the principal paradigms for solar-energy science and engineering. Charge transfer (CT) is at the core of life-sustaining biological processes, including the natural energy conversion and storage. Concurrently, nanoscale CT governs the performance of electronic and energy-conversion devices. Electric fields are invaluable for guiding charge movement. Therefore, as electrostatic analogues of magnets, electrets have unexplored potential for generating local electric fields for accelerating desired CT processes while suppressing undesired ones. This publication presents the design principles and the development of CT bioinspired molecular electrets, along with some of the synthetic challenges for obtaining the optimal motifs. We discuss these advances in the contexts of how dipoles affect CT and excited-state dynamics. The underlying complexity of dipole-induced effects reveals unexplored paradigms for CT science, as well as for electronic and optical engineering. The ubiquity of electric dipoles underlines the immense potentials that electrets have for electronics, photonics and energy-conversion.

Solvent-induced selectivity of Williamson etherification in the pursuit of amides resistant against oxidative degradation.

This article reports two discoveries. (1) 2-Methoxyethanol induces unprecedented selectivity for etherification of 5-hydroxy-2-nitrobenzic acids without forming undesired esters. (2) Such compounds are precursors for amides showing unusual robustness against oxidative degradation, essential for molecular electrets that transfer strongly oxidizing holes at about −6.4 eV vs. vacuum.

Biomimetic and bioinspired molecular electrets. How to make them and why does the established peptide chemistry not always work?

Abstract “Biomimetic” and “bioinspired” define different aspects of the impacts that biology exerts on science and engineering. Biomimicking improves the understanding of how living systems work, and builds tools for bioinspired endeavors. Biological inspiration takes ideas from biology and implements them in unorthodox manners, exceeding what nature offers. Molecular electrets, i.e. systems with ordered electric dipoles, are key for advancing charge-transfer (CT) science and engineering. Protein helices and their biomimetic analogues, based on synthetic polypeptides, are the best-known molecular electrets. The inability of native polypeptide backbones to efficiently mediate long-range CT, however, limits their utility. Bioinspired molecular electrets based on anthranilamides can overcome the limitations of their biological and biomimetic counterparts. Polypeptide helices are easy to synthesize using established automated protocols. These protocols, however, fail to produce even short anthranilamide oligomers. For making anthranilamides, the residues are introduced as their nitrobenzoic-acid derivatives, and the oligomers are built from their C- to their N-termini via amide-coupling and nitro-reduction steps. The stringent requirements for these reduction and coupling steps pose non-trivial challenges, such as high selectivity, quantitative yields, and fast completion under mild conditions. Addressing these challenges will provide access to bioinspired molecular electrets essential for organic electronics and energy conversion.

(Invited) Organic Molecular Electrets: Multifaceted Complexity of Dipole Effects on Charge Transfer

Electric dipoles are everywhere. While they provide potentially important paradigms for controlling charge transfer (CT), dipole effects on CT still remain largely unexplored. CT is among the most fundamental processes responsible for sustaining life on earth and for ensuring the functionality of a range of energy and electronic devices that are an intricate part of everyday life. Therefore, we pioneer the concept of bioinspired molecular electrets that, in addition to possessing large macrodipoles, also can mediate long-range CT. (Electrets are systems with ordered electric dipoles, i.e., they are the electrostatic analogies of magnets.) Our electret designs are based on polypeptides composed of non-native aromatic beta-amino acids. We observe that even a single electret residue induces rectification of CT that surpasses what is reported for similar systems utilizing polypeptide helices composed of native amino acids. While dipoles generate enormous fields around them, the strength of these localized fields significantly decays with distance. Therefore, we incorporate the dipoles within the participants in the CT steps. It allows us to observe unprecedentedly large dipole effects on the CT kinetics.5 A decrease in solvent polarity increases the rates of CT. It appears counterintuitive, and it should be the other way around: polar media stabilizes the formed charged CT states and thus increases the rates of photoinduced CT in the Marcus normal region. Concurrently, polar media screens the dipole-generated fields and suppresses their effects on CT. For non-polar solvents, changing the dipole orientation can alter the reduction potentials of the photosensitizers with hundreds of millivolts. Therefore, a careful balance between the two opposing polarity effects allows us to demonstrate CT in lipophilic media, such as toluene. For non-polar solvent, we observe enhancement of electron-transfer (ET) rates along the dipole, while ET against the dipole is completely suppressed. By modifying the semi-classical CT theory, we quantify such experimentally observed dipole effects for the first time. In addition to affecting the reduction potentials and thus the Franck-Condon aspects of the CT kinetics, the molecular electrets reveal that dipoles can also drastically change the CT rates by altering the donor-acceptor electronic coupling. This discovery contradicts the accepted notion about dipole effects on CT, where the focus is on the Franck-Condon nuclear aspects of the kinetics and the electronic contributions are considered insignificant. Our findings not only lead to rethinking the fundamentals of dipole effects on CT, but also open new avenues for development of unprecedented paradigms for organic electronics and energy materials.

Dipole-induced effects on charge transfer and charge transport. Why do molecular electrets matter?

Charge transfer (CT) and charge transport (CTr) are at the core of life-sustaining biological processes and of processes that govern the performance of electronic and energy-conversion devices. Electric fields are invaluable for guiding charge movement. Therefore, as electrostatic analogues of magnets, electrets have unexplored potential for generating local electric fields for accelerating desired CT processes and suppressing undesired ones. The notion about dipole-generated local fields affecting CT has evolved since the middle of the 20th century. In the 1990s, the first reports demonstrating the dipole effects on the kinetics of long-range electron transfer appeared. Concurrently, the development of molecular-level designs of electric junctions has led the exploration of dipole effects on CTr. Biomimetic molecular electrets such as polypeptide helices are often the dipole sources in CT systems. Conversely, surface-charge electrets and self-assembled monolayers of small polar conjugates are the preferred sources for modifying interfacial electric fields for controlling CTr. The multifaceted complexity of such effects on CT and CTr testifies for the challenges and the wealth of this field that still remains largely unexplored. This review outlines the basic concepts about dipole effects on CT and CTr, discusses their evolution, and provides accounts for their future developments and impacts.

Bioinspired approach toward molecular electrets: synthetic proteome for materials

Abstract Molecular-level control of charge transfer (CT) is essential for both, organic electronics and solar-energy conversion, as well as for a wide range of biological processes. This article provides an overview of the utility of local electric fields originating from molecular dipoles for directing CT processes. Systems with ordered dipoles, i.e. molecular electrets, are the centerpiece of the discussion. The conceptual evolution from biomimicry to biomimesis, and then to biological inspiration, paves the roads leading from testing the understanding of how natural living systems function to implementing these lessons into optimal paradigms for specific applications. This progression of the evolving structure-function relationships allows for the development of bioinspired electrets composed of non-native aromatic amino acids. A set of such non-native residues that are electron-rich can be viewed as a synthetic proteome for hole-transfer electrets. Detailed considerations of the electronic structure of an individual residue prove of key importance for designating the points for optimal injection of holes (i.e. extraction of electrons) in electret oligomers. This multifaceted bioinspired approach for the design of CT molecular systems provides unexplored paradigms for electronic and energy science and engineering.

Bioinspired Molecular Electrets: Bringing Proteomic Approaches to Charge-Transfer Systems

Controlling charge transfer (CT) at a molecular level is immensely important for energy and electronic applications. Electrets, which are inherently dielectrics, possess ordered electric dipoles. That is, electrets are electrostatic analogies of magnets and they offer key paradigms for steering photogenerated charge carriers, usually away from each other, improving charge separation efficiencies. As bioinspired molecular electrets, anthranilamides possess an intrinsic dipole moment originating from their ordered amide and hydrogen bonds. The aromatic residues and extended π-conjugation along the backbone provides a means for long-range CT. Composed of non-native amino acids, i.e., derivatives of the anthranilica acid, these electrets are polypeptides and provide a means to employ principles of proteomics in their design and development. Rational functionalization of the non-native residues allows for accessing a wide range of electronic properties, as reveled by varying their reduction potentials. Development of such non-native redox-active non-native amino acids sets the foundation for introducing synthetic proteome to the field of charge transfer.

Bioinspired Molecular Electrets: Bringing Proteomic Approaches to Charge-Transfer Systems

The ability to control charge transfer at molecular and nanometer scales represents the ultimate level of electronic mastery, and its impacts on electronic, energy and other applications cannot be overstated. As electrostatic analogues of magnets, electrets possess ordered electric dipoles that present key paradigms for directing transduction of electrons and holes. We undertake a bioinspired approach in the design of molecular electrets based on anthranilamides.[J. Phys. Chem. Lett. 2011, 2, 503–508] Similar to protein helices, the anthranilamides possess intrinsic dipoles originating from ordered amide and hydrogen bonds, i.e., they are composed of non-native anthranilic residues.[J. Org. Chem. 2013, 78, 1994-2004; Pure Appl. Chem. 2015, 87, 779-792] Unlike proteins, however, the bioinspired molecular electrets have extended pi-conjugation along their backbones providing a means for efficient charge transfer. The anthranilic molecular electrets are polypeptides of non-native amino acids. This feature offers unexplored routes for bringing principles of proteomics into the de novo designs of electronic molecular systems and materials. In analogy, less than a couple of dozen transcribable native alpha-amino acids, each with a different single side chain, are the building blocks of proteins with countless structural and functional characteristics yielding the amazing diversity of life on our planet. Therefore, a similar set of Aa residues with different electronic properties should prove most essential for the design of molecular electrets with a wide range of charge-transfer properties. Unlike the native amino acids, each of the anthranic residues has two side chains that we selectively alter. Electrochemical and spectroelectrochemical studies revealed that the electronic properties of an anthranilic residue depend not only on the type of a substituent, but also on its exact position [J. Phys. Chem. Lett. 2016, 7, 758-764]. This regio-dependence of properties offers a larger diversity in anthranilamide residues than what native-type amino acids can offer, demonstrating once again the clear advantage of bioinspired over biomimetic or biomediated approaches. The bioinspired molecular electrets, indeed, rectify the kinetics of charge separation, i.e., they act as molecular diodes.[J. Am. Chem. Soc. 2014, 136, 12966-12973] The rates of electron transfer along the electret dipoles are faster than the rates against the dipoles. For processes with small driving forces, the modulation of the energies of the charge-transfer states accounts for the observed rectification trends. Conversely, for processes with a large graving force, such as charge-recombination, it is the asymmetry in the electronic coupling between the electret residues and the axillary photosensitizer. These effects become more pronounced as the length of the anthranilic oligomers increases. Another feature that is affected by the electret length is the kinetic isotope effect on charge-transfer kinetics. Replacing all amide protons with a deuterium significantly slows down charge separation and charge recombination. While the amide protons are not on the charge-transfer pathways, most likely, their modes of vibration couple with charge-transfer transition-state modes affecting its kinetics. Currently, we have developed more than 15 non-native anthranilic residues. Controlling the sequence of these residues in the molecular electrets allows for introducing barriers, wells, and rectifying junctions on the charge-transfer pathways. These findings and developments provide unexplored paradigms for energy science and organic electronics.

Fluorinated aminoanthranilamides: non-native amino acids for bringing proteomic approaches to charge-transfer systems.

The ability to control charge transfer at molecular and nanometer scales represents the ultimate level of electronic mastery, and its impacts cannot be overstated. As electrostatic analogues of magnets, electrets possess ordered electric dipoles that present key paradigms for directing transduction of electrons and holes. Herein we describe the design and development of fluorinated aminoanthranilamides, derivatives of non-native aromatic beta-amino acids, as building blocks for hole-transfer molecular electrets. A highly regio-selective nucleophilic aromatic substitution of difluorinated nitrobenzoic acid provides the underpinnings for an array of unprecedented anthranilamide structures. Spin density distribution and electrochemical analyses reveal that fluorine induces about 200 mV positive shifts in reduction potentials without compromising the stability of the oxidized residues, making them invaluable building blocks for hole-transfer systems. These findings open unexplored routes to novel amino-acid structures, setting a foundation for bringing principles of proteomics to designs of charge-transfer systems.

Electrochemical Properties of Residues for Bioinspired Molecular Electrets

Anthranilamides are a class of bioinspired molecular electrets that have promising characteristics for organic electronics and energy applications. Derivatives of anthranilic acid, i.e., non-native aromatic beta-amino acids, are the principal building blocks for the anthranilamide conjugates. Derivatization with three types of electron-donating substituents results in anthranilamide residues exhibiting a diverse range of reduction potentials (that correlate with their ionization energies). The electrochemical trends reveal a structural and functional means for course- and fine-tuning of the electronic properties of anthranilamides. These findings set a foundation of design principles for easily modifiable organic electrets, and in particular for electrets that mediate charge transfer.

Building blocks for bioinspired electrets: molecular-level approach to materials for energy and electronics

Abstract In biology, an immense diversity of protein structural and functional motifs originates from only 20 common proteinogenic native amino acids arranged in various sequences. Is it possible to attain the same diversity in electronic materials based on organic macromolecules composed of non-native residues with different characteristics? This publication describes the design, preparation and characterization of non-native aromatic β-amino acid residues, i.e. derivatives of anthranilic acid, for polyamides that can efficiently mediate hole transfer. Chemical derivatization with three types of substituents at two positions of the aromatic ring allows for adjusting the energy levels of the frontier orbitals of the anthranilamide residues over a range of about one electronvolt. Most importantly, the anthranilamide residues possess permanent electric dipoles, adding to the electronic properties of the bioinspired conjugates they compose, making them molecular electrets.

Bioinspired molecular electrets: bottom-up approach to energy materials and applications

© 2015 Society of Photo-Optical Instrumentation Engineers (SPIE). The diversity of life on Earth is made possible through an immense variety of proteins that stems from less than a couple of dozen native amino acids. Is it possible to achieve similar engineering freedom and precision to design electronic materials? What if a handful of nonnative residues with a wide range of characteristics could be rationally placed in sequences to form organic macromolecules with specifically targeted properties and functionalities? Referred to as molecular electrets, dipolar oligomers and polymers composed of non-native aromatic beta-amino acids, anthranilamides (Aa) provide venues for pursuing such possibilities. The electret molecular dipoles play a crucial role in rectifying charge transfer, e.g., enhancing charge separation and suppressing undesired charge recombination, which is essential for photovoltaics, photocatalysis, and other solar-energy applications. A set of a few Aa residues can serve as building blocks for molecular electrets with widely diverse electronic properties, presenting venues for bottom-up designs. We demonstrate how three substituents and structural permutations within an Aa residue widely alter its reduction potential. Paradigms of diversity in electronic properties, originating from a few changes within a basic molecular structure, illustrate the promising potentials of biological inspiration for energy science and engineering.

Bioinspired Molecular Electrets for Organic Electronics and Energy Applications

Controlling charge transfer at nanometer scale is crucial for electronic materials and energy conversion. Because local electric fields provide a means for “steering” charge carriers, molecular electrets present an important paradigm for energy science and molecular electronics. An “electret,” or specifically, a “dipole-polarization electret,” is an electrostatic analogue of a magnet, i.e., an electret contains co-directionally ordered permanent electric dipoles. While it is paramount for the electrets to be dielectrics (free charge carriers will screen the fields generated by the intrinsic dipoles and suppress their effects), adjusting their reduction and oxidation properties provides a means for attaining electron or hole transduction through them. We undertake a bioinspired approach in the design of molecular electrets based on anthranilamides.[J. Phys. Chem. Lett. 2011, 2, 503–508] Similar to protein helices, the anthranilamides possess intrinsic dipoles originating from ordered amide and hydrogen bonds, i.e., they are composed of non-native anthranilic residues.[J. Org. Chem. 2013, 78, 1994-2004] Unlike their biological counterparts, however, the bioinspired molecular electrets have extended π-conjugation along their backbones providing a means for efficient charge transfer. Following principles of proteomics, the bioinspired electrets provide venues for attaining an immensely broad diversity of electronic and photonic functionalities by altering the sequences of the non-native anthranilic residues. In biological systems, 20 native amino acids (arranges in various sequences) dictate the countless functionalities of proteins. The single side chain of a native amino acid determines the structural and functional features it introduces to the biomolecules. The anthranilic residues have two side chains that we can alter. Electrochemical and spectroelectrochemical studies revealed that the electronic properties of an anthranilic residue depend not only on the type of a side-chain substituents, but also on its exact position. This regio-dependence of properties offers a much larger diversity in the anthranilic resideus than what native-type amino acids with single side chains provide, demonstrating the clear advantage of bioinspired over biomimetic or biomediated approaches. The bioinspired molecular electrets, indeed, rectify the kinetics of charge separation, i.e., they act as molecular diodes.[J. Am. Chem. Soc. 2014, 136, 12966-12973] The rates of electron transfer along the electret dipoles are faster than the rates against the dipoles. (The direction of the dipoles is defined from their negative to their positive poles.) For processes with small driving forces, the modulation of the energies of the charge-transfer states accounts for the observed rectification trends. Conversely, the same explanation could not account for the observed rectification of charge recombination. Regioselectivity of electron trasnfer proved to prevail the charge-recombination kinetics. These new mechanistic findings provide unexplored paradigms and venues for energy science and organic electronics.

Anthranilamides as Bioinspired Molecular Electrets: Experimental Evidence for a Permanent Ground-State Electric Dipole Moment

As electrostatic equivalents of magnets, organic electrets offer unparalleled properties for impacting energy conversion and electronic applications. While biological systems have evolved to efficiently utilize protein α-helices as molecular electrets, the synthetic counterparts of these conjugates still remain largely unexplored. This paper describes a study of the electronic properties of anthranilamide oligomers, which proved to be electrets based on their intrinsic dipole moments as evident from their spectral and dielectric properties. NMR studies provided the means for estimating the direction of the intrinsic electric dipoles of these conjugates. This study sets the foundation for the development of a class of organic materials that are de novo designed from biomolecular motifs and possess unexplored electronic properties.