Organic Electronic Materials Research Articles

“Rim‐Differentiated” Pillar[6]arenes

AbstractA “rim‐differentiated” pillar[6]arene (RD‐P[6]) was obtained successfully, with the assistance of a dimeric silver trifluoroacetate template, among eight different constitutional isomers in a direct and regioselective manner. The solid‐state conformation of this macrocycle could switch from the 1,3,5‐alternate to a truly rim‐differentiated one upon guest inclusion. This highly symmetric RD‐P[6] not only hosts metal‐containing molecules inside its cavity, but also can form a pillar[6]arene‐C60 adduct through co‐crystallization on account of donor‐acceptor interactions. The development of synthetic strategies to desymmetrize pillararenes offers new opportunities for engineering complex molecular architectures and organic electronic materials.

Using automated synthesis to understand the role of side chains on molecular charge transport

The development of next-generation organic electronic materials critically relies on understanding structure-function relationships in conjugated polymers. However, unlocking the full potential of organic materials requires access to their vast chemical space while efficiently managing the large synthetic workload to survey new materials. In this work, we use automated synthesis to prepare a library of conjugated oligomers with systematically varied side chain composition followed by single-molecule characterization of charge transport. Our results show that molecular junctions with long alkyl side chains exhibit a concentration-dependent bimodal conductance with an unexpectedly high conductance state that arises due to surface adsorption and backbone planarization, which is supported by a series of control experiments using asymmetric, planarized, and sterically hindered molecules. Density functional theory simulations and experiments using different anchors and alkoxy side chains highlight the role of side chain chemistry on charge transport. Overall, this work opens new avenues for using automated synthesis for the development and understanding of organic electronic materials.

Interplay of band occupation, localization, and polaron renormalization for electron transport in molecular crystals: Naphthalene as a case study

Understanding electronic properties and charge transport in organic semiconductors is important for improving organic electronic materials and devices. Here we investigate the impact of electronic band occupation, charge-carrier concentration, and symmetry of phonon modes on the electron mobility in naphthalene crystals for various temperatures. Our theoretical approach is based on the description of the electron-phonon coupling (EPC), where the coupling to low-frequency modes is treated by an effective vibrational disorder potential with local and nonlocal contributions and the coupling to high-frequency modes is included by a polaron treatment. Surprisingly, the coupling to high-frequency modes leads to an increase in the mobility in presence of the low-frequency modes, which is explained by localization and band occupation effects that further depend on the carrier density. A symmetry analysis sheds additional light on the energy dependence of the EPC, which is important to describe transport properties as a function of charge density and temperature. We also find that coupling to low-frequency phonons together with band occupation effects can lead to a vanishing slope of the mobility versus temperature that is known from experiments.

Anomalous Interfacial Electronic Structure of Solid Films of Non‐Planar π‐Conjugated Molecule 6‐Cycloparaphenylene

Abstractn‐Cycloparaphenylene ([n]CPP) is a non‐planar π‐conjugated molecule with a macrocyclic, hoop‐like molecular structure that allows for control of the electronic structure unlike planar molecules. Thus, [n]CPP has the potential as a novel functional material in organic electronics. In this study, the unique electronic structure of [6]CPP is revealed, which originates from its macrocyclic molecular structure by comparison with p‐sexiphenyl, a linear molecule also consisting of six phenylenes. The morphology of the vapor‐deposited [6]CPP polycrystalline film is found to be highly dependent on the evaporation rate and the type of substrate. It is also found that a [6]CPP molecular layer is formed at the interface between the film and gold substrate, which has a much smaller energy gap than that of the bulk [6]CPP. This anomalous molecular layer is composed of [6]CPP molecules with a smaller dihedral angle than that in the bulk because of the interaction with a gold substrate. When [6]CPP is used as a semiconducting layer in electronic devices, this layer can play a role in improving the charge injection efficiency from the electrode to the [6]CPP film.

A PVP-silica-titania hybrid film for low-voltage organic field-effect transistor

A PVP-silica-titania hybrid film with excellent dielectric properties was fabricated by the sol–gel method at low temperature (180 °C). The film has a high dielectric constant (k = 13), a low leakage current density (<20nA cm−2 at 2 V), and a smooth surface (Rq < 0.4 nm). The organic field-effect transistor (OFET), with the PVP-silica-titania hybrid film, had been fabricated and compared with the OFET, which was fabricated with SiO2 dielectric. The electrical performance of the device had obviously improved in the operating voltage (−1.5 V), threshold voltage (−0.35 V), and mobility (0.7 cm2 V−1 s−1). To investigate the suitability of the PVP-silica-titania hybrid film in flexible applications, a low-voltage flexible OFET was fabricated. The device had been tested in the bending state and exhibited good performance under the ambition conditions. The result demonstrated that the PVP-titania hybrid film is a promising candidate material for flexible organic electronics with low power consumption.

Updated Calibrated Model for the Prediction of Molecular Frontier Orbital Energies and Its Application to Boron Subphthalocyanines.

A diverse range of computational methods have been used to calibrate against available data and to compare against the correlation for the prediction of frontier orbital energies and optical gaps of novel boron subphthalocyanine (BsubPc) derivatives and related compounds. These properties are of fundamental importance to organic electronic material applications and development, making BsubPcs ideal candidates in pursuit of identifying promising materials for targeted applications. This work employs a database of highly accurate experimental data from materials produced and characterized in-house. The models presented herein calibrate these properties with R2 values > 0.95. We find that computationally inexpensive semiempirical methods such as PM6 and PM7 outperform most density functional theory methods for calibration. We are excited to share these results with the field as it empowers the community to determine key physical properties of BsubPcs with confidence using free software and a standard laptop prior to the arduous synthesis and purification thereof. This study is a follow up to our previous work calibrating PM3, RM1, and B3LYP-6-31G(d), which used a smaller set of BsubPc derivatives at a past point when less data were available.

The Bifurcated σ-Hole···σ-Hole Stacking Interactions.

The bifurcated σ-hole···σ-hole stacking interactions between organosulfur molecules, which are key components of organic optical and electronic materials, were investigated by using a combined method of the Cambridge Structural Database search and quantum chemical calculation. Due to the geometric constraints, the binding energy of one bifurcated σ-hole···σ-hole stacking interaction is in general smaller than the sum of the binding energies of two free monofurcated σ-hole···σ-hole stacking interactions. The bifurcated σ-hole···σ-hole stacking interactions are still of the dispersion-dominated noncovalent interactions. However, in contrast to the linear monofurcated σ-hole···σ-hole stacking interaction, the contribution of the electrostatic energy to the total attractive interaction energy increases significantly and the dispersion component of the total attractive interaction energy decreases significantly for the bifurcated σ-hole···σ-hole stacking interaction. Another important finding of this study is that the low-cost spin-component scaled zeroth-order symmetry-adapted perturbation theory performs perfectly in the study of the bifurcated σ-hole···σ-hole stacking interactions. This work will provide valuable information for the design and synthesis of novel organic optical and electronic materials.

Active Learning Accelerates Design and Optimization of Hole-Transporting Materials for Organic Electronics.

Data-driven methods are receiving increasing attention to accelerate materials design and discovery for organic light-emitting diodes (OLEDs). Machine learning (ML) has enabled high-throughput screening of materials properties to suggest new candidates for organic electronics. However, building reliable predictive ML models requires creating and managing a high volume of data that adequately address the complexity of materials’ chemical space. In this regard, active learning (AL) has emerged as a powerful strategy to efficiently navigate the search space by prioritizing the decision-making process for unexplored data. This approach allows a more systematic mechanism to identify promising candidates by minimizing the number of computations required to explore an extensive materials library with diverse variables and parameters. In this paper, we applied a workflow of AL that accounts for multiple optoelectronic parameters to identify materials candidates for hole-transport layers (HTL) in OLEDs. Results of this work pave the way for efficient screening of materials for organic electronics with superior efficiencies before laborious simulations, synthesis, and device fabrication.

Design of Organic Electronic Materials With a Goal-Directed Generative Model Powered by Deep Neural Networks and High-Throughput Molecular Simulations.

In recent years, generative machine learning approaches have attracted significant attention as an enabling approach for designing novel molecular materials with minimal design bias and thereby realizing more directed design for a specific materials property space. Further, data-driven approaches have emerged as a new tool to accelerate the development of novel organic electronic materials for organic light-emitting diode (OLED) applications. We demonstrate and validate a goal-directed generative machine learning framework based on a recurrent neural network (RNN) deep reinforcement learning approach for the design of hole transporting OLED materials. These large-scale molecular simulations also demonstrate a rapid, cost-effective method to identify new materials in OLEDs while also enabling expansion into many other verticals such as catalyst design, aerospace, life science, and petrochemicals.

Band-like Transport of Charge Carriers in Oriented Two-Dimensional Conjugated Covalent Organic Frameworks

A tunable topology and a porous network make π-conjugated covalent organic frameworks (COFs) a new class of organic semiconductors for optoelectronic, smart sensing, and catalytic applications. Although some of the COFs exhibit enhanced electric conductivity with a high charge carrier mobility, the nature and pathways of charge transport still remain elusive. In order to unveil the transport mechanism, herein, we have developed crystalline π-conjugated COFs using planar building blocks, and a wafer-scale self-supporting thin film was grown, which could be transferred onto any of the desired substrates. The COF film was found to be highly oriented and exhibited a high in-plane electronic conductivity. The conductivity was almost independent of temperature with an ultra-low activation energy of 14.3 meV, approaching a band-like transport of charge carriers within the crystalline domains. The COF films also showed a high photoresponsivity in electronic conduction against a complete visible range, demonstrated as a flexible photodetector device. This work represents a thorough investigation of the mechanism and direction of charge transport in crystalline π-conjugated COF semiconductors, which suggests their feasibility as key active materials in multi-functional organic electronics.

(Dimesityl)boron Benzodithiophenes: Synthesis, Electrochemical, Photophysical and Theoretical Characterization.

Triarylboranes containing linear or angular benzodithiophene moieties and bearing one or two dimesitylboron units were synthesized. The electrochemical and optical features of these compounds were investigated by cyclic voltammetry, UV/Vis and fluorescence spectroscopy while DFT calculations were run to analyze the energetic landscape of these systems. For both linear and angular benzodithiophenes, symmetrical disubstitution leads to the highest photoluminescence yields. The linear benzodithiophene disubstituted with two dimesitylboron units proved to be the most interesting and promising molecule as an electron‐transport material for organic electronics owing to its LUMO energy level of −2.84 eV which is close to those of commonly used electron transport materials like bathocuproine or bathophenantroline.

In operando nanomechanical mapping of PEDOT:PSS thin films in electrolyte solutions with bimodal AFM.

Understanding the interplay between the nanomechanical properties of organic electronic materials and their electronic properties is central to developing sensors and transducers for applications ranging from immunosensing to e-skin. Controlling organic device operations in aqueous electrolyte solutions and their mechanical compliance with the host tissue or living systems, as for instance in active implants for the recording or stimulation of neural signals, is still largely unexplored. Here, we implemented bimodal AFM to map the nanomechanical and structural properties of thin films made from poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS), the most widely used conducting polymer blend, during operation as a microelectrode in an electrolyte solution. Nanomechanical maps showed the film consisting of a granular structure made from PEDOT:PSS regions embedded in the PSS matrix. The film swelled upon immersion in an aqueous solution. In operando bimodal AFM data obtained by applying a sequence of doping/de-doping bias cycles showed a significant decrease in the modulus (70%) that saturated after about 10 cycles. A similar sequence of biases at the opposite polarity did not significantly influence the mechanical behaviour of PEDOT:PSS. The decrease in the modulus was explained by the development of persistent hydration, which was enhanced by the cations trapped inside the organic electronic material.

Dithieno[3,2-b:2',3'-d]thiophene (DTT): an emerging heterocyclic building block for future organic electronic materials & functional supramolecular chemistry.

Heterocyclic compounds being potent biochemical materials are ubiquitous molecules in our life. Amongst, the five membered aromatic ring systems, thiophene has emerged as a remarkable entity in organic electronics owing to its (i) high resonance energy, (ii) more electrophilic reactivity than benzene, (iii) high π-electron density, (iv) planar structure and, (v) presence of vacant d-orbital in addition to the presence of loosely bind lone-pairs of electrons on sulfur atoms. In recent past, thiophene-fused molecule namely, dithienothiophene (DTT) has attracted a tremendous attention of the researchers worldwide due to their potential applicability in organic electronics such as in solar cells, electrochromic devices (ECDs), organic field effect transistors (OFETs), organic limiting diodes (OLEDs), fluorescent probes, redox switching and so forth because of their (i) higher charge mobility, (ii) extended π-conjugation, and (iii) better tuning of band gaps, etc. In this particular review article, we envisioned to report the recent advancements made on the DTT-based architectures not only because of the potential applicability of this valuable scaffold in organic electronic but also to motivate the young researchers worldwide to look for the challenging opportunities related to this privileged building block in both material sciences and functional supramolecular chemistry.

High-Performance Organic Electronic Materials by Contorting Perylene Diimides.

Perylene diimide (PDI) is a workhorse of the organic electronics community. However, the vast majority of designs that include PDI substitute the core with various functional groups to encourage intimate cofacial contacts between largely planar PDIs. Over the past several years, we have observed the counterintuitive result that contorting the planar aromatic core of PDI leads to higher performing photovoltaics, photodetectors, batteries, and other organic electronic devices. In this Perspective, we describe how different modes of contortion can be reliably installed into PDI-based molecules, oligomers, and polymers. We also describe how these different contortions modify the observed optical and electronic properties of PDI. For instance, contorting PDIs into bowls leads to high-efficiency singlet fission materials, while contorting PDIs into helicene-like structures leads to nonlinear amplification of Cotton effects, culminating in the highest g-factors so far observed for organic compounds. Finally, we show how these unique optoelectronic properties give rise to higher performance organic electronic devices. We specifically note how the three-dimensional structure of these contorted aromatic molecules is responsible for the enhancements in performance we observe. Throughout this Perspective, we highlight opportunities for continued study in this rapidly developing organic materials frontier.

A Supramolecular Polymer Constituted of Antiaromatic NiII Norcorroles

AbstractFor the creation of next‐generation organic electronic materials, the integration of π‐systems has recently become a central theme. Such functional materials can be assembled by supramolecular polymerization when aromatic π‐systems are used as monomers, and the properties of the resulting supramolecular polymer strongly depend on the electronic structure of the monomers. Here, we demonstrate the construction of a supramolecular polymer consisting of an antiaromatic π‐system as the monomer. An amide‐functionalized NiII norcorrole derivative formed a one‐dimensional supramolecular polymer through π‐π stacking and hydrogen‐bonding interactions, ensuring the persistency of the conducting pathway against thermal perturbation, which results in higher charge mobility along the tightly bound linear aggregates than that of the aromatic analogue composed of ZnII porphyrins.

A Supramolecular Polymer Constituted of Antiaromatic NiII Norcorroles.

For the creation of next-generation organic electronic materials, the integration of π-systems has recently become a central theme. Such functional materials can be assembled by supramolecular polymerization when aromatic π-systems are used as monomers, and the properties of the resulting supramolecular polymer strongly depend on the electronic structure of the monomers. Here, we demonstrate the construction of a supramolecular polymer consisting of an antiaromatic π-system as the monomer. An amide-functionalized NiII norcorrole derivative formed a one-dimensional supramolecular polymer through π-π stacking and hydrogen-bonding interactions, ensuring the persistency of the conducting pathway against thermal perturbation, which results in higher charge mobility along the tightly bound linear aggregates than that of the aromatic analogue composed of ZnII porphyrins.

Plant Bioelectronics and Biohybrids: The Growing Contribution of Organic Electronic and Carbon-Based Materials.

Life in our planet is highly dependent on plants as they are the primary source of food, regulators of the atmosphere, and providers of a variety of materials. In this work, we review the progress on bioelectronic devices for plants and biohybrid systems based on plants, therefore discussing advancements that view plants either from a biological or a technological perspective, respectively. We give an overview on wearable and implantable bioelectronic devices for monitoring and modulating plant physiology that can be used as tools in basic plant science or find application in agriculture. Furthermore, we discuss plant-wearable devices for monitoring a plant’s microenvironment that will enable optimization of growth conditions. The review then covers plant biohybrid systems where plants are an integral part of devices or are converted to devices upon functionalization with smart materials, including self-organized electronics, plant nanobionics, and energy applications. The review focuses on advancements based on organic electronic and carbon-based materials and discusses opportunities, challenges, as well as future steps.

Organic Bioelectronics for In Vitro Systems.

Bioelectronics have made strides in improving clinical diagnostics and precision medicine. The potential of bioelectronics for bidirectional interfacing with biology through continuous, label-free monitoring on one side and precise control of biological activity on the other has extended their application scope to in vitro systems. The advent of microfluidics and the considerable advances in reliability and complexity of in vitro models promise to eventually significantly reduce or replace animal studies, currently the gold standard in drug discovery and toxicology testing. Bioelectronics are anticipated to play a major role in this transition offering a much needed technology to push forward the drug discovery paradigm. Organic electronic materials, notably conjugated polymers, having demonstrated technological maturity in fields such as solar cells and light emitting diodes given their outstanding characteristics and versatility in processing, are the obvious route forward for bioelectronics due to their biomimetic nature, among other merits. This review highlights the advances in conjugated polymers for interfacing with biological tissue in vitro, aiming ultimately to develop next generation in vitro systems. We showcase in vitro interfacing across multiple length scales, involving biological models of varying complexity, from cell components to complex 3D cell cultures. The state of the art, the possibilities, and the challenges of conjugated polymers toward clinical translation of in vitro systems are also discussed throughout.

How side‐chain hydrophilicity modulates morphology and charge transport in mixed conducting polymers

AbstractOrganic mixed ionic‐electronic conductors (OMIECs) are a developing class of organic electronic materials distinguished by their dual modes of conduction. The side‐chains of OMIEC polymers are responsible for forming a percolating electrolyte phase that mediates doping and ionic conduction. Despite this critical role, design rules for OMIEC side‐chains are still nascent and their effects on OMIEC morphology and charge transport have yet to be systematically studied. Here we perform the first dedicated coarse‐grained molecular dynamics study of OMIECs where the side‐chain identity and distribution are systematically varied using a random copolymer architecture. The simulations recapitulate the nonlinear progression of the morphology from an interfacially gated electrolyte when large fractions of hydrophobic side‐chains are incorporated, to an electrolyte swelled morphology after crossing a threshold of approximately 40% polar side‐chains. Kinetic Monte Carlo simulations were used to characterize the charge transport behaviors in these systems, revealing two interesting maxima in the mobility at 40% and 100% polar side‐chain fractions, respectively. With respect to maximizing the charge mobility and conductivity, these simulations suggest that a uniform hydrophilic side‐chain distribution is optimal and that there are few advantages to using mixed side‐chains in a random copolymer architecture. These results also suggest several alternative side‐chain engineering strategies for optimizing OMIEC performance.

DNA-mediated molecular assembly of a triphenylene–surfactant complex monolayer

Bottom-up DNA-guided molecular assembly is a technique of choice in modern organic device engineering. DNA, being a polyelectrolyte, is capable of mediating electrostatic interactions between different molecular building blocks. Here, we report DNA-mediated molecular assembly of an electrostatically driven triphenylene-surfactant complex monolayer. The monolayer, formed at air-water interface with DNA in the subphase, was studied using surface manometry and Brewster angle microscopy. Unlike that of the pristine system, the monolayer in presence of DNA showed enhanced stability and a remarkably high efficiency of deposition on solid supports. The morphologies of deposited films with controlled number of layers were studied using atomic force microscopy. In addition, UV–visible spectroscopy and grazing incidence X-ray diffraction (GIXRD) technique confirmed the presence of DNA in the deposited films. Since both triphenylene and surfactant are technologically relevant molecules, the DNA-mediated molecular assemblies deposited on solid substrates could find applications as functional materials in organic electronics.