Molecular Design Principles Research Articles

Molecular Design Principles to Elongate the Metal-to-Ligand Charge Transfer Excited-State Lifetimes of Square-Planar Nickel(II) Complexes.

Square-planar NiII complexes and their electronically excited states play key roles in cross-coupling catalysis and could offer new opportunities to complement well-known isoelectronic PtII luminophores. Metal-to-ligand charge transfer (MLCT) excited states and their deactivation pathways are particularly relevant in these contexts. We sought to extend the lifetimes of 3MLCT states in square-planar NiII complexes by creating coordination environments that seemed particularly well adapted to the 3d8 valence electron configuration. Using a rigid tridentate chelate ligand, in which a central cyclometalated phenyl unit is flanked by two coordinating N-heterocyclic carbenes, along with a monodentate isocyanide ligand, a very strong ligand field is created. Bulky substituents at the isocyanide backbone furthermore protect the NiII center from nucleophilic attack in the axial directions. UV-Vis transient absorption spectroscopies reveal that upon excitation into 1MLCT absorption bands and ultrafast intersystem crossing to the 3MLCT excited state, the latter relaxes onward into a metal-centered triplet state (3MC). A torsional motion of the tridentate ligand and a NiII-carbon bond elongation facilitate 3MLCT relaxation to the 3MC state. The 3MLCT lifetime gets longer with increasing ligand field strength and improved steric protection, thereby revealing clear design guidelines for square-planar NiII complexes with enhanced photophysical properties. The longest 3MLCT lifetime reached in solution at room temperature is 48 ps, which is longer by a factor of 5-10 compared to previously investigated square-planar NiII complexes. Our study contributes to making first-row transition metal complexes with partially filled d-orbitals more amenable to applications in photophysics and photochemistry.

Achieving dual-state emission and high-contrast mechanochromic luminescence based on donor-acceptor type phenothiazine methanone derivatives

The development of fluorescent materials that can achieve dual-state emission (DSE) and high-contrast mechanochromic luminescence has attracted more and more attention. However, such materials are very rare, and there are no clear molecular design principles to follow. In current work, two novel DSE emitters are designed and synthesized by modifying phenothiazine and benzene or naphthalene rings on both sides of the methanone. Their photophysical properties in both dilute solution and the solid state are studied and compared. Optical researches have shown that D-A type phenothiazine methanone derivatives exhibit well-defined ICT (intramolecular charge transfer) characteristics. At the same time, they show intense luminescent efficiencies in both solution (ΦPTZ-CO-B = 0.645; ΦPTZ-CO-NA = 0.624) and solid states (ΦPTZ-CO-B = 0.386; ΦPTZ-CO-NA = 0.357), indicating their DSE properties. It is noteworthy that compounds PTZ-CO-B and PTZ-CO-NA exhibit significant fluorescence response to mechanical force, and the emitting wavelengths are red-shifted by 33 nm and 46 nm, respectively. XRD patterns and DSC curves confirm that the mechanochromism results from reversible changes in stacking pattern. The current work develops materials that can achieve DSE emission and high-contrast mechanochromism, laying the foundation for the development of such materials.

Rational Design of Sulfonamide‐Based Additive Enables Stable Solid Electrolyte Interphase for Reversible Zn Metal Anode

AbstractThe solid electrolyte interphase (SEI)‐forming additives strategy is of great significance for improving the cycle stability of zinc (Zn) anodes. Although various additives have been reported, the relationship between their molecular structures and SEI chemistries is poorly understood. Herein, a molecular design principle for sulfonamide‐containing additives that endow Zn anodes with a robust SEI layer is proposed. The incorporation of the benzene ring and amino group (−NH2) leads to high adsorption energy, low lowest unoccupied molecular orbital lowest unoccupied molecular orbital (LUMO), and a small highest occupied molecular orbital‐LUMO (HOMO‐LUMO) gap, facilitating the reduction process of sulfanilamide (SA) additives. Coupled with SA/ZnSO4 electrolytes, Zn|Zn symmetric cells deliver an ultralong cycle life of 4800 h (200 days) at 2 mA cm−2 and 2 mAh cm−2. Additionally, a high cumulative plated capacity (CPC) of 6000 mAh cm−2 and 2700 mAh cm−2 is also achieved at a capacity per cycle of 10 mAh cm−2 and 30 mAh cm−2, respectively. More importantly, the versatility of SA additives is also demonstrated in Zn‐V2O5, Zn‐I2, and Zn‐MnO2 full cells at a low N/P ratio (the theoretical capacity ratio between the negative and positive electrode) of 5.3, 8.3, and 4.5, respectively. This molecular structure strategy provides a promising path to develop effective SEI‐forming additives.

(Invited) Conducting Polymers As Dual Charge Conductors for Electrochemical Systems

Electrically conductive polymers are a class of polymers, which can conduct electricity. Conductive polymers have found niche applications such as anti-statics. The electrochemical energy storage devices, especially lithium-ion rechargeable batteries, has grown significantly in the past two decades. Recently multifunctional conductive polymers have been designed as dual ion and electron transport materials, and synthesized through a thermal process. These class of dual charge conducting polymers play a significant role as electrode binders for Silicon (Si) and Tin (Sn) alloy based anode electrode. Si is an attractive candidate for lithium-ion batteries because it delivers 10 times greater theoretical (∼4200 mAh/g) specific capacity than that of a traditional graphite anode (∼370 mAh/g). However, the widespread application of silicon materials has remained a significant challenge because of the large volume change during lithium insertion and extraction processes, disrupting both the Si electrode surface and electrode mechanical integrity. This large volume change causes electrode failure, leading to loss of the electrical contact and drastic capacity fading. Nanosizing the Si and Sn based anode materials provides better performance, but poses significant challenges to manufacturing of the electrode, including particle aggregation, and difficulties in maintaining constant electrical contacts to the nanoparticles, and excessive surface area. Conductive polymer binders can play multiple functions for Si electrode, including improved adhesion and connectivity, lithium ion compensation, better ion and electric conductivity as well as surface and interface modification. Organic and polymer chemistry has provided almost infinity possibilities to modify the polymeric binders to include the desired functionalities. This presentation will discuss the specific molecular design principles and synthetic steps to realize the structures and functionalities of the binders, how these binders interact with different alloy materials, and the electrochemical performances of the electrodes based on these binders.

Hydrogen-bonds mediate liquid-liquid phase separation of mussel derived adhesive peptides

Marine mussels achieve strong underwater adhesion by depositing mussel foot proteins (Mfps) that form coacervates during the protein secretion. However, the molecular mechanisms that govern the phase separation behaviors of the Mfps are still not fully understood. Here, we report that GK-16*, a peptide derived from the primary adhesive protein Mfp-5, forms coacervate in seawater conditions. Molecular dynamics simulations combined with point mutation experiments demonstrate that Dopa- and Gly- mediated hydrogen-bonding interactions are essential in the coacervation process. The properties of GK-16* coacervates could be controlled by tuning the strength of the electrostatic and Dopa-mediated hydrogen bond interactions via controlling the pH and salt concentration of the solution. The GK-16* coacervate undergoes a pH induced liquid-to-gel transition, which can be utilized for the underwater delivery and curing of the adhesives. Our study provides useful molecular design principles for the development of mussel-inspired peptidyl coacervate adhesives with tunable properties.

Regioregular Pyridal[1,2,3]Triazole‐Based Polymer Semiconductors for High Mobility and Near‐Infrared Light Emission Applications

AbstractThe development of polymer semiconductors with strong luminescence and high mobility has always been challenging, and achieving both properties together is considered paradoxical according to molecular design principles. Studies on novel molecular species with unique optoelectrical characteristics are limited. Herein, a novel fully rigid coplanar thiophene‐pyridal[1,2,3]triazole‐thiophene (T‐PT‐T) class structure with considerable intramolecular noncovalent interactions is proposed. To balance luminescence and carrier transport properties, a series of non‐conventional regioregular conjugated polymers (PPTs) with suitable energy gaps (Egs ≈ 1.8 eV) are developed, which exhibit near‐infrared (NIR, 700–1000 nm) light emission and high mobility. One of the synthesized polymers, PPT‐2, exhibits an excellent photoluminescence quantum yield (PLQY) of 9% in the solid‐state and a high mobility reaching 0.35 cm2 V−1 s−1, indicating superior performance compared to other NIR light‐emitting polymer semiconductors. The authors believe that the class structure of fully rigid coplanar “T‐PT‐T” could be harnessed to develop high‐mobility NIR light‐emitting polymer semiconductors.

Functionalizing aromatic compounds with optical cycling centres

Molecular design principles provide guidelines for augmenting a molecule with a smaller group of atoms to realize a desired property or function. We demonstrate that these concepts can be used to create an optical cycling centre, the Ca(I)-O unit, that can be attached to a number of aromatic ligands, enabling the scattering of many photons from the resulting molecules without changing the molecular vibrational state. Such capability plays a central role in quantum state preparation and measurement, as well as laser cooling and trapping, and is therefore a prerequisite for many quantum science and technology applications. We provide further molecular design principles that indicate the ability to optimize and expand this work to an even broader class of molecules. This represents a great step towards a quantum functional group, which may serve as a generic qubit moiety that can be attached to a wide range of molecular structures and surfaces.

Recent Advances in the Molecular Design and Delivery Technology of mRNA for Vaccination Against Infectious Diseases.

Vaccines can prevent many millions of illnesses against infectious diseases and save numerous lives every year. However, traditional vaccines such as inactivated viral and live attenuated vaccines cannot adapt to emerging pandemics due to their time-consuming development. With the global outbreak of the COVID-19 epidemic, the virus continues to evolve and mutate, producing mutants with enhanced transmissibility and virulence; the rapid development of vaccines against such emerging global pandemics becomes more and more critical. In recent years, mRNA vaccines have been of significant interest in combating emerging infectious diseases due to their rapid development and large-scale production advantages. However, their development still suffers from many hurdles such as their safety, cellular delivery, uptake, and response to their manufacturing, logistics, and storage. More efforts are still required to optimize the molecular designs of mRNA molecules with increased protein expression and enhanced structural stability. In addition, a variety of delivery systems are also needed to achieve effective delivery of vaccines. In this review, we highlight the advances in mRNA vaccines against various infectious diseases and discuss the molecular design principles and delivery systems of associated mRNA vaccines. The current state of the clinical application of mRNA vaccine pipelines against various infectious diseases and the challenge, safety, and protective effect of associated vaccines are also discussed.

Electrocatalytic Reduction of Furfural Using Single-Atom Molecular Catalysts

The electrochemical conversion of bio-based platform chemicals is an effective way of moving away from a crude oil reliant chemical source. Lignocellulosic biomass such as hemicellulose provides a sustainable chemicals source which can yield important platform chemicals including furfural, which can be upgraded into higher valued chemicals for biofuels, renewable polymers, and pharmaceuticals. In this study, we investigate the electrochemical reduction of furfural using Cu and Co single-atom molecular catalysts on carbon electrodes in a mild basic electrolyte (pH 10) for selective production of hydrofuroin, a promising precursor to sustainable drop-in jet fuels. Using density functional theory, we show that the selectivity of furfural reduction products on transition metals could be generally described by the adsorption energies of furfural and hydrogen (Fig 1a). In particular, we predict that the weak-binding molecular catalysts could give rise to a facile reaction path towards coupling product. Based on theoretical calculations, we synthesized Cu and Co-doped phthalocyanines and show that those single-atom molecular catalysts display up to 92% Faradaic efficiency for hydrofuroin production with suppressed hydrogen evolution in pH 10 at -0.50 V vs. the reversible hydrogen electrode (RHE). Combining experiment and theory, we show that the rate-determining step for hydrofuroin formation on single-atom molecular catalysts is the first proton-coupled electron transfer rather than the chemical coupling step. Furthermore, a single-atom molecular design principle is briefly proposed by tuning the adsorption strength of furfural. Figure 1

(Invited) The Application of Conducting Polymers for Nanomaterials Based Alloy Anodes in Battery Manufacturing

Electrically conductive polymers are a class of polymers, which can conduct electricity. Conductive polymers have found niche applications such as anti-statics. The electrochemical energy storage devices, especially lithium-ion rechargeable batteries, has grown significantly in the past two decades. Multifunctional conductive polymers may play a significant role as electrode binders for Silicon (Si) and Tin (Sn) alloy based anode electrode. Si is an attractive candidate for lithium-ion batteries because it delivers 10 times greater theoretical (∼4200 mAh/g) specific capacity than that of a traditional graphite anode (∼370 mAh/g). However, the widespread application of silicon materials has remained a significant challenge because of the large volume change during lithium insertion and extraction processes, disrupting both the Si electrode surface and electrode mechanical integrity. This large volume change causes electrode failure, leading to loss of the electrical contact and drastic capacity fading. Nanosizing the Si and Sn based anode materials provides better performance, but poses significant challenges to manufacturing of the electrode, including particle aggregation, and difficulties in maintaining constant electrical contacts to the nanoparticles, and excessive surface area. Conductive polymer binders can play multiple functions for Si electrode, including improved adhesion and connectivity, lithium ion compensation, better ion and electric conductivity as well as surface and interface modification. Organic and polymer chemistry has provided almost infinity possibilities to modify the polymeric binders to include the desired functionalities. This presentation will discuss the specific molecular design principles and synthetic steps to realize the structures and functionalities of the binders, how these binders interact with different nano Si materials, and the electrochemical performances of the electrodes based on these binders.

Fluorinating the Solid Electrolyte Interphase by Rational Molecular Design for Practical Lithium-Metal Batteries.

The lifespan of practical lithium (Li)-metal batteries is severely hindered by the instability of Li-metal anodes. Fluorinated solid electrolyte interphase (SEI) emerges as a promising strategy to improve the stability of Li-metal anodes. The rational design of fluorinated molecules is pivotal to construct fluorinated SEI. Herein, design principles of fluorinated molecules are proposed. Fluoroalkyl (-CF2 CF2 -) is selected as an enriched F reservoir and the defluorination of the C-F bond is driven by leaving groups on β-sites. An activated fluoroalkyl molecule (AFA), 2,2,3,3-tetrafluorobutane-1,4-diol dinitrate is unprecedentedly proposed to render fast and complete defluorination and generate uniform fluorinated SEI on Li-metal anodes. In Li-sulfur (Li-S) batteries under practical conditions, the fluorinated SEI constructed by AFA undergoes 183 cycles, which is three times the SEI formed by LiNO3 . Furthermore, a Li-S pouch cell of 360 Wh kg-1 delivers 25 cycles with AFA. This work demonstrates rational molecular design principles of fluorinated molecules to construct fluorinated SEI for practical Li-metal batteries.

Fluorinating the Solid Electrolyte Interphase by Rational Molecular Design for Practical Lithium‐Metal Batteries

AbstractThe lifespan of practical lithium (Li)‐metal batteries is severely hindered by the instability of Li‐metal anodes. Fluorinated solid electrolyte interphase (SEI) emerges as a promising strategy to improve the stability of Li‐metal anodes. The rational design of fluorinated molecules is pivotal to construct fluorinated SEI. Herein, design principles of fluorinated molecules are proposed. Fluoroalkyl (−CF2CF2−) is selected as an enriched F reservoir and the defluorination of the C−F bond is driven by leaving groups on β‐sites. An activated fluoroalkyl molecule (AFA), 2,2,3,3‐tetrafluorobutane‐1,4‐diol dinitrate is unprecedentedly proposed to render fast and complete defluorination and generate uniform fluorinated SEI on Li‐metal anodes. In Li–sulfur (Li−S) batteries under practical conditions, the fluorinated SEI constructed by AFA undergoes 183 cycles, which is three times the SEI formed by LiNO3. Furthermore, a Li−S pouch cell of 360 Wh kg−1 delivers 25 cycles with AFA. This work demonstrates rational molecular design principles of fluorinated molecules to construct fluorinated SEI for practical Li‐metal batteries.

Metal-Free Organic Phosphors toward Fast and Efficient Room-Temperature Phosphorescence.

ConspectusMetal-free purely organic phosphors (POPs) are promising materials for display technologies, solid-state lighting, and sensors platforms because of their advantageous properties such as large design windows, easy processability, and economic material cost. Unlike inorganic semiconductors, creating the conditions for triplet excitons to produce light in organic materials is a demanding task because of the presence of electron spin configurations that undergo spin-forbidden transitions, which is usually facilitated by spin-orbit coupling (SOC). In the absence of heavy metals, however, the SOC efficiency in POPs remains low, and consequently, external nonradiative photophysical processes will also severely affect triplet excitons. Addressing these challenges requires the development of rational molecular design principles to accurately account for how all conceivable structural, electronic, chemical, compositional factors affect materials performance.This Account summarizes important molecular design and matrix engineering strategies to tackle the two key challenges for POPs─boosting SOC efficiencies and suppressing nonradiative decays. We start by reviewing the fundamental understanding of internal and external factors affecting the emission efficiencies of POPs, including the theory behind SOC and the origin of nonradiative decays. Subsequently, we discuss the design of contemporary POP systems on the basis of research insights from our group and others, where SOC is mostly promoted by heavy atom effects and the El-Sayed rule. On one hand, nonmetal heavy atoms including Br, I, or Se provide the heavy atom effects to boost SOC. On the other hand, the El-Sayed rule addresses the necessity of orbital angular momentum change in SOC and the general utilization of carbonyl, heterocyclic rings, and other moieties with rich nonbonding electrons. Because of the slow-decaying nature of triplet excitons, engineering the matrices of POPs is critical to effectively suppress collisional quenching as the major nonradiative decay route, thus achieving POPs with decent room temperature quantum efficiency. For that purpose, crystalline or rigid amorphous matrices have been implemented along with specific intermolecular forces between POPs and their environment.Despite the great efforts made in the past decade, the intrinsic SOC efficiencies of POPs remain low, and their emission lifetimes are pinned in the millisecond to second regime. While this is beneficial for POPs with ultralong emission, designing high-SOC POPs with simultaneous fast decay and high quantum efficiencies is particularly advantageous for display systems. Following the design of contemporary POPs, we will discuss molecular design descriptors that could potentially break the current limit to boost internal SOC in purely organic materials. Our recently developed concept of "heavy atom oriented orbital angular momentum manipulation" will be discussed, accompanied by a rich and expanded library of fast and efficient POP molecules, which serves as a stepping stone into the future of this field. We will conclude this Account by discussing the noteworthy application of POPs in organic light-emitting diodes (OLEDs), solid-state lighting, and sensors, as well as the remaining challenges in the design of fast and efficient POPs.

Near-Amorphous Conjugated Polymers: An Emerging Class of Semiconductors for Flexible Electronics

The emergence of flexible electronics with broad application prospects has put forward new requirements for organic semiconductors, i.e., not only the electrical properties but also the mechanical properties. Among the strategies to achieve synergy between mechanical and electrical properties, near-amorphous polymers are competitive candidates due to their advantages of intrinsic flexibility, high mobility, and high reproducibility. Near-amorphous polymers with excellent performance are continuing to attract attention, and recently, some significant progress has been achieved in this field. In this perspective, we summarize these excellent advances on near-amorphous polymers from the point of view of their molecular design principles, their resulting charge transport and mechanical properties, and their potential applications in flexible electronic devices. Finally, we give a brief discussion on the challenges and opportunities of near-amorphous materials in flexible electronics from the perspectives of semiconductor layer materials themselves, devices, and multifunctional integration.

A Molecular Design Principle for Pure-Blue Light-Emitting Polydiarylfluorene with Suppressed Defect Emission by the Side-Chain Steric Hindrance Effect

A Molecular Design Principle for Pure-Blue Light-Emitting Polydiarylfluorene with Suppressed Defect Emission by the Side-Chain Steric Hindrance Effect

Rational molecular design of electrochromic conjugated polymers: Toward high-performance systems with ultrahigh coloration efficiency

• Three fused aryl amine-based π-conjugated polymers (CPs) are rationally designed. • Electrochromic devices (ECDs) containing synthesized polymers are demonstrated. • The ECDs exhibit fast switching, high coloration efficiency, and stable operation. • The achieved high-performance is explained through kinetics and structural analysis. • Density functional theory reveals the origin of the device stability. Organic electrochromic devices (ECDs) based on π-conjugated polymers (CPs) have notable advantages including relatively fast switching time and abundant colors with wide tunability. However, their practical applications are still limited due to low coloration efficiency and poor long-term stability. Here, we propose a series of reasonably designed CPs for high-performance ECDs. The principle of molecular design includes the introduction of a fused aryl amine (e.g., indolocarbazole or indoloindole) to effectively modify the optical and electrical characteristics of CPs, leading to high redox stability with fast response and high coloration efficiency ( η ). The resulting ECD with the fluorinated indoloindole-based CP exhibits the highest overall performance with rapid coloration and bleaching dynamic responses of ∼5.6 and ∼0.6 s, respectively, a large transmittance contrast of ∼46%, an outstanding η of ∼1563.3 cm 2 /C, and a reliable operation stability with an initial transmittance loss of less than 0.6% even after 6,500 cycles. We elucidate the origin of this superb performance by calculating redox rate constants of each CP via the Nicholson method and determining their thin film crystalline structures using grazing-incidence X-ray diffraction. In addition, the ultrahigh cyclic stability is explained by predicting the degree of molecular distortions of CPs during redox reactions through density functional theory.

Interface Engineering in Solution-Processed Thin-Film Solar Cells

ConspectusAlong with the rapid industrialization of human society over the past century, incessant energy consumption and endless damage to the environment have aroused growing attention for seeking clean and renewable energy sources. Photovoltaics (PV) that can directly harvest and transform sunlight into electricity have shown great potential in achieving this goal. Especially for solution-processed thin-film solar cells, their extremely cost-effective and facile processing methods compatible with different substrates at large scales exhibit unique advantages over conventional PVs based on crystalline silicon. Various types of solution-processed thin-film PVs have been achieving or already exceeded 15% power conversion efficiency (PCE) through the numerous efforts of researchers. Organic solar cells (OSCs) and organic–inorganic hybrid perovskite solar cells (PVSCs) are the most well-known emerging solution-processed thin-film solar cells that have attracted great interest recently (the PCE of PVSCs soared form 3.8% to over 25% in the past decade). Usually, photogenerated excitons will form as a response to illumination in the active layer, then dissociate into charge carriers, travel in between layers, and finally get collected by electrodes of the device. Besides the broad exploration of active layer materials, suitably matched charge-transporting layers and electrodes also play a vital role in achieving high PCE and stability in PV devices. Furthermore, interfaces between different functional layers created during solution processing need to be carefully addressed to ensure efficient charge transport and prevent degradation. The utilization of proper interfacial materials to modify the chemical and electrical properties at interfaces has become an effective strategy to enhance the performance of PV. Therefore, it is important to develop a comprehensive understanding into the correlation between interfacial properties and charge carrier dynamics and establish molecular design principles for interfacial materials to realize commercialization of emerging PVs.In this Account, we first introduce the fundamental roles of interfaces in PVs, including the modulation of film formation, together with management of charge transport and recombination. Detailed analysis of interfaces and related surface science are also discussed to provide better understanding. Then, we highlight our research on interfacial materials with different functionalities including self-assembled monolayers, dopants, functional molecules for post-treatment, and composite materials. Combining with the fundamental mechanisms and design criteria of interfacial materials, new materials and interface engineering strategies can be integrated into other PV systems to achieve high PCE and stability. In the end, the main challenges and future perspectives toward the commercialization of solution-processed thin-film PVs are discussed to inspire more novel material designs and interface engineering.

Stimuli-Responsive Antibacterial Materials: Molecular Structures, Design Principles, and Biomedical Applications.

Infections are regarded as the most severe complication associated with human health, which are urgent to be solved. Stimuli‐responsive materials are appealing therapeutic platforms for antibacterial treatments, which provide great potential for accurate theranostics. In this review, the advantages, the response mechanisms, and the key design principles of stimuli‐responsive antibacterial materials are highlighted. The biomedical applications, the current challenges, and future directions of stimuli‐responsive antibacterial materials are also discussed. First, the categories of stimuli‐responsive antibacterial materials are comprehensively itemized based on different sources of stimuli, including external physical environmental stimuli (e.g., temperature, light, electricity, salt, etc.) and bacterial metabolites stimuli (e.g., acid, enzyme, redox, etc.). Second, structural characteristics, design principles, and biomedical applications of the responsive materials are discussed, and the underlying interrelationships are revealed. The molecular structures and design principles are closely related to the sources of stimuli. Finally, the challenging issues of stimuli‐responsive materials are proposed. This review will provide scientific guidance to promote the clinical applications of stimuli‐responsive antibacterial materials.

Nitrogen-Containing Perylene Diimides: Molecular Design, Robust Aggregated Structures, and Advances in n-Type Organic Semiconductors.

ConspectusOrganic semiconductors (OSCs) have attracted much attention because of their potential applications for flexible and printed electronic devices and thus have been extensively investigated in a variety of research fields, such as organic chemistry, solid-state physics, and device physics and engineering. Organic thin-film transistors (OTFTs), a class of OSC-based devices, have been expected to be an alternative of silicon-based metal oxide semiconductor field-effect transistors (MOSFETs), which is the indispensable element for most of the current electronic devices. However, the noncovalently aggregated, van der Waals solid nature of the OSCs, by contrast to covalently bound silicon, conventionally exhibits lower carrier mobilities, limiting the practical applications of OTFTs. In particular, electron-transporting (i.e., n-type) OSCs lag behind their hole-transporting (p-type) counterparts in carrier mobility and ambient stability as OTFTs. This is primarily because of the difficulty in achieving compatibility between the aggregated structure exhibiting excellent carrier mobility and that with enough electron affinity. Recent understandings of carrier transport in OSCs explain that large and two-dimensionally isotropic transfer integrals coupled with small fluctuations are crucial for high carrier mobilities. In addition, from a practical point of view, the compatibility with practical device processes is highly required. Rational molecular design principles, therefore, are still demanded for developing OSCs and OTFTs toward high-end device applications.Herein, we will show our recent progress in the development of n-type OSCs with the key π-electron core (π-core) of benzo[de]isoquinolino[1,8-gh]quinolinetetracarboxylic diimide (BQQDI) on the basis of single-crystal OTFT technologies and the band-transport model enabled by two-dimensional molecular packing arrangements. The critical point is the introduction of electronegative nitrogen atoms into the π-core: the nitrogen atoms in BQQDI not only deepen the molecular orbital energies but also allow hydrogen-bonding-like attractive intermolecular interactions to control the aggregated structures, unlike the conventional role of the nitrogen introduced into OSCs only for the former role. Hence, the BQQDI analogues exhibit air-stable OTFT behavior and two-dimensional brickwork packing structures. Specifically, phenethyl-substituted analogue (PhC2-BQQDI) has been shown as the first principal BQQDI-based material, demonstrating solution-processable thin-film single crystals, fewer anisotropic transfer integrals, and an effective suppression of molecular motions, leading to band-like electron-transport properties and stress-durable n-channel OTFT performances, in conjunction with the support of computational calculations. Insights into more fundamental points of view have been found by side-chain derivatization and OTFT studies on polycrystalline and single-crystal films. We hope that this Account provides readers with new strategies for designing high-performance OSCs by two-dimensional control of the aggregated structures.

HOFs Built from Hexatopic Carboxylic Acids: Structure, Porosity, Stability, and Photophysics.

Hydrogen-bonded organic frameworks (HOFs) have attracted renewed attention as another type of promising candidates for functional porous materials. In most cases of HOF preparation, the applied molecular design principle is based on molecules with rigid π-conjugated skeleton together with more than three H-bonding groups to achieve 2D- or 3D-networked structures. However, the design principle does not always work, but results in formation of unexpected structures, where subtle structural factors of which we are not aware dictate the entire structure of HOFs. In this contribution, we assess recent advances in HOFs, focusing on those composed of hexatopic building block molecules, which can provide robust frameworks with a wide range of topologies and properties. The HOFs described in this work are classified into three types, depending on their H-bonded structural motifs. Here in, we focus on: (1) the chemical aspects that govern their unique fundamental chemistry and structures; and (2) their photophysics at the ensemble and single-crystal levels. The work addresses and discusses how these aspects affect and orient their photonic applicability. We trust that this contribution will provide a deep awareness and will help scientists to build up a systematic series of porous materials with the aim to control both their structural and photodynamical assets.