Charge carrier mobility in organic molecular materials probed by electromagnetic waves

π-Extended 9b-Boraphenalenes: Synthesis, Structure, and Physical Properties

Boraphenalenes, compounds in which one carbon atom in the phenalenyl skeleton is replaced with a boron atom, have attracted attention for their solid-state and electronic structures; however, the construction of boraphenalene skeletons remains challenging because of the lack of suitable methods. Through this study, we showed that the tandem borylative cyclization of C3-symmetric dehydrobenzo[12]annulenes produces a new class of fully fused boron-atom-embedded polycyclic hydrocarbons possessing a 9b-boraphenalene skeleton. The obtained compounds exhibited high electron-accepting characteristics, and their two-step redox process was reversible in the reductive region, involving interconversion of 9b-boraphenalene between Hückel aromaticity and antiaromaticity. Notably, the benzo[b]fluorene-fused derivative exhibited a stepwise single-crystal-to-single-crystal (SCSC) phase transition triggered by thermal annealing. Intermolecular electron coupling calculation of the crystal structures suggested a significant improvement of charge transporting ability associated with the SCSC phase transition. Moreover, adequate photoconductivity was observed for the single crystals before and after the SCSC phase transition through flash photolysis-time-resolved microwave conductivity.

Substitution-pattern- and counteranion-dependent ion-pairing assemblies of heteroporphyrin-based π-electronic cations

Metal complexation and peripheral modifications of thiaporphyrins have been investigated for preparing polarized π-electronic cations with anion-dependent ion-pairing assembling modes, including charge-segregated structures exhibiting electric conductive properties.

Nonclassical Crystal Growth of Supramolecular Polymers in Aqueous Medium

A mechanistic understanding of the principles governing the hierarchical organization of supramolecular polymers offers a paradigm for tailoring synthetic molecular architectures at the nano to micrometric scales. Herein, the unconventional crystal growth mechanism of a supramolecular polymer of superbenzene(coronene)-diphenylalanine conjugate (Cr-FFOEt ) is demonstrated. 3D electron diffraction (3D ED), a technique underexplored in supramolecular chemistry, is effectively utilized to gain a molecular-level understanding of the gradual growth of the initially formed poorly crystalline hairy, fibril-like supramolecular polymers into the ribbon-like crystallites. The further evolution of these nanosized flat ribbons into microcrystals by oriented attachment and lateral fusion is probed by time-resolved microscopy and electron diffraction. The gradual morphological and structural changes reveal the nonclassical crystal growth pathway, where the balance of strong and weak intermolecular interactions led to a structure beyond the nanoscale. The role of distinct π-stacking and H-bonding interactions that drive the nonclassical crystallization process of Cr-FFOEt supramolecular polymers is analyzed in comparison to analogous molecules, Py-FFOEt and Cr-FF forming helical and twisted fibers, respectively. Furthermore, the Cr-FFOEt crystals formed through nonclassical crystallization are found to improve the functional properties.

Alkyl-C60 liquid electrets as deformable mechanoelectric generators

Special attention is being paid to the potential applicability of various soft electronics in deformable/wearable devices. These devices must be constantly connected to energy sources to ensure their uninterrupted operation. Electrets, which are capable of retaining quasi-permanent electric charges inside or on the surface of materials, are expected to be a battery-less power source. Here, we present a strategy for harvesting the charges in alkyl-C60 liquids. Suitable substitution of bulky yet flexible branched long-alkyl chains generated C60-mono-adducts and regioisomeric bis-adducts as room-temperature solvent-free liquids. These alkyl-C60 liquids were negatively poled by the corona-discharging and soaked in nylon fabric. The liquid of the C60 bis-adduct exhibited better charge retention in comparison to the liquid of the C60 mono-adduct. This suggests that the bulky long-alkyl chains provided proper insulation for the C60 core and charge trapping in the liquid. This charge-trapping behaviour and the inherent fluidity of the alkyl-C60 liquids enabled their fabrication into deformable mechanoelectric generator (MEG) devices. The MEG exhibited applicability as a deformable micropower source or vibration sensor by generating output voltage pulses even under folded/twisted/rolled conditions. The alkylated-liquid-based MEGs worked at frequencies similar to human body motion, showing promising potential for body motion sensors and healthcare applications.

Two-Step Synthesis of B2 N2 -Doped Polycyclic Aromatic Hydrocarbon Containing Pentagonal and Heptagonal Rings with Long-Lived Delayed Fluorescence

Pentagon-heptagon embedded polycyclic aromatic hydrocarbons (PAHs) have aroused increasing attention in recent years due to their unique physicochemical properties. Here, for the first time, this report demonstrates a facile method for the synthesis of a novel B2 N2 -doped PAH (BN-2) containing two pairs of pentagonal and heptagonal rings in only two steps. In the solid state of BN-2, two different conformations, including saddle-shaped and up-down geometries, are observed. Through a combined spectroscopic and calculation study, the excited-state dynamics of BN-2 is well-investigated in this current work. The resultant pentagon-heptagon embedded B2 N2 -doped BN-2 displays both prompt fluorescence and long-lived delayed fluorescence components at room temperature, with the triplet excited-state lifetime in the microsecond time region (τ = 19 µs). The triplet-triplet annihilation is assigned as the mechanism for the observed long-lived delayed fluorescence. Computational analyses attributed this observation to the small energy separation between the singlet and triplet excited states, facilitating the intersystem crossing (ISC) process which is further validated by the ultrafast spectroscopic measurements.

Flattened 1D fragments of fullerene C60 that exhibit robustness toward multi-electron reduction

Fullerenes are compelling molecular materials owing to their exceptional robustness toward multi-electron reduction. Although scientists have attempted to address this feature by synthesizing various fragment molecules, the origin of this electron affinity remains unclear. Several structural factors have been suggested, including high symmetry, pyramidalized carbon atoms, and five-membered ring substructures. To elucidate the role of the five-membered ring substructures without the influence of high symmetry and pyramidalized carbon atoms, we herein report the synthesis and electron-accepting properties of oligo(biindenylidene)s, a flattened one-dimensional fragment of fullerene C₆₀. Electrochemical studies corroborated that oligo(biindenylidene)s can accept electrons up to equal to the number of five-membered rings in their main chains. Moreover, ultraviolet/visible/near-infrared absorption spectroscopy revealed that oligo(biindenylidene)s exhibit enhanced absorption covering the entire visible region relative to C₆₀. These results highlight the significance of the pentagonal substructure for attaining stability toward multi-electron reduction and provide a strategy for the molecular design of electron-accepting π-conjugated hydrocarbons even without electron-withdrawing groups.

Charge-Segregated Stacking Structure with Anisotropic Electric Conductivity in NIR-Absorbing and Emitting Positively Charged π-Electronic Systems

Squarylium-based π-electronic cation with an augmented dipole was synthesized by methylation of zwitterionic squarylium. The cation formed various ion pairs in combination with anions, and the ion pairs exhibited distinct photophysical properties in the dispersed state, ascribed to the formation of J- and H-aggregates. The ion pairs provided solid-state assemblies based on cation stacking. It is noteworthy that complete segregation of cations and anions was observed in a pseudo-polymorph of the ion pair with pentacyanocyclopentadienide as a π-electronic anion. In the crystalline state, the ion pairs exhibited photophysical properties and electric conductivity derived from cation stacking. In particular, the charge-segregated ion-pairing assembly induces an electric conductive pathway along the stacking axis. The charge-segregated mode and fascinating properties were derived from the reduced electrostatic repulsion between adjacent π-electronic cations via dipole-dipole interactions.

Supramolecular Recognition within a Nanosized "Buckytrap" That Exhibits Substantial Photoconductivity

We report here a nanosized "buckytrap", 1, constructed from two bis-zinc(II) expanded-TTF (exTTF) porphyrin subunits. Two forms, 1a and 1b, differing in the axial ligands, H₂O vs tetrahydrofuran (THF), were isolated and characterized. Discrete host-guest inclusion complexes are formed upon treatment with fullerenes as inferred from a single-crystal X-ray structural analyses of 1a with C₇₀. The fullerene is found to be encapsulated within the inner pseudohexagonal cavity of 1a. In contrast, the corresponding free-base derivative (2) was found to form infinite ball-and-socket type supramolecular organic frameworks (3D-SOFs) with fullerenes, (2•C₆₀)_n or (2•C₇₀)_n. This difference is ascribed to the fact that in 1a and 1b the axial positions are blocked by a H₂O or THF ligand. Emission spectroscopic studies supported a 1:1 host-guest binding stoichiometry, allowing association constants of (2.0 ± 0.5) × 10⁴ M^-1 and (4.3 ± 0.9) × 10⁴ M^-1 to be calculated for C₆₀ and C₇₀, respectively. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) studies of solid films of the Zn-complex 1a revealed that the intrinsic charge carrier transport, i.e., pseudo-photoconductivity (ϕ∑μ), increases upon fullerene inclusion (e.g., ϕ∑μ = 1.53 × 10^-4 cm² V^-1 s^-1 for C₆₀⊂(1a)₂ and ϕ∑μ = 1.45 × 10^-4 cm² V^-1 s^-1 for C₇₀⊂(1a)₂ vs ϕ∑μ = 2.49 × 10^-5 cm² V^-1 s^-1 for 1a) at 298 K. These findings provide support for the notion that controlling the nature of self-assembly supramolecular constructs formed from exTTF-porphyrin dimers through metalation or choice of fullerene can be used to regulate key functional features, including photoconductivity.

Structural Transformable Coulomb Lattice of n-Type Semiconductors for Guest Sorption

In recent years, highly designable organic porous materials have attracted considerable attention in the development of new types of molecular adsorption-desorption materials. The adsorption-desorption process also changes the electronic structure via the existence of guest molecules. Therefore, it is possible to change the physical property during the guest adsorption-desorption cycle using an appropriate chemical design of the host crystal lattice. As the development of n-type organic semiconductors has been limited, we focused on designing an n-type organic semiconductor material to control the host crystal lattice, electronic dimensionality, chemical stability, and high electron mobility using an ionic naphthalenediimide (NDI) derivative. Low symmetrical dianionic bis(benzene-m-sulfonate)-naphthalenediimide (m-BSNDI^2-) forms various types of single-crystal (M⁺)₂(m-BSNDI^2-)·n(guest) with a combination of M⁺ = Na⁺, K⁺, Rb⁺, and guest = H₂O, CH₃OH. Four crystals of (K⁺)₂(m-BSNDI^2-)·n(H₂O), (K⁺)₂(m-BSNDI^2-)·n(CH₃OH), α-(K⁺)₂(m-BSNDI^2-), and β-(K⁺)₂(m-BSNDI^2-) were transformable using the guest adsorption-desorption cycle. Two kinds of single-crystal (K⁺)₂(m-BSNDI^2-)·n(CH₃OH) with n = 0 and 2.0 showed a single-crystal to single-crystal (SCSC) transformation through CH₃OH desorption. On the contrary, five kinds of single crystals with n = 0, 3.0, 3.3, 4.75, and 5.5 were identified in the single-crystal X-ray structural analyses of (K⁺)₂(m-BSNDI^2-)·n(H₂O). Systematic change of the ionic radii in (M⁺)₂(m-BSNDI^2-) modified the crystal lattice flexibility for the guest adsorption-desorption cycles.

Impact of the Hydrogen-Bonding Functional Group on Hydrogelation of Amphiphilic Naphthalene-diimide Derivatives and Nonspecific Protein Adsorption

This manuscript reports the effect of hydrogen-bonding functionality on the supramolecular assembly of naphthalene-diimide (NDI)-derived amphiphilic building blocks in water. All the molecules contain a central NDI chromophore, functionalized with a hydrophilic oligo-oxyethylene (OE) wedge in one arm and a phenyl group on the opposite arm. They differ by a single H-bonding functionality, which links the NDI chromophore and the phenyl moiety. The H-bonding functionalities are amide, thioamide, urea, and urethane in NDI-A, NDI-TA, NDI-U, and NDI-UT, respectively. All of these molecules exhibit π-stacking in water, as evident from their distinct UV/vis absorption spectra when compared to that of the monomeric dye in THF. However, among these four, only NDI-A and NDI-TA show hydrogelation, while the other two precipitate out of the medium. The NDI-A hydrogel also exhibits transient stability and leads to a crystalline precipitate within ∼5 h. Only NDI-TA produces stable transparent hydrogel with the entangled fibrillar morphology that is typical for gelators. Both NDI-A and NDI-TA showed a thermoresponsive property with a lower critical solution temperature of about 41-42 °C. Powder XRD studies show a parallel orientation for NDI-A and an antiparallel orientation for NDI-TA. Computational studies support this experimental observation and indicate that the NDI-A assembly is highly stabilized by strong H-bonding among the amide groups and π-stacking interaction in the parallel orientation. On the other hand, due to weak H-bonding among the thioamide groups, the binding energy of the parallelly oriented NDI-TA was significantly lower and the optimized structure was disordered. Instead, its antiparallel orientation was more stable, with criss-cross aligned H-bonding interactions and π-π interactions between adjacent aromatic rings. The NDI-TA hydrogel with less ordered OE chains on the surface showed prominent adsorption of serum protein BSA. In sharp contrast, NDI-A did not exhibit any notable interaction with BSA, as evident from the ITC studies.

Microscopic Structures, Dynamics, and Spin Configuration of the Charge Carriers in Organic Photovoltaic Solar Cells Studied by Advanced Time-Resolved Spectroscopic Methods

Organic photovoltaics (OPVs) are promising solutions for renewable energy and sustainable technologies and have attracted much attention in recent years. Two types of organic semiconductors are used as donor materials to fabricate OPV cells. One type is a photoconductive polymer, and the other type is a small-molecule-based compound. The discovery of a bulk-heterojunction (BHJ) structure using a mixture of p- and n-type organic semiconductors has dramatically increased the power conversion efficiency (PCE) of OPV cells. In this feature article, we review our recent studies on organic BHJ thin films and OPVs by using advanced time-resolved spectroscopic techniques. Two topics regarding the microscopic behaviors of the charge carriers are discussed. The first topic is focused on how to quantify the local mobility of the charge carriers. Here, we discuss charge carrier dynamics in diketopyrrolopyrrole-linked tetrabenzoporphyrin (DPP-BP) BHJ thin films studied by time-resolved terahertz spectroscopy on a subpicosecond to several tens of picoseconds time scale and by transient photocurrent measurements on a microsecond time scale. The second topic concerns the spin configuration and interaction of the electron and hole of the polaron pairs in polymer-based BHJ thin films and OPV cells studied by the time-resolved electron paramagnetic resonance method, time-resolved simultaneous optical and electrical detection, and measurement of the magnetoconductance effect.

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.

Facile synthesis of an ambient stable pyreno[4,5-b]pyrrole monoanion and pyreno[4,5-b:9,10-b']dipyrrole dianion: from serendipity to design

The stability of singly or multiply negatively charged π-conjugated organic compounds is greatly influenced by their electronic delocalization. Herein, we report a strategic methodology for isolation of a mysterious compound. The isolated compounds, a pyreno[4,5-b]pyrrole monoanion and pyreno[4,5-b:9,10-b′]dipyrrole dianion, were highly stable under ambient conditions due to high delocalization of the negative charge over multiple electron deficient C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N groups and pyrene π-scaffolds and allowed purification by column chromatography. To our knowledge, this is the first report on TCNE type reductive condensation of malononitrile involving pyrene di- and tetraone and formation of pyrenopyrrole. All compounds were characterized by spectroscopic methods and X-ray crystallography. A UV-vis spectroscopic study shows an intense low energy absorption band with a large absorption coefficient (ε).

An ambient stable pyreno[4,5-b]pyrrole monoanion and pyreno[4,5-b:9,10-b′]dipyrrole dianion have been isolated and characterized, showing a low energy intense absorption band with the absorption coefficient reaching 7.1 × 10⁴ dm³ mol⁻¹ cm⁻¹.

Ultrahigh anisotropic carrier mobility in ZnSb monolayers functionalized with halogen atoms

The experimental fabrication of novel two-dimensional ZnSb inspires us to explore the tunability of its fundamental physical properties. In this work, we present the density functional theory simulations on the mechanical, electronic and transport properties of the two-dimensional ZnSb monolayers functionalized with halogen atoms. It is found that the halogen atoms prefer to form ionic bonds with Sb atoms and these ZnSbX (X = Cl, Br and I) monolayers are very flexible with Young's moduli ranging from 24.02 N m⁻¹ to 30.16 N m⁻¹ along the armchair and zigzag directions. The pristine ZnSb monolayer sheet exhibits metallic phase while the functionalization can lead to a metal-to-semiconductor transition with band gaps as large as 0.55 eV. The transport study reveals a large tunability with the hole mobility reaching 43.44 × 10³ cm² V⁻¹ s⁻¹ along the armchair direction and the electron mobility as high as 36.99 × 10³ cm² V⁻¹ s⁻¹ along the zigzag direction. In contrast, the electron mobility along the armchair direction and the hole mobility along the zigzag direction are of relatively small magnitude. The ultrahigh carrier mobility together with the directional anisotropy can boost the separation of photo-excited electron–hole pairs. The finite band gaps and exceptional transport property of ZnSbX monolayers render them new materials with promising applications in flexible optoelectronic and nanoelectronic devices.

Ultrahigh carrier mobility and transport anisotropy in ZnSb monolayers functionalized with halogen atoms.

Structurally directed thienylenevinylene self–assembly for improved charge carrier mobility: 2D sheets vs 1D fibers

High charge carrier mobility is a prerequisite for organic electronics for which molecular arrangement and morphology plays a vital role. Herein, we report how the self-assembly of thienylenevinylenes T1 and...

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 Ni^II 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 Zn^II porphyrins.

A structural parameter to link molecular geometry to macroscopic orientation in discotic liquid crystals: study of metalloporphyrin tapes

Novel liquid crystalline (LC) molecules were prepared from a dimeric porphyrin tape. A series of metal complexes (1Zn, 1Pd, 1Cu, and 1Ni) and the free-base form (12H) of the porphyrin tape formed a columnar LC phase. Although only the central metal ions were different among these compounds, 1Ni, 12H, and 1Cu aligned homeotropically in a sandwiched glass cell, while 1Zn and 1Pd exhibited a random orientation at a macroscopic scale. The strength of the π-π interactions, tunable by the distortion of the porphyrin cores through metallation, is a key factor for the observed macroscopic orientation difference.

Charge Mobility in Discotic Liquid Crystals

Discotic (disk-shaped) molecules or molecular aggregates may form, within a certain temperature range, partially ordered phases, known as discotic liquid crystals, which have been extensively studied in the recent past. On the one hand, this interest was prompted by the fact that they represent models for testing energy and charge transport theories in organic materials. However, their long-range self-assembling properties, potential low cost, ease of processability with a variety of solvents and the relative ease of tailoring their properties via chemical synthesis, drove the attention of researchers also towards the exploitation of their semiconducting properties in organic electronic devices. This review covers recent research on the charge transport properties of discotic mesophases, starting with an introduction to their phase structure, followed by an overview of the models used to describe charge mobility in organic substances in general and in these systems in particular, and by the description of the techniques most commonly used to measure their charge mobility. The reader already familiar or not interested in such details can easily skip these sections and refer to the core section of this work, focusing on the most recent and significant results regarding charge mobility in discotic liquid crystals.

Thiophene-Fused Naphthodiphospholes: Modulation of the Structural and Electronic Properties of Polycyclic Aromatics by Precise Fusion of Heteroles

For polycyclic aromatics with heterole-fused structures, the orientation of fused heterole rings as well as the geometry of their fused structures has a large impact on the physicochemical properties. In this study, a series of isomers of thiophene-fused naphthodiphospholes was designed and synthesized. Systematic investigation unveiled the explicit impact of heterole-fused structures on their structural and electronic properties. The isomers with 1,2/5,6-fused structure display phosphorescence due to enhanced spin-orbit coupling, whereas the isomers with 2,3/6,7-fused structure exhibit intense fluorescence. The trans isomers exhibited 1D slip π-stacked arrangement. In contrast, the cis isomers displayed 2D herringbone structure or columnar structure with a cavity. Therefore, the precisely controlled fusion of heterole rings is a universal approach to uncover their intrinsic properties for versatile applications as organic functional materials.

Chlorophylls in thin-film photovoltaic cells, a critical review

Self-assembly and electrical properties of chlorophyll-type dyes are reviewed with emphasis on their potential applications in thin-film solar cells.

Ultrathin Two Dimensional (2D) Supramolecular Assembly and Anisotropic Conductivity of an Amphiphilic Naphthalene-Diimide

Two-dimensional (2D)-supramolecular assemblies of π-conjugated chromophores are relatively less common compared to a large number of recent examples on their low dimensional (0D or 1D) assemblies or 3D architectures. This article reports a rational design for the 2D supramolecular assembly of an amphiphilic core-substituted naphthalene-diimide derivative (cNDI-1). The building block contains a naphthalene-diimide (NDI) chromophore, symmetrically substituted with two dodecyl chains from the aromatic core while the imide positions are functionalized with two hydrophilic wedges containing oligo-oxyethylene chains. In water, it exhibits entropically favorable self-assembly with a critical aggregation concentration of 1.5 × 10^-5 M and a lower critical solution temperature of 55 °C. The UV/vis absorption spectrum in water shows bathochromically shifted absorption bands compared to that of the monomeric dye in THF, indicating offset π-stacking among the NDI chromophores. C-H symmetric and asymmetric stretching frequencies in the FT-IR spectrum support the presence of organized hydrocarbon chains in trans conformation in the self-assembled state, similar to that in the crystalline n-alkanes, which is further supported by studying the general polarization (GP) values of a noncovalently entrapped Laurdan dye. The atomic force microscopy (AFM) image shows the formation of ultrathin (height < 2.0 nm) ribbons for the spontaneously assembled sample which eventually produces a large-area 2D nanosheet by the lateral organization. The powder X-ray diffraction pattern of the drop-casted film, prepared from the preformed aggregates, reveals sharp peaks that indicate a crystalline lamellar packing along the direction of the 2D growth. Differential scanning calorimetry trace shows the melting of the crystalline alkyl chain domain at T > 75 °C, which destroys the 2D assembly. Local-scale photoconductivity of the ordered 2D assembly, studied by the flash-photolysis time-resolved microwave conductivity (FP-TRMC) technique, reveals an anisotropic conductivity with ∼3 times larger conductivity along the parallel direction compared to that along the perpendicular one.

Evaluation-oriented exploration of photo energy conversion systems: from fundamental optoelectronics and material screening to the combination with data science

Light is a form of energy that can be converted to electric and chemical energies. Thus, organic photovoltaics (OPVs), perovskite solar cells (PSCs), photocatalysts, and photodetectors have evolved as scientific and commercial enterprises. However, the complex photochemical reactions and multicomponent materials involved in these systems have hampered rapid progress in their fundamental understanding and material design. This review showcases the evaluation-oriented exploration of photo energy conversion materials by using electrodeless time-resolved microwave conductivity (TRMC) and materials informatics (MI). TRMC with its unique options (excitation sources, environmental control, frequency modulation, etc.) provides not only accelerated experimental screening of OPV and PSC materials but also a versatile route toward shedding light on their charge carrier dynamics. Furthermore, MI powered by machine learning is shown to allow extremely high-throughput exploration in the large molecular space, which is compatible with experimental screening and combinatorial synthesis.

Switching of Electron and Ion Conductions by Reversible H2O Sorption in n-Type Organic Semiconductors

Polar H₂O molecules generally act as trapping sites and suppress the electron mobility of n-type organic semiconductors, making chemical design of H₂O-tolerant and responsive n-type semiconductors an important step toward multifunctional electron-ion coupling devices. The introduction of effective electrostatic interactions between potassium ions (K⁺) and carboxylate (-COO^-) anions into the electron-transporting naphthalenediimide π-framework enables the design of high-performance H₂O-tolerant n-type semiconductors with a reversible H₂O adsorption-desorption ability, where the electron mobility and K⁺ ionic conductivity were coupled with the reversible H₂O sorption behavior. The reversible H₂O adsorption into the crystals enhanced the electron mobility from 0.04 to 0.28 cm² V^-1 s^-1, whereas the K⁺ ionic conductivity decreased from 3.4 × 10^-5 to 4.7 × 10^-7 S cm^-1. Because this reversible electron-ion conducting switch is responsive to H₂O sorption behavior, it is a strong candidate for H₂O gating carrier transport systems.

Noncovalent Functionalization of Few-Layered Antimonene with Fullerene Clusters and Photoinduced Charge Separation in the Composite

Few-layered antimonene (FLSb) nanosheets were noncovalently functionalized with fullerene C₆₀ clusters by quick addition of a poor solvent (i.e., acetonitrile) into a mixed dispersion of FLSb and C₆₀ in a good solvent (i.e., toluene). In a flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurement, the FLSb-C₆₀ composite, (FLSb+C₆₀ )_m , showed a rapid rise in transient conductivity, whereas no conductivity signal was observed in the single components, FLSb and C₆₀ . This demonstrated the occurrence of photoinduced charge separation between FLSb and C₆₀ in (FLSb+C₆₀ )_m . Furthermore, a photoelectrochemical device with an electrophoretically deposited (FLSb+C₆₀ )_m film exhibited an enhanced efficiency of photocurrent generation, compared to those of the single-components, FLSb and C₆₀ , due to the photoinduced charge separation between FLSb and C₆₀ . This work provides a promising approach for fabrication of antimonene-organic molecule composites and paves the way for their application in optoelectronics.

Microfluidic out-of-equilibrium control of molecular nanotubes

The bottom-up fabrication of functional nanosystems for light-harvesting applications and excitonic devices often relies on molecular self-assembly. Gaining access to the intermediate species involved in self-assembly would provide valuable insights into the pathways via which the final architecture has evolved, yet difficult to achieve due to their intrinsically short-lived nature. Here, we employ a lab-on-a-chip approach as a means to obtain in situ control of the structural complexity of an artificial light-harvesting complex: molecular double-walled nanotubes. Rapid and stable dissolution of the outer wall was realized via microfluidic mixing thereby rendering the thermodynamically unstable inner tubes accessible to spectroscopy. By measurement of the linear dichroism and time-resolved photoluminescence of both double-walled nanotubes and isolated inner tubes we show that the optical (excitonic) properties of the inner tube are remarkably robust to such drastic perturbation of the system's supramolecular structure as removal of the outer wall. The developed platform is readily extendable to a broad range of practical applications such as e.g. self-assembling systems and molecular photonics devices.

Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?

The field of organic electronics has been prolific in the last couple of years, leading to the design and synthesis of several molecular semiconductors presenting a mobility in excess of 10 cm² V^-1 s^-1 . However, it is also started to recently falter, as a result of doubtful mobility extractions and reduced industrial interest. This critical review addresses the community of chemists and materials scientists to share with it a critical analysis of the best performing molecular semiconductors and of the inherent charge transport physics that takes place in them. The goal is to inspire chemists and materials scientists and to give them hope that the field of molecular semiconductors for logic operations is not engaged into a dead end. To the contrary, it offers plenty of research opportunities in materials chemistry.

Protein-directed crystalline 2D fullerene assemblies

Water soluble 2D crystalline monolayers of fullerenes grow on planar assemblies of engineered consensus tetratricopeptide repeat proteins. Designed fullerene-coordinating tyrosine clamps on the protein introduce specific fullerene binding sites, which facilitate fullerene nucleation. Through reciprocal interactions between the components, the hybrid material assembles into two-dimensional 2 nm thick structures with crystalline order, that conduct photo-generated charges. Thus, the protein-fullerene hybrid material is a demonstration of the developments toward functional materials with protein-based precision control of functional elements.

Controlled supramolecular polymerization of π-systems

Externally-initiated controlled supramolecular polymerization of the kinetically trapped aggregated state in a chain growth mechanism can produce well-defined living supramolecular polymers and copolymers.

Synthesis and electron-transport properties of a stable antiaromatic NiII norcorrole with the smallest meso-substituent

meso-Dimethylnorcorrole NiII complex exhibited enough stability under ambient conditions despite the distinct antiaromaticity. The small methyl substituents realized a dense and long-range π-stacking in its solid state, which resulted in the superior electron-transporting ability to previously reported NiII norcorroles.

Supramolecular Nanowires from an Acceptor-Donor-Acceptor Conjugated Chromophore

Oligothiophene derivatives have been extensively studied as p-type semiconducting materials in organic electronics applications. This work reports the synthesis, self-assembly and photophysical properties of acceptor-donor-acceptor (A-D-A)-type oligothiophene derivatives by end-group engineering of quaterthiophene (QT) with naphthalene monoimide (NMI) chromophores that are further connected to a trialkoxy benzamide wedge. Conjugation to the NMI units reduces the HOMO-LUMO gap significantly, and consequently the absorption spectrum exhibits a bathochromic shift of about 50 nm compared with QT. Furthermore, extended H-bonding interactions among the amido groups of the peripheral wedges produce entangled fibrillar nanostructures and gelation in hydrocarbon solvents such as methylcyclohexane, wherein the A-D-A chromophore exhibits typical H-aggregation. On the contrary, the fact that the same chromophore, lacking only the amido units, does not produce gels or H-aggregates indicates strong impact of H-bonding on the self-assembly. Computational studies revealed the electronic properties of the chromophore and predicted the geometry of a dimer in the H-aggregate that reasonably matches with the experimental results. Bulk electrical conductivity measurements determined an excellent conductivity of 2.3×10^-2  S cm^-1 for the H-aggregated system (OT-1), which is two orders of magnitude higher than that of the same chromophore lacking the amido groups (OT-2).

Repurposing DNA-binding agents as H-bonded organic semiconductors

Organic semiconductors are usually polycyclic aromatic hydrocarbons and their analogs containing heteroatom substitution. Bioinspired materials chemistry of organic electronics promises new charge transport mechanism and specific molecular recognition with biomolecules. We discover organic semiconductors from deoxyribonucleic acid topoisomerase inhibitors, featuring conjugated backbone decorated with hydrogen-bonding moieties distinct from common organic semiconductors. Using ellipticine as a model compound, we find that hydrogen bonds not only guide polymorph assembly, but are also critical to forming efficient charge transport pathways along π-conjugated planes when at a low dihedral angle by shortening the end-to-end distance of adjacent π planes. In the π-π stacking and hydrogen-bonding directions, the intrinsic, short-range hole mobilities reach as high as 6.5 cm2V-1s-1 and 4.2 cm2V-1s-1 measured by microwave conductivity, and the long-range apparent hole mobilities are up to 1.3 × 10-3 cm2V-1s-1 and 0.4 × 10-3 cm2V-1s-1 measured in field-effect transistors. We further demonstrate printed transistor devices and chemical sensors as potential applications.

Charge Carrier Polarity Modulation in Diketopyrrolopyrrole-Based Low Band Gap Semiconductors by Terminal Functionalization

Organic semiconductors with variable charge carrier polarity are required for optoelectronic applications. Herein, we report the synthesis of three novel diketopyrrolopyrrole (DPP)-based D-A molecules having three different terminal groups (amide, ester, and dicyano) and study their electronic properties. An increase in electron acceptor strength from amide to dicyano leads to a bathochromic shift in absorption. Photoconductivity and field effect transistor (FET) measurements confirmed that a small increase in acceptor strength can result in a large change in the charge transport properties from p-type to n-type. The molecule with an amide group, DPP-amide, exhibited a moderate p-type mobility (1.3 × 10^-2 cm² V^-1 s^-1), whereas good n-type mobilities were observed for molecules with an ester moiety, DPP-ester (1.5 × 10^-2 cm² V^-1 s^-1), and with a dicyano group, DPP-DCV (1 × 10^-2 cm² V^-1 s^-1). The terminal functional group modification approach presented here is a simple and efficient method to alter the charge carrier polarity of organic semiconductors.

Preparation, Electronic and Liquid Crystalline Properties of Electron-Accepting Azaacene Derivatives

A series of electron-accepting azaacene-type materials 1-4 with different kinds and degrees of intermolecular interactions were synthesized. Simple modification of the terminal substituents significantly modulated the photophysical and electrochemical properties. The degree of weak intermolecular interaction determined the emergence of a liquid crystalline (LC) phase for each compound. Dipole-dipole interaction, π-π interaction, and van der Waals interaction all contributed to stabilize the LC phase of 1 and 2. The introduction of strong hydrogen bonding interaction enabled the formation of a highly ordered LC phase in 4. Charge-transport properties of 1, 2, and 4 were also investigated.

Conjugated Nanowire Sensors via High-Energy Single-Particle-Induced Linear Polymerization of 9,9'-Spirobi[9 H-fluorene] Derivatives

Nanostructures composed of conjugated polymers or π-conjugated molecules provide sensing platforms with large specific surface areas. One of the feasible approaches to accessing such nanostructured miniaturized sensors with ultrahigh sensitivity is to develop a network of organic nanowires with optical/electronic properties that can measure signals upon interacting with the analytes at their surfaces. In this work, organic nanowires with controlled number density and uniform length were fabricated by one-dimensional solid-state polymerization of 9,9'-spirobi[9 H-fluorene] (SBF) derivatives triggered by high-energy single particles. SBF was chosen as a conjugated molecular motif with the interplay of high density of π-electrons, high solubility, and uniform solid-state structures, allowing us to fabricate sensing platforms via solution processing. The as-deposited energy density in linear polymerization nanospace was theoretically analyzed by a collision model, interpreting nanowire sizes at subnanometer levels. The substitution of bromine atoms was confirmed to be effective not only for the higher collision probability of the incident particles but also for the remarkable increase in radiolytic neutral radical yield via C-Br cleavages or electron-dissociative attachments onto the bromine atoms. The fluorescence spectra of SBF-based nanowires were different from those of SBF derivatives due to extended bond formation as a result of polymerization reactions. Fluorescence was quenched by the addition of nitrobenzene, indicating the potential use of our nanowires for fluorometric sensing applications. Microwave-based conductivity measurements revealed that the SBF-based nanowires exhibited charge carrier transport property upon photoexcitation, and that the conductivity was changed upon treatment with nitrobenzene vapors. The presented strategy of bromination of aromatic rings for efficient fabrication of controlled nanowire networks with favorable fluorescent and charge transport properties of nanowires advances the development of nanostructured sensing systems.

Landscape of Charge Carrier Transport in Doped Poly(3-hexylthiophene): Noncontact Approach Using Ternary Combined Dielectric, Paramagnetic, and Optical Spectroscopies

We report on a comprehensive measurement system for mobility and energy states of charge carriers in matter under dynamic chemical doping. The temporal evolution of the iodine doping process of poly(3-hexylthiophene) (P3HT) was monitored directly through electron paramagnetic resonance (EPR) and optical absorption spectroscopy, as well as differential electrical conductivity by the microwave conductivity measurement. The increase in conductivity was observed after the EPR intensity reached a maximum and declined thereafter, and the conductivity finally reached ∼80 S cm^-1. The carrier species changed from a paramagnetic polaron with an estimated mobility of μ_P+ ≈ 2 × 10^-3 cm² V^-1 s^-1 to an antiferromagnetic polaron pair with μ_PP+ ≈ 0.6 cm² V^-1 s^-1. The technique presented here can be a ubiquitous method for rapid and direct observation of charge carrier mobility and energy states in p-type semiconducting materials as a completely noncontact, experimental, and quantitative technique.