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The chemical doping of molecular semiconductors is based on electron-transfer reactions between the semiconductor and dopant molecules; here, the redox potential of the dopant is key to control the Fermi level of the semiconductor 1 , 2 . The tunability and reproducibility of chemical doping are limited by the availability of dopant materials and the effects of impurities such as water. Here we focused on proton-coupled electron-transfer (PCET) reactions, which are widely used in biochemical processes 3 , 4 ; their redox potentials depend on an easily handled parameter, that is, proton activity. We immersed p-type organic semiconductor thin films in aqueous solutions with PCET-based redox pairs and hydrophobic molecular ions. Synergistic reactions of PCET and ion intercalation resulted in efficient chemical doping of crystalline organic semiconductor thin films under ambient conditions. In accordance with the Nernst equation, the Fermi levels of the semiconductors were controlled reproducibly with a high degree of precision—a thermal energy of about 25 millielectronvolts at room temperature and over a few hundred millielectronvolts around the band edge. A reference-electrode-free, resistive pH sensor based on this method is also proposed. A connection between semiconductor doping and proton activity, a widely used parameter in chemical and biochemical processes, may help create a platform for ambient semiconductor processes and biomolecular electronics. Proton-coupled electron-transfer reactions are used to achieve efficient chemical doping of organic semiconductor thin films under ambient conditions, and a reference-electrode-free, resistive pH sensor based on the method is proposed.

The formation of excitons in organic molecules by charge injection is an essential process in organic light-emitting diodes (OLEDs) 1 – 7 . According to a simple model based on spin statistics, the injected charges form spin-singlet (S 1 ) excitons and spin-triplet (T 1 ) excitons in a 1:3 ratio 2 – 4 . After the first report of a highly efficient OLED 2 based on phosphorescence, which is produced by the decay of T 1 excitons, more effective use of these excitons has been the primary strategy for increasing the energy efficiency of OLEDs. Another route to improving OLED energy efficiency is reduction of the operating voltage 2 – 6 . Because T 1 excitons have lower energy than S 1 excitons (owing to the exchange interaction), use of the energy difference could—in principle—enable exclusive production of T 1 excitons at low OLED operating voltages. However, a way to achieve such selective and direct formation of these excitons has not yet been established. Here we report a single-molecule investigation of electroluminescence using a scanning tunnelling microscope 8 – 20 and demonstrate a simple method of selective formation of T 1 excitons that utilizes a charged molecule. A 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) molecule 21 – 25 adsorbed on a three-monolayer NaCl film atop Ag(111) shows both phosphorescence and fluorescence signals at high applied voltage. In contrast, only phosphorescence occurs at low applied voltage, indicating selective formation of T 1 excitons without creating their S 1 counterparts. The bias voltage dependence of the phosphorescence, combined with differential conductance measurements, reveals that spin-selective electron removal from a negatively charged PTCDA molecule is the dominant formation mechanism of T 1 excitons in this system, which can be explained by considering the exchange interaction in the charged molecule. Our findings show that the electron transport process accompanying exciton formation can be controlled by manipulating an electron spin inside a molecule. We anticipate that designing a device taking into account the exchange interaction could realize an OLED with a lower operating voltage. Recombination of excitons to produce molecular light emission is made more efficient by controlling electron spin within the molecule to produce spin-triplet excitons only, without the usual accompanying spin-singlet excitons.

The development of science and technology of advanced materials using nanoscale units can be conducted by a novel concept involving combination of nanotechnology methodology with various research disciplines, especially supramolecular chemistry. The novel concept is called 'nanoarchitectonics' where self-assembly processes are crucial in many cases involving a wide range of component materials. This review of self-assembly processes re-examines recent progress in materials nanoarchitectonics. It is composed of three main sections: (1) the first short section describes typical examples of self-assembly research to outline the matters discussed in this review; (2) the second section summarizes self-assemblies at interfaces from general viewpoints; and (3) the final section is focused on self-assembly processes at interfaces. The examples presented demonstrate the strikingly wide range of possibilities and future potential of self-assembly processes and their important contribution to materials nanoarchitectonics. The research examples described in this review cover variously structured objects including molecular machines, molecular receptors, molecular pliers, molecular rotors, nanoparticles, nanosheets, nanotubes, nanowires, nanoflakes, nanocubes, nanodisks, nanoring, block copolymers, hyperbranched polymers, supramolecular polymers, supramolecular gels, liquid crystals, Langmuir monolayers, Langmuir-Blodgett films, self-assembled monolayers, thin films, layer-by-layer structures, breath figure motif structures, two-dimensional molecular patterns, fullerene crystals, metal-organic frameworks, coordination polymers, coordination capsules, porous carbon spheres, mesoporous materials, polynuclear catalysts, DNA origamis, transmembrane channels, peptide conjugates, and vesicles, as well as functional materials for sensing, surface-enhanced Raman spectroscopy, photovoltaics, charge transport, excitation energy transfer, light-harvesting, photocatalysts, field effect transistors, logic gates, organic semiconductors, thin-film-based devices, drug delivery, cell culture, supramolecular differentiation, molecular recognition, molecular tuning, and hand-operating (hand-operated) nanotechnology.

The efficiency with which polymeric semiconductors can be chemically doped—and the charge carrier densities that can thereby be achieved—is determined primarily by the electrochemical redox potential between the π-conjugated polymer and the dopant species 1 , 2 . Thus, matching the electron affinity of one with the ionization potential of the other can allow effective doping 3 , 4 . Here we describe a different process—which we term ‘anion exchange’—that might offer improved doping levels. This process is mediated by an ionic liquid solvent and can be pictured as the effective instantaneous exchange of a conventional small p-type dopant anion with a second anion provided by an ionic liquid. The introduction of optimized ionic salt (the ionic liquid solvent) into a conventional binary donor–acceptor system can overcome the redox potential limitations described by Marcus theory 5 , and allows an anion-exchange efficiency of nearly 100 per cent. As a result, doping levels of up to almost one charge per monomer unit can be achieved. This demonstration of increased doping levels, increased stability and excellent transport properties shows that anion-exchange doping, which can use an almost infinite selection of ionic salts, could be a powerful tool for the realization of advanced molecular electronics. The limitations of conventional chemical doping of polymeric semiconductors can be overcome by adding a second ionic species into the system, leading to enhanced doping, electrical conductivity and stability.

Nanoarchitectonics is a new paradigm to combine and unify nanotechnology with other sciences and technologies, such as supramolecular chemistry, self-assembly, self-organization, materials technology for manipulation of the size of material objects, and even biotechnology for hybridization with bio-components. The nanoarchitectonic concept leads to the synergistic combination of various methodologies in materials production, including atomic/molecular-level control, self-organization, and field-controlled organization. The focus of this review is on soft 2D nanoarchitectonics. Scientific views on soft 2D nanomaterials are not fully established compared with those on rigid 2D materials. Here, we collect recent examples of 2D nanoarchitectonic constructions of functional materials and systems with soft components. These examples are selected according to the following three categories on the basis of 2D spatial density and motional freedom: (i) well-packed and oriented organic 2D materials with rational design of component molecules and device applications, (ii) well-defined assemblies with 2D porous structures as 2D network materials, and (iii) 2D control of molecular machines and receptors on the basis of certain motional freedom confined in two dimensions.

Singlet fission, in which a singlet exciton is converted to two triplet excitons, is a process that could be beneficial in photovoltaic applications. A full understanding of the dynamics of singlet fission in molecular systems requires detailed knowledge of the relevant potential energy surfaces and their (conical) intersections. However, obtaining such information is a nontrivial task, particularly for molecular aggregates. Here we investigate singlet fission in rubrene crystals using transient absorption spectroscopy and state-of-the-art quantum chemical calculations. We observe a coherent and ultrafast singlet-fission channel as well as the well-known and conventional thermally assisted incoherent channel. This coherent channel is accessible because the conical intersection for singlet fission on the excited-state potential energy surface is located very close to the equilibrium position of the ground-state potential energy surface and also because of the excitation of an intermolecular symmetry-breaking mode, which activates the electronic coupling necessary for singlet fission. Singlet fission — converting a singlet exciton to two triplet excitons — may be useful for improving photovoltaic efficiency. Ultrafast spectroscopic measurements and quantum chemical calculations have now uncovered aspects of the process critical to it occurring efficiently, including the role of intermolecular vibrations and symmetry breaking, and the location of a conical intersection on the excited-state potential-energy surface.

Coherent charge transport can occur in organic semiconductor crystals thanks to the highly periodic electrostatic potential—despite the weak van der Waals bonds. And as spin–orbit coupling is usually weak in organic materials, robust spin transport is expected, which is essential if they are to be exploited for spintronic applications. In such systems, momentum relaxation occurs via scattering events, which enables an intrinsic mobility to be defined for band-like charge transport, which is >10 cm 2  V −1  s −1 . In contrast, there are relatively few experimental studies of the intrinsic spin relaxation for organic band-transport systems. Here, we demonstrate that the intrinsic spin relaxation in organic semiconductors is also caused by scattering events, with much less frequency than the momentum relaxation. Magnetotransport measurements and electron spin resonance spectroscopy consistently show a linear relationship between the two relaxation times over a wide temperature range, clearly manifesting the Elliott–Yafet type of spin relaxation mechanism. The coexistence of an ultra-long spin lifetime of milliseconds and the coherent band-like transport, resulting in a micrometre-scale spin diffusion length, constitutes a key step towards realizing spintronic devices based on organic single crystals. A linear relationship between spin and momentum relaxation shows that the spin relaxation in an organic semiconductor crystal that has ultra-long spin lifetimes and coherent band-like transport is governed by the Elliott–Yafet mechanism.

Transistors, the most important logic elements, are maintained under dynamic influence during circuit operations. Practically, circuit design protocols and frequency responsibility should stem from a perfect agreement between the static and dynamic properties. However, despite remarkable improvements in mobility for organic semiconductors, the correlation between the device performances achieved under static and dynamic circumstances is controversial. Particularly in the case of organic semiconductors, it remains unclear whether parasitic elements that relate to their unique molecular aggregates may violate the radiofrequency circuit model. Thus, we herein report the manufacture of micrometre-scale transistor arrays composed of solution-processed organic semiconductors, which achieve near very high-frequency band operations. Systematic investigations into the device geometrical factors revealed that the radiofrequency circuit model established on a solid-state continuous medium is extendable to organic single-crystal field-effect transistors. The validity of this radiofrequency circuit model allows a reliable prediction of the performances of organic radiofrequency devices. Though literature reports improvements in organic electronic device performance, understanding the correlation between static and dynamic device responses remains a challenge. Here, the authors correlate static and dynamic parameters in organic transistors by using the radiofrequency circuit model.

Organic semiconductor (OSC) single crystals feature flexibility, solution processability, and high-mobility coherent carrier transport, which are advantageous for printed flexible electronic applications. A mechanical strain sensor is a target device whose high sensitivity and wide measurement range have been demonstrated when OSC single crystals were employed as the active channel. However, there have been limited reports on scalable fabrication of devices and reliable measurements, which limits the use of strain sensors in a wide range of applications. In this study, we present a comprehensive approach to address these issues through advanced device processing, design, and measurements. Our resistive strain sensors showed a small drift owing to the stable and effective p-type chemical doping of the OSC single crystals. A Wheatstone bridge circuit and compact lock-in amplifier were designed to accurately measure resistance changes at low noise levels. The experimental results demonstrated a substantial reduction in noise and achieved high-precision measurements with precision of ± 1.8 ppm. These results demonstrate the scalable fabrication of organic semiconductor strain sensors with high precision and reliability, which opens up the possibility of employing them in various industrial sectors.

Organic molecular semiconductors are solution processable, enabling the growth of large-area single-crystal semiconductors. Improving the performance of organic semiconductor devices by increasing the charge mobility is an ongoing quest, which calls for novel molecular and material design, and improved processing conditions. Here we show a method to increase the charge mobility in organic single-crystal field-effect transistors, by taking advantage of the inherent softness of organic semiconductors. We compress the crystal lattice uniaxially by bending the flexible devices, leading to an improved charge transport. The mobility increases from 9.7 to 16.5 cm 2  V −1  s −1 by 70% under 3% strain. In-depth analysis indicates that compressing the crystal structure directly restricts the vibration of the molecules, thus suppresses dynamic disorder, a unique mechanism in organic semiconductors. Since strain can be easily induced during the fabrication process, we expect our method to be exploited to build high-performance organic devices. The mobility of organic semiconductors can be tuned by modifying their chemical composition or crystalline properties. Here, the authors show that bending organic single crystals increases their field effect transistor mobility due to restrained molecular vibrations and subsequently reduced dynamic disorder.