Enantiomerization of five-membered-heterocycle-embedded helicenes: A DFT study.

In this work, DFT theoretical calculations were employed to investigate the enantiomerization of helicenes embedded with five-membered heterocycles. The original benzene rings in the helicene backbone were replaced by heterocycles such as furan, thiophene, pyrrole, or phosphole to create [n]helicenes with n ranging from 4 to 7. The impact of the type, position, and number of heterocycles on the enantiomerization barrier was systematically evaluated. Notably, the enantiomerization barrier was found to be significantly dependent on the rotatory angle and the position of the heterocycles, particularly for [4, 5]helicenes. With less rotatory angle of heterocycle, the enantiomerization barrier of helicenes was revealed to be lower, while when the heterocycle was close to the central part of the helicene chain, the barrier was also lower. Furthermore, the number of thiophene rings also had a marked effect on enantiomerization, showing a decrease of the barrier with more thiophene rings placed on the helicenes backbone. We expect this work would deliver new perspective on the relative studies for the helicene conformational conversion.

Photocatalytic Three-Component Reaction for the Synthesis of Multifunctional Diaryl Sulfides.

A photocatalytic three-component reaction of a nitroarene, a thiophenol, and a ketone for the synthesis of multifunctional diaryl sulfides was reported using a nitro group as the nitrogen source and thiophenol as the sulfur source. Thiophenol also serves as a proton donor to reduce nitroarene to arylamine as a key intermediate for the formation of C-N and C-S bonds. Good functional group tolerance and mild reaction conditions make this method have practical synthetic value for diversified multifunctional diaryl sulfides.

High-energy-density flexible graphene-based supercapacitors enabled by atypical hydroquinone dimethyl ether.

Supercapacitor is an electrochemical energy-storage technology that can meet the green and sustainable energy needs of the future. However, a low energy density was a bottleneck that limited its practical application. To overcome this, we developed a heterojunction system composed of two-dimensional (2D) graphene and hydroquinone dimethyl ether- an atypical redox-active aromatic ether. This heterojunction displayed a large specific capacitance (Cs) of 523 F g-1 at 1.0 A g-1, as well as good rate capability and cycling stability. When assembled in symmetric and asymmetric two-electrode configuration, respectively, supercapacitors can work in voltage windows of 0 âˆ¼ 1.0 V and 0 âˆ¼ 1.6 V, accordingly, and exhibited attractive capacitive characteristics. The best device can deliver an energy density of 32.4 Wh Kg-1 and a power density of 8000 W Kg-1, and suffered a small capacitance degradation. Additionally, the device showed low self-discharge and leakage current behaviors during long time. This strategy may inspire exploration of aromatic ether electrochemistry and pave a way to develop electrical double-layer capacitance (EDLC)/pseudocapacitance heterojunctions to boost the critical energy density.

Reduced Graphene Oxide-Doped Porous Thermoplastic Polyurethane Sponges for Highly Efficient Oil/Water Separation.

In this paper, a porous polyurethane sponge with excellent hydrophobicity was prepared through thermal phase separation. Preparation condition modified experiments were systematically carried out, and a sponge with a saturated oil absorption capacity (13.3 g g-1) and a rapid absorption rate (achieving absorption equilibrium within 20 s) was achieved. The thermoplastic polyurethane (TPU) sponge as an oil absorbent is capable of selectively absorbing various oils/organic solvents from oil/water mixtures with a high recovery rate. To further enhance the hydrophobicity and mechanical properties of the porous sponge, 3% reduced graphene oxide was doped to this material. The morphological investigation indicated that the three-dimensional composite sponges have uniformly distributed micropores and nanopores, and the hydrophobicity and mechanical properties were improved. The composite as a whole exhibited remarkable superelasticity, excellent reversible compressibility, and fatigue resistance (strength up to 186 kPa at 80% strain), which allows it to re-absorb oil by simple manual extrusion. The abovementioned properties make this TPU porous material a promising candidate for practical application in water pollution treatment.

Structure-Performance relationship guided design and strategic synthesis of lithiated oxa-graphene for high lithium storage.

Graphene derivative materials are widely used as anode component in lithium-ion batteries. However, there is still a lack of reliable and foresighted guides helpful for designing high-performance graphene-based electrode materials. To this end, we strategically chose challenging graphite fluoride as starting material for the derivatization of graphene in order to exclude interference factors. As a result, graphene framework was functionalized with oxygen-containing carboxylate and sulfonate groups and oxygen-free aniline units at a similar functionalization degree. Due to the strong effect of lithiation, out-of-plane p-aminobenzoic acid blocks boosted the lithium-storage capacity of graphene matrix to 636 mAh g-1 at 0.1 A/g, and sulfanilic acid blocks maximized this value to 873 mAh g-1. Sadly, oxygen-free aniline functionalized graphene material only delivered a specific capacity of 88 mAh g-1. Meanwhile, spatial lithiated carboxylate and sulfonate units endowed graphene framework with better rate capability and cycling stability. Such a structure-performance relationship established herein was beneficial for the design and preparation of high-performance graphene derivative electrode materials.

Effective Approach toward Selective Near-Infrared Dyes: Rational Design, Synthesis, and Characterization of Thieno[3,4-b]thiophene-Based Quinoidal Oligomers.

This paper describes syntheses, photophysical properties, and electrochemical characteristics of three thieno[3,4-b]thiophene (TT)-based quinoidal oligomers OnTTO. The rigid planar backbones of these oligomers give the molecules narrow absorption bands, and the main absorption bands were significantly red-shifted when the TT units were extended and demonstrated wide transparent windows. The compound O4TTO was found to possess strong absorption in the near-infrared (NIR) region approaching 1200 nm but remained transparent in the visible region. Electrochemical experiments have shown that the energy band gaps gradually narrow when the TT units are increased. Optical properties predicted by density functional theory calculations are in good agreement with the experimental optical results. These dye molecules could be promising candidates for future NIR photodetectors, filters, and bioimaging technologies.

Energy Dissipation from Confined States in Nanoporous Molecular Networks.

Crystalline nanoporous molecular networks are assembled on the Ag(111) surface, where the pores confine electrons originating from the surface state of the metal. Depending on the pore sizes and their coupling, an antibonding level is shifted upward by 0.1-0.3 eV as measured by scanning tunneling microscopy. On molecular sites, a downshifted bonding state is observed, which is occupied under equilibrium conditions. Low-temperature force spectroscopy reveals energy dissipation peaks and jumps of frequency shifts at bias voltages, which are related to the confined states. The dissipation maps show delocalization on the supramolecular assembly and a weak distance dependence of the dissipation peaks. These observations indicate that two-dimensional arrays of coupled quantum dots are formed, which are quantitatively characterized by their quantum capacitances and resonant tunneling rates. Our work provides a method for studying the capacitive and dissipative response of quantum materials with nanomechanical oscillators.

Quantum Interference-Controlled Conductance Enhancement in Stacked Graphene-like Dimers.

Stacking interactions are of significant importance in the fields of chemistry, biology, and material optoelectronics because they determine the efficiency of charge transfer between molecules and their quantum states. Previous studies have proven that when two monomers are π-stacked in series to form a dimer, the electrical conductance of the dimer is significantly lower than that of the monomer. Here, we present a strong opposite case that when two anthanthrene monomers are π-stacked to form a dimer in a scanning tunneling microscopic break junction, the conductance increases by as much as 25 in comparison with a monomer, which originates from a room-temperature quantum interference. Remarkably, both theory and experiment consistently reveal that this effect can be reversed by changing the connectivity of external electrodes to the monomer core. These results demonstrate that synthetic control of connectivity to molecular cores can be combined with stacking interactions between their π systems to modify and optimize charge transfer between molecules, opening up a wide variety of potential applications ranging from organic optoelectronics and photovoltaics to nanoelectronics and single-molecule electronics.

Flexible Superlubricity Unveiled in Sidewinding Motion of Individual Polymeric Chains.

A combination of low temperature atomic force microcopy and molecular dynamic simulations is used to demonstrate that soft designer molecules realize a sidewinding motion when dragged over a gold surface. Exploiting their longitudinal flexibility, pyrenylene chains are indeed able to lower diffusion energy barriers via on-surface directional locking and molecular strain. The resulting ultralow friction reaches values on the order of tens of pN reported so far only for rigid chains sliding on an incommensurate surface. Therefore, we demonstrate how molecular flexibility can be harnessed to realize complex nanomotion while retaining a superlubric character. This is in contrast with the paradigm guiding the design of most superlubric nanocontacts (mismatched rigid contacting surfaces).

Exploiting Cooperative Catalysis for the On-Surface Synthesis of Linear Heteroaromatic Polymers via Selective C-H Activation.

Regiospecific C-H activation is a promising approach to achieve extended polymers with tailored structures. While a recent on-surface synthetic approach has enabled regioselective homocoupling of heteroaromatic molecules, only small oligomers have been achieved. Herein, selective C-H activation for dehydrogenative C-C couplings of hexaazatriphenylene by Scholl reaction is reported for the first time. By combining low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM), we revealed the formation of one-dimensional polymers with a double-chain structure. The details of the growth process are rationalized by density functional theory (DFT) calculations, pointing out a cooperative catalytic action of Na and Ag adatoms in steering the C-H selectivity for the polymerization.

Synergy between Photoluminescence and Charge Transport Achieved by Finely Tuning Polymeric Backbones for Efficient Light-Emitting Transistor.

The lack of design principle for developing high-performance polymer materials displaying strong fluorescence and high ambipolar charge mobilities limited their performance in organic light-emitting transistors (OLETs), electrically pumped organic laser, and other advanced electronic devices. A series of semiladder polymers by copolymerization of weak acceptors (TPTQ or TPTI) and weak donors (fluorene (F) or carbazole (C)) have been developed for luminescent and charge transporting properties. It was found that enhanced planarity, high crystallinity, and a delicate balance in interchain aggregation obtained in the new copolymer, TPTQ-F, contributed to high ambipolar charge mobilities and photoluminescent quantum yield. TPTQ-F showed excellent performance in solution-processed multilayered OLET devices with an external quantum efficiency (EQE) of 5.3%.

On-Surface Synthesis of Nitrogen-Doped Kagome Graphene.

Nitrogen-doped Kagome graphene (N-KG) has been theoretically predicted as a candidate for the emergence of a topological band gap as well as unconventional superconductivity. However, its physical realization still remains very elusive. Here, we report on a substrate-assisted reaction on Ag(111) for the synthesis of two-dimensional graphene sheets possessing a long-range honeycomb Kagome lattice. Low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM) with a CO-terminated tip supported by density functional theory (DFT) are employed to scrutinize the structural and electronic properties of the N-KG down to the atomic scale. We demonstrate its semiconducting character due to the nitrogen doping as well as the emergence of Kagome flat bands near the Fermi level which would open new routes towards the design of graphene-based topological materials.

Finely Designed P3HT-Based Fully Conjugated Graft Polymer: Optical Measurements, Morphology, and the Faraday Effect.

In this work, our group synthesized and characterized a fully conjugated graft polymer comprising of a donor-acceptor molecular backbone and regioregular poly(3-hexylthiophene) (RRP3HT) side chains. Here, our macromonomer (MM) was synthesized via Kumada catalyst transfer polycondensation reaction based on ditin-benzodithiophene (BDT) initiator. The tin content of MM was then investigated by inductively coupled plasma-mass spectrometry (ICP-MS), which allowed for accurate control of donor/acceptor monomer ratio of 1:1 for the following Stille coupling polymerization toward our graft polymer (BP). The structures of the polymers were then characterized by gel permeation chromatography (GPC), NMR, and elemental analysis. This was followed by the characterization of optical, electrochemical, and physical properties. The magneto-optical activity of graft polymer BP was then measured. It was found that, despite the presence of the acceptor backbone, the characteristic large Faraday rotation of RRP3HT was maintained in polymer BP, which exhibited a Verdet constant of 2.39 ± 0.57 (104) °/T·m.

Bottom-up Synthesis of Nitrogen-Doped Porous Graphene Nanoribbons.

Although methods for a periodic perforation and heteroatom doping of graphene sheets have been developed, patterning closely spaced holes on the nanoscale in graphene nanoribbons is still a challenging task. In this work, nitrogen-doped porous graphene nanoribbons (N-GNRs) were synthesized on Ag(111) using a silver-assisted Ullmann polymerization of brominated tetrabenzophenazine. Insights into the hierarchical reaction pathways from single molecules toward the formation of one-dimensional organometallic complexes and N-GNRs are gained by a combination of scanning tunneling microscopy (STM), atomic force microscopy (AFM) with CO-tip, scanning tunneling spectroscopy (STS), and density functional theory (DFT).

Design of High-Performance Organic Light-Emitting Transistors.

Organic light-emitting transistors (OLETs) integrate the light-emitting and gate-modulated electrical switching functions in a single device. Over the past decades, progress has been made in developing new fluorescent semiconductors and device engineering that pushed efficiencies of OLET devices to 8%. However, this efficiency of transistors is still too low to be competitive with organic light-emitting diodes (OLEDs). Currently, there are relatively few suitable organic fluorescent semiconductors suitable for OLETs, and the mechanism of electroluminescence in the devices is still not fully understood. In this mini-review, we discuss the state of highly efficient OLETs and plausible approaches to those unsettled problems. Since this is a mini-review, we will not be able to cover all the excellent work in the literature. Readers are encouraged to read other excellent reviews published earlier.

Foldable semi-ladder polymers: novel aggregation behavior and high-performance solution-processed organic light-emitting transistors.

A critical issue in developing high-performance organic light-emitting transistors (OLETs) is to balance the trade-off between charge transport and light emission in a semiconducting material. Although traditional materials for organic light-emitting diodes (OLEDs) or organic field-effect transistors (OFETs) have shown modest performance in OLET devices, design strategies towards high-performance OLET materials and the crucial structure-performance relationship remain unclear. Our research effort in developing cross-conjugated weak acceptor-weak donor copolymers for luminescent properties lead us to an unintentional discovery that these copolymers form coiled foldamers with intramolecular H-aggregation, leading to their exceptional OLET properties. An impressive external quantum efficiency (EQE) of 6.9% in solution-processed multi-layer OLET devices was achieved.

Sequential Bending and Twisting around C-C Single Bonds by Mechanical Lifting of a Pre-Adsorbed Polymer.

Bending and twisting around carbon-carbon single bonds are ubiquitous in natural and synthetic polymers. Force-induced changes were so far not measured at the single-monomer level, owing to limited ways to apply local forces. We quantified down to the submolecular level the mechanical response within individual poly-pyrenylene chains upon their detachment from a gold surface with an atomic force microscope at 5 K. Computer simulations based on a dedicated force field reproduce the experimental traces and reveal symmetry-broken bent and rotated conformations of the sliding physisorbed segment besides steric hindrance of the just lifted monomer. Our study also shows that the tip-molecule bond remains intact but remarkably soft and links force variations to complex but well-defined conformational changes.

Robust graphene-based molecular devices.

One of the main challenges to upscale the fabrication of molecular devices is to achieve a mechanically stable device with reproducible and controllable electronic features that operates at room temperature1,2. This is crucial because structural and electronic fluctuations can lead to significant changes in the transport characteristics at the electrode-molecule interface3,4. In this study, we report on the realization of a mechanically and electronically robust graphene-based molecular junction. Robustness was achieved by separating the requirements for mechanical and electronic stability at the molecular level. Mechanical stability was obtained by anchoring molecules directly to the substrate, rather than to graphene electrodes, using a silanization reaction. Electronic stability was achieved by adjusting the π-π orbitals overlap of the conjugated head groups between neighbouring molecules. The molecular devices exhibited stable current-voltage (I-V) characteristics up to bias voltages of 2.0 V with reproducible transport features in the temperature range from 20 to 300 K.

Dirac-cone induced gating enhancement in single-molecule field-effect transistors.

Using graphene as electrodes provides an opportunity for fabricating stable single-molecule field-effect transistors (FETs) operating at room temperature. However, the role of the unique graphene band structure in charge transport of single-molecule devices is still not clear. Here we report the Dirac-cone induced electrostatic gating effects in single-molecule FETs with graphene electrodes and a solid-state local bottom gate. With the highest occupied molecular orbital (HOMO) as the dominating conduction channel and the graphene leads remaining intrinsic at zero gate voltage, electrostatic gating on the HOMO and the density of states of graphene at the negative gate polarity reinforces each other, resulting in an enhanced conductance modulation. In contrast, gating effects on the HOMO and the graphene states at the positive gate polarity are opposite. Depending on the gating efficiencies, the conductance can decrease, increase or remain almost unchanged when a more positive gate voltage is applied. Our observations can be well understood by a modified single-level model taking into account the linear dispersion of graphene near the Dirac point. Single-molecule FETs with Dirac-cone enhanced gating have shown high performances, with the modulation of a wide range of current over one order of magnitude. Our studies highlight the advantages of using graphene as an electrode material for molecular devices and pave the way for single-molecule FETs toward circuitry applications.

Overcoming Steric Hindrance in Aryl-Aryl Homocoupling via On-Surface Copolymerization.

On-surface synthesis is a unique tool for growing low-dimensional carbon nanomaterials with precise structural control down to the atomic level. This novel approach relies on carefully designed precursor molecules, which are deposited on suitable substrates and activated to ultimately form the desired nanostructures. One of the most applied reactions to covalently interlink molecular precursors is dehalogenative aryl-aryl coupling. Despite the versatility of this approach, many unsuccessful attempts are also known, most of them associated to the poor capability of the activated precursors to couple to each other. Such failure is often related to the steric hindrance between reactants, which may arise due to their coplanarity upon adsorption on a surface. Here, we propose a copolymerization approach to overcome the limitations that prevent intermolecular homocoupling. We apply the strategy of using suitable linkers as additional reactants to the formation of fully conjugated polycyclic nanowires incorporating non-benzenoid rings.