Steric Effects on Single-Molecule Conductance in Flat-Lying Phenanthrene.

A previous combined experimental and theoretical study found that the position of anchoring groups on a phenanthrene (PHE) backbone played a large role in determining the single-molecule conductance of the PHE derivative. However, a consistent 0.1 G0 feature was found across all PHE derivatives. To understand this, the previously investigated PHE derivatives were placed flat on a simulated Au substrate with a scanning tunneling microscope (STM) tip over PHE and conductance was calculated using the non-equilibrium Green's function technique in conjunction with density functional theory (NEGF-DFT). The location of the tip was varied to find the most conductive and most energetically favorable arrangements, which did not coincide. Furthermore, the variation in conductance found in erect junctions was not present when PHE derivatives were lying flat, with all derivatives calculated to have conductance values around 0.1 G0.

A Three-Dimensional Non-Fullerene Acceptor with Contorted Hexabenzocoronene and Perylenediimide for Organic Solar Cells.

Although fullerene derivatives such as [6,6]-phenyl-C61/C71-butyric acid methyl ester (PC61BM/PC71BM) have dominated the the photoactive acceptor materials in bulk heterojunction organic solar cells (OSCs) for decades, they have several drawbacks such as weak absorption, limited structural tunability, prone to aggregation, and high costs of production. Constructing non-fullerene small molecules with three-dimensional (3D) molecular geometry is one of the strategies to replace fullerenes in OSCs. In this study, a 3D molecule, contorted hexa-cata-hexabenzocoronene tetra perylenediimide (HBC-4-PDI), was designed and synthesized. HBC-4-PDI shows a wide and strong light absorption in the whole UV-vis region as well as suitable energy levels as an acceptor for OSCs. More importantly, the 3D construction effectively reduced the self-aggregation of c-HBC, leading to an appropriate scale phase separation of the blend film morphology in OSCs. A preliminary power conversion efficiency of 2.70 % with a champion open-circuit voltage of 1.06 V was obtained in OSCs with HBC-4-PDI as the acceptor, which was the highest among the previously reported OSCs based on c-HBC derivatives. The results indicated that HBC-4-PDI may serve as a good non-fullerene acceptor for OSCs.

Microwave assisted synthesis of PQ-GDY@NH2-UIO-66(Zr) for improved photocatalytic removal of NOx under visible light.

Pyrazinoquinoxaline-based graphdiyne (PQ-GDY) contains a fixed number of sp-sp2 hybridized carbon atoms and pyrazine-like sp2 hybridized N atoms. In this paper, NH2-UIO-66(Zr) on PQ-GDY substrate was successfully constructed with the help of microwave-assisted heating. PQ-GDY surface acts as a microwave antenna under microwave irradiation to rapidly absorb microwave energy and form hot spots (hot spot effect), which facilitates the formation of well-dispersed NH2-UIO-66(Zr) with good crystallinity. Transient absorption spectra show that high hole transport property of PQ-GDY can accelerate the migration of photogenerated holes from NH2-UIO-66(Zr) to PQ-GDY and greatly reduce the recombination rate of photogenerated electrons and holes due to the strong interaction between PQ-GDY and NH2-UIO-66(Zr). Under visible light (λ ≥ 420 nm), PQ-GDY@NH2-UIO-66(Zr) shows high photocatalytic stability and high NOx removal rate up to 74%, which is 44% higher than that of primitive NH2-UIO-66(Zr). At the same time, it inhibits the formation of toxic by-products (NO2) and limits its concentration to a low level.

Three Isomeric Tetraphenylethylene-pyridine Compounds: Synthesis, Crystal Structures, and Photophysical Properties.

Many aggregation-induced emission (AIE) molecules based on tetraphenylethylene (TPE) structure have been synthesized, but a clear understanding of the photophysical difference between different isomeric pyridyl-based tetraphenylethylene molecules remains elusive. Herein, we designed a series of isomeric tetraphenylethylene-pyridines (o-Py-TPE, m-Py-TPE, p-Py-TPE) to investigate the influence of the position of N atoms in the pyridine subunit on the photophysical property of the whole molecule by detailed DFT calculations and single-crystal structures analysis. All compounds show typical AIE properties, and notably, the meta pyridyl isomer (m-Py-TPE) shows the highest solid photoluminescence quantum yield (PLQY) up to 64.56 %. Further investigation and DFT calculations indicate that the center C=C bond dihedral angles of the TPE subunit in the solid state of these compounds, which are affected by C-H⋅⋅⋅π interaction, play a vital role in their emission and PLQY properties. This work provides underlying principles for the design of pyridyl-based TPE molecules with high photoluminescent performance in the future.

Distinct armchair and zigzag charge transport through single polycyclic aromatics.

In aromatic systems with large π-conjugated structures, armchair and zigzag configurations can affect each material's electronic properties, determining their performance and generating certain quantum effects. Here, we explore the intrinsic effect of armchair and zigzag pathways on charge transport through single hexabenzocoronene molecules. Theoretical calculations and systematic experimental results from static carbon-based single-molecule junctions and dynamic scanning tunneling microscope break junctions show that charge carriers are preferentially transported along the hexabenzocoronene armchair pathway, and thus, the corresponding current through this pathway is approximately one order of magnitude higher than that through the zigzag pathway. In addition, the molecule with the zigzag pathway has a smaller energy gap. In combination with its lower off-state conductance, it shows a better field-effect performance because of its higher on-off ratio in electrical measurements. This study on charge transport pathways offers a useful perspective for understanding the electronic properties of π-conjugated systems and realizing high-performance molecular nanocircuits toward practical applications.

Divergent Synthesis of Contorted Polycyclic Aromatics Containing Pentagons, Heptagon, and/or Azulene.

Divergent synthesis of four contorted aromatics containing pentagons, a heptagon, and/or an azulene from the same difluorenyl pentacenediene precursor were realized in one step. The subtle differences in molecular structure were confirmed by X-ray crystallography. The mechanisms for the control of different products, which involve a ring-expansion rearrangement, Scholl reactions, and/or Mallory cyclization were proposed on the basis of control experiments and DFT calculations. Such development adds good structure versatility and materials accessibility to the study of contorted aromatics.

Regio- and Steric Effects on Single Molecule Conductance of Phenanthrenes.

Here, six phenanthrene (the smallest arm-chair graphene nanoribbon) derivatives with dithiomethyl substitutions at different positions as the anchoring groups were synthesized. Scanning tunneling microscopy break junction technique was used to measure their single molecule conductances between gold electrodes, which showed a difference as much as 20-fold in the range of ∼10-2.82 G0 to ∼10-4.09 G0 following the trend of G2,7 > G3,6 > G2,6 > G1,7 > G1,6 > G1,8. DFT calculations agree well with this measured trend and indicate that the single molecule conductances are a combination of energy alignment, electronic coupling, and quantum effects. This significant regio- and steric effect on the single molecule conductance of phenanthrene model molecules shows the complexity in the practice of graphene nanoribbons as building blocks for future carbon-based electronics in one hand but also provides good conductance tunability on the other hand.

Tunable Symmetry-Breaking-Induced Dual Functions in Stable and Photoswitched Single-Molecule Junctions.

The aim of molecular electronics is to miniaturize active electronic devices and ultimately construct single-molecule nanocircuits using molecules with diverse structures featuring various functions, which is extremely challenging. Here, we realize a gate-controlled rectifying function (the on/off ratio reaches ∼60) and a high-performance field effect (maximum on/off ratio >100) simultaneously in an initially symmetric single-molecule photoswitch comprising a dinuclear ruthenium-diarylethene (Ru-DAE) complex sandwiched covalently between graphene electrodes. Both experimental and theoretical results consistently demonstrate that the initially degenerated frontier molecular orbitals localized at each Ru fragment in the open-ring Ru-DAE molecule can be tuned separately and shift asymmetrically under gate electric fields. This symmetric orbital shifting (AOS) lifts the degeneracy and breaks the molecular symmetry, which is not only essential to achieve a diode-like behavior with tunable rectification ratio and controlled polarity, but also enhances the field-effect on/off ratio at the rectification direction. In addition, this gate-controlled symmetry-breaking effect can be switched on/off by isomerizing the DAE unit between its open-ring and closed-ring forms with light stimulus. This new scheme offers a general and efficient strategy to build high-performance multifunctional molecular nanocircuits.

Detecting Individual Bond Switching within Amides in a Tunneling Junction.

Amides are essential in the chemistry of life. Detecting the chemical bond states within amides could unravel the nature of amide stabilization and planarity, which is critical to the structure and reactivity of such molecules. Yet, so far, no work has been reported to detect or measure the bond changes at the single-molecule level within amides. Here, we show that a transition between single and double bonds between N and C atoms in an amide can be monitored in real time in a nanogap between gold electrodes via the generation of distinctive conductance features. Density functional theory simulations show that the switching between amide isomers proceeds via a proton transfer process facilitated by a water molecule bridge, and the resulting molecular junctions display bimodal conductance states with a difference as much as nine times.

ZnS-SnS@NC Heterostructure as Robust Lithiophilicity and Sulfiphilicity Mediator toward High-Rate and Long-Life Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries are severely hindered by the low sulfur utilization and short cycling life, especially at high rates. One of the effective solutions to address these problems is to improve the sulfiphilicity of lithium polysulfides (LiPSs) and the lithiophilicity of the lithium anode. However, it is a great challenge to simultaneously optimize both aspects. Herein, by incorporating the merits of strong absorbability and high conductivity of SnS with good catalytic capability of ZnS, a ZnS-SnS heterojunction coated with a polydopamine-derived N-doped carbon shell (denoted as ZnS-SnS@NC) with uniform cubic morphology was obtained and compared with the ZnS-SnS2@NC heterostructure and its single-component counterparts (SnS@NC and SnS2@NC). Theoretical calculations, ex situ XANES, and in situ Raman spectrum were utilized to elucidate rapid anchoring-diffusion-transformation of LiPSs, inhibition of the shuttling effect, and improvement of the sulfur electrochemistry of bimetal ZnS-SnS heterostructure at the molecular level. When applied as a modification layer coated on the separator, the ZnS-SnS@NC-based cell with optimized lithiophilicity and sulfiphilicity enables desirable sulfur electrochemistry, including high reversibility of 1149 mAh g-1 for 300 cycles at 0.2 C, high rate performance of 661 mAh g-1 at 10 C, and long cycle life with a low fading rate of 0.0126% each cycle after 2000 cycles at 4 C. Furthermore, a favorable areal capacity of 8.27 mAh cm-2 is maintained under high sulfur mass loading of 10.3 mg cm-2. This work furnishes a feasible scheme to the rational design of bimetal sulfides heterostructures and boosts the development of other electrochemical applications.

High Quality Pyrazinoquinoxaline-Based Graphdiyne for Efficient Gradient Storage of Lithium Ions.

N-doping of graphdiyne with atomic precision is very important for the study of heteroatom doping effect and the structure-properties relationships of graphdiyne. Here we report the bottom-up synthesis and characterizations of high-quality pyrazinoquinoxaline-based graphdiyne (PQ-GDY) film. First-principle studies of the layered structure were performed to examine the stacking mode, lithium binding affinity, and bulk lithium storage capacity. Three-stage insertion of 14 lithium atoms with binding affinities in the order of pyrazine nitrogen > diyne carbon > central aromatic ring were confirmed by both lithium-ion half-cell measurements and DFT calculations. More than half of the lithium atoms preferentially bind to pyrazine nitrogen, and a reversible capacity of 570.0 mA h g-1 at a current density of 200 mA g-1 after 800 cycles was achieved. Such a high capacity utilization rate of 97.2% provides a good case study of N-doped GDY with atomic precision.

Stringing the Perylene Diimide Bow.

This study explores a new mode of contortion in perylene diimides where the molecule is bent, like a bow, along its long axis. These bowed PDIs were synthesized through a facile fourfold Suzuki macrocyclization with aromatic linkers and a tetraborylated perylene diimide that introduces strain and results in a bowed structure. By altering the strings of the bow, the degree of bending can be controlled from flat to highly bent. Through spectroscopy and quantum chemical calculations, it is demonstrated that the energy of the lowest unoccupied orbital can be controlled by the degree of bending in the structures and that the energy of the highest occupied orbital can be controlled to a large extent by the constitution of the aromatic linkers. The important finding is that the bowing results not only in red-shifted absorptions but also more facile reductions.

Permethylation Introduces Destructive Quantum Interference in Saturated Silanes.

The single-molecule conductance of silanes is suppressed due to destructive quantum interference in conformations with cisoid dihedral angles along the molecular backbone. Yet, despite the structural similarity, σ-interference effects have not been observed in alkanes. Here we report that the methyl substituents used in silanes are a prerequisite for σ-interference in these systems. Through density functional theory calculations, we find that the destructive interference is not evident to the same extent in nonmethylated silanes. We find the same is true in alkanes as the transmission is significantly suppressed in permethylated cyclic and bicyclic alkanes. Using scanning tunneling microscope break-junction method we determine the single-molecule conductance of functionalized cyclohexane and bicyclo[2.2.2]octane that are found to be higher than that of equivalent permethylated silanes. Rather than the difference between carbon and silicon atoms in the molecular backbones, our calculations reveal that it is primarily the difference between hydrogen and methyl substituents that result in the different electron transport properties of nonmethylated alkanes and permethylated silanes. Chemical substituents play an important role in determining the single-molecule conductance of saturated molecules, and this must be considered when we improve and expand the chemical design of insulating organic molecules.

Controlling Singlet Fission by Molecular Contortion.

Singlet fission, the generation of two triplet excited states from the absorption of a single photon, may potentially increase solar energy conversion efficiency. A major roadblock in realizing this potential is the limited number of molecules available with high singlet fission yields and sufficient chemical stability. Here, we demonstrate a strategy for developing singlet fission materials in which we start with a stable molecular platform and use strain to tune the singlet and triplet energies. Using perylene diimide as a model system, we tune the singlet fission energetics from endoergic to exoergic or iso-energetic by straining the molecular backbone. The result is an increase in the singlet fission rate by 2 orders of magnitude. This demonstration opens a door to greatly expanding the molecular toolbox for singlet fission.

Conjugated Macrocycles in Organic Electronics.

This Account describes a body of research on the design, synthesis, and application of a new class of electronic materials made from conjugated macrocycles. Our macrocyclic design takes into consideration the useful attributes of fullerenes and what properties make fullerenes efficient n-type materials. We identified four electronic and structural elements: (1) a three-dimensional shape; (2) a conjugated and delocalized π-space; (3) the presence of an interior and exterior to the π-surface; and (4) low-energy unoccupied molecular orbitals allowing them to accept electrons. The macrocyclic design incorporates some of these properties, including a three-dimensional shape, an interior/exterior to the π-surface, and low-lying LUMOs maintaining the n-type semiconducting behavior, yet we also install synthetic flexibility in our approach in order to tune the properties further. Each of the macrocycles comprises perylenediimide cores wound together with linkers. The perylenediimide building block endows each macrocycle with the ability to accept electrons, while the synthetic flexibility to install different linkers allows us to create macrocycles with different electronic properties and sizes. We have created three macrocycles that all absorb well into the visible range of the solar spectrum and possess different shapes and sizes. We then use these materials in an array of applications that take advantage of their ability to function as n-type semiconductors, absorb in the visible range of the solar spectrum, and possess intramolecular cavities. This Account will discuss our progress in incorporating these new macrocycles in organic solar cells, organic photodetectors, organic field effect transistors, and sensors. The macrocycles outperform acyclic controls in organic solar cells. We find the more rigid macrocyclic structure results in less intrinsic charges and lower dark current in organic photodetectors. Our macrocyclic-based photodetector has the highest detectivity of non-fullerene acceptors. The macrocycles also function as sensors and are able to recognize nuanced differences in analytes. Perylenediimide-based fused oligomers are efficient materials in both organic solar cells and field effect transistors. We will use the oligomers to construct macrocycles for use in solar energy conversion. In addition, we will incorporate different electron-rich linkers in our cycles in an attempt to engineer the HOMO/LUMO gap further. Looking further into the future, we envision opportunities in applying these conjugated macrocycles as electronic host/guest materials, as concatenated electronic materials by threading the macrocycles with electroactive oligomers, and as a locus for catalysis that is driven by light and electric fields.

Synthesis, Regioselective Bromination, and Functionalization of Coronene Tetracarboxydiimide.

A new method for the effective synthesis of coronene tetracarboxydiimide (CDI) was developed by utilizing inexpensive and nontoxic potassium vinyltrifluoroborate. Controllable brominations of CDI were accomplished to yield CDI mono-, di-, tri-, and tetra-bromides, which could be used as synthon and functionalized by aromatic nucleophilic substitution and the Sonogashira coupling reaction.

Large Variations in the Single-Molecule Conductance of Cyclic and Bicyclic Silanes.

Linear silanes are efficient molecular wires due to strong σ-conjugation in the transoid conformation; however, the structure-function relationship for the conformational dependence of the single-molecule conductance of silanes remains untested. Here we report the syntheses, electrical measurements, and theoretical characterization of four series of functionalized cyclic and bicyclic silanes including a cyclotetrasilane, a cyclopentasilane, a bicyclo[2.2.1]heptasilane, and a bicyclo[2.2.2]octasilane, which are all extended by linear silicon linkers of varying length. We find an unusual variation of the single-molecule conductance among the four series at each linker length. We determine the relative conductance of the (bi)cyclic silicon structures by using the common length dependence of the four series rather than comparing the conductance at a single length. In contrast with the cyclic π-conjugated molecules, the conductance of σ-conjugated (bi)cyclic silanes is dominated by a single path through the molecule and is controlled by the dihedral angles along this path. This strong sensitivity to molecular conformation dictates the single-molecule conductance of σ-conjugated silanes and allows for systematic control of the conductance through molecular design.

Comprehensive suppression of single-molecule conductance using destructive σ-interference.

The tunnelling of electrons through molecules (and through any nanoscale insulating and dielectric material 1 ) shows exponential attenuation with increasing length 2 , a length dependence that is reflected in the ability of the electrons to carry an electrical current. It was recently demonstrated3-5 that coherent tunnelling through a molecular junction can also be suppressed by destructive quantum interference 6 , a mechanism that is not length-dependent. For the carbon-based molecules studied previously, cancelling all transmission channels would involve the suppression of contributions to the current from both the π-orbital and σ-orbital systems. Previous reports of destructive interference have demonstrated a decrease in transmission only through the π-channel. Here we report a saturated silicon-based molecule with a functionalized bicyclo[2.2.2]octasilane moiety that exhibits destructive quantum interference in its σ-system. Although molecular silicon typically forms conducting wires 7 , we use a combination of conductance measurements and ab initio calculations to show that destructive σ-interference, achieved here by locking the silicon-silicon bonds into eclipsed conformations within a bicyclic molecular framework, can yield extremely insulating molecules less than a nanometre in length. Our molecules also exhibit an unusually high thermopower (0.97 millivolts per kelvin), which is a further experimental signature of the suppression of all tunnelling paths by destructive interference: calculations indicate that the central bicyclo[2.2.2]octasilane unit is rendered less conductive than the empty space it occupies. The molecular design presented here provides a proof-of-concept for a quantum-interference-based approach to single-molecule insulators.

A graphyne-like porous carbon-rich network synthesized via alkyne metathesis.

The synthesis of graphyne has been considered challenging, especially when it comes to adopting new topologies and obtaining thinner layers. Herein, we report the synthesis and characterization of a graphyne-like porous carbon-rich network via alkyne metathesis reactions, which resulted in a sp2/sp hybridized 2D thin film structure with a layer to layer distance of 0.37 nm. This graphyne-like porous carbon-rich network is an n-type semiconductor with a low work function of 3.9 eV and a reduction potential of -0.54 V vs. SHE, which could be applied as an excellent reducing agent for metal electroless deposition. In addition, this material has a narrow pore size distribution of 2 to 4 nm, a high surface area of 675 m2 g-1 and a large pore volume of 0.795 cm3 g-1 favoring gas adsorption. It shows the selective absorption of CO2 over N2 owing to the strong affinity between CO2 and the carbon-carbon triple bond.