Air-stable, long-length, solution-based graphene nanoribbons.

Within the context of nanoelectronics, general strategies for the development of electronically tunable and air stable graphene nanoribbons are crucial. Previous studies towards the goal of processable nanoribbons have been complicated by ambient condition instability, insolubility arising from aggregation, or poor cyclization yield due to electron deficiency. Herein, we present a general strategy for the elongation of smaller graphene nanoribbon fragments into air-stable, easily processed, and electronically tunable nanoribbons. This strategy is facilitated by the incorporation of electron-rich donor units between electron-poor acceptor perylene diimide oligomeric units. The ribbons are processed in solution via a visible-light flow photocyclization using LEDs. The resulting long nanoribbons can be solution-cast and imaged, which are necessary characteristics for device fabrication. The ribbons become conductive after thermolysis of the pendent side-chains. The electron-accepting character of these nanoribbons in solution is reversible, and the conductivity of the thermolyzed species as a solid remains stable. This work highlights our general strategy for the mild and reliable fabrication of tunable and ambient-stable graphene nanoribbons, and charts a straightforward route for facile device incorporation.

Electrophotocatalysis with a Trisaminocyclopropenium Radical Dication.

Visible-light photocatalysis and electrocatalysis are two powerful strategies for the promotion of chemical reactions. Here, these two modalities are combined in an electrophotocatalytic oxidation platform. This chemistry employs a trisaminocyclopropenium (TAC) ion catalyst, which is electrochemically oxidized to form a cyclopropenium radical dication intermediate. The radical dication undergoes photoexcitation with visible light to produce an excited-state species with oxidizing power (3.33 V vs. SCE) sufficient to oxidize benzene and halogenated benzenes via single-electron transfer (SET), resulting in C-H/N-H coupling with azoles. A rationale for the strongly oxidizing behavior of the photoexcited species is provided, while the stability of the catalyst is rationalized by a particular conformation of the cis-2,6-dimethylpiperidine moieties.

Dimensional Control in Contorted Aromatic Materials.

This Account details key developments in dimensional control of contorted aromatics for organic electronics. Coronene, perylene, pyrene, and [4]helicene, which are fragments of graphene, can be contorted using facile synthetic chemistry into large nanoribbons and nano-architectures. In comparing contorted or higher-dimensional graphene architectures to planar or lower-dimensional species, the materials properties are reliably enhanced for the contorted aromatics. Examples of enhanced properties include optical absorptivity, conductivity, device photoconversion efficiency, and solubility. These enhancements are exemplified in organic photovoltaics, photodetectors, field effect transistors, and perovskite solar cells. Described herein are key advances in dimensional control of contorted aromatics that have resulted in world record photoconversion efficiencies, photodetection capabilities matching inorganic state-of-the-art devices, and ∼5 nm long ultrathin soluble graphene nanoribbons.

Designing Three-Dimensional Architectures for High-Performance Electron Accepting Pseudocapacitors.

By storing energy from electrochemical processes at the electrode surface, pseudocapacitors bridge the performance gap between electrostatic double-layer capacitors and batteries. In this context, molecular design offers the exciting possibility to create tunable and inexpensive organic electroactive materials. Here we describe a porous structure composed of perylene diimide and triptycene subunits and demonstrate its remarkable performance as a pseudocapacitor electrode material. The material exhibits capacitance values as high as 350 F/g at 0.2 A/g as well as excellent stability over 10 000 cycles. Moreover, we can alter the performance of the material, from battery-like (storing more charge at low rates) to capacitor-like (faster charge cycling), by modifying the structure of the pores via flow photocyclization. Organic materials capable of stable electron accepting pseudocapacitor behavior are rare and the capacitance values presented here are among the highest reported. More broadly, this work establishes molecular design and synthesis as a powerful approach for creating tunable energy storage materials.

Three-Dimensional Graphene Nanostructures.

This Communication details the implementation of a new concept for the design of high-performance optoelectronic materials: three-dimensional (3D) graphene nanostructures. This general strategy is showcased through the synthesis of a three-bladed propeller nanostructure resulting from the coupling and fusion of a central triptycene hub and helical graphene nanoribbons. Importantly, these 3D graphene nanostructures show remarkable new properties that are distinct from the substituent parts. For example, the larger nanostructures show an enhancement in absorption and decreased contact resistance in optoelectronic devices. To show these enhanced properties in a device setting, the nanostructures were utilized as the electron-extracting layers in perovskite solar cells. The largest of these nanostructures achieved a PCE of 18.0%, which is one of the highest values reported for non-fullerene electron-extracting layers.

Molecular Materials for Nonaqueous Flow Batteries with a High Coulombic Efficiency and Stable Cycling.

This manuscript presents a working redox battery in organic media that possesses remarkable cycling stability. The redox molecules have a solubility over 1 mol electrons/liter, and a cell with 0.4 M electron concentration is demonstrated with steady performance >450 cycles (>74 days). Such a concentration is among the highest values reported in redox flow batteries with organic electrolytes. The average Coulombic efficiency of this cell during cycling is 99.868%. The stability of the cell approaches the level necessary for a long lifetime nonaqueous redox flow battery. For the membrane, we employ a low cost size exclusion cellulose membrane. With this membrane, we couple the preparation of nanoscale macromolecular electrolytes to successfully avoid active material crossover. We show that this cellulose-based membrane can support high voltages in excess of 3 V and extreme temperatures (-20 to 110 °C). These extremes in temperature and voltage are not possible with aqueous systems. Most importantly, the nanoscale macromolecular platforms we present here for our electrolytes can be readily tuned through derivatization to realize the promise of organic redox flow batteries.

Cove-Edge Nanoribbon Materials for Efficient Inverted Halide Perovskite Solar Cells.

Two cove-edge graphene nanoribbons hPDI2-Pyr-hPDI2 (1) and hPDI3-Pyr-hPDI3 (2) are used as efficient electron-transporting materials (ETMs) in inverted planar perovskite solar cells (PSCs). Devices based on the new graphene nanoribbons exhibit maximum power-conversion efficiencies (PCEs) of 15.6 % and 16.5 % for 1 and 2, respectively, while a maximum PCE of 14.9 % is achieved with devices based on [6,6]-phenyl-C61 -butyric acid methyl ester (PC61 BM). The interfacial effects induced by these new materials are studied using photoluminescence (PL), and we find that 1 and 2 act as efficient electron-extraction materials. Additionally, compared with PC61 BM, these new materials are more hydrophobic and have slightly higher LUMO energy levels, thus providing better device performance and higher device stability.

Long, Atomically Precise Donor-Acceptor Cove-Edge Nanoribbons as Electron Acceptors.

This Communication describes a new molecular design for the efficient synthesis of donor-acceptor, cove-edge graphene nanoribbons and their properties in solar cells. These nanoribbons are long (∼5 nm), atomically precise, and soluble. The design is based on the fusion of electron deficient perylene diimide oligomers with an electron rich alkoxy pyrene subunit. This strategy of alternating electron rich and electron poor units facilitates a visible light fusion reaction in >95% yield, whereas the cove-edge nature of these nanoribbons results in a high degree of twisting along the long axis. The rigidity of the backbone yields a sharp longest wavelength absorption edge. These nanoribbons are exceptional electron acceptors, and organic photovoltaics fabricated with the ribbons show efficiencies of ∼8% without optimization.

Helical Nanoribbons for Ultra-Narrowband Photodetectors.

This Communication describes a new molecular design that yields ultranarrowband organic photodetectors. The design is based on a series of helically twisted molecular ribbons as the optoelectronic material. We fabricate charge collection narrowing photodetectors based on four different helical ribbons that differ in the wavelength of their response. The photodetectors made from these materials have narrow spectral response with full-width at half maxima of <20 nm. The devices reported here are superior by approximately a factor of 5 to those from traditional organic materials due to the narrowness of their response. Moreover, the active layers for the helical ribbon-based photodetectors are solution-cast but have performance that is comparable to the state-of-the-art narrowband photodetectors made from methylammonium lead trihalide perovskite single crystals. The ultranarrow bandwidth for detection results from the helical ribbons' high absorption coefficient, good electron mobility, and sharp absorption edges that are defined by the twisted molecular conformation.

Towards pi-extended cycloparaphenylenes as seeds for CNT growth: investigating strain relieving ring-openings and rearrangements.

Despite significant multidisciplinary effort over many years, the preparation of uniform carbon nanotubes (CNTs) is still an unsolved problem in the scientific community. This inaccessibility hampers the commercial use of CNTs in electronic devices due to the sensitive connection between their electronic properties and molecular structure. The [n]cycloparaphenylenes ([n]CPPs), the smallest horizontal segment of an armchair CNT, hold great promise as "seeds", or templates, for the preparation of homogenous batches of CNTs. Initial reports towards this goal, however, suggest that it would be advantageous to pi-extend these structures through traditional organic synthesis before their use in CNT growth. Towards this, several strategies have been reported attempting to utilize the Scholl reaction on aryl-substituted cycloparaphenylenes to yield a small CNT for use as a template for larger tubes. Prominently used in polyaromatic hydrocarbon chemistry, the Scholl reaction has afforded numerous extraordinary targets, such as graphene nanoribbons and graphene propellors. In this work, both experimental and computational studies are provided to unravel the complex cationic rearrangements and ring-openings associated with the Scholl reaction in the context of the cycloparaphenylenes-systems that are thermodynamically and kinetically different from flat graphene fragments. Additionally, this work demonstrates the unique reactivity of cycloparaphenylenes in the context of cationic or radical cationic intermediates, which are common reaction pathways for numerous transformations.

The effects of cyclic conjugation and bending on the optoelectronic properties of paraphenylenes.

Cycloparaphenylenes (CPPs) have optoelectronic properties that are unique when compared to their acyclic oligoparaphenylene counterparts. The synthesis and characterization of two bent heptaphenyl-containing macrocycles has been achieved in order to probe the effects of bending and cyclic conjugation on the properties of the CPPs. The study suggests that both bending and cyclic conjugation play a role in the novel properties of the CPPs.

Selective syntheses of [7]-[12]cycloparaphenylenes using orthogonal Suzuki-Miyaura cross-coupling reactions.

The divergent, selective syntheses of [7]-[12]cycloparaphenylenes have been accomplished utilizing sequential, orthogonal Suzuki-Miyaura cross-coupling reactions from two late-stage intermediates. Quantum yields decrease dramatically as cycloparaphenylene size decreases, highlighting the unique photophysical behavior of the smaller cycloparaphenylenes.

Synthesis of tetraphenyl-substituted [12]cycloparaphenylene: toward a rationally designed ultrashort carbon nanotube.

The first phenyl-substituted [n]cycloparaphenylene (1) has been synthesized. The preparation of this structure addresses several challenges toward a more elaborate phenyl-substituted [n]cycloparaphenylene (2), a molecule that may lead to the homogeneous synthesis of armchair carbon nanotubes.

Selective synthesis of strained [7]cycloparaphenylene: an orange-emitting fluorophore.

[n]Cycloparaphenylenes, which are short fragments of carbon nanotubes, have unique size-dependent optical properties. In this communication, we describe the first synthesis of [7]cycloparaphenylene ([7]CPP), the smallest cycloparaphenylene prepared to date. In order to access this structure, we have developed a synthetic route that capitalizes on successive orthogonal Suzuki-Miyaura coupling reactions. [7]CPP has 83 kcal/mol of strain energy and an orange emission at 592 nm.