Air-Stable Perylene Diimide Trimer Material for N-Type Organic Electrochemical Transistors.

A new method is reported to make air-stable n-type organic mixed ionic-electronic conductor (OMIEC) films for organic electrochemical transistors (OECTs) using a solution-processable small molecule helical perylene diimide trimer, hPDI[3]-C11. Alkyl side chains are attached to the conjugated core for processability and film making, which are then cleaved via thermal annealing. After the sidechains are removed, the hPDI[3] film becomes less hydrophobic, more ordered, and has a deeper lowest unoccupied molecular orbital (LUMO). These features provide improved ionic transport, greater electronic mobility, and increased stability in air and in aqueous solution. Subsequently, hPDI[3]-H is used as the active material in OECTs and a device with a transconductance of 44 mS, volumetric capacitance of ≈250 F cm-3, µC* value of 1 F cm-1 V-1 s-1, and excellent stability (> 5 weeks) is demonstrated. As proof of their practical applications, a hPDI[3]-H-based OECTs as a glucose sensor and electrochemical inverter is utilized. The approach of side chain removal after film formation charts a path to a wide range of molecular semiconductors to be used as stable, mixed ionic-electronic conductors.

Near-Infrared, Organic Chiroptic Switch with High Dissymmetry Factors.

Here we unveil a chiral molecular redox switch derived from PDI-based twistacenes─chPDI[2] that has the remarkable attributes of high-intensity and a broadband chiral response. This material exhibits facile, stable, and reversible multistate chiroptical switching behavior over a broad active wavelength range close to 700 nm, encompassing ultraviolet, visible, and near-infrared regions. Upon reduction, chPDI[2] exhibits a substantial increase in the amplitude of its circular dichroic response, with an outstanding |ΔΔε| > 300 M-1 cm-1 and a high dissymmetry factor of 3 × 10-2 at 960 nm. DFT calculations suggest that the long wavelength CD signal for doubly reduced chPDI[2] originates from excitation of the PDI backbone to the π* orbital of the bridging alkene. Importantly, the dimer's molecular contortion facilitates ionic diffusion, enabling chiral switching in solid state films. The high dissymmetry factors and near-infrared response establish chPDI[2] as a unique chiroptic switch.

Fusing perylene diimide with helicenes.

Incorporating perylene diimide (PDI) units into helicene structures has become a useful strategy for giving access to non-planar electron acceptors as well as a method of creating molecules with unique and intriguing chiroptical properties. This minireview describes this fusion of PDIs with helicenes.

Coaxially Conductive Organic Wires Through Self-Assembly.

Here, we describe the synthesis of the hexameric macrocyclic aniline (MA[6]), which spontaneously assembles into coaxially conductive organic wires in its oxidized and acidified emeraldine salt (ES) form. Electrical measurements reveal that ES-MA[6] exhibits high electrical conductivity (7.5 × 10-2 S·cm-1) and that this conductivity is acid-base responsive. Single-crystal X-ray crystallography reveals that ES-MA[6] assembles into well-defined trimeric units that then stack into nanotubes with regular channels, providing a potential route to synthetic nanotubes that are leveraged for ion or small molecule transport. Ultraviolet-visible-near-infrared absorbance spectroscopy and electron paramagnetic spectroscopy showcase the interconversion between acidic (conductive) and basic (insulating) forms of these macrocycles and how charge carriers are formed through protonation, giving rise to the experimentally observed high electrical conductivity.

A few-layer covalent network of fullerenes.

The two natural allotropes of carbon, diamond and graphite, are extended networks of sp3-hybridized and sp2-hybridized atoms, respectively1. By mixing different hybridizations and geometries of carbon, one could conceptually construct countless synthetic allotropes. Here we introduce graphullerene, a two-dimensional crystalline polymer of C60 that bridges the gulf between molecular and extended carbon materials. Its constituent fullerene subunits arrange hexagonally in a covalently interconnected molecular sheet. We report charge-neutral, purely carbon-based macroscopic crystals that are large enough to be mechanically exfoliated to produce molecularly thin flakes with clean interfaces-a critical requirement for the creation of heterostructures and optoelectronic devices2. The synthesis entails growing single crystals of layered polymeric (Mg4C60)∞ by chemical vapour transport and subsequently removing the magnesium with dilute acid. We explore the thermal conductivity of this material and find it to be much higher than that of molecular C60, which is a consequence of the in-plane covalent bonding. Furthermore, imaging few-layer graphullerene flakes using transmission electron microscopy and near-field nano-photoluminescence spectroscopy reveals the existence of moiré-like superlattices3. More broadly, the synthesis of extended carbon structures by polymerization of molecular precursors charts a clear path to the systematic design of materials for the construction of two-dimensional heterostructures with tunable optoelectronic properties.

Remote Control of Dynamic Twistacene Chirality.

We report a reliable way to manipulate the dynamic, axial chirality in perylene diimide (PDI)-based twistacenes. Specifically, we reveal how chiral substituents on the imide position induce the helicity in a series of PDI-based twistacenes. We demonstrate that this remote chirality is able to control the helicity of flexible [4]helicene subunits by UV-vis, CD spectroscopy, X-ray crystallography, and TDDFT calculations. Furthermore, we have discovered that both the chiral substituent and the solvent each has a strong impact on the sign and intensity of the CD signals, highlighting the control of the dynamic helicity in this flexible system. DFT calculations suggest that the steric interaction of the chiral substituents is the important factor in how well a particular group is at inducing a preferred helicity.

Single-Handed Helicene Nanoribbons via Transfer of Chiral Information.

Here we show the access to single-handed helicene nanoribbons by utilizing a [6]helicene building block to induce diastereoselective, photochemical formation of [5]helicene units. Specifically, we have synthesized nanoribbons P1 and P2 with different ratios of [6]helicene "sergeants" to [5]helicene "soldiers", which on average consist of between ∼50 and 60 ortho-annulated benzene rings. These are the longest, optically active helicene backbones that have been prepared to date. The chiroptic properties of P1 and P2 reveal the transfer of stereochemical information from "sergeants" to "soldiers". To gain further insight into the stereo-information relay, we apply the same molecular design to discrete, model oligomers 1-5 and confirm that they also preferentially adopt homochiral geometries.

Highly Selective N-Alkylation of Pyrazoles: Crystal Structure Evidence for Attractive Interactions.

Inspired by crystal structures, we designed and achieved a catalyst-free Michael reaction for the preparation of an N1-alkyl pyrazole in a high yield (>90%) with excellent regioselectivity (N1/N2 > 99.9:1). The scope of this protocol has been extended to accomplish the first general regioselective N1-alkylation of 1H-pyrazoles to give di-, tri-, and tetra-substituted pyrazoles in a single step. The resulting pyrazoles bear versatile functional groups such as bromo, ester, nitro, and nitrile, offering opportunities for late-stage functionalization. This efficient methodology will have an impact on drug discovery, as several Food and Drug Administration-approved drugs are pyrazole derivatives. A working hypothesis for the regioselectivity is proposed. X-ray crystal structures of the products that highlight the attractive interactions are discussed. This report provides a rare source for the further elucidation of the attractive interactions because the isomeric ratios and the crystal structures are directly related.

Iterative Synthesis of Contorted Macromolecular Ladders for Fast-Charging and Long-Life Lithium Batteries.

We report here an iterative synthesis of long helical perylene diimide (hPDI[n]) nanoribbons with a length up to 16 fused benzene rings. These contorted, ladder-type conjugated, and atomically precise nanoribbons show great potential as organic fast-charging and long-lifetime battery cathodes. By tuning the length of the hPDI[n] oligomers, we can simultaneously modulate the electrical conductivity and ionic diffusivity of the material. The length of the ladders adjusts both the conjugation for electron transport and the contortion for lithium-ion transport. The longest oligomer, hPDI[6], when fabricated as the cathode in lithium batteries, features both high electrical conductivity and high ionic diffusivity. This electrode material exhibits a high power density and can be charged in less than 1 min to 66% of its maximum capacity. Remarkably, this material also has exceptional cycling stability and can operate for up to 10,000 charging-discharging cycles without any appreciable capacity decay. The design principles described here chart a clear path for organic battery electrodes that are sustainable, fast-charging, and long lasting.

π-Conjugated redox-active two-dimensional polymers as organic cathode materials.

Redox-active two-dimensional polymers (RA-2DPs) are promising lithium battery organic cathode materials due to their regular porosities and high chemical stabilities. However, weak electrical conductivities inherent to the non-conjugated molecular motifs used thus far limit device performance and the practical relevance of these materials. We herein address this problem by developing a modular approach to construct π-conjugated RA-2DPs with a new polycyclic aromatic redox-active building block PDI-DA. Efficient imine-condensation between PDI-DA and two polyfunctional amine nodes followed by quantitative alkyl chain removal produced RA-2DPs TAPPy-PDI and TAPB-PDI as conjugated, porous, polycrystalline networks. In-plane conjugation and permanent porosity endow these materials with high electrical conductivity and high ion diffusion rates. As such, both RA-2DPs function as organic cathode materials with good rate performance and excellent cycling stability. Importantly, the improved design enables higher areal mass-loadings than were previously available, which drives a practical demonstration of TAPPy-PDI as the power source for a series of LED lights. Collectively, this investigation discloses viable synthetic methodologies and design principles for the realization of high-performance organic cathode materials.

Extraterrestrial prebiotic molecules: photochemistry vs. radiation chemistry of interstellar ices.

In 2016, unambiguous evidence for the presence of the amino acid glycine, an important prebiotic molecule, was deduced based on in situ mass-spectral studies of the coma surrounding cometary ice. This finding is significant because comets are thought to have preserved the icy grains originally found in the interstellar medium prior to solar system formation. Energetic processing of cosmic ices via photochemistry and radiation chemistry is thought to be the dominant mechanism for the extraterrestrial synthesis of prebiotic molecules. Radiation chemistry is defined as the "study of the chemical changes produced by the absorption of radiation of sufficiently high energy to produce ionization." Ionizing radiation in cosmic chemistry includes high-energy particles (e.g., cosmic rays) and high-energy photons (e.g., extreme-UV). In contrast, photochemistry is defined as chemical processes initiated by photon-induced electronic excitation not involving ionization. Vacuum-UV (6.2-12.4 eV) light may, in addition to photochemistry, initiate radiation chemistry because the threshold for producing secondary electrons is lower in the condensed phase than in the gas phase. Unique to radiation chemistry are four phenomena: (1) production of a cascade of low-energy (<20 eV) secondary electrons which are thought to be the dominant driving force for radiation chemistry, (2) reactions initiated by cations, (3) non-uniform distribution of reaction intermediates, and (4) non-selective chemistry leading to the production of multiple reaction products. The production of low-energy secondary electrons during radiation chemistry may also lead to new reaction pathways not available to photochemistry. In addition, low-energy electron-induced radiation chemistry may predominate over photochemistry because of the sheer number of low-energy secondary electrons. Moreover, reaction cross-sections can be several orders of magnitude larger for electrons than for photons. Discerning the role of photochemistry vs. radiation chemistry in astrochemistry is challenging because astrophysical photon-induced chemistry studies have almost exclusively used light sources that produce >10 eV photons. Because a primary objective of chemistry is to provide molecular-level mechanistic explanations for macroscopic phenomena, our ultimate goal in this review paper is to critically evaluate our current understanding of cosmic ice energetic processing which likely leads to the synthesis of extraterrestrial prebiotic molecules.