Exploiting the NADP+/NADPH-like Hydride-Transfer Redox Cycle with Bis-Imidazolium-Embedded Heterohelicene for Electrocatalytic Hydrogen Evolution Reaction

Orthogonal Synthesis of Cationic Azatriphenylene Derivatives for Aggregation-Induced Emission (AIE) and Aggregation-Caused Quenching (ACQ) Property Switching

Herein, we report a divergent synthesis of cationic azatriphenylene derivatives using orthogonal control of thermal and electro-oxidative pyridination, which transforms the single precursor into controlled products with perfect selectivity. Simultaneously, two switchable reactions afford the corresponding pyridinium salts with different optical properties such as aggregation-induced emission (AIE) and aggregation-caused quenching (ACQ) effects.

Rhenium coordination-induced conformational modulation in nitrogen-doped nanographene

The conformation of nanographene plays a pivotal role in determining its electronic, mechanical, and optical properties. Analyzing the conformation of nanographene is crucial because of its significant implications in various scientific and technological domains. In this work, we examined the changes in the structural conformation of nanographene upon coordination with a metal. We synthesized a series of nitrogen-doped nanographenes (2, 3, and 4) that differed by one C-C bond and their corresponding nanographene-Re complexes (2Re, 3Re, and 4Re). The conformations of the metal complexes were thoroughly analyzed using various spectroscopic techniques. A comparison of the X-ray crystal structures of nanographene before and after rhenium coordination revealed that rhenium coordination induced conformational changes in the nanographene moiety. Additionally, we assessed the catalytic activity of the nanographene-Re complexes toward the electrocatalytic hydrogen evolution reaction (HER). In the presence of a small amount of weak acid (1.5 mM acetic acid), the nanographene-Re complex exhibited hydrogen production at a low overpotential of 133 mV.

High-Throughput Screening of Molecule/Polymer Photocatalysts for the Hydrogen Evolution Reaction

Although there has been progress in designing organic photocatalysts, identifying and designing structurally distinct polymeric or molecular photocatalysts with high performance is still challenging. Using the properties of a set of well-known polymer photocatalysts, we performed a virtual screening of a large data set of around 50 000 organic semiconductors. In the initial stage, we looked for candidates with electronic properties similar to those of the best-performing photocatalysts. Next, we screened the data set using reactivity descriptors based on mechanisms derived from quantum chemical calculations for selected cases. We identified 33 candidates with high potential as photocatalysts for the hydrogen evolution reaction.

Sym- and Asym-Expanded Heterohelicene Isomers Featuring Extended Multi-Resonance Skeleton for Narrowband Deep-Blue Fluorescence

Expanded heterohelicenes composed of alternating linearly and angularly fused multi-resonance (MR) skeletons have gained wide interest owing to their promising narrowband emissions. Herein, a pair of sym- and asym-expanded heterohelicene isomers was obtained by merging boron/oxygen (B/O)-embedded MR triangulene and indolo[3,2,1-jk]carbazole units via one-pot synthesis. Owing to their fully resonating extended helical skeleton, the target heterohelicenes exhibit a significantly narrowed spectra bandwidth while emission red-shifting, thus affording deep-blue narrowband emission with a peak at approximately 460 nm, full-width-at-half-maximum (FWHM) of only 18 nm, and near-unity photoluminescence quantum yields. In comparison to the symmetrical structure, the asym-expanded heterohelicene displays suppressed aggregation within doped films, thereby showing superior narrowband electroluminescence in devices with CIE coordinates of (0.12, 0.18) and a high external quantum efficiency of up to 25 %, which retains 18.1 % even at a high luminance of 10,000 cd m-1. Overall, this work provides a novel paradigm for the development of narrowband expanded heterohelicenes.

Thermodynamics-inspired identification of privileged organocatalysts for hydrogen evolution reactions

The 7π-aromaticity observed in N-heterocyclic phosphinyl radicals (NHP˙) can offset the energy penalty associated with the disruption of 6π-aromaticity in NHP cations. This property endows NHP hydrides with facile regeneration, rendering NHP privileged organocatalysts for electrochemical hydrogen evolution reactions.

Hydride Transfer-Based CO2 Reduction Catalysis: Navigating Metal Hydride to Organic Hydride in the Catalytic Loop

ConspectusThe reductive conversion of carbon dioxide (CO2) into value-added products is a process of immense importance. In the context of rising CO2 concentration in the atmosphere and the detrimental effects it is having on the biosphere, use of alternative fuels which can offer a low-carbon or carbon-neutral pathway for storage and utilization of low-carbon energy by maintaining the net atmospheric CO2 concentration might be a prospective solution. Among the wide variety of reduced products that can be obtained from CO2, formic acid and formate salts are particularly important due to their ability to be used as an alternative fuel or a reversible hydrogen storage material. Utilization of molecular catalysts for CO2 conversion offers several advantages such as high selectivity, mechanistic clarity, versatility, and stability, making them attractive for thermochemical and electro/photochemical CO2 reduction processes. The presence of N-heterocyclic carbene (NHC) ligands in transition-metal-based molecular catalysts enhances the stability of the catalysts under harsh reaction conditions, such as high pressure, high temperature, and reductive environments, providing crucial benefits for sustained catalytic activity and longevity. Though the development of metal complex-based catalysts is essential to addressing the challenge of CO2 reduction, the possibility of using purely organic compounds as catalysts for this transformation is lucrative from the aspect of developing a truly sustainable protocol with photosynthesis being its biggest inspiration. We begin this Account by examining our systematic development of molecular metal complexes based on NHC ligands for the chemical upgradation of CO2 to formic acid/formate salt. In such cases, the ability of NHCs to act as strong σ-donor ligands for a greater hydride transfer propensity is discussed and analyzed. The reports range from catalytic ambient- and high-pressure CO2 hydrogenation to CO2 transfer-hydrogenation. Coupling of CO2 capture methodologies with CO2 conversion is also discussed. A case is made for the heterogenization of one of the highly efficient metal-NHC catalysts to develop a self-supported single-site catalyst for practical applications. Finally, our recent success of developing a novel organic catalyst system inspired from the natural NADP+/NADPH-based hydride-transfer redox couple that is active in photosynthetic CO2 reduction has been discussed. This catalyst is designed based on a bis-imidazolium-embedded heterohelicene with a central pyridine ring and is capable of electrocatalytically converting CO2 to HCO2H with TON values 100-1000 times greater than the existing reported values achieved so far by organic catalysts. Overall, we believe that the results of hydride transfer-based CO2 reduction catalysis presented in this Account hold significant implications beyond our work and have the potential for motivating future research toward further development in this important field.

Annulated oxazolium anion-π+ AIEgens

A new class of anion-π+ AIEgens based on ring-fused annulated oxazolium architectures is reported, which can be readily synthesized via a single-step dual C-H activation annulative π-extension (APEX) protocol from simple oxazolium salts. The crucial and decisive role of anion-π+ interactions in their solid-state structural arrangement was analyzed to correlate their tunable AIE features and solid-state quantum yields.

Organocatalytic Hydrogen Evolution Reaction by Diazaphospholenes

The electrochemical hydrogen evolution reaction (HER) is currently recognized as a prospective way to obtain clean energy. The electrocatalysts used currently are dominantly based on transition metals. In this work, we have demonstrated a diazaphospholene (N-heterocyclic phosphine (NHP))-type small molecular organocatalyst that can catalyze the HER with a maximum current density of 130 mA·cm-2, an overpotential of 354 mV, and a faradaic efficiency of 90%. Mechanistic studies verify a Heyrovsky-type process with NHP, whereas its hydricity and aromaticity favor hydrogen release and catalyst regeneration.

Orchestrated Octuple C-H Activation: A Bottom-Up Topology Engineering Approach toward Stimuli-Responsive Double-Heptagon-Embedded Wavy Polycyclic Heteroaromatics

Curiosity-driven innovations on the design and synthesis of nonplanar polycyclic aromatic/heteroaromatic compounds with new molecular topologies unfold exciting opportunities for harnessing their intriguing supramolecular properties and thereby the development of novel functional organic materials. This work presents such an innovative synthetic concept of a bottom-up molecular topology engineering through a unique orchestrated octuple C-H activation reaction, toward the rapid synthesis of a novel class of double heptagon-incorporated nitrogen-doped laterally-fused polycyclic compounds with rarely reported wavy structural configuration. The profound impact of the molecular wavy structures of these compounds on their properties is manifested by weak and tunable solid-state intermolecular interactions controlling the electronic properties of the materials, leading to reversibly switchable fluorochromism in the solid state and thin films with mechanical force and solvent vapors as external stimuli, thereby indicating their potential applicability in rewritable fluorescent optical recording media, security papers, mechanosensors, volatile organic compound (VOC) sensors etc.

Unveiling N-Fused Nitreniums as Potent Catalytic Photooxidants

The first example of a hitherto-unknown facet of catalytic photooxidant capability of nitrenium cations is reported herein. The fundamental limitation of inability of the traditional and reported nitreniums to achieve the excited-state redox potential beyond +2.0 V (vs Ag/AgCl), the primary requirement for a powerful photooxidant, is addressed in this work by developing a structurally unique class of N-fused nitrenium cations, with the required structural engineering involving extensive π-conjugation through ring fusion at the nitrenium site, which enabled significant lowering of the LUMO energy and easy reduction at the excited state (excited-state redox potential up to +2.5 V vs Ag/AgCl), facilitated by effective delocalization/stabilization of the generated radical. This finding opens a new way to discover novel and useful (photo)catalytic properties of nitrenium cations beyond just Lewis acidity.

Small Organic Molecular Electrocatalysts for Fuels Production

In recent years, heterocyclic organic compounds have been explored as molecular electrocatalysts in relevant reactions for energy conversion and storage. Merging mimetics of biological systems that perform hydride transfer with rational synthetic chemical design has opened many opportunities for organic molecules to be tuned at the atomic level conferring them interesting reactivities. These molecular electrocatalysts represent an alternative to traditional metallic materials and metal complexes employed for water oxidation, hydrogen production, and carbon dioxide reduction. This minireview describes recent reports concerning design, catalytic activity and the mechanism of synthetic molecular electrocatalysts towards solar fuels production.

Directing Group Strategy for the Isolation of Organoselenium(VI) Benzoselenonates: Metal-Free Catalysts for Hydrogen Evolution Reaction

The exploration of fourth-period organoelements, particularly organoseleniums in their highest VI oxidation state, is limited owing to their stability and synthesis. Herein, the isolation of a new class of quinolinyl-embedded, hexavalent selenium(VI) benzoselenonates has been discussed and further evaluated for a metal-free electrocatalytic hydrogen evolution reaction (HER). The Se(VI) benzoselenonates exhibited high Faradaic efficiency (F.E.) of metal-free H2 gas production up to 86% with a very good turnover number (TON) up to 43 and moderate overpotential (η) of 500 mV; in the presence of mild acetic acid source in a less deprotonating DMF solvent. Taken together with various (NMR, UV-vis, and EPR) spectroscopic and DFT computation studies, a plausible HER pathway is proposed, which suggests that the electrochemical reduction of quinolinyl ring is the initiation step and Se(VI) acts as the reaction site by involving a hydridic type of intermediate for the electrochemical H2 gas generation.