Extent of carbon nitride photocharging controls energetics of hydrogen transfer in photochemical cascade processes.

Graphitic carbon nitride is widely studied in organic photoredox catalysis. Reductive quenching of carbon nitride excited state is postulated in many photocatalytic transformations. However, the reactivity of this species in the turn over step is less explored. In this work, we investigate electron and proton transfer from carbon nitride that is photocharged to a various extent, while the negative charge is compensated either by protons or ammonium cations. Strong stabilization of electrons by ammonium cations makes proton-coupled electron transfer uphill, and affords air-stable persistent carbon nitride radicals. In carbon nitrides, which are photocharged to a smaller extent, protons do not stabilize electrons, which results in spontaneous charge transfer to oxidants. Facile proton-coupled electron transfer is a key step in the photocatalytic oxidative-reductive cascade - tetramerization of benzylic amines. The feasibility of proton-coupled electron transfer is modulated by adjusting the extent of carbon nitride photocharging, type of counterion and temperature.

Green Light Photoelectrocatalysis with Sulfur-Doped Carbon Nitride: Using Triazole-Purpald for Enhanced Benzylamine Oxidation and Oxygen Evolution Reactions.

Materials dictate carbon neutral industrial chemical processes. Visible-light photoelectrocatalysts from abundant resources will play a key role in exploiting solar irradiation. Anionic doping via pre-organization of precursors and further co-polymerization creates tuneable semiconductors. Triazole derivative-purpald, an unexplored precursor with sulfur (S) container, combined in different initial ratios with melamine during one solid-state polycondensation with two thermal steps yields hybrid S-doped carbon nitrides (C3 N4 ). The series of S-doped/C3 N4 -based materials show enhanced optical, electronic, structural, textural, and morphological properties and exhibit higher performance in organic benzylamine photooxidation, oxygen evolution, and similar energy storage (capacitor brief investigation). 50M-50P exhibits the highest photooxidation conversion (84 ± 3%) of benzylamine to imine at 535 nm - green light for 48 h, due to a discrete shoulder (≈700) nm, high sulfur content, preservation of crystal size, new intraband energy states, structural defects by layer distortion, and 10-16 nm pores with arbitrary depth. This work innovates by studying the concomitant relationships between: 1) the precursor decomposition while C3 N4 is formed, 2) the insertion of S impurities, 3) the S-doped C3 N4 property-activity relationships, and 4) combinatorial surface, bulk, structural, optical, and electronic characterization analysis. This work contributes to the development of disordered long-visible-light photocatalysts for solar energy conversion and storage.

Saving the Energy Loss in Lithium-Mediated Nitrogen Fixation by Using a Highly Reactive Li3 N Intermediate for C-N Coupling Reactions.

Direct synthesis of N-containing organic compounds from dinitrogen (N2 ) can make synthetic chemistry more sustainable. Previous bottlenecks in lithium-mediated N2 fixation were resolved by loading Li-metal anodes covered with the typical Li+ ion-conducting solid electrolyte interface, which are subsequently allowed to react with N2 . The developed strategy allowed us to reach high Faradaic efficiencies toward Li3 N. These reactive Li3 N were then contacted with acylchlorides. Surface nitride ions are more nucleophilic than amines which direct the two C-N coupling reactions toward formation of imides rather than amides, and an integrated current efficiency of 57-77 % could be realized. This study thereby not only provides a feasible electrochemical Li3 N synthesis, but also delineates an economical and green synthesis of highly valuable N-containing compounds from N2 under mild conditions, just using commercial spare parts and processes from the omnipresent Li battery technology.

Synthesis of Sulfonyl Chlorides from Aryldiazonium Salts Mediated by a Heterogeneous Potassium Poly(heptazine imide) Photocatalyst.

Visible light photocatalysis is a tool in synthetic chemistry that allows us to utilize the energy of photons via photoinduced electron transfer to promote diverse organic reactions. Herein, a heterogeneous transition metal-free material, a type of carbon nitride photocatalyst, potassium poly(heptazine imide), is employed to produce sulfonyl chlorides from arenediazonium salts under mild conditions (visible light irradiation, room temperature) with 50-95% yields. The method is suitable for the synthesis of both electron rich and electron deficient compounds, and it shows high tolerance toward different functional groups (halides, ester, nitro, cyano groups). Thus, a sustainable photocatalytic alternative to the Meerwein chlorosulfonylation reaction is offered.

Chromoselective Synthesis of Sulfonyl Chlorides and Sulfonamides with Potassium Poly(heptazine imide) Photocatalyst.

Among external stimuli used to promote a chemical reaction, photocatalysis possesses a unique one-light. Photons are traceless reagents that provide an exclusive opportunity to alter chemoselectivity of the photocatalytic reaction varying the color of incident light. This strategy may be implemented by using a sensitizer capable to activate a specific reaction pathway depending on the excitation light. Herein, we use potassium poly(heptazine imide) (K-PHI), a type of carbon nitride, to generate selectively three different products from S-arylthioacetates simply varying the excitation light and otherwise identical conditions. Namely, arylchlorides are produced under UV/purple, sulfonyl chlorides with blue/white, and diaryldisulfides at green to red light. A combination of the negatively charged polyanion, highly positive potential of the valence band, presence of intraband states, ability to sensitize singlet oxygen, and multi-electron transfer is shown to enable this chromoselective conversion of thioacetates.

Enhanced Organic Photocatalysis in Confined Flow through a Carbon Nitride Nanotube Membrane with Conversions in the Millisecond Regime.

Bioinspired nanoconfined catalysis has developed to become an important tool for improving the performance of a wide range of chemical reactions. However, photocatalysis in a nanoconfined environment remains largely unexplored. Here, we report the application of a free-standing and flow-through carbon nitride nanotube (CNN) membrane with pore diameters of 40 nm for confined photocatalytic reactions where reactants are in contact with the catalyst for <65 ms, as calculated from the flow. Due to the well-defined tubular structure of the membrane, we are able to assess quantitatively the photocatalytic performance in each of the parallelized single carbon nitride nanotubes, which act as spatially isolated nanoreactors. In oxidation of benzylamine, the confined reaction shows an improved performance when compared to the corresponding bulk reaction, reaching a turnover frequency of (9.63 ± 1.87) × 105 s-1. Such high rates are otherwise only known for special enzymes and are clearly attributed to the confinement of the studied reactions within the one-dimensional nanochannels of the CNN membrane. Namely, a concave surface maintains the internal electric field induced by the polar surface of the carbon nitride inside the nanotube, which is essential for polarization of reagent molecules and extension of the lifetime of the photogenerated charge carriers. The enhanced flow rate upon confinement provides crucial insight on catalysis in such an environment from a physical chemistry perspective. This confinement strategy is envisioned not only to realize highly efficient reactions but also to gain a fundamental understanding of complex chemical processes.

Unconventional Photocatalysis in Conductive Polymers: Reversible Modulation of PEDOT:PSS Conductivity by Long-Lived Poly(Heptazine Imide) Radicals.

In photocatalysis, small organic molecules are converted into desired products using light responsive materials, electromagnetic radiation, and electron mediators. Substitution of low molecular weight reagents with redox active functional materials may increase the utility of photocatalysis beyond organic synthesis and environmental applications. Guided by the general principles of photocatalysis, we design hybrid nanocomposites composed of n-type semiconducting potassium poly(heptazine imide) (K-PHI), and p-type conducting poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the redox active substrate. Electrical conductivity of the hybrid nanocomposite, possessing optimal K-PHI content, is reversibly modulated combining a series of external stimuli ranging from visible light under inert conditions and to dark conditions under an O2 atmosphere. Using a conductive polymer as the redox active substrate allows study of the photocatalytic processes mediated by semiconducting photocatalysts through electrical conductivity measurements.

Potassium Poly(Heptazine Imide): Transition Metal-Free Solid-State Triplet Sensitizer in Cascade Energy Transfer and [3+2]-cycloadditions.

Polymeric carbon nitride materials have been used in numerous light-to-energy conversion applications ranging from photocatalysis to optoelectronics. For a new application and modelling, we first refined the crystal structure of potassium poly(heptazine imide) (K-PHI)-a benchmark carbon nitride material in photocatalysis-by means of X-ray powder diffraction and transmission electron microscopy. Using the crystal structure of K-PHI, periodic DFT calculations were performed to calculate the density-of-states (DOS) and localize intra band states (IBS). IBS were found to be responsible for the enhanced K-PHI absorption in the near IR region, to serve as electron traps, and to be useful in energy transfer reactions. Once excited with visible light, carbon nitrides, in addition to the direct recombination, can also undergo singlet-triplet intersystem crossing. We utilized the K-PHI centered triplet excited states to trigger a cascade of energy transfer reactions and, in turn, to sensitize, for example, singlet oxygen (1 O2 ) as a starting point to synthesis up to 25 different N-rich heterocycles.

Solar Reforming of Biomass with Homogeneous Carbon Dots.

A sunlight-powered process is reported that employs carbon dots (CDs) as light absorbers for the conversion of lignocellulose into sustainable H2 fuel and organics. This photocatalytic system operates in pure and untreated sea water at benign pH (2-8) and ambient temperature and pressure. The CDs can be produced in a scalable synthesis directly from biomass itself and their solubility allows for good interactions with the insoluble biomass substrates. They also display excellent photophysical properties with a high fraction of long-lived charge carriers and the availability of a reductive and an oxidative quenching pathway. The presented CD-based biomass photoconversion system opens new avenues for sustainable, practical, and renewable fuel production through biomass valorization.