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.

Red edge effect and chromoselective photocatalysis with amorphous covalent triazine-based frameworks.

Chromoselective photocatalysis offers an intriguing opportunity to enable a specific reaction pathway out of a potentially possible multiplicity for a given substrate by using a sensitizer that converts the energy of incident photon into the redox potential of the corresponding magnitude. Several sensitizers possessing different discrete redox potentials (high/low) upon excitation with photons of specific wavelength (short/long) have been reported. Herein, we report design of molecular structures of two-dimensional amorphous covalent triazine-based frameworks (CTFs) possessing intraband states close to the valence band with strong red edge effect (REE). REE enables generation of a continuum of excited sites characterized by their own redox potentials, with the magnitude proportional to the wavelength of incident photons. Separation of charge carriers in such materials depends strongly on the wavelength of incident light and is the primary parameter that defines efficacy of the materials in photocatalytic bromination of electron rich aromatic compounds. In dual Ni-photocatalysis, excitation of electrons from the intraband states to the conduction band of the CTF with 625 nm photons enables selective formation of C‒N cross-coupling products from arylhalides and pyrrolidine, while an undesirable dehalogenation process is completely suppressed.

"Red Carbon": A Rediscovered Covalent Crystalline Semiconductor.

Carbon suboxide (C3 O2 ) is a unique molecule able to polymerize spontaneously into highly conjugated light-absorbing structures at temperatures as low as 0 °C. Despite obvious advantages, little is known about the nature and the functional properties of this carbonaceous material. In this work, the aim is to bring "red carbon," a forgotten polymeric semiconductor, back to the community's attention. A solution polymerization process is adapted to simplify the synthesis and control the structure. This allows one to obtain this crystalline covalent material at low temperatures. Both spectroscopic and elemental analyses support the chemical structure represented as conjugated ladder polypyrone ribbons. Density functional theory calculations suggest a crystalline structure of AB stacks of polypyrone ribbons and identify the material as a direct bandgap semiconductor with a medium bandgap that is further confirmed by optical analysis. The material shows promising photocatalytic performance using blue light. Moreover, the simple condensation-aromatization route described here allows the straightforward fabrication of conjugated ladder polymers and can be inspiring for the synthesis of carbonaceous materials at low temperatures in general.

Multisite PCET with photocharged carbon nitride in dark.

A combination of photochemistry and proton coupled electron transfer (PCET) is a primary strategy employed by biochemical systems and synthetic chemistry to enable uphill reactions under mild conditions. Degenerate nanometer-sized n-type semiconductor nanoparticles (SCNPs) with the Fermi level above the bottom of the conduction band are strongly reducing and act more like metals than semiconductors. Application of the degenerate SCNPs is limited to few examples. Herein, we load microporous potassium poly(heptazine imide) (K-PHI) nanoparticles with electrons (e‒) and charge balancing protons (H+) in an illumination phase using sacrificial agents. e‒/H+ in the K-PHI nanoparticles are weakly bound and therefore could be used in a range of PCET reactions in dark, such as generation of aryl radicals from aryl halides, ketyl radicals from ketones, and 6e‒/6H+ reduction of nitrobenzene to aniline. The integration of several features that until now were intrinsic for plants and natural photosynthesis into a transition metal free nanomaterial composed of abundant elements (C, N, and K) offers a powerful tool for synthetic organic chemistry.

Emerging Concepts in Carbon Nitride Organic Photocatalysis.

Carbon nitrides encompass a class of transition-metal-free materials possessing numerous advantages such as low cost (few Euros per gram), high chemical stability, broad tunability of redox potentials and optical bandgap, recyclability, and a high absorption coefficient (>105  cm-1 ), which make them highly attractive for application in photoredox catalysis. In this Review, we classify carbon nitrides based on their unique properties, structure, and redox potentials. We summarize recently emerging concepts in heterogeneous carbon nitride photocatalysis, with an emphasis on the synthesis of organic compounds: 1) Illumination-Driven Electron Accumulation in Semiconductors and Exploitation (IDEASE); 2) singlet-triplet intersystem crossing in carbon nitride excited states and related energy transfer; 3) architectures of flow photoreactors; and 4) dual metal/carbon nitride photocatalysis. The objective of this Review is to provide a detailed overview regarding innovative research in carbon nitride photocatalysis focusing on these topics.

Dichloromethylation of enones by carbon nitride photocatalysis.

Small organic radicals are ubiquitous intermediates in photocatalysis and are used in organic synthesis to install functional groups and to tune electronic properties and pharmacokinetic parameters of the final molecule. Development of new methods to generate small organic radicals with added functionality can further extend the utility of photocatalysis for synthetic needs. Herein, we present a method to generate dichloromethyl radicals from chloroform using a heterogeneous potassium poly(heptazine imide) (K-PHI) photocatalyst under visible light irradiation for C1-extension of the enone backbone. The method is applied on 15 enones, with γ,γ-dichloroketones yields of 18-89%. Due to negative zeta-potential (-40 mV) and small particle size (100 nm) K-PHI suspension is used in quasi-homogeneous flow-photoreactor increasing the productivity by 19 times compared to the batch approach. The resulting γ,γ-dichloroketones, are used as bifunctional building blocks to access value-added organic compounds such as substituted furans and pyrroles.