Tri-s-triazines (s-heptazines)—From a “mystery molecule” to industrially relevant carbon nitride materials

Tripodal Heptazine Core-Based C3-Symmetric Multi-Azo(Hetero)Arenes: Photoswitching, Supramolecular Sol-Gel Behavior, and Base Sensing Prospects

Electron-deficient heptazine-based molecular and polymeric materials are a fast-growing area of research with their exciting electronic and optical properties, finding potential applications in catalysis, optoelectronics etc. However, the continuous thirst for the design of heptazine-core-based molecular systems has grown in interest in the search for a new class of materials. Herein, we have synthesized two heptazine core-based C3-symmetric tripodal molecular systems decorated with three azobenzene and phenylazo-3,5-dimethylisoxazole as photoswitchable units connected through ─NH linker. The photoswitching characteristics and thermal half-lives of photoswitched states were studied using UV-vis and nuclear magnetic resonance (NMR) spectroscopy. In addition, for azobenzene derivative 1, intriguing properties such as reversible sonication-induced gelation and base sensing ability have been demonstrated and supported by computations.

Pyrrolation of Melem: A Facile Gateway into the Field of Monomeric s-Heptazine Chemistry

Monomeric s-heptazines are an intriguing class of compounds with many attractive properties for various areas of application such as photocatalysis or organic light-emitting diodes. However, research into these properties has so far been challenging, as only a few synthetic routes for the preparation of monomeric s-heptazines are known in the literature. Furthermore, these few reported synthetic pathways generally require the use of specialized equipment that may not be available to all laboratories interested in studying monomeric s-heptazines. For this reason, a more accessible synthetic route for the preparation of monomeric s-heptazines has been developed in the course of this work. The central compound of this new approach is 2,5,8-tri(1H-pyrrol-1-yl)-s-heptazine, which could be conveniently synthesized via an acid catalyzed pyrrolation of melem with bench-stable 2,5-dimethoxytetrahydrofuran in a simple one-pot synthesis in air. This compound was shown to be a potent starting material for the synthesis of numerous other monomeric s-heptazines by reaction with both nucleophiles and electrophiles. The monomeric s-heptazines thus accessible were analyzed for their crystal structures by single crystal X-ray diffraction and for their optical properties by ultraviolet/visible and photoluminescence spectroscopy.

Stretchable Full-Color Phosphorescent PVA-Based Ionogels for Multimodal Sensing-Visual Integration Applications

Exploring ionogels with superior conductivity, mechanical properties, and long-lasting room temperature phosphorescence (RTP) offers considerable potential for new-generation optoelectronics. However, reports on ionogels remain limited owing to the contradiction between the flexibility required for stretching and the rigidity necessary for RTP and load-bearing within the same ionogels. Here, a facile strategy is reported to enhance the toughness and extend the RTP of ionogels by salting-out-induced microphase separation, which results in the formation of an IL-rich phase (soft) for stretching and ionic conduction and a polymer-rich phase (stiff) for energy dissipation and clustering-triggered phosphorescence. The obtained ionogels exhibit high stretchability (≈400% strain), toughness (≈∼20 MJ m-3), ionic conductivity (8.4 mS cm-1), and ultralong afterglow lifetime (112.4 ms). This strategy is applicable to chromophores with color-tunable phosphorescence. By leveraging observable full-color RTP and real-time electrical signals in response to diverse stimuli (i.e., stretching and pressing), an intelligent grasping strategy is developed for robust hand pose reconstruction. In addition, a tactile-visual fusion recognition keyboard is created with dual functionality of information encryption and signal transmission. The ease of fabrication, wide tunability, and multifunctionality will help expand the scope of ionogels for smart devices.

Novel Graphitic Oxynitrides as Photocatalysts for Sustainable H2 Production and CO2 Valorization. The Importance of Self-Assembly for Catalytic Activity

The field of carbocatalysis, often portrayed by paradigmatic graphitic carbonaceous structures, has become a booming topic tailored for multiple applications. To this end, a new metal-free carbocatalyst has been constructed from simple prebiotic monomers such as cyanamide and glyoxal. The resulting material shows an excellent performance as photocatalyst for H2 production and CO2 valorization, thus unveiling its real value to tackle sustainable goals. The unique oxygen-rich carbonaceous structure has been characterized in detail, which is consistent with a graphitic layered network. The described performance in two major societal concerns along with a facile preparation from C1/C2 platforms, makes this type of overlooked oxynitride carbocatalysts promising for real-life environmental endeavors.

Synthesis, Characterization, and Roles of Vacancy Defects in Polymer and Graphitized Carbon Nitride Photocatalysts: A Comprehensive Review

Vacancy defect graphitic carbon nitride (g-C3N4) and conjugated polyimide (PI) polymer photocatalysts have become increasingly recognized as metal-free photocatalysts featuring an appropriate bandgap. The narrow absorption spectrum of visible light and the rapid recombination rate of the photoexcited charge carriers in PI polymers and g-C3N4 impede its photocatalytic performance. The presence of oxygen vacancies (OVs) in PI polymer photocatalysts, as well as nitrogen vacancies (NVs) and carbon vacancies (CVs) in g-C3N4, can significantly enhance the migration of photogenerated electrons. Adding vacancies to improve the electronic structure and band gap width can greatly enhance the photocatalytic efficiency of PI polymers and g-C3N4. Defect engineering is important for increasing the photocatalytic ability of PI-polymer and g-C3N4. There remains a notable absence of thorough review papers covering the synthesis, characterization, and applications of vacancy-rich PI-polymer and g-C3N4 in photocatalysis. This review paper examines the roles of OVs in PI-polymer, NVs, and CVs in g-C3N4 and thoroughly summarizes the preparation approaches employed before and after, as well as during polymerization. This review scrutinizes spectroscopic characterization techniques, such as EPR, XPS, PAS, XRD, FTIR, and NMR, for vacancy defect analysis. We also reviewed the role of vacancies, which include light absorption, photogenerated charge carrier separation, and transfer dynamics. This review could serve as a comprehensive understanding, a vacancy-engineered design framework, and a practical guide for synthesizing and characterizing.

MnCl2(C6N10H6): Insights into a Luminescent Transition Metal-Melem Complex

In this work, the (MnCl2(C6N10H6) complex has been synthesized via solid-state reaction between manganese (II) chloride and melamine in the molar ratio of 1:2. A similar synthesis has been repeated with CoCl2, and FeCl2, resulting in two new metal-melam complexes (FeCl2(C6N11H9) and CoCl2(C6N11H9)). MnCl2(C6N10H6) crystallizes in the monoclinic crystal system with the space group I2/a. The crystalline powder of MnCl2(C6N10H6) was studied by X-ray diffraction, infrared spectroscopy, and thermogravimetric analysis to examine its structure and properties. MnCl2(C6N10H6) also shows good thermal stability up to 370 °C; however, the complete decomposition occurred at 900 °C, yielding Mn7C3. This paper presents an easy synthesis of the first luminescent transition metal-melem complex, providing new insights into the reactivity of melamine at elevated temperatures in the presence of transition metal chlorides.

Designing C9N10 Anchored Single Mo Atom as an Efficient Electrocatalyst for Nitrogen Fixation

Electrochemical nitrogen reduction reaction (NRR) is a promising route for realizing green and sustainable ammonia synthesis under ambient conditions. However, one of the major challenges of currently available Single-atom catalysts (SACs) is poor catalytic activity and low catalytic selectivity, which is far away from the requirements of industrial applications. Herein, first-principle calculations within the density functional theory were performed to evaluate the feasibility of a single Mo atom anchored on a g-C9N10 monolayer (Mo@g-C9N10) as NRR electrocatalysts. The results demonstrated that the gas phase N2 molecule can be sufficiently activated on Mo@g-C9N10, and N2 reduction dominantly occurs on the active Mo atom via the preferred enzymatic mechanism, with a low limiting potential of -0.48 V. In addition, Mo@g-C9N10 possesses a good prohibition ability for the competitive hydrogen evolution reaction. More impressively, good electronic conductivity and high electron transport efficiency endow Mo SACs with excellent activity for electrocatalytic N2 reduction. This theoretical research not only accelerates the development of NRR electrocatalysts but also increases our insights into optimizing the catalytic performance of SACs.

Atomically dispersed low-valent Au boosts photocatalytic hydroxyl radical production

Providing affordable, safe drinking water and universal sanitation poses a grand societal challenge. Here we developed atomically dispersed Au on potassium-incorporated polymeric carbon nitride material that could simultaneously boost photocatalytic generation of ·OH and H2O2 with an apparent quantum efficiency over 85% at 420 nm. Potassium introduction into the poly(heptazine imide) matrix formed strong K-N bonds and rendered Au with an oxidation number close to 0. Extensive experimental characterization and computational simulations revealed that the low-valent Au altered the materials' band structure to trap highly localized holes produced under photoexcitation. These highly localized holes could boost the 1e- water oxidation reaction to form highly oxidative ·OH and simultaneously dissociate the hydrogen atom in H2O, which greatly promoted the reduction of oxygen to H2O2. The photogenerated ·OH led to an efficiency enhancement for visible-light-response superhydrophilicity. Furthermore, photo-illumination in an onsite fixed-bed reactor could disinfect water at a rate of 66 L H2O m-2 per day.

Thermal deprotonation and condensation of melamine in the presence of indium(III)chloride

The thermal condensation of melamine into molecules melam, melem, and the one-dimensional polymer melon has already been reported. An interesting question arises about the impact of other compounds being present in this process of thermal conversion. The solid-state reaction of C3N6H6 with InCl3 leads to a novel compound featuring deprotonated melam units in a supramolecular assembly, based on the [C12N20H8]4- anion that is interconnected in the structure via N-In-N bonding. The reaction pathway of the formation of this compound is investigated by thermal analysis and the crystal structure of unique (NH4)[(InCl2)3(C12N20H8)]·⅔[InCl3(NH3)] is reported as well as its photoluminescence properties.

Exclusive Coordination between Melem and Silver(I) Ions: From Irregular Aggregates to Nanofibers to Crystal Cubes

There is growing focus on metal-free molecules and polymers owing to their potential applications in various energy and catalysis-related applications. Melem (2,5,8-triamino-s-heptazine, C6H6N10) has emerged as a metal-free material for solar-to-fuel conversion. However, its reactivity with metal ions or organic molecules has never been reported although it possesses multiple supramolecular interaction sites. In this work, we report on the synthesis of a novel metal-organic coordination framework (melem-Ag) by simply introducing Ag+ into the aqueous suspension of aggregated melem particles. Notably, as the reaction progresses, the melem disappears, and the morphology of the newly formed complex spontaneously evolves from nanofibers to single-crystalline blocks, which possess the same chemical structure, indicating that the morphology evolution is driven by Ostwald ripening. The structure of melem-Ag displays infinite nanocages of triangular pyramids consisting of melem molecules and Ag+, linked via Ag-N coordinate bonding and Ag-Ag argentophilic interactions. It is noteworthy that Ag+ is the only transition-metal cation that reacts with melem suspensions, even in the presence of other transition-metal cations (Co2+, Ni2+, Cu2+, and Zn2+). The coordination of Ag+ to melem results in metal-to-ligand charge transfer (MLCT), resulting in a quenched photoluminescence and enhanced light absorption. Exposing the melem-Ag crystals to UV light for varying time intervals results in the formation of colorful powders, which may be used for Ag-decorated photocatalysts.

Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single Atom Sites on Carbon Nitride for Selective Photooxidation of Methane into Methanol

Direct selective transformation of greenhouse methane (CH4) to liquid oxygenates (methanol) can substitute energy-intensive two-step (reforming/Fischer-Tropsch) synthesis while creating environmental benefits. The development of inexpensive, selective, and robust catalysts that enable room temperature conversion will decide the future of this technology. Single-atom catalysts (SACs) with isolated active centers embedded in support have displayed significant promises in catalysis to drive challenging reactions. Herein, high-density Ni single atoms are developed and stabilized on carbon nitride (NiCN) via thermal condensation of preorganized Ni-coordinated melem units. The physicochemical characterization of NiCN with various analytical techniques including HAADF-STEM and X-ray absorption fine structure (XAFS) validate the successful formation of Ni single atoms coordinated to the heptazine-constituted CN network. The presence of uniform catalytic sites improved visible absorption and carrier separation in densely populated NiCN SAC resulting in 100% selective photoconversion of (CH4) to methanol using H2O2 as an oxidant. The superior catalytic activity can be attributed to the generation of high oxidation (NiIII═O) sites and selective C─H bond cleavage to generate •CH3 radicals on Ni centers, which can combine with •OH radicals to generate CH3OH.

Small variation induces a big difference: the effect of polymerization kinetics of graphitic carbon nitride on its photocatalytic activity

Graphitic carbon nitride (g-CN) has emerged as a promising visible-light-responsive photocatalyst, and its activity is highly sensitive to synthesis conditions. In this work, we attempt to correlate the photocatalytic activity of g-CN with its production yield, which is kinetically determined by the specific condensation process. Bulk g-CN samples were synthesized by the conventional condensation procedure, but in static air and flowing air, respectively. The one synthesized in static air showed a lower production yield with an increased specific surface area and preferential surface chemical states, corresponding to a significantly improved activity for photocatalytic hydrogen evolution (PHE) and dye degradation. We further synthesized a series of g-CN samples by merely changing the synthetic atmosphere, the ramping rate, and the loading amount of the precursor, and the difference in their PHE performance was found to be as high as 7.05 times. The notable changes in their production yields as well as the photocatalytic activities have been discussed from the point of view of polymerization reaction kinetics, and the self-generated NH3 atmosphere plays a crucial role.

Atomically isolated Sb(CN)3 on sp2-c-COFs with balanced hydrophilic and oleophilic sites for photocatalytic C-H activation

Activation of carbon-hydrogen (C-H) bonds is of utmost importance for the synthesis of vital molecules. Toward achieving efficient photocatalytic C-H activation, our investigation revealed that incorporating hydrophilic C≡N-Sb(CN)3 sites into hydrophobic sp2 carbon-conjugated covalent organic frameworks (sp2-c-COFs) had a dual effect: It simultaneously enhanced charge separation and improved generation of polar reactive oxygen species. Detailed spectroscopy measurements and simulations showed that C≡N-Sb(CN)3 primarily functioned as water capture sites, which were not directly involved in photocatalysis. However, the potent interaction between water molecules and the Sb(CN)3-modified framework notably enhanced charge dynamics in hydrophobic sp2-c-COFs. The reactive species ·O2- and ·OH (ad) subsequently combined with benzyl radical, leading to the formation of benzaldehyde, benzyl alcohol, and lastly benzyl benzoate. Notably, the Sb(CN)3-modified sp2-c-COFs exhibited a 54-fold improvement in reaction rate as compared to pristine sp2-c-COFs, which achieved a remarkable 68% conversion rate for toluene and an 80% selectivity for benzyl benzoate.

Maximizing the linear and nonlinear optical responses of alkaline tricyanomelaminate

High-performance bi-functional materials are in urgent demand for the next-generation integrated optical devices. In this work, we successfully synthesized the first tricyanomelaminate with bi-functional optical responses, namely Cs3C6N9•H2O (I), from its analogue Na3C6N9•3H2O by a facile ion exchange method. In contrast to Na3C6N9•3H2O, I realizes an optimal arrangement of π-conjugated (C6N9)3- anion groups in its crystal structure. As a result, the second-order nonlinear optical (NLO) response is greatly enhanced from nearly zero of Na3C6N9•3H2O to ∼9.8 × KH2PO4 of I. Furthermore, I exhibits a giant linear optical anisotropic response (i.e. birefringence) of 0.52 at the wavelength of 550 nm. Both responses are almost the largest among the inorganic compounds of π-conjugated rings, which indicates that I has great potential as a bi-functional optical crystal. Structural and theoretical analyses reveal the microscopic origin of excellent optical properties. This work would attract a lot of interest to the persistently neglected potential of tricyanomelaminates as linear optical and NLO crystals.

B, P, and S heteroatom doped, bio- and hemo-compatible 2D graphitic-carbon nitride (g-C3N4) with antioxidant, light-induced antibacterial, and bioimaging endeavors

The synthesis of two-dimensional (2D) graphiticg-C3N4and heteroatom-doped graphitic H@g-C3N4(H: B, P, or S) particles were successfully done using melamine as source compounds and boric acid, phosphorous red, and sulfur as doping agents. The band gap values of 2Dg-C3N4, B50@g-C3N4, P50@g-C3N4, and S50@g-C3N4structures were determined as 2.90, 3.03, 2.89, and 2.93 eV, respectively. The fluorescent emission wavelengths of 2Dg-C3N4, B50@g-C3N4, P50@g-C3N4, and S50@g-C3N4structures were observed at 442, 430, 441, and 442 nm, respectively upon excitation atλEx= 325 nm. There is also one additional new emission wavelength was found at 345 nm for B50@g-C3N4structure. The blood compatibility test results ofg-C3N4, B50@g-C3N4, P50@g-C3N4, and S50@g-C3N4structures revealed that all materials are blood compatible with <2% hemolysis and >90% blood clotting indices at 100μg ml-1concentration. The cell toxicity of the prepared 2D graphitic structures were also tested on L929 fibroblast cells, and even the heteroatom doped hasg-C3N4structures induce no cytotoxicity was observed with >91% cell viability even at 250μg ml-1particle concentration with the exception of P50@g-C3N4which as >75 viability. Moreover, for 2Dg-C3N4, B50@g-C3N4, and S50@g-C3N4constructs, even at 500μg ml-1concentration, >90% cell viabilities was monitored. As a diagnostic material, B50@g-C3N4was found to have significantly high penetration and distribution abilities into L929 fibroblast cells granting a great potential in fluorescence imaging and bioimaging applications. Furthermore, the elemental doping with B, P, and S ofg-C3N4were found to significantly increase the photodynamic antibacterial activity e.g. more than half of bacterial elimination by heteroatom-doped forms ofg-C3N4under UVA treatment was achieved.

On-Surface Carbon Nitride Growth from Polymerization of 2,5,8-Triazido-s-heptazine

Carbon nitrides have recently come into focus for photo- and thermal catalysis, both as support materials for metal nanoparticles as well as photocatalysts themselves. While many approaches for the synthesis of three-dimensional carbon nitride materials are available, only top-down approaches by exfoliation of powders lead to thin-film flakes of this inherently two-dimensional material. Here, we describe an in situ on-surface synthesis of monolayer 2D carbon nitride films as a first step toward precise combination with other 2D materials. Starting with a single monomer precursor, we show that 2,5,8-triazido-s-heptazine can be evaporated intact, deposited on a single crystalline Au(111) or graphite support, and activated via azide decomposition and subsequent coupling to form a covalent polyheptazine network. We demonstrate that the activation can occur in three pathways, via electrons (X-ray illumination), via photons (UV illumination), and thermally. Our work paves the way to coat materials with extended carbon nitride networks that are, as we show, stable under ambient conditions.

Preparation and Characterization of a Coordination Polymer Based on Iron (III)-Cyamelurate as a Superior Catalyst for Heterogeneous Fenton-Like Processes

We report on a new iron (iii)-cyamelurate-based coordination polymer. The new material based on a heptazine derivative was prepared in aqueous medium and characterized by a variety of techniques including TGA, FTIR, XRD, HRTEM, and STEM. Due to the high structural stability of the complex in aqueous media, its heterogeneous Fenton-like catalytic activity was evaluated using a model molecule. The results obtained showed a high catalytic activity in both in basic and acid media. The pseudo-first-order rate constants normalized by iron(III) concentrations was approximately 1000 times higher than the result obtained for traditional heterogeneous catalysts based on iron(III) oxyhydroxides. The best observed catalytic activities were attributed to the increase in the binding sites of Fe³⁺ ions, in parallel with the increased exposure of the catalytic sites, leading to a higher atomic efficiency of the reaction.

Organocatalysis with carbon nitrides

Carbon nitrides, a distinguished class of metal-free catalytic materials, have presented a good potential for chemical transformations and are expected to become prominent materials for organocatalysis. This is largely possible due to their low cost, exceptional thermal and chemical stability, non-toxicity, ease of functionalization, porosity development, etc. Especially, the carbon nitrides with increased porosity and nitrogen contents are more versatile than their bulk counterparts for catalysis. These N-rich carbon nitrides are discussed in the earlier parts of the review. Later, the review highlights the role of such carbon nitride materials for the various organic catalytic reactions including Knoevenagel condensation, oxidation, hydrogenation, esterification, transesterification, cycloaddition, and hydrolysis. The recently emerging concepts in carbon nitride-based organocatalysis have been given special attention. In each of the sections, the structure-property relationship of the materials was discussed and related to their catalysis action. Relevant comparisons with other catalytic materials are also discussed to realize their real potential value. The perspective, challenges, and future directions are also discussed. The overall objective of this review is to provide up-to-date information on new developments in carbon nitride-based organic catalysis reactions that could see them rising as prominent catalytic materials in the future.

First Example of a Heptazine-Porphyrin Dyad; Synthesis and Spectroscopic Properties

We have prepared the first example of a porphyrin linked to an heptazine photoactive antenna. The two entities, linked with an alkyl spacer, demonstrate the activity of both active moieties. While they behave electrochemically independantly, on the other hand the spectroscopy shows the existence of energy transfer between both partners.

Two-Dimensional Covalent Heptazine-Based Framework Enables Highly Photocatalytic Performance for Overall Water Splitting

Screening high-efficiency 2D conjugated polymers toward visible-light-driven overall water splitting (OWS) is one of the most promising but challenging research directions to realize solar-to-hydrogen (STH) energy conversion and storage. "Mystery molecule" heptazine is an intriguing hydrogen evolution reaction (HER) building block. By covalently linking with the electron-rich alkynyl and phenyl oxygen evolution reaction (OER) active units, 10 experimentally feasible 2D covalent heptazine-based frameworks (CHFs) are constructed and screened four promising visible-light-driven OWS photocatalysts, which are linked by p-phenyl (CHF-4), p-phenylenediynyl (CHF-7), m-phenylenediynyl (CHF-8), and phenyltriynyl (CHF-9), respectively. Their HER and OER active sites achieve completely spatially separated, where HER active sites focus on heptazine units and OER active sites located on alkynyl or phenyl units. Their lower overpotentials allow them to spontaneously trigger the surface OWS reaction under their own light-induced bias without using any sacrificial agents and cocatalysts. Among them, CHF-7 shows the best photocatalytic performance with an ideal STH energy conversion efficiency estimated at 12.04%, indicating that it is a promising photocatalyst for industrial OWS. This work not only provides an innovative idea for the exploration of novel polymer photocatalysts for OWS but also supplies a direction for the development of heptazine derivatives.

S-Heptazine N-ligand based luminescent coordination materials: synthesis, structural and luminescent studies of lanthanide-cyamelurate networks

Various series of lanthanide metal-organic networks denoted Ln-Cy (Ln = La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb), were synthesized under solvothermal conditions using potassium cyamelurate (K₃Cy) and lanthanide nitrate salts. All obtained materials were fully characterized, and their crystal structures were solved by single-crystal X-ray diffraction. Four types of coordination modes were elucidated for the Ln-Cy series with different Ln³⁺ coordination geometries. Structural studies were performed to compare the various coordination compounds of the Ln-Cy series. Moreover, the cyamelurate linkers of rich π-conjugated and uncoordinated Lewis basic sites were used as an absorbing chromophore to enhance the luminescence quantum efficiency, the band emission and the luminescence lifetime of the coordinated Ln metal centers. Solid-state UV-visible measurements combined with density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were performed to further explore luminescent features of the Ln-Cy series and their origins.

Chemical reactions of graphitic carbon nitride films with glass surfaces and their impact on photocatalytic activity

Thin films of graphitic carbon nitride (g-CN), a visible-light-driven photocatalyst, have recently attracted interest for application in photoelectrochemical cells for water splitting and high-throughput photocatalysis. In typical syntheses, g-CN films are formed by heating the nitrogen-rich precursor and substrate to 500-600 °C. The heated substrate should affect the polycondensation of the precursor and thereby alter the properties of the g-CN film. In this paper, we demonstrate that soda-lime glass, such as commercial glass slides, modifies the chemical structure of g-CN. The terminal amino groups of g-CN are partially substituted with cyanamide and hydroxyl groups. The electron-withdrawing groups provide the energy offsets of the frontier orbitals between the modified and unmodified molecules, facilitating exciton dissociation. After alkali metals are removed, the modified g-CN film exhibits a faster photodegradation of methyl orange compared with a melon film. The simple protocol to activate a g-CN film without co-catalysts paves a new way to enhance photocatalytic activity via selections of substrates, including waste glass.

Growth and characterization of melem hydrate crystals with a hydrogen-bonded heptazine framework

In carbon nitride (CN) compounds, hydrogen bonds play a major role in cohesion, in addition to dispersion forces. The crystal structures of CN compounds produced via thermal polymerization are complex, but they possess unique and attractive features. Melem is a well-known building unit of CN compounds such as melon and g-C₃N₄, which have recently attracted attention as photocatalysts. Melem hydrate (Mh) forms hexagonal prismatic crystals that are sufficiently porous to accommodate small molecules. In this study, we grew and characterized single crystals of Mh and investigated their optical properties and hygroscopicity. By precisely adjusting the hydration conditions, we succeeded in growing a well-formed hexagonal prismatic single crystal of Mh (Mhr) with a length measuring several tens of micrometers. Furthermore, we discovered a parallelogram-shaped Mh single crystal (Mhp), which possessed a different crystal structure and optical properties from those of Mh and melem crystals. Although the crystal structure of Mh was greatly disrupted by dehydration, it exhibited hygroscopicity and could absorb moisture even in air, restoring the crystal structure of Mh. In addition, Mh demonstrated a high photoluminescence quantum yield and long lifetime delayed fluorescence, similar to melem crystal. The high quantum yield of Mh can be attributed to the effect of the strong anchoring of the melem molecule by several hydrogen bonds in the Mh crystal, since the strongly anchored molecule is less likely to undergo radiation-free deactivation due to the small displacement of atomic positions in the excited state after light absorption. The findings obtained in this study shed light not only on the application of CN compounds as photocatalysts, but also on a wider range of applications based on their optoelectronic functions.

Ab Initio Electronic Structure Study of the Photoinduced Reduction of Carbon Dioxide with the Heptazinyl Radical

The photocatalytic conversion of carbon dioxide to liquid fuels with electrons taken from water with solar photons is one of the grand goals of renewable energy research. Polymeric carbon nitrides recently emerged as metal-free materials with promising functionalities for hydrogen evolution from water as well as the activation of carbon dioxide. Molecular heptazine (Hz), the building block of polymeric carbon nitrides, is one the strongest known organic photo-oxidants and has been shown to be able to photo-oxidize water with near-visible light, resulting in reduced (hydrogenated) heptazine (HzH) and OH radicals. In the present work, we explored with ab initio computational methods whether the HzH chromophore is able to reduce carbon dioxide to the hydroxy-formyl (HOCO) radical in hydrogen-bonded HzH-CO₂ complexes by the absorption of a photon. In remarkable contrast to the high barrier for carbon dioxide activation in the electronic ground state, the excited-state proton-coupled electron transfer (PCET) reaction is nearly barrierless, but requires the diabatic passage of three conical intersections. The possibility of barrierless carbon dioxide activation by excited-state PCET has so far not been taken into consideration in the interpretation of photocatalytic carbon dioxide reduction on carbon nitride materials.

Water Oxidation and Hydrogen Evolution with Organic Photooxidants: A Theoretical Perspective

In this Perspective, we discuss a novel water-splitting scenario, namely the direct oxidation of water molecules by organic photooxidants in hydrogen-bonded chromophore-water complexes. In comparison with the established scenario of semiconductor-based water splitting, the distance of electron transfer processes is thereby reduced from mesoscopic scales to the Ångström scale, and the time scale is reduced from milliseconds to femtoseconds, which suppresses competing loss processes. The concept is illustrated by computational studies for the heptazine-H₂O complex. The excited-state landscape of this complex has been characterized with ab initio electronic-structure methods and the proton-coupled electron-transfer dynamics has been explored with nonadiabatic dynamics simulations. A unique feature of the heptazine chromophore is the existence of a low-lying and exceptionally long-lived ¹ππ* state in which a substantial part of the photon energy can be stored for hundreds of nanoseconds and is available for the oxidation of water molecules. The calculations reveal that the absorption spectra and the photochemical functionalities of heptazine chromophores can be systematically tailored by chemical substitution. The options of harvesting hydrogen and the problems posed by the high reactivity of OH radicals are discussed.

Two-dimensional biomaterials: material science, biological effect and biomedical engineering applications

To date, nanotechnology has increasingly been identified as a promising and efficient means to address a number of challenges associated with public health. In the past decade, two-dimensional (2D) biomaterials, as a unique nanoplatform with planar topology, have attracted explosive interest in various fields such as biomedicine due to their unique morphology, physicochemical properties and biological effect. Motivated by the progress of graphene in biomedicine, dozens of types of ultrathin 2D biomaterials have found versatile bio-applications, including biosensing, biomedical imaging, delivery of therapeutic agents, cancer theranostics, tissue engineering, as well as others. The effective utilization of 2D biomaterials stems from the in-depth knowledge of structure-property-bioactivity-biosafety-application-performance relationships. A comprehensive summary of 2D biomaterials for biomedicine is still lacking. In this comprehensive review, we aim to concentrate on the state-of-the-art 2D biomaterials with a particular focus on their versatile biomedical applications. In particular, we discuss the design, fabrication and functionalization of 2D biomaterials used for diverse biomedical applications based on the up-to-date progress. Furthermore, the interactions between 2D biomaterials and biological systems on the spatial-temporal scale are highlighted, which will deepen the understanding of the underlying action mechanism of 2D biomaterials aiding their design with improved functionalities. Finally, taking the bench-to-bedside as a focus, we conclude this review by proposing the current crucial issues/challenges and presenting the future development directions to advance the clinical translation of these emerging 2D biomaterials.

Donor-Acceptor Conjugated Heptazine Polymers with Highly Efficient Photocatalytic Degradations towards Tetracyclines

Photocatalysis is an efficient and green technology in the environmental protection. Due to the high charge separation and transfer, donor-acceptor (D-A) conjugated polymers attract much attention for their photocatalytic degradations towards organic pollutants. Herein, the authors reported three novel D-A conjugated polymers, named as HPBP, HPTP, and HPF, with heptazine moieties as electron acceptors, while biphenyl, terphenyl, or fluorene moieties as electron donors, respectively, which indeed exhibit a highly efficient photocatalytic degradation towards tetracyclines upon the visible-light irradiation. Among them, the photocatalytic performance of HPF is especially noticeable with the degradation rate up to 87% within 30 min, almost 11 times in comparison to those of pristine g-C₃ N₄ , which is mainly attributed to its high crystallinity and conjugation. For their photocatalytic mechanism, the •O₂ ^- radical anions are regarded as the active species.

New carbon nitride close to C6N7 with superior visible light absorption for highly efficient photocatalysis

The rational design and construction of novel two-dimensional (2D) carbon nitrides (CNs) beyond g-C₃N₄ is a hot topic in the fields of chemistry and materials. Inspired by the polymerisation of urea, we have prepared a series of novel C-C bridged heptazine CNs UO_x (where x is the ratio of urea to oxamide, x = 1, 1.5, 2, 2.5, and 3), which are similar to (C₆N₇)_n, upon the introduction of oxamide. As predicted using density functional theory (DFT) calculations, the conjugated structure of UO_x was effectively extended from an individual heptazine to the entire material. Consequently, its bandgap was reduced to 2.05 eV, and its absorption band edge was significantly extended to 600 nm. Furthermore, its carrier transfer and separation were significantly enhanced, establishing its superior photocatalytic activity. The optimised UO₂ exhibits a superior photocatalytic hydrogen production rate about 108.59 μmol h^-1 (using 10 mg of catalyst) with an apparent quantum efficiency (AQE) of 36.12% and 0.33% at 420 and 600 nm, respectively, which is one of the most active novel CNs reported to date. Moreover, UO₂ exhibits excellent photocatalytic activity toward the oxidation of diphenylhydrazine to azobenzene with conversion and selectivity reaching ~100%, which represents a promising highly efficient 2D CN material. Regarding phenols degradation, UO₂ also displayed significantly higher activity and durability during the degradation of phenol when compared to traditional g-C₃N₄, highlighting its significant potential for application in energy, environment and photocatalytic organic reactions.

Efficient Deep-Blue Electroluminescence Employing Heptazine-Based Thermally Activated Delayed Fluorescence

We report an efficient deep-blue organic light-emitting diode (OLED) based on a heptazine-based thermally activated delayed fluorescent (TADF) emitter, 2,5,8-tris(diphenylamine)-tri-s-triazine (HAP-3DPA). The deep-blue-emitting compound, HAP-3DPA, was designed and synthesized by combining the relatively rigid electron-accepting heptazine core with three electron-donating diphenylamine units. Due to the rigid molecular structure and intramolecular charge transfer characteristics, HAP-3DPA in solid state presented a high photoluminescence quantum yield of 67.0% and obvious TADF nature with a short delayed fluorescent lifetime of 1.1 μs. Most importantly, an OLED incorporating HAP-3DPA exhibited deep-blue emission with Commission Internationale de l’Eclairage (CIE) coordinates of (0.16, 0.13), a peak luminance of 10,523 cd/m−2, and a rather high external quantum efficiency of 12.5% without any light out-coupling enhancement. This finding not only reports an efficient deep-blue TADF molecule, but also presents a feasible pathway to construct high-performance deep-blue emitters and devices based on the heptazine skeleton.

Triangular boron carbon nitrides: an unexplored family of chromophores with unique properties for photocatalysis and optoelectronics

It has recently been shown that cycl[3.3.3]azine and heptazine (1,3,4,6,7,9,9b-heptaazaphenalene) as well as related azaphenalenes exhibit inverted singlet and triplet states, that is, the energy of the lowest singlet excited state (S1) is below the energy of the lowest triplet excited state (T1). This feature is unique among all known aromatic chromophores and is of outstanding relevance for applications in photocatalysis and organic optoelectronics. Heptazine is the building block of the polymeric material graphitic carbon nitride which is an extensively explored photocatalyst in hydrogen evolution photocatalysis. Derivatives of heptazine have also been identified as efficient emitters in organic light emitting diodes (OLEDs). In both areas, the inverted singlet-triplet gap of heptazine is a highly beneficial feature. In photocatalysis, the absence of a long-lived triplet state eliminates the activation of atmospheric oxygen, which is favourable for long-term operational stability. In optoelectronics, singlet-triplet inversion implies the possibility of 100% fluorescence efficiency of electron-hole recombination. However, the absorption and luminescence wavelengths of heptazine and the S1-S0 transition dipole moment are difficult to tune for optimal functionality. In this work, we employed high-level ab initio electronic structure theory to devise and characterize a large family of novel heteroaromatic chromophores, the triangular boron carbon nitrides. These novel heterocycles inherit essential spectroscopic features from heptazine, in particular the inverted singlet-triplet gap, while their absorption and luminescence spectra and transition dipole moments are widely tuneable. For applications in photocatalysis, the wavelength of the absorption maximum can be tuned to improve the overlap with the solar spectrum at the surface of earth. For applications in OLEDs, the colour of emission can be adjusted and the fluorescence yield can be enhanced.

Ionic liquid-assisted synthesis of porous boron-doped graphitic carbon nitride for photocatalytic hydrogen production

This work presents a simple way to prepare boron-doped graphitic carbon nitride (B/g-C₃N₄), exhibiting an enhanced photocatalytic performance to split water for hydrogen production. B/g-C₃N₄ was synthesized via the pyrolysis of urea and 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF₄), which was adopted as the boron source. The aggregate of B/g-C₃N₄ nanosheets shows a porous structure since some bubbles are generated under the heat decomposition of ionic liquids. The porous structure is conducive to the exposure of more active sites. Moreover, B-doping will form some localized electronic energy levels in the band gap of g-C₃N₄, thereby extending its visible light response. As impacted by the porous structure of B/g-C₃N₄ aggregate and the narrow the band gap, the photocatalytic hydrogen generation rate (901 μmol h^-1 g^-1) is increased, almost 3 times faster than g-C₃N₄ (309 μmol h^-1 g^-1). This work proposed a simple method to prepare the aggregate of B/g-C₃N₄ nanosheets exhibiting pores under ionic liquid assistance. It can be a novel method to design porous polymer photocatalysts.

Carbon nitride derivatives as photocatalysts for the CO2 reduction reaction: computational study

Photocatalytic reduction of CO₂ to hydrocarbons is considered to be a promising strategy to solve the energy crisis and environmental problems. Herein, the electronic and optical properties, and catalytic performance of g-C₃N₄ derivatives [C₆N₇(C₆H₄)_1.5]_n (systems 1 and 2), and [C₆N₇(C₁₂H₈)_1.5]_n (system 3) were studied by density functional theory (DFT) computations. Compared to g-C₃N₄ the band gaps of systems 1-3 are smaller, and the absorption intensities of the three derivatives in the visible light region increase, indicating that these derivatives can produce more electrons under visible light irradiation and enhance the photocatalytic performance. The computational results show that the main products of CO₂ reduction catalyzed by system 1 are HCOOH and CH₃OH. The rate-determining step is CO₂→ COOH* with a ΔG of 1.22 eV. Therefore, system 1 is predicted to be a promising catalyst for the CO₂ reduction reaction.

What does graphitic carbon nitride really look like?

Graphitic carbon nitrides (g-CNs) have become popular light absorbers in photocatalytic water splitting cells. Early theoretical work on these structures focused on fully polymerized g-C3N4. Experimentally, it is known that the typically employed melamine polycondensation does not go toward completion, yielding structures with ∼15 at% hydrogen. Here, we study the conformational stability of "melon", with the [C6N9H3]n structural formula using DFT. Referencing to a 2D melon sheet, B3LYP-dDsC and PBE-MBD computations revealed the same qualitative trend in stability of the 3D structures, with several of them within 5 kJ mol-1 per tecton. Fina's orthorhombic melon is the most stable of the studied conformers, with Lotsch' monoclinic melon taking an intermediate value. Invoking a simple Wannier-Mott-type approach, Fina's and Lotsch' structures exhibited the lowest optical gaps (2.8 eV), within the error margin of the experimental value (2.7 eV). All conformers yielded gaps below that of the monolayer's (3.2 eV), suggesting Jelley-type ("J") aggregation effects.

Infrared Multiple Photon Dissociation Spectroscopy of Protonated Cyameluric Acid

The present study reports the first structural characterization of protonated cyameluric acid ([CA + H]⁺) in the gas phase, which paves the way for prospective bottom-up research on the condensed-phase chemistry of CA in the protonated form. A number of [CA + H]⁺ keto-enol isomers can a priori be produced as a result of protonation at available N and O positions of precursor neutral CA tautomers, yet ab initio computations predict different reduced [CA + H]⁺ isomer populations dominating the solution and gas phases that are involved in the ion generation process (i.e., electrospray ionization). Infrared multiple photon dissociation spectra were recorded in the 990-1900 and 3300-3650 cm^-1 regions and compared with theoretical [B3LYP/6-311++G(d,p)] IR absorption spectra of several [CA + H]⁺ isomers, providing a satisfactory agreement for the most stable monohydroxy form in the gas phase, [1358a]⁺, yet the contribution of its nearly isoenergetic OH rotamer, [1358b]⁺, cannot be neglected. This is indicative of the occurrence of [CA + H]⁺ isomer interconversion reactions, assisted by protic solvent molecules, during their transfer into the gas phase. The results suggest that available O positions on neutral CA are energetically favored protonation sites in the gas phase.

State of the Art in the Preparation and Properties of Molecular Monomeric s-Heptazines: Syntheses, Characteristics, and Functional Applications

This review gives an account on the fast expanding field of monomeric (or molecular) heptazines, at the exclusion of their various polymeric forms, often referred to as carbon nitrides. While examples of monomeric heptazines were extremely limited until the beginning of this century, the field has started expanding quickly since then, as has the number of reports on polymeric materials, though previous reviews did not separate these fields. We provide here a detailed report on the synthetic procedures for molecular heptazines. We also extensively report on the different achievements realized from these new molecules, in the fields of physical chemistry, spectroscopy, materials preparation, (photo)catalysis, and devices. After a comprehensive summary and discussion on heptazines syntheses and characteristics, we show that starting from well-defined molecules allows a versatility of approaches and a wide tunability of the expected properties. It comes out that the field of monomeric heptazines is now emerging and possibly heading toward maturity, while diverging from the one of polymeric carbon nitrides. It is likely that this area of research will quickly surge to the forefront of the search for active organic molecules, with special attention to the domains of catalysis and organic-based functional materials and devices.

Density functional theory study on a nitrogen-rich carbon nitride material C3N5 as photocatalyst for CO2 reduction to C1 and C2 products

A new-type nitrogen-rich carbon nitride material C₃N₅ has been synthesized recently, in which the C:N ratio increases from 3:4 in g-C₃N₄ to 3:5 due to the introduction of azo linkage (NN) connecting segments in two C₆N₇ units. Herein, C₃N₅ as a photocatalyst for CO₂ reduction was investigated by density functional theory methods. The electronic and optical properties indicate that C₃N₅ has a longer visible-light region with 2.0 eV of band gap in comparison with g-C₃N₄. The spatial distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) show that the π network of C₃N₅ is extended by introducing -NN- linkage, which results in much higher photocatalytic efficiency than g-C₃N₄. The Gibbs free energies for possible CO₂ reaction paths on C₃N₅ were computed. The results show that CO₂ can be reduced to CH₄ with a low limiting potential of -0.54 V and to CH₃CH₂OH with a low limiting potential of -0.61 V, which all driven by solar energy. The present work is expected to provide useful guide for new-type nitrogen-rich C₃N₅ as promising photocatalyst for CO₂ reduction reaction (CO₂RR).

Biuret-A Crucial Reaction Intermediate for Understanding Urea Pyrolysis To Form Carbon Nitrides: Crystal-Structure Elucidation and In Situ Diffractometric, Vibrational and Thermal Characterisation

The crystal structure of biuret was elucidated by means of XRD analysis of single crystals grown through slow evaporation from a solution in ethanol. It crystallises in its own structure type in space group C2/c (a=15.4135(8) Å, b=6.6042(3) Å, c=9.3055(4) Å, Z=8). Biuret decomposition was studied in situ by means of temperature-programmed powder XRD and FTIR spectroscopy, to identify a co-crystalline biuret-cyanuric acid phase as a previously unrecognised reaction intermediate. Extensive thermogravimetric studies of varying crucible geometry, heating rate and initial sample mass reveal that the concentration of reactive gases at the interface to the condensed sample residues is a crucial parameter for the prevailing decomposition pathway. Taking these findings into consideration, a study on the optimisation of carbon nitride synthesis from urea on the gram scale, with standard solid-state laboratory techniques, is presented. Finally, a serendipitously encountered self-coating of the crucible inner walls by graphite during repeated synthetic cycles, which prove to be highly beneficial for the obtained yields, is reported.

Intrinsic Defects in Polymeric Carbon Nitride for Photocatalysis Applications

Introducing intrinsic defects in polymeric carbon nitride (PCN) without the addition of exotic atoms have been verified as an available strategy to boost the photocatalytic performance. This minireview focuses on the fundamental classifications and positive roles of intrinsic defects in PCN for photocatalysis applications. The intrinsic defects in PCN are classified into several types, such as nitrogen vacancy, carbon vacancy and derivative functional groups such as cyano, amino and cyanamide groups. The critical roles of these defects on the electronic configuration, charge transfer and surface properties of PCN are also carefully classified and elaborated. Furthermore, the photocatalysis applications of the defective PCN including photocatalytic water splitting, N₂ fixation, H₂ O₂ production, CO₂ reduction and NO removal are summarized. In the end, the challenges and opportunities of defect chemistry in PCN for photocatalysis field are presented.

An Infrared and Thermal Decomposition Study on Solid Deposits Originating from Heavy-Duty Diesel SCR Urea Injection Fluids

In this study, we present the effects of time and temperature on the formation of urea deposits and their composition under conditions realistic to mobile heavy-duty SCR applications. To this end, various synthesis times were evaluated (1 h, 4 h and 24 h), as well as temperatures (298–673 K). The formed urea deposits were qualitatively evaluated using ATR FTIR to elucidate their molecular composition. Furthermore, the effect of dry and moist synthesis conditions were evaluated to simulate water rich and deficient exhaust gas composition. This was achieved by conducting the synthesis in open or closed containers to simulate dry and humid conditions, respectively. The findings are presented in this paper in two maps covering dry and moisture conditions. The trend reveals that at low temperatures the deposits mainly consist of urea derived compounds, like biuret and cyanuric acid. Increasing the temperature leads to an increase in more aminated mono-triazine compounds. At the highest synthesis temperature of 673 K, the main constituents are cyamuleric compounds, consisting of fused triazine rings like melem. Humid synthesis conditions hampers the formation of highly aminated compounds up to a synthesis temperature of 573 K, even after a synthesis period of 24 h. Temperatures higher than 573 K, and a long synthesis period of 24 h, results only in minor differences and are observed between samples prepared under dry or humid conditions. The decomposition properties of the synthesized materials are optimal for the samples prepared at 523 K, whereas the lowest decomposition rates were observed for samples prepared at 623 K and 673 K. A humid air gas flow was shown to be beneficial for the decomposition rate under these conditions.

Elucidating the structure of the graphitic carbon nitride nanomaterials via X-ray photoelectron spectroscopy and X-ray powder diffraction techniques

By using the most popular method of thermal condensation of dicyandiamide in a semi-closed system, graphitic carbon nitrides (gCNs) were synthesized at 500, 550, and 600 °C. The resulting materials were comprehensively analyzed via X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRD)techniques. We show that the use of routine analytical methods provides an insight into the structure of the carbon nitride materials. The analysis of geometric linear structures and fully condensed structure of polymeric carbon nitrides was performed and the ranges within which the contents of different nitrogen species (pyridine, amine, imine and quaternary nitrogen) can change were determined. This analysis, in combination with quantitative XPS studies, permits to state that the carbon nitride structure prepared by the thermal condensation of dicyandiamide is closer to the structure in which poly(aminoimino)heptazine subunits are linked into chains rather than the structure involving fully-condensed polyheptazine network. The XRD analysis proved that the 3D crystal structure of carbon nitride is described more correctly by the orthorhombic cell and space group P21212 applied to condensed chains of poly(aminoimino)heptazine (melon) and not by the hexagonal cell with the space group P6m2.

A straightforward synthesis of a new family of molecules: 2,5,8-trialkoxyheptazines. Application to photoredox catalyzed transformations

We have prepared several 2,5,8-trialkoxyheptazines starting from the soluble precursor 2,5,8-tris(3,5-diethylpyrazolyl)-heptazine. We present their synthesis along with their promising spectroscopic and electrochemical properties, which demonstrate large band gaps and high reduction potentials altogether. Subsequently, we provide a short assessment of the promising ability of one of these molecules to perform catalytic oxidation test-reactions.

Revisiting the Limiting Factors for Overall Water-Splitting on Organic Photocatalysts

In pursuit of inexpensive and earth abundant photocatalysts for solar hydrogen production from water, conjugated polymers have shown potential to be a viable alternative to widely used inorganic counterparts. The photocatalytic performance of polymeric photocatalysts, however, is very poor in comparison to that of inorganic photocatalysts. Most of the organic photocatalysts are active in hydrogen production only when a sacrificial electron donor (SED) is added into the solution, and their high performances often rely on presence of noble metal co-catalyst (e.g. Pt). For pursuing a carbon neutral and cost-effective green hydrogen production, unassisted hydrogen production solely from water is one of the critical requirements to translate a mere bench-top research interest into the real world applications. Although this is a generic problem for both inorganic and organic types of photocatalysts, organic photocatalysts are mostly investigated in the half-reaction, and have so far shown limited success in hydrogen production from overall water-splitting. To make progress, this article exclusively discusses critical factors that are limiting the overall water-splitting in organic photocatalysts. Additionally, we also have extended the discussion to issues related to stability, accurate reporting of the hydrogen production as well as challenges to be resolved to reach 10 % STH (solar-to-hydrogen) conversion efficiency.

Characterization and Electronic Properties of Heptazine Layers: Towards Promising Interfacial Materials for Organic Optoelectronics

For the first time, an original compound belonging to the heptazine family has been deposited in the form of thin layers, both by thermal evaporation under vacuum and spin-coating techniques. In both cases, smooth and homogeneous layers have been obtained, and their properties evaluated for eventual applications in the field of organic electronics. The layers have been fully characterized by several concordant techniques, namely UV-visible spectroscopy, steady-state and transient fluorescence in the solid-state, as well as topographic and conductive atomic force microscopy (AFM) used in Kelvin probe force mode (KPFM). Consequently, the afferent energy levels, including Fermi level, have been determined, and show that these new heptazines are promising materials for tailoring the electronic properties of interfaces associated with printed electronic devices. A test experiment showing an improved electron transfer rate from a tris-(8-hydroxyquinoline) aluminum (Alq3) photo-active layer in presence of a heptazine interlayer is finally presented.