The photoredox activation of organic substrates with visible light is a powerful methodology that generates reactive radical species under very mild conditions. When combined with another catalytic process in a dual catalytic system, novel, visible‐light‐promoted transformations have been realized that do not proceed using either catalyst in isolation. In this minireview, the state of the art in organic reactions mediated by dual catalytic systems merging photoredox activation with organo‐, acid or metal catalysis is discussed. De(light)ful catalysis! The merger of photoredox catalysis with another catalytic mode can result in novel, visible‐light‐promoted reactions that do not proceed by using either catalyst independently. Herein, the different ways that two catalytic modes can operate in tandem are highlighted, focusing on dual‐catalyzed organic processes that merge photoredox with organo‐, acid, and transition‐metal catalysis.
Na‐ion batteries are an attractive alternative to Li‐ion batteries for large‐scale energy storage systems because of their low cost and the abundant Na resources. This Review provides a comprehensive overview of selected anode materials with high reversible capacities that can increase the energy density of Na‐ion batteries. Moreover, we discuss the reaction and failure mechanisms of those anode materials with a view to suggesting promising strategies for improving their electrochemical performance. Anode materials with high reversible capacities that can increase the energy density of Na‐ion batteries are the focus of this Minireview. Moreover, the reaction and failure mechanisms of those anode materials are discussed with the aim of targeting promising strategies for improving their electrochemical performance.
This article aims to show the identity of “circularly polarized luminescent active simple organic molecules” as a new concept in organic chemistry due to the potential interest of these molecules, as availed by the exponentially growing number of research articles related to them. In particular, it describes and highlights the interest and difficulty in developing chiral simple (small and non‐aggregated) organic molecules able to emit left‐ or right‐circularly polarized light efficiently, the efforts realized up to now to reach this challenging objective, and the most significant milestones achieved to date. General guidelines for the preparation of these interesting molecules are also presented. Will it be possible to generate circularly polarized luminescence (CPL) efficiently from simple organic molecules (SOM) in solution? This Concept article highlights the interest of this difficult question, the efforts realized up to now to solve it, and the most significant milestones achieved to date. The article postulates the important role that molecular designs based on helical structures could play in overcoming limiting factors.
A facile, economic and green one‐step hydrothermal synthesis route using dopamine as source towards photoluminescent carbon nanoparticles (CNPs) is proposed. The as‐prepared CNPs have an average size about 3.8 nm. The emission spectra of the CNPs are broad, ranging from approximately 380 (purple) to approximately 525 nm (green), depending on the excitation wavelengths. Due to the favorable optical properties, the CNPs can readily enter into A549 cells and has been used for multicolor biolabeling and bioimaging. Most importantly, the as‐prepared CNPs contain distinctive catechol groups on their surfaces. Due to the special response of catechol groups to Fe3+ ions, we further demonstrate that such wholly new CNPs can serve as a very effective fluorescent sensing platform for label‐free sensitive and selective detection of Fe3+ ions and dopamine with a detection limit as low as 0.32 μM and 68 nM, respectively. The new “mix‐and‐detect” strategy is simple, green, and exhibits high sensitivity and selectivity. The present method was also applied to the determination of Fe3+ ions in real water samples and dopamine in human urine and serum samples successfully. Mix and detect makes sense: A new preparative route toward distinctive fluorescent carbon nanoparticles (CNPs) by using dopamine (see figure) is described. Such CNPs can be used for multicolor bioimaging and also serve as a very effective fluorescent sensors for label‐free sensitive and selective detection of Fe3+ ions and dopamine with a detection limit as low as 0.32 μM and 68 nM, respectively. This work may provide new insights into the design of CNPs and their application in bioimaging and sensing.
The equilibrium association free enthalpies ΔGa for typical supramolecular complexes in solution are calculated by ab initio quantum chemical methods. Ten neutral and three positively charged complexes with experimental ΔGa values in the range 0 to −21 kcal mol−1 (on average −6 kcal mol−1) are investigated. The theoretical approach employs a (nondynamic) single‐structure model, but computes the various energy terms accurately without any special empirical adjustments. Dispersion corrected density functional theory (DFT‐D3) with extended basis sets (triple‐ζ and quadruple‐ζ quality) is used to determine structures and gas‐phase interaction energies (ΔE), the COSMO‐RS continuum solvation model (based on DFT data) provides solvation free enthalpies and the remaining ro‐vibrational enthalpic/entropic contributions are obtained from harmonic frequency calculations. Low‐lying vibrational modes are treated by a free‐rotor approximation. The accurate account of London dispersion interactions is mandatory with contributions in the range −5 to −60 kcal mol−1 (up to 200 % of ΔE). Inclusion of three‐body dispersion effects improves the results considerably. A semilocal (TPSS) and a hybrid density functional (PW6B95) have been tested. Although the ΔGa values result as a sum of individually large terms with opposite sign (ΔE vs. solvation and entropy change), the approach provides unprecedented accuracy for ΔGa values with errors of only 2 kcal mol−1 on average. Relative affinities for different guests inside the same host are always obtained correctly. The procedure is suggested as a predictive tool in supramolecular chemistry and can be applied routinely to semirigid systems with 300–400 atoms. The various contributions to binding and enthalpy–entropy compensations are discussed. Unprecedented accuracy for computed association free enthalpies of supramolecular host–guest complexes (some examples are shown here) in solution has been achieved by a combination of high‐level quantum chemical procedures. The approach is quite general, includes all basic physical effects quantitatively and requires no special empirical adjustments.
Aromatic substrates with oxygen‐ and nitrogen‐containing substituents undergo oxidative coupling with alkynes and alkenes under rhodium catalysis through regioselective CH bond cleavage. Coordination of the substituents to the rhodium center is the key to activate the CH bonds effectively. Various fused‐ring systems can be constructed through these reactions. Controlled fusion: Aromatic substrates with oxygen‐ and nitrogen‐containing substituents undergo oxidative coupling with alkynes and alkenes under rhodium catalysis through regioselective CH bond cleavage. Coordination of the substituents to the rhodium center is the key to activate the CH bonds effectively. Various fused‐ring systems can be constructed through these reactions.
Research on natural products containing hexahydropyrrolo[2,3‐b]indole (HPI) has dramatically increased during the past few years. Newly discovered natural products with complex structures and important biological activities have recently been isolated and synthesized. This review summarizes the structures, biological activities, and synthetic routes for natural compounds containing HPI, emphasizing the different strategies for assembling this motif. It covers a broad range of molecules, from small alkaloids to complex peptides. Hippee or Hippic: Nature is replete with compounds containing either a hexahydropyrrolo[2,3‐b]indole (HPI) unit, or the corresponding 2‐carboxylate or 2‐carboxamide (both abbreviated HPIC). This review summarizes the structures, biological activities, and synthetic routes towards compounds containing HPI and HPIC units.
The concept of using amide bond distortion to modulate amidic resonance has been known for more than 75 years. Two classic twisted amides (bridged lactams) ingeniously designed and synthesized by Kirby and Stoltz to feature fully perpendicular amide bonds, and as a consequence emanate amino‐ketone‐like reactivity, are now routinely recognized in all organic chemistry textbooks. However, only recently the use of amide bond twist (distortion) has advanced to the general organic chemistry mainstream enabling a host of highly attractive N−C amide bond cross‐coupling reactions of broad synthetic relevance. In this Minireview, we discuss recent progress in this area and present a detailed overview of the prominent role of amide bond destabilization as a driving force in the development of transition‐metal‐catalyzed cross‐coupling reactions by N−C bond activation. Twist of faith! Recently the use of amide bond twist (distortion) has advanced to the general organic chemistry mainstream enabling a host of highly attractive N−C amide bond cross‐coupling reactions of broad synthetic relevance. In this Minireview, recent progress in this area is discussed and a detailed overview of the prominent role of amide bond destabilization as a driving force in the development of transition‐metal‐catalyzed cross‐coupling reactions by N−C amide bond activation is presented.
We present an investigation on the influence of benzoic acid, acetic acid, and water on the syntheses of the Zr‐based metal–organic frameworks Zr–bdc (UiO‐66), Zr–bdc–NH2 (UiO‐66–NH2), Zr–bpdc (UiO‐67), and Zr–tpdc–NH2 (UiO‐68–NH2) (H2bdc: terephthalic acid, H2bpdc: biphenyl‐4,4′‐dicarboxylic acid, H2tpdc: terphenyl‐4,4′′‐dicarboxylic acid). By varying the amount of benzoic or acetic acid, the synthesis of Zr–bdc can be modulated. With increasing concentration of the modulator, the products change from intergrown to individual crystals, the size of which can be tuned. Addition of benzoic acid also affects the size and morphology of Zr–bpdc and, additionally, makes the synthesis of Zr–bpdc highly reproducible. The control of crystal and particle size is proven by powder XRD, SEM and dynamic light scattering (DLS) measurements. Thermogravimetric analysis (TGA) and Ar sorption experiments show that the materials from modulated syntheses can be activated and that they exhibit high specific surface areas. Water proved to be essential for the formation of well‐ordered Zr–bdc–NH2. Zr–tpdc–NH2, a material with a structure analogous to that of Zr–bdc and Zr–bpdc, but with the longer, functionalized linker 2′‐amino‐1,1′:4′,1′′‐terphenyl‐4,4′′‐dicarboxylic acid, was obtained as single crystals. This allowed the first single‐crystal structural analysis of a Zr‐based metal–organic framework. Modulated crystallinity: Nanocrystals of varying sizes of UiO‐66 (Zr–bdc metal–organic framework (MOF)), individual microcrystals of UiO‐67 (Zr–bpdc MOF) and the first single crystal of a Zr‐based MOF containing a long linker (Zr–tpdc–NH2 MOF) were obtained (see figure). The last one allowed, for the first time, a single‐crystal diffraction study of a Zr‐based MOF. (H2bdc: terephthalic acid, H2bpdc: biphenyl‐4,4′‐dicarboxylic acid, H2tpdc: terphenyl‐4,4′′‐dicarboxylic acid.)
Nanoporous carbons (NPCs) have large specific surface areas, good electrical and thermal conductivity, and both chemical and mechanical stability, which facilitate their use in energy storage device applications. In the present study, highly graphitized NPCs are synthesized by one‐step direct carbonization of cobalt‐containing zeolitic imidazolate framework‐67 (ZIF‐67). After chemical etching, the deposited Co content can be completely removed to prepare pure NPCs with high specific surface area, large pore volume, and intrinsic electrical conductivity (high content of sp2‐bonded carbons). A detailed electrochemical study is performed using cyclic voltammetry and galvanostatic charge–discharge measurements. Our NPC is very promising for efficient electrodes for high‐performance supercapacitor applications. A maximum specific capacitance of 238 F g−1 is observed at a scan rate of 20 mV s−1. This value is very high compared to previous works on carbon‐based electric double layer capacitors. Highly graphitized nanoporous carbons (NPCs) are synthesized by one‐step direct carbonization of cobalt‐containing zeolitic imidazolate framework‐67 (ZIF‐67). After chemical etching, the deposited Co nanoparticles are completely removed to prepare pure NPCs with high specific surface area, large pore volume, and intrinsic electrical conductivity. The specific energy of the NPC‐based supercapacitor reached 19.6 W h kg−1 at a specific power of 700 W kg−1 (see figure).
Halogen bonding is a noncovalent interaction similar to hydrogen bonding, which is based on electrophilic halogen substituents. Hydrogen‐bonding‐based organocatalysis is a well‐established strategy which has found numerous applications in recent years. In light of this, halogen bonding has recently been introduced as a key interaction for the design of activators or organocatalysts that is complementary to hydrogen bonding. This Concept features a discussion on the history and electronic origin of halogen bonding, summarizes all relevant examples of its application in organocatalysis, and provides an overview on the use of cationic or polyfluorinated halogen‐bond donors in halide ion reactions or in the activation of neutral organic substrates. Catalytic bonds: Halogen bonding is a noncovalent interaction similar to hydrogen bonding, which is based on electrophilic halogen substituents. Recently, halogen bonding has been introduced as a key interaction for the design of activators or organocatalysts. This Concept provides an overview on the use of cationic or polyfluorinated halogen‐bond donors in halide ion reactions or in the activation of neutral organic substrates.
Continuous‐flow photochemistry is used increasingly by researchers in academia and industry to facilitate photochemical processes and their subsequent scale‐up. However, without detailed knowledge concerning the engineering aspects of photochemistry, it can be quite challenging to develop a suitable photochemical microreactor for a given reaction. In this review, we provide an up‐to‐date overview of both technological and chemical aspects associated with photochemical processes in microreactors. Important design considerations, such as light sources, material selection, and solvent constraints are discussed. In addition, a detailed description of photon and mass‐transfer phenomena in microreactors is made and fundamental principles are deduced for making a judicious choice for a suitable photomicroreactor. The advantages of microreactor technology for photochemistry are described for UV and visible‐light driven photochemical processes and are compared with their batch counterparts. In addition, different scale‐up strategies and limitations of continuous‐flow microreactors are discussed. Light up your chemistry! Continuous‐flow photochemistry is used increasingly by researchers in academia and industry to facilitate photochemical processes and their subsequent scale‐up. This Review provides an up‐to‐date overview of both technological and chemical aspects associated with photochemical processes in microreactors.
An advanced supercapacitor material based on nitrogen‐doped porous graphitic carbon (NPGC) with high a surface area was synthesized by means of a simple coordination–pyrolysis combination process, in which tetraethyl orthosilicate (TEOS), nickel nitrate, and glucose were adopted as porogent, graphitic catalyst precursor, and carbon source, respectively. In addition, melamine was selected as a nitrogen source owing to its nitrogen‐enriched structure and the strong interaction between the amine groups and the glucose unit. A low‐temperature treatment resulted in the formation of a NPGC precursor by combination of the catalytic precursor, hydrolyzed TEOS, and the melamine–glucose unit. Following pyrolysis and removal of the catalyst and porogent, the NPGC material showed excellent electrical conductivity owing to its high crystallinity, a large Brunauer–Emmett–Teller surface area (SBET=1027 m2 g−1), and a high nitrogen level (7.72 wt %). The unusual microstructure of NPGC materials could provide electrochemical energy storage. The NPGC material, without the need for any conductive additives, showed excellent capacitive behavior (293 F g−1 at 1 A g−1), long‐term cycling stability, and high coulombic efficiency (>99.9 % over 5000 cycles) in KOH when used as an electrode. Notably, in a two‐electrode symmetric supercapacitor, NPGC energy densities as high as 8.1 and 47.5 Wh kg−1, at a high power density (10.5 kW kg−1), were achieved in 6 M KOH and 1 M Et4NBF4‐PC electrolytes, respectively. Thus, the synthesized NPGC material could be a highly promising electrode material for advanced supercapacitors and other conversion devices. Energy storage made easy: Nitrogen‐doped porous graphitic carbon (NPGC) materials with large surface areas (1027 m2 g−1) and high nitrogen levels (7.72 wt %) were produced by a robust coordination–pyrolysis process (see figure). The prepared NPGC materials exhibit excellent capacitive behavior, superior cycling stability, and high energy and power densities as electrode material for advanced supercapacitors.
The doping of carbon quantum dots with nitrogen provides a promising direction to improve fluorescence performance and broaden their applications in sensing systems. Herein we report a one‐pot solvothermal synthesis of N‐doped carbon quantum dots (NCQDs) and the synthesis of a series of NCQDs with different nitrogen contents. The as‐prepared NCQDs were compared with carbon quantum dots (CQDs); the introduction of nitrogen atoms largely increased the quantum yield of NCQDs and highest emission efficiency is up to 36.3 %. The fluorescence enhancement may originate from more polyaromatic structures induced by incorporated nitrogen atoms and protonation of nitrogen atoms on dots. It was found that NCQDs can act as a multifunctional fluorescence sensing platform because they can be used to detect pH values, AgI, and FeIII in aqueous solution. The fluorescence intensity of NCQDs is inversely proportional to pH values across a broad range from 5.0 to 13.5, which indicates that NCQDs can be devised as an effective pH indicator. Selective detection of AgI and FeIII was achieved based on their distinctive fluorescence influence because AgI can significantly enhance the fluorescence whereas FeIII can greatly quench the fluorescence. The quantitative determination of AgI can be accomplished with NCQDs by using the linear relationship between fluorescence intensity of NCQDs and concentration of AgI. The sensitive detection of H2O2 was developed by taking advantage of the distinct quenching ability of FeIII and FeII toward the fluorescence of NCQDs. Cellular toxicity test showed NCQDs still retain low toxicity to cells despite the introduction of a great deal of nitrogen atoms. Moreover, bioimaging experiments demonstrated that NCQDs have stronger resistance to photobleaching than CQDs and more excellent fluorescence labeling performance. Going dotty! The doping of nitrogen into carbon nanodots can not only greatly improve their fluorescence performance, but also acts as an active site to selectively and sensitively detect pH values, AgI, FeIII, and H2O2 in aqueous solution (see figure).
Lithium–sulfur batteries are among the most promising electrochemical energy storage devices of the near future. Especially the low price and abundant availability of sulfur as the cathode material and the high theoretical capacity in comparison to state‐of‐the art lithium‐ion technologies are attractive features. Despite significant research achievements that have been made over the last years, fundamental (electro‐) chemical questions still remain unanswered. This review addresses ten crucial questions associated with lithium–sulfur batteries and critically evaluates current research with respect to them. The sulfur–carbon composite cathode is a particular focus, but its complex interplay with other hardware components in the cell, such as the electrolyte and the anode, necessitates a critical discussion of other cell components. Modern in situ characterisation methods are ideally suited to illuminate the role of each component. This article does not pretend to summarise all recently published data, but instead is a critical overview over lithium–sulfur batteries based on recent research findings. Promising storage: This review addresses ten crucial questions associated with lithium–sulfur batteries and critically evaluates current research with respect to them. The sulfur–carbon composite cathode is in the particular focus, but its complex interplay with other hardware components, such as the electrolyte and the anode, necessitates a critical discussion as well.
The recent advent of transition‐metal mediated CH activation is revolutionizing the synthetic field and gradually infusing a “CH activation mind‐set” in both students and practitioners of organic synthesis. As a powerful testament of this emerging synthetic tool, applications of CH activation in the context of total synthesis of complex natural products are beginning to blossom. Herein, recently completed total syntheses showcasing creative and ingenious incorporation of CH activation as a strategic manoeuver are compared with their “non‐CH activation” counterparts, illuminating a new paradigm in strategic synthetic design. The total synthesis of natural products has been carried out showcasing the creative and ingenious application of CH activation in the construction of complex molecular architectures that are compared with their “non‐CH activation” counterparts. These accomplishments stood as true testaments of the growing potential of CH activation, and further cementing the power of this emerging synthetic technology (see figure).
Methylammonium lead halide (MAPbX3) perovskites exhibit exceptional carrier transport properties. But their commercial deployment as solar absorbers is currently limited by their intrinsic instability in the presence of humidity and their lead content. Guided by our theoretical predictions, we explored the potential of methylammonium bismuth iodide (MBI) as a solar absorber through detailed materials characterization. We synthesized phase‐pure MBI by solution and vapor processing. In contrast to MAPbX3, MBI is air stable, forming a surface layer that does not increase the recombination rate. We found that MBI luminesces at room temperature, with the vapor‐processed films exhibiting superior photoluminescence (PL) decay times that are promising for photovoltaic applications. The thermodynamic, electronic, and structural features of MBI that are amenable to these properties are also present in other hybrid ternary bismuth halide compounds. Through MBI, we demonstrate a lead‐free and stable alternative to MAPbX3 that has a similar electronic structure and nanosecond lifetimes. Call me MA‐Bi‐I! A lead‐free alternative to hybrid lead perovskites is explored for solar cells: methylammonium bismuth iodide (see figure). This material exhibits higher air stability than hybrid lead perovskites, while demonstrating optoelectronic properties promising for solar absorbers.
Three new electron‐rich metal–organic frameworks (MOF‐1–MOF‐3) have been synthesized by employing ligands bearing aromatic tags. The key role of the chosen aromatic tags is to enhance the π‐electron density of the luminescent MOFs. Single‐crystal X‐ray structures have revealed that these MOFs form three‐dimensional porous networks with the aromatic tags projecting inwardly into the pores. These highly luminescent electron‐rich MOFs have been successfully utilized for the detection of explosive nitroaromatic compounds (NACs) on the basis of fluorescence quenching. Although all of the prepared MOFs can serve as sensors for NACs, MOF‐1 and MOF‐2 exhibit superior sensitivity towards 4‐nitrotoluene (4‐NT) and 2,4‐dinitrotoluene (DNT) compared to 2,4,6‐trinitrotoluene (TNT) and 1,3,5‐trinitrobenzene (TNB). MOF‐3, on the other hand, shows an order of sensitivity in accordance with the electron deficiencies of the substrates. To understand such anomalous behavior, we have thoroughly analyzed both the steady‐state and time‐resolved fluorescence quenching associated with these interactions. Determination of static Stern–Volmer constants (KS) as well as collisional constants (KC) has revealed that MOF‐1 and MOF‐2 have higher KS values with 4‐NT than with TNT, whereas for MOF‐3 the reverse order is observed. This apparently anomalous phenomenon was well corroborated by theoretical calculations. Moreover, recyclability and sensitivity studies have revealed that these MOFs can be reused several times and that their sensitivities towards TNT solution are at the parts per billion (ppb) level. Competing size and electronic effects: Luminescent Zn‐MOFs can be prepared by employing π‐electron‐rich isophthalic acid derivatives bearing fluorescent tags. These MOFs can be used for sensing nitroaromatic explosives on the basis of fluorescence quenching (see graphic; 4‐NT=4‐nitrotoluene; TNT=trinitrotoluene). For the more porous MOFs, the fluorescence quenching is dictated by size selection. For MOFs in which the pores are blocked by pyrenyl tags, the fluorescence quenching thus occurs on the surface and conforms to the electronic properties of the analytes.
In this minireview, squaramides are presented from their roots as artificial anion receptors in molecular recognition studies to their recent advances as powerful bifunctional hydrogen‐bonding catalysts in asymmetric organocatalysis. The main features of the squaramido functionality and the direct comparison with the analogous ureas and thioureas are also discussed. New dimensions for squaramides: In this minireview, squaramides are presented from their roots as artificial anion receptors in molecular recognition studies to the new advances in their application in asymmetric organocatalysis as powerful bifunctional hydrogen‐bonding catalysts. The main features of the squaramido functionality and direct comparison with analogous ureas and thioureas are also discussed.
Tremendous development in the field of portable electronics and hybrid electric vehicles has led to urgent and increasing demand in the field of high‐energy storage devices. In recent years, many research efforts have been made for the development of more efficient energy‐storage devices such as supercapacitors, batteries, and fuel cells. In particular, supercapacitors have great potential to meet the demands of both high energy density and power density in many advanced technologies. For the last half decade, graphene has attracted intense research interest for electrical double‐layer capacitor (EDLC) applications. The unique electronic, thermal, mechanical, and chemical characteristics of graphene, along with the intrinsic benefits of a carbon material, make it a promising candidate for supercapacitor applications. This Review focuses on recent research developments in graphene‐based supercapacitors, including doped graphene, activated graphene, graphene/metal oxide composites, graphene/polymer composites, and graphene‐based asymmetric supercapacitors. The challenges and prospects of graphene‐based supercapacitors are also discussed. Charged up: This Review focuses on recent research developments in graphene‐based supercapacitors, including doped graphene, activated graphene, graphene/metal oxide composites, graphene/polymer composites, and graphene‐based asymmetric supercapacitors. The challenges and prospects of graphene‐based supercapacitors are also discussed.