Electron-withdrawing Properties Research Articles

Single-Atom Doped Fullerene (MN4-C54) as Bifunctional Catalysts for the Oxygen Reduction and Oxygen Evolution Reactions.

Development of high-performance oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts is crucial to realizing the electrolytic water cycle. C60 is an ideal substrate material for single atom catalysts (SACs) due to its unique electron-withdrawing properties and spherical structure. In this work, we screened for a novel single-atom catalyst based on C60, which anchored transition metal atoms in the C60 molecule by coordination with N atoms. Through first-principles calculations, we evaluated the stability and activity of MN4-C54 (M = Fe, Co, Ni, Cu, Rh, Ru, Pd, Ag, Pt, Ir, Au). The results indicate that CuN4-C54, which is based only on earth-abundant elements, exhibited low overpotentials of 0.46 and 0.47 V for the OER and ORR, respectively, and was considered a promising bifunctional catalyst, showing better performance than the noble-metal ones. In addition, according to the linear relationship of intermediates, we established volcano plots to describe the activity trends of the OER and ORR on MN4-C54. Finally, d-band center and crystal orbital Hamiltonian populations methods were used to explain the catalytic origin. Suitable d-band centers lead to moderate adsorption strength, further leading to good catalytic performances.

Leap from Diradicals to Tetraradicals by Topological Control of π-Conjugation.

In this work, we explore the series of diradical(oid)s based on 2,2'-(5,11-dihydroindolo[3,2-b]carbazole-3,9-diyl)dimalononitrile (further referred to as PH). Hydrogen atoms in the central benzenoid (CB) ring of PH are substituted by the series of substituents with various lengths of π-conjugated chain and electron-donating or electron-withdrawing properties to study how they modulate the diradical character of the parent compound. The diradical character of molecules increases up to 88-89% by two groups doubly bonded to both sides of the CB ring of PH in para relative positions. This breaks the direct π-conjugation between unpaired electrons that gives rise to two radical centers and restricts the minimal polyradical identity of the compound to diradical. We show that diradicals and tetraradicals can be designed, and their polyradical character can be modulated by controlling the topology of π-conjugation as long as there is sufficient aromatic stabilization. Henceforth, the bridge between diradicals and tetraradicals is established, leading to the tetraradical(oid) molecule, which has been predicted to have narrow low-spin to high-spin energy gaps in our recent Letter.

The effect of fluorinated conjugated side chains on the photovoltaic performance of polymers based on benzodithiophene (BDT) and carbazolobistriazole (CTA)

Reasonable regulation of the structure-performance relationship is extremely crucial to enhancing the device performance of organic solar cells (OSCs). As a novel fused-benzotriazole (BTA) electron-accepting (A) unit, carbazolobistriazole (CTA) has excellent luminescent properties, and the alkyl chains on the three N atoms allow flexible modulation of polymer solubility. However, higher molecular energy levels and weak crystallinity restrict the enhancement of device performance. Herein, a new donor polymer PE97 with CTA as A unit and 4,8-bis(4-(2-ethylhexyl)-3,5-difluorophenyl)benzo[1,2-b:4,5-b’]dithiophene (BDT-P2F) as electron-donating (D) unit was designed and synthesized. In comparison with the first CTA-based polymer PE93 with 4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene (BDT-TF) as D unit, PE97 realizes stronger molecular stacking conducive to modulation of the active layer morphology and a deeper highest occupied molecular orbital (HOMO) level, which are favorable to increase short-circuit current density (JSC) and open-circuit voltage (VOC) of OSCs. When paired with Y-series non-fullerene acceptor eC9-2F, PE97-based OSC devices have achieved a higher power conversion efficiency (PCE) of 15.5 %, with a VOC of 0.81 V, a JSC of 25.3 mA cm−2, and a fill factor (FF) of 75.7 %, which figures significantly surpass efficiency (13.6 %) of PE93-based devices. These results indicate that the BDT-P2F unit can excellently modulate the optoelectronic properties of CTA-based polymers. This highlights the potential of the BDT-P2F as a promising D unit for polymers with weak crystallinity and limited electron-withdrawing properties.

Boosting photocatalytic performance of polymeric carbon nitride by adjusting its donor-acceptor structure via functional group modification strategy

Solar-driven photodegradation of pollutant is attractive for environmental remediation. Herein, we designed and synthetized a new kind of group-modified polymeric carbon nitride (PCN) photocatalyst with urea and 4-Nitro-o-phenylenediamine by one-pot method and applied to degrade bisphenol A (BPA) in aqueous solution. The light response range of photocatalyst had been extended a lot due to conjugation and electron-withdrawing properties of nitrobenzene. Physical analysis shows that 4-Nitro-o-phenylenediamine grafting brings an improved charge separation capacity. EPR and DFT results demonstrate the charge separation is significantly affected by the donor-acceptor structure of PCN, which can be altered via aromatic electron-withdrawing group. The kinetic constant of photocatalytic degradation for BPA was promoted by 8.8-times greater than unmodified PCN and a good recyclability was achieved. To verify the universality of group modification strategies, we prepared other two kinds of photocatalysts via electron-withdrawing group modification strategy and their photocatalytic performance all had been improved obviously.

Weakened functional group activity enables the uniform distribution of the gel electrolyte to achieve the high-performance of Li-ion batteries

Electrolyte gelation is considered to be one of the main routes to solve the safety problem of liquid electrolytes. However, the distribution of gel polymer electrolyte (GPE) on the surface of electrodes and the mechanism of their effect on the performance of battery are still unknown. Here, methyl methacrylate (MMA) containing methyl (-CH3) and methyl-2-cyanoacrylate (MCA) containing cyanide (-CN) are employed as representative monomers to explore the relationship between gel distribution and battery performance. Due to the stronger electron-withdrawing properties, PMCA gel has shorter chain length and greater shrinkage, showing many cracks on the electrode surface, while PMMA gel can evenly cover the surfaces of electrodes. As a result, the capacity retention rate of 1.4 Ah NCM811/Gr pouch cells with PMMA is 93.5% for 500 cycles at 25 °C and 91.5% for 600 cycles at 60 °C, which these of the cells with PMCA are 58.8% for 266 cycles at 25 °C and 69.1% for 327 cycles at 60 °C. XPS analysis of the electrode sheets before and after cycling reveal that the PMCA-electrode has a large number of rzero valent lithium element precipitation, whereas the PMMA-electrode has the more stable interface film. This study indicates that the uniform distribution of gel electrolyte with -CH3 functional group on the electrode surface can improve the electrochemical performance of NCM811/Gr battery, which has the guiding significance.

Highly and Deep Red-luminescent Bisphenylamine-appended Benzocarcogendiazole Fluorophores.

Benzocarcogendiazole units have been frequently utilized for optoelectronics such as organic solar cells because of their robustness, rigidity,and band-gap tunability based on the strong electron-withdrawing properties. Focusing on the luminescent characteristics, these molecules have been utilized to demonstrate highly sensitive chromisms because of the potential of charge transfer. Here, we demonstrate deep red-emissions inbis(4-tert-butylphenyl)amine-appended benzocarcogendiazole-based donor-acceptor-donor (D-A-D) fluorophores, namely1and2. Because benzocarcogendiazole andbis(4-tert-butylphenyl)amineserve as strong electron acceptor and donor, respectively, strong intramolecular charge transfer (ICT) enables long wavelength of photoluminescence (PL) even in the small molecular weight. Although photoluminescence (PL) in long wavelength tends to exhibit quite low PL quantum efficiency (ΦPL), the values of solutions1and2are quite high (up to 50 %). According to X-ray crystallographic characterizations and DFT calculations, these highΦPLvalues are attributable to the segregatedπ-planes of benzocarcogendiazole units, which is induced by the bulky substituents ofbis(4-tert-butylphenyl)amines.

Integrated Cyano Groups into the Skeleton of Conjugated Polymers to Activate Molecular Oxygen for Boosting Photocatalytic H2O2 Efficiency.

Conjugated polymers (CPs) have shown promising potential in the field of hydrogen peroxide (H2O2) photosynthesis. However, a deeper understanding of the interactions between building units and specific functional groups within the molecular skeleton is necessary to elucidate the mechanisms driving H2O2 generation. Herein, a series of typical donor-acceptor (D-A) conjugated polymers (B-B, B-CN, B-DCN) were synthesized by introducing different amounts of cyano groups (-CN) into the molecular skeleton. The strong electron withdrawing properties of cyano can greatly promote the effective separation and transfer of photogenerated charges between building units, resulting in an impressive efficiency of H2O2 generation (2128.5 μmol g-1 h-1) for B-DCN, representing a 96-fold enhancement compared to B-B. More importantly, experimental results and theoretical calculations further revealed that the introduction of -CN can markedly reduce the adsorption energy (Ead) of O2, while serving as an active site to induce the conversion of crucial intermediate superoxide anions (.O2-) into singlet oxygen (1O2), achieving dual-channel H2O2 generation (O2→.O2-→H2O2, O2→.O2-→1O2→H2O2). This work provides valuable insights into the design of efficient H2O2 photosynthesis materials.

The electron-withdrawing properties of butanesultone-based polymer electrolytes facilitate stable operation at 4.6 V for solid-state lithium batteries with NCM811 cathodes

Polymer electrolyte (PEs) based solid-state lithium metal batteries (SSLMBs) featuring LiNi0.8Co0.1Mn0.1O2 (NCM811) cathodes offer high energy density and safety. However, their development is hampered by the poor anti-oxidation ability and severe interfacial reactions between the PEs and NCM811 cathode. Herein, the butanesultone-based polymer electrolytes (PBS-PEs) were synthesized via ring-opening polymerization, with different terminal groups tailored to optimize anti-oxidation ability and interface compatibility. The electron-withdrawing properties of sulfonate groups readily complex with nickel atoms, enhancing the anti-oxidation resistance of the PEs. Furthermore, PBS-PEs containing terminal –OCH3 and trifluoromethanesulfonate groups, exhibit superior electrochemical performance compared to those with terminal -OH groups. These variants offer a wide electrochemical stability window of 5.1 V (vs. Li+/Li) and ionic conductivity reaching up to 1.25 × 10−3 S cm−1 at 25 °C. The improved interfacial compatibility with NCM811 cathodes exhibiting a good capacity retention of 80 % after 150 cycles at a current density of 2 C when charged to 4.6 V. Thus, this study not only introduces a novel electron-withdrawing sulfonate groups PEs for pairing with high-voltage cathodes, but also advances the application of NCM811 cathode material in high-energy SSLMBs.

Design and synthesis of fluorescent BODIPY derivatives with D-A structure for cancer cell imaging

Boron-dipyrromethene (BODIPY) as a traditional fluorescent dye has received increasing attention because of its high molar extinction coefficient, easy modification, and good light stability. In this work, three BODIPY compounds (BDP-1, BDP-2 and BDP-3) were designed and synthesized by introducing PEG chains and a naphthalimide (NI) group onto the parent structure of BODIPY. Due to the electron-withdrawing properties of NI, the compounds of BDP-2 and BDP-3 formed a typical donor (D)-acceptor (A) structure, which led to the red-shifted absorption and fluorescence spectrum. The synthesized BODIPY exhibited intense green emission, with fluorescence quantum yield of 0.66, 0.29, and 0.19 for BDP-1, BDP-2 and BDP-3 respectively. The decrease in fluorescence of BDP-2 and BDP-3 was probably ascribed to the enhanced intramolecular charge transfer because of the introduction of the NI group. The good biocompatibility and low cytotoxicity of the synthesized BODIPY compounds were verified by the CCK-8 assay. The subcellular localization of the three compounds was further evaluated through a confocal laser scanning microscope. The results showed that these BODIPY derivatives were mainly localized in the lysosomes of cancer cells. Thus, the synthesized BODIPY fluorescent dyes show promising applications in tumor imaging benefiting from their good biocompatibility and strong fluorescence.

Substituent effects of halogens on the excited-state intermolecular proton transfer reactions.

Fluorescent aromatic urea compounds undergo excited-state intermolecular proton transfer (ESPT) in the presence of acetate anions to produce an excited state of the tautomer (T*) from the excited state of the complex (N*), resulting in dual fluorescence. Herein, we performed spectroscopic measurements of anthracen-1-yl-3-phenylurea derivatives with substituents, -CF3, -F, or -Cl, at the p-position of the phenyl group in the presence of acetate to investigate the substituent effects on the ESPT reaction and the deactivation processes of N* and T*. Kinetic analysis showed that the reverse ESPT rate constant (k-PT) depended on the respective substituents, suggesting that each substituent may influence the reverse ESPT process differently. In particular, since the electron-withdrawing properties of -F are estimated by the - I and + Iπ effects, it is plausible that -F has a slight electron-donating property and influences the reverse process from T* to N* in the excited state. This study shows that it is possible to control emission by selecting specific substituents in the ESPT system.

On substituent effect in 1,n–homodisubstituted polyenes

AbstractThe all-trans and all-cis polyenes homodisubstituted at the ends were calculated at the B3LYP/6-31G** level. The disubstitution gives rise to three end-types of the conformers: trans-trans, trans-cis, and cis-cis, denoted as EE, EZ, and ZZ. The symmetry of the EE or ZZ all-cis isomers depended on the double bond parity. Twelve substituents used: H, BeH, BH2, BF2, Br, CH3, Cl, CN, F, NH2, NO2, OH, and SiH3 were chosen to exhibit different σ- and π-electron donating and electron withdrawing properties. For polyenes composed up to ca. 20 C-atoms, the π-electron donating and withdrawing character of the end groups matters and differently acting substituents play significantly different roles. Unexpectedly, the intramolecular interactions between the substituents and the neighboring chain CH groups near appeared more decisive for the compound’s stability than the substituent electron donating/withdrawing properties. The substituent-chain interplay was consonant in the all-trans and all-cis polyenes. Still, they were always more destabilizing in the latter than in all-trans isomers.

Graphene-Molecule Hybridization at a Ferromagnetic Interface.

The introduction of a graphene (Gr) buffer layer between a ferromagnetic substrate and a metallorganic molecule is known to mediate the magnetic coupling between them, an effect attributed to a weak hybridization between graphene and molecule. In this paper, we present experimental evidence of this effect through a detailed investigation of the frontier electronic properties of iron phthalocyanine deposited on cobalt-supported graphene. Despite being physisorbed, the molecular adsorption on Gr/Co induces a sizeable charge transfer from graphene to the molecular macrocycle leading to the partial occupation of the LUMO and the appearance of an energetically localized hybrid state, which can be attributed to the overlap between the graphene pz state and the molecular macrocycle. Graphene is not inert either; the adsorption of the molecule induces doping and alters the Fermi velocity of both the hybrid minicone state and the Dirac cone. Similar effects are observed when the molecular periphery is decorated with fluorine atoms, known for their electron-withdrawing properties, with minimal changes in the energy alignment.

Achieving Record External Quantum Efficiency of 11.5 % in Solution-Processable Deep-Blue Organic Light-Emitting Diodes Utilizing Hot Exciton Mechanism.

High performance solution-processable deep-blue emitters with a Commission International de l'Eclairage (CIE) coordinate of CIEy≤0.08 are highly desired in ultrahigh-definition display. Although, deep-blue materials with hybridized local and charge-transfer (HLCT) excited-state feature are promising candidates, their rigidity and planar molecular structures limit their application in solution-processing technique. Herein, four novel deep-blue solution-processable HLCT emitters were first proposed by attaching rigid imide aliphatic rings as functional units onto the HLCT emitting core. The functional units not only improve solubility, enhance thermal properties and morphological stability of the emitting core, but also promote photoluminescence efficiency, balance charge carrier transport, and inhibit aggregation-caused quenching effect due to the weak electron-withdrawing property as well as steric hindrance. The corresponding solution-processable organic light-emitting diodes (OLEDs) substantiate an unprecedented maximum external quantum efficiency (EQEmax) of 11.5 % with an emission peak at 456 nm and excellent colour purity (full width at half maximum=56 nm and CIEy=0.09). These efficiencies represent the state-of-the-art device performance among the solution-processable blue OLEDs based on the "hot exciton" mechanism. This simple strategy opens up a new avenue for designing highly efficient solution-processable deep-blue organic luminescent materials.

The Influence of the Alkylamino Group on the Solvatochromic Behavior of 5-(4-substituted-arylidene)-1,3-dimethylpyrimidine-2,4,6-triones: Synthesis, Spectroscopic and Computational Studies.

Advances in electronics and medical diagnostics have made organic dyes extremely popular as key functional materials. From a practical viewpoint, it is necessary to assess the spectroscopic and physicochemical properties of newly designed dyes. In this context, the condensation of 1,3-dimethylbarbituric acid with electron-rich alkylaminobenzaldehyde derivatives has been described, resulting in a series of merocyanine-type dyes. These dyes exhibit intense blue-light absorption but weak fluorescence. An electron-donating alkylamino group at position C4 is responsible for the solvatochromic behavior of the dyes since the lone electron pair of the nitrogen atom is variably delocalized toward the barbituric ring, which exhibits electron-withdrawing properties. This was elucidated, taking into account the different geometry of the amino group. The intramolecular charge transfer in the molecules is responsible for the relatively high redshift in absorption and fluorescence spectra. Additionally, an increase in solvent polarity moves the absorption and fluorescence to lower energy regions. The observed solvatochromism is discussed in terms of the four-parameter Catalán solvent polarity scale. The differences in the behavior of the dyes were quantified with the aid of time-dependent density functional theory calculations. The obtained results made it possible to find regularities linking the basic spectroscopic properties of the compounds with their chemical structure. This is important in the targeted search for new, practically important dyes.

Hydrogen peroxide production in pure water on a pyrrole-containing hydrothermal carbon photocatalyst

Herein, a pyrrole-containing hydrothermal carbon (N-SHTC) was developed by a simple one-step hydrothermal carbonization approach using starch and nitric acid as starting reactants. The introduction of nitric acid promoted the dehydration and carbonization of starch and led to the synthesis of N-SHTC at lower hydrothermal temperature, and also resulted in the incorporation of N into the furan structure to form pyrrole units. The charge transfer efficiency of N-SHTC was significantly improved due to the introduced pyrrole units as potential electron acceptor. With no scavengers or O2 bubbling, N-SHTC exhibited a H2O2 production rate of 1132 μmol g−1 h−1 in pure water under visible light. Notably, the H2O2 photoproduction performance of N-SHTC was further enhanced in the presence of metal salt ions due to the strengthened electron-withdrawing property of pyrrole units caused by the metal salt ions. These merits enabled N-SHTC the potentially practical applications in photocatalytic production of H2O2.

Tuning structural and electronic properties of metal-organic framework 5 by metal substitution and linker functionalization.

The chemical flexibility of metal-organic frameworks (MOFs) offers an ideal platform to tune structure and composition for specific applications, from gas sensing to catalysis and from photoelectric conversion to energy storage. This variability gives rise to a large configurational space that can be efficiently explored using high-throughput computational methods. In this work, we investigate from first principles the structural and electronic properties of MOF-5 variants obtained by replacing Zn with Be, Mg, Cd, Ca, Sr, and Ba and by functionalizing the originally H-passivated linkers with CH3, NO2, Cl, Br, NH2, OH, and COOH groups. To build and analyze the resulting 56 structures, we employ density-functional theory calculations embedded in an in-house developed library for automatized calculations. Our findings reveal that structural properties are mainly defined by metal atoms and large functional groups, which distort the lattice and modify coordination. The formation energy is largely influenced by functionalization and enhanced by COOH and OH groups, which promote the formation of hydrogen bonds. The charge distribution within the linker is especially influenced by functional groups with electron-withdrawing properties, while the metal nodes play a minor role. Likewise, the bandgap size is crucially determined by ligand functionalization. The smallest gaps are found with NH2 and OH groups, which introduce localized orbitals at the top of the valence band. This characteristic makes these functionalizations particularly promising for the design of MOF-5 variants with enhanced gas uptake and sensing properties.

Precisely Engineering Asymmetric Atomic CoN4 by Electron Donating and Extracting for Oxygen Reduction Reaction.

The development of nonpyrolytic catalysts featuring precisely defined active sites represents an effective strategy for investigating the fundamental relationship between the catalytic activity of oxygen reduction reaction (ORR) catalysts and their local coordination environments. In this study, we have synthesized a series of model electrocatalysts with well-defined CoN4 centers and nonplanar symmetric coordination structures. These catalysts were prepared by a sequential process involving the chelation of cobalt salts and 1,10-phenanthroline-based ligands with various substituent groups (phen(X), where X=OH, CH3, H, Br, Cl) onto covalent triazine frameworks (CTFs). By modulating the electron-donating or electron-withdrawing properties of the substituent groups on the phen-based ligands, the electron density surrounding the CoN4 centers was effectively controlled. Our results demonstrated a direct correlation between the catalytic activity of the CoN4 centers and the electron-donating ability of the substituent group on the phenanthroline ligands. Notably, the catalyst denoted as BCTF-Co-phen(OH), featuring the electron-donating OH group, exhibited the highest ORR catalytic activity. This custom-crafted catalyst achieved a remarkable half-wave potential of up to 0.80 V vs. RHE and an impressive turnover frequency (TOF) value of 47.4×10-3 Hz at 0.80 V vs. RHE in an alkaline environment.

Terminal-substituted benzoxazole based Schiff-base Mesogens and their Cu(II) complexes: Synthesis and structural characterization

Three homologous series of 3‑hydroxy-4-(((-4-benzoxazol-2-yl)phenyl)imino)methyl)phenyl-4-alkoxybenzoate (with ester linkage) and 5-(alkoxy)-2-(((4-(5-benzoxazol-2-yl)phenyl)imino)methylphenol (without ester linkage) Schiff based derivatives, bearing different substituents at 5 position i.e., 5-methylbenzoxazole, benzoxazole and 5-chlorobenzoxazole units at the end of the molecule and their Copper complexes were synthesised. The molecular structure of all the compounds were elucidated by different spectroscopic techniques such as FT-IR, 1H & 13C NMR and mass spectrometry together with elemental analysis. Their phase transition behaviour was investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. The structure-property relationships, compounds bearing a variety of polar substituents and changing the spacer from ester linked to without ester-linked has been studied. Substituents with stronger electron-withdrawing properties gave a wider mesomorphic temperature domain, in the order Cl > CH3>H.

Development of 1,3a,6a-triazapentalene derivatives as practical erasable fluorescent cell staining reagents

1,3a,6a-Triazapentalene (TAP) derivatives were developed as erasable fluorescent cell staining reagents that can be used with a confocal laser microscope. To enhance the utility of previously developed erasable fluorescent cell staining reagents such as 2-biphenyl-TAP (1a) for practical applications in confocal laser microscopy, a carbonyl group with strong electron-withdrawing properties was introduced between the TAP core skeleton and the biphenyl group, leading to a longer wavelength shift in absorption and emission maxima. While the analog 1b, featuring only the carbonyl group insertion, declined significantly in fluorescence intensity, the two-carbon extended ketone analog 1d showed a desirable shift in absorption maximum towards longer wavelengths, green fluorescence, and high fluorescence intensity. Cell staining experiments using 1d demonstrated its usefulness, including rapid staining, erasable fluorescent staining, and compatibility with confocal laser microscopy employing a standard filter.

Late-Stage Diversification of Pyrazoles as Antileishmanial Agents.

N-Pyrazolylcarboxamides and N-pyrazolylureas represent promising lead compounds for the development of novel antileishmanial drugs. Herein, we report the late-stage diversification of 3-bromopyrazoles 10 A/B and 14 A by Pd-catalyzed Sonogashira and Suzuki-Miyaura cross coupling reactions. The electron-withdrawing properties of the cyano moiety in 4-position of the pyrazole ring limited the acylation of the primary amino moiety in 5-position. A large set of pyrazoles bearing diverse aryl and alkynyl substituents in 3-position was prepared and the antileishmanial and antitrypanosomal activity was recorded. The urea 38 lacking the electron withdrawing cyano moiety in 4-position and containing the large 4-benzylpiperidinoo moiety exhibited a modest antileishmanial (IC50=19 μM) and antitrypanosomal activity (IC50=7.9 μM)). However, its considerable toxicity against the PMM and MRC-5 cells indicates low selectivity, i. e. a small gap between the desired antiparasitic activity and undesired cytotoxicity of <2- to 4-fold.