Influence Of Electron-withdrawing Groups Research Articles

Preparation and properties of PPO-based anion exchange membranes with flexible side chains for fuel cells

Poor alkali resistance and low ionic conductivity are the crux of the matter that hinder the commercialization applications of anion exchange membranes (AEMs). Here, we designed and synthesized a range of poly(phenylene oxide) (PPO)-based AEMs with benzyl free side-chain type. After that, the influences of different cationic functional groups, such as quaternary ammonium, imidazole and piperidine, on the membrane performances were examined systemically. The introduction of ether bond-containing side chains to build micro-phase separation structures enhanced the conductivity of AEMs (64 mS·cm−1 at 80 °C). At the same time, the side-chain without benzyl structures effectively avoids the influence of electron withdrawing groups on PPO main chains. Moreover, the steric hindrance effect of the benzene ring can further alleviate the attack of OH−, thereby improving the alkali resistance of the membrane. Molecular dynamics simulations revealed that a well-developed hydrogen bonding network was formed between ether linkages and water molecules in the side-chain structures, which enhanced the electrical conductivity of the membrane. Meanwhile, the prepared membranes owned a good water uptake and a certain dimensional stability. These results provide a basis for the subsequent enhancement of membrane performances.

Influence of electron-withdrawing groups in two-photon absorption of imidazopyridines derivatives

This work investigates the influence of different electron-withdrawing groups (Cl, Br, fluorobenzonitrile), on the two-photon absorption cross-section of six imidazo[4,5-b]pyridine derivatives. The two-photon absorption cross-section spectra were obtained by ultra-fast Z-scan technique from 470 nm up to 700 nm. The Sum-Over-States approach was applied to model the two-photon absorption spectra, using a three-level energy system. Photophysical properties, such as transition dipole moment, the difference of permanent dipole moments, and anisotropy coefficient were determined through the analysis of one-photon absorption spectra, solvatochromism, and fluorescence anisotropy, respectively. Besides, the excited state absorption spectra were measured through ultra-fast transient absorption, allowing the excited state lifetime and spectral position determination. Two-photon absorption cross-sections of about 160 GM were observed when two electron-withdrawing groups are linked to the imidazo[4,5-b]pyridine core, elucidating a path to achieve high optical nonlinearities in imidazopyridine derivatives. Furthermore, a increase in the two-photon cross-section was observed when chloride is linked at the C-6 position (90 GM) instead of the C-5 position (50 GM), which is related to the proximity of a higher excited state.

Skillfully tuning 1-hydroxy-9H-fluoren-9-one forward-backward ESIPT processes by introducing electron-withdrawing groups: A theoretical exploration

Judiciously tuning the forward-backward excited state intramolecular proton transfer (ESIPT) barriers can effectively influence the generation of white light. Harnessing the 1-hydroxy-9H-fluoren-9-one (HHF) ESIPT characteristics as the prototype, two electron-withdrawing substituents (nitro group and formyl group) were introduced. The influence of electron-withdrawing group on the energy barrier of the forward-backward ESIPT processes has been studied by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The frontier molecular orbitals, infrared spectra, non-covalent interactions, electrostatic potential, electronic spectra, molecular structure have cross-validated our results. These results indicate that the electron-withdrawing groups dramatically promote the occurrence of the forward ESIPT process and hinder the backward ESIPT. The potential curves revealed that the forward ESIPT energy barrier was obviously reduced to 5.73 kcal/mol (HHF-CHO) and 4.55 kcal/mol (HHF-NO2) compared with HHF (7.64 kcal/mol). Meanwhile, the backward ESIPT energy barrier is availably improved to 3.57 kcal/mol (HHF-CHO) and 3.84 kcal/mol (HHF-NO2) compared with HHF (2.51 kcal/mol). Through the introduction of the electron-withdrawing groups, the energy barrier of the forward-backward ESIPT is efficaciously modulated. Furthermore, the more stable and better white fluorescence has been achieved. Our work will be conducive to improve molecular ESIPT properties and exploit white luminescent molecules.

Substituent Effect on the Thermodynamics and Kinetics of Carbyne Complex [(η5-C5H5)(CO)(COMe)Re≡CC6H4X] Isomerization to Carbene Complex [(η5-C5H5)(CO)2Re=C(Me)(para-C6H4X]: A Theoretical Study

This study investigates the effect of different substituents on the isomerization reaction of the [(η5‑C5H5)(CO)(Me)Re≡CC6H5] carbyne complex to the [(η5-C5H5)(CO)2Re=C(Me)(C6H5)] carbene complex using MPW1PW91-based quantum mechanical calculations. In this respect, the study evaluates the influence of electron withdrawing groups (EWGs) and electron donating groups (EDGs) groups on the relative energies, thermodynamics parameters and the rate constant of this reaction and correlates the calculated parameters with the Hammett and Brown’s constants for the para-substituted functional groups (σp and $$\sigma _{{\text{p}}}^{ + }$$, respectively). Also, the calculated Wiberg indices of the Re–C bond values were used to illustrate the cleavage and formation of π-bonds along with the Re–C bonds in the isomerization transition state. The rate constants of the isomerization reaction were computed from 300 to 1200 K, in gas phase.

Thieme Chemistry Journal Awardees – Where are They Now? The Influence of Electron-Withdrawing Groups at the 2- and 2′-Positions of Dibenzothienylethenes on Molecular Switching

Three dibenzothienylethenes that carry a methyl, a trifluoromethyl, or a fluoro substituent in the 2- and 2′-position were synthesised in short multistep syntheses, respectively. The trifluoromethyl and fluoromethyl substituents significantly improve the absorption properties of the corresponding open and closed isomers (both in the UV-A and in the visible range). The increasing electron deficiency (i) enlarges the portion of the closed and coloured diarylethene isomer in the corresponding photostationary states and slows down the closing and opening isomerisation, (ii) the switching reactions, both closing and opening, are slowed down, and (iii) the switching quantum yields are lowered.

Cover Picture: Synthesis of Unsymmetrically Substituted Phosphane Oxides (R1 R2 P(O)H) and Phosphinous Acids (R1 R2 POH) (Chem. Eur. J. 28/2014)

Keeping the balancewith electron-withdrawing groups! The unusual form of a phosphinous acid can be stabilized with respect to the corresponding phosphane oxide tautomer by the influence of electron-withdrawing groups. In their Full Paper on page 8615 ff., B. Hoge et al. describe an efficient strategy for the synthesis of phosphinous acids and phosphane oxides featuring one pentafluoroethyl group with an additional trifluoromethyl, pentafluorophenyl, or phenyl substituent. The differing stabilization of the phosphinous acid form is evidenced in the tautomeric equilibrium.

Synthesis and Kinetic Study of Thermal Cycloimidization of Novel Poly(Amide Amic Acid) to Poly(Amide Imide) by Thermogravimetric Analysis

A new series of fluorinated poly(amide imide)s (PAIs) were prepared by polycondensation of diamine amic acid (DAA) with isophthaloyl, terephthaloyl, or trimellitic anhydride chloride in dimethylformamide, and their subsequent thermal cycloimidization. DAAs were obtained by reacting 4, 4′ (hexafluoroisopropylidene) diphthalic anhydride (6FDA) with diamines in dimethylformamide at 0–5°C. The kinetics of thermal cycloimidization of poly(amide amic acid) (PAA) to PAIs was investigated by thermogravimetric analysis under isothermal conditions at four different temperatures, i.e. 175, 200, 225, and 260°C, for 75 min. A generalized equation to calculate the theoretical amount of weight loss upon complete imidization, i.e. Tw % = 18Nw/R Mw, was developed. In the above equation numeral 18 corresponds to the molecular weight of water, N is the number of water molecules, which are eliminated per repeat unit of PAA upon cycloimidization, w is the weight of PAA taken for TGA, and R Mw is the molecular weight of the repeat unit of PAA. The polymers were characterized by FT-IR spectra. Complete cycloimidization process is discussed in terms of monomer structure (diamines and acid chlorides), effect of imidization temperature and time, types of linkages (meta or para) present, and influence of electron withdrawing groups, such as sulfone, in the monomer structure. It was observed that cycloimidization occurred at a faster rate at the beginning, up to about 30 min, and then proceeded at a slower rate till the end of curing. The thermal and thermooxidative stabilities of PAIs were also investigated in nitrogen and oxygen atmosphere.

A Series of Thienylene/Phenylene-Based Polymers Functionalized with Electron-Withdrawing or -Donating Groups: Synthesis and Characterization

A series of polymers comprising alternating phenylene and thienylene repeating units and with electron-donating or -withdrawing groups attached on thienylene units, i.e., poly[1,4-bis(3-X-2,5-thienylene)phenylene-alt-2,5-dioctyl-1,4-phenylene] (PBTX, X = OMe, H, Cl, Br, CN), has been synthesized and characterized. These polymers are highly fluorescent, among which PBTH shows the highest solution quantum yield, up to 94% relative to quinine sulfate. The absorption and emission peak wavelengths of PBTOMe are bathochromically shifted and band gap (Eg) is lowered by the presence of electron-donating OMe group, in comparison with PBTH. The influence of electron-withdrawing groups, Br, Cl, and CN, on the absorption peak wavelength and Eg, on the other hand, is not so great. Nevertheless, the film of PBTCN shows an emission maximum near to that of PBTOMe due to the strong interchain interactions. The band structures as deduced from electrochemistry give information supporting the optical measurements. The IP of ...

Carbonyl- and carboxyl-substituted enediynes: synthesis, computations, and thermal reactivity.

The influence of electron-withdrawing groups (carbonyl and carboxyl) at the alkyne termini on the reactivity of enediynes was investigated by a combination of experimental and computational techniques. While the general chemical reactivity of such enediynes, especially if non-benzannelated, is increased markedly, the thermal cyclization, giving rise to Bergman cyclization products, is changed little relative to the parent enediyne system. This is evident from kinetic measurements and from density functional theory (DFT, BLYP/6-31G + thermal corrections) computations of the experimental systems which show that the Bergman cyclization barriers slightly (3-4 kcal/mol) increase, in contrast to earlier theoretical predictions. The effect on the endothermicities is large (DeltaDeltaH(r) = 7-12 kcal/mol). Hence, the increased reactivity of the substituted enediynes is entirely due to nucleophiles or radicals present in solution. This was demonstrated by quantitative experiments with diethylamine and tetramethyl piperidyl oxide (TEMPO) which both give fulvenes through 5-exo-dig cyclizations.

Spirobipyridopyrans and indolinospiropyridopyrans: Synthesis, X-ray crystal structure, separation of enantiomers, and barriers to thermal cleavage of the C(sp(3))-O bond.

The novel chiral spirobipyridopyrans 1 and 2 were synthesized by the acid catalyzed aldol type condensation of 5-deoxypyridoxal with the appropriate ketone and subsequent reaction of the resulting pyrylium salt with base. The indolinospiropyridopyrans 3-5, which contain the modified B(6) unit, were prepared by aldol reaction of 5-deoxypyridoxal with 1,3,3-trimethyl-2-methylenindolines. Analytical separation of enantiomers was accomplished by low-pressure liquid chromatography (LPLC) on triacetylcellulose. The barriers to thermal racemization were determined by on-line measurements of the enriched enantiomers after LPLC. Gibbs energies of activation DeltaG superset, not equals for reversible cleavage of the C(spiro)-O bond in 1, 3, and 4 were in the range 103-108 kJ/mol. The lower limits of the barriers in 2 and 5 were estimated to be greater than 102 and 109 kJ/mol by attempted thermal racemizations. The increase of the barriers from 3 to 4 and 5 was explained by the influence of electron withdrawing groups, which reduce the stability of the ring-opened transition states to C(sp3)-O bond cleavage. Geometrical data from X-ray structure analysis showed that the angle [C3-C2-C3'] around the spiro carbon atom increases with elongation of the chain in the C3-C3' bridge. This angle widening is explained by a ring-strain effect, which is greater in the five-membered ring in the skeleton of 7 than in the six- and seven-membered rings of 1 and 2.