Polar Media Research Articles

Photoinduced electron transfer across the polymer-capped CsPbBr3 interface in a polar medium.

In-situ polymer capping of cesium lead bromide (CsPbBr3) nanocrystals with polymethyl acrylate is an effective approach to improve the colloidal stability in the polar medium and thus extends their use in photocatalysis. The photoinduced electron transfer properties of polymethyl acrylate (PMA)-capped CsPbBr3 nanocrystals have been probed using surface-bound viologen molecules with different alkyl chains as electron acceptors. The apparent association constant (Kapp) obtained for the binding of viologen molecules with PMA-capped CsPbBr3 was 2.3 × 107 M-1, which is an order of magnitude greater than that obtained with oleic acid/oleylamine-capped CsPbBr3. Although the length of the alkyl chain of the viologen molecule did not show any impact on the electron transfer rate constant, it influenced the charge separation efficiency and net electron transfer quantum yield. Viologen moieties with a shorter alkyl chain length exhibited a charge separation efficiency of 72% compared with 50% for the longer chain alkyl chain length viologens. Implications of polymer-capped CsPbBr3 perovskite nanocrystals for carrying out photocatalytic reduction in the polar medium are discussed.

Mechanism of chlorotriflamidation of vinylsilanes with <i>N</i>,<i>N</i>-dichlorotriflamide

The mechanism of the reaction of vinylsilanes with N , N -dichlorotriflamide and the effect of a substituent at the silicon atom on the reaction course and on the charge distribution in substrates and their carbon analogues were studied by DFT method. The C=C bond in vinylsilanes and alkenes is polarized in the opposite way. The reaction proceeds in two stages: chlorination of the substrate with the formation of a chloronium ion, and its opening at the Cβ-Cl bond by the N -chlorotriflamide anion. Transition states of two stages were calculated. The reaction products are hydrolyzed to NH-derivatives; their IR spectra and supramolecular structure, including cyclic and linear dimers, calculated in the gas phase and in a polar medium, were studied.

Free‐radical propagation rate coefficients

AbstractThe pulsed laser polymerization‐size‐exclusion chromatography (PLP‐SEC) technique has become the International Union of Pure and Applied Chemistry (IUPAC)‐recommended method for the accurate determination of propagation rate coefficients, kp, for radical polymerization. The fundamental insight provided by Heuts, Gilbert, and Radom into the relevance of transition state dynamics allows for an adequate understanding of the dependence of kp on radical chain length, solvent environment, and monomer conversion. The associated entropic effects are particularly pronounced in aqueous solution, but are also found in less polar media and with non‐polar monomers. Moreover, these arguments are applicable towards the interpretation of copolymerization reactivity ratios. The addition of sodium hydroxide to aqueous solutions of (meth)acrylic acid yields partially and even fully ionized systems with clearly reduced kp, which indicates the action of repulsive forces between the equally charged monomer and radical species. Increasing the concentration of monomer as well as the addition of salts may reverse this effect and increase kp due to counterion activity.

Tight Bounds and the Role of Optical Loss in Polariton-Mediated Near-Field Heat Transfer

We introduce an analytical framework for near-field radiative heat transfer in bulk plasmonic and polar media. Considering material dispersion, we derive a closed-form expression for the radiative thermal conductance, which disentangles the role of optical loss from other material dispersion characteristics, such as the spectral width of the Reststrahlen band in polar dielectrics, as well as from the temperature. We provide a universal condition for maximizing heat transfer that defines the optimal interplay between a material's optical loss and polariton resonance frequency, based on which we introduce tight bounds to near-field heat transfer. With this formalism, one can quantitatively evaluate all polaritonic materials in terms of their performance as near-field thermal emitters.

Metadynamics simulations of R–NHC reductive elimination in intermediate palladium complexes of cross-coupling and Mizoroki–Heck reactions

Exploring the free energy surface of the R–NHC coupling reaction in the key intermediates of the Mizoroki–Heck and cross-coupling catalytic cycles has been conducted by the methods of biased and unbiased molecular dynamics. Molecular dynamics simulations were carried out both in vacuum and in a polar solvent, with the following main observations on the influence of the media: (1) the solvent prevents the dissociation of the solvate ligand, so the R–NHC coupling proceeds in a four-coordination complex (rather than in a three-coordination one, as in the case of a gas-phase reaction); (2) in the condensed phase, the potential barrier of the reaction is significantly higher compared to the same process in vacuum (17.7 vs. 21.8 kcal mol-1); (3) polar solvent stabilizes the R–NHC coupling product. The reaction in a polar medium is exergonic (ΔG = −3.9 kcal mol-1), in contrast to the in vacuum modeling, where the process is endergonic (ΔG = 0.4 kcal mol-1).

Understanding of Intramolecular Charge Transfer Dynamics of a Push–Pull Dimethylamino-phenylethynylphenyl-dicyanoimidazole by Steady-State and Ultrafast Spectroscopic Studies

Photophysical behaviors of D−π–A compound 2-{4-[4-(N,N-dimethylamino)phenylethynyl]phenyl-1-methyl-1H-imidazole-4,5-dicarbonitrile (DMAPPIDCN) were explored using steady-state absorption, fluorescence emission, and femtosecond time-resolved absorption and emission spectroscopic techniques at room temperature along with computational time-dependent density functional theory (TD-DFT) calculation. The spectroscopic studies were carried out in different solvents of varying polarities including binary solvent mixtures. The role of the solvent polarity, viscosity, and temperature on the relaxation mechanism of DMAPPIDCN is disclosed. The observed steady-state and time-resolved spectroscopic features were attributed to intramolecular charge transfer (ICT) dynamics. The ICT in DMAPPIDCN is rationalized to a sequential twisted motion of both N(CH3)2 and whole N,N-dimethylaminophenyl moieties around the molecular axis interconnecting the adjacent imidazolephenyl moiety leading to the TICT1 and TICT2 (σ*) states. The increased solvent polarity affected mostly the fluorescence emission spectra pointing to a significant increase in the excited state dipole moment. This result clearly reveals formation of the TICT2 (σ*) involving efficient charge transfer from the (N,N-dimethylamino)phenyl (DMAP) donor to the phenyl-1-methyl-1H-imidazole-dicarbonitrile (PIDCN) acceptor in the excited state in a polar environment. In the TICT2 (σ*) state, the planes of electron-withdrawing and electron-donating moieties are perpendicular with the angle (DMAP)C–C≡C being 141.1°. This nonplanar arrangement accounts for the observed large Stokes shift. Time-resolved fluorescence spectroscopic studies unveil the excited state relaxation processes confirming the increase in the nonradiative decay rate in aprotic medium with increase in the solvent dielectric constants. Femtosecond transient spectroscopic studies unambiguously confirmed the existence of well separated LE and TICT states and their ensuing kinetics in polar medium. In nonpolar solvents, DMAPPIDCN shows strong fluorescence which emits from the LE (ππ*) state, whereas in polar solvents, formation of two consecutive TICT states occurs from the LE (ππ*) in a sub picosecond to few picosecond time domain depending on polarity of the solvents and the non-radiative decay from the TICT states.

Vibrational spectroscopy simulation of solvation effects on a G-quadruplex

It is commonly believed that solvation effects on the vibrational properties of a solute are easily accounted for by simple rules of thumbs, that is, solvating a polar molecule in a polar medium has the only effect of red shifting all its spectroscopical features and, similarly, solvating a polar molecule in a nonpolar medium has the opposite effect. In this work, we use theoretical vibrational spectroscopy at quasi-classical and quantum approximate semiclassical level to gain atomistic insights about solvent–solute interactions for 2′-deoxyguanosine and the G-quadruplex. We employ the quasi-classical trajectory method to include full anharmonicity into our calculated spectra, and then introduce quantum nuclear effects by means of divide-and-conquer semiclassical spectroscopy calculations. Solvation is treated explicitly leading to a good reproducibility of the available experimental data and reliable predictions when an experimental reference is missing.Communicated by Ramaswamy H. Sarma

High-speed counter-current chromatographic separation and thermodynamic mechanism of an antioxidant from Morus alba leaves

Morus alba leaves have a long history as a plant medicine and possess many biological activities. Antioxidant activity is the basis of its many biological activities. In order to explore the antioxidant effect of Morus alba leaves, antioxidant and its mechanism of scavenging free radicals have been investigated in this study. As results, rutin with the EC50 of 27.94 ± 1.18 μg/mL and 25.86 ± 0.09 μg/mL to DPPH· and ABTS·+ respectively was enriched by liquid-liquid extraction, and isolated by high-speed counter-current chromatography and identified by NMR, and was recognized as the main antioxidant in Morus alba leaves. The results of reaction thermodynamics and frontier molecular orbital theory indicated that 4′-OH in rutin was the most active group, which scavenged free radicals by hydrogen atom transfer in non-polar media and single electron transfer followed by proton transfer in polar media. Polar solvents were more helpful for rutin to exert its antioxidant effect. This study not only explored the antioxidant basis of Morus alba leaves, but also revealed its mechanism of scavenging free radicals.

Polarity-Driven Isomerization of a Hydroxynaphthalimide-Containing Spiropyran at Room Temperature.

Design of spiropyrans showing spontaneous isomerization driven by the polarity of solvents is an important consideration for the synthesis of optical sensory materials. Although some spiropyrans undergo polarity-driven isomerization, they must be heated owing to the high activation energy required for isomerization. In this study, we describe that a spiropyran containing a hydroxynaphthalimide unit (1) exhibits a polarity-driven isomerization at room temperature. It exists as a colorless spirocyclic (SP) form in less polar solvents but is isomerized to a colored merocyanine (MC) form in polar solvents. The equilibrium amount of the MC form increases with an increase in the polarity of solvents. The MC form involves two resonance structures-the quinoidal and zwitterionic forms. In polar media, the zwitterionic form dominates mainly owing to solvation by polar molecules. Solvation stabilizes the negative charge of the zwitterionic form and decreases its ground state energy, thereby enhancing SP → MC isomerization. The SP ⇌ MC isomerization terminates within barely 30 s even at room temperature because the naphthol moiety with high π-electron density lowers the activation energy for the rate-determining rotational step.

Microstructure realization of a lattice-based polar solid for arbitrary elastic waveguiding

The ability to precisely directing and controlling longitudinal (P) and transverse (S) waves in 2D solids along an arbitrary trajectory has attracted significant research interest and is crucial for practical applications such as imaging, cloaking, and wave focusing. Here, we report, design and examine an inhomogeneous lattice-based polar medium for ideal elastic waveguide, whose microstructures are inversely determined by the discrete transformation elasticity (DTE). Microstructures of the suggested medium, which are realized through global linear transformation and local affine transformation, enables arbitrary waveguides to transport elastic waves with minimal energy loss. Numerical simulation is then conducted to demonstrate that the lattice-based polar waveguide can efficiently steer both in-plane P and S wave modes over a broad frequency band. We also leverage the medium for Rayleigh wave control on curved surfaces. The constructed polar surface can break the conventional limit of the Rayleigh wave propagation on both concave and convex surfaces with extreme curvatures. This study is not only a concrete manifestation of the polar material, discrete transform elasticity, and their advantages but also provides a great potential in engineering applications such as signal detection, vibration control, and earthquake protection.

Effect of ferulic acid derivative concentration on the release kinetics, antioxidant capacity, and thermal behaviour of different polymeric films

Ferulic acid displays poor thermal resistance during extrusion and compression moulding, slow 2,2-diphenyl-1-picrylhydrazyl (DPPH) reaction kinetics, and undetected release from polylactide (PLA) and polyhydroxyalkanoates (PHA)-based films into polar media. Thus, in this study, a ferulic acid derivative Bis-O-dihydroferuloyl-1,4-butanediol (BDF) was used as an active additive (up to 40 w%) in PLA, poly(3-hydroxybutyrate) (PHB), and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) matrices to produce blends by extrusion. These blends were then used to prepare films by solvent casting. The BDF displayed good stability with 86–93% retention. The release kinetics in Food Simulant A revealed higher BDF release amounts (1.16–3.2%) for PHA-based films as compared to PLA. The BDF displayed faster DPPH reaction kinetics as compared to ferulic acid. The PHA-based films containing BDF displayed > 80% of DPPH inhibition. The growth of crystals inside polymer matrix had a nucleation effect which reduced the glass transition temperature of the films.

Aggregation patterns of curcumin and piperine mixtures in different polar media.

This work reports a thorough molecular dynamics investigation on the aggregation patterns of curcumin and piperine in water, ethanol and a mixture of both solvents. The low solubility of curcumin in water results in a rapid formation of very stable dimers for both keto and enol tautomers. In agreement with a higher solubility, piperine molecules move closer and farther apart several times during the simulation, which indicates the formation of a less stable dimer in water. In contrast, both curcumin and piperine are soluble in ethanol and, thus, dimers can hardly be formed in this media. In comparison with a pure-water solvent, a 30 : 70 mixture of ethanol and water significantly reduces the probability of formation of most dimers of curcumin and piperine molecules. The simulations show that larger clusters may be complex structures, but the formation of stacks (in the case of piperine and enol tautomer of curcumin) and cages (when the keto tautomer of curcumin is involved) are not rare. Furthermore, it is shown that each single molecule presents a certain degree of mobility in the cluster, especially on the surface, but without leading to dissociation.

Theoretical study on the radical scavenging activity of gallic acid

The global descriptors of the chemical activity: ionization potential IP, electron affinity EA, chemical potential μ, absolute electronegativity χ, molecular hardness η and softness S, electrophilicity index ω, electro-donating ω-, electro-accepting ω+ powers as well as Ra and Rd indexes for gallic acid (GA) in the gas phase and water medium have been determined. To this aim, the HOMO and LUMO energies were calculated using the DFT method at the B3LYP, M06–2X, LC-ωPBE, BHandLYP, ωB97XD/cc-pVQZ theory levels using C-PCM, IEF-PCM and SMD solvation models, enabling more accurate descriptor calculations than those carried out so far. Quantum-chemical computations were also applied to investigate the GA structure and thermodynamic parameters characterizing its radical scavenging properties. To this aim, the full optimization of the neutral GA and its radical, cationic and anionic forms in vacuum and water medium has been performed, and then the bond dissociation enthalpy BDE, adiabatic ionization potential AIP, proton dissociation enthalpy PDE, proton affinity PA, electron transfer enthalpy ETE, gas phase acidity Hacidity and free Gibbs acidity Gacidity in water have been determined. The calculations revealed that GA in vacuum scavenges free radicals via hydrogen atom transfer (HAT), whereas in water (polar) medium by sequential proton loss electron transfer (SPLET). Analysis of the global activity descriptors of GA indicates that only B3LYP method combined with different solvation models satisfactory reproduces LUMO-HOMO energies and provides the smallest value of the total electron energy of GA. Among the parameters of chemical activity, the indexes Ra and Rd are the most independent of the computational method and the solvation model used. They can be recommended as a reliable source of information on the antioxidant activity of chemical compounds.

Computational assessment of the primary and secondary antioxidant potential of alkylresorcinols in physiological media.

Alkylresorcinols are a group of natural phenolic compounds found in various foods such as whole grain cereals, bread, and certain fruits. They are known for their beneficial health effects, such as anti-inflammatory and anti-cancer properties. This study aimed to evaluate the antioxidant activity of two typical alkylresorcinols namely olivetol and olivetolic acid (Oli and OliA) under physiological conditions. The free radical scavenging capacity of Oli and OliA toward oxygenated free radicals (HO˙ and HOO˙ radicals) was investigated using thermodynamic and kinetic calculations. The results revealed that Oli and OliA are potent scavengers of HO˙ radical in both polar and lipid media, acting exclusively via the FHT (formal hydrogen transfer) mechanism. Moreover, they demonstrated excellent scavenging activity toward HOO˙ radical in water via the SET (single electron transfer) mechanism, outperforming the common antioxidant BHT. In lipid media, Oli and OliA showed moderate scavenging activity toward HOO˙ radical via the FHT mechanism. Significant prooxidant potential of OliA- was also demonstrated through the formation of complexes with copper ions. Additionally, docking studies indicate that the compounds exhibited a good affinity for ROS-producing enzymes, including myeloperoxidase (MP), cytochrome P450 (CP450), lipoxygenase (LOX), and xanthine oxidase (XO), highlighting their potential as natural antioxidants with promising therapeutic applications.

Effects of donor and acceptor substituents on the photophysics of 4-ethynyl-2,1,3-benzothiadiazole derivatives.

The present work explores the photophysical, electrochemical, and fluorescence polarization properties of a group of π-conjugated phenylethynyl-2,1,3-benzothiadiazole derivatives (BTDs) bearing different electron-donating (ED) or electron-withdrawing (EW) substituents at the para position of the phenylethynyl moiety. The BTDs were synthesized through the Sonogashira cross-coupling reaction between 4-bromo-2,1,3-benzothiadiazole and the respective para-substituted phenylethynyl derivatives. The BTDs with the EW-substituents show relatively weak solvatochromic behavior, while the BTDs with the strong ED-substituents like methoxy and N,N-dimethylamino-based substituents (BTDPhOMe and BTDPhNMe2) exhibit a pronounced solvatochromic behavior. The change in dipole moments in the excited states of the derivatives was calculated using Lippert-Mataga plots. The conclusions drawn on the spectral behavior of the molecules could be rationalized by TD-DFT calculations involving electron density difference (EDD) maps that correlate with the ICT characteristics of the molecules. The experimental and theoretical calculations reveal that the BTDs with the strong ED-substituents (strong push-pull type BTDs) have a strong ICT character in the excited state. These strong push-pull type BTDs show high fluorescence quantum yield (ΦF) in apolar solvents and low ΦF in polar solvents. In contrast, the BTDs with the weak ED-substituents (weak push-pull type BTDs) and EW-substituents (pull-pull type BTDs) have a weaker ICT character with low ΦF in apolar and high ΦF in polar solvent media. There is good a agreement among the HOMO-LUMO band gaps obtained from absorption spectroscopy and electrochemical studies and theoretical calculations. The fluorescence anisotropy measurement in the glycerol medium shows that the studied BTDs generally exhibit higher sensitivity towards microviscosity than the traditional DPH fluorescence anisotropy probe.

A squaramide cage capable of binding and extracting H2PO4- and HP2O73- in highly polar protic media.

A novel squaramide cage (2) binds H2PO4- and HP2O73- with high selectivity and affinity in a highly polar protic solvent system. Receptor 2 is also able to extract these hydrophilic anions into a chloroform phase from water. The X-ray crystal structure demonstrated that compound 2 forms a complex with H2PO4- with 1 : 1 stoichiometry in the solid state.

Computational insights on the antioxidant, antinitrosant, and xanthine oxidase inhibitory capacities of neobavaisoflavone

Neobavaisoflavone is a potent scavenger of free radicals in polar physiological media, with higher rate constants than common antioxidants such as BHT. It is also a potential inhibitor of xanthine oxidase and follows the same pathway as quercetin.

SULFONIC ACID POLYIMIDES AND THEIR SALTS: PROPERTIES OF THEIR PARTICLES IN SOLUTION AND SUSPENSION

A high-temperature one-step synthesis resulted in two new polymers PI-I and PI-II (polyimides), which have functional groups as SO<sub>3</sub>H and SO<sub>3</sub>Na in their chain. Their molecular-mass molecular characterization revealed that PIs are polydisperse systems, and show thermal stability up to 400°C. The dynamic characteristics of polyimide particles in polar and nonpolar media were studied. The particle size of PI-I is significantly affected by the polarity of the medium. Due to intermolecular interactions associated with the free proton, the average particles diameters in dimethylformamide (DMF) was 10 μm, while in polymethylsiloxane (PMS-5) it was 1.4 μm. The size particles of PI-II in different media were 246 and 606 nm. Viscoelastic and mechanical properties of polymer systems were studied using microrheology and classical rheology. Hysteresis loss coefficients (κ) at various temperatures (T) were relatively small and stable for PI-I at T < 60°C with insignificant changes at increasing frequency. PI-II coefficient changes markedly but smoothly as a function of ω (angular frequency) at T < 60°C. With increasing temperature, the values of κ are almost independent of ω, taking quite large values compared to the corresponding values for the PI-I. It was determined that the temperature increases the interaction between the particles of the dispersed phase in the electric field. And the decrease in the viscosity of the suspension is due to a decrease in the viscosity of the medium. The electrorheological effect is most pronounced for the PI-I suspension.

Magnetic fluid method for sealing liquid media

Magnetic fluid seals for sealing gas environments are widely used in various industries due to their undeniable advantages. However, such seals are not capable of reliable sealing of liquid media with different polarities. The paper analyses physicochemical processes that lead to destructing magnetic fluid in a seal under the influence of a liquid medium in contact with it. There are results of experimental studies on sealing using magnetic seals of non-magnetic fluids with different polarity. The authors studied the tightness of a magnetic fluid seal capacity in contact with weakly polar liquids: MVP instrument oil, vaseline oil, and water as a highly polar liquid. For sealing water, the authors chose magnetic fluids with liquid siloxanes as the basis; they are immiscible with water and hydrophobic. Weakly polar liquids were sealed using magnetic fluid with a dispersion medium of triethanolamine, which is almost insoluble in hydrocarbon liquids and has a high dielectric permittivity and surface tension comparable in magnitude. It is established that magnetic fluid based on triethanolamine reliably seals the experimental bearing from penetrating of weakly polar liquids at an overpressure of 10 kPa and below. To seal polar liquid media, it seems promising to use oleophobic magnetic fluids based on PES-5, containing a large amount of filler in the form of ferrite particles. A magnetic fluid should have the smallest possible contact area with the sealed fluid and maintain a laminar flow regime.

Evolution of vibrational bands upon gradual protonation/deprotonation of arsinic acid H2As(O)OH in media of different polarity.

This computational work is devoted to the investigation (MP2/def2-TZVP) of the geometry and IR parameters of arsinic acid H2AsOOH and its hydrogen-bonded complexes under vacuum and in media with different polarity. The medium effects were accounted for in two ways: (1) implicitly, using the IEFPCM model, varying the dielectric permittivity (ε) and (2) explicitly, by considering hydrogen-bonded complexes of H2As(O)OH with various hydrogen bond donors (41 complexes) or acceptors (38 complexes), imitating a gradual transition to the As(OH)2+ or AsO2- moiety, respectively. It was shown that the transition from vacuum to a medium with ε > 1 causes the As(O)OH fragment to lose its flatness. The solvent polar medium introduces significant changes in the geometry and IR spectral parameters of hydrogen-bonded complexes too: as the polarity of a medium increases, weak hydrogen bonds become weaker, and strong and medium hydrogen bonds become stronger; in the case of a complex with two hydrogen bonds cooperativity effects are observed. In almost all cases the driving force of these changes appears to be preferential solvation of charge-separated structures. In the limiting case of complete deprotonation (or conversely complete protonation) the vibrational frequencies of νAsO and νAs-O turn into νAs-O(asym) and νAs-O(sym), respectively. In the intermediate cases the distance between νAsO and νAs-O is sensitive to both implicit solvation and explicit solvation and the systematic changes of this distance can be used for estimation of the degree of proton transfer within the hydrogen bond.