A combined experimental and theoretical study on photoinduced intramolecular charge transfer in trans-ethyl p-(dimethylamino)cinamate

Intramolecular charge transfer (ICT) behavior of trans-ethyl p-(dimethylamino)cinamate (EDAC) in various solvents has been studied by steady-state absorption and emission, picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption experiments as well as time-dependent density functional theory (TDDFT). Large fluorescence spectral shift in more polar solvents indicates an efficient charge transfer from the donor site to the acceptor moiety in the excited state compared to the ground state. The energy for 0,0 transition (ν_0,0) for EDAC shows very good linear correlation with static solvent dielectric property. The relaxation dynamics of EDAC in the excited state can be effectively described by a “three state” model where, the locally excited (LE) state converts into the ICT state within 350 ± 100 fs. A combination of solvent reorganization and intramolecular vibrational relaxation within 0.5–6 ps populates the relaxed ICT state which undergoes fluorescence decay within few tens to hundreds of picoseconds.     

Photoinduced nitrosyl linkage isomers in complexes based on the photochromic cation [RuNO(NH3)5] with the paramagnetic anion [Cr(CN)6] and the diamagnetic anions [Co(CN)6] and [ZrF6]

Two metastable nitrosyl linkage isomers SI and SII are generated by light irradiation in the spectral range 370–500 nm in the two diamagnetic compounds [RuNO(NH₃)₅][Co(CN)₆] and [RuNO(NH₃)₅]₂[ZrF₆]₃ as well as in the paramagnetic compound [RuNO(NH₃)₅][Cr(CN)₆]. The frequencies of the ν(NO) stretching vibrations of SI and SII identify SI as the isonitrosyl Ru–O–N isomer and SII as the side-on η² isomer of NO. The population, i.e., the number of generated linkage isomers, is determined from the decrease of the area of the fundamental ν(NO) and of the higher harmonic 2 · ν(NO) of the ν(NO) stretching vibration of the ground state. Using differential scanning calorimetry (DSC) the heat release during the thermal decay of the metastable linkage isomers is determined. The activation energies, frequency factors, and the energetic position of the metastable linkage isomers are determined from the DSC and infrared spectroscopic experiments. It is found that the exchange of the counter ion significantly influences the energetic positions of the linkage isomers, while the activation energy and frequency factor are much less affected.     

The lipid-mediated hypothesis of fumonisin B1 toxicodynamics tested in model membranes

The disruption of lipidic metabolism was considered a good candidate to explain FB1 toxicity mechanism. In the present work we investigated molecular organizational changes induced by FB1–biomembrane interaction possibly involved in mycotoxic effects.

FB1 was self-aggregated with a critical micellar concentration of 1.97 mM. FB1 (0–81.4 μM), decreased in a dose-dependent manner, the fluorescence anisotropy of TMA-DPH (from 0.349 ± 0.003 to 0.1720 ± 0.0035) in dpPC bilayers, whilst no differences were registered with DPH. At 5.6 μM in the subphase, FB1 increased the lateral surface pressure (π) of a Langmuir film to an extent that depended on the monolayer composition (Δπ_{dpPC:DOTAP 3:1} > Δπ_dpPC:dpPA3:1 > Δπ_dpPC), the molecular packing (Δπ decreased linearly as a function of the initial π) and the subphase pH (Δπ_{pH 2.6} > Δπ_{pH 7.4} and maximal π allowing the drug penetration π_cut-off was 34.3 and 27.7 mN/m at pH 2.63 and 7.4, respectively). FB1 increased the surface potential of dpPC and dpPC:DOTAP monolayers and decreased that of dpPC:dpPA. This suggested that FB1 acquired different orientations and/or foldings depending on the surface electrostatics and the toxin charge state. Moreover, FB1–lipid interactions were transduced into long-range effects at the mesoscopic level affecting the lipidic self-separated lateral domains shape and density.     

Release properties on gelatin-gum arabic microcapsules containing camphor oil with added polystyrene

In this study, gelatin blended with arabic gum microcapsules containing camphor oil with added polystyrene were fabricated by a compound coacervation method. The parameters of oil/wall volume ratio, emulsification stirring speed, concentration of cross-linking agent, treated time and oil release properties were investigated. In order to improve the constant release effect of camphor oil, oil-soluble polystyrene (PS) was used as a sustained release agent. The camphor oil release curves were expressed by the exponential equation: ψ(t) = C_eq(1–e^−t/τ), where ψ(t) represent the variant of camphor oil concentration in the operation environment, C_eq as the equilibrium state, t as the release time and τ as time constant. C_eq and τ are significant factors pertaining to the camphor oil release properties. The results indicated that, for the microcapsules, the optimal oil/wall volume ratio was 0.75 to achieve the encapsulation efficiency of 99.6 wt.%. The average particle size were 294.7 ± 14.2 μm, 167.2 ± 11.2 μm, 85.7 ± 8.7 μm at the homogenization stirring speed of 500, 1000, and 2000 rpm, respectively. The effect of sustained oil release will increase whereas the stirring speed decreases and the concentration of glutaraldehyde (GA) and treated time increases. Along with the increasing of quantity of polystyrene added, C_eq decreased and τ increased, indicating that the sustained oil release amount and the release rate depend on the quantity of PS considerably.     

An accurate expression for radial distribution function of the Lennard-Jones fluid

A simple and accurate expression for radial distribution function (RDF) of the Lennard-Jones fluid is presented. The expression explicitly states the RDF as a continuous function of reduced interparticle distance, temperature, and density. It satisfies the limiting conditions of zero density and infinite distance imposed by statistical thermodynamics. The distance dependence of this expression is expressed by an equation which contains 11 adjustable parameters. These parameters are fitted to 353 RDF data, obtained by molecular dynamics calculations, and then expressed as functions of reduced distance, temperature and density. This expression, having a total of 65 constants, reproduces the RDF data with an average root-mean-squared deviation of 0.0152 for the range of state variables of 0.5  T^*  5.1 and 0.35  ρ^*  1.1 (T^*=kT/ε and ρ^* = ρσ³ are reduced temperature and density, respectively). The expression predicts the pressure and the internal energy of the Lennard-Jones fluid with an uncertainty that is comparable to that obtained directly from the molecular dynamics simulations.     

Application of the ISM EoS for polymer solutions and blends

A method for predicting an analytical equation of state for polymer mixtures and blends from surface tension and liquid state density at normal (ordinary) temperature (γ_n, ρ_n), as scaling constants, is presented. B₂(T) follows a promising corresponding-states principle. Calculation of (T) and b(T), the two other temperature-dependent constants of the equation of state, are made possible by scaling. As a result, γ_n and ρ_n are sufficient for determination of thermophysical properties of polymer mixtures and blends.

We applied the procedure to predict liquid density of poly(ethylene glycol) (PEG-200) + 1-octanol solutions and poly(propylene glycol) (PPG) + poly(ethylene glycol) (PEG-200) blends at compressed state with temperature range from 298.15 to 338.15 K and pressures up to 40 MPa. In this work, the ISM EoS is extended to polymer mixtures and blends as well as pure case without proposing any mixing rule.     

A density functional study of N NQR parameters of imidazole derivatives and extrapolation to PfHRP2–Fe-PPIX complex

In this paper, density functional theory (DFT) was used to calculate ¹⁴N nuclear quadrupole coupling constants (NQCC), χ, and asymmetry parameters, η, for a series of imidazole derivatives: imidazole, 5-methylimidazole and histidine. These calculations were carried out with the PW91P86 method via the Gaussian 98 package. A systematic theoretical investigation of the different environmental effects on (χ, η) values of amino ¹⁴N1 and imino ¹⁴N2 of imidazole ring of these compounds, reveals that the local surrounding of nitrogen atoms play an important role in determining their χ and η values. Our calculations in solution show that adding explicit solvent molecules to the polarizable continuum model (PCM) has a strong effect on (χ, η) values, thereby indicating that for long-range effects, PCM, is not sufficient to describe the whole solvent effects. We also evaluate the influence of [Fe³⁺ (S = 1/2)] on the (χ, η) values of proximal and remote nitrogens of an axial ligand and compare with those of free ligands. The results show that Fe³⁺ has a strong effect on the (χ, η) values of proximal nitrogen unlike remote nitrogen. Finally, our results predict (χ = 1.56 MHz, η = 0.690) for proximal nitrogen and (χ = 2.75 MHz, η = 0.169) for remote nitrogen in PfHRP2–Fe³⁺-PPIX complex.     

Photophysical and photochemical properties of tetrasulfonated silicon and germanium phthalocyanine in aqueous and non-aqueous media

The photophysical and photochemical properties of tetrasulfonated silicon and germanium phthalocyanine (SiPcS₄ and GePcS₄) in aqueous solution (phosphate-buffered saline (PBS) solution, pH 7.4) (in the presence and absence of cremophore EL (CEL)) and in dimethylsulphoxide (DMSO) were studied. The complexes have intense absorption in the visible/near-IR region though they highly aggregate in aqueous solution with a dimerization constant of 2 × 10⁴ dm³ mol⁻¹. The fluorescence excitation spectra however have only one band suggesting that only the monomer fluoresces. Both the quantum yields of the triplet state (Φ_T) and the triplet lifetimes (τ_T) were found to be higher in DMSO compared to in aqueous solution. Aggregation is hindered by addition of cremophore EL in aqueous solution and this induced disaggregation caused an increased Φ_T and τ_T probably due to the reduced interaction of the phthalocyanines with the aqueous medium in the presence of CEL.     

Determination of concentration-dependent transport coefficients in nanofiltration: Defining an optimal set of coefficients

The currently used equation for solute transport in nanofiltration contains two parameters (ω and 1 – σ) that may be concentration-dependent. A force balance equation allows interpretation of these parameters (as well as L_p) in terms of distribution and friction coefficients, as was demonstrated for a neutral solute and a single 1:1 salt. It is generally assumed in model calculations that it is the distribution coefficient that determines the concentration dependence of ω and 1 – σ. This suggests that a more practically convenient form of the equation may be proposed, in which only one concentration-dependent parameter, ω, appears, while the other is replaced with the ratio of the two, A, which has the meaning of the membrane Peclét number divided by the volume flux and may be assumed to be constant. This may facilitate the analysis of flux–rejection curves and parameter evaluation including concentration dependence, which is a crucial and unavoidable step towards predictive NF modeling. The direct connection between transport parameters and distribution coefficients also suggests that experimentally measured concentration dependence may help to discriminate between different exclusion mechanisms. An approximate analysis based on the connection between A and solute–water friction shows that for presently used NF membranes and realistic fluxes the expected contribution of convection to solute flow cannot become dominant so that the limiting value for salt rejection, R = σ, cannot be reached.     

Amphiphile bilayer films from DPPC: bilayer lipid membranes and Newton black films

Amphiphile bilayer films are obtained from 1,2 dipalmitoyl-glycero-3-phosphocholine (DPPC): bilayer lipid membranes (BLM) and Newton black films (NBF), through thinning of the respective thin liquid films, thus allowing for a very precise determination of the moment of their formation. Stability (or rupture) and formation of BLM and NBF are considered from a unified point of view with the microscopic theory of Kashchiev–Exerowa [J. Colloid Interface Sci., 77 (1980) 501–511], based on the formation of nanoscopic holes in them. BLM and NBF are obtained and studied with the microinterferometric method of Scheludko–Exerowa in its contemporary version. The equivalent thickness of both BLM (in benzene solution between two water phases with 0.1 M NaCl) and NBF in aqueous DPPC solution (in the presence of 0.1 M NaCl) is determined as being h_w = 7.0 nm for BLM and h_w = 7.8 nm for NBF. By means of the dependences: BLM lifetime versus DPPC concentration and probability for BLM formation versus DPPC concentration, it is established that there exist metastable BLM and stable NBF. The good fit between the experimental results of τ(C) dependence and theory in the case of BLM allow to determine the three constants: pre-exponential factor A = 1.5 × 10⁻³ s, related to the process kinetics; constant B = 20.2 ± 0.2, related to the specific hole energy γ = 1.7 × 10⁻¹¹ J/m and the equilibrium concentration C_e = 6 × 10⁻⁴ ± 7.2 × 10⁻⁶ m/l. The specific hole linear energy γ = 1.7 × 10⁻¹¹ J/m determined as well as the binding energy Q between first neighbor molecules in the bilayers Q = 1.48 × 10⁻¹⁹ J (36 kT) are lower than the ones determined for DPPC foam bilayer in gel state γ = 9.1 × 10⁻¹¹ J/m and Q = 55 kT. This means that interaction is weaker in the case of BLM. The critical concentration C_c at which bilayer formation starts is: for BLM C_c = 30 μg/ml and for NBF C_c = 70 μg/ml. This concentration characterizes quantitatively the formation of the amphiphile bilayer and is a very useful parameter that can be used for various purposes.     

Effect of salts on the electrical conductance of a fluorine-containing poly(carboxylic acid), PPFNA

Electrical conductance and other solution properties of aqueous solutions of a fluorine-containing poly(carboxylic acid), (poly(9H,9H-perfluoro-2,5-dimethyl-3,6-dioxa-8-nonenoic acid), PPFNA) were studied with special attention to the salt effect. This polymer dissociated strongly resulting in a low pH value in unneutralized state (β = 0, β: degree of neutralization). The specific conductance was the highest at β = 0 and decreased as β increased. A considerable increase in conductance was observed by titrating NaCl at low β, because large amounts of bound protons were released by addition of NaCl. The amounts of released protons exceeded those originally dissociated at β = 0. Such an anomalous proton liberation suggests that this polymer is a fairly strong polyacid but not a typical one such as poly(styrene sulfonic acid). Under fully neutralized state (β = 1), however, the solution conductance was lower than the sum of the polymer and NaCl added, due to polyion–salt ion interaction.     

Structure modification in hen egg yolk low density lipoproteins layers between 30 and 45 mN/m observed by AFM

We have studied the structure of films made by low density lipoproteins (LDL) from hen egg yolk, which are composed of apoproteins, neutral lipids and phospholipids. These LDL have been deposited on air–water interface to form a monolayer which has been compressed to measure an isotherm using Langmuir balance. This isotherm presented three transitions (neutral lipid (surface pressure, π = 19 mN/m), apoprotein–lipid (π = 41 mN/m) and phospholipid (π = 51 mN/m) transitions). We have studied only the apoprotein–lipid transition. In order to observe the LDL film structure before (π = 30 mN/m) and after (π = 45 mN/m) the apoprotein–lipid transition, the formed films were transferred and visualised by atomic force microscopy (AFM). Our results have shown that the structures observed in the LDL film were different depending on the surface pressure. The apoproteins and neutral lipids appeared to be miscible up to the apoprotein–lipid transition, when demixing occured. The structures observed after the apoprotein–lipid transition should be due to the demixing between apoproteins and neutral lipids. On the other hand, apoproteins and phospholipids seemed miscible whatever the surface pressure. Hence, the first transition (π = 19 mN/m) should be attributed to the free neutral lipid collapse; the second transition (π = 41 mN/m) should be attributed to the demixing of apoprotein–neutral lipid complexes; and the last transition (π = 51 mN/m) should be attributed to phospholipid collapse or to demixing of apoprotein–phospholipid complexes.     

Kinetic investigation of electronic energy transfer processes following the pulsed dye-laser generation of excited atomic barium, Ba[6s6p(P1)], in the presence of atomic strontium

The collisional behaviour of Ba[6s5d(³D_J)], 1.151 eV above the 6s²(¹S₀) electronic ground state, in the presence of atomic strontium, has been investigated in the ‘long-time domain' (ca. 100 μs–1 ms) following the pulsed dye-laser excitation of barium vapour at elevated temperature at λ = 553.5 nm (Ba[6s6p(¹P₁)] ← Ba[6s²(¹S₀)]. Ba(³D_J) is subsequently produced from the short-lived ¹P₁ state (τ_e = 8.37 ± 0.38 ns) by a number of radiative and collisional processes. It may then be monitored in the ‘long-time domain' by atomic spectroscopic marker methods involving either collisional activation of Ba(³D_J) by Ba(¹S₀) and He buffer gas to yield Ba[6s6p(³P_J)] with subsequent emission from the ³P₁ state (τ_e = 1.2 ± 0.1 μs): Ba[6s6p(³P₁)] → Ba[6s²(¹S₀)] + hv (λ = 791.1 nm). Alternatively, emission from Ba(¹P₁) may be monitored at long times following the generation of this short-lived state by energy pooling following self-annihilation of Ba(³D_J) + Ba(³D_J) from Ba[6s6p(¹P₁)] → Ba[6s²(¹S₀)] + hv (λ = 553.5 nm). The generation of Ba(³D_J) in the presence of atomic strontium yields emission in the long-time domain from Sr[5s5p(³P₁)] (τ_e = 19.6 μs): Sr[5s5p(³P₁)] → Sr[5s²(¹S₀)]  + hv (λ = 689.3 nm). Whilst the decay profiles at short times are complex in form, at long times all these atomic profiles show first-order kinetic removal with the decay coefficients for λ = 791.1 nm, 689.3 nm and 553.5 nm emissions in the ratio 1 : 2 : 2, consistent with overall third-order activation of the form: Ba(³D_J) + Ba(³D_J) + Sr(¹S₀) → Sr(³P_J) + 2Ba(¹S₀). The mechanism is modelled in detail, including measurement of integrated emission intensities, yielding kinetic data for fundamental collisional processes. The overall rate constant for the third-order collisional activation of Sr[5s5p(³P_J])from 2Ba[6s5d(³D_J)] + Sr[5s²(¹S₀)] takes the upper limit of 5.8 × 10⁻²⁷ cm⁶ atom⁻² s⁻¹ (T = 900 K). The rate constant for the two body collisional quenching of Ba[6s5d(³D_J)] by ground state atomic strontium, Sr[5s²(¹S₀)], is found to be (2.0 ± 0.1) × 10⁻¹² cm³ atom⁻¹ s⁻¹ (T = 900 K).     

Dynamics of 4,4'-di-n-heptylazoxybenzene (HAB) studied using dielectric and 2H NMR relaxation measurements

Results of studies of 4,4'-di-n-heptylazoxybenzene (HAB) in the isotropic, nematic and smectic A phases are presented. Two experimental methods were employed: broad band dielectric spectroscopy and nuclear magnetic resonance spectroscopy. The complex dielectric permittivity, ε*(ω)=ε'(ω)-iε'(ω), was measured in the frequency range 1 kHz-4 GHz. This allowed two main relaxation processes to be separated in all the phases studied: the low frequency (l. f.) process connected with molecular reorientations around the short axes, and the high frequency (h. f.) process connected with the rotations around the long axes. The corresponding relaxation times and activation enthalpies were obtained. The l. f. relaxation time changes step-wise at the phase transitions, whereas the h. f. relaxation time passes smoothly through all the phases. The measurement of ²H spin-lattice relaxation times was carried out throughout the mesophase range at 61.38 MHz. These data were analysed together with the relaxation times measured at 10.00 and 46.04 MHz, available from previous studies. Using suitable theoretical models the principal components of the diffusional tensor, D_‖ and D_⊥, as well as the diffusion coefficients D_R relative to the internal rotation of the phenyl rings, were determined. The results of both studies are compared and discussed.     

Interfacial characteristics of β-casein spread films at the air–water interface

Casein is well known to be a good protein emulsifier and β-casein is the major component of casein and commercial sodium caseinate. This work studies the behaviour of β-casein at the interface. The interfacial characteristics (structure and stability) of β-casein spread films have been examined at the air–water interface in a Langmuir-type film balance, as a function of temperature (5–40°C) and aqueous phase pH (pH 5 and 7). From surface pressure–area isotherms (π–A isotherms) as a function of temperature we can draw a phase diagram. β-Casein spread films present two structures and the collapse phase. That is, there is a critical surface pressure and a surface concentration at which the film properties change significantly. This transition depends on the temperature and the aqueous phase pH. The film structure was observed to be more condensed and β-casein interfacial density was higher at pH 5. β-Casein films were stable at surface pressures lower than equilibrium surface pressure. In fact, no hysteresis was observed in π–A isotherms after continuous compression-expansion cycles or over time. The relative area relaxation at constant surface pressure (10 or 20 mN m⁻¹) and the surface pressure relaxation at constant area near the monolayer collapse, can be fitted by two exponential equations. The characteristic relaxation times in β-casein films can be associated with conformation–organization changes, hydrophilic group hydration and/or surface rheology, as a function of pH.     

Excited state dipole moments of N,N-dimethylaniline from thermochromic effect on electronic absorption and fluorescence spectra

The effect of temperature on absorption and fluorescence spectra of N,N-dimethylaniline (DMA) in ethyl acetate has been studied for temperature ranging from 293 to 388 K. The permittivity ε and refractive index n of the solvent decrease with temperature increase and the absorption and fluorescence bands are blue shifted (so-called “thermochromic shift”). Based on this phenomenon, the dipole moment μ_e in the excited singlet state and the Onsager interaction radius a for DMA were determined using the Bilot and Kawski theory [L. Bilot, A. Kawski, Z. Naturforsch. 17a (1962) 621; 18a (1963) 10, 256].

For the known dipole moment in the ground state μ_g = 1.61 D and for /a³ = 0.54 ( is the polarizability of the solute) the average value of μ_e = 3.55 D and a = 3.1 Å were determined. The obtained values for DMA are compared with the experimental values determined by other authors.     

H NMR relaxation in urea

Proton NMR spin–lattice relaxation times T₁ were measured for urea as a function of temperature. An activation energy of 46.3 ± 4.7 kJ/mol was extracted and compared with the range of 38–65 kJ/mol previously reported in the literature as measured by different magnetic resonance techniques. In addition, proton NMR spin–lattice relaxation times in the rotating frame T_1ρ were measured as a function of temperature. These measurements provide acquisition conditions for the ¹³C and ¹⁵N CP/MAS spectra of pure urea in the crystalline phase.     

Thermogravimetric study on perovskite-like oxygen permeation ceramic membranes

The oxygen permeation properties of mixed-conducting ceramics SrFeCo_0.5O_3−δ (SFCO), Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO), La_0.2Sr_0.8Co_0.8Fe_0.2O_3−δ (LSCFO) and Ba_0.95Ca_0.05Co_0.8Fe_0.2O_3−δ (BCCFO) were studied by thermogravimetric method in the temperature range 600–900 °C. The results show that the oxygen adsorption rate constants k_a of all material are larger than oxygen desorption rate constants k_d and both k_a and k_d are not strongly dependent on temperature in the studied temperature range. The oxygen vacancy contents δ(N₂) and δ(O₂) in nitrogen and oxygen and their difference Δδ = δ(N₂) − δ(O₂) play an important role in determining the temperature behavior of oxygen permeation flux J_O2.     

Stress relaxation behaviour of dipalmitoyl phosphatidylcholine monolayers spread on the surface of a pendant drop

Dipalmitoyl phosphatidylcholine (DPPC) monolayers were characterised by surface pressure/area isotherms (π/A) and surface dilational rheological parameters at temperatures 20–40°C. The methods used were the Langmuir trough and the pendant drop micro-film balance. The latter allows accurate measurements at higher temperatures and transient drop deformation. Stable DPPC monolayers were found only for low surface pressures, π<15 mN m⁻¹. At higher monolayer compression π decreases over a long time, mainly caused by molecular rearrangement processes in the monolayer starting in the coexisting region. At π>25 mN m⁻¹ and 20°C relaxation experiments give evident of rupturing, brittle monolayer structures. At higher temperatures the monolayers became more fluid-like. π/A-isotherms determined by using both methods principally agree with each other, but show also remarkable differences, which cannot be explained so far satisfactory. Transient drop relaxation experiments were analysed for the short time range (600 s). At 20°C the dilational modulus (_r) and the surface dilational viscosity (ξ_r) passes a stationary maximum at 0.54 nm² molecule⁻¹ and increase strongly at higher surface coverage, thus indicating crystalline monolayer structure. Increasing temperature from 20 to 30°C causes a rapid decrease of _r and ξ_r and a shift of the stationary maximum to lower surface coverage. No evidence for crystalline structure is found. Further increase of temperature causes _r and ξ_r increase again. This increase is caused by a rising relaxation time, while the elasticity does not change in the same manner. Such intermediate decrease of _r and ξ_r in the range 30–40°C appears to be unusual and can be interpreted as a consequence of strong DPPC interactions and strongly pronounced retardation of monolayer deformation. The study is discussed in connection to the physiology of breathing. For pulmonary surfactants the observed behaviour seems to be understandable. It is however interesting that such complex behaviour is observed for monolayers consisting of DPPC only.