Inverting the Solvatochromism of Pyridinium-N-phenolate Dyes by the Addition of a Second Pyridinium Unit.

Three bipyridinium phenolates were synthesized, and their spectral behavior was recorded at various solvent polarities and compared to a classic pyridinium phenolate dye possessing only one pyridinium acceptor ring in its structure. The addition of a second pyridinium unit to the classic solvatochromic core results in an unexpected change in the spectral behavior from negative solvatochromism (displacement of the absorption band to shorter wavelengths) to inverted solvatochromism, characterized by the transition from negative to positive solvatochromism (displacement of the absorption band to longer wavelengths) at moderate solvent polarities.

A simple method to estimate the mean number of lipophilic molecules on nanoparticle surfaces by fluorescence measurements.

Measurements of fluorescence intensity of the hydrophobic pyridinium salt (DTPSH) remaining in the organic phase after partition experiments in the DCM/H2O system allowed an approximate method to be developed to estimate the mean number of molecules (N = 942) on the surface of 22.8 nm gold nanoparticles and the separation (1.89 nm) between these organic molecules. This protocol is based on the ability that the organic molecules possess to coat the surface of the nanoparticle, which can migrate from the organic to the aqueous phase as a result of the driving force of the strong binding of sulfur to gold. To validate our estimation, we used a projection of the results obtained by Wales and Ulker to solve the Thomson problem, a mathematicians' challenge, used as a model to calculate the mean distance (1.82 nm) separating particles on the surface, in excellent agreement with the results obtained by our method. The quality of results, the simplicity of calculations, the low fluorescence detection limit, and the inexpensive materials, recommend this procedure for rapid estimates of the mean number of molecules on the surface of nanoparticles.

An Intervention Delivered by Mobile Phone Instant Messaging to Increase Acceptability and Use of Effective Contraception Among Young Women in Bolivia: Randomized Controlled Trial.

BACKGROUND: Although the most effective methods of contraception are available in Bolivia, unmet need for contraception among women aged 15 to 19 years is estimated to be 38% (2008), and the adolescent fertility rate is 71 per 1000 women (2016). Mobile phones are a popular mode to deliver health behavior support. We developed a contraceptive behavioral intervention for young Bolivian women delivered by mobile phone and guided by behavioral science. The intervention consists of short instant messages sent through an app over 4 months. OBJECTIVE: This trial aimed to evaluate the effect of the intervention on young Bolivian women's use of and attitudes toward the effective contraceptive methods available in Bolivia. METHODS: This was a parallel group, individually randomized superiority trial with a 1:1 allocation ratio. Women were eligible if they were aged 16 to 24 years, owned a personal Android mobile phone, lived in La Paz or El Alto, reported an unmet need for contraception, and could read Spanish. The target sample size was 1310 participants. Participants allocated to the intervention had access to an app with standard family planning information and intervention messages. Participants allocated to the control group had access to the same app and control messages. Coprimary outcomes were use of effective contraception and acceptability of at least one method of effective contraception at 4 months. Secondary outcomes were use of effective contraception during the study, acceptability of the individual methods, service uptake, unintended pregnancy, and abortion. Process outcomes included knowledge, perceived norms, personal agency, and intention. Outcomes were analyzed using logistic and linear regression. We also asked participants about physical violence. RESULTS: A total of 640 participants were enrolled, and 67.0% (429) of them contributed follow-up data for the coprimary outcome, the use of effective contraception. There was no evidence that use differed between the groups (33% control vs 37% intervention; adjusted odds ratio [OR] 1.19, 95% CI 0.80 to 1.77; P=.40). There was a borderline significant effect regarding acceptability (63% control vs 72% intervention; adjusted OR 1.49, 95% CI 0.98 to 2.28; P=.06). There were no statistically significant differences in any of the secondary or process outcomes. The intervention dose received was low. In the control group, 2.8% (6/207) reported experiencing physical violence compared with 1.9% (4/202) in the intervention group (Fisher exact test P=.75). CONCLUSIONS: This trial was unable to provide definitive conclusions regarding the effect of the intervention on use and acceptability of effective contraception because of under recruitment. Although we cannot strongly recommend implementation, the results suggest that it would be safe and may increase the acceptability of effective contraception if the intervention messages were offered alongside the download of the app. TRIAL REGISTRATION: ClinicalTrials.gov NCT02905526; https://clinicaltrials.gov/ct2/show/NCT02905526.

Hydroxyl Radical Generation and DNA Nuclease Activity: A Mechanistic Study Based on a Surface-Immobilized Copper Thioether Clip-Phen Derivative.

Coordination compounds of copper have been invoked as major actors in processes involving the reduction of molecular oxygen, mostly with the generation of radical species the assignment for which has, so far, not been fully addressed. In the present work, we have carried out studies in solution and on surfaces to gain insights into the nature of the radical oxygen species (ROS) generated by a copper(II) coordination compound containing a thioether clip-phen derivative, 1,3-bis(1,10-phenanthrolin-2-yloxy)-N-(4-(methylthio)benzylidene)propan-2-amine (2CP-Bz-SMe), enabling its adsorption/immobilization to gold surfaces. Whereas surface plasmon resonance (SPR) and electrochemistry of the adsorbed complex indicated the formation of a dimeric Cu(I) intermediate containing molecular oxygen as a bridging ligand, scanning electrochemical microscopy (SECM) and nuclease assays pointed to the generation of a ROS species. Electron paramagnetic resonance (EPR) data reinforced such conclusions, indicating that radical production was dependent on the amount of oxygen and H2 O2 , thus pointing to a mechanism involving a Fenton-like reaction that results in the production of OH(.) .

The effect of micellization on the EPR spectra and reactivity of 2,2,4,4-tetramethylpiperidinoxyl (TEMPO) radicals.

A series of 4-alkanoyloxy-2,2,6,6-tetramethylpiperidinoxyl radicals was prepared, and their reactivity in water vis-à-vis antioxidant Trolox was compared. Spectral (electron paramagnetic resonance) and dynamic-light-scattering measurements suggested the formation of micelles for the more hydrophobic members of the series. The observed increase in reactivity for the micelle-forming radicals reflected the increased local concentration of the radical fragment on the micellar interface. Copyright © 2016 John Wiley & Sons, Ltd.

TEMPO-Attached Pre-fluorescent Probes Based on Pyridinium Fluorophores.

The photophysical behavior of three pyridinium-derived fluorophores, the N-aryl-2,4,6-triphenylpyridinium, the N-aryl-5,6-dihydro-2,4-diphenylbenzo[h]quinolinium and the N-aryl-5,6,8,9-tetrahydro-7-phenyldibenzo[c,h]acridinium perchlorates, was investigated. Comparison of their fluorescence quantum yields led to the preparation of a novel, more sensitive pyridinium-based, TEMPO-attached prefluorescent probe for H-abstraction processes, the N-{4-[4-(N-oxyl-2,2,6,6-tetramethylpiperidinyl)carbonylamino]phenyl}-5,6,8,9-tetrahydro-7-phenyldibenzo[c,h]-acridinium perchlorate.

Governing the emissive properties of 4-aminobiphenyl-2-pyrimidine push-pull systems via the restricted torsion of N,N-disubstituted amino groups.

Donor-acceptor-substituted biphenyl derivatives are particularly interesting model compounds, which exhibit intramolecular charge transfer because of the extent of charge transfer between both substituents. The connection of a 4-[1,1'-biphenyl]-4-yl-2-pyrimidinyl) moiety to differently disubstituted amino groups at the biphenyl terminal can offer push-pull compounds with distinctive photophysical properties. Herein, we report a comprehensive study of the influence of the torsion angle of the disubstituted amino group on the emissive properties of two pull-push systems: 4-[4-(4-N,N-dimethylaminophenyl)phenyl]-2,6-diphenylpyrimidine (D1) and 4-[4-(4-N,N-diphenylaminophenyl)phenyl]-2,6-diphenylpyrimidine (D2). The torsion angle of the disubstituted amino group, either N,N-dimethyl-amine or N,N-diphenyl-amine, at the biphenyl end governs their emissive properties. A drastic fluorescence quenching occurs in D1 as the solvent polarity increases, whereas D2 maintains its emission independently of the solvent polarity. Theoretical calculations on D1 support the presence of a twisted geometry for the lowest energy, charge-transfer excited state (S1,90), which corresponds to the minimum energy structure in polar solvents and presents a small energy barrier to move from the excited to the ground state, thereby favoring the non-radiative pathway and reducing the fluorescence efficiency. In contrast, this twisted structure is absent in D2 due to the steric hindrance of the phenyl groups attached to the amine group, making the non-radiative decay less favorable. Our findings provide insights into the crucial role of the substituent in the donor moiety of donor-acceptor systems on both the singlet excited state and the intramolecular charge-transfer process.

A simple method for the determination of the partitioning of nitroxide probes in microheterogeneous media.

The use of the ESR g-factor of spin labels for the easy and clean assessment of their partitioning in microheterogeneous media is described. The method constitutes a valuable alternative in those cases where the classical Benesi-Hildebrand treatment, based on UV-visible measurements, is difficult or not feasible.

Exchange interactions through π-π stacking in the lamellar compound [{Cu(bipy)(en)}{Cu(bipy)(H2O)}{VO3}4]n.

Structural, magnetic, and powder and single-crystal electron paramagnetic resonance (EPR) studies were performed on [{Cu(bipy)(en)}{Cu(bipy)(H(2)O)}{VO(3)}(4)](n) (bipy = 2,2'-bipyridine, en = ethylenediamine), which is a new copper-vanadium hybrid organic-inorganic compound containing Cu(II) and V(V) centers. The oxovanadium units provide an anionic scaffolding to the structure, where two types of Cu(II) coordination modes, octahedral (Cu1) and square pyramidal (Cu2), contribute to the magnetic properties. The crystal structure contains layers including Cu1 and Cu2 ions, separated by stacked arrangements of 2,2'-bipyridine molecules. Each type of Cu(II) ion in these layers forms parallel spin chains described by exchange coupling parameters J(1) and J(2) for Cu1 and Cu2, respectively (exchange couplings defined as H(ex)(i,j) = -J(ij)S(i)S(j)), which, for necessity, are assumed to be equal to J. These chains are coupled by much weaker Cu1-Cu2 exchange interactions J(3) connecting neighbor Cu1 and Cu2 ions within a layer, through paths acting as rungs of a ladder chain structure. The average coupling J, which is antiferromagnetic (J < 0), according to the susceptibility data, is estimated with similar results with a mean field approximation (J = -1.4 cm(-1)), and with a uniform chain model (J = -1.7 cm(-1)). The EPR spectra of powdered samples and oriented single crystals are shown to be independent of J(1) and J(2), but are dependent on the weak coupling J(3), and the data allow a lower limit to be established: |J(3)| > 0.04 cm(-1). The spectra are also strongly sensitive to extremely weak coupling interactions with average magnitude J(4) between copper atoms in neighboring layers, separated by ∼10 Å, using the stacked 2,2'-bipyridine molecules, which produce a 2D-to-3D quantum phase transition. This is observed in single-crystal samples when the energy levels are changed with the orientation of the magnetic field. From the characteristics of these transitions, we estimate a value of |J(4)| = 0.0034 ± 0.0004 cm(-1) between Cu(II) ions in neighboring layers. This work emphasizes the important possibilities of EPR to evaluate extremely small exchange couplings between metal ions in a solid material, even in the presence of other much larger couplings.

Interfacial kinetics in olive oil-in-water nanoemulsions: Relationships between rates of initiation of lipid peroxidation, induction times and effective interfacial antioxidant concentrations.

HYPOTHESIS: A detailed quantitative description of the effects of antioxidants in inhibiting lipid peroxidation in oil-in-water emulsions can be achieved by determining the relationships between the rates of initiation of the lipid peroxidation reaction, the length of the induction period preceding the propagation step of the radical oxidation process and the effective antioxidant interfacial concentrations. EXPERIMENTS: We successfully prepared and characterized a series of olive oil-in-water nanoemulsions and allowed them to spontaneously oxidize. Their oxidative stability was evaluated by carrying out in the presence, and absence, of antioxidants derived from gallic acid, by monitoring the formation of primary oxidation products with time, by determining the corresponding induction periods, and by determining the effective interfacial concentrations of the antioxidants in the intact emulsions. FINDINGS: Results show that both, the length of the induction periods and the antioxidant interfacial concentrations change concomitantly, increasing with the hydrophobicity of the antioxidant up to a maximum at the octyl derivative; longer aliphatic chains decrease their efficiency. The ratio between the interfacial antioxidant concentration and the induction period remains constant independently of the antioxidant, demonstrating that the effective concentrations of antioxidant at the interface control their efficiencies in emulsions.

Oxygen Reduction Reaction at Penta-Coordinated Co Phthalocyanines.

From the early 60s, Co complexes, especially Co phthalocyanines (CoPc) have been extensively studied as electrocatalysts for the oxygen reduction reaction (ORR). Generally, they promote the 2-electron reduction of O2 to give peroxide whereas the 4-electron reduction is preferred for fuel cell applications. Still, Co complexes are of interest because depending on the chemical environment of the Co metal centers either promote the 2-electron transfer process or the 4-electron transfer. In this study, we synthetized 3 different Co catalysts where Co is coordinated to 5 N atoms using CoN4 phthalocyanines with a pyridine axial linker anchored to carbon nanotubes. We tested complexes with electro-withdrawing or electro-donating residues on the N4 phthalocyanine ligand. The catalysts were characterized by EPR and XPS spectroscopy. Ab initio calculations, Koutecky-Levich extrapolation and Tafel plots confirm that the pyridine back ligand increases the Co-O2 binding energy, and therefore promotes the 4-electron reduction of O2. But the presence of electron withdrawing residues, in the plane of the tetra N atoms coordinating the Co, does not further increase the activity of the compounds because of pull-push electronic effects.

The location of amphiphobic antioxidants in micellar systems: The diving-swan analogy.

A protocol for determining the location of antioxidants (AOs) in a micro-heterogeneous medium was applied to three series of AOs with increasing hydrophobicities: chromancarboxylic acid ("Trolox") esters, caffeic acid and its esters, and gallic acid and its esters. The observed paradoxical behaviour of these and other commonly encountered antioxidants was rationalized with the aid of a pictorial simile, the "diving-swan" analogy, that explains the orientation and location of an amphiphobic AO when it reacts with a radical probe in the micellar interface.

Hydrogen-transfer reactions from phenols to TEMPO prefluorescent probes in micellar systems.

A nitroxide prefluorescent probe has been used to evaluate local reactivity of antioxidants in micellar systems. An apparent rate constant that directly reflects the relevance of antioxidant hydrophobicity on the reaction toward nitroxide radicals has been defined. Dramatic increases in this parameter for quercetin are shown on moving from methanol to micellar media: 90 and 230 fold enhancements for SDS and Triton X100 micelles, respectively. This is a clear consequence of the favorable partition of reactants in the micelles.

Antioxidant-spotting in micelles and emulsions.

A simple protocol is described for locating the site of action of an antioxidant (AO) in a micro-heterogeneous mixture, based on the pattern of the reactivity curve towards the AO of a series of 4-alkanoyl TEMPO radicals. The resulting cut-off curves yield information regarding the hydrophobic microenvironment surrounding the reactive AO group, and its accessibility by the probe. Convex curves are an indication of an AO located in a more hydrophilic environment, while concave plots originate from AOs in a more hydrophobic location in the micro-heterogeneous system.

The Solvatofluorochromism of 2,4,6-Triarylpyrimidine Derivatives.

Seven new 2,4,6-triarylpyrimidines were synthesized and their solvatofluorochromism investigated in 12 solvents and in an aqueous micellar solution of reduced Triton X-100. A multiparametric analysis of their emission band showed that the solvent dipolarity and basicity were mainly responsible for their solvatofluorochromism, which arose from an internal charge-transfer from a donor fragment to the pyrimidine acceptor, confirmed by theoretical calculations. In the micellar system, quenching of their fluorescence by addition of derivatives of 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) radical was investigated and the results were consistent with the spectral changes brought about by the micro-heterogeneous system.

Transient enol isomers of dibenzoylmethane and avobenzone as efficient hydrogen donors toward a nitroxide pre-fluorescent probe.

Dibenzoylmethane and avobenzone photochemistry involves the formation of transient enol isomers (Z and E). Conjugation of the OH group with the carbonyl group in these transient isomers reduces the OH bond energy. A fast reduction of the pre-fluorescent probe (C343T) was observed after addition to photolysed DBM samples in nonpolar solvents. This can be attributed to a hydrogen transfer reaction from the transient E accumulated. While no quenching for Z by TEMPO was detected in the laser flash photolysis timescale. Theoretical calculations of spin densities distribution of the radical formed from Z and E showed a more delocalized distribution that would indicate a low reactivity towards oxygen. Our results suggest that DBM and avobenzone can effectively behave as photoantioxidants or photoactivated antioxidants under conditions where its enol isomers can be accumulated.

Cut-off effect of radical TEMPO derivatives in olive oil-in-water emulsions.

Three oil-in-water emulsions were prepared from mixtures of olive oil and Tween 20 in water. The effectiveness of a series of radical 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) derivatives of variable lipophilicity in reactions with antioxidant Trolox, and as pyrene-fluorescence quenchers, was compared in the three emulsions. A "cut-off" effect was observed for the pyrene quenching by the probes, but not for their reaction with Trolox. The results were rationalized in terms of the amphiphobic nature of the probes, and the different locations of probe, pyrene and Trolox in the three-phase microheterogeneous systems.

Magnetic field control of photoinduced silver nanoparticle formation.

The micellar photoreduction of benzophenone in the presence of Ag+ leads to very rapid and efficient formation of silver nanoperticles. External magnetic fields can be used to control the rate of formation and properties of silver nanoparticles generated by reaction of ketyl radicals formed in the photoreduction of benzophenone in surfactant micelles. The effect is attributed to Zeeman splitting of the triplet sublevels of the confined radical pair that causes a reduction in the rate of geminate processes and increases the availability of ketyl radicals (through escape processes) for metal ion reduction.

"Cut-off" effect of antioxidants and/or probes of variable lipophilicity in microheterogeneous media.

The activities of two hydrophilic (ascorbic acid and Trolox) and two hydrophobic (α-tocopherol and BHT) antioxidants were measured by reaction with a series of 4-alkanoyloxyTEMPO radical probes 1 in buffered (pH 7), aqueous, micellar solutions of reduced Triton-X 100. In all cases, a cut-off effect was observed, in line with previous observations of the same effect for the partitioning of probe series 1 in this medium. These results support an interpretation of the cut-off effect in food emulsions, based on the "amphiphobic" nature of either the antioxidants or probes: competition between two molecular moieties, for the micellar hydrophobic core, tends to expose a reacting fragment differently to a more hydrophilic microenvironment, as the probe or antioxidant hydrophobicity increases.