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ABSTRACT: A comprehensive study on the enantioseparation of racemic bis[1-phenylethyl]amine (PEA) on a series of molecularly imprinted polymers (MIPs) prepared using the chiral functional monomer (S)-2-(2-methyl-acryloylamino)-3-phenyl propionic acid (MAPP) is reported. MIP-R, MIP-S and MIP-RS, were synthesized separately by imprinting the pure enantiomers (R-, S-PEA) and racemic PEA, respectively, MAPP, EDGMA as crosslinker and chloroform as the porogen. It was found that all MIPs prepared were able to resolve the PEA racemate. Residence times (t(r)) and enantioselectivity factors (α) were estimated from typical elution chromatography experiments. Frontal chromatography experiments were conducted to acquire the adsorption isotherms for both enantiomers on the different MIPs (and on the non-imprinted polymer, NIP). The adsorption isotherms were analyzed using the affinity spectrum (AS) and the expectation-maximization (EM) methods. The study also involved the theoretical evaluation of the MAPP/enantiomers interactions in the pre-polymer mixture. The EM method predicts mono- and bimodal distribution of affinity binding sites depending upon the polymer analyzed. Apparently, the enantioseparation process depends on relatively small differences in the stabilization of the diasteroisomeric ion-pairs PEA/MAPP complexes on the surface of the polymers.
Journal of chromatography. A 09/2012; · 4.19 Impact Factor
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ABSTRACT: The photophysics of Safranine-O (3,6-diamino-2,7-dimethyl-5 phenyl phenazinium chloride) (SfH(+)Cl(-)) was investigated in reverse micelles (RMs) of AOT (sodium bis(2-ethylhexyl)sulfosuccinate) with special emphasis on the triplet state processes. The triplet is formed in its monoprotonated form, independently of the pH of the water used to prepare the RMs. While the intersystem crossing quantum yields in RMs are similar to those in organic solvents, the triplet lifetime is much longer. Since the pH in the water pool of AOT RMs is close to 5 and the triplet state of the dye is subjected to proton quenching, the long lifetime indicates that the dye resides in a region where it cannot be reached by protons during its lifetime. All the measurements indicate that the dye is localized in the interface, sensing a medium of micropolarity similar to EtOH : water (3:1) mixtures. The quenching by aliphatic amines was also investigated. While the quenching by the hydrophobic tributylamine is similar to that in methanol, the hydro-soluble triethanolamine is one order of magnitude more effective in RMs than in homogeneous solution. In the latter case the quenching process is interpreted by a very fast intramicellar quenching, the overall kinetics being controlled by the exchange of amine molecules between RMs. Semireduced dye is formed in the quenching process in RMs in the di-protonated state with a comparable quantum yield to the monoprotonated state formed in homogeneous solvents. The results point to the advantage of the reverse micellar system for the generation of active radicals for the initiation of vinyl polymerization, since a much lower concentration of amine can be employed with similar quantum yields.
Photochemical and Photobiological Sciences 11/2011; 11(2):302-8. · 2.58 Impact Factor
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01/2010: pages 281 - 324; , ISBN: 9780470584118
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ABSTRACT: The photophysics and photochemistry of (1-biphenyl-4-yl-1-methyl-ethyl)-tert-butyl diazene were thoroughly studied by laser flash photolysis from the picosecond to the microsecond time domain. The compound has favorable features as a radical photoinitiator and as a probe for cage effect studies in liquids, supercritical fluids, and compressed gases. The biphenyl moiety acts as an antenna efficiently transferring electronic energy to the dissociative (1)n,pi* state centered on the azo moiety. By picosecond experiments irradiating at the biphenyl- and at the azo-centered transitions, we were able to demonstrate this fact as well as determine a lifetime of 0.7 ps for the buildup of 1-biphenyl-4-yl-1-methyl-ethyl radicals (BME*). The sum of in-cage reaction rate constants of BME* radicals by combination and disproportionation is 5 x 10(10) s(-1). The free radical quantum yield in solution is 0.21 (phi(BME*)) in n-hexane at room temperature, whereas the dissociation quantum yield approaches 50%. The symmetric ketone, 2,4-bis-biphenyl-4-yl-2,4-dimethyl-pentan-2-one, was used as a reference compound for the production and reaction of BME* radicals. Transient IR measurements show CO stretching bands of the excited (3)pi,pi* and (1)n,pi* states but no dissociation up to 0.5 ns. A fluorescence lifetime of 1 ns for this ketone is consistent with this observation. By transient actinometry and kinetic decays in the microsecond time range, we measured epsilon(BME*) = (2.3 +/- 0.2) x 10(4) M(-1) cm(-1) at 325 nm and a second-order rate constant of 5.8 x 10(9) M(-1) s(-1) for the consumption of BME* radicals.
The Journal of Physical Chemistry A 05/2009; 113(19):5531-9. · 2.95 Impact Factor
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ABSTRACT: We studied the photolysis of (1-biphenyl-4-yl-1-methyl-ethyl)-tert-butyl diazene in supercritical CO(2) and Xe, as well as in compressed Kr. The compound has good solubility in the mentioned fluids, allowing the photolysis measurements to be performed in CO(2) at 1.4 K above T(c) and at pressures as low as 70 bar. We monitored relative cage effect after nanosecond laser pulses by measuring the absorbance at 320 nm (DeltaA(t-->0)) corresponding to the total amount of out-of-cage 1-biphenyl-4-yl-1-methyl-ethyl radical (BME.) produced after nitrogen loss of the diazene. In supercritical CO(2) and Xe, isothermal values of DeltaA(t-->0) showed an increase-decrease behavior with increasing pressure at constant temperature, a typical feature of the transition from the solvent energy transfer to the friction controlled regimes. The comparison of the behavior of DeltaA(t-->0) in CO(2) at reduced temperatures between 1.004 and 1.027, in Xe, and in Kr points to an absence of enhanced cage effect near the critical point. Compatibility with spectroscopic data is analyzed.
The Journal of Physical Chemistry A 04/2009; 113(18):5289-95. · 2.95 Impact Factor
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ABSTRACT: The fluorescence quenching of pyrene (Py) by a series of N-methyl and N-H substituted indoles was studied in isooctane at 298 K. The fluorescence quenching rate constants were evaluated by mean of steady-state and time-resolved measurements. In all cases, the quenching process involves a charge-transfer (CT) mechanism. The I(o)/I and tau(o)/tau Stern-Volmer plots obtained for the N-H indoles show a very unusual upward deviation with increasing concentration of the quenchers. This behavior is attributed to the self-quenching of the CT intermediates by the free indoles in solution. The efficiency of quenching of the polyaromatic by the N-H indoles increases abruptly in the presence of small amount of added pyridine (or propanol). A detailed analysis of the experimental data obtained in the presence of pyridine provides unambiguous evidence that the self-quenching process involves proton transfer from the CT states to indoles.
The Journal of Physical Chemistry A 02/2008; 112(4):589-93. · 2.95 Impact Factor
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ABSTRACT: The effect of added salts on the efficiency for photoinduced charge separation in two typical electron acceptor (A)/electron donor (D) systems was studied by the technique of laser flash photolysis. We investigate the exciplex-forming pyrene/p-dicyanobenzene (Py/DCB) and pyrene/N,N-dimethylaniline (Py/DMA) systems in ethyl acetate. The salts selected for this study are tetrabutylammonium chloride, tetrahexylammonium chloride, lithium perchlorate, sodium perchlorate, tetrabutylammonium perchlorate, sodium tetraphenylborate and tetrabutylammonium tetraphenyl-borate. In most cases, the salts quench the emission of the exciplexes with rate constants near the diffusional rate limit in ethyl acetate. An apparent red shift of the fluorescence maximum of the exciplexes with increasing salt concentration is also generally observed. Laser flash photolysis experiments showed that in the absence of the salts both A/D systems yield exclusively the triplet excited state of the polyaromatic. However, in the presence of many of the electrolytes studied, induced free radical ion formation is observed. The experimental efficiencies for induced charge separation (n) depend on the A/D system and on the nature of the salt. The measured n values vary between 0 and 0.5. The most striking variation corresponds to the lithium and sodium perchlorates. These salts are almost totally inefficient in quenching the Py/DCB exciplex, while they quench and induce charge separation from the Py/DMA exciplex with a high yield. The effect of the different salts on both exciplexes may be rationalized by using the concept of the soft/hard character of the interacting ions.
Photochemistry and Photobiology 01/2008; 68(4):481 - 486. · 2.41 Impact Factor
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ABSTRACT: Tetraalkylammonium salts, characterized by an aromatic system pending from one of the alkyl chains, are taken as model systems to study the spectroscopic and redox properties of the aromatic centre under the field effects exerted by the charged group through alkyl bridges of varying length. The changes in the aromatic's redox properties, due to the net field effect and its different components, are interpreted on theoretical bases.
Physical Chemistry Chemical Physics 01/2008; 9(45):5988-96. · 3.57 Impact Factor
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ABSTRACT: Electron-transfer rate constants were determined by means of lifetime measurements for the fluorescence quenching of 9,10-dicyanoanthracene by aromatic amines and methoxybenzenes as electron donors, and for the quenching of the synthetic dyes eosin Y and phenosafranine by a series of p-benzoquinones as electron acceptors. All determinations were carried out in acetonitrile at 298 K. The quenching rate constants (k(q)) in the region of -1.9 eV < DeltaG(et) < -0.2 eV do not decrease as predicted by Marcus theory, but they show a small increase with decreasing DeltaG(et). Although this behaviour is in qualitative agreement with the current theories for reactive systems in the diffusion limit region, a closer analysis of the experimental data showed that several aspects of the dependence of the k(q) on DeltaG(et) are not entirely explained, suggesting that new, refined theoretical models may be required.
Photochemical and Photobiological Sciences 01/2008; 7(1):104-8. · 2.58 Impact Factor
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ABSTRACT: Steady-state and laser-pulsed irradiations of dibenzyl ketone (ACOB0) and derivatives with a p-methyl or a p-hexadecyl chain (ACOB1 and ACOB16, respectively) have been conducted in polyethylene films with 0, 46, and 68% crystallinities. Calculation of the fractions of in-cage combinations of the triplet benzylic radical-pair intermediates based on photoproduct yields, Fc, from ACOB16 are shown to be incorrect as a result of the kinetic consequences of drastically different diffusion coefficients for the benzyl and p-hexadecylbenzyl radicals. Careful analyses of the transient absorption traces, based upon a new model developed here, allow the correct cage effects to be determined even from ACOB0. The model also permits the rate constants for radical-pair combinations and escape from their cage of origin to be calculated using either an iterative fitting procedure or a very simple one which requires only k-CO and the intensities of the transient absorption immediately after the flash and after the in-cage portion of reaction by the benzylic radicals is completed. Values of the rate constant for decarbonylation of the initially formed arylacetyl radicals, k-CO, have been measured from the rise portions of the laser-flash transient absorption traces. They confirm the assertion from results in liquid alkane media that decarbonylation rates are independent of microviscosity. The data separate components of a reaction from an (in-cage) "cage effect" and an (out-of-cage) "persistent radical effect" that are responsible for formation of AB-type (i.e., decarbonylated) products. The effects here are a consequence of vastly different rates of diffusion for coreacting A. and B. benzylic radicals rather than segregation of the radicals in different parts of a hetereogeneous environment (which leads to an excess of AA and BB products). Heretofore, observation of exclusive formation of AB products has been attributed to in-cage combinations of geminate radical pairs. We show that not to be the case here and provide methodologies which may be used for testing the importance of the "persistent radical effect" component of reaction.
Journal of the American Chemical Society 04/2007; 129(16):5012-22. · 9.91 Impact Factor
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ABSTRACT: The photochemistry of 1-(4-methylphenyl)-3-phenyl-2-propanone (MeDBK) in polyethylene (PE) films of varying crystallinity and at different temperatures has been investigated. Specifically, the selectivity of combination of the initially formed triplet pairs of benzylic radicals derived from irradiation of MeDBK has been used to determine the natures of the PE reaction cavities and how they influence the motions of the radicals. The results indicate that in-cage combinations are favored at lower temperatures and in PE of higher crystallinity. At temperatures significantly above the glass transition of PE, the fraction of radical pairs combining in-cage (F(c)) is independent of the extent of conversion of MeDBK. The data are interpreted according to a structural model that considers the shape, free volume, wall stiffness, and permeability of the reaction cavities and a dynamic model that includes the competition between the rates of in-cage combination and cage escape by the radical pairs. Near the glass transition temperature, F(c) decreases as the temperature is reduced and the data are interpreted using a stochastic model that considers the slow relaxation of the walls of the reaction cavities. The results demonstrate that the cavities afforded by PE can be subtly 'tuned' by temperature changes and microscopic variations in the polymer structure and morphology.
Photochemical and Photobiological Sciences 04/2004; 3(3):287-95. · 2.58 Impact Factor
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ABSTRACT: The quenching of the excited singlet of indole and its methyl derivatives by monosubstituted benzenes has been studied in three solvents of different polarities and bimolecular quenching rate constants have been determined. Below the diffusion limit the rate constants decrease when solvent polarity increases, with the exception of those for the quenching by benzonitrile. For the latter the rate constants are near the diffusion limit in all cases. The decrease may be understood in terms of a lower energy of the indole excited state in the more polar solvents. In cyclohexane the remnant emission in the presence of high concentration of the quencher is clearly red shifted, indicating the presence of a new emitting species that can be ascribed to the presence of an exciplex. In some cases the emission of the exciplex can be clearly separated from the molecular fluorescence. In the quenching by chlorobenzene in ethyl acetate and acetonitrile a new highly fluorescent product is formed. This photoreaction is observed with all the indole derivatives and the kinetics of the reaction was followed by the increase in fluorescence intensity at a wavelength were the reactants do not emit. Three major products were identified as phenyl substituted indoles by GC-MS and their fluorescence emission and excitation spectra.
Photochemical and Photobiological Sciences 09/2003; 2(8):893-7. · 2.58 Impact Factor
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Journal of Polymer Science Part A Polymer Chemistry 02/2002; 40(7):901 - 913. · 3.92 Impact Factor
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Journal of the Brazilian Chemical Society 01/1995; 6:145-154. · 1.43 Impact Factor
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ABSTRACT: Methylene Green (MG) is a cationic phenothiazine dye that can be considered as a nitro derivative of methylene blue (MB). The photophysical and spectroscopical properties of MG were investigated in a wide variety of solvents, including protic and aprotic ones. The absorption and fluorescence maxima, fluorescence quantum yields and emission lifetimes were measured and correlated with the properties of the media by using the Bakhshiev’s model and the empirical solvent parameter . Fairly good linear correlations were obtained in both cases indicating the absence of specific interaction between the dye and the solvents studied. The fluorescence quantum yields were found to be in the range 0.004–0.06. Quantum mechanical calculations at the level of Density Functional Theory (DFT) and its Time-Dependent treatment for excited states (TD-DFT) performed on the ground and singlet excited states of MG (and MB) allowed finding a reasonable interpretation of the solvent effects observed on the photophysical properties of the dyes.The triplet excited state properties of the dye were investigated by laser flash photolysis in both protic and non-protic media. In all cases, the triplet quantum yields (ΦT) measured for MG were very small (<0.01). Triplet–triplet sensitization by duroquinone was required for the spectroscopic characterization of the triplet excited state spectrum of MG and the determination of the ΦT in the different media. The triplet state shows important absorptions at 410, 488, and 865 nm. The molar absorption coefficient at 865 nm in acetonitrile, estimated using the depletion method, is ∼30,000 M−1 cm−1. Comparing the calculated values of the deactivation rate constants for the singlet excited states of MG and MB, it is concluded that the incorporation of the nitro group in the thiazine chromophore produces a large (∼30 times) decrease of the intersystem crossing rate constant. With the aid of quantum mechanical calculations it is concluded that the intersystem crossing mechanism in both dyes differ significantly.Research highlights► Methylene Green shows very low fluorescence and triplet quantum yields. ► The singlet excited state decays mainly through an internal conversion pathway. ► Comparing with Methylene Blue, the presence of the nitro group in the phenothiazine chromophore produces significant changes in the electronic structure of the excited states and in their photophysical and spectroscopical properties. ► This interpretation is supported by quantum mechanical calculations.
Dyes and Pigments 90(1):28-35. · 3.13 Impact Factor
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ABSTRACT: The photochemical processes of two N,N-dialkyl phenyl-alpha-oxoamides, N,N-diisopropyl phenyl-alpha-oxoamide (1) and N,N-dibenzyl phenyl-alpha-oxoamide (2), are investigated at different temperatures in methanol and ethylene glycol (to probe the influences of H-bonding and viscosity), in the solid phase of D-sorbitol at room temperature (to compare with the results in the liquid alcohols and to assess the influence of a poorly organized "stiff" environment), in the neat solid phase (to probe the influence of well-ordered, "stiff" matrices), and in the solid, smectic A2 and isotropic phases of methyl-tris-tetradecylphosphonium tetrafluoroborate (1P14BF4) (to assess the ability of the intermediates to respond to subtle changes in the order and polarity of their local environments). From differences between the activation parameters for product pathways from irradiations in methanol and in 1P14BF4, we conclude that the zwitterionic pre-product intermediate from 1 is much more sensitive to the polarity, viscosity and order of its local environment than is the isomeric pre-product biradicaloid intermediate or either of the pre-product intermediates from 2. A very sensitive balance among the medium parameters, as well as internal steric and electronic factors of 1 and 2, controls the reactive pathways of the photochemically generated intermediates.
Photochemistry and Photobiology 83(3):570-83. · 2.41 Impact Factor
