Hernán A. Montejano

A mechanistic and laser flash photolysis investigation of acrylamide polymerization photoinitiated by the three component system safranine-T/triethanolamine/diphenyliodonium chloride

Abstract The photopolymerization of acrylamide (AA) initiated by the synthetic dye safranine-T in the presence of triethanolamine (TEOHA) as co-initiator has been investigated in aqueous solution. It was found that the addition of diphenyliodonium chloride (DPIC) to the system has a marked accelerating effect on the polymerization rate. In the absence of DPIC an inhibiting effect of TEOHA is observed at high concentrations of the amine. This effect is suppressed by the presence of the onium salt. At the same time the photobleaching of the dye caused by the amine is suppressed by the presence of AA or DPIC. Laser flash photolysis experiments show that the presence of the onium salt increases the yield of triplet state of the dye. Also the yield of radicals in the quenching of the dye triplet by TEOHA increases by the presence of DPIC. The semi-reduced form of the dye decays faster in the presence of DPIC, suggesting a possible way of generating extra active radicals at the same time that the dye is regenerated. It is concluded that are several factors that contribute to the accelerating effect of DPIC on the polymerization rate.

Solvent effects on the photoinduced electron transfer reaction between excited singlet pyrene and indole

Abstract The quenching of the excited singlet of pyrene was studied in acetonitrile (MeCN), propionitrile (PrCN), methanol (MeOH) and ethanol (EtOH). Rate constants for the quenching processes and their activation parameters were determined. The activation enthalpies are −2.9 kcal mol −1 (MeCN), −1.4 kcal mol −1 (PrCN), −0.2 kcal mol −1 (MeOH) and 0.0 kcal mol −1 (EtOH). The activation entropies are −30.0 cal K −1 mol −1 (MeCN), −26.4 cal K −1 mol −1 (PrCN), −21.8 cal K −1 mol −1 (MeOH) and −22.8 cal K −1 mol −1 (EtOH). The electron transfer nature of the quenching was confirmed by laser flash photolysis. The radical anion and triplet state of pyrene are both produced in the quenching process. The quantum yields of these species were determined in the four solvents. The quantum yield of radical ions follows a good correlation with the cage escape rate constants calculated with the Eigen equation. The rate constants for back electron transfer to the triplet and ground state were estimated from the quantum yields and the cage escape rate constants. They are independent of the solvent.

Experimental and theoretical studies on the enantioselectivity of molecularly imprinted polymers prepared with a chiral functional monomer

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.

Medium effects on the photochemical reaction between pyrene and indole. A laser flash photolysis study

A laser flash photolysis study of the photoinduced electron-transfer reaction between pyrene and indole is reported. The pyrene radical anion and the hydropyrenil radical were confirmed as reaction intermediates from their transient absorption spectra. The photobleaching of pyrene was observed in the presence of indole but not in the presence of 1-methylindole. The quantum yields of the reaction and of the transient species are highly dependent on the solvent. The reaction was also carried out in the presence of tetrabutylammonium perchlorate, and an inhibitory effect by the salt was observed. At the same time, the quantum yield of the free ions increased. The results were interpreted by a mechanism that involves a proton transfer following the initial electron transfer. The effect of micelles on the reaction was also investigated.

Photoinduced electron-transfer quenching of excited singlet states of polycyclic aromatic hydrocarbons by organic acceptors

The electron-transfer quenching of the singlet excited state of polycyclic aromatic hydrocarbons (PAHs) by organic electron acceptors (cyano-and nitrobenzenes) has been investigated in acetonitrile and methanol. Intermolecular electron-transfer rate constants were determined. They follow a Rehm–Weller type correlation with the overall Gibbs energy of reaction. When the rate constants are below the diffusion control limit, in nearly all cases they are higher in methanol than in acetonitrile. The triplet state and free radical ions are formed in the quenching reaction. The charge-separation efficiency was also determined in both solvents. Using Eigens equation for the cage-escape rate constants the global back electron transfer was estimated and correlated with the Gibbs energy change for the back-electron-transfer reaction, to the triplet state.

Specific solvent effects on the charge separation efficiency in photoinduced electron transfer processes

Abstract The photoinduced electron transfer (PET) quenching of singlet excited pyrene and 1,2,5,6-dibenzanthracene (DBA) by 3-cyanopyridine and o -dicyanobenzene, respectively, was investigated in several protic and aprotic solvents. The triplet quenching of DBA by p -nitrobenzaldehyde was also studied in the same solvents. In all cases free radical ions yield were measured by laser flash photolysis. In the case of the triplet reaction, forward electron transfer quenching rate constants were also measured. Charge separation efficiencies, ϕ cs were determined from the free radical ions yields. The solvent effect on ϕ cs in singlet state PET may be explained in aprotic solvents by a model based on the macroscopic properties of the solvent. For alcohols the efficiencies are lower than the corresponding quantities for aprotic solvents of similar viscosities and dielectric constant. For triplet mediated PET reactions, the forward rate constants are scarcely sensitive to the solvent, while strong specific solvent effects are important in determining the back-electron-transfer to ground state.

Solvent and temperature effects in the electron transfer quenching of triplet states of aromatic hydrocarbons by nitrobenzenes

Abstract The triplet quenching of anthracene and a,h-dibenzanthracene by four nitrobenzenes was studied by laser flash photolysis. Bimolecular quenching rate constants were determined in acetonitrile and methanol. They follow the expected free energy correlation for an outer sphere electron transfer process. The rate constants in methanol are generally higher than those in acetonitrile. The activation enthalpies and entropies were also measured in both solvents. The activation entropies are more negative in acetonitrile and the activation enthalpies are lower in acetonitrile with negative values being observed in some cases. Transient absorption spectra show that in the quenching process free radical ions are formed in methanol, but in acetonitrile free ions are almost absent. These results are compared with the current models for outer sphere electron transfer reactions. They cannot be explained if the solvent is considered as a dielectric continuum.

Solvent effects on intermolecular photoinduced electron transfer reactions of aromatic molecules

Abstract The electron transfer quenching of excited singlet of pyrene by indole and 3-cyanopyridine, and a,h-dibenzanthracene by o-dicyanobenze was investigated in solvents of different polarity. The electron transfer nature of the quenching reaction in all the solvents was confirmed by the transient absorption spectra of the radical ions formed. Quenching rate constants were determined and their variation with the solvent was compared with the prediction of the current electron transfer theories. It was found that for aprotic solvents the trend may be explained by the semiclassical theory for non-adiabatic electron transfer if a dielectric continuum model is used for the solvent. In hydroxylic solvents the rate constants do not fall on the same correlation, probably due to specific effects not taken into account by the model.

SOLVENT EFFECTS IN THE ELECTRON TRANSFER QUENCHING OF THE TRIPLET STATE OF ANTHRACENE BY p-BENZOQUINONES

Se presenta aqui una investigacion de la desactivacion del estado triplete de antraceno por p-benzoquinonas, en dos solventes de propiedades dielectricas macroscopicas aproximadamente iguales, metanol y acetonitrilo. Las constantes de velocidad bimoleculares de desactivacion fueron medidas por laser flash fotolisis. Las eficiencias cuanticas de separacion de carga fueron determinadas en el solvente protico. Las constantes de velocidad en acetonitrilo siguen una correlacion de Rehm-Weller tipica para un proceso de transferencia de electrones. Aunque se han observado iones radicales libres en metanol, las constantes de velocidad son aproximadamente independientes de la fuerza impulsora, contrariamente al comportamiento esperado. En este solvente, el rendimiento cuantico de separacion de cargas disminuye cuando disminuye la energia del par ionico, en acuerdo con la ley del salto de energia

Comparison of the Kinetics of Electron Transfer in the Diffusion Limit for the Singlet and Triplet Quenching of Eosin Y by Quinones

Electron transfer (ET) rate constants were determined by means of lifetime measurements for the fluorescence quenching and by laser flash photolysis for the triplet quenching of the dye eosin Y by benzoquinones in acetonitrile. The results represent a new aspect of the dependence of the rate constants with the driving force in the diffusion limit region. That is, the rate constants for singlet quenching in the highly negative region of ΔGet do not decrease as predicted by Marcus theory, but rather show a small positive dependence on the driving force. However, it is found that, in the same free energy range, the triplet rate constants are lower than those for the singlet process. They also increase with the exergonicity of the reaction, but the dependence with ΔGet is less marked than that found for the singlet reaction. Even at a Gibbs energy change of −1.0 eV the triplet quenching rate constants do not reach the theoretical diffusion limit. The results are analyzed using the current theories for diffusion‐mediated ET reactions.