Cationic Exciplexes: Role of Hydrogen Bonding in Deactivation and Electronic Coupling.

Emissive properties for the cationic exciplex (A+ */D→A. D.+ ) of an isoquinolinium cation tethered to a substituted arene (1+ ) are strongly affected by hydrogen bonding solvents. At equal dielectric constant (ϵ), the ground-to-excited state energy gaps (ΔG) and solvent reorganization energies (λs ) decrease from nitriles to aliphatic alcohols. The corresponding decrease from aliphatic alcohols to high hydrogen bond acidity solvents is ∼3 times larger. The exciplex decay (kEx ), largely determined by unfolding of the exciplex to a stretched conformer, changes in a complex way depending on the strength of the hydrogen bond ability of these solvents. In contrast, the electronic couplings between the exciplex ground, excited, and charge transfer states do not show a solvent functionality dependence.

Solvent Dependence of Cationic-Exciplex Emission: Limitation of Solvent Polarity Functions and the Role of Hydrogen Bonding.

Conventional exciplexes are products of excited-state charge generation reactions between neutral reactants (e.g., A* + D → A•-D•+), whereas cationic exciplexes are products of charge shift reactions of cations with neutral donors (e.g., A+* + D → A•D•+). Compared herein is the solvent-dependent fluorescence of a cationic exciplex with extant data for conventional exciplexes. Although linear correlations of conventional exciplex emission maxima with the Lippert-Mataga solvent polarity function are well documented in low to moderate polarity solvents, the correlations are often poor in more polar solvents. A number of such plots in moderate to high polarity solvents show a strong curvature. Intriguingly, for these same cases, plots of emission maxima versus the solvent dielectric constant (ε) are remarkably linear. Interestingly, emission maxima for the cationic exciplex of 1+ in nitrile and alcohol solvents also correlate linearly with ε. The solvent dependency for cationic exciplex emission maxima on ε is ca 1/3 of that for conventional exciplexes, which is ascribed to solvent stabilization of both the excited state and the ground state for cationic exciplexes. Differences in exciplex emissions between nitrile and alcohol solvents for 1+ are attributed to hydrogen bonding, with larger differences in higher hydrogen-bond acidity solvents.

Emissive Organic Exciplexes in Water.

Unlike numerous known examples of exciplexes (products of charge formation reactions), we reported recently that cationic exciplexes (products of charge shift reactions) can be formed with N-methylisoquinolinium as an excited acceptor and alkyl benzene donors. We have now synthesized five intramolecular analogues (isoquinolinium linked by a trimethylene tether to alkyl benzenes) that proved to be well suited to demonstrating that emissive exciplexes can be formed in water from purely organic components. Three conformers (anti, gauche, and folded) leading to electron transfer were identified using a combination of absorption spectroscopy, fluorometry, and time-correlated single photon counting. The hydrophobicity of the donor moiety was found to enhance the formation of the folded conformer, which leads directly to exciplex formation. Electronic coupling matrix elements between ground, charge-transfer, and locally excited states were determined from correlations between radiative rate constants and average emission frequencies. The charge transfer (CT) character of the exciplexes (88-97%) was calculated from the electronic coupling. In spite of such a high CT character in a highly polar solvent, exciplex fluorescence quantum yields up to 0.03 and lifetimes up to 17 ns were observed.

Cationic (Charge Shift) Exciplexes.

Of the many known examples of exciplexes, those formed from bimolecular encounter between a cationic, excited state electron acceptor and a neutral donor in fluid media have not been previously reported. We now show that emissive exciplexes formed from excited N-methyl isoquinolinium cation (NMiQ+) with alkyl benzene donors are readily detected in acetonitrile. These cationic exciplexes result in a charge shift (A+* + D → A•D•+) with no net change in charge, which differs fundamentally from the charge-generation of conventional exciplex formation (A* + D → A•-D•+). We find that cationic and conventional exciplexes show similar trends, e.g., bathochromic shifts and decreases in fluorescence quantum yields with decreasing oxidation potentials of the donors. In the presented examples of NMiQ+ exciplexes, the fluorescence quantum yield decreases by a factor of 30 and the radiative rate constant by 6.6 as the fractional CT character of the exciplex increases from ∼0.79 to ∼0.95. Interestingly, the electronic coupling matrix elements for the NMiQ+ exciplexes, derived from a correlation of the radiative rate constants with the average emission frequencies, are similar to those of related conventional exciplexes, in spite of the absence of Coulombic stabilization in the cationic exciplexes.

Multiple Intermolecular Exciplexes in Highly Polar Solvents.

Exciplexes of 2,6,9,10-tetracyanoanthracene (TCA) with alkylbenzenes were investigated in solvents ranging from cyclohexane to acetonitrile. Plots of the reduced emission maxima or the average emission frequency (hνav) versus redox potential differences (Eredox) were linear with a slope of ∼1 in all solvents, which is consistent with the highly ionic character of the exciplexes. The exciplex spectra were analyzed in terms of the energy gap between the exciplex minimum and the AD pair (ΔG), the energy difference between ΔG and Eredox (δEx), and the total reorganization energy (Σλ). A plot of (Eredox - hνav), equivalent to (Σλ - δEx), versus a solvent polarity function showed a linear dependency for the low-to-moderate polarity solvents, whereas highly polar solvents deviated significantly. δEx showed a smooth linear dependency for all solvents. Thus, the deviation of the polar solvents is due to a larger-than-expected Σλ. Additionally, the full width at half-maximum (fwhm) of the emission spectra in polar solvents deviates significantly from the extrapolated trend in less-polar solvents. The deviations of Σλ and fwhm in highly polar solvents can plausibly be explained by composite emissions from two exciplex structures, with the donor overlapping with the inner or outer ring of TCA.

Feminicidio y derecho penal / Femicide and criminal law / Feminicídio e direito penal

El delito de homicidio tiene un agravante, que se configura cuando se comete por el hecho de ser mujer. En Colombia, solo hasta el 4 de marzo del 2015 la Corte Suprema de Justicia se ocupó por primera vez de un caso, en el cual se daba aplicación al agravante, mediante la determinación de los elementos que son importantes para su configuración. Se analiza el concepto de feminicidio, dentro de un contexto más amplio de violencia contra la mujer, al igual que los conceptos de violencia de género, de violencia contra la mujer y, finalmente, violencia sexual y feminicidio, todo ello con el propósito de mostrar los diferentes elementos que están alrededor de este fenómeno.

Homicide as a crime has an aggravating factor when it is committed simply and solely when and because the victim is a woman. In Colombia, only until March 4 2015 the Supreme Court of Justice for the first time took into account a case where the application of this aggravating factor took place, by determining the elements that are essential in the configuration of this type of crime. The recently introduced concept of femicide ("the killing of a woman" as opposed to homicide, i.e. 'he killing of a man') is analyzed within a broader concept of violence against women, just as other notions of gender-based violence, and, ultimately, sex-and/or-gender-oriented violent conducts and murder, all this for the purpose of revealing the different elements lying around and behind the femicide phenomenon afflicting us today.

O crime do homicídiotem um agravante, que é configurado quando é cometido pelo fato de ser mulher. Na Colômbia, só até o 4 de março de 2015 o Supremo Tribunal de Justiçaocupou-se pela primeira vez de um caso, no qual aplicava-se o agravante, por meio da determinação dos elementos que são importantes para sua configuração. O conceito do feminicídioé analisado, dentro de um contexto mais amplo da violência contra a mulher, assim como os conceitos da violência de gênero, da violência contra a mulher e, finalmente, a violência sexual e feminicídio, tudo a fim de mostrar os diferentes elementos que tem relaçãocom este fenômeno.

Transition from charge-transfer to largely locally excited exciplexes, from structureless to vibrationally structured emissions.

Exciplexes of 9,10-dicyanoanthracene (DCA) with alkylbenzene donors in cyclohexane show structureless emission spectra, typical of exciplexes with predominantly charge-transfer (CT) character, when the donor has a relatively low oxidation potential (Eox ), e.g. hexamethylbenzene (HMB). With increasing Eox and stronger mixing with a locally excited (LE) state, vibrational structure begins to appear with 1,2,3,5-tetramethylbenzene and becomes prominent with p-xylene (p-Xy). A simple theoretical model reproduces the spectra and the radiative rate constants, and it reveals several surprises: Even in this nonpolar solvent, the fractional CT character of a highly mixed exciplex varies widely in response to fluctuations in the microscopic environment. Environments that favor the LE (or CT) state contribute more to the blue (or red) side of the overall spectrum. It is known that sparsely substituted benzene radical cations, e.g., p-Xy(•+) , are stabilized more in acetonitrile than the heavily substituted HMB(•+) . Remarkably, ion pairing with DCA(•-) in cyclohexane leads to even larger differences in the stabilization of these radical cations. The spectra of the low-Eox donors are almost identical except for displacements that approximately equal the differences in Eox , even though the exciplexes have varying degrees of CT character. These similarities result from compensation among several nonobvious, but quantified factors.

Accurate oxidation potentials of 40 benzene and biphenyl derivatives with heteroatom substituents.

The redox equilibrium method was used to determine accurate oxidation potentials in acetonitrile for 40 heteroatom-substituted compounds. These include methoxy-substituted benzenes and biphenyls, aromatic amines, and substituted acetanilides. The redox equilibrium method allowed oxidation potentials to be determined with high precision (≤ ±6 mV). Whereas most of the relative oxidation potentials follow well-established chemical trends, interestingly, the oxidation potentials of substituted N-methylacetanilides were found to be higher than those of the corresponding acetanilides. Density functional theory calculations provided insight into the origin of these surprising results in terms of the preferred conformations of the amides versus their cation radicals.

Chain-amplified photochemical fragmentation of N-alkoxypyridinium salts: proposed reaction of alkoxyl radicals with pyridine bases to give pyridinyl radicals.

Photoinduced electron transfer to N-alkoxypyridiniums, which leads to N­O bond cleavage and alkoxyl radical formation, is highly chain amplified in the presence of a pyridine base such as lutidine. Density functional theory calculations support a mechanism in which the alkoxyl radicals react with lutidine via proton-coupled electron transfer (PCET) to produce lutidinyl radicals (BH•). A strong electron donor, BH• is proposed to reduce another alkoxypyridinium cation, leading to chain amplification, with quantum yields approaching 200. Kinetic data and calculations support the formation of a second, stronger reducing agent: a hydrogen-bonded complex between BH• and another base molecule (BH•···B). Global fitting of the quantum yield data for the reactions of four pyridinium salts (4-phenyl and 4-cyano with N-methoxy and N-ethoxy substituents) led to a consistent set of kinetic parameters. The chain nature of the reaction allowed rate constants to be determined from steady-state kinetics and independently determined chain-termination rate constants. The rate constant of the reaction of CH3O• with lutidine to form BH•, k1, is ~6 × 10(6) M(­1) s(­1); that of CH3CH2O• is ~9 times larger. Reaction of CD3O• showed a deuterium isotope effect of ~6.5. Replacing lutidine by 3-chloropyridine, a weaker base, decreases k1 by a factor of ~400.

Bimolecular electron transfers that deviate from the Sandros-Boltzmann dependence on free energy: steric effect.

As we reported recently, endergonic to mildly exergonic electron transfer between neutral aromatics (benzenes and biphenyls) and their radical cations in acetonitrile follows a Sandros-Boltzmann (SB) dependency on the reaction free energy (ΔG); i.e., the rate constant is proportional to 1/[1 + exp(ΔG/RT)]. We now report deviations from this dependency when one reactant is sterically crowded: 1,4-di-tert-butylbenzene (C1), 1,3,5-tri-tert-butylbenzene (C2), or hexaethylbenzene (C3). Obvious deviation from SB behavior is observed with C1. Stronger deviation is observed with the more crowded C2 and C3, where steric hindrance increases the interplanar separation at contact by ~1 Å, significantly decreasing the π orbital overlap. Consequently, electron transfer (k(et)) within the contact pair becomes slower than diffusional separation (k(-d)), causing deviation from the SB dependency, especially near ΔG = 0. Fitting the data to a standard electron-transfer theory gives small matrix elements (~5-7 meV) and reasonable reorganization energies. A small systematic difference between reactions of C3 with benzenes vs biphenyls is rationalized in terms of small differences in the electron-transfer parameters that are consistent with previous data. The influence of solvent viscosity on the competition between k(et) and k(-d) was investigated by comparing reactions in acetonitrile and propylene carbonate.

Reexamination of the Rehm-Weller data set reveals electron transfer quenching that follows a Sandros-Boltzmann dependence on free energy.

In a landmark publication over 40 years ago, Rehm and Weller (RW) showed that the electron transfer quenching constants for excited-state molecules in acetonitrile could be correlated with the excited-state energies and the redox potentials of the electron donors and acceptors. The correlation was interpreted in terms of electron transfer between the molecules in the encounter pair (A*/D ⇌ A(•-)/D(•+) for acceptor A and donor D) and expressed by a semiempirical formula relating the quenching constant, k(q), to the free energy of reaction, ΔG. We have reinvestigated the mechanism for many Rehm and Weller reactions in the endergonic or weakly exergonic regions. We find they are not simple electron transfer processes. Rather, they involve exciplexes as the dominant, kinetically and spectroscopically observable intermediate. Thus, the Rehm-Weller formula rests on an incorrect mechanism. We have remeasured k(q) for many of these reactions and also reevaluated the ΔG values using accurately determined redox potentials and revised excitation energies. We found significant discrepancies in both ΔG and k(q), including A*/D pairs at high endergonicity that did not exhibit any quenching. The revised data were found to obey the Sandros-Boltzmann (SB) equation k(q) = k(lim)/[1 + exp[(ΔG + s)/RT]]. This behavior is attributed to rapid interconversion among the encounter pairs and the exciplex (A*/D ⇌ exciplex ⇌ A(•-)/D(•+)). The quantity k(lim) represents approximately the diffusion-limited rate constant, and s the free energy difference between the radical ion encounter pair and the free radical ions (A(•-)/D(•+) vs A(•-) + D(•+)). The shift relative to ΔG for the overall reaction is positive, s = 0.06 eV, rather than the negative value of -0.06 eV assumed by RW. The positive value of s involves the poorer solvation of A(•-)/D(•+) relative to the free A(•-) + D(•+), which opposes the Coulombic stabilization of A(•-)/D(•+). The SB equation does not involve the microscopic rate constants for interconversion among the encounter pairs and the exciplex. Data that fit this equation contain no information about such rate constants except that they are faster than dissociation of the encounter pairs to (re-)form the corresponding free species (A* + D or A(•-) + D(•+)). All of the present conclusions agree with our recent results for quenching of excited cyanoaromatic acceptors by aromatic donors, with the two data sets showing indistinguishable dependencies of k(q) on ΔG.

Bimolecular electron transfers that follow a Sandros-Boltzmann dependence on free energy.

Rate constants (k) for exergonic and endergonic electron-transfer reactions of equilibrating radical cations (A(•+) + B ⇌ A + B(•+)) in acetonitrile could be fit well by a simple Sandros-Boltzmann (SB) function of the reaction free energy (ΔG) having a plateau with a limiting rate constant k(lim) in the exergonic region, followed, near the thermoneutral point, by a steep drop in log k vs ΔG with a slope of 1/RT. Similar behavior was observed for another charge shift reaction, the electron-transfer quenching of excited pyrylium cations (P(+)*) by neutral donors (P(+)* + D → P(•) + D(•+)). In this case, SB dependence was observed when the logarithm of the quenching constant (log k(q)) was plotted vs ΔG + s, where the shift term, s, equals +0.08 eV and ΔG is the free energy change for the net reaction (E(redox) - E(excit)). The shift term is attributed to partial desolvation of the radical cation in the product encounter pair (P(•)/D(•+)), which raises its free energy relative to the free species. Remarkably, electron-transfer quenching of neutral reactants (A* + D → A(•-) + D(•+)) using excited cyanoaromatic acceptors and aromatic hydrocarbon donors was also found to follow an SB dependence of log k(q) on ΔG, with a positive s, +0.06 eV. This positive shift contrasts with the long-accepted prediction of a negative value, -0.06 eV, for the free energy of an A(•-)/D(•+) encounter pair relative to the free radical ions. That prediction incorporated only a Coulombic stabilization of the A(•-)/D(•+) encounter pair relative to the free radical ions. In contrast, the results presented here show that the positive value of s indicates a decrease in solvent stabilization of the A(•-)/D(•+) encounter pair, which outweighs Coulombic stabilization in acetonitrile. These quenching reactions are proposed to proceed via rapidly interconverting encounter pairs with an exciplex as intermediate, A*/D ⇌ exciplex ⇌ A(•-)/D(•+). Weak exciplex fluorescence was observed in each case. For several reactions in the endergonic region, rate constants for the reversible formation and decay of the exciplexes were determined using time-correlated single-photon counting. The quenching constants derived from the transient kinetics agreed well with those from the conventional Stern-Volmer plots. For excited-state electron-transfer processes, caution is required in correlating quenching constants vs reaction free energies when ΔG exceeds ∼+0.1 eV. Beyond this point, additional exciplex deactivation pathways-fluorescence, intersystem crossing, and nonradiative decay-are likely to dominate, resulting in a change in mechanism.

Accurate oxidation potentials of benzene and biphenyl derivatives via electron-transfer equilibria and transient kinetics.

Nanosecond transient absorption methods were used to determine accurate oxidation potentials (E(ox)) in acetonitrile for benzene and a number of its alkyl-substituted derivatives. E(ox) values were obtained from a combination of equilibrium electron-transfer measurements and electron-transfer kinetics of radical cations produced from pairs of benzene and biphenyl derivatives, with one member of the pair acting as a reference. Using a redox-ladder approach, thermodynamic oxidation potentials were determined for 21 benzene and biphenyl derivatives. Of particular interest, E(ox) values of 2.48 +/- 0.03 and 2.26 +/- 0.02 V vs SCE were obtained for benzene and toluene, respectively. Because of a significant increase in solvent stabilization of the radical cations with decreasing alkyl substitution, the difference between ionization and oxidation potentials of benzene is approximately 0.5 eV larger than that of hexamethylbenzene. Oxidation potentials of the biphenyl derivatives show an excellent correlation with substituent sigma+ values, which allows E(ox) predictions for other biphenyl derivatives. Significant dimer radical cation formation was observed in several cases and equilibrium constants for dimerization were determined. Methodologies are described for determining accurate electron-transfer equilibrium constants even when dimer radical cations are formed. Additional equilibrium measurements in trifluoroacetic acid, methylene chloride, and ethyl acetate demonstrated that solvation differences can substantially alter and even reverse relative E(ox) values.

Bonded exciplex formation: electronic and stereoelectronic effects.

As recently proposed, the singlet-excited states of several cyanoaromatics react with pyridine via bonded-exciplex formation, a novel concept in photochemical charge transfer reactions. Presented here are electronic and steric effects on the quenching rate constants, which provide valuable support for the model. Additionally, excited-state quenching in poly(vinylpyridine) is strongly inhibited both relative to that in neat pyridine and also to conventional exciplex formation in polymers, consistent with a restrictive orientational requirement for the formation of bonded exciplexes. Examples of competing reactions to form both conventional and bonded exciplexes are presented, which illustrate the delicate balance between these two processes when their reaction energetics are similar. Experimental and computational evidence is provided for the formation of a bonded exciplex in the reaction of the singlet excited state of 2,6,9,10-tetracyanoanthracene (TCA) with an oxygen-substituted donor, dioxane, thus expanding the scope of bonded exciplexes.

Quantum amplified isomerization in polymers based on triplet chain reactions.

Photoinitiated triplet quantum amplified isomerizations (QAI) of substituted Dewar benzene derivatives in polymeric media are reported. The quantum efficiencies and the ultimate extents of reactant-to-product conversions increase significantly with the incorporation of appropriate co-sensitizers; compounds whose triplet energies are similar to or lower than that of the sensitizer and close to that of the reactant. These co-sensitizers serve to promote chain-propagating energy transfer processes and thereby increase the action sphere of photosensitization. Isomerization quantum yields increase, as predicted, with increasing concentrations of the reactants and the co-sensitizers. Chain amplifications as large as approximately 16 and extents of conversion that approach 100% have been achieved. Mechanistic schemes are proposed to account for the dynamics of the inherent energy transfer processes and provide a predictively useful model for the design of a new class of photoresponsive polymers based on changes in the refractive index of the materials.

Bonded exciplexes. A new concept in photochemical reactions.

Charge-transfer quenching of the singlet excited states of cyanoaromatic electron acceptors by pyridine is characterized by a driving force dependence that resembles those of conventional electron-transfer reactions, except that a plot of the log of the quenching rate constants versus the free energy of electron transfer is displaced toward the endothermic region by 0.5-0.8 eV. Specifically, the reactions with pyridine display rapid quenching when conventional electron transfer is highly endothermic. As an example, the rate constant for quenching of the excited dicyanoanthracene is 3.5 x 10(9) M(-1)s(-1), even though formation of a conventional radical ion pair, A*-D*+, is endothermic by approximately 0.6 eV. No long-lived radical ions or exciplex intermediates can be detected on the picosecond to microsecond time scale. Instead, the reactions are proposed to proceed via formation of a previously undescribed, short-lived charge-transfer intermediate we call a "bonded exciplex", A- -D+. The bonded exciplex can be formally thought of as resulting from bond formation between the unpaired electrons of the radical ions A*- and D*+. The covalent bonding interaction significantly lowers the energy of the charge-transfer state. As a result of this interaction, the energy decreases with decreasing separation distance, and near van der Waals contact, the A- -D+ bonded state mixes with the repulsive excited state of the acceptor, allowing efficient reaction to form A- -D+ even when formation of a radical ion pair A*-D*+ is thermodynamically forbidden. Evidence for the bonded exciplex intermediate comes from studies of steric and Coulombic effects on the quenching rate constants and from extensive DFT computations that clearly show a curve crossing between the ground state and the low-energy bonded exciplex state.

Radiationless decay in exciplexes with variable charge transfer.

Rate constants for radiative decay, radiationless decay, and intersystem crossing are reported for a series of excited states formed by reaction of cyanoanthracene acceptors with alkylbenzenes as donors in several solvents of moderate to low polarity. The excited states have widely varying degrees of charge transfer, from essentially pure electron transfer states to pure locally excited states. The data illustrate the fundamental factors that control the contrasting relative efficiencies of radiative and radiationless processes in electron transfer compared to locally excited states. The radiationless decay rate constants can be described quantitatively as a function of the extent of charge transfer using weighted contributions from a locally excited decay mechanism and a pure electron-transfer type mechanism. The factors that control the rate constants for radiationless decay in excited states with intermediate charge-transfer character are discussed.

Highly efficient triplet chain isomerization of Dewar benzenes: adiabatic rate constants from cage kinetics.

Quantum yields as high as 120 were achieved for triplet-sensitized photoisomerizations of several Dewar benzene reactants, R, to the corresponding benzene products, P. Considerable chain amplification is maintained even at high conversion. All relevant rate constants of this triplet chain reaction were extracted from laser flash photolysis plus steady-state photolysis experiments. The crucial rate constant ka for adiabatic isomerization of the triplet reactant to triplet product (R* --> P*) cannot be directly measured because it is so large relative to the bimolecular rate of R* formation via sensitization. However, ka was obtained indirectly using a cage/encounter complex model to analyze the competition between the dissociation of encounter pairs with the sensitizer, e.g., S/R* --> S + R*, and the in-cage processes, S/R* --> S/P* --> S*/P, in nonviscous and viscous solvents. These measurements yielded ka values of (approximately 4-9) x 10(9) s(-1), which suggests that only a small (approximately 3 kcal/mol) energy barrier exists along the potential energy surface from R* to P*. Steady-state data indicated that the chain-terminating rate constant R* --> R is negligibly small, an ideal condition for chain amplification. Triplet energy transfer from a series of sensitizers to the Dewar benzene derivatives shows a nonclassical falloff in rate constants with decreasing sensitizer triplet energy, suggesting energy transfer to thermally distorted configurations having lower singlet-triplet energy gaps. As a result of distorted geometries of R* and P*, the chain-propagating energy transfer from P* to R proceeds with a rate constant of only approximately 2 x 10(7) M(-1) s(-1), despite strong exothermicity. The isomerization reaction can release over 100 kcal/kcal of absorbed photons due to the high-energy content of the reactant together with the large chain length.

Kinetic determinations of accurate relative oxidation potentials of amines with reactive radical cations.

Accurate oxidation potentials for organic compounds are critical for the evaluation of thermodynamic and kinetic properties of their radical cations. Except when using a specialized apparatus, electrochemical oxidation of molecules with reactive radical cations is usually an irreversible process, providing peak potentials, E(p), rather than thermodynamically meaningful oxidation potentials, E(ox). In a previous study on amines with radical cations that underwent rapid decarboxylation, we estimated E(ox) by correcting the E(p) from cyclic voltammetry with rate constants for decarboxylation obtained using laser flash photolysis. Here we use redox equilibration experiments to determine accurate relative oxidation potentials for the same amines. We also describe an extension of these experiments to show how relative oxidation potentials can be obtained in the absence of equilibrium, from a complete kinetic analysis of the reversible redox kinetics. The results provide support for the previous cyclic voltammetry/laser flash photolysis method for determining oxidation potentials.

Enhancement of chain amplification in photoreactions of N-methoxypyridinium salts with alcohols.

Recently we reported a chain-amplified photochemical reaction, initiated by electron transfer from an excited sensitizer to N-methoxypyridinium salts, which leads to N-O bond cleavage (26). Hydrogen atom abstraction by the methoxy radical from an alcohol yields an alpha-hydroxy radical, which reduces another N-methoxypyridinium molecule and propagates the chain. We now report that the chain amplification can be significantly enhanced in the presence of water. Detailed kinetic studies of the reaction of 4-cyano-N-methoxypyridinium salt (CMP) with benzhydrol (BH) showed that the rate constant for reduction of CMP by the diphenyl ketyl radical (1.1 x 10(6) M(-1) s(-1)) increases by more than an order of magnitude in the presence of water. This increase in the rate constant is the result of coupling of the electron transfer to a proton transfer from the ketyl radical to water, which decreases the endothermicity of the reaction. Unfortunately, this increase in the rate constant for one of the two propagation steps is accompanied by a larger increase in the rate constant(s) of the competing termination reaction(s) of the ketyl radical. The observed enhancement in chain amplification is the result of a significant increase in the ratio of propagation to termination rate constants of the reactions of the methoxy radical. The main chain-terminating reactions of the methoxy radical are deuterium abstraction from the solvent, CD(3)CN, and reaction with the sensitizer, thioxanthone. The effect of increase in the ratios of the propagation rate constant of the methoxy radical (hydrogen abstraction from BH) to those of both termination reactions is larger than the unfavorable effect of water on the reactions of the ketyl radical. The increase in chain amplification depends on the concentration of the reactants; at 0.037 M of both reactants, the quantum yield increases form approximately 16 to approximately 45 in the presence of <1% water. The reaction of 4-phenyl-N-methoxypyridinium (PMP) with 4-methoxybenzyl alcohol does not proceed via chain amplification because of large endothermicity for electron transfer from the alpha-hydroxy radical to the pyridinium salt. However, chain amplification could be induced, simply by addition of water, where at approximately 10% water content, a quantum yield of approximately 5 was obtained. Water-induced, proton-coupled electron transfer increases the rate constant for reduction of PMP from a negligible level to becoming the dominant path.