Marcus Cross-Relationship Probed by Time-Resolved CIDNP.

The time-resolved CIDNP method can provide information about degenerate exchange reactions (DEEs) involving short-lived radicals. In the temperature range from 8 to 65 °C, the DEE reactions of the guanosine-5'-monophosphate anion GMP(-H)- with the neutral radical GMP(-H)•, of the N-acetyl tyrosine anion N-AcTyrO- with a neutral radical N-AcTyrO•, and of the tyrosine anion TyrO- with a neutral radical TyrO• were studied. In all the studied cases, the radicals were formed in the reaction of quenching triplet 2,2'-dipyridyl. The reorganization energies were obtained from Arrhenius plots. The rate constant of the reductive electron transfer reaction in the pair GMP(-H)•/TyrO- was determined at T = 25 °C. Rate constants of the GMP(-H)• radical reduction reactions with TyrO- and N-AcTyrO- anions calculated by the Marcus cross-relation differ from the experimental ones by two orders of magnitude. The rate constants of several other electron transfer reactions involving GMP(-H)-/GMP(-H)•, N-AcTyrO-/N-AcTyrO•, and TyrO-/TyrO• pairs calculated by cross-relation agree well with the experimental values. The rate of nuclear paramagnetic relaxation was found for the 3,5 and ß-protons of TyrO• and N-AcTyrO•, the 8-proton of GMP(-H)•, and the 3,4-protons of DPH• at each temperature. In all cases, the dependences of the rate of nuclear paramagnetic relaxation on temperature are described by the Arrhenius dependence.

Hole Transfer Processes in meta- and para-Conjugated Mixed Valence Compounds: Unforeseen Effects of Bridge Substituents and Solvent Dynamics.

To address the question whether donor substituents can be utilized to accelerate the hole transfer (HT) between redox sites attached in para- or in meta-positions to a central benzene bridge, we investigated three series of mixed valence compounds based on triarylamine redox centers that are connected to a benzene bridge via alkyne spacers at para- and meta-positions. The electron density at the bridge was tuned by substituents with different electron donating or accepting character. By analyzing optical spectra and by DFT computations we show that the HT properties are independent of bridge substituents for one of the meta-series, while donor substituents can strongly decrease the intrinsic barrier in the case of the para-series. In stark contrast, temperature-dependent ESR measurements demonstrate a dramatic increase of both the apparent barrier and the rate of HT for strong donor substituents in the para-cases. This is caused by an unprecedented substituent-dependent change of the HT mechanism from that described by transition state theory to a regime controlled by solvent dynamics. For solvents with slow longitudinal relaxation (PhNO2, oDCB), this adds an additional contribution to the intrinsic barrier via the dielectric relaxation process. Attaching the donor substituents to the bridge at positions where the molecular orbital coefficients are large accelerates the HT rate for meta-conjugated compounds just as for the para-series. This effect demonstrates that the para-meta paradigm no longer holds if appropriate substituents and substitution patterns are chosen, thereby considerably broadening the applicability of meta-topologies for optoelectronic applications.

Insights into the Hydrogen-Atom Transfer of the Blue Aroxyl.

An experimental and theoretical study on hydrogen-atom transfer dynamics in the hydrogen-bonded substituted phenol/phenoxyl complex of the blue aroxyl (2,4,6-tri-tert-butylphenoxyl) is presented. The experimental exchange dynamics is determined in different organic solvents from the temperature-dependent alternating line-width effect in the continuous-wave ESR spectrum. From bent Arrhenius plots, effective tunnelling contributions with parallel heavy-atom motion are concluded. To clarify the transfer mechanism, reaction paths for different conformers of the substituted phenol/phenoxyl complex are modelled theoretically. Various DFT and post-Hartree-Fock methods including multireference methods are applied. From the comparison of experimental and theoretical data it is concluded that the system favours concerted hydrogen-atom transfer along a parabolic reaction path caused by heavy-atom motion.

A Binary Mixture of Ionic Liquids as a New Approach for an Experimental Diffusion Correction in the Study of Activated Processes.

A binary system of two ionic liquids, 1-ethyl-3-methylimidazolium and trioctylmethylammonium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf2 ] and [OMA][NTf2 ], respectively), with varying molar fractions, is introduced. It allows the dynamic viscosity to remain constant over a range of almost 60 K; this means that any activated process can be studied independent of the temperature dependence of viscosity itself. This principle is proven upon reinvestigation of electron self-exchange kinetics of tetrathiafulvalene by continuous-wave ESR line-broadening experiments. From these results, it is also confirmed that this process is totally diffusion controlled.

Influence of the excitation light intensity on the rate of fluorescence quenching reactions: pulsed experiments.

The effect of multiple light excitation events on bimolecular photo-induced electron transfer reactions in liquid solution is studied experimentally. It is found that the decay of fluorescence can be up to 25% faster if a second photon is absorbed after a first cycle of quenching and recombination. A theoretical model is presented which ascribes this effect to the enrichment of the concentration of quenchers in the immediate vicinity of fluorophores that have been previously excited. Despite its simplicity, the model delivers a qualitative agreement with the observed experimental trends. The original theory by Burshtein and Igoshin (J. Chem. Phys., 2000, 112, 10930-10940) was created for continuous light excitation though. A qualitative extrapolation from the here presented pulse experiments to the continuous excitation conditions lead us to conclude that in the latter the order of magnitude of the increase of the quenching efficiency upon increasing the light intensity of excitation, must also be on the order of tens of percent. These results mean that the rate constant for photo-induced bimolecular reactions depends not only on the usual known factors, such as temperature, viscosity and other properties of the medium, but also on the intensity of the excitation light.

Characterization of dimethylsulfoxide/glycerol mixtures: a binary solvent system for the study of "friction-dependent" chemical reactivity.

The properties of binary mixtures of dimethylsulfoxide and glycerol, measured using several techniques, are reported. Special attention is given to those properties contributing or affecting chemical reactions. In this respect the investigated mixture behaves as a relatively simple solvent and it is especially well suited for studies on the influence of viscosity on chemical reactivity. This is due to the relative invariance of the dielectric properties of the mixture. However, special caution must be taken with specific solvation, as the hydrogen-bonding properties of the solvent change with the molar fraction of glycerol.

Visible light mediated cyclization of tertiary anilines with maleimides using nickel(II) oxide surface-modified titanium dioxide catalyst.

Surface-modified titanium dioxides by highly dispersed NiO particles have an extended absorption in the visible light region and a reduced hole-electron pair recombination than unmodified TiO2. They have now been successfully applied as highly active heterogeneous photocatalysts in the visible light mediated direct cyclization of tertiary anilines with maleimides to give tetrahydroquinoline products in moderate to high yields at ambient temperature. In contrast with unmodified titanium dioxide catalysts that are conventionally used in a stoichiometric amount in combination with UVA light, only a catalytic amount (1 mol %) of the surface-modified TiO2 catalyst is needed along with visible light to efficiently catalyze the reaction. Compared with transition-metal complexes such as Ru(bpy)3Cl2 or Ir(ppy)2(dtbbpy)PF6, advantages of these surface-modified titanium dioxides as photocatalyst include high catalytic activity, low cost, ease of recovering, and being able to be used for at least nine times without significant decay of catalytic activity.

Influence of the medium's viscosity on the kinetics of hydrogen atom self-exchange for N-hydroxy phthalimide/piperidine-N-oxyl (NHPI/PINO˙) measured by CW-ESR spectroscopy.

CW-ESR line broadening experiments are used to investigate the kinetics of fast hydrogen atom self-exchange reactions. The system NHPI/PINO˙ was studied in five different aprotic organic solvents at room temperature with a focus on the influence of the viscosity of the medium. Our findings support the theoretical descriptions derived from the statistical dynamics of the impact of the reorganization of the medium. In an Arrhenius type description the influence appears in the preexponential factor as a linear dependence on the dynamic viscosity.

Electron spin relaxation of C60 monoanion in liquid solution: applicability of Kivelson-Orbach mechanism.

We report the results of our investigation on the electron spin relaxation mechanism of the monoanion of C60 fullerene in liquid solution. The solvent chosen was carbon disulfide, which is rather uncommon in EPR spectroscopy but proved very useful here because of its liquid state over a wide temperature range. The conditions for exclusive formation of the monoanion of C60 in CS2 were first determined using electrochemical measurements. Using these results, only the monoanion of C60 was prepared by chemical reduction using Hg2I2/Hg as the reducing agent. The EPR line width was measured over a wide temperature range of 120-290 K. The line widths show weak dependence on temperature, changing by a factor of only about 2, over this temperature range. We show that the observed temperature dependence does not obey the Kivelson-Orbach mechanism of electron spin relaxation in liquids, applicable for radicals with low-lying, thermally accessible excited electronic states. The observed temperature dependence can be empirically fitted to an Arrhenius type of exponential function, from which an activation energy of 74 ± 3 cm(-1) is obtained. From the qualitative similarities in the characteristics of the spin relaxation rates of C60 monoanion radical and the cyclohexane type of cation radicals reported in the literature, we propose that a pseudorotation-induced electron spin relaxation process could be operating in the C60 monoanion radical in liquid solution. The low activation energy of 74 cm(-1) observed here is consistent with the pseudorotation barrier of C60 monoanion, estimated from reported Jahn-Teller energy levels.

Effect of amino group charge on the photooxidation kinetics of aromatic amino acids.

The kinetics of the photooxidation of aromatic amino acids histidine (His), tyrosine (Tyr), and tryptophan (Trp) by 3,3',4,4'-benzophenonetetracarboxylic acid (TCBP) has been investigated in aqueous solutions using time-resolved laser flash photolysis and time-resolved chemically induced dynamic nuclear polarization. The pH dependence of quenching rate constants is measured within a large pH range. The chemical reactivities of free His, Trp, and Tyr and of their acetylated derivatives, N-AcHis, N-AcTyr, and N-AcTrp, toward TCBP triplets are compared to reveal the influence of amino group charge on the oxidation of aromatic amino acids. The bimolecular rate constants of quenching reactions between the triplet-excited TCBP in the fully deprotonated state and tryptophan, histidine, and tyrosine with a positively charged amino group are kq = 2.2 × 10(9) M(-1) s(-1) (4.9 < pH < 9.4), kq = 1.6 × 10(9) M(-1) s(-1) (6.0 < pH < 9.2), and kq = 1.5 × 10(9) M(-1) s(-1) (4.9 < pH < 9.0), respectively. Tryptophan, histidine, and tyrosine with a neutral amino group quench the TCBP triplets with the corresponding rate constants kq = 8.0 × 10(8) M(-1) s(-1) (pH > 9.4), kq = 3.0 × 10(8) M(-1) s(-1) (pH > 9.2), and kq = (4.0-10.0) × 10(8) M(-1) s(-1) (9.0 < pH < 10.1) that are close to those for the N-acetylated derivatives. Thus, it has been established that the presence of charged amino group changes oxidation rates by a significant factor; i.e., His with a positively charged amino group quenches the TCBP triplets 5 times more effectively than N-AcHis and His with a neutral amino group. The efficiency of quenching reaction between the TCBP triplets and Tyr and Trp with a positively charged amino group is about 3 times as high as that of both Tyr and Trp with a neutral amino group, N-AcTyr and N-AcTrp.

Oxidation of purine nucleotides by Triplet 3,3',4,4'-benzophenone tetracarboxylic acid in aqueous solution: pH-dependence.

The photo-oxidation of purine nucleotides adenosine-5'-monophosphate (AMP) and guanosine-5'-monophosphate (GMP) by 3,3',4,4'-benzophenone tetracarboxylic acid (TCBP) has been investigated in aqueous solutions using nanosecond laser flash photolysis (LFP) and time-resolved chemically induced dynamic nuclear polarization (CIDNP). The pH dependences of quenching rate constants and of geminate polarization are measured within a wide range of pH values. As a result, the chemical reactivity of reacting species in different protonation states is determined. In acidic solution (pH < 4.9), the quenching rate constant is close to the diffusion-controlled limit: kq = 1.3 × 10(9) M(-1) s(-1) (GMP), and kq = 1.2 × 10(9) M(-1) s(-1) (AMP), whereas in neutral and basic solutions it is significantly lower: kq = 2.6 × 10(8) M(-1) s(-1) (GMP, 4.9 < pH < 9.4), kq = 3.5 × 10(7) M(-1) s(-1) (GMP, pH > 9.4), kq = 1.0 × 10(8) M(-1) s(-1) (AMP, pH > 6.5). Surprisingly, the strong influence of the protonation state of the phosphoric group on the oxidation of adenosine-5'-monophosphate is revealed: the deprotonation of the AMP phosphoric group (6.5) decreases the quenching rate constant from 5.0 × 10(8) M(-1) s(-1) (4.9 < pH < 6.5) to 1.0 × 10(8) M(-1) s(-1) (pH > 6.5).

Time-resolved magnetic field effects distinguish loose ion pairs from exciplexes.

We describe the experimental investigation of time-resolved magnetic field effects in exciplex-forming organic donor-acceptor systems. In these systems, the photoexcited acceptor state is predominantly deactivated by bimolecular electron transfer reactions (yielding radical ion pairs) or by direct exciplex formation. The delayed fluorescence emitted by the exciplex is magnetosensitive if the reaction pathway involves loose radical ion pair states. This magnetic field effect results from the coherent interconversion between the electronic singlet and triplet radical ion pair states as described by the radical pair mechanism. By monitoring the changes in the exciplex luminescence intensity when applying external magnetic fields, details of the reaction mechanism can be elucidated. In this work we present results obtained with the fluorophore-quencher pair 9,10-dimethylanthracene/N,N-dimethylaniline (DMA) in solvents of systematically varied permittivity. A simple theoretical model is introduced that allows discriminating the initial state of quenching, viz., the loose ion pair and the exciplex, based on the time-resolved magnetic field effect. The approach is validated by applying it to the isotopologous fluorophore-quencher pairs pyrene/DMA and pyrene-d10/DMA. We detect that both the exciplex and the radical ion pair are formed during the initial quenching stage. Upon increasing the solvent polarity, the relative importance of the distant electron transfer quenching increases. However, even in comparably polar media, the exciplex pathway remains remarkably significant. We discuss our results in relation to recent findings on the involvement of exciplexes in photoinduced electron transfer reactions.

Solvation dynamics of a radical ion pair in micro-heterogeneous binary solvents: a semi-quantitative study utilizing MARY line-broadening experiments.

This work aims at elucidating the mechanism of solvation of a radical ion pair (RIP) in a micro-heterogeneous binary solvent mixture using magnetically affected reaction yield (MARY) spectroscopy. For the exciplex-forming 9,10-dimethylanthracene/N,N-dimethylaniline system a comparative, composition-dependent MARY line-broadening study is undertaken in a heterogeneous (toluene/dimethylsulfoxide) and a quasi-homogenous (propyl acetate/butyronitrile) solvent mixture. The half-saturation field extrapolated to zero-quencher concentration, B(1/2), and the self-exchange rate constants are analyzed in the light of solvent dynamical properties of the mixtures and a dielectric continuum solvation model. The dependence of B(1/2) on the solvent composition is explained by cluster formation giving rise to shortened RIP lifetimes. The results are in qualitative agreement with the continuum solvation model suggesting that it could serve as a theoretical basis for quantitative modeling.

Absorption and fluorescence emission attributes of a fluorescent dye: 2,3,5,6-tetracyano-p-hydroquinone.

Four cyano groups have been substituted on the aromatic ring of p-hydroquinone (2,3,5,6-tetracyanohydroquinone) in order to study the enhanced photoacidity of this molecule. The acid-base equilibria have been studied using absorption (for ground state pKa) and fluorescence (excited state pKa) spectra. Three distinct species (neutral, anionic and dianionic forms) were observed in the ground state and only two species (anionic and dianionic forms) were found in the excited state when studied at different pH/Ho in water. Absorption and emission characteristics were studied in various organic solvents, including protic and aprotic solvents. Deprotonation was also investigated using binary mixtures. It has been revealed that absorption and emission spectra are considerably changed with change in media. Proton transfer to the solvent has been observed in various solvents.

Kinetics of the oxidation of thymine and thymidine by triplet 2,2'-dipyridyl in aqueous solutions at different pH values.

The photo-oxidation of the nucleobase, thymine (Thy), and nucleoside, thymidine (dThy), by dipyridyl (DP) has been investigated in aqueous solution using time-resolved laser flash photolysis. The pH dependence of the oxidation rate constants is measured within a large pH scale. As a consequence, the chemical reactivity of the reactants existing in solution at a certain range of pH is predicted. Bimolecular rate constants of the quenching reactions between triplet dipyridyl and thymine and thymidine are, respectively, kq = 2.4 × 10(7) M(-1) s(-1) (pH < 5.8) and kq = 1.0 × 10(7) M(-1) s(-1) (5.8 < pH < 9.8). Cyclic voltammetry was used to measure the potentials of thymine oxidation and dipyridyl reduction in water at pH < 7. Both results give hints for a proton coupled electron-transfer (PCET) reaction from thymine to triplet dipyridyl.

Experimental observation of preferential solvation on a radical ion pair using MARY spectroscopy.

The effect of preferential solvation on the exciplex luminescence detected magnetic field effect has been studied using magnetic-field-effect-on-reaction-yield (MARY) spectroscopy. By designing solvent mixtures which can provide a micro-environment around the magneto-sensitive radical ion pair (RIP) from highly heterogeneous to quasi-homogenous, the effect of the polarity scan on an absolute magnetic field effect (χ(E)) and B(1/2) (the field value marking half saturation) has been studied on the system 9,10-dimethylanthracene (fluorophore)/N,N'-dimethylaniline (quencher). While the trend in χ(E) (although with subtle differences) follows the usual norm of passing through maxima with increasing polarity, the B(1/2) values show either a large monotonic decrease (for heterogeneous solvents) or remain constant (for quasi-homogenous systems) with increasing polarity. The observations have been interpreted invoking the concept of amplification of the "cage-effect" as a result of preferential solvation in binary solvents and its influence on the decaying exciplex. The use of ternary solvents further confirms the proposed mechanism. Additionally electron hopping from the radical ion pair to the surrounding neutral donor molecules could also possibly contribute to the observed trend.

High pressure ESR studies of electron self-exchange reactions of organic radicals in solution.

Simple electron self-exchange reactions are often used to study the role of the reaction medium on a chemical process, commonly implying the use of various solvents with different physical properties. In principle, similar studies may be conducted using a single solvent, changing its physical properties by application of elevated pressures, but so far only little information is available on pressure dependent exchange reactions. In this work, we have used a recently constructed high pressure apparatus for use with electron spin resonance (ESR) spectroscopy to investigate simple electron self-exchange reactions involving 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and tetracyanoethylene (TCNE) and their respective radical anions as well as TMPPD and its radical cation in three different solvents. The self-exchange was observed by ESR line broadening experiments, yielding rate constants and volumes of activation. The experimental results were compared to theoretical calculations based on Marcus theory and taking into account solvent dynamic effects. The use of elevated pressures has enabled the study of solvent effects without commonly encountered problems like solubility issues or chemical reactions between solvent and solute which sometimes limit the range of useable solvents.

Kinetics of photoinduced electron transfer between DNA bases and triplet 3,3',4,4'-benzophenone tetracarboxylic acid in aqueous solution of different pH's: proton-coupled electron transfer?

The kinetics of triplet state quenching of 3,3',4,4'-benzophenone tetracarboxylic acid (BPTC) by DNA bases adenine, adenosine, thymine, and thymidine has been investigated in aqueous solution using time-resolved laser flash photolysis. The observation of the BPTC ketyl radical anion at λ(max) = 630 nm indicates that one electron transfer is involved in the quenching reactions. The pH-dependence of the quenching rate constants is measured in detail. As a result, the chemical reactivity of the reactants is assigned. The bimolecular rate constants of the quenching reactions between triplet BPTC and adenine, adenosine, thymine, and thymidine are k(q) = 2.3 × 10(9) (4.7 < pH < 9.9), k(q) = 4.0 × 10(9) (3.5 < pH < 4.7), k(q) = 1.0 × 10(9) (4.7 < pH < 9.9), and k(q) = 4.0 × 10(8) M(-1) s(-1) (4.7 < pH < 9.8), respectively. Moreover, it reveals that in strong basic medium (pH = 12.0) a keto-enol tautomerism of thymine inhibits its reaction with triplet BPTC. Such a behavior is not possible for thymidine because of its deoxyribose group. In addition, the pH-dependence of the apparent electrochemical standard potential of thymine in aqueous solution was investigated by cyclic voltammetry. The ΔE/ΔpH ≈ -59 mV/pH result is characteristic of proton-coupled electron transfer. This behavior, together with the kinetic analysis, leads to the conclusion that the quenching reactions between triplet BPTC and thymine involve one proton-coupled electron transfer.

Magnetic field effects on exciplex-forming systems: the effect on the locally excited fluorophore and its dependence on free energy.

This study addresses magnetic field effects in exciplex forming donor-acceptor systems. For moderately exergonic systems, the exciplex and the locally excited fluorophore emission are found to be magneto-sensitive. A previously introduced model attributing this finding to excited state reversibility is confirmed. Systems characterised by a free energy of charge separation up to approximately -0.35 eV are found to exhibit a magnetic field effect on the fluorophore. A simple three-state model of the exciplex is introduced, which uses the reaction distance and the asymmetric electron transfer reaction coordinate as pertinent variables. Comparing the experimental emission band shapes with those predicted by the model, a semi-quantitative picture of the formation of the magnetic field effect is developed based on energy hypersurfaces. The model can also be applied to estimate the indirect contribution of the exchange interaction, even if the perturbative approach fails. The energetic parameters that are essential for the formation of large magnetic field effects on the exciplex are discussed.

On the coherent description of diffusion-influenced fluorescence quenching experiments II: early events.

In a previous article we showed how to perform and analyze steady-state and nanosecond time-resolved experiments on fluorescence quenching by electron transfer in a coherent manner. Now, by making use of a superior time resolution, we explore the first stages of this kind of reaction. The novel information gained enables us to merge the results on the viscosity and the driving-force dependencies of the reaction rate. A unique set of parameters for a single reaction channel suffices to describe all the results in the frame of differential encounter theory for diffusion-influenced, bimolecular, remote electron-transfer reactions. The inclusion of the solvent structure is crucial for the understanding of the reaction kinetics. To the authors' best knowledge, this is the first time that such a comprehensive set of data has been successfully and jointly explained in the field, with physically sound parameters for electron-transfer reactions.