Luminescence quenching by reversible ionization or exciplex formation/dissociation

The kinetics of fluorescence quenching by both charge transfer and exciplex formation is investigated, with an emphasis on the reversibility and nonstationarity of the reactions. The Weller elementary kinetic scheme of bimolecular geminate ionization and the Markovian rate theory are shown to lead to identical results, provided the rates of the forward and backward reactions account for the numerous recontacts during the reaction encounter. For excitation quenching by the reversible exciplex formation, the Stern-Volmer constant is specified in the framework of the integral encounter theory. The bulk recombination affecting the Stern-Volmer quenching constant makes it different for pulse excited and stationary luminescence. The theory approves that the free energy gap laws for ionization and exciplex formation are different and only the latter fits properly the available data (for lumiflavin quenching by aliphatic amines and aromatic donors) in the endergonic region.     

Non-markovian theory of diffusion-controlled excitation transfer

The non-markovian encounter theory is extended to kinetic calculations of an exciton (electron) contact transfer between an energy (charge) donor and acceptor. This process is shown to be non-stationary from the beginning to the end if the donor state relaxation time is shorter than the encounter time. The theory is applied to calculations of hot luminescence quantum yield in liquid solutions with a quenching impurity.     

Photoinduced electron transfer and geminate recombination in liquids on short time scales: Experiments and theory

The coupled processes of intermolecular photoinduced forward electron transfer and geminate recombination between the (hole) donor (Rhodamine 3B) and (hole) acceptors (N,N-dimethylaniline) are studied in three molecular liquids: acetonitrile, butyronitrile, and benzonitrile. Two color pump-probe experiments on time scales from approximately 100 fs to hundreds of picoseconds give information about the depletion of the donor excited state due to forward electron transfer and the survival kinetics of the radicals produced by forward electron transfer. The data are analyzed with a model presented previously that includes distance dependent forward and back electron transfer rates, donor and acceptor diffusion, solvent structure, and the hydrodynamic effect in a mean-field theory of through solvent electron transfer. The forward electron transfer is in the normal regime, and the Marcus equation for the distance dependence of the transfer rate is used. The forward electron transfer data for several concentrations in the three solvents are fitted to the theory with a single adjustable parameter, the electronic coupling matrix element Jf at contact. Within experimental error all concentrations in all three solvents are fitted with the same value of Jf. The geminate recombination (back transfer) is in the inverted region, and semiclassical treatment developed by Jortner [J. Chem. Phys. 64, 4860 (1976)] is used to describe the distance dependence of the back electron transfer. The data are fitted with the single adjustable parameter Jb. It is found that the value of Jb decreases as the solvent viscosity increases. Possible explanations are discussed.     

蒈烯芳氰敏化光氧化反应的研究——Ⅰ.蒈烯对9,10-二腈基蒽荧光的猝灭特性

本文通过研究在不同溶剂中蒈烯对9,10-二腈基蒽(DCA)荧光猝灭的光物理特性及溶剂极性对猝灭速度的影响,温度效应的测定及其在乙腈中双分子猝灭速率常数k_q值与计算所得自由能的变化(△G)之间的关系符合 RehmWeller关系,证明了菇烯对DCA荧光的猝灭是一个电子转移的动态猝灭过程。     

Radical heterolysis reactions. Dynamics of formation, collapse, and solvation of ion pairs determined by a competition kinetic method

The kinetics of radical heterolysis reactions, including rate constants for radical cation-anion contact ion pair formation, collapse of the contact pair back to the parent radical, and separation of the contact pair to a solvent-separated ion pair or free ions were obtained in several solvents for a beta-mesyloxy radical. Rate constants were determined from indirect kinetic studies using thiophenol as both a radical trapping agent via H-atom transfer and an alkene radical cation trapping agent via electron transfer. [reaction: see text].     

State-to-state rotational relaxation rate constants for CO+Ne from IR-IR double-resonance experiments: comparing theory to experiment

IR-IR double-resonance experiments were used to study the state-to-state rotational relaxation of CO with Ne as a collision partner. Rotational levels in the range Ji=2-9 were excited and collisional energy transfer of population to the levels Jf=2-8 was monitored. The resulting data set was analyzed by fitting to numerical solutions of the master equation. State-to-state rate constant matrices were generated using fitting law functions. Fitting laws based on the modified exponential gap (MEG) and statistical power exponential gap (SPEG) models were used; the MEG model performed better than the SPEG model. A rate constant matrix was also generated from scattering calculations that employed the ab initio potential energy surface of McBane and Cybulski [J. Chem. Phys. 110, 11 734 (1999)]. This theoretical rate constant matrix yielded kinetic simulations that agreed with the data nearly as well as the fitted MEG model and was unique in its ability to reproduce both the rotational energy transfer and pressure broadening data for Ne-CO. The theoretical rate coefficients varied more slowly with the energy gap than coefficients from either of the fitting laws.     

Fluorescence quenching of biologically active carboxamide by aniline and carbon tetrachloride in different solvents using Stern-Volmer plots

Fluorescence quenching of biologically active carboxamide namely (E)-2-(4-chlorobenzylideneamino)-N-(2-chlorophenyl)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxamide [ECNCTTC] by aniline and carbon tetrachloride (CCl4) quenchers in different solvents using steady state method and time resolved method using only one solvent has been carried out at room temperature to understand the role of quenching mechanisms. The Stern-Volmer plot has been found to be linear for all the solvents studied. The probability of quenching per encounter p (p') was determined in all the solvents and was found to be less than unity. Further, from the studies of rate parameters and life time measurements in n-heptane and cyclohexane with aniline and carbon tetrachloride as quenchers have been shown that, the phenomenon of quenching is generally governed by the well-known Stern-Volmer (S-V) plot. The activation energy Ea (or E'a) of quenching was determined using the literature values of activation energy of diffusion Ed and the experimentally determined values of p (or p'). It has been found that, the activation energy Ea (E'a) is greater than the activation energy for diffusion Ed in all solvents. Hence, from the magnitudes of Ea (or E'a) as well as p (or p') infer that, the quenching mechanism is not solely due to the material diffusion, but there is also contribution from the activation energy.     

Kinetics of diffusion-limited catalytically activated reactions: an extension of the Wilemski-Fixman approach

We study the kinetics of diffusion-limited catalytically activated A+B-->B reactions taking place in three-dimensional systems, in which an annihilation of diffusive A particles by diffusive traps B may happen only if the encounter of an A with any of the Bs happens within a special catalytic subvolumen: these subvolumens being immobile and uniformly distributed within the reaction bath. Suitably extending the classical approach of Wilemski and Fixman [J. Chem. Phys. 58, 4009 (1973)] to such three-molecular diffusion-limited reactions, we calculate analytically an effective reaction constant and show that it comprises several terms associated with the residence and joint residence times of Brownian paths in finite domains. The effective reaction constant exhibits a nontrivial dependence on the reaction radii, the mean density of catalytic subvolumens, and particles' diffusion coefficients. Finally, we discuss the fluctuation-induced kinetic behavior in such systems.     

The dependence of quinine fluorescence quenching on ionic strength

The Stern-Volmer constant for the quenching of quinine fluorescence by chloride ions has been found to be markedly dependent on acid concentration. Steady-state and time-resolved fluorescence measurements under different acid and salt concentrations have further shown that the decrease in quenching arises from the influence of increasing ionic strength on the diffusion-controlled rate constant for the bimolecular quenching process. Two possible mechanisms for this dependence are discussed: a decrease in the intrinsic rate constant for the reaction due to the kinetic salt effect, and a decrease in the effective encounter distance due to screening of the charges on the reactants. © 1996 John Wiley & Sons, Inc.     

Structural and environmental requirements for quenching of singlet oxygen by cyanine dyes

Singlet-oxygen quenching constants were measured for 19 cyanine dyes in acetonitrile. The most efficient quenchers were 1-butyl-2-[2-[3-[(1-butyl-6-chlorobenz-[cd]indol-2(1H)- ylidene)ethylidene]-2-chloro-1-cyclohexen-1-yl]ethenyl]-6-chlorobenz[cd] indolium and 6-chloro-2-[2-[3-(6-chloro-1-ethylbenz[cd]indol-2(1H)-ylidene) ethylidene]-2-phenyl-1-cyclopenten-1-yl]ethenyl]-1-ethyl-benz[cd]indolium, having quenching constants with diffusion-controlled values of 2.0 +/- 0.1 x 10(10) and 1.5 +/- 0.1 x 10(10) M-1 s-1, respectively. There was a trend toward increased quenching constants for cyanine dyes with the absorption band maxima at longer wavelengths. However, the quenching constants correlated better with the oxidation potentials of the cyanine dyes, suggesting that quenching proceeds by charge transfer rather than energy transfer. The quenching constants for 1,1',3,3,3',3'-hexamethylindotricarbocyanine perchlorate and 1,1'-diethyl-4,4'-carbocyanine iodide were measured in several solvents as well as in aqueous solutions of detergent micelles. In different solvents, the quenching constants varied by as much as a factor of 50. The quenching constants were largest in solvents with the highest values on the pi* scale of Kamlet, Abboud, Abraham and Taft. This was consistent with quenching occurring by charge transfer. Within cells, cyanine dyes concentrate in membrane-bound organelles. The quenching constants were substantial within detergent micelles. To the extent that micelles are models for biological membranes, cyanine dyes may be effective biological singlet-oxygen quenchers.     

Mechanistic insight from activation parameters for the reaction of a ruthenium hydride complex with CO2 in conventional solvents and an ionic liquid

Detailed kinetic studies were performed on the reaction of [Ru(II)(terpy)(bpy)H](+) (terpy = 2,2',6',2″-terpyridine; bpy = 2,2'-bipyridine) with CO(2) in conventional solvents (water, methanol, and ethanol) and in the ionic liquid [emim][NTf(2)] ([emim] = 1-ethyl-3-methyl-imidazolium; [NTf(2)] = bistrifluoromethylsulfonylamide). Second-order rate constants and activation parameters (ΔH(?), ΔS(?), and ΔV(?)) were determined for the reaction in all solvents. The second-order rate constants correlate with the acceptor number of the solvent, whereas the activation parameters support the associative nature of the reaction. The results in water, especially the activation entropy (+14 ± 2 J K(-1) mol(-1)) and activation volume (-5.9 ± 0.6 cm(3) mol(-1)), differ significantly from those found for the other solvents.     

On the influence of solvents on the rate of ion-transfer reactions

The literature data on the kinetics of cation electrodeposition on mercury in different solvents were analysed. For all cations considered in different solvents there was a linear decrease of the logarithm of the standard charge-transfer rate constant with increasing basicity of solvent and with more negative formal potential of the electrode reaction expressed in the scale of a solvent-independent electrode, as well as a linear dependence of the activation energy on the Gibbs energy of cation transfer. No dependence of the logarithm of the heterogeneous rate constant on the rate of exchange of solvent molecules from the first solvation sphere was observed. For the different electrode systems studied in one solvent, the dependences of the activation energy on (i) the cation solvation energy, (ii) the Gibbs energy of metal amalgamation, and (iii) metal solubility in mercury were analysed.     

Equilibrium, kinetics, mechanism, and process design for the sorption of methylene blue onto rice husk

Batch experiments were carried out for the sorption of methylene blue onto rice husk particles. The operating variables studied were initial solution pH, initial dye concentration, adsorbent concentration, and contact time. Equilibrium data were fitted to the Freundlich and Langmuir isotherm equations and the equilibrium data were found to be well represented by the Langmuir isotherm equation. The monolayer sorption capacity of rice husks for methylene blue sorption was found to be 40.5833 mg/g at room temperature (32 degrees C). The sorption was analyzed using pseudo-first-order and pseudo-second-order kinetic models and the sorption kinetics was found to follow a pseudo-second-order kinetic model. Also the applicability of pseudo second order in modeling the kinetic data was also discussed. The sorption process was found to be controlled by both surface and pore diffusion with surface diffusion at the earlier stages followed by pore diffusion at the later stages. The average external mass transfer coefficient and intraparticle diffusion coefficient was found to be 0.01133 min(-1) and 0.695358 mg/g min0.5. Analysis of sorption data using a Boyd plot confirms that external mass transfer is the rate limiting step in the sorption process. The effective diffusion coefficient, Di was calculated using the Boyd constant and was found to be 5.05 x 10(-04) cm2/s for an initial dye concentration of 50 mg/L. A single-stage batch-adsorber design of the adsorption of methylene blue onto rice husk has been studied based on the Langmuir isotherm equation.     

Quenching of singlets and triplets by reversible ionization followed by charge recombination

The reversible electron transfer from donor to excited molecule (acceptor of electron) is shown to be the irreversible energy quenching, if it is completed by subsequent irreversible recombination radical-ions which are produced. The Stern-Volmer constant of fluorescence as well as the Markovian rate constant of triplet quenching are calculated analytically, assuming the electron transfer is contact. The multiple Rehm-Weller effect is shown to be peculiar to both constants.     

Study of the electron transfer reaction between [Co(EDTA)]− and [Ru(NH3)5py]2+ in methanol–water mixtures

The title reaction was studied in different water–cosolvent (methanol) mixtures. The results have been rationalized employing the Marcus–Hush treatment. To apply this treatment, the true, unimolecular, electron‐transfer rate constants (k_et) were obtained from the experimentally measured rate constants after calculation of the equilibrium constant for the processes of formation of the encounter complex. This calculation was carried out using Eigen–Fuoss (EF) and exponential mean spherical (EMSA) approaches employing effective values of the solvent dielectric constant. These effective values were obtained from the measured association constants corresponding to other ion pairs. The results reveal that in these media there is an additional component of reorganization energy, absent in neat solvents. An explanation of the origin of this component is given. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 658–666, 2009     

Kinetics and mechanism of the reactions of platinum(II)‐dipicolinate and platinum(II)‐glycylglycine with oxalate ion

The kinetics of the reactions of [Pt(dipic)(H₂O)] and [Pt(digly)(H₂O)] (where H₂dipic = pyridine‐2,6‐dicarboxylic acid and H₂digly = glycylglycine) with oxalate ion were studied at 25°C in aqueous medium by UV–vis spectroscopy at I = 0.1 mol dm^?3 over an wide range of pH. A probable associative pathway may involve a five‐coordinate intermediate leading to the formation of an unidentate oxalate species, which converts to bidentate chelate in subsequent fast steps. The products are isolated and characterized by CHN analysis, IR, and ¹H NMR spectra. The kinetic data from pH variation experiments are fitted by a computer program to a sequence of reactions and the different rate constants are evaluated. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 327–333, 2003     

Kinetics of salicylic acid adsorption on activated carbon

The adsorption and desorption of salicylic acid from water solutions was investigated in HPLC microcolumns packed with activated carbon. The adsorption isotherm was obtained by the step-up frontal analysis method in a concentration range of 0-400 mg/L and was well fitted with the Langmuir equation. The investigation of rate aspects of salicylic acid adsorption was based on adsorption/desorption column experiments where different inlet concentrations of salicylic acid were applied in the adsorption phase and desorption was conducted with pure water. The concentration profiles of individual adsorption/desorption cycles data were fitted using several single-parameter models of the fixed-bed adsorption to assess the influence of different phenomena on the column behavior. It was found that the effects of axial dispersion and extraparticle mass transfer were negligible. A rate-determining factor of fixed-bed column dynamics was the kinetics of pore surface adsorption. A bimodal kinetic model reflecting the heterogeneous character of adsorbent pores was verified by a simultaneous fit of the column outlet concentration in four adsorption/desorption cycles. The fitted parameters were the fraction of mesopores and the adsorption rate constants in micropores and mesopores, respectively. It was shown that the former rate constant was an intrinsic one whereas the latter one was an apparent value due to the effects of pore blocking and diffusional hindrances in the micropores.     

Modulation of electron transfer kinetics by protein conformational fluctuations during early-stage photosynthesis

The kinetics of electron transfer during the early stages of the photosynthetic reaction cycle has recently been shown in transient absorption experiments carried out by Wang et al. [Science 316, 747 (2007)] to be strongly influenced by fluctuations in the conformation of the surrounding protein. A model of electron transfer rates in polar solvents developed by Sumi and Marcus using a reaction-diffusion formalism [J. Chem. Phys. 84, 4894 (1986)] was found to be successful in fitting the experimental absorption curves over a roughly 200 ps time interval. The fits were achieved using an empirically determined time-dependent function that described protein conformational relaxation. In the present paper, a microscopic model of this function is suggested, and it is shown that the function can be identified with the dynamic autocorrelation function of intersegment distance fluctuations that occur in a harmonic potential of mean force under the action of fractional Gaussian noise.     

The mechanism of fluorescence quenching of aromatic compounds by acids: ZDO calculations

The fluorescence quenching of cyano, hydroxy, methoxy, and amino derivatives of naphthalene, anthracene and pyrene by acids in polar solvents has been studied by quantum-chemistry methods in semi-empirical approximation. Quenching mechanisms, including protonation of the aromatic nucleus and electron transfer have been considered. It is shown that the mechanism of radiationless deactivation in encounter complexes of reagents consists in the specific interaction of the solvated proton with certain carbon atoms of the aromatic nucleus, not in the electron transfer. It is found that the rate constant of radiationless deactivation correlates with the charge at the corresponding carbon atom of the excited molecule. It is shown that the fluorescence quenching is determined mainly by the nature of the fluorescent state and electron donor-acceptor properties of the aromatic nucleus and the substituent. The present model makes it possible to predict the fluorescent properties of some aromatic compounds in the presence of acids.     

Kinetics of solvolysis and cyanide attack at 2-benzoylpyridine-iminemolybdenum(0) tetracarbonyl and related compounds

Summary The kinetics of reaction of [Mo(CO)₄(bpami)], where bpami is the Schiff base derived from 2-benzoyl pyridine and ammonia, with cyanide in several solvents show parallel solvolysis and cyanide attack. Rate laws and activation parameters are consistent with an associative mechanism for the predominant pathway, cyanide attack. From kinetic and solubility measurements it is possible to analyse solvent effects on reactivity for these reactions into initial state and transition state contributions. Rate constant trends in binary mixtures of non-aqueous solvents can be understood in terms of preferential solvation of the molybdenum compound and of cyanide. Finnlly, ligand effects on reactivity have been established for a number of [Mo(CO)₄(diimine)] complexes; over a 5×10⁴ times range in rate constants for cyanide attack there is a marked correlation of reactivity with MLCT frequencies for the metal to diimine charge-transfer band.