Electrogenerated chemiluminescence reactions between the [Ru(bpy)3](2+) complex and PAMAM GX.0 dendrimers in an aqueous medium.

Electrogenerated chemiluminescence, ECL, reactions between tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)3](2+), and PAMAM GX.0 (X=1 and 2) dendrimers in an aqueous medium were carried out at pH10 (fully deprotonated dendrimer surface). ECL was detected in the presence of GX.0 dendrimers without addition of any known coreactant. Atomic force microscopy, AFM, measurements for GX.0 dendrimers in the presence of the [Ru(bpy)3](2+) complex were also done. AFM images showed the existence of aggregates (pillars) of globular shape, as well as interdendrimer networks forming fibers in the x-y direction for dendrimer aqueous solutions. ECL and AFM results in cooperation suggest that the coreactant effect of the end amine groups is improved by both the dendritic branched shells and the globular z-type aggregates. The ECL efficiency trends as a function of [GX.0] (whole range) can be interpreted taking into account the coreactant effect modulated by the presence of the z and x-y type aggregates. Importantly, ECL efficiency values can be taken as a measure of the change induced on the dendrimer aggregation in aqueous solutions when their concentrations rise. Redox potentials of the [Ru(bpy)3](3+/2+) couple in the presence of the G1.0 and G2.0 dendrimers were also determined.

Free energy of binding of cationic metal complexes to AuNPs through electron-transfer processes.

AuNP effects on the oxidation reaction of [Ru(NH3)5pz](2+) with S2O8(2-) were studied. Experimental results show the effects produced by gold nanoparticles of different sizes, which are then discussed by using an approach based on the two-state model. Changes in the observed reactivity are explained by a change in the degree of association of the reactants to the nanoclusters, which depends on the negative surface potential of the gold nanoparticles. TEM and zeta potential measurements show that this potential determines the binding strength of one of the reactants ([Ru(NH3)5pz](2+)) to the citrate gold surface. The number of binding sites on a citrate nanoparticle receptor has also been determined. The experimental results confirm that an electron transfer reaction can be used as a probe for the determination of the free energy of binding of cationic metal complexes to gold nanoparticles.

Electrochemiluminescence of the [Ru(bpy)3]2+ complex: the coreactant effect of PAMAM dendrimers in an aqueous medium.

Electrogenerated chemiluminescence (ECL) from aqueous solutions of tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)(3)](2+), in the presence of PAMAM G1.5 and G4.5 dendrimers, was observed without the addition of coreactants. The ECL efficiency, Φ(ECL), was enhanced with the addition of increasing amounts of G1.5 dendrimer. Indeed, the ECL efficiency for the [Ru(bpy)(3)](2+)/G1.5 dendrimer became about 10 times higher than that for the [Ru(bpy)(3)](2+)/ oxalate anion system. However, the ECL efficiency in the presence of the G4.5 dendrimer was smaller than that for the G1.5 dendrimer at concentrations similar to those for G1.5 with identical medium conditions. Besides, the addition of NaCl at a given concentration of G1.5 dendrimer decreased the ECL efficiency. The results of Φ(ECL) were interpreted by taking into account the coreactant effect and the electrostatic (long-range and short-range) interactions between the ruthenium(II) complex and the electric field of the dendrimer surface. Standard formal potentials of the [Ru(bpy)(3)](3+/2+) couple in the presence of G1.5 and G4.5 dendrimers were also determined.

Photoinduced electron-transfer reactions: a study of the diffusion-controlled and activation-diffusion-controlled processes.

The diffusion-controlled electron transfer rate constants (k(d)) of several quenching reactions of ruthenium complexes [Ru(L)(3)](2+*) (L = bpy, phen, and 4,7-(CH(3))(2)phen) with [Fe(CN)(6)](3-) were experimentally determined at different concentrations of NaNO(3). From these rate constants, the effective values of viscosity coefficients for NaNO(3) solutions were calculated using EMSA (exponential mean spherical approximation) and EF (Eigen-Fuoss) approaches in order to take into account the mean force potential between reactants. The reliability of the effective parameters were checked through calculations of the rate constants of the reaction [IrCl(6)](2-)+ [Ru(bpy)(3)](2+)* in these NaNO(3) solutions. The rate constants of this reaction were also obtained by fluorescence quenching measurements. The agreement between the two sets of data (experimental and predicted) is excellent. The trends of association (k(d)) and dissociation (k(-d)) rate constants for 2+/3-, 2+/2-, and 2+/2+ reactions in NaNO(3) solutions are discussed. The use of effective diffusion coefficients for estimating k(d) and k(-d) allowed us to obtain the intrinsic electron transfer rate constant (k(et)) for the activation-diffusion-controlled process between [Ru(bpy)(3)](2+)* and [Co(NH(3))(5)Cl](2+) complexes from the observed (quenching) rate constant. The trend of electron-transfer rate constant in NaNO(3) for this reaction was rationalized by using the Marcus electron-transfer treatment.

Reaction and reorganization free energies of electron-transfer reactions under restricted geometry conditions.

Electron-transfer reactions between iron and cobalt complexes were studied in beta-cyclodextrin (betaCD), 2-hydroxypropyl-beta-cyclodextrin (HbetaCD), and 18-crown-6 ether (18C6) solutions. The results were rationalized taking as the basis the Marcus-Hush formalism. We employed two different approaches, depending on the kind of receptor and solvent, to obtain the reorganization and reaction free energies that determine the reaction rate constant. The opposite trends in reactivity observed in betaCD and HbetaCD solutions and the behavior in solutions of 18C6 are explained.

Salt and solvent effects on the kinetics and thermodynamics of the inclusion of the ruthenium complex [Ru(NH3)5(4,4'-bpy)]2+ in beta-cyclodextrin.

The influences of solvents (in water-cosolvent mixtures) and salts on the kinetics and thermodynamics of the inclusion of [Ru(NH3)5(4,4'-bpy)]2+ in beta-cyclodextrin (beta-CD) have been studied. Solvent effects on the kinetics can be described as a consequence of the competition of the cosolvent for the beta-CD cavity. The salt effects on the kinetics depend on the ion pairing of the anions with the [Ru(NH3)5(4,4'-bpy)]2+ complex. On the other hand, the solvent effects on the equilibrium constant depend on the stabilization of the 4,4'-bipyridine ligand in the water-cosolvent mixture relative to water. Finally, salt effects on the equilibrium constant are interpreted as a consequence of ion pairing between the anion of the salt and the inclusion complex.

Salt and solvent effects on the kinetics of the oxidation of the excited state of the [Ru(bpy)3]2+ complex by S2O8(2-).

The title reaction was studied in different reaction media: aqueous salt solutions (NaNO3) and water-cosolvent (methanol) mixtures. The observed rate constants, k(obs), show normal behavior in the solutions containing the electrolyte, that is, a negative salt effect. However, the solvent effect is abnormal, because a decrease of the rate constant is observed when the dielectric constant of the reaction medium decreases. These effects (the normal and the abnormal) can be explained using the Marcus-Hush treatment for electron transfer reactions. To apply this treatment, the true, unimolecular, electron-transfer rate constants, k(et), have been obtained from k(obs) after calculation of the rate constants corresponding to the formation of the encounter complex from the separate reactants, k(D), and the dissociation of this complex, k(-D). This calculation has been carried out using an exponential mean spherical approach (EMSA).

Method for the evaluation of the reorganization energy of electron transfer reactions produced under restricted geometry conditions.

The kinetics of the electron transfer reactions between S(2)O(8)(2-) and the complexes (Ru(NH(3))(5)L)(2+) (L = pyridine, pyrazine, and 4-cyanopyridine) have been studied in micellar (SDS) solutions. A method for the evaluation of the reorganization energy of these reactions, based on the comparison of their rate constants, is proposed. From the results obtained, we concluded that the observed rate constants go through a minimum for the surfactant concentration in which the reorganization energy goes through a maximum. The method can be applied to any kind of restricted geometry conditions.

Strength and character of peptide/anion interactions.

The binding free energy of complex [Co(C(2)O(4))(3)](3-) to three peptides H-Lys-Gly-Lys-Gly-Lys-Gly-Lys-NH(2) (P-1), H-(Lys-Gly-Lys-Gly-Lys-Gly-Lys)(2)-NH(2) (P-2), H-(Lys-Gly-Lys-Gly-Lys-Gly-Lys)(3)-NH(2) (P-3) and to the monomers (amino acids) forming the peptides has been obtained using the kinetics of the electron-transfer reaction between [Ru(NH(3))(5)py](2+) and [Co(C(2)O(4))(3)](3-) as the probe. The polymerization of the monomers increases the negative free energy of binding and changes its character, noncooperative for the monomers and anticooperative for the peptides. This increase in the negative free energy represents a driving force for the polymerization process. The magnitude of the gain in negative free energy, as a consequence of the anticooperative character of the binding of the cobalt complex to the peptide, depends on the ratio of [complex]/[monomers].

Estimation of Electron Transfer Rate Constant from Static (Optical and Thermodynamic) Measurements: A Study of Ru(NH(3))(5)pz(2+) + Fe(CN)(6)(3)(-) right harpoon over left harpoon Ru(NH(3))(5)pz(3+) + Fe(CN)(6)(4)(-) Electron Transfer Reactions.

A kinetic study of the electron-transfer reactions Ru(NH(3))(5)pz(2+) + Fe(CN)(6)(3-) right harpoon over left harpoon Ru(NH(3))(5)pz(3+) + Fe(CN)(6)(4-) was carried out in several water-organic solvent mixtures at 298.2 K. The free energies of activation for these thermal electron-transfer reactions were calculated from a combination of spectroscopic and thermodynamic data and are compared with those obtained from the kinetic study. Quantitative agreement is found between the two series of data. This shows the possibility of estimating activation free energies for electron-transfer reactions from these (static) measurements.