Luminescence quenching of tris(2,2'-bipyridine)ruthenium(II) by 2,6-dimethylphenol and 4-bromo-2,6-dimethylphenol in sol-gel-processed silicate thin films.

The luminescence properties of tris(1,2-bipyridine)ruthenium(II) (Ru(bpy)(3)(2+)), included in different organically modified silicate gel matrixes were investigated. Spin and dip-coated thin films were prepared from methyltrimethoxysilane (MTMOS) and methyltriethoxysilane (MTEOS). A blue shift in the emission spectrum of the MLCT excited state of Ru(bpy)(3)(2+) with respect to the aqueous solution was observed in all the films, practically independent of the reaction pH used to prepare the "sol," silane-derived precursor, and procedure used (dip-coating or spin-coating) to obtain the film. A bimodal distribution of probe sites in the films was obtained from modeling of the emission decays by a double exponential and from application of the exponential series method. The parameters of the decay components depended principally on the thermal treatment used in the processing of the films. The lifetimes decreased with the increase in the drying temperature of the films; at the same time, the emission spectra showed a red shift and the luminescence efficiency decreased. A luminescence quenching of the ruthenium complex in the films by 4-bromo-2,6-dimethylphenol and 2,6-dimethylphenol in aerated aqueous solution at pH 12 in contact with the film was also observed. The quenching plots obtained from luminescence intensities or luminescence intensity decay measurements showed a downward curvature. These plots could be fitted satisfactorily by a sum of two Stern-Volmer terms with quenching constants K(SV1) and K(SV2) associated with two different binding sites of the ruthenium complex. This result is indicative of the matrix microheterogeneity in the films and is fully consistent with the biexponential nature of the luminescence intensity decay profiles. The Stern-Volmer parameter values for both sites in the films suggest that only a low percentage of the probe is accessible to the quencher and its respective constant K(SV1) is lower than in water.

Luminescent properties of tris(2,2'-bipyridine)ruthenium(II) in sol-gel-processed dip-coated thin films.

The luminescence decay and spectral behavior of ruthenium(II)-tris-1,2-bipyridine dichloride dissolved in different organically modified silicate gel matrixes were investigated. Dip-coated thin films were synthesized from tetraethoxysilane (TEOS), methyltriethoxysilane (MTEOS), ethyltriethoxysilane (ETEOS), and methyl- trimethoxysilane (MTMOS). A blue shift in the ruthenium complex emission spectrum with respect to the aqueous solution was observed for all the films on the sol to gel conversion. This spectral shift was slightly dependent on the precursor used to obtain the films and independent of the reaction pH to prepare the "sol". In the data treatment of the time-resolved luminescence measurements, it was assumed that the distribution of the luminophore in the films was nonhomogeneous. The analysis of the luminescence decay profiles was based on a multisite model. All decay curves are best described by a double-exponential model. The parameters of the decay components depended principally on the thermal treatment used in the processing of the films. The lifetimes decreased and the emission espectra showed a red shift with the increase in the drying temperature. A luminescence quenching of the ruthenium complex in the films by dissolved oxygen in aqueous solution was also observed. The quenching rate constant obtained from the preexponential amplitude-weighted mean lifetimes (tau(M)) was in the order of 10(9) M(-1) s(-1). When a phenolic derivative was used as quencher the process rate was greatly reduced compared to the quenching in water. It would seem that the metallic complex sequestered within the film is placed either into a higher microviscosity microenvironment or in a location which the phenolic quencher cannot access. In both cases, the quenching plot based on tau(o)(M)/tau(M) could be fitted satisfactorily by a sum of two terms of Stern-Volmer. This fact is indicative of the matrix microheterogeneity for the films and is fully consistent with the biexponential nature of the luminescence intensity decay profiles.

The Effect of Adsorbed Sodium Dodecylsulfate at the Alumina-Water Interface on the Luminescence Quenching of Tris(2,2'-bipyridine) ruthenium(II) by Nitrophenols.

The luminescence quenching of tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)(3)(2+)) by 2-, 3-, and 4-nitrophenol (NO(2)PhOH) was studied in the absence and presence of aggregates of sodium dodecyl sulfate (SDS) in the bulk solution and in the adsorbed state on alpha-alumina at pH 2. At this pH, the alumina has a superficial excess of positive charge. The uncorrected luminescence spectrum of Ru(bpy)(3)(2+) did not show any spectral shift in alumina suspension in the absence of SDS in relation to the spectrum in aqueous solution at pH 2 (lambda(max)=609 nm). In the presence of 1 g/L alumina and concentrations of SDS between 0.6 and 0.8 mM and the critical micellar concentration (CMC), the surfactant forms surface aggregates-hemimicelles or admicelles. In these surface aggregates, the emission spectrum showed a red shift (lambda(max)=637 nm). At SDS concentrations higher than the CMC, the surfactant forms micelles in the bulk solution and surface aggregates onto alpha-alumina. However, from the spectral shift observed with the increase in SDS concentration, it seems that, at high surfactant concentrations in the bulk, the metallic complex prefers to remain in the micellar instead of in the hemimicellar phases. The emission maximum in micelles was in 627 nm. The Stern-Volmer constants (K(SV)) of luminescence quenching were determined from steady-state emission intensity measurements at the maximum of emission in each case. The luminescence quenching of Ru(bpy)(3)(2+) showed that the NO(2)PhOHs are incorporated into both types of aggregates. On the basis of the K(SV) values as a function of the SDS concentration, the process of luminescence quenching of Ru(bpy)(3)(2+) by these compounds is more efficient in premicelles/micelles than in the surface aggregates onto alpha-alumina. This was observed up to a concentration of 5 mM SDS. These results can be interpreted as an effect of the increase in polarity of the micellar microenvironment on the electron-transfer mechanism for the quenching process. At SDS concentrations higher than 5 mM, the plot of K(SV) vs SDS concentration in the presence of 1 g/L alumina is shifted to higher SDS concentrations in relation to the observed plot in the absence of 1 g/L alumina. This fact can be explained in terms of the surfactant that forms surface aggregates onto alumina which do not contain the metallic complex. Copyright 2000 Academic Press.

Association of Nitrophenols to Sodium Dodecylsulfate Micelles

The luminescence quenching of tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)32+) by 2-, 3-, and 4-nitrophenol (NO2PhOH), in their protonated forms, has been studied by static and time-resolved techniques in water and sodium dodecyl sulfate (SDS) aqueous solution. In water, the bimolecular rate constants were obtained separately from both techniques and they were coincident. The values were close to the diffusional limit. In the presence of SDS micelles at a fixed detergent concentration, the Stern-Volmer plots of tau0/tau vs quencher total concentration ([Q]t) were linear in the whole range of [Q]t used. However, the Stern-Volmer plots of the ratio I0/I vs [Q]t in SDS micelles showed an upward deviating curve. From these plots of I0/I, the concentrations of the NO2PhOHs in the micellar pseudophase were measured. The treatment of the data of both types of measurements let us determine the association constants (K) of the NO2PhOHs to SDS micelles and the exit (k-) and entrance (k+) rate constants of NO2PhOHs to the micelles. The K values obtained from either static or time-resolved luminescence techniques were in agreement within experimental error. An electron-transfer mechanism was also proposed for the quenching reaction of the excited state Ru(bpy)32+ by nitrophenols in which the metallic complex is oxidized. Copyright 1997 Academic Press. Copyright 1997Academic Press

Influence of the pH on the photodynamic effect in lysozyme A comparative kinetic study with the sensitized photooxidation of isolated amino acids.

The kinetics of the eosin-sensitized photooxidation ([O2((1)Δg)]-mediated) of the protein lysozyme (Lyso) was investigated under two different pH conditions (pH 7 and pH 11). Rates of oxygen consumption and the fade in the protein fluorescence spectrum upon sensitized irradiation were monitored. Parallel studies on both denatured Lyso (absence of the four-S-S- bridges in the protein) and different mixtures of the photooxidizable amino acids of Lyso were also carried out. The mixtures maintained the same molar ratio as in the native protein, and were selected just in order to throw into relief the preferential amino acids that were being photooxidized at both pH values.Under work conditions Lyso was only photooxidizable at pH 7, whereas the opposite accounted for the denatured protein: only measurable oxygen consumption was detected at pH 11. Nevertheless, Lyso at pH 11, evidenced an important physical quenching of O2((1)Δg) due to the Tyr and Trp residues.The results for the native protein were interpreted on the basis of a previously described dark complex Eosin-Lyso, which selectively favours the photooxidation of the bounded protein. The Trp residues were the main reactive entities in the native protein. The photodinamic effect in denatured Lyso was characterized by the prevalence of Tyr residues as photooxidizable targets.In the discussion of the results, a comparisson with the photooxidation kinetics of the mixtures of free amino acids was made.