Enrique San Román

Continuous measurement of gaseous pollutants in Buenos Aires city

Abstract Data on CO, NO, NO 2 and O 3 concentrations measured in Buenos Aires city using a continuous monitoring station are reported. This is the first systematic study of this kind carried out in the city, which is, together with its surroundings, the third more populated in Latin America. Measurements were performed during 12 months in one of the principal avenues near downtown. Results indicate that vehicular traffic is the principal source of CO and NO x . The concentration of O 3 is generally quite low and results from the mixing of clean air masses with exhaust gases containing high amounts of NO. The monthly averages of CO and NO decrease from Winter to Summer in correlation with the increase of the mean wind speed and average temperature. These results are compared with previous measurements on the spatial distribution of NO 2 in the whole city using passive diffusion tubes and with the concentration of CO, which is being continuously registered since several years in the downtown area. Measurements performed at a green, windy, low traffic area beneath the La Plata river are also shown.

Sensitization of TiO2 with phthalocyanines. Part 1.—Photo-oxidations using hydroxoaluminium tricarboxymonoamidephthalocyanine adsorbed on TiO2

Hydroxoaluminium tricarboxymonoamidephthalocyanine has been adsorbed on TiO2(Degussa P-25) particles. Water suspensions and solid samples were analysed by SEM and by IR, visible extinction, diffuse reflectance and fluorescence spectroscopy. Visible spectra showed that the dye is not highly aggregated on the surface. Fluorescence was completely quenched by the semiconductor. The sample is stable toward visible irradiation. The photocatalytic activity of the modified semiconductor for transformation of different substrates under visible irradiation in the phthalocyanine Q-band has been tested. While EDTA, oxalic acid and benzoquinone did not show any changes after irradiation, photooxidation of KI, phenol, thiophenol, 4-chlorophenol, hydroquinone and salicylic acid was observed. The analysis of the results as a function of the substrate redox potential suggests that the radical cation of the phthalocyanine, produced by electron injection of the dye into the TiO2 conduction band, is the species responsible for the oxidation of the substrates. No reaction was found, at least for phenol, when TiO2 free of AlTCPc was irradiated under the same conditions.

CARBOXYLATED ZINC‐PHTHALOCYANINES–II* DIMERIZATION AND SINGLET MOLECULAR OXYGEN SENSITIZATION IN HEXADECYLTRIMETHYLAMMONIUM BROMIDE MICELLES

The dimerization of the diamide of zinc‐tetracarboxyphthalocyanine was studied spectro‐scopically in hexadecyltrimethylammonium bromide (CTAB) micelles at surfactant concentrations from 0.026 to 0.1 M and dye concentrations between 0.1 and 10 μM. The apparent dimerization constant in CTAB 0.1 M is 8.6 × 105M−1, while the intramicellar dimerization constant is 1.8 × 103. The dimer absorption spectrum was also obtained. Singlet molecular oxygen sensitization was studied by steady state photolysis using 1,3‐diphenylisobenzofurane as scavenger in 0.1 M CTAB. The usual sensitization mechanism is extended to include dimer reactions. Singlet molecular oxygen sensitization yields for monomer and dimer in the micelles are 0.7 and 0.1, respectively. With the reported values it is possible to calculate the average yield of singlet molecular oxygen production at any surfactant and dye concentrations.

CARBOXYLATED ZINC-PHTHALOCYANINE, INFLUENCE OF DIMERIZATION ON THE SPECTROSCOPIC PROPERTIES. AN ABSORPTION, EMISSION, AND THERMAL LENSING STUDY

Monomer and dimer absorption and emission spectra, and dimerization constants are reported for the diamide of the zinc‐tetracarboxyphthalocyanine in pure dimethylformamide (DMF) and in H2O/DMF mixtures at room temperature. The dimerization constant increases steadily with the water content. The monomer absorption Q‐band is insensitive to the solvent composition, whereas dimer spectra show great variations with the water content. Stationary emission measurements show that fluorescence originates exclusively from the monomers. The fluorescence spectrum as well as its absolute fluorescence quantum yield, measured by steady‐state thermal lensing, are also insensitive to the solvent composition. The thermal lensing method is discussed for the case of two absorbing species in equilibrium.

Modeling of fluorescence quantum yields of supported dyes

Aggregation equilibrium and the fluorescence properties of hydroxoaluminium tricarboxymonoamidephthalocyanine adsorbed on microgranular cellulose have been studied at different dye loadings. Up to a concentration of nearly 3 × 10−6 mol phthalocyanine (g cellulose)−1 diffuse reflectance spectra may be interpreted on the basis of a simple monomer–dimer equilibrium. Monomer and dimer spectra are similar to the spectra of the monomeric dye in solution. The solid-state dimer spectrum is red-shifted with respect to that of the monomer and this is attributed to the coplanarity of dimers. Fluorescence spectra and quantum yields show typical effects of re-absorption and re-emission of light. In particular, the observed fluorescence quantum yields depend on concentration and span the range 0.29–0.07. To account for these effects as well as the effect of aggregation on fluorescence quantum yields and to obtain corrected fluorescence spectra a model based on the Kubelka–Munk theory of diffuse reflectance is developed. The application of this model to the case under study yields a true fluorescence quantum yield ϕ = 0.46 ± 0.02 in the whole range of concentrations, which is slightly higher than the value found for the same dye in dimethyl sulfoxide solution.

Quantum yield of singlet molecular oxygen sensitization by copper(II) tetracarboxyphthalocy anine

Abstract The quantum yield φ Δ for sensitization of singlet molecular oxygen, O 2 ( 1 Δ g ), by copper(II) tetracarboxyphthalocyanine (CuTCPc) in dimethyl sulphoxide was determined under stationary illumination, using diphenyl-isobenzofurane as chemical quencher. A constant value of φ Δ = 0.16 ± 0.03 was obtained, independent of sensitizer concentration in the range 2–26 μM even though dimers and possibly oligomers are present at the highest concentrations. The monomer and the aggregates of CuTCPc physically quench O 2 ( 1 Δ g ) with a nearly diffusional rate constant.

Visible and near-IR spectroscopic and photochemical characterization of substituted metallophthalocyanines

Abstract The UV—visible spectra of copper(II) tetracarboxyphthalocyanine (CuTCPc), copper(II) tetrapentyloxycarbonyl- phthalocyanine (CuTPOCPc) and hydroxyaluminium tricarboxymonoamidephthalocyanine (AlTCPc) were studied as a function of concentration in various solvents. Dye aggregation equilibria were investigated and pure monomet and dimer spectra were obtained. Near-IR absorption maxima for the above compounds and for cobalt(II) dicarboxydiamidephthalocyanine (Co(II)TCPc) and dicyanocobalt(III) dicarboxydiamidephthalocyanine (Co(III)TCPc) were also obtained and their assignment to triplet—multiplet transitions is discussed. The analysis of the monomer and dimer spectra on the basis of exciton theory and results obtained from molecular mechanics computational calculations allows conclusions to be drawn on the monomer and dimer structures and observed differences in aggregation ability to be interpreted. Quantum yields of singlet molecular oxygen photoproduction by monomers and dimers were determined for aluminium and copper dyes by steady state irradiation using diphenylisobenzofuran as singlet oxygen quencher. The lack of activity in dimers, in accordance with the literature available, is interpreted in terms of the vibronic coupling between monomeric units in the dimer (exciton theory).

Rose Bengal adsorbed on microgranular cellulose: evidence on fluorescent dimers

Rose Bengal adsorbed on microgranular cellulose was studied in the solid phase by total and diffuse reflectance and steady-state emission spectroscopy. A simple monomer-dimer equilibrium fitted reflectance data up to dye loadings of 4 x 10(-7) mol (g cellulose)(-1) and allowed calculation of monomer and dimer spectra. Further increase of dye loading resulted in the formation of higher aggregates. Observed emission and excitation spectra and quantum yields were corrected for reabsorption and reemission of luminescence, using a previously developed model, within the assumption that only monomers are luminescent [M. G. Lagorio, L. E. Dicelio, M. I. Litter and E. San Roman, J. Chem. Soc., Faraday Trans., 1998, 94, 419]. An apparent increase of fluorescence quantum yield with dye loading was found, which was attributed to the occurrence of dimer fluorescence. Extension of the model to two luminescent species (i.e. monomer and dimer) yielded constant fluorescence quantum yields for the monomer, phiM= 0.120 +/- 0.004, and for the dimer, phiD= 0.070 +/- 0.006. The monomer quantum yield is close to the value found for the same dye in basic ethanol. The presence of fluorescent dimers and calculated quantum yields are supported by analysis of the excitation spectra and other experimental evidence. The possible occurrence of non-radiative energy transfer and the effect of surface charge on the properties of the dimer are analyzed.

Silicon Nanoparticle Photophysics and Singlet Oxygen Generation

The effect of molecular oxygen and water on the blue photoluminescence of silicon nanoparticles synthesized by anodic oxidation of silicon wafers and surface functionalized with 2-methyl 2-propenoic acid methyl ester is investigated. The particles of 3 +/- 1 nm diameter and a surface composition of Si(3)O(6)(C(5)O(2)H(8)) exhibit room-temperature luminescence in the wavelength range 300-600 nm upon excitation with 300-400 nm light. The luminescence shows vibronic resolution and high quantum yields in toluene suspensions, while a vibronically unresolved spectrum and lower emission quantum yields are observed in aqueous suspensions. The luminescence intensity, though not the spectrum features, depends on the presence of dissolved O(2). Strikingly, the luminescence decay time on the order of 1 ns does not depend on the solvent or on the presence of O(2). To determine the mechanisms involved in these processes, time-resolved and steady-state experiments are performed. These include low-temperature luminescence, heavy atom effect, singlet molecular oxygen ((1)O(2)) phosphorescence detection, reaction of specific probes with (1)O(2), and determination of O(2) and N(2) adsorption isotherms at 77 K. The results obtained indicate that physisorbed O(2) is capable of quenching nondiffusively the particle luminescence at room temperature. The most probable mechanism for (1)O(2) generation involves the energy transfer from an exciton singlet state to O(2) to yield an exciton triplet of low energy (<0.98 eV) and (1)O(2). In aqueous solutions, excited silicon nanoparticles are able to reduce methylviologen on its surface.

Dye-polyelectrolyte layer-by-layer self-assembled materials: molecular aggregation, structural stability, and singlet oxygen photogeneration.

The interaction of rose Bengal (RB) and fluorescein (FL) with poly[diallyldimethylammonium] chloride (PDDA) was studied in layer-by-layer self-assembled thin films and in solution. The spectroscopic behavior is explained in terms of dye-dye, dye-polyelectrolyte, and in solution, dye-solvent interactions. A correlation among dye hydrophobicity, aggregation tendency, polymer folding in solution, and the stability of self-assembled films is obtained. In spite of the very high dye concentration (approximately 1 M), RB-PDDA multilayer thin films are able to photogenerate singlet molecular oxygen, as demonstrated by chemical monitoring and IR phosphorescence detection.