Antonio E. H. Machado

Investigation of Ground- and Excited-State Photophysical Properties of 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphyrin with Ruthenium Outlying Complexes

The present work employs a set of complementary techniques to investigate the influence of outlying Ru(II) groups on the ground- and excited-state photophysical properties of free-base tetrapyridyl porphyrin (H(2)TPyP). Single pulse and pulse train Z-scan techniques used in association with laser flash photolysis, absorbance and fluorescence spectroscopy, and fluorescence decay measurements, allowed us to conclude that the presence of outlying Ru(II) groups causes significant changes on both electronic structure and vibrational properties of porphyrin. Such modifications take place mainly due to the activation of nonradiative decay channels responsible for the emission quenching, as well as by favoring some vibrational modes in the light absorption process. It is also observed that, differently from what happens when the Ru(II) is placed at the center of the macrocycle, the peripheral groups cause an increase of the intersystem crossing processes, probably due to the structural distortion of the ring that implies a worse spin-orbit coupling, responsible for the intersystem crossing mechanism.

Degradation of the commercial herbicide glyphosate by photo-fenton process: evaluation of kinetic parameters and toxicity

Parameters that influence the efficiency of the degradation of glyphosate (addition of Fe2+, simultaneous addition of Fe2+ and Fe3+ at a 1:1 (Fe2+/Fe3+) molar ratio, addition of oxalate and of H2O2) were evaluated at lab-scale. Synergic effects on its degradation and release of phosphate were observed using Fe2+/Fe3+, as well as adding oxalate. On the other hand, the concentration increase of Fe2+/Fe3+, oxalate and H2O2 did not promote a linear increase of glyphosate mineralization and release of phosphate. Using high concentrations of these species, the efficiency of glyphosate mineralization and release of phosphate was constant or even decreased. Under optimized conditions (0.27 mmol L-1 of Fe2+/Fe3+; 1.13 mmol L-1 of oxalate and 10.3 mmol L-1 of H2O2), close results for mineralization and release of phosphate were obtained in lab-scale and using a solar pilot plant. A direct ratio between reducing the toxicity and glyphosate concentration was also observed.

Photophysics and spectroscopic properties of zinc phthalocyanine revisited using quantum chemistry

This study provides a theoretical evaluation of the photophysical behavior of zinc phthalocyanine (ZnPc) using density functional theory (DFT) and its time-dependent (TD-DFT) approach, based on the combination of the B3LYP hybrid functional and the 6-311+G(d,p) basis set. The influence of solvation was estimated using IEFPCM (integral equation formalism approach of polarizable continuum model) considering DMSO (dimethyl sulfoxide) as solvent. The simulated absorption spectra were based in the calculation of the first 40 excited states being that the eight most intense peaks could be assigned to the Q and Soret (B) bands. Fluorescence emission wavelength and radiative lifetime were calculated using the TD-DFT approach and compared to experimental data. The ZnPc intersystem crossing (ISC) phenomena was attributed to spin-orbit coupling induced by Zn2+ since the adjacent singlet and triplet states have the same orbital symmetry. The phosphorescence data in gas phase were also compared.

Molecular modeling study on the possible polymers formed during the electropolymerization of 3-hydroxyphenylacetic acid

The compound 3-hydroxyphenylacetic acid (3-HPA) has been used as a monomer in the synthesis of polymeric films by electropolymerization; these films serve as supports for the immobilization of biomolecules in electrochemical biosensors. To assist in the elucidation of the mechanism of 3-HPA electropolymerization, a systematic quantum mechanical study was conducted. In addition to the monomer, all possible intermediates and the probable oligomers formed during the electropolymerization were investigated using a density functional theory (DFT) method combined with a previous conformational analysis performed with the aid of the RM1 semi-empirical method or a Monte Carlo conformational analysis with the force field OPLS-2005. From the data analysis combined with the experimental results, a mechanism was proposed for the main route of formation of the polymeric films. The mechanism involves the formation of polyethers from the coupling of phenoxide radicals and radicals based on the aromatic ring.

Photolytic degradation of chloramphenicol in different aqueous matrices using artificial and solar radiation: reaction kinetics and initial transformation products

The photodegradation of cloramphenicol (CAP) in ultrapure water (UW), untreated surface water (USW), and treated effluent from sewage treatment plant (TESTP) in laboratory scale and pilot scale, was evaluated using solar and artificial radiation. The results show, in all cases, that the CAP degradation occurs according to pseudo-first order kinetics, with the apparent degradation rate constants (kapp) following the order UW ≡ USW > TESTP. The kapp and half-life were strongly influenced by the radiation source. Mono- and di-hydroxyl transformation products were identified in UW after 40 min of solar irradiation, while the acute toxicity to Artemia salina increased from 35% to 100%, respectively after 180 and 1440 min of artificial and solar irradiation (94 and 132 kJ L-1), when 99.2 and 97.7% of CAP degradation occurred. The transformation products did not present antimicrobial activity.

Applications of Mesoporous Ordered Semiconductor Materials — Case Study of TiO2

TiO2 is a promising material for technological applications for its versatility [1-4], abundance, low toxicity, good chemical stability, photosensitivity and photostability [5-8]. In nature, it is found mainly in the mineral ilmenite [9], that can be processed industrially by two different routes [10]: The first, involves the reaction of the concentrate of ilmenite with hot sulphuric acid, resulting in the formation of sulphates of titanium, Fe(II) and Fe(III), being these last eliminat‐ ed by centrifugation, after cooling. The final solution is then purified and hydrolyzed to produce pure TiO2 [11]. The other usual way of obtaining consists in combining the ore with coke and gaseous chlorine under heating, resulting in CO2 and a spongy material rich in TiCl4. The reaction product is subjected to successive fractional distillation, with the formation of TiCl2 and TiCl3, due to stability of titanium in other degrees of oxidation. The different precursors of titanium are hydrolyzed, forming titanium dioxide [12].

Rapid Preparation of (BiO) 2 CO 3 Nanosheets by Microwave-Assisted Hydrothermal Method with Promising Photocatalytic Activity Under UV-Vis Light

Crystalline (BiO)2CO3 nanosheets were synthesized by a rapid one-step reaction via microwaveassisted hydrothermal method using urea as a morphology mediator and carbon source. The hydrothermal method combined with microwave heating allowed to obtain sheet-like (BiO)2CO3 particles at shorter reaction times when compared to the conventional heating hydrothermal method. The photocatalytic activity of the as prepared samples was evaluated towards degradation of Ponceau 4R (C.I. 16255) under artificial UV-Vis light irradiation. The results show that good photocatalytic efficiency can be obtained for powders prepared with reaction times as low as 2 minutes.

Potential Applications for Solar Photocatalysis: From Environmental Remediation to Energy Conversion

Antonio Eduardo Hora Machado1,*, Lidiaine Maria dos Santos1, Karen Araujo Borges1, Paulo dos Santos Batista2, Vinicius Alexandre Borges de Paiva1, Paulo Souza Muller Jr.1, Danielle Fernanda de Melo Oliveira1 and Marcela Dias Franca1 1Universidade Federal de Uberlândia, Instituto de Quimica, Laboratorio de Fotoquimica, Uberlândia, Minas Gerais, 2Universidade Federal de Goias, Campus Catalao, Departamento de Quimica, Catalao, Goias, Brazil

Dye Degradation Enhanced by Coupling Electrochemical Process and Heterogeneous Photocatalysis

In this study, we evaluated the combination between an electrochemical process, occurring in the dark, and a heterogeneous photocatalytic process for dye degradation, using the azo dye tartrazine as model of oxidizable substrate. TiO2 P25 and an Ag-doped TiO2 were used as photocatalysts in suspensions containing 50 mg L–1 of tartrazine. The best result, 74% of dye mineralization in 120 min of reaction, was obtained using TiO2 P25 as photocatalyst and a current density of 10 mA cm–2 in the electrochemical cell, a value 30% higher than the sum of the results obtained by heterogeneous photocatalysis (44%) and electrochemical oxidation (13%). The use of Ag-doped TiO2 did not result in significant improvement on tartrazine mineralization, due to the aggregation of these nanoparticles. Our results suggest that this process can be an alternative for a complete treatment (discoloration and mineralization) of tartrazine and most likely other azo dyes.

Spectroscopic properties and singlet oxygen production by the compound ethyl 3,12-dioxopyran[3,2-a]xanthone-2-carboxylate.

The steady state and time resolved experiments together with absorption and emission spectroscopies and quantum chemical calculations have been employed to investigate spectroscopic properties of a xanthone-type compound (ethyl 3,12-dioxopyran[3,2-a]xanthone-2-carboxylate). The spectroscopic data show good agreement with results obtained from quantum chemical calculations. Additionally, this compound shows expressive quantum efficiency for triplet population and a quantum efficiency of singlet oxygen generation very close to unity. Correlations between the nature of singlet and triplet excited states and spectroscopic properties were performed in order to understand the high quantum efficiency of singlet oxygen generation by this compound.