P. Jacques

Photodegradation of the aminoazobenzene acid orange 52 by three advanced oxidation processes: UV/H2O2, UV/TiO2 and VIS/TiO2: Comparative mechanistic and kinetic investigations

Abstract This paper presents the results obtained from the oxidation of the aminoazobenzene dye AO52 by the UV/H2O2, UV/TiO2 and VIS/TiO2 systems. In the former case, we investigated the formation of first by-products by means of GC/MS, HPLC and 1H NMR. spectroscopy. We conclude that hydroxyl radicals are added to aromatic rings in the ipso position with respect to the sulfonate group or to the azo-linkage-bearing carbon. The reaction of the inorganic radical with the N,N-dimethylamino substituent, leading to demethylation, adds to the multiplicity of the possible pathways. Degradation by the UV/TiO2 system is pH dependent. Whereas hydroxyl radicals are the main oxidative agent in neutral and alkaline solutions, positive hole-induced oxidation competes with the reduction of the protonated dye molecules in acid media. Moreover, FTIR spectroscopy of AO52/TiO2 wafers provided an insight to the nature of the photoproducts. This process is very efficient since only ultimate breakdown products, i.e. aliphatic acids and inorganic salts, are detected. Similar results were obtained using visible light as the irradiation source in the case of wafers whereas in heterogeneous solutions, the dye seems to be resistant to degradation.

Photooxidation of the phenylazonaphthol AO20 on TIO2: kinetic and mechanistic investigations

The results of the photocatalytic and photosensitized degradations of the monoazo dye AO20 in aqueous solution using suspended titanium dioxide are presented. Kinetic and mechanistic details have been elucidated using UV/Vis, FTIR and TCO techniques. It was proven that adsorption of dye molecules onto the support material is essential for the processes to be effective. Moreover, even if their mechanisms differ during the early stages, photocatalysis and photosensitization lead to very similar ultimate breakdown products. Indeed, the original dye anchored to the oxide surface systematically undergoes fast decomposition until it is transformed into CO2 or aliphatic acids (formic, acetic, oxalic acids), which react rather slowly with hydroxyl radicals or trapped holes.

Photooxidation of an azo dye induced by visible light incident on the surface of TiO2

Oxidation of Acid Orange 7 (AO7) induced by visible light on TiO2 surfaces has been probed by FT-IR spectroscopy for identification and kinetic studies of degradation products. The tautomerism has been considered for this type of azo dye. The hydrazone fo

PHOTOCHEMICAL AND PHOTOCATALYTIC DEGRADATION OF AN INDIGOID DYE: A CASE STUDY OF ACID BLUE 74 (AB74)

Abstract A detailed investigation of the photochemical and photocatalytic degradations of the indigoid dye AB74 is presented. H2O2 and C-UV light have negligible effect when they are used on their own. However, joint UV/H2O2 treatment more efficiently decomposes this organic compound. Surprisingly, reaction order depends strongly on the initial dye concentration. On the other hand, 1 H NMR analysis reveals that the original dye is first converted into isatinsulfonic acid, then into aliphatic acids. Under band gap excitation of semiconductors, AB74 undergoes irreversible oxidation too. The photodegradation kinetics, on various catalysts, are discussed in term of the Langmuir–Hinshelwood model. 1 H NMR, UV–VIS and FTIR spectroscopic techniques provide an insight into the nature of the photoproducts. It was possible then to present a more complete degradation mechanism than supposed until now.

Investigation of the interaction between a sulfonated azo dye (AO7) and a TiO2 surface

Abstract The adsorption of an azo dye, Acid Orange 7 (AO7), on a TiO 2 surface was studied using FT–IR spectroscopy and adsorption isotherm parameters. The major FT–IR bands of the dye have been assigned in the range 1000–1900 cm −1 . Within the AO7–TiO 2 complex, the dye appears to exist in its hydrazone form. The interaction of AO7 with the TiO 2 surface is strong, leading to a surface inner-sphere-type complex. The dye is linked to three Ti IV surface metallic cations through two oxygen atoms from the sulfonate group and the oxygen atom of the carbonyl group of the hydrazone tautomer.

Visible light induced photopolymerization : Speeding up the rate of polymerization by using co-initiators in dye/amine photoinitiating systems

The activity of six newly designed three-component systems (containing a dye, an amine and a triazine derivative) for the initiation of the photopolymerization of multifunctional acrylates under visible light has been evaluated. The selection of the dyes was based on thermodynamic considerations. A discussion of the photochemical reactivity of these systems reveals the role played by thermodynamics and outlines different aspects concerned with kinetics.

Solvatochromism of a typical merocyanine dye. A theoretical investigation through the CNDO/SCI method including solvation

Solvatochromism of a typical merocyanine dye is tentatively explained by a semi-empirical method including σ and π electrons (the CNDO/SCI method) for the first time. The procedure used makes allowance for a variation in geometry induced by the solvent, and stabilisation of the energy levels is introduced through the solvaton model proposed by Constanciel and Tapia. The calculations show that the solvatation results in a modification of the geometry from a ‘quinonoid’ to a ‘benzenoid’ structure with a concomitant general blue shift of the n→π* and π→π* transitions which fits the experimental results. Moreover, the results indicate a weak red shift of the π→π* transition for the lowest polarities. A detailed analysis of the composition of the π→π* transition made by considering the dipole moments of the ground and excited states substantiates the above-mentioned results. An attempt to explain the importance of the oscillator strength in the solvatochromism background and to correlate the net charges with the 13C shifts as the solvatation changes is proposed.

Solvent dependence of the intersystem crossing kinetics of thioxanthone

Abstract Intersystem crossing (ISC) of thioxanthone is investigated in a large number of solvents by employing picosecond pump–probe spectroscopy. The measurement of ISC rate constants for this particular molecule is somewhat difficult due to the overlap between the T 1 –T n and S 1 –S n transitions and to the solvatochromism of the triplet–triplet absorption. The S 1 –S n transition, as well as the excited state lifetime has been assigned. Indeed, the population kinetics of the triplet state consists in a fast component k 2 presumably higher than 10 11 s −1 and a slow component k 1 which is dramatically solvent dependent; k 1 decreases from 7×10 10 s −1 in toluene to 4×10 8 s −1 in methanol. A strong correlation between 1/ k 1 and the fluorescence lifetime is observed. A four excited state level diagram is proposed which accounts for the complex kinetics of the S→T conversions.

Investigation of the triplet quantum yield of thioxanthone by time-resolved thermal lens spectroscopy: solvent and population lens effects

Abstract The triplet quantum yield Φ T of thioxanthone (TX) was measured in different solvents by means of time-resolved thermal lens spectroscopy. It was observed that the experimental signal depends on the probe light wavelength. This effect which was mainly ascribed to the population lens (PL) arises from the change in solution composition after excitation, and adds its contribution to the thermal lens signal. From the transient absorption spectrum of the triplet state of TX, the change in refractive index associated with this PL was calculated as a function of the wavelength. Therefore, probing the photolens at an appropriate wavelength allowed accurate measurement of Φ T . It was found that besides departing seriously from unity, Φ T decreases from 0.85 in hexane to 0.56 in methanol.

Determination of Fluorescence Yields, Singlet Lifetimes and Singlet Oxygen Yields of Water-Insoluble Porphyrins and Metalloporphyrins in Organic Solvents and in Aqueous Media

Abstract— Fluorescence quantum yields and singlet lifetimes for a wide range of hydrophilic to hydrophobic porphyrins and metalloporphyrins have been determined in toluene, methanol or acetone. Photosensitized singlet oxygen yields have been determined in the same solvents. For some porphyrins, the same quantities were determined in an aqueous medium, through use of an amphiphilic polymer to solubilize the porphyrin sensitizer and target molecule, 1, 3‐diphenylisobenzofuran. Because rate constants for the deactivation of singlet oxygen (kd) and for its reaction with a target molecule (ka) are unknown in such aqueous polymer systems, a new method was developed for evaluating yields of singlet oxygen formation that also provides a value for the ratio kd/ka. A variation observed in quantum yield of singlet oxygen production for the aqueous polymer system with variation in initial concentration of the target molecule is discussed.