M. A. Brusa

Mechanism of chlorine dioxide photodissociation in condensed media

The photobleaching of OClO dissolved in CCl_4 was investigated by kinetic electron spin resonance spectrometry as a function of [OClO], [O_2], photon flux, irradiation wavelength (λ = 303, 365 and 436 nm) and temperature (233 K ≤ T ≤ 298 K). OClO decays non-exponentially, the apparent kinetic order increasing with conversion. The initial quantum yields of OClO disappearance Φ are larger than one under anoxic conditions, but decrease in the presence of O_2, except at 365 nm. The rates are directly proportional to the absorbed photon flux and depend weakly on temperature. Remarkably, OClO is partially regenerated after irradiation, even in fully bleached solid samples. We show that a minimal mechanism comprising 21 pseudo-elementary steps can account for these observations, within experimental error, if the primary products of OClO photodecomposition change from (O + ClO) at 303 and 436 nm to (Cl + O_2) at 365 nm. The photolysis of ClOClO_2, one of the putative intermediate species, into (Cl + OClO) also contributes to the initial rates at 303 nm.

Electron Transfer from Photoexcited TiO2 to Chelating Alizarin Molecules: Reversible Photochromic Effect in Alizarin@TiO2 under UV Irradiation

Reduction of alizarin molecules coupled to TiO(2) nanoparticles (A@TiO(2)) occurs on UV irradiation in the presence of a sacrificial electron donor. Evidence is presented that reduction is mediated by conduction-band electrons and yields a 1,2,9,10-tetrahydroxyanthracene species which remains coupled to the TiO(2) nanoparticles. The spectrum of the reduced complex displays two overlapping broad bands centred at 480 and 650 nm which can harvest visible photons besides 900 nm, in agreement with theoretical predictions by TDDFT. The potential relevance of the dual-redox behaviour of strongly TiO(2) coupled anthraquinone dyes in the field of photocatalysis and in connection with their utilization in the development of dye-sensitized TiO(2) solar cells is briefly discussed.

ESR kinetic study of the photobleaching of chlorine dioxide solutions at 303, 365 and 436 nm

Initial quantum yields for the photobleaching of OClO dissolved in CCl_4 at 298 K are measured by electron spin resonance spectrometry at three wavelengths, under 1 atm of either N_2 or O_2. Yields are unaffected at 365 nm, but increase by about 45% at 303 and 436 nm, when O_2 is replaced by N_2. These results, at variance with gas-phase photolysis, suggest a qualitative change in the mechanism of photofragmentation at ≈365 nm. We show that OClO solvatochromic shifts are controlled by the dynamic polarization of the medium, rather than by a modification of the energies of the states involved.

Time-resolved photoacoustic calorimetry of aqueous peroxodisulfate photolysis in the presence of nitrite anions

The photodissociation of aqueous peroxodisulfate in the presence of nitrite anions was studied between 282 and 292 K by time-resolved photoacoustic calorimetry at 266 nm. Aqueous peroxodisulfate was chosen as the calorimetric reference, thus enabling the measurement of the enthalpy and volume changes associated with the reaction of the photoproduced sulfate radicals with nitrite anions to form SO4−2 and NO2 in the submicrosecond time range. The enthalpy change obtained (−192 ± 19 kJ mol−1) is consistent with available thermochemical data for related species. The interpretation of the measured reaction volume (−13.8 ± 0.7 mL mol−1) points to the ionic charge accumulation on the sulfate anion as the dominating change causing the contraction of the solvent. The comparative analysis of our experimental result and data for partial molar volumes and electrostriction volumes of electrolytes, reinforces the concept that particularly in the case of water, more realistic models for the solvent are needed involving structure rather than continuous features.

Application of photoacoustic calorimetry to the determination of volume changes in reactions involving radical anions in aqueous solutions

The particular features of the application of photoacoustic calorimetry to the determination of volume changes in reactions involving radical anions in aqueous solutions are addressed. Analysis of recent literature data on volume changes of redox pairs, including our previous results on the couples NO2(-)/NO2 and SO4(2-)/(SO4)*-, together with known values of molal volume changes for ionization reactions, allow us to derive an empirical correlation for these categories of reactions. Finally, the pertinence of describing the volume changes of complex processes as a simple sum of separable terms is critically examined.