Vittorio Carassiti

Photodeposition of uranium oxides onto TiO2 from aqueous uranyl solutions

The photoreduction of uranyl solutions on illuminated TiO2 suspensions and electrodes has been investigated. The photoreduction leads to the formation, on TiO2, of an uranium oxide having a stoichiometry close to U3O8.Adsorption of uranyl species in the dark was examined. It was found that the degree of adsorption depended on pH and on the formation of uranyl complexes in solution.Initial rates of photoreduction are given for the cases when propan-2-ol, sodium acetate or sodium formate are used as scavengers of the photogenerated holes.With the use of microelectrode theory, it was possible to calculate the photoreduction rates from photoelectrochemical data obtained with TiO2 electrodes. The reduction rates calculated in this way were from 3.5 to 4 times lower with the electrode than with the suspension, in agreement with the larger area available in the latter case.

Photochemistry of iron-porphyrin complexes. Biomimetics and catalysis

Abstract An intramolecular electron transfer can occur in iron porphyrin complexes that are irradiated with light of suitable wavelength corresponding to axial ligand-to-metal charge transfer transitions. This process leads to the reduction of Fe(III) to Fe(II) and to the oxidation of the axial ligand to a radical species. The efficiency of the photoredox process is increased in the presence of species able of trapping the ferrous complex and/or the radical in competition with the back electron transfer process. Photoredox reactions of iron porphyrins and heme-proteins (cytochrome c) are examined in the framework of the fundamental role of electron transfer processes in biological systems. The products of the primary photoprocess can induce reactions leading to reduction or oxidation of various substrates with catalytic efficiency. This biomimetic aspect of the photoredox behavior of iron porphyrins is pointed out examining the results obtained in investigations concerning the iron-porphyrin based photoreduction of CCl4 in homogeneous solution, and photooxygenation of alkanes in both homogeneous and heteterogeneous systems.

Photocatalyzed Oxidation of Cyclohexene and Cyclooctene with (nBu4N)4W10O32 and (nBu4N)4W10O32/FeIII[meso-Tetrakis(2,6-dichlorophenyl)porphyrin] in Homogeneous and Heterogeneous Systems

Photoexcitation of (nBu4N)4W10O32 is a suitable mean of oxidizing cyclohexene and cyclooctene with O2 at room temperature and pressure. This process can be carried out in homogeneous solution as well as using the decatungstate in a dispersed form after its heterogenisation on silica. Cyclohexene and cyclooctene are mainly oxidized to the corresponding hydroperoxides as a consequence of primary photoprocesses which lead to the formation of allylic radicals. The presence of the FeIII[meso-tetrakis(2,6-dichlorophenyl)porphyrin] chloride as cocatalyst strongly affects the photocatalytic properties of (nBu4N)4W10O32, playing a key role in the allylic-hydroperoxide dependent oxidation of the cycloalkenes. In the photooxidation of cyclohexene, the porphyrin increases the photocatalytic efficiency of the decatungstate in terms of total turnover number and catalyses the decomposition of cyclohexenyl hydroperoxide with the selective formation of cyclohex-2-en-1-ol. On the other hand, its presence during the photoinduced oxidation of cyclooctene favours the formation of cyclooctene epoxide by addition of ROO· and RO· radicals to the double bond. In the case of cyclooctene, the heterogenisation of the decatungstate on the solid support also affects the chemoselectivity of the photocatalytic process in the absence of the iron porphyrin complex.

CATALYTIC OXYGENATION OF CYCLOHEXANE BY PHOTOEXCITED (NBU4N)4W10O32: THE ROLE OF RADICALS

Abstract Photochemical excitation of (nBu4N)4W10O32 in the presence of oxygen (760 torr) leads to the formation of C6H11− and OH− radicals which can be revealed by ESR spin-trapping investigations. Cyclohexane is transformed in cyclohexanone and cyclohexanol with an overall photooxidation quantum yield of 0.35 at 325 nm photoexcitation wavelength. A strong influence of oxygen pressure on the concentration ratio of these products is observed; in that, at low oxygen pressure (15 torr), the yield of cyclohexane is negligible and the photocatalytic oxidation of cyclohexane mainly leads to the formation of cyclohexanol. The photocatalyst investigated does not suffer irreversible modifications allowing the oxygenation of cyclohexane by O2 itself under mild conditions with good turnover values.

INTEGRATED PHOTOCATALYSTS FOR HYDROCARBON OXIDATION : POLYOXOTUNGSTATES/ IRON PORPHYRINS SYSTEMS IN THE REDUCTIVE ACTIVATION OF MOLECULAR OXYGEN

Abstract The photochemical and photocatalytic properties of (nBu4N)3PW12O40(PW12O403).(nBu4N)4W10O32 and (Et4N)4W10O32(W10O324) were investigated in the presence of several Fe(III) meso-tetraarylporphyrins (Fe(III)Por). Laser flash photolysis experiments show that in the (nBu4N) 3PW12O40/Fe(III)Por integrated systems the electron transfer from the photoreduced polyoxotungstate to the Fe(III) Por is the first step for the subsequent reductive activation of O2 by the ferrous complex. In contrast, the direct reduction of O2 by the photoreduced polyoxotungstate is the kinetically favored process in the (nBu4N)4W10O32/Fe(III) Por systems. A comparison between the results obtained using (nBu4N)4W10O12 and (Et4N)4W10O32 shows that the decrease in the steric hindrance of the counterion, surrounding the photochemically active polyoxoanion, increases the rate of electron transfer between W10O32 and Fe(III)Por, thus enabling the decatungstate to contribute to the activation of dioxygen through the reduction of the Fe(III) Por. Continuous irradiation of the POT11/Fe(III) Por integrated systems under controlled aerobic conditions leads to the oxidation of cyclohexane to cyclohexanol and cyclohexanone. In comparisn with POT11 a slone a higher cyclohexanol/cyclohexanone concentration ratio is obtained, while in comparison with Fe(III)Por alone, higher yields of oxidation products and a higher stability of the photocatalysts are observed. The effect of the iron porphyrin on the products distribution can be ascribed to the ability of the complex in both the ferrous and the ferric forms to generate monooxygenating intermediates as a consequence of its reactions with O2 alkylhydroperoxide and radical species.

Crystal structure and spectroscopic analysis of melamine hydrobromide. Evidence for iso-melamine cations and charge-transfer complexes in the solid state

The crystal structure and vibrational analyses of melamine hydrobromide by FTIR and FT-Raman spectra, are presented. The crystallographic data show π-electron delocalization towards the amino substituents with ring nitrogen protonation in the solid state. Additionally, the presence of intermolecular hydrogen bonding interactions occurring between amino substituents and ring nitrogen lone-pairs gives rise to charge–transfer complexes, as demonstrated by UV–VIS reflectance in the solid state. FTIR and FT-Raman spectra allow the assignments of the vibrational modes in melamine hydrobromide by comparison with the corresponding deuteriated molecules, and show the existence of iso-melamine cations in the solid state.

Photo-oxidative cyanation of aromatics on semiconductor powder suspensions I: oxidation processes involving radical species

Abstract Illumination of platinized WO 3 and TiO 2 suspensions in CH 3 CN/H 2 O mixed solvents containing dimethoxybenzene (DMB) and cyanide leads to the formation of cyanoanisole together with side products. Laser flash photolysis of the transparent colloidal dispersions shows the formation of DMB +. . The amount of DMB + formed decreases in the presence of CN − , which indicates the competitive oxidation of DMB and Cn − rather than the reaction of DMB + and CN − . Using the electron spin resonance spin trapping technique we report for the first time evidence of CN and CH 2 CN radical formation in illuminated semiconductor suspensions. Use of dry CH 3 CN, DMB and (Bu) 4 + CN − decreases further the yield of cyanoanisole due to formation of a cyanated derivative of tributylamine which, in turn, originates from (Bu) 4 + after cyanide oxidation to CN .

Photoreduction of chlorohemin in pure pyridine

Abstract Chlorohemin (Fe(III)PPCl) undergoes photoreduction when irradiated in pure pyridine solution with 400–450 nm light. A thermal reduction is observed to occur simultaneously with the photochemical one, but after a one hour irradiation about 75% of the reduction product is formed in a photochemical way. Both five and six-coordinated species are observed to be present in solution; however, only the Fe(III)PPpy+ five coordinated complex is photoreducible. A mechanism is proposed whereby the primary photochemical act is an axial pyridine → iron electron transfer process yielding Fe(II)PP and py+ species. The Fe(II)PP moiety gives rise to the formation of the spectrophotometrically detectable Fe(II)(PP(py)2 complex. ESR spin trapping results are consistent with the formation of 2-pyridyl radicals from py+ cation by fast transfer of a proton to a pyridine molecule.

The role of the excited-state geometry in the photochemical isomerization of bis(glycinato) platinum (II)

Abstract The mechanisms of the thermal and photochemical isomerizations of cis -Pt(gly) 2 have been investigated. The thermal isomerization has been found to occur only in the presence of free glycine. On the contrary, the photoisomerization takes place both in the presence and in the absence of free glycine, with constants quantum yield (0.13). When the two isomerization reactions are carried out in the presence of labeled free glycine, the thermal isomerization leads to the formation of labeled trans isomer as well as to an extensive labeling on the starting cis form; whereas the photochemical isomerization occurs without any labeling of the two isomers. These results show that the thermal isomerization takes place by an intermolecular mechanism, whereas the photochemical isomerization must be an intramolecular process. A twisting mechanism is discussed for the photochemical isomerization. This mechanism is related to the recent spectroscopic evidence for tetrahedral stable conformations of some excited states of square planar complexes.

An ESR spin trapping investigation on the photoreduction of chlorohemin in mixed solvents

Abstract Radical scavenging by nitrone compounds is used in conjunction with ESR spectroscopy in order to obtain evidence for radical formation in the photoreduction of chlorohemin in aqueous alcoholic mixed solvents. The formation of adducts between hydrogen atoms and the spin traps appears to be the most important process in aerated and in pyridine-containing solutions. Evidence for the formation of hydroxyethyl adducts is obtained when the alcoholic component of the solvent is ethanol. In deaerated solutions not containing pyridine, no evidence is obtained for radical formation. The results are interpreted in terms of hydrogen abstraction by the nitrone from an alkoxy radical formed together with Fe(II) in the primary photoreduction. The abstraction process strongly competes with cage re-oxydation, as well as with the diffusion of the alkoxy radical in the solvent bulk where it would rapidly react to give hydroxy radicals. The role of pyridine and oxygen may respectively be stabilizing and oxidizing the Fe(II) species before it undergoes cage re-oxidation.