No abstract

No abstract

No abstract

The photochemical and photocatalytic properties of iron meso-tetraarylporphyrins bearing an OH(-) axial ligand and different substituents in the beta-positions of the porphyrin ring are reported. Irradiation (lambda = 365 nm) in the absence of dioxygen leads to the reduction of Fe(III) to Fe(II) with the formation of OH(*) radicals. Substituents at the pyrrole beta-positions are found to markedly affect the photoreduction quantum yields. Under aerobic conditions, this photoreaction can induce the subsequent oxidation of cyclohexane to cyclohexanone and cyclohexanol by O(2) itself. The process occurs under mild conditions (22 degrees C; 760 Torr of O(2)) and without the consumption of a reducing agent. The polarity of the solvent and the nature of the porphyrin ring have a remarkable effect on the selectivity of the photooxidation process, likely controlling the cleavage of O-O bonds of possible iron peroxoalkyl intermediates. In particular, in pure cyclohexane, oxidation occurs with the selective formation of cyclohexanone; in contrast, in dichloromethane/cyclohexane mixed solvent, the main oxidation product is cyclohexanol. Phenyl-tert-butylnitrone (pbn) has been found to quench the radical chain autooxidation of the substrate thus increasing the yield of cyclohexanol. This becomes the only oxidation product when iron 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin hydroxide (Fe(III)(TDCPP)(OH)) is used as photocatalyst.

Vanadium-doped TiO2 nanoparticles (V-TiO2 NPs) with a V/Ti ratio of 3.0 at. % were prepared by gas-phase condensation and subsequent oxidation at elevated temperature. Both photocatalytic activity for -NO2 reduction and photoelectrochemical water splitting were induced by V-doping in the visible spectral range > 450 nm, where undoped TiO2 NPs are completely inactive. The photocatalytic properties were correlated with the ultrafast dynamics of the photoexcited charge carriers studied by femtosecond transient absorption (TA) spectroscopy with three different excitation wavelengths, i.e. e = 330, 400, and 530 nm. Only in V-doped NPs, the photoexcitation of electrons into the conduction band by sub-bandgap irradiation (e = 530 nm) was detected by TA spectroscopy. This observation was associated with electronic transitions from an intra-gap level localized on V 4+ cations. The photoexcited electrons subsequently relaxed, with characteristic times of 200-500 ps depending on e, into Ti-related surface traps that possessed suitable energy to promote -NO2 reduction. The photoexcited holes migrated to long-lived surface traps with sufficient overpotential for the oxidization of both 2-propanol and water. On the basis of TA spectroscopy and photocurrent measurements, the position of the dopant-induced intra-gap level was estimated as 2.2 eV below the conduction band minimum.

Proper reaction conditions have been found for the conversion of geraniol, citronellol, trans-2-penten-1-ol and 1-pentanol to the corresponding aldehydes with good chemo-selectivity (>70%) by photochemical excitation of suspensions of P25-TiO(2). It is demonstrated that adsorption of the alcohol on the surface as an alcoholate is necessary for its oxidation. ESR-spin trapping experiments point out that oxidation of alcohols starts with the formation of alkoxide radicals. Water content in the dispersing medium strongly inhibits alcohol adsorption and subsequent oxidation. In fact, water increases the polarity of the dispersing medium favouring the affinity between the polar alcohol and the CH(3)CN-H(2)O mixture itself; moreover, water competitive adsorption with the alcohol causes the removal of the latter from the photocatalytic surface with consequent difficult oxidation, as evidenced by ESR-spin trapping investigation. The reactivity of the alcohol on the surface of photoexcited P25-TiO(2) is also affected by the nature of its hydrophobic aliphatic chain: geraniol and citronellol are more susceptible to the water content than their short analogues trans-2-penten-1-ol and 1-pentanol. Moreover, in anhydrous CH(3)CN, specific interaction between the surface and the OH group enhances the reactivity of the primary aliphatic alcohols towards their partial oxidation to aldehyde, which can be accumulated in the reaction environment.

We address the role of the energetics of photogenerated electrons in the reduction of 4-nitrobenzaldehyde on TiO2. This model molecule bears two functional groups featuring different reducibilities. Electrochemistry shows that reduction to 4-aminobenzyl alcohol occurs in entirely distinct potential ranges. Partial reduction of the -NO2 group, affording 4-aminobenzaldehyde, takes place through surface states at potentials positive of the flatband potential (E(fb)). Dark currents caused by reduction of the aldehyde group are observed only at potentials more negative than E(fb), and the process requires an electron accumulation regime. Photocatalysis with TiO2 suspensions agrees with the electrochemical data. In particular, reduction of the nitro group is a relatively fast process (k=0.059 s(-1)), whereas that of the aldehyde group is slower (k=0.001 s(-1)) and requires electron photoaccumulation. Control of the photogenerated charge is a prospective means for achieving chemoselective reductions.

No abstract