Ivan P. Pozdnyakov

New Insight into Photochemistry of Ferrioxalate

Optical spectroscopy and nanosecond flash photolysis (Nd:YAG laser, 355 nm, pulse duration 5 ns, mean energy 5 mJ/pulse) were used to study the photochemistry of Fe(III)(C2O4)3(3-) complex in aqueous solutions. The main photochemical process was found to be intramolecular electron transfer from the ligand to Fe(III) ion with formation of a primary radical complex [(C2O4)2Fe(II)(C2O4(*))](3-). The yield of radical species (i.e., CO2(*-) and C2O4(*-)) was found to be less than 6% of Fe(III)(C2O4)3(3-) disappeared after flash. [(C2O4)2Fe(II)(C2O4(*))](3-) dissociates reversibly into oxalate ion and a secondary radical complex, [(C2O4)Fe(II)(C2O4(*))](-). The latter reacts with the initial complex and dissociates to Fe(II)(C2O4) and oxalate radical. In this framework, the absorption spectra and rate constants of the reactions of all intermediates were determined.

Mechanism of Fe(OH)2+(aq) photolysis in aqueous solution (technical report)

Experiments on laser flash photolysis (308 nm) of Fe(OH)2+(aq) complex in aqueous solution with addition of nitrobenzene demonstrate the formation of hydroxyl radical in the primary photochemical process.

Redox processes in photochemistry of Pt( iv ) hexahaloid complexes

Ultrafast pump–probe spectroscopy (λpump = 405 nm) was applied to study the primary photochemical processes for PtCl62− and PtBr62− complexes in aqueous and alcohol solutions. For PtCl62−, an intermediate with a lifetime of ca. 200 ps was registered and identified as an Adamson radical pair [PtIIICl52−⋯Cl˙]. The transformations of the primary intermediate give rise to successive formation of different Pt(III) species. The reactions of Pt(III) results in chain photoaquation in aqueous solutions and reduction of Pt(IV) to Pt(II) in alcohol solutions. For PtBr62− complex, the previously reported (I. L. Zheldakov, M. N. Ryazantsev and A. N. Tarnovsky, J. Phys. Chem. Lett., 2011, 2, 1540; I. L. Zheldakov, PhD thesis, Bowling Green State University, 2010) formation of active 3PtBr5− intermediate is followed by very fast (15 ps) aquation of Pt(IV) in aqueous solutions and parallel reactions of solvation and reduction of Pt(IV) to Pt(II) in alcohol solutions. All the processes in alcohols are finished within 0.5 ns. The data of ultrafast experiments are supported by nanosecond laser flash photolysis and stationary photolysis.

Photodegradation of organic pollutants in aqueous solutions caused by Fe(OH)aq 2+ photolysis: Evidence of OH radical formation

Photochemistry of Fe(III) hydroxocomplexes has a great interest because of possibility of hy- droxyl radical formation at the primary photochemical step. This mechanism is based mainly on the analysis of the final products of photochemical reactions between light-excited Fe(III) complexes and organic com- pounds. However, realization of an alternative mechanism of photolysis, based on the intermolecular electron transfer from organic molecules located in the second coordination sphere to the light-excited complex could take place. To distinguish these mechanisms, the experiments on the nanosecond laser flash photolysis of Fe(OH)aq 2+ in water solutions (pH 3) with organic additives; namely nitrobenzene, N,N-dimethylformamide, and phenol have been performed. And the formation of the organic radicals Ph( • OH)NO2, HCON(CH2 • )(CH3), and Ph(OH)2 • , was observed, respectively. The rate constants of the formation and decay of the radicals co- incide with those found in literature. The results support the formation of the hydroxyl radical upon the photolysis of Fe(OH)aq 2+ .

Chain processes in the photochemistry of PtIV halide complexes in aqueous solutions

The mechanisms of the photoaquation of PtCl62− and PtBr62− complexes were compared by the experimental results on stationary photolysis, nanosecond laser flash photolysis, and ultrafast pump-probe spectroscopy. The formation of the photoaquation product of the bromide complex, viz., PtBr5(H2O)−, was shown to proceed via the mechanism of heterolytic cleavage of the Pt-Br bond, and the platinum cation remained tetravalent in the course of the whole process. For the chloride complex, the Pt-Cl bond cleavage was found to be homolytic, and precursors of the photoaquation product, viz., PtCl5(H2O)− complex, are intermediates of trivalent platinum sequentially transforming into each other. The reactions of these intermediates determine the chain character of the photoaquation process.

Mechanistic study of fulvic acid assisted propranolol photodegradation in aqueous solution.

Laser flash (355 nm) and stationary (365 nm) photolysis were used to study the mechanisms of propranolol photolysis in the presence of fulvic acid in aqueous solutions. The FA-assisted photodegradation of propranolol was observed using UV-A irradiation (where propranolol is stable). Direct evidence indicated that the photodegradation resulted from the static quenching of the FA triplet state by propranolol via the electron transfer mechanism. The triplet state yield (ϕT≈0.6%) and the T-T absorption coefficient (ɛT(620 nm)≈5×10(4) M(-1) cm(-1)) were estimated for the first time by modeling the yields of the FA triplet state in the presence of propranolol. Thus, fulvic acid is a promising agent for accelerating propranolol photodegradation in aqueous solutions under UV-A light irradiation.

Primary photophysical and photochemical processes upon UV excitation of PtBr62– and PtCl62– complexes in water and methanol

Primary photophysical and photochemical processes were studied for PtIVBr62– and PtIVCl62– complexes in water and methanol by ultrafast kinetic spectroscopy upon excitation in the band region of charge transfer from the ligand-centered group π-orbitals to the eg*-orbital of PtIV complex anion (LMCT bands). The data obtained earlier upon excitation in the region of d—d bands were compared. Irrespective of the excitation wavelength, the photochemical properties of complexes are caused by the reactions of intermediates proceeding in the picosecond time range. These intermediates were identified as PtIVBr5– upon photolysis of PtIVBr62– and, presumably, the Adamson radical pair [PtIIICl52–(C4v)...Cl•] upon photolysis of PtIVCl62–. The difference in the exciting light wavelengths has an impact only on the first step of these processes, i.e., transition from the Franck—Condon excited state to intermediates.

Investigation of fulvic acid photochemistry in aqueous solutions by stationary and laser flash photolysis techniques

Photochemistry of fulvic acid (FA, Henan ChangSneng Corporation) in aqueous solutions was studied using stationary and nanosecond laser (355 nm) flash photolysis. UV-excitation leads to formation of FA triplet state which is characterized by wide unstructured absorption band with maximum at 620 nm. The yield of FA triplet state depends on pH: intermediate absorption signal is maximal at neutral pH (6–7) and decreases in basic and acidic media. Kinetics of triplet state decay does not depend on solution pH and exhibits multiexponential character with characteristic times t1 = 4.3 ± 2.2, t2 = 54 ± 28, t3 = 830 ± 240 μs.

Photochemistry of iron(iii)-lactic acid complex in aqueous solutions

Photochemistry of a 1: 1 FeIII-lactic acid complex, [Fe(Lact)]+, in aqueous solutions was studied by stationary photolysis, nanosecond laser flash photolysis (355 nm, 6 ns), and femtosecond pump-probe spectroscopy (400 nm, 200 fs). The quantum yield of photolysis of [Fe(Lact)]+ upon excitation at 355 nm is 0.4 and 0.22 in the deoxygenated and air-saturated solutions, respectively. Weak transient absorption in the range 500–750 nm was observed in the nanosecond experiments. It was assigned to a [FeII...-O-CH(Me)-COO·]+ radical complex. The spectral properties of the ligand-to-metal charge transfer excited state and the characteristic time of formation of the radical complex (1.5 ps) were determined in the femtosecond spectroscopy experiments. A reaction mechanism was proposed, which involves inner-sphere electron transfer in the excited complex with the formation of a radical complex [FeII...-O-CH(Me)-COO·]+ and its subsequent transformation to the end product of the photochemical reaction.

Aqueous photochemistry of bisphenol E in the presence of β-cyclodextrin

Nanosecond laser flash photolysis and time-resolved fluorescence were used to study photochemistry of bis(4-hydroxyphenyl)ethane (Bisphenol E, BPE) and complex of BPE with β-cyclodextrin (BPE-CD) in aqueous solutions. For both systems the primary photochemical process was found to be photoionization with the formation of a hydrated electron—phenoxyl radical pair. Inclusion of BPE in cyclodextrin cavity leads to the increase of photoionization and fluorescence quantum yield (from 0.009 to 0.16) as well as fluorescence lifetime (from 0.07 to 2.5 ns) due to decreasing of the quenching rate of the singlet excited state of complexed BPE by solvent molecules.