E. M. Glebov

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.

Laser flash photolysis of IrCl62− in aqueous solutions

Abstract The method of laser flash photolysis (308 nm) is used to study the photochemistry of IrCl 6 2− complex in aqueous solutions with and without free Cl − ions. Photolysis in aqueous solutions lead to photoaquation of initial complex within less than 20 ns with a quantum yield of 0.01. In aqueous solutions containing free Cl − ions, the photoreduction of IrCl 6 2− and the appearance of an absorption band of Cl 2 •− radical ion with a maximum at 350 nm are observed. An increase in the Cl − ion concentration causes an increase in both the quantum yield of photoreduction and Cl 2 •− yield. These results allow one to conclude that a chlorine atom, precursor of Cl 2 •− radical ion, arises due to the electron transfer from an outerspheric Cl − ion to the excited complex. The obtained experimental data have made it possible to estimate the rate constants of direct and back electron transfer in the (IrCl 6 2− ) ∗ –Cl − pair.

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.

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.

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.

Photolysis of [PtBr6]2- complex in frozen methanol matrix

Photochemistry of the [PtBr6]2– complex in the low-temperature methanol matrix (77 K) was studied by low-temperature spectrophotometry and ESR spectroscopy. The [PtBr4]2– complex is the main photolysis product. The mechanism of two-electron reduction of the platinum(IV) ion is proposed. The primary photochemical process is electron transfer from the solvent molecule to the photoexcited initial complex to form the intermediate radical complex ([PtBr6]3–...·CH2OH). The transfer of the second electron in the radical complex produces the final reaction products.

Kinetics and mechanism of photochromic transformations of a 2,3-diarylcyclopentenone

Photochemistry of recently synthesized 2,3-bis-(2,5-dimethylthiophen-3-yl-cyclopent-2-en-1-one) (diarylcyclopentenone, 1A) as a characteristic representative of photochromic 2,3-diarylcyclopent-2-en-1-ones (DCPs) was examined by means of stationary an nanosecond laser flash photolysis. Quantum yields of forward and backward photochemical reactions and molar absorption coefficient of the closed form were measured. Contradictions in the literature values of these parameters were eliminated. In laser flash photolysis experiments the formation of the 1A open form triplet state was exhibited, and the rate constants of the reactions of its decay were measured. 1A demonstrates rather low fatigue resistance, as any of other DCPs. In spite of the triplet state formation, photodegradation of 1A is not caused by the reactions of singlet oxygen.

Primary photophysical and photochemical processes for Pt(SCN) 6 2– complex

Photosolvation of a PtIV hexathiocyanate complex Pt(SCN)62– in water and ethanol was studied by steady-state photolysis, nanosecond laser flash photolysis, and ultrafast kinetic spectroscopy. Complexes Pt(SCN)5(H2O)– and Pt(SCN)5(C2H5OH)– were found to be the only reaction products. The quantum yields of photosolvation are independent of the excitation wavelength, being equal to 0.25 and 0.5 for the solutions of the complex in water and ethanol, respectively. Photosolvation proceeds by the mechanism of heterolytic metal—ligand bond dissociation without involvement of redox processes. The characteristic time of formation of the end products for both solvents is about 10 ps. Three successive intermediates detected on the picosecond time scale were interpreted as PtIV complexes. The nature of the intermediates and possible mechanisms of photosolvation are discussed.

Two-quantum photochemistry of the complex cis,trans-[PtIV(en)(I)2(CH3COO)2]

The two-quantum photochemistry of aqueous solutions of cis,trans-[PtIV(en)(I)2(CH3COO)2] (complex 1) has been studied by laser flash photolysis using an irradiation wavelength of 355 nm. This compound can be considered as a model representative of the mixed-ligand Pt(IV) complexes tested for use in photodynamic therapy. The appearance of transient absorption, presumably due to two consecutively produced Pt(III) complexes, has been revealed. The spectral and kinetic characteristics of the intermediates have been determined. A mechanism of two-quantum photolysis of complex 1 is proposed on the basis of the data obtained.

Photophysics and photochemistry of uranyl ions in aqueous solutions: Refining of quantitative characteristics

The photochemistry and photophysics of aqueous solutions of uranyl nitrate have been investigated by nanosecond laser photolysis with excitation at 266 and 355 nm and by time-resolved fluorescence spectroscopy. The quantum yield has been determined for (UO22+)* formation under excitation with λ = 266 and 355 nm light (φ = 0.35). The quantum yield of uranyl luminescence under the same conditions is 1 × 10–2 and 1.2 × 10–3, respectively, while the quantum yield of luminescence in the solid state is unity, irrespective of the excitation wavelength. The decay of (UO22+)* in the presence of ethanol is biexponential. The rate constants of this process at pH 3.4 are k1 = (2.7 ± 0.2) × 107 L mol–1 s–1 and k2 = (5.4 ± 0.2) × 106 L mol–1 s–1. This biexponential behavior is explained by the existence of different complex uranyl ion species in the solution. The addition of colloidal TiO2 to the solution exerts no effect on the quantum yield of (UO22+)* formation or on the rate of the reaction between (UO22+)* and ethanol. The results of this study have been compared with data available from the literature.