Victor F. Plyusnin

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.

Intermediates formed by laser flash photolysis of [PtCl6]2− in aqueous solutions

The stationary photolysis of [PtCl(6)](2-) in aqueous solutions (10(-5)-10(-4) M) at the region of 313 nm leads to its photoaquation with a quantum yield of 0.19. Laser flash photolysis experiments (308 nm) provided evidence of the formation of Pt(iii) intermediates, namely [PtCl(4)(OH)(H(2)O)](2-) and [PtCl(4)](-), and Cl(2) (-) radical anions. The Pt(iii) complexes formed as a result of an intrasphere electron transfer from Cl(-) ligands to the excited Pt(iv) ion. However, the main ( approximately 90%) photolysis channel was not accompanied by the transfer of Cl atoms to the solvent bulk. The photoaquation of [PtCl(6)](2-) results from the back electron transfer in the secondary geminate pair, [PtCl(5)(H(2)O)](2-)-Cl. The relative yield of Pt(iii) intermediates, recorded after the completion of all processes in the geminate pair, was less than 10% of the number of disappearing initial [PtCl(6)](2-) complexes.

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.

Photochemistry of dithiocarbamate Cu(II) complex in CCl4.

Laser pulse photolysis was used to study the nature and reactions of intermediates in the photochemistry of the flat dithiocarbamate complex Cu(Et(2)dtc)(2) in CCl(4). A nanosecond laser pulse (355 nm) is shown to induce intermediate absorption bands of bivalent copper complex whose coordination sphere contains a dithiocarbamate radical Et(2)dtc(•) and a chloride ion at the axial position ([(Et(2)dtc)Cu(Et(2)dtc(•))Cl(a)]). At room temperature during some microseconds after the laser pulse, this intermediate interacts with the initial complex to form presumably a dimer [Cu(2)(Et(2)dtc)(3)(Et(2)dtc(•))Cl]. The latter vanishes in the second-order reaction. Analysis of kinetic and spectral features gives the arguments for the formation of a cluster [Cu(2)(Et(2)dtc)(3)Cl-tds-Cu(2)(Et(2)dtc)(3)Cl], which produces a new absorption band at 345 nm. The cluster decomposes in ∼5 ms into final products, a binuclear complex [Cu(2)(Et(2)dtc)(3)Cl] and tetraethylthiuramdisulfide (Et(4)tds).

Spectral and luminescent properties of ZnO–SiO2 core–shell nanoparticles with size-selected ZnO cores

Deposition of silica shells onto ZnO nanoparticles (NPs) in dimethyl sulfoxide was found to be an efficient tool for terminating the growth of ZnO NPs during thermal treatment and producing stable core–shell ZnO NPs with core sizes of 3.5–5.8 nm. The core–shell ZnO–SiO2 NPs emit two photoluminescence (PL) bands centred at ∼370 and ∼550 nm originating from the direct radiative electron–hole recombination and defect-mediated electron–hole recombination, respectively. An increase of the ZnO NP size from 3.5 to 5.8 nm is accompanied by a decrease of the intensity of the defect PL band and growth of its radiative life-time from 0.78 to 1.49 μs. FTIR spectroscopy reveals no size dependence of the FTIR-active spectral features of ZnO–SiO2 NPs in the ZnO core size range of 3.5–5.8 nm, while in the Raman spectra a shift of the LO frequency from 577 cm−1 for the 3.5 nm ZnO core to 573 cm−1 for the 5.8 nm core is observed, which can indicate a larger compressive stress in smaller ZnO cores induced by the SiO2 shell. Simultaneous hydrolysis of zinc(II) acetate and tetraethyl orthosilicate also results in the formation of ZnO–SiO2 NPs with the ZnO core size varying from 3.1 to 3.8 nm. However, unlike the case of the SiO2 shell deposition onto the pre-formed ZnO NPs, individual core–shell NPs are not formed but loosely aggregated constellations of ZnO–SiO2 NPs with a size of 20–30 nm are. The variation of the synthetic procedures in the latter method proposed here allows the size of both the ZnO core and SiO2 host particles to be tuned.

Optical spectroscopy of perfluorothiophenyl, perfluorothionaphthyl, xanthate and dithiophosphinate radicals

Laser flash photolysis has been used to record the optical spectra of sulfur-containing radicals forming from photodissoci- ation of diphenyl disulfide, perfluorodiphenyl disulfide, perfluoro-2,2 X -dinaphthyl disulfide, diisopropyldixantogene and . bis diisobutylthio-phosphoryl-disulfane . The extinction coefficients of absorption bands were determined from the reaction of S-radicals with a stable nitroxyl radical. The rate constant of this reaction was for all radicals close to 10 9 M y1 s y1 and successfully competes with the reaction of recombination. The presence of a narrow and strong absorption band in the optical spectrum of a nitroxyl radical and the absence of absorption in the reaction products allows one to accurately determine the extinction coefficients of the absorption bands of S-radicals. q 2000 Published by Elsevier Science B.V.

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.

Photochemistry of IrCl62− complex in alcohol solutions

Abstract Both stationary and laser flash photolysis were used to study the photochemistry of IrCl 6 2− in alcohol solutions. The primary photochemical photolysis process at 308 nm was demonstrated to involve electron transfer from the solvent to the excited complex. When the photolysis was performed at 248 nm, the photodissociation of the excited complex, with the elimination of a chlorine atom from the coordination sphere of iridium, was accompanied by electron transfer. At room temperature, the rate constants of the reaction between the primary alcohol radicals and the IrCl 6 2− complex were determined to be (3.2±0.1) × 10 9 M −1 s −1 for hydroxymethyl and (2.3±0.1) × 10 9 M −1 s −1 for α-hydroxyethyl radicals. The dark reactions of the IrCl 6 2− complex arising from photolysis are discussed.

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.