Photochemistry of (n-Bu4N)2[Pt(NO3)6] in acetonitrile.

Photochemistry of the (n-Bu4N)2[Pt(NO3)6] complex in acetonitrile was studied by means of stationary photolysis and nanosecond laser flash photolysis. The primary photochemical process was found to be an intramolecular electron transfer followed by an escape of an •NO3 radical to the solution bulk. The spectra of two successive Pt(III) intermediates were detected in the microsecond time domain, and their spectral and kinetic characteristics were determined. These intermediates were identified as PtIII(NO3)52- and PtIII(NO3)4- complexes. Disproportionation of Pt(III) species resulted in formation of final Pt(II) products.

Primary processes in photophysics and photochemistry of a potential light-activated anti-cancer dirhodium complex.

Photophysics and photochemistry of a potential light-activated cytotoxic dirhodium complex [Rh2(µ-O2CCH3)2(bpy)(dppz)](O2CCH3)2, where bpy = 2,2'-bipyridine, dppz = dipyrido[3,2-a:2',3'-c]phenazine (Complex 1 or Rh2) in aqueous solutions was studied by means of stationary photolysis and time-resolved methods in time range from hundreds of femtoseconds to microseconds. According to the literature, Complex 1 demonstrates both oxygen-dependent (due to singlet oxygen formation) and oxygen-independent cytotoxicity. Photoexchange of an acetate ligand to a water molecule was the only observed photochemical reaction, which rate was increased by oxygen removal from solutions. Photoexcitation of Complex 1 results in the formation of the lowest triplet electronic excited state, which lifetime is less than 10 ns. This time is too short for diffusion-controlled quenching of the triplet state by dissolved oxygen resulting in 1O2 formation. We proposed that singlet oxygen is produced by photoexcitation of weakly bound van der Waals complexes [Rh2…O2], which are formed in solutions. If this is true, no oxygen-independent light-induced cytotoxicity of Complex 1 exists. Residual cytotoxicity deaerated solutions are caused by the remaining [Rh2…O2] complexes.

Multifunctional fluorescent diarylethene: time-resolved study of photochemistry.

A study of luminescence and photochromic properties of (E)-2,3-bis(2,5-dimethylthiophen-3-yl)-5-(4-(pyrrolidin-1-yl)benzylidene)cyclopent-2-en-1-one, which is a diarylethene with a push-pull system between carbonyl and dimethylamino groups, was performed using time-resolved methods. The intramolecular charge transfer (ICT) process as well as 6π-electrocyclization and E-/Z-isomerization contribute to the complex light-induced properties of this molecule. Formation of unexpected short-lived intermediates was detected in the time range from 100 fs to 100 µs. A model based on two processes (additional photocyclization and interconversion between conformers) was proposed to rationalize this result. The key intermediates existing in the picosecond time domain are so-called precursors, which are proposed for both parallel (p) and anti-parallel (ap) isomers of the open form. In general, fast light-induced processes for the fluorescent diarylcyclopentenones are much more complicated than for the parent cyclopentenone-based DAE.

Photochemistry of hexachloroosmate(IV) in ethanol.

The photochemistry of the OsIVCl62- complex in ethanol was studied by means of stationary photolysis, nanosecond laser flash photolysis, ultrafast pump-probe spectroscopy and quantum chemistry. The direction of the photochemical process was found to be wavelength-dependent. Irradiation in the region of the d-d and LMCT bands results in the photosolvation (with the wavelength-dependent quantum yield) and photoreduction of Os(iv) to Os(iii), correspondingly. The characteristic time of photosolvation is ca. 40 ps. Photoreduction occurs in the micro- and millisecond time domains via several Os(iii) intermediates. The nature of intermediates and the possible mechanisms of photoreduction are discussed. We believe that the lability of the photochemically produced Os(iv) and Os(iii) intermediates determines the synthetic potential of OsIVCl62- photochemistry.

Mechanistic study of the trans,cis,cis-[RuCl2(DMSO)2(H2O)2] complex photochemistry in aqueous solutions.

It is known that trans,cis,cis-[RuCl2(DMSO)2(H2O)2] (1a) complexes, which are formed upon dissolution of trans-[RuCl2(DMSO)4] in water, demonstrate light-induced cytotoxicity. The mechanistic study of 1a photochemistry has been performed using ultrafast pump-probe spectroscopy, laser flash photolysis and stationary photolysis. The first stage of 1a photochemistry is the photoexchange of a DMSO ligand to a water molecule; its quantum yield is wavelength-dependent (estimating by values 0.3 and 0.04 upon irradiation at 308 and 430 nm, respectively). The mechanism of photoexchange is complicated involving at least four Ru(ii) intermediates. Two tentative mechanisms of the process are proposed.

Mechanism of photochromic transformations and photodegradation of an asymmetrical 2,3-diarylcyclopentenone.

A mechanistic study of the photochromic properties and photodegradation processes of an asymmetrical diarylcyclopentenone bearing thiophene and benzothiophene units using stationary photolysis, nanosecond laser flash photolysis and time-resolved luminescence was performed. It was found that the light-induced reversible isomerization of (3-(2,5-dimethyltiophen-3-il)-2-(2-methyl-1-benzylthiophen-3-il)cyclopent-2-en-1-one, compound 1) from open to closed form is a common photochromic transformation inherent to diarylethenes, while the photodegradation process proceeds in two ways. The first is a formal 1,2-dyotropic rearrangement, proceeding without the participation of oxygen. The second is the oxygen-dependent mechanism involving the excitation of the open form 1A into the triplet state, quenching of the latter by dissolved oxygen, and oxidation of the initial compound by singlet oxygen.

Photoinduced inhibition of DNA repair enzymes and the possible mechanism of photochemical transformations of the ruthenium nitrosyl complex [RuNO(ß-Pic)2(NO2)2OH].

In this work we have demonstrated that the ruthenium nitrosyl complex [RuNO(ß-Pic)2(NO2)2OH] is suitable for investigation of the inactivation of DNA repair enzymes in vitro. Photoinduced inhibition of DNA glycosylases such as E. coli Endo III, plant NtROS1, mammalian mNEIL1 and hNEIL2 occurs to an extent of ≥90% after irradiation with the ruthenium complex. The photophysical and photochemical processes of [RuNO(ß-Pic)2(NO2)2OH] were investigated using stationary and time-resolved spectroscopy, and mass spectrometry. A possible mechanism of the photo-processes was proposed from the combined spectroscopic study and DTF calculations, which reveal that the photolysis is multistage. The primary and secondary photolysis stages are the photo-induced cleavage of the Ru-NO bond with the formation of a free nitric oxide and RuIII complex followed by ligand exchange with solvent. For E. coli Endo III, covalent interaction with the photolysis product was confirmed by UV-vis and mass spectrometric methods.

Mechanistic Aspects of Photoinduced Rearrangement of 2,3-Diarylcyclopentenone Bearing Benzene and Oxazole Moieties.

Photoinduced rearrangement of diarylethenes to naphthalenes or isoelectronic benzoannulated heterocycles is a novel reaction in preparative organic photochemistry. Recently it was shown that unsymmetrical diarylethenes containing benzene and oxazole derivatives efficiently undergo this transformation leading to amide derivatives of naphthalene. Mechanistic study of skeletal rearrangement for a typical representative of these compounds, namely 3-(5-methyl-2-phenyl-1,3-oxazol-4-yl)-2-phenylcyclopent-2-en-1-one, was performed by stationary and laser flash photolysis as well as density functional theory (DFT) calculations. The mechanism of the rearrangement was found to comprise several thermal stages. Both singlet and triplet states of the initial compound can be transformed to the reaction product, which results in the dependence of the quantum yield vs concentration of dissolved oxygen. Three reactive intermediates were registered in the laser flash photolysis experiment; the predicted structures were in accordance with DFT calculations of the electronic absorption spectra. In addition to the previously proposed mechanism of skeletal rearrangement based on a sigmatropic shift of hydrogen, two new parallel pathways based on formation of a carbanion/carbocation were determined.

A cis,fac-[RuCl2(DMSO)3(H2O)] complex exhibits ultrafast photochemical aquation/rearrangement.

It is known that both cis,fac-[RuCl2(DMSO)3(H2O)] (1a) and trans,cis,cis-[RuCl2(DMSO)2(H2O)2] (2a) complexes, which are formed on the dissolution of trans and cis-isomers of [RuCl2(DMSO)4] in water, demonstrate light-induced anticancer activity. The first stage of 1a photochemistry is its transformation to 2a occurring with a rather high quantum yield, 0.64 ± 0.17. The mechanism of the 1a → 2a phototransformation was studied by means of nanosecond laser flash photolysis and ultrafast pump-probe spectroscopy. The reaction occurs in the picosecond time range via the formation and decay of two successive intermediates interpreted as Ru(ii) complexes with different sets of ligands. A tentative mechanism of phototransformation is proposed.

Photooxidation of herbicide amitrole in the presence of fulvic acid.

Fulvic acid (Henan ChangSheng Corporation) photoinduced degradation of non-UVA-absorbing herbicide amitrole (3-amino-1,2,4-triazole, AMT) as a way for its removal from polluted water was investigated in details. It was shown that the main primary species generated by fulvic acid under UVA radiation, triplet state and hydrated electron, are not directly involved in the herbicide degradation. AMT decays in reactions with secondary intermediates, reactive oxygen species, formed in reactions of the primary ones with dissolved oxygen. Singlet oxygen is responsible for 80% of herbicide oxidation, and •OH and O2-• radicals-for the remaining 20% of AMT. It was found that quantum yield of AMT photodegradation (ϕ 365nm) decreases linearly from 2.2 × 10-3 to 1.2 × 10-3 with the increase of fulvic acid concentration from 1.1 to 30 mg L-1. On the contrary, the increase of AMT concentration from 0.8 to 25 mg L-1 leads to practically linear growth of ϕ 365nm value from 1.8 × 10-4 to 4 × 10-3. Thus, the fulvic acid exhibits a good potential as UVA photooxidizer of organic pollutants sensitive to the singlet oxygen (ϕ 532nm(1O2) = 0.025 at pH 6.5).

Primary photochemical processes for Pt(iv) diazido complexes prospective in photodynamic therapy of tumors.

Diazide diamino complexes of Pt(iv) are considered as prospective prodrugs in oxygen-free photodynamic therapy (PDT). Primary photophysical and photochemical processes for cis,trans,cis-[Pt(N3)2(OH)2(NH3)2] and trans,trans,trans-[Pt(N3)2(OH)2(NH3)2] complexes were studied by means of stationary photolysis, nanosecond laser flash photolysis and ultrafast kinetic spectroscopy. The process of photolysis is multistage. The first stage is the photosubstitution of an azide ligand to a water molecule. This process was shown to be a chain reaction involving redox stages. Pt(iv) and Pt(iii) intermediates responsible for the chain propagation were recorded using ultrafast kinetic spectroscopy and nanosecond laser flash photolysis. The mechanism of photosubstitution is proposed.

Primary photophysical and photochemical processes for hexachloroosmate(iv) in aqueous solution.

The photoaquation of the OsIVCl62- complex was studied by means of stationary photolysis, nanosecond laser flash photolysis and ultrafast kinetic spectroscopy. The OsIVCl5(OH)2- complex was found to be the only reaction product. The quantum yield of photoaquation is rather low and wavelength-dependent. No impact of redox processes on photoaquation was revealed. The total characteristic lifetime of the process is about 80 ps. Three intermediates were recorded in the femto- and picosecond time domains and assigned to different Os(iv) species. The nature of intermediates and possible mechanisms of photoaquation are discussed.

Photochemistry of Dithiocarbamate Cu(S2CNEt2)2 Complex in CHCl3. Transient Species and TD-DFT Calculations.

Nanosecond laser flash photolysis was used to study the mechanism of photochemical transformations of the diethyldithiocarbamate Cu(II) complex (Cu(dtc)2, where dtc- ≡ -S2CNEt2 anion) in chloroform solutions. The electron transfer from the excited Cu(dtc)2 complex to a solvent molecule leads to the appearance of the primary intermediate, the [ClCu(dtc)(dtcCHCl2)] complex, where a dtcCHCl2 molecule is coordinated with a copper ion via one sulfur atom. In the fast reaction (k = 2.1 × 109 M-1 s-1) with Cu(dtc)2, this complex forms a long-lived dimer [ClCu(dtc)(dtcCHCl2)Cu(dtc)2]. This intermediate decays during several seconds (k = 5.6 × 10-2 s-1) into the final product, i.e., a diamagnetic dimer [ClCu(dtc)Cu(dtc)2]. To determine the structure of intermediate complexes the quantum chemical calculations were carried out using DFT, TD-DFT, and PCM (Polarizable Continuum Model) methods.

Photochemical transformation of an iron(III)-arsenite complex in acidic aqueous solution.

Surface complexation between arsenious acid anions (As(III)) and ferric (hydr)oxides in water is important for the transformation and transfer of inorganic arsenic species. The mechanisms of formation and the photochemistry of dissolved Fe(III)-As(III) complexes in acidic aqueous solution are still unclear. Here, the photooxidation of As(III) in the presence of Fe(III) ions in acidic media has been investigated by laser flash and steady-state photolysis. At low arsenite concentrations (<1 mM), As(III) is oxidized by the ˙OH radical generated by photolysis of the FeOH(2+) complex. At higher arsenite concentrations (>10 mM), photoactive Fe(III)-As(III) complexes are formed (ϕ≈ 0.012). At all arsenite concentrations, a white FeAsO4 colloid is formed during As(III) photolysis in the presence of Fe(III) ions. Solid Fe(III)-As(III) complexes have been prepared and characterized, and the photochemical transformation of As(III) into As(V) in solid Fe(III)-As(III) complexes has been confirmed. These findings are important for a better understanding of the evolution of As(III) species under environmental conditions and should provide guidance for detoxification of As(III)-polluted water systems.

Femtosecond spectroscopy of the dithiolate Cu(II) and Ni(II) complexes.

Femtosecond spectroscopy was applied to study the ultrafast dynamics for the excited states of dithiolate Cu(ii) and Ni(ii) complexes. The detailed information on the initial steps after the absorption of a photon by the metal complexes is of fundamental importance to understand the mechanism of photochemical reactions. The fast processes for the dithiolate complexes have hardly been studied. In this review the spectra of transients and their lifetimes will be presented. For example, the xanthogenate Ni(S2COEt)2 complex in acetonitrile and CCl4 after the pulse of the second harmonic (100 fs, 400 nm) of a Ti:S laser moves to the excited (1)LMCT state which decays in 0.76 ps to the excited (3)LF state. In 6.8 ps the (3)LF state undergoes vibrational cooling and then it slowly decays in 550 ps to the ground state. However, for many dithiolate complexes the kinetic curves can be well treated in a two-exponential approximation. A short time (less than 1 ps) may include several processes (relaxation of the Franck-Condon state, redistribution of vibrational energy (IVR), internal conversion (IC) and intersystem crossing (ISC)). A long time (a few picoseconds) usually reflects the vibrational cooling of the ground state. The quantum yields of the dithiolate and dithiolene complex disappearance in halogen containing solvents have a strong dependence on the wavelength of irradiation. It is very likely that electron transfer to the acceptor becomes effective when the electron in the excited complex moves to antibonding ligand orbitals localized at the periphery of the complex close to the acceptor molecule (halogenated solvent).

Laser flash photolysis study of photocatalytic properties of pillared interlayered clays and Fe,Al-silica mesoporous catalysts.

Laser flash photolysis was applied to determine the primary photochemical processes over iron-containing clay (montmorillonite KSF), pillared interlayered clays (PILCs) and mesoporous mesophase iron silicate materials (MMMs). For KSF, the homogeneous photochemical reaction of Fe(III) leached from the clay material resulted in the formation of OH radicals, which were monitored by means of their reaction with methyl viologen dication (MV(2+)). For PILCs and MMMs, no leaching of Fe(III) to the solution nor hydroxyl radical formation were observed. Nevertheless, these catalysts were found to exhibit a sufficient effect on phenol photoionization. The increase in quantum yields of PhO radicals is caused by the effect of PILCs and MMMs and is explained by heterogeneous processes on the surface of catalyst particles.

Intermediates in photochemistry of Fe(III) complexes with carboxylic acids in aqueous solutions.

The primary processes in the photochemistry of Fe(III) complexes with carboxylic acids (glyoxalic, tartaric, pyruvic and lactic) were studied by means of laser flash photolysis. The inner-sphere electron transfer with the formation of Fe(II) complex and an escape of an organic radical to the solution bulk was shown to be a minor channel of the photolysis. The main channel was proposed to be the formation of a long-lived radical complex [Fe(II)···Ë™OOC-R](2+). Spectral and kinetic parameters of the radical complexes are determined.

Photophysics and photochemistry of 2-aminobenzoic acid anion in aqueous solution.

Nanosecond laser flash photolysis and absorption and fluorescence spectroscopy were used to study photochemical processes of 2-aminobenzoic acid anion (ABA(-)) in aqueous solutions. Excitation of this species gives rise to the ABA(-) triplet state to the ABA* radical and to the hydrated electron (e(aq)(-)). The last two species result from two-photon processes. In a neutral medium, the main decay channels of ABA(-) triplet state, the ABA* radical, and e(aq)(-) are T-T annihilation, recombination, and capture by the ABA(-) anion, respectively.

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.

Photoinduced electron transfer via nonbuttressed metal-metal bonds. The photochemical study of binuclear complexes with platinum-thallium bonds.

The photochemistry of binuclear metal-metal bonded complexes [(NC) 5Pt-Tl(solv) x ] (solv is water or dimethylsulfoxide) has been studied in aqueous and dimethylsulfoxide solutions. Both stationary and nanosecond laser flash photolysis have been carried out on the species. The metal-metal bonded complexes have been photolyzed by irradiation into the corresponding intense MMCT absorption bands. Photoexcitation results in the cleavage of the platinum-thallium bond and the formation of a solvated thallous ion and a cyano complex of platinum(IV), [Pt(CN) 5(solv)] (-), in both cases. The species have been characterized by multinuclear NMR and optical spectroscopy. The products of the photoreaction indicate a complementary two-electron transfer occurring between platinum and thallium ions in the binuclear Pt-Tl species. Quantum yield values for the photodecomposition of the species have been determined. The intermediates of the photoinduced metal-to-metal electron transfer have been detected and characterized by optical spectroscopy. The kinetics of transient formation and decomposition have been studied, and mechanisms of the photoactivated redox reaction have been suggested.