Photochemistry of (η5)-C5H5)Mn(CO)2[C(C6H5)2]. Generation of the diphenylcarbene radical anion

The electronic absorption spectrum of (η⁵-C₅H₅)Mn(CO)₂[C(C₆H₅)₂]shows an intense maximum which is assigned to a MLCT transition in which the empty p_π orbital on the carbene carbon is populated. Upon irradiation of this band, the complex undergoes a decomposition with a disappearance quantum yield Φ = 0.10 ± 0.01 independent of solvent. In the CT excited state, the complex can be roughly described as containing d⁵ Mn^II and a diphenylcarbene radical anion ligand C(C₆H₅)₂^?. Due to the kinetic lability, the complex decomposes producing a Mn^II species and the free carbene radical anion, which then undergoes secondary reactions. In addition, small amounts of substitution product are observed. It is proposed that prior to total decomposition of the excited state, a radical pair (η⁵-C₅H₅)Mn(CO)₂S⁺/C(C₆H₅)₂^?forms (S = solvent). A back electron transfer from C(C₆H₅)₂^?to the labile cation competes with decomposition to produce the substituted complex and free carbene.     

PHOTOPHYSICAL AND PHOTOSENSITIZING PROPERTIES OF BENZOPORPHYRIN DERIVATIVE MONOACID RING A (BPD-MA)*

The photophysical properties of benzoporphyrin derivative monoacid ring A (BPD-MA), a second-generation photosensitizer currently in phase II clinical trials, were investigated in homogeneous solution. Absorption, fluorescence, triplet-state, singlet oxygen (O₂(¹Δ_g)) sensitization studies and photobleaching experiments are reported. The ground state of this chlorin-type molecule shows a strong absorbance in the red (λ≈ 688 nm, ?≈ 33 000 M^?1 cm^?1 in organic solvents). For the singlet excited state the following data were determined in methanol: energy level, E_s= 42.1 kcal mol^?1, lifetime, Φ_f= 5.2 ns and fluorescence quantum yield, Φ_f= 0.05 in air-saturated solution. The triplet state of BPD-MA has a lifetime, τ_f >. 25 ns, an energy level, E_T= 26.9 kcal mol^?1 and the molar absorption coefficient is ?_T= 26 650 M^?1 cm^?1 at 720 nm. A dramatic effect of oxygen on the fluorescence (φ_f) and intersystem crossing (φ_T) quantum yields has been observed. The BPD-MA presents rather high triplet (φ_T= 0.68 under N₂-saturated conditions) and singlet oxygen (φ_Δ= 0.78) quantum yields. On the other hand, the presence of oxygen does not significantly modify the photobleaching of this photostable compound, the photodegradation quantum yield (φ_Pb) of which was found to be on the order of 5 × 10^?5 in organic solvents.     

ESR of O− and O2− stabilized on95Mo ions in a molybdenum-silica gel support system

The O^? and O₂^? anion radicals adsorbed at surface molybdenum ions were investigated by the EPR method by using the⁹⁵Mo isotope. From the valuesg_∥ = 2.006,g= 2.021,a_∥ = 7.6G anda= 8.0G for the O^? anion-radical the unpaired electron density (≈2%) was calculated. It is concluded that the unpaired electron in O^? is also delocalized onto the lattice oxygen ions.     

Comparative Study of Ru(bpz)32+Ru(bipy)32+and Ru(bpz)2CI2 as Photosensitizers of DNA Cleavage and Adduct Formation

Abstract— The efficiency of ruthenium complexes for photosensitizing DNA damage depends on the oxidizing character of their ligands. Here we report on the difference in behavior of tris(2.2'-bipyrazyl)ruthenium(II) (Ru[bpz]₃²+), tris(2,2′-bipyridyl)ruthenium(II) (Ru[bipy]₃²+) and cis-dichlorobis(2,2′-bipyrazyl)ruthenium(II) (Ru[bpz]₂Cl₂). Upon irradiation at 436 nm, Ru(bpz)₃²+was far less stable than Ru(bipy)₃²+. Ru(bpz)₃²+in phosphate buffer containing NaCl undergoes a photoanation reaction leading to the formation of Ru(bpz)₂Cl₂, as previously reported also in organic media. In the presence of phage φX174 DNA, Ru(bpz)₃²+photosensitized the formation of single strand breaks with an efficiency that was, at the beginning of irradiation, similar to that of Ru(bipy)₃²+. After 8 min of irradiation, the cleavage efficiency of Ru(bpz)₃²+reached a plateau that may correspond to its photode-composition. For the same conditions, Ru(bpz)₂Cl₂ did not induce DNA breakage. Scavenging experiments showed that, in the presence of oxygen, DNA cleavage induced by Ru(bpz)₃²+partly resulted from the formation of singlet oxygen and hydroxyl radical while in the absence of oxygen an additionnal mechanism involving electron transfer between the excited state of the ruthenium complex and DNA is proposed. The ICP measurement showed that Ru(bpz)₃²+and Ru(bpz)₂Cl₂ gave rise to covalent binding onto DNA in contrast with Ru(bipy)₃²+, which did not bind to DNA under the experimental conditions. The results are discussed with regard to the potential use of these photosensitizers in phototherapy.     

Zero kinetic energy ions in dissociative attachment on triatomic molecules: S−/OCS,O−/CO2

The zero kinetic energy yield of S^? ions in OCS and O^? ions in CO₂, produced by dissociative electron attachment through the lowest shape resonance, has been measured in a quadrupole mass spectrometer with electrostatic filter for translational energy analysis. The relative cross sections for S^? and O^? ion formation associated with CO fragments excited up to the vibrational level υ = 4 are obtained.     

A Highly Efficient and Selective Aerobic Cross‐Dehydrogenative‐Coupling Reaction Photocatalyzed by a Platinum(II) Terpyridyl Complex

Thanks to the superior redox potential of platinum(II) complex compared with that of Ru(bpy)₃²⁺ in the excited state, an efficient and selective visible‐light‐induced CDC reaction has been developed by using a catalytic amount (0.25 %) of 1 . With the aid of FeSO₄ (2 equiv), the corresponding amide could not be detected under visible‐light irradiation (λ=450 nm), but the desired cross‐coupling product was exclusively obtained under ambient air conditions. A spectroscopic study and product analysis revealed that the CDC reaction is initiated by photoinduced electron‐transfer from N‐phenyltetrahydroisoquinoline to the complex. An EPR (electron paramagnetic resonance) experiment provides direct evidence on the generation of superoxide radical anion (O₂^{? .} ) rather than singlet oxygen (¹O₂) under irradiation of the reaction system, in contrast to that reported in the literature. Combined, the photoinduced electron‐transfer and subsequent formation of superoxide radical anion (O₂^{? .} ) results in a clean and facile transformation.     

Photogeneration of superoxide and decarboxylated peptide radicals by carboquone, mitomycin C and streptonigrin. An electron spin resonance and spin trapping study

Visible light (405–615 nm) excitation of carboquone, mitomycin C, and streptonigrin dissolved in dimethylsulfoxide in the presence of oxygen generates superoxide anion radicals (O₂^?). The quantum yields for these reactions range from 4.2 times 10^?2 (carboquone, λ= 615 ± 10 nm) to 7.3 times 10^?6 (streptonigrin, λ=545 ± 10 nm). O₂^? radicals were spin trapped with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) and identified by electron spin resonance (ESR). The efficiency of DMPO to spin trap O₂^? in dimethylsulfoxide was determined and indicated that 91% of the O₂^? present in dimethylsulfoxide is trapped by DMPO. The oxidation of the photoexcited drug molecules occurs via a direct electron transfer to dissolved oxygen in solution. Ultraviolet irradiation (λ= 313 ± 10 nm) of the aminoquinone drug solutions (80% H₂O, 20% dimethylsulfoxide) in the presence of peptides results in the decarboxylation of the peptides. In this case the photoexcited drugs are reduced, abstracting an electron from the C-terminal carboxyl group of the peptides. The reaction is specific to the C-terminal amino acid of the peptide. The decarboxylated peptide radicals were spin trapped with 2-methyl-2-nitrosopropane (MNP) and identified by ESR.     

Effects of Light Energy and Reducing Agents on C60‐Mediated Photosensitizing Reactions

Many biomolecules contain photoactive reducing agents, such as reduced nicotinamide adenine dinucleotide (NADH) and 6‐thioguanine (6‐TG) incorporated into DNA through drug metabolism. These reducing agents may produce reactive oxygen species under UVA irradiation or act as electron donors in various media. The interactions of C₆₀ fullerenes with biological reductants and light energy, especially via the Type‐I electron‐transfer mechanism, are not fully understood although these factors are often involved in toxicity assessments. The two reductants employed in this work were NADH for aqueous solutions and 6‐TG for organic solvents. Using steady‐state photolysis and electrochemical techniques, we showed that under visible light irradiation, the presence of reducing agents enhanced C₆₀‐mediated Type‐I reactions that generate superoxide anion (O₂^.?) at the expense of singlet oxygen (¹O₂) production. The quantum yield of O₂^.? production upon visible light irradiation of C₆₀ is estimated below 0.2 in dipolar aprotic media, indicating that the majority of triplet C₆₀ deactivate via Type‐II pathway. Upon UVA irradiation, however, both C₆₀ and NADH undergo photochemical reactions to produce O₂^.?, which could lead to a possible synergistic toxicity effects. C₆₀ photosensitization via Type‐I pathway is not observed in the absence of reducing agents.     

Photooxidation of Tryptophan: O2(1Δg) versus Electron-Transfer Pathway

Abstract— Tris (2,2'-bipyridyl)ruthenium(II)chloride hexahydrate (Ru[bpy]₃²+) free in solution and adsorbed onto antimony-doped SnO₂ colloidal particles was used as a photosensitizer for a comparison of the O₂(¹Δ_g) and electron-transfer-mediated photooxidation of tryptophan (TRP), respectively. Quenching of excited Ru(bpy)₃²+ by O₂(³σ_g^?) in an aerated aqueous solution leads only to the formation of O₂(¹Δ_g) (φ₄= 0.18) and this compound was used as a type II photosensitizer. Excitation of Ru(bpy)₃²+ adsorbed onto Sb/SnO₂ results in a fast injection of an electron into the conduction band of the semiconductor and accordingly to the formation of Ru(bpy)₃²+ and was used for the sensitization of the electron-transfer-mediated photooxidation. The Ru(bpy)₃³+ is reduced by TRP with a bimolecular rate constant k_Q= 5.9 × 10⁸M^?1 s^?1, while O₂(¹Δ_g) is quenched by TRP with k_t= 7.1 × 10⁷M^?1 s^?1 (chemical + physical quenching). Relative rate constants for the photooxidation of TRP (k_c) via both pathways were determined using fluorescence emission spectroscopy. With N_p, the rate of photons absorbed, being constant for both pathways we obtained k_c= (372/N_p) M^?1 s^?1 for the O₂(¹Δ_g) pathway and k_c≥ (25013/N_p) M^?1 s^?1 for the electron-transfer pathway, respectively. Thus the photooxidation of Trp is more than two orders of magnitude more efficient when it is initiated by electron transfer than when initiated by O₂(¹Δ_g).     

Selected fluorescence and dissociation lifetimes on N2O+ in its Ã(2Σ+),000 and 100 states

Radiative and radiationless lifetimes together with the quantum yield towards fluorescence of individual vibronic states of molecular ions can be determined by threshold electron—fluorescence photon coincidence measurements. This method is here applied to the X?²Π_{$\text{3}\text{2,1,2}$} and òΣ⁺ states of N₂O⁺ which were also studied by high resolution photoionization resonance spectroscopy.     

ESR studies on the reactions of sulphite radical anion,SO3.̄−, with nitroalkane compounds in aqueous solutions

The reactions of the sulphite radical anion, SO₃^{$\text{.}\text{?}$?} (generated from the Ti³⁺-H₂O₂-Na₂SO₃ system at pH 9), in aqueous solutions with some nitroalkane compounds were investigated by using a rapid-mixing flow technique coupled with electron spin resonance (ESR) which can detect the radicals having a lifetime of 5–100 ms. The SO₃^{$\text{.}\text{?}$?} radical added to the double bond of CN in the nitroalkane aci-anions which are the main form of nitroalkanes in aqueous alkaline solutions. From the observed hyperfine splitting constants of the SO₃^{$\text{.}\text{?}$?} adducts of nitroalkane aci-anions, the preferred conformation of the adducts was deduced.     

Electron spin density distribution in the CO+, CO2− and O2− radicals as a function of geometry

The standard INDO method has been used to calculate the electron spin density distribution and isotropic Fermi contact hyperfine coupling in the radicals CO⁺, CO₂^?, and O₂^?. Bond lengths and angle (for CO₂^?) have been varied over a reasonable range. Geometrical effects are found to have a relatively small influence on the calculated values which do agree closely with the experimental data.     

Reactions of cisplatin hydrolytes, cis-[Pt(15NH3)2(H2O) 2]2+, with N-acetyl-L-cysteine

The reaction of cis-[Pt(¹⁵NH₃)2(H₂O) ₂] ²⁺ (3) with N-acetylcysteine [H₃accys] was investigated in aqueous solution. In this reaction, the ammine in the platinum complex formed was liberated. A mono-dentate sulfur-boundplatinum(II) product cis-[Pt(¹⁵NH₃)₂(H₂O)(H2accys-S)]⁺ (7) and six-membered che-late ring complex cis-[Pt(¹⁵NH₃)₂ (Haccys-S,O)] (8) were formed in solution. The dinuclear sulfur-bridged complex 9, giving a broad peak in ¹⁵N NMR, was also observed, but only present in very tiny amounts. The mass spectrometry (ES-MS) was undertaken from this re action, and the product detected was only the dinuclear sulfur bridged platinum species and species related to it by ammine loss.     

LIGHT-INDUCED FORMATION OF O-2˙ OXYGEN RADICALS IN SYSTEMS CONTAINING CHLOROPHYLL

It has been shown recently that photosystem 1 particles, photosystem 1 lipid vesicles and chlorophyll-a lipid vesicles show identical photochemical reactions in the presence of oxygen e.g. H⁺-and O₂-uptake (Van Ginkel, 1979). Therefore, spin-trapping experiments were done to identify the oxygen radicals formed. The spintrap phenyltertiarybutylnitrone (PBN) failed to yield information about oxygen radicals. With the spintrap 5,5-dimethyl-1-pyrroline-1-oxide (DMPO), however, we obtained a mixed spectrum of O^-_2˙ and OH·-adducts generated in chloroplasts, photosystem 1 particles or chlorophyll-a lipid vesicles. These data indicate that chlorophyll-a in an artificial membrane can also catalyze O^-_2˙-formation. Chlorophyll-a lipid vesicles catalyze light-induced formation of the Tiron-semiquinone free radical, which has been proposed as a specific O^-_2˙-probe (Greenstock and Miller, 1975). However, OH· scavengers strongly reduce the formation of this radical, whereas superoxide dismutase does not. Pulse-radiolysis measurements showed that the rate constant for the reaction of Tiron with OH· is 8.2 · 10⁹M^-1 s^-1, which is considerably higher than the published Tiron/O^-_2˙ rate constants. Therefore, Tiron is a better spin probe for OH· than for O^-_2˙. We suggest that light-induced H⁺-and O^-_2˙-uptake in membranes containing chlorophyll-a in the presence of ascorbate is caused mainly by the very rapid reaction of OH· with ascorbate.     

Oxidation of Chiral 1,4-Cyclohexadienes: A Comparison of Chemically and Photochemically Generated Singlet Oxygen1O2(1Δg)

Chiral alkyl-substituted 2,5-cyclohexadiene-l-carboxyIic acids la-c have been oxidized in water and in methanol with singlet oxygen, ¹O₂ (¹Δ_g), generated either photochemically or chemically from the catalytic system hydrogen peroxide/sodium molybdate. These methods were compared in terms of chemo-, regio- and diastereoselec-tivities and the chemical (k_T) and physical (k_q) quenching rate constants of ¹O₂ were determined. The ratio of the cis and trans isomers of the hydroperoxides 2a-c is not influenced by the source of ¹O₂ but, on the other hand, it depends slightly on the solvent and greatly on the steric hindrance of the substituents linked to the chiral carbon. The results may be interpreted on the basis of the successive formation of an exciplex and a perepoxide that evolves either by giving the final allylic hydroperoxide or by dissociating into the starting substrate and singlet or triplet oxygen.     

Oxidation of Ascorbate by Ni(III) Complexes with Tetraaza-macrocyclic Ligands in Neutral Aqueous Solutions. A Pulse-Radiolysis Study

Abstract

The mechanisms and kinetics of oxidation of ascorbate, AH^?, by Ni(III)Lⁱ _aq and by LⁱNi(III) (HPO₄)₂ ^? complexes (L¹ = meso-(5,12)-7,7,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; L² = 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) in neutral aqueous solutions have been investigated.

The oxidation of ascorbate by the LⁱNi(III) (HPO₄)₂ ^? and Ni(III)L¹ _aq proceeds via two consecutive reactions well separated in time. The products of the first reaction are the A^.? radical anion and the corresponding Ni(II) complex. The oxidations by the LⁱNi(III)(HPO₄)₂ ^? complexes proceed via the outer sphere mechanism, whereas the detailed mechanism of reaction of Ni(III)L¹ _aq cannot be determined. The rate of reaction decreases with the increase in the concentration of phosphate, thus indicating that LⁱNi(III)(HPO₄)(H₂O)⁺ and LⁱNi(III)OH²⁺ are stronger oxidizing agents than LⁱNi(III)(HPO₄)^? ₂.

The oxidation of ascorbate by Ni(III)L² _aq proceeds via three consecutive reactions which are well separated in time. Thus the results clearly point out that this process occurs via the inner sphere mechanism. The first transient observed is tentatively identified as L²(H₂O)Ni(II)(A^.?)²⁺, i.e., an unexpected complex of the ascorbate anion radical. Also in this process the last transient observed is the A^.? anion radical. The stabilization of the ascorbyl radical in a transient complex might be of biological significance.     

The (CF3S)2N. radical : preparation,dimerization, ESR-studies

The Bis (trifluormethylmercapto) amino radical (CF₃S)₂N· has been prepared by oxidation of Bis (trifluormethylmercapto) amine, (CF₃S)₂NH with activated lead diooxide in CCl₃F solution at room temperature. The 10^?5 – 10^?6 molar solution of the radical shows in the ESR- spectrum a triplett of septetts with a isotropic nitrogen hyperfine splitting constant a(¹⁴N) = 13,2 G, a(¹⁹F) = 1.95 G, indicating that the radical is a planar sp² species where the unpaired electron is located predominantly in the nitrogen 2p orbital. It is suggested that in solution an thermal equilibrium exists between the (CF₃S)₂ N radical and its dimer Tetrakis (trifluormethylmercapto)-hydrazine:

Aspects of preparation as well as thermodynamic and kinetic details of the hydrazine (NN bond strength 7 kcal/mol^?1) and the equilibrium, which can be followed by ESR-spectroscopy will be given.     

Zinc(II) and Cadmium(II) Complexes of the 3‐(2‐Pyridyl)‐5,6‐diphenyl‐1,2,4‐triazine (PDPT) Ligand,Structural Studies of [Zn(PDPT)2Cl(ClO4)] and [Cd(PDPT)2(NO3)(ClO4)]

Two new mixed‐anion zinc(II) and cadmium(II) complexes of 3‐(2‐pyridyl)‐5,6‐diphenyl‐1,2,4‐triazine (PDPT) ligand, [Zn(PDPT)₂Cl(ClO₄)] and [Cd(PDPT)₂(NO₃)(ClO₄)], have been synthesized and characterized by elemental analysis, IR‐ and ¹H NMR spectroscopy. The single crystal X‐ray analyses show that the coordination number in these complexes is six with four N‐donor atoms from two “PDPT” ligand and two of the anionic ligands, ZnN₄ClO_perchlorate, CdN₄O_nitrateO_perchlorate. Self‐assembly of these compounds in the solid state via π–π‐stacking interactions is discussed.     

Esr studies on the reactive character of the radical anions,so−2, so−3 and so−4 in aqueous solution

The reactive characteristics of the oxy anion radicals of sulphur, SO^?₂, SO^?₃ and SO^?₄ were investigated by use of the rapid-mixing flow technique coupled with electron spin resonance (ESR) which can detect the radicals having a lifetime of 5–100 msec. The SO^?₂ reduced the aromatic nitro compounds to the corresponding anion radicals, but did not abstract the hydrogen from the saturated compounds nor add to the unsaturated compounds. The SO^?₃ could add to the compounds having CC bond, but did not abstract the hydrogen from the saturated compounds nor reduce the aromatic nitro compounds. The SO^?₄ could abstract the hydrogen from the saturated compounds and also add to the unsaturated compounds having CC bond, but did not reduce the aromatic nitro compounds. These differences of the reactivity towards the organic substrates were discussed on the basis of the difference in the distribution of the unpaired electron density of each radical anion.     

Theoretical description of the 2A′' and 2A′ states of the peroxyformyl radical

The equilibrium geometries and spin density distributions for some phosphoranyl radicals and the radical anion PH₃O^? were calculated by the UHF CNDO/2 method. Barriers to permutational isomerization in PF₄ and PH₃OH were estimated.