Reversible excited state metal-to-metal electron transfer across a heterocyclic σ, π-bridge

A laser flash photolysis study of a binuclear methyl (η¹-azaferrocene)cobaloxime is reported. Excitation at 355 nm (4 ns, approximately 60 mJ) results in the reversible electron transfer between cobalt and iron centres CO^III/Fe^II CO^II/Fe^III. This constitutes the first example of photoinduced metal-to-metal electron transfer across a heterocyclic σ, π-bridge     

193 nm Laser dissociation of CS2: prompt emission from CS and internal energy distribution of CS(XΣ)

Single and multiple photon processes are identified in the 193 nm excimer laser photolysis of CS₂. CS(X¹Σ⁺, υ = 1 to 5, J = 5 to 45) is observed by dye laser induced fluorescence of the A¹Π ↔ ; X¹Σ⁺ transition, following the single photon 193 nm photolysis of CS₂. Multiple photon 193 nm generation of CS fragment emission from 620 to 170 nm is also reported. A partial assignment of the emission spectrum identifies fluorescence from the CS A′¹Σ⁺ and A¹Π states.     

Electron transfer from triplet thymine and thymidine to lipoic acid

Electron transfer from the triplet excited state of thymine or thymidine to the disulphide compound lipoic acid (RSSR) was studied using KrF laser flash photolysis (248 nm, 20 ns). The electron transfer reaction rate constants, measured at 310 nm, were determined to be 1.3×10¹⁰ M⁻¹ s⁻¹ and 6.9×10⁹ M⁻¹ s⁻¹ for thymine and thymidine respectively. The transient absorbance at 400 nm in the presence of the quencher is attributed to the anion radical of lipoic acid.     

Persulphate quenching of the excited state of ruthenium(II) tris-bipyridyl dication: thermal reactions

The rate constant for the reaction between the sulphate radical (SO₄^√−) and the ruthenium (II) tris-bipyridyl dication (Ru(bipy)₃²⁺) is (3.3±0.2)×10⁹ mol⁻¹ dm³ s⁻¹ in 1 mol dm⁻³ H₂SO₄ and (4.9±0.5)×10⁹ mol⁻¹ dm³ s⁻¹ in 0.1 mol dm⁻³, pH 4.7 acetate buffer. The SO₄^√−radical produced by the electron transfer quenching of Ru(bipy)₃^2+* by S₂O₈²⁻ reacts rapidly with both acetate buffer and chloride ions. These side reactions result in a reduction in the overall quantum yield of Ru(bipy)₃³⁺ production and reduced reaction selectivity when Ru(bipy)₃^2+* is quenched by persulphate.     

Triplet excited-state characterization and determination of the photoionization mechanism of the antitumoral drug pazelliptine

The triplet properties of the excited triplet state of pazelliptine (PZE), an antitumoral drug derived from ellipticine, were investigated in dioxane, ethanol and buffer aqueous solutions using the laser flash photolysis technique. The triplet absorption spectra and the kinetic parameters associated with the excited state decay were quite similar in the different solvents. ³PZE reacted with unexcited PZE in deaerated solutions (k = 6 × 10¹⁰ M⁻¹ s⁻¹) and was quenched by oxygen (k ≈ 2 × 10⁷ s⁻¹). The extinction coefficients of the triplet transition were estimated and used to calculate the singlet-triplet intersystem crossing quantum yields of about 5%.

A biphotonic ionization of PZE in buffer aqueous solution has been demonstrated in a previous work. This process was also observed in ethanol but not in dioxane. Mixed yttrium aluminum garnet laser harmonics (355 nm + 532 nm) and delayed-pulse experiments were carried out in order to determine the intermediate excited state involved in this photoionization process. The results indicate that pazelliptine radical cation and e⁻_s are formed via a consecutive two-photon absoprtion in which the first excited singlet state is the only intermediate.     

All electron ab initio investigations of the electronic states of the MoN molecule

The low lying electronic states of the molecule MoN were investigated by performing all electron ab initio multi-configuration self-consistent-field (CASSCF) calculations. The relativistic corrections for the one electron Darwin contact term and the relativistic mass-velocity correction were determined in perturbation calculations. The electronic ground state is confirmed as being ⁴∑⁻. The chemical bond of MoN has a triple bond character because of the approximately fully occupied delocalized bonding π and σ orbitals. The spectroscopic constants for the ground state and ten excited states were derived. The excited doublet states ²∑⁻, ²Γ, ²Δ, and ²∑⁺ are found to be lower lying than the ⁴Π state that was investigated experimentally. Elaborate multi-configuration configuration-interaction (MRCI) calculations were carried out for the states ⁴∑⁻ and ⁴∏ using various basis sets. The spectroscopic constants for the ⁴∑⁻ ground state were determined as r_e=1.636 Å and ω_e=1109 cm⁻¹, and for the ⁴∏ state as r_e=1.662 Å and ω_e=941 cm⁻¹. The values for the ground state are in excellent agreement with available experimental data. The MoN molecule is polar with a charge transfer from Mo to N. The dipole moment was determined as 2.11 D in the ⁴∑⁻ state and as 4.60 D in the ⁴∏ state. These values agree well with the revised experimental values determined from molecular Stark spectroscopic measurements. The dissociation energy, D_e, is determined as 5.17 eV, and D₀ as 5.10 eV.     

Photophysics of ruthenium(II) complexes with 2-(2′-pyridyl) pyrimidine and 2,2′-bipyridine ligands in fluid solution

The photophysics of three complexes of the form Ru(bpy)₃₋(pypm)²⁺ (where bpy2,2′-bipyridine, pypm 2-(2′-pyridyl)pyrimidine and P=1, 2 or 3) was examined in H₂O, propylene carbonate, CH₃CN and 4:1 (v/v) C₂H₅OH---CH₃OH; comparison was made with the well-known photophysical behavior of Ru(bpy)₃²⁺. The lifetimes of the luminescent metal-to-ligand charge transfer (MLCT) excited states were determined as a function of temperature (between −103 and 90 °C, depending on the solvent), from which were extracted the rate constants for radiative and non-radiative decay and ΔE, the energy gap between the MLCT and metal-centered (MC) excited states. The results indicate that ^*Ru(bpy)₂(pypm)²⁺ decays via a higher lying MLCT state, whereas ^*Ru(pypm)₃²⁺ and ^*Ru(pypm)₂(bpy)²⁺ decay predominantly via the MC state.     

Photophysical Properties of the Cationic Form of Neutral Red

Abstract— Photophysical properties of the cationic form of neutral red (NRH⁺), a phenazine-based dye of biological importance, have been investigated in several protic and aprotic solvents using optical absorption, steady-state and time-resolved fluorescence and picosecond laser flash photolysis techniques. Absorption and fluorescence characteristics of the dye in protic solvents indicate the existence of intermolecular hydrogen bonding between the NRH⁺ and solvent molecules in the ground state as well as in the excited state. Measurements of the fluorescence lifetime in normal and heavy water also support the formation of intermolecular hydrogen bonding. Time-resolved transient absorption spectra obtained in the picosecond laser flash photolysis experiments show only the absorption band due to the S_n← S₁ absorption. The picosecond transient absorption results do not indicate any spectral shifts attributable to the hydrogen bond formation dynamics between the excited NRH⁺ and the protic solvent molecules. It is inferred that the hydrogen bonding dynamics are much faster than the time resolution of our picosecond setup (∼35 ps).     

Highly efficient blue up-conversion of Tm in Nd–Yb–Tm co-doped ZrF4-based fluoride glass

Up-conversion luminescence and energy-transfer processes in Nd³⁺, Yb³⁺ and Tm³⁺ co-doped ZrF₄-based fluoride glasses have been studied under 800 nm light excitation. Blue up-converted emission around 478 nm which can be assigned to the Tm³⁺:¹G₄→³H₆ transition, was strongly observed. Up-conversion luminescence intensity exhibited an YbF₃-concentration dependence. Among the Nd³⁺, Yb³⁺ and Tm³⁺, Nd³⁺ and Tm³⁺ have ground state absorption bands due to the (²H_9/2,⁴F_5/2)←⁴I_9/2 and ³F₄←³H₆ transitions, respectively, which can be directly pumped by 800 nm radiation. However, no emissions were observed in Tm³⁺ singly-doped and Tm³⁺–Yb³⁺ doubly-doped glasses under 800 nm excitation. Therefore, a possible up-conversion mechanism may be proposed as follows: energy-transfer firstly occurs from Nd³⁺ to Yb³⁺ when Nd³⁺ is excited by 800 nm light, then the energy is transferred from Yb³⁺ to Tm³⁺ which is in the excited state and, finally, blue up-conversion emission of Tm³⁺ is observed through the Tm³⁺:¹G₄→³H₆ transition.     

Evidence for two light-induced metastable states in Cl3[Ru(NH3)5NO]H2O

Differential Scanning Calorimetry measurements on irradiated Cl₃[Ru(NH₃)₅NO]H₂O reveal the existence of two light-induced long-lived metastable states SI, SII. Irradiation with light in the spectral range 400–500 nm leads to the excitation of SI. For the first time we report experimental evidence for the state SII in this compound, which can be excited by transferring SI into SII with irradiation of light in the spectral range 1000–1200 nm. The excitation and transfer of the metastable states is described and the exponential decays are evaluated according to Arrhenius' law yielding activation energies of E_A(SI)=0.73(3) eV, E_A(SII)=0.66(3) eV and frequency factors of Z(SI)=1 × 10¹² s⁻¹, Z(SII) = 5 × 10¹² s⁻¹.     

Photochemical reaction of gadolinium(III) tetraphenylporphyrin in toluene solution containing an electron acceptor or donor

In this paper we have studied by absorption spectroscopy the course of the steady state photolysis of gadolinium(III) complex with 5,10,15,20-tetraphenylporphyrin. The result has been compared with the photolysis of free base tetraphenylporphyrin. Irradiation of GdTPP(acac) and H₂TPP (H₂TPP ≡ tetraphenylporphyrin; Hacac ≡ acetylacetone) with visible light in the presence of triethylamine as an electron donor, leads to their photoreduction. Formation of the gadolinium tetraphenylchlorin complex and tetraphenylchlorin is observed. The process of irradiation in the presence of CCI₄ as an electron acceptor has different pathways for GdTPP(acac) and H₂TPP. The gadolinium complex is mono oxidized, giving rise to the π⁺ radical cation while, for unmetallated porphyrin, dication H₄(TPP)²⁺ formation is observed.     

吡啶离子液体[BPy][BF4]与乙腈混合体系中杜醌的光化学反应动力学

用激光闪光光解方法研究了杜醌(DQ)在吡啶型离子液体N-丁基吡啶四氟硼酸盐([BPy][BF₄])与乙腈(MeCN)组成的共混溶剂中的光化学反应机理与动力学. 实验结果表明, 离子液体[BPy][BF₄]对混合体系中杜醌激发三线态(³DQ^*)的瞬态吸收峰位置和吸光度大小都没有产生明显影响. 在N₂饱和条件下, 无论是在乙腈溶液中还是在[BPy][BF₄]/MeCN混合溶液中³DQ^*的衰减都遵循一级反应动力学规律. 而[BPy][BF₄]的存在对³DQ^*与三乙胺(TEA)之间的电子转移影响显著. 随着[BPy][BF₄]/MeCN 体系中离子液体比例的增加, 杜醌三线态³DQ^*与TEA间的瞬态反应机理没有改变, 但它们之间的光诱导电子转移反应速率和生成自由基的量子产额逐渐降低, 通过改变离子液体的比例可以调节该体系中光诱导电子转移反应的速率和效率.     

Rate constant for the reaction of the t-butoxyl radical with Fe(II) ion

The rate of the reaction of the tert-butoxyl radical (t-BuO^√) with Fe²⁺ was measured using laser flash photolysis of methanolic solutions at room temperature. t-BuO^√ were generated by homolytic photodecomposition of di-tert-butyl peroxide. The rate constant for oxidation of Fe²⁺ with t-BuO^√ radicals was studied under pseudo-first order conditions. On the basis of competitive kinetics the quantum yield of oxidation, Φ(Fe³⁺), was determined as function of Fe²⁺ concentration by measuring the absorbance of Fe³⁺ as [FeCl]²⁺ complex. By using the literature values of the rate constants of relevant competing reactions, the desired rate constant was determined to be 3.0×10⁸ M⁻¹ s⁻¹.     

PHOTOIONIZATION OF CRYSTAL VIOLET IN AQUEOUS SOLUTION

Abstract— Laser flash photolysis studies were carried out on a triphenylmethane dye, crystal violet (CV⁺), at 248 nm in aqueous solutions. The results show that CV⁺ undergoes photoionization and the resulting transients CV^-2+, hydrated electrons (e^-_aq) and CV⁺ radical formed by the reaction of e^-_aq with CV⁺ have been characterized. Studies using suitable scavengers were done to support the characterization of the transient species. Laser intensity effects show that the ionization is biphotonic. Two mechanisms are proposed to explain the observed photoionization involving higher excited singlet state and/ or another long-lived excited state of the dye.     

Hydrogen peroxide photoproduction by the semicarbazide—tris(2,2′-bipyridine)ruthenium(II)—oxygen system

A light-driven system consisting of tris(2,2′-bipyridine)ruthenium(II) (Ru(bpy)₃²⁺) as the photosensitizer, semicarbazide as the electron donor and molecular oxygen as the electron acceptor has been employed for hydrogen peroxide production. The efficiency of this photosystem markedly depends on pH: while the peroxide yield is almost negligible at acid, neutral or slightly alkaline pH, it reaches significant values at high hydroxide concentrations, the initial rate of H₂O₂ formation drastically increasing from pH 12 to pH 14. In 1 M NaOH solutions containing Ru(bpy)₃²⁺ and semicarbazide at optimum concentrations, the number of catalytic cycles (or turnover number) undergone by the ruthenium complex over the complete course of the photochemical reaction is as high as 1.1 × 10⁴.

Spectrofluorometric and laser flash photolysis techniques were used to study the primary photochemical reactions involving the excited state of the ruthenium complex as well as the photochemically generated species Ru(bpy)₃³⁺ and Ru(bpy)₃⁺. It is proposed that at pH 14 a sequence of reactions leading to O₂ photoreduction by electrons from semicarbazide takes place, with the concomitant formation of H₂O₂; the excited state of Ru(bpy)₃²⁺ appears to react via oxidative quenching by oxygen rather than via reductive quenching by semicarbazide. At neutral pH, in contrast, there is no H₂O₂ formation owing to the fact that semicarbazide is unable to reduce (Ru(bpy)₃³⁺ to Ru(bpy)₃²⁺, although the photoexcited ruthenium complex is quenched equally by oxygen.     

Fraction of excited-state oxygen formed as b Σg in solution-phase-photosensitized reactions II. Effects of sensitizer substituent

The fraction F_Σ of excited-state oxygen formed as b ¹Σ_g⁺ was determined for a series of triplet-state photosensitizers in CCl₄ solutions. F_Σ was determined by monitoring the intensities of (a) O₂(b ¹Σ_g⁺) fluorescence at 1926 nm (O₂(b ¹Σ_g⁺)→O₂(a ¹Δ_g) and (b) O₂(a ¹ Δ_g) phosphorescence at 1270 nm (O₂(a ¹Δ_g) → O₂(X³Σ_g⁻)). Oxygen excited states were formed by energy transfer from substituted benzophenones and acetophenones. The data indicate that F_Σ depends on several variables including the orbital configuration of the lowest triplet state and the triplet-state energy. The available data indicate that the sensitizer-oxygen charge transfer (CT) state is not likely to influence F_Σ strongly by CT-mediated mixing of various sensitizer-oxygen states.     

Oxidation of crystal violet and malachite green in aqueous solutions — a kinetic spectrophotometric study

The reactions of two triphenyl methane (TPM) dyes—crystal violet (CV⁺) and malachite green (MG⁺)—with N₃^• and OH^• radicals were studied by pulse radiolytic kinetic spectrophotometry. The rate constants for the reaction of the cationic dyes (D⁺) with N₃^• are (9.0±0.6)×10⁹ and (3.0±0.2)×10⁹ dm³ mol⁻¹ s⁻¹ respectively and those for the reaction with OH^• are obtained as (8.0±0.6)×10⁹ and (1.1±0.1)×10⁹ dm³ mol⁻¹ s⁻¹ respectively. The transient spectra resulting from the oxidation of the dyes were characterized. The time-resolved spectra indicate that the reaction with OH^• radicals initially generates an adduct which subsequently dissociates to form the radical dication D^•2+. The D^•2+ species decay by further reaction with the parent dye.     

Redox reactions of 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) in aqueous solution

Pulse radiolysis technique has been employed to study the reactions of oxidizing (^√OH, N₃^√) and reducing radicals (e⁻_aq, CO₂^√−, acetone ketyl radical) with 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) at different pH. Hydroxyl radicals react mostly by addition reaction forming radical adducts (λ_max=420 nm) and the oxidation is only a minor process even in the alkaline region. The reaction with azide radicals produced phenoxyl radicals (λ_max=340 nm), which are formed on fast deprotonation of solute radical cation. Using PMZ^√+/PMZ and ABTS^√−/ABTS²⁻ as the reference couple, different methods are employed to determine the one-electron reduction potential of o-vanillin and the average value is estimated to be 1.076±0.004 V vs. NHE at pH 6. The phenoxyl radicals of o-vanillin were able to oxidize ABTS²⁻ quantitatively. The e_aq⁻ is observed to react with o-vanillin with rate constant value of 2×10¹⁰ dm³ mol⁻¹ s⁻¹. CO₂^√− and acetone ketyl radical are also observed to react with o-vanillin by electron transfer mechanism and showed the formation of transient absorption bands with λ_max at 350 and 390 nm at pH 4.5 and 9.7, respectively. The pK_a of the one-electron reduced species was determined to be 8.1. The results indicate that the aldehydic group is the most preferred site for electron addition.     

Photoinduced electron transfer reactions of rose bengal and selected electron donors

The photoinduced electron transfer reactions of the triplet state of rose bengal (RB) and several electron donors were investigated by the complementary techniques of steady state and time-resolved electron paramagnetic resonance (EPR) and laser flash photolysis (LFP). The yield of radicals varied with the light fluence rate, RB concentration and, in particular, the electron donor used. Thus for L-dopa (dopa, dihydroxyphenylalanine) only 10% of RB anion radical (RB√−) was produced, with double the yield observed with NADH (NAD, nicotinamide adenine dinucleotide) as quencher and more than three times the yield observed with ascorbate as quencher. Quenching of the RB triplet was both reactive and physical with total quenching rate constants of 4 × 10⁸ mol⁻¹ dm³ s⁻¹ and 8.5 × 10⁸ mol⁻¹ dm³ s⁻¹ for ascorbate and NADH respectively. The rate constant for the photoinduced electron transfer from ascorbate to RB triplet was 1.4 × 10⁸ mol⁻¹ dm³ s⁻¹ as determined by Fourier transform EPR (FT EPR). FT EPR spectra were spin polarized in emission at early times indicating a radical pair mechanism for the chemically induced dynamic electron polarization. Subsequent to the initial electron transfer production of radicals, a complex series of reactions was observed, which were dominated by processes such as recombination, disproportionation and secondary (bleaching) reactions.

It was observed that back electron transfer reactions could be prevented by mild oxidants such as ferric compounds and duroquinone, which were efficiently reduced by RB√−.     

Far-UV laser flash photolysis in solution. A study of the chemistry of 1,1-dimethylsilene in hydrocarbon solvents

The far-UV (193 nm) laser flash photolysis of nitrogen-saturated isooctane solutions of 1,1-dimethylsiletane allows the direct detection of 1,1-dimethylsilene as a transient species, which (at low laser intensities) decays with pseudo-first-order kinetics (τ 10 μs) and exhibits a UV absorption spectrum with λ_max 255 nm. Characteristic rapid quenching is observed for the silene with methanol (k_McOH = (4.9 ± 0.2) × 10⁹ M⁻¹ s⁻¹), tert-butanol (k_BuOH = (1.8 ± 0.1) × 10⁹ M⁻¹ s⁻¹) and oxygen (k_O2 = (2.0 ± 0.5) × 10⁸ M⁻¹ s⁻¹). The Arrhenius activation parameters for the reaction with methanol have been determined to be E_a = −2.6 ± 0.6 kcal mol⁻¹ and log A = 7.7 ± 0.3.