Reactions of oxidizing radicals with 2- and 3-aminopyridines: a pulse radiolysis study

Reactions of OH radicals and some one-electron oxidants with 2-aminopyridine (2-AmPy) and 3-aminopyridine (3-AmPy) were studied in aqueous solutions using pulse radiolysis technique. The OH adduct of 2-AmPy at pH 9 has an absorption maximum at 360 nm along with a weak absorption band in the visible region and was found to be reactive with oxygen. The rate constant for its reaction with O₂ was determined to be 1.0×10⁸ dm³ mol⁻¹ s⁻¹. At pH 4 also, the OH adduct of 2-AmPy has an absorption band at 360 nm. However, there are differences in the absorption at other wavelengths. From the plot of ΔOD vs. pH at 340 nm, the pK_a of the OH adduct was determined to be 6.5. Among the specific oxidants, only SO₄^−√ radicals were able to oxidize 2-AmPy. In the case of 3-aminopyridine (3-AmPy), the transient species formed by OH radical reaction at pH 9 has an absorption maximum at 410 nm with shoulder bands on both the sides. Its absorption spectrum at pH 4 was different indicating the existence of a pK value for the OH adduct. pK_a of 3-AmPy-OH radical adduct species was evaluated to be 5.7. This adduct species was also found to be reactive with oxygen (k=7.6×10⁶ dm³ mol⁻¹ s⁻¹). Specific one-electron oxidants like N₃^√, Br₂^−√ C₂^−√ and SO₄^−√ were able to oxidize 3-AmPy indicating that it is easier to oxidize 3-AmPy as compared to 2-AmPy.     

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.     

RADICAL CATION PRODUCTION IN THE PHOTOLYSIS OF 5-M-ETHOXY-1-M-ETHYLINDOLE

Abstract— The flash photolysis of 5-m-ethoxy-1-m-ethylindole in aqueous media was studied for the purpose of assigning the absorption spectrum of the radical cation. Transients produced in this study were analogous to those formed in the photolysis of 5-m-ethoxyindole. The major transient observed with an absorption maximum of 460 nm was O₂-s_-ensitive and had a lifetime of 20 μs in nitrogen saturated solutions. One radical species is produced with absorption maxima at 445 and 530 nm. Ionic strength effects on the reaction of this species with I⁻ confirms that it is the radical cation of 5-m-ethoxy-l-methylindole. The effect of H⁺ and Br⁻ on the fluorescence, radical cation and triplet yields is discussed in relation to the mechanism of transient formation.     

Photochemistry of dicyanopyridines

The photochemistry of a variety of dicyanopyridines (2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dicyanopyridine) in solution at room temperature was investigated. Pulsed UV (308 nm) laser irradiation in deoxygenated acetonitrile yields the triplet state with lifetimes between 4 and 10 μs and absorption bands in the 400 and 320 nm regions. In the presence of added HCl an air-insensitive transient (τ ≈ 10–12 μs, λ_max ≈ 360–380 nm) was observed, suggesting the formation of a protonated excited state.

Irradiation in the presence of amines resulted in the production of the pyridyl radical anion (τ ≈ 40–80 μs, air sensitive, λ_max ≈ 360–380 nm) formed by electron transfer from the amine to the pyridine triplet excited state. Stern-Volmer analysis gave electron transfer rate constants in the range (1–8) × 10⁻⁸ M⁻¹ s⁻¹.

In methanol solvent, irradiation yielded an air-insensitive transient assigned as the neutral pyridyl radical (τ ≈ 30–200 μs, λ_max ≈ 370–385 nm). The formation of these transients is discussed in the context of previous photochemical electron spin resonance and product studies.     

A millimeter/sub-millimeter-wave spectroscopic study of the FeCl radical (XΔi)

Pure rotational spectroscopy of the FeCl radical (X⁶Δ_i) has been carried out using millimeter/sub-millimeter direct absorption techniques. The species was created by the reaction of chlorine with iron vapor. All six spin-orbit components were observed in the majority of the twenty-one rotational transition recorded. Chlorine hyperfine structure was resolved in the Ω = 9/2 and 7/2 components, and lambda-type doubling observed in the Ω = 3/21/2, and −1/2 ladders. The data were analyzed with a ⁶Δ Hamiltonian, and rotational, fine structure, lambda-doubling, and hyperfine parameters determined. The hyperfine and lambda-doubling interactions in FeCl appear to be different from those in the FeF radical.     

CO2 absorption of perovskites as seen by positron lifetime spectroscopy

The CO₂ absorption of several ABO₃ type perovskites was studied by positron lifetime spectroscopy. The longer positron lifetime was associated with positrons trapped by A site vacancies. The evaluated positron lifetime data indicated the relative stability of the crystal structure of Sr(Co_0.5Fe_0.5)O_3−δ against Ca doping at low Ca concentrations. Oxygen desorption and CO₂ absorption/desorption could also be followed by positron lifetime spectroscopy. It was shown that the concentration of oxygen vacancies has a large effect on positron lifetime data through the electron density of A site vacancies.     

1,3,6,8-Tetraethynylpyrene and 1,3,6,8-tetrakis (trimethylsilylethynyl) pyrene: Photophysical properties in homogeneous media

The photophysical properties of two new tetra substituted derivatives of pyrene: 1,3,6,8-tetraethynylpyrene (TEP) and 1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene (TEP-TMS) have been studied. Studies were done with respect to mirror image symmetry in the absorption and emission spectra and permissive or forbidden nature of S₀–S₁ transition, solvent sensitivity of the first and third vibronic bands and fluorescence anisotropy. Both the derivatives exhibited a strongly allowed S₀–S₁ transition, high fluorescence quantum yield, shorter fluorescence lifetime compared to pyrene and invariance of the vibronic band intensity ratio to solvent polarity. The behavior of the two pyrene derivatives validates the hypothesis “solvent polarity mediates vibronic coupling and therefore the emission band intensities, for forbidden S₀–S₁ transitions”. The trimethylsilyl derivative (TEP-TMS) was characterized by a strong fluorescence in solid state. The tetraethynyl derivative (TEP) showed high fluorescence anisotropy comparable to the well-known anisotropy probe DPH in glycerol at 0 °C. The fluorescence intensities of TEP and TEP-TMS did not show any significant change in the temperature ranger 0–40 °C for a low viscous solvent like ethanol and in the range 0–60 °C in glycerol. Unlike pyrene, no excimer emission was observed even up to 10⁻³ M for TEP and TEP-TMS.     

Photophysical, electrochemical and electrochromic properties of copper-bis(4,4′-dimethyl-6,6′-diphenyl-2,2′-bipyridine) complexes

The synthesis, solution and solid state structural characterization, photophysical and electrochemical properties of two redox forms of an electrochromic copper-bis(4,4′-dimethyl-6,6′-diphenyl-2,2′-bipyridine) complex, [Cu(3)₂]ⁿ (n=+1, +2), are presented. Both complexes were characterized in the solid state by X-ray diffraction methods on single-crystals showing that both forms exist in a pseudo-tetrahedral coordination, and a comparison with other structures was made. Like most copper(I) complexes, the red [Cu(3)₂]⁺ complex shows a rather weak emission (Φ_em=2.7×10⁻⁴, dichloromethane). The lifetime of the emitting MLCT state is 34±1 ns, as observed with time resolved emission, and transient absorption (in deoxygenated dichloromethane). Typical emission and transient absorption spectra are presented. The transient absorption spectra indicate that the MLCT state absorbs stronger than the ground state, which is relatively uncommon for metal bipyridine complexes, i.e. no ground state bleaching is observed. The green [(3)₂Cu]²⁺ complex does not show any observable emission or transient absorption, which is a common feature for Cu(II) complexes of this type. The electronic absorption spectra of the chemically and electrochemically produced copper(I/II) complexes are identical. The repeated electrochemical conversion of the Cu(I) center into Cu(II) and vice versa does not cause any decomposition. This is consistent with a fully reversible Cu(I)/Cu(II) redox couple in the corresponding cyclic voltammogram, (E_1/2 (Cu(I)/Cu(II))=+0.68 V vs. SCE=+0.23 V vs. Fc/Fc⁺). These observations indicate that no large structural reorganization occurs upon electrochemical timescales (sub second), and that the different ways of generating the complexes does not effect their final structure, apart from the small differences observed in the X-ray structures of both forms. These characteristics make these complexes rather well suited for their incorporation into an electrochromic display configuration.     

Pulse radiolysis studies of 4,7-phenanthroline(II)

The one-electron reduction of 4,7-phenanthroline (P) in aqueous solutions at neutral pH has been further studied by pulse radiolysis. The spectral and kinetic properties of the transient formed due to the reaction of 4,7-phenanthroline with hydrated electron were investigated. The transient absorption spectrum obtained 5μs after the pulse exhibits a broad band with a λ_max at 420 nm. The λ_max is 10 nm blue shift compared with the absorption spectrum obtained at pH 2.9 where the reactant was the protonated form. The bimolecular'rate constant of the reaction of 4,7-phenanthroline with hydrated electron was 0etermined to be (2.2±0.1)×10¹⁰ dm³ mol⁻¹ s⁻¹. It was found that the decay of the transient was mainly following a first-order kinetics. The first-order decay rate constant was determined to be (1.25±0.1)×10⁴s⁻¹.     

Electric magnetic resonance and spectrophotometry evidence on the photodynamic activity of a new perylenequinonoid pigment

Di-cysteine substituted hypocrellin B (DCHB) is a new water-soluble photosensitizer with significantly enhanced red absorption at wavelengths longer than 600 nm over the parent compound hypocrellin B (HB). The photosensitizing properties (Type I and/or Type II mechanisms) of DCHB have been investigated in dimethylsulfoxide (DMSO) and aqueous solution (pH 7.4) using electron paramagnetic resonance (EPR) and spectrophotometric methods. In anaerobic DMSO solution, the semiquinone anion radical of DCHB (DCHB^•−) is predominantly photoproduced via self-electron transfer between excited- and ground-state DCHB species. The presence of an electron donor significantly promotes the formation of the reduced form of DCHB. When a deoxygenated aqueous solution of DCHB and an electron donor are irradiated with 532 nm light, the hydroquinone of DCHB (DCHBH₂) is formed via the disproportionation of the first-formed DCHB^•− and second electron transfer to DCHB^•− from the electron donor. When oxygen is present, singlet oxygen (¹O₂), superoxide anion radical (O₂^•−) and hydroxyl radical (^•OH) are produced. The quantum yield of ¹O₂ generation by DCHB photosensitization is estimated to be 0.54 using Rose Bengal as a reference, a little lower than that of HB (0.76). The superoxide anion radical is also significantly enhanced by the presence of electron donors. Moreover, (O₂^•−) upon disproportionation generated H₂O₂ and ultimately the highly reactive ^•OH via the Haber-Weiss reaction pathway. The efficiency of (O₂^•−) generation by DCHB is obviously enhanced over that of HB. These findings suggest that the photodynamic actions of DCHB may proceed via Type I and Type II mechanisms and that this new photosensitizer retains photosensitizing activity after photodynamic therapy-oriented chemical modification.     

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.     

Photophysical properties of lumichromes in water

The photophysics of lumichrome, 1-methyllumichrome, and lumiflavin in water solutions have been investigated. Fluorescence lifetimes of 2.7 and 2.2 ns were observed for lumichrome and 1-methyllumichrome, respectively, the corresponding triplet state lifetimes of 17 and 18 μs have been obtained from the transient absorption spectra. Evidence for long lived species with absorption maxima near 450 nm and lifetimes of ca. 400 μs has been found in the transient absorption spectra of both lumichromes. Quantum yields for the sensitised production of singlet oxygen, φ_Δ, are 0.36 and 0.41 for lumichrome and 1-methyllumichrome, respectively, in D₂O.     

Highly sensitive detection of transient absorption in dye-doped ultrathin polymer films by the TiO(2)/K(+) composite optical waveguide method upon pulsed laser excitation

A composite optical waveguide (OWG) composed of a 10–18 nm thick titanium dioxide (TiO₂) film sputtered on a conventional K⁺-doped optical waveguide was first applied to detect transient absorption of organic dyes in ultrathin polymer films upon excitation with ns laser. The thickness of the TiO₂ film considerably affected the relative sensitivity of the composite OWG. The composite OWG with 10 nm thick TiO₂ gave much stronger transient absorption for 30–415 nm thick polymer films containing organic dyes than that with 18 nm TiO₂. Transient absorption of phthalocyanine and spiropyran in 20–135 nm thick polymer films was detected 3–20 times more sensitively by the composite OWG with 10 nm TiO₂ than the conventional K⁺-doped OWG which showed a 150-fold sensitivity as compared with the usual normal incidence method. The relative sensitivity of the composite waveguide was also affected by the thickness and refractive index of polymers.     

PREPARATION AND PHOTOPHYSICAL STUDIES OF PORPHYRIN-C60 DYADS

Abstract Porphyrin-C₆₀ dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels-Alder reaction. The porphyin singlet lifetimes of both the zinc (Pz_n-C₆₀) and free base (P-C₆₀) dyads, determined by time-resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet-singlet energy transfer to C₆₀ The lifetime of Pzn-¹C₆₀ is -5 ps in toluene, whereas the singlet lifetime of an appropriate C₆₀ model compound is 1.2 ns. This quenching is attributed to electron transfer to yield P_zn^bull;+-C₆₀^bull;-. In toluene, P-¹C₆₀ is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of-10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C₆₀ triplet. In the more polar benzonitrile, P-¹C₆₀ underoes photoinduced electron transfer to give P^•+-C₆₀^bull;-. The electron transfer rate constant is −2 × 10¹¹ s⁻¹.     

A study of the B2 excited state geometries of the metal-metal quadruply bonded compounds Mo2X4(PMe3)4 (X = Cl, Br or I)

The excited state geometries of the metal-metal quadruply bonded compounds Mo₂X₄(PMe₃)₄ (X = Cl, Br or I) have been studied by means of resonance Raman and absorption spectroscopy. A fit of the parameters of a simple theoretical model to the experimental data indicates that the metal-metal bond increases some 10 pm on excitation to the ¹B₂ (δδ^*) state, whereas other geometric changes are small. Furthermore, the phenomenological lifetime factor of the excited state, Γ, is found to be dependent on the vibrational quantum number, ν, of this state.     

One-electron oxidation of mitomycin C and its corresponding peroxyl radicals. A steady-state and pulse radiolysis study

The one-electron oxidation of Mitomycin C (MMC) as well as the formation of the corresponding peroxyl radicals were investigated by both steady-state and pulse radiolysis. The steady-state MMC-radiolysis by OH-attack followed at both absorption bands showed different yields: at 218 nm G_i (-MMC) = 3.0 and at 364 nm G_i (-MMC) = 3.9, indicating the formation of various not yet identified products, among which ammonia was determined, G(NH₃) = 0.81. By means of pulse radiolysis it was established a total κ (OH + MMC) = (5.8 ± 0.2) × 10⁹ dm³ mol⁻¹ s⁻¹. The transient absorption spectrum from the one-electron oxidized MMC showed absorption maxima at 295 nm (ε = 9950 dm³ mol⁻¹ cm^t-1), 410 nm (ε = 1450 dm³ mol⁻¹ cm⁻¹) and 505 nm ( ε = 5420 dm³ mol⁻¹ cm⁻¹). At 280–320 and 505 nm and above they exhibit in the first 150 μs a first order decay, κ₁ = (0.85 ± 0.1) × 10³ s⁻¹, and followed upto ms time range, by a second order decay, 2κ = (1.3 ± 0.3) × 10⁸ dm³ mol^-1 s⁻¹. Around 410 nm the kinetics are rather mixed and could not be resolved.

The steady-state MMC-radiolysis in the presence of oxygen featured a proportionality towards the absorbed dose for both MMC-absorption bands, resulting in a G_i (-MMC) = 1.5. Among several products ammonia-yield was determined G(NH₃) = 0.52. The formation of MMC-peroxyl radicals was studied by pulse radiolysis, likewise in neutral aqueous solution, but saturated with a gas mixture of 80% N₂O and 20% O₂. The maxima of the observed transient spectrum are slightly shifted compared to that of the one-electron oxidized MMC-species, namely: 290 nm (ε = 10100 dm³ mol⁻¹ cm⁻¹), 410 nm (ε = 2900 dm³ mol⁻¹ cm⁻¹) and 520 nm (ε = 5500 dm³ mol⁻¹ cm⁻¹). The O₂-addition to the MMC-one-electron oxidized transients was found to be at 290 to 410 nm gk(MMC·OH + O₂) = 5 × 10⁷ dm³ mol⁻¹ s⁻¹, around 480 nm κ = 1.6 × 10⁸ dm³ mol⁻¹ s⁻¹ and at 510 nm and above, κ = 3 × 10⁸ dm³ mol⁻¹ s⁻¹. The decay kinetics of the MMC-peroxyl radicals were also found to be different at the various absorption bands, but predominantly of first order; at 290–420 nm κ₁ = 1.5 × 10³ s⁻¹ and at 500 nm and above, κ = 7.0 × 10³ s⁻¹.

The presented results are of interest for the radiation behaviour of MMC as well as for its application as an antitumor drug in the combined radiation-chemotherapy of patients.     

Ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pd(en)2] [PdCl2(en)2] (ClO4)4

The ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pd(en)₂] [PdCl₂(en)₂] (ClO₄)₄ (en = ethylenediamine) has been investigated by the use of a femtosecond absorption spectrum, calculated from a pump—probe reflection spectrum at room temperature by the Kramers—Kronig relations. The time dependence of the transient photoinduced absorption around 1.7 eV and the bleaching from 1.9 to 2.5 eV were calculated for three decay components. They are free excitons with a lifetime of about 800 fs, self-trapped excitons with a lifetime of about 3 ps, and polaron pairs which relax within a 100 ps time period.     

On the kinetics of polymer degradation in solution—XII. Radiolysis of poly(2,2,2-trichloroethyl methacrylate)

The title compound (PTCMA) was irradiated in O₂-free dioxane solution by ⁶⁰Co-γ-rays or by 100 nsec-pulses of 16 MeV electrons. At concentrations below ca 0.04 base mol/l, main-chain scission occurred as was concluded from the decrease of the light scattering intensity (LSI). G(S) = 3.7 ± 0.4 was independent of the polymer concentration and equal to G(S) observed with solid PTCMA. This result indicates that in dilute solution main-chain scission is induced by the direct action of radiation on the polymer. The free radical [presumably ---C(CH₃)---(CH₂)---] giving rise to main-chain scission has a lifetime of 2.6 msec. It reacts with ethane thiol (k = 6.6 × 10⁴M⁻¹sec⁻¹). At concentrations above 0.04 base mol/l, the polymer crosslinked (gel formation, increase of the LSI according to 2nd order kinetics). It is assumed that crosslinking is due to the combination of radicals of the type ---CH₂---C(CH₃)(COOCH₂CCl₂)--- formed via dissociative electron capture processes involving mainly electrons in spurs that otherwise recombine with parent ions. This conclusion was inferred from the finding that [(τ_1/2)₁]⁻¹ ∞ (c_polymer)²[(τ_1/2)₁): 1st half-life of LSI increase after the pulse, c_polymer: polymer concentration].     

The discrimination between triplet state and radical cation in the pulse radiolysis of bithiophene in CCl4

The triplet state (³2T) and the radical cation (2T⁺√) of 2,2′-bithiophene (2T) are characterized by pulse radiolysis in CCl₄. Two main absorption bands at 360 and 420 nm are respectively attributed to ³2T* and to 2T⁺√. The triplet, induced in an excited state through a Förster mechanism, undergoes a conformational rearrangement (k₆=(6.8±0.9)×10⁶ s⁻¹). The radical cation is produced both through a resonance charge transfer and a second diffusional process; the two oxidizing species are respectively CCl₄⁺√ and (CCl⁺₃Cl⁻)_solv through the mediation of a singlet excited state, ¹2T*.     

The temperature dependence of the two positronium bubble states in liquid SF6

Positron lifetime measurements have been performed on liquid SF₆ in the temperature range from −45°C to 71°C (T_c = 45.65°C). The positron lifetime spectra were resolved into four lifetime components. In the order of increasing lifetimes the four lifetime components are associated with the decay of para-positronium (p-Ps), free positrons, ortho-positronium (o-Ps) from a small bubble state, and o-Ps from a large bubble state. The lifetime of o-Ps annihilating from the large bubble state τ₄ increases from 5.7 ns at −45°C to 19.5 ns at 53°C. The lifetime of o-Ps annihilating from the small bubble state τ₃ was found to be 2–2.5 ns in the main part of the temperature range studied. Apparently, this is the first observation of two different o-Ps states in a liquid. The intensity I₄ (I₃) increases (decreases) from 16.9% (16%) at −45°C to 47.2% (6.4%) at the critical point while above I₃ and I₄ are essentially temperature independent. The large Ps bubble state seems to be similar to the Ps bubble state found in most liquids.