Showing papers on "Flash photolysis published in 1975"

pH dependence of singlet oxygen production in aqueous solutions using thiazine dyes as photosensitizers.

Abstract: — The production of singlet oxygen by thiazine dye photosensitization, as measured by the rate of photooxidation of tryptophan, was found to be very sensitive to changes of pH in the range 5–9. For methylene blue in aerated solutions, the production of 1O2* is approximately five times more efficient in basic than in acidic medium. This was shown to be related to the pK's of the triplet dyes, by evaluating the yields of 1O2* from the lifetimes and the quenching rate constants for the two ionic species of sensitizer triplets measured by laser flash photolysis. Changes in the quenching rate constants of the thiazine triplet states can be correlated with the triplet energies.

Kinetic investigation of ground state carbon atoms, C(23PJ)

Abstract: A kinetic study of C(23PJ), generated by the repetitive pulsed irradiation of carbon suboxide in the Schumann region, is presented. The ground state carbon atoms were monitored photoelectrically by resonance absorption at λ≈ 166 nm in the time-resolved mode. The resulting photoelectric signals were translated into kinetic data by means of rapid response precision logarithmic circuitry, signal averaging and computerised analysis of the raw data. Rate data for the removal of C(23PJ) in the presence of the added reactant gases H2, O2, NO, N2O, CO2, H2O and C3O2 are reported and these are compared with those derived from other studies involving vacuum ultra-violet absorption spectroscopy, principally “single-shot” flash photolysis experiments involving oscilloscopic recording of attenuation of resonance radiation and plate photometry. The advantages of the present method, premitting detailed investigation of the atomic decays, are demonstrated.

Laser flash photolysis of aqueous tryptophan

Abstract: Laser flash photolysis of aqueous tryptophan at 265 nm generates the neutral radical product of indole ring N-H bond photolysis (lambda/sub max/ 510 nm, 1800 M/sup -1/ cm/sup -1/; 330 nm, 3100 M/sup -1/ cm/sup -1/), the triplet state (lambda/sub max/ 460 nm), and the hydrated electron as initial products. The photoionization quantum yields for tryptophan, 1-methyltryptophan, tyrosine, and glycine-tryptophan peptides have been determined using ferrocyanide as the reference system. The electron and aromatic radical decay by a previously unreported first-order back reaction of 0.80-..mu..sec half-time, leading to residual yields of the separated products after several microseconds significantly smaller than the photoionization yield. The kinetics results are consistent with the formation of a loose complex between the hydrated electron and the radical in which recombination competes with separation. The implications of this process for tryptophyl residue destruction in proteins are considered. A comparable first-order electron decay stage was observed with tryptophan derivatives, tyrosine, I/sup -/, and Fe(CN)/sub 6//sup 4 -/.

DETERMINATION OF THE pk VALUES OF THE LUMIFLAVIN TRIPLET STATE BY FLASH PHOTOLYSIS

Abstract: — The triplet-triplet absorption spectra in aqueous solution of the acid (3LfH2+), the neutral (3LfH) and the basic (3Lf-) forms of lumifiavin (6,7,9-trimethylisoalloxazine) were measured by flash photolysis. The pKa values of the corresponding protolytic equilibria of the lumifiavin triplet were found to be 4.45±0.1 and 9.8±0.15.

Rates of OH Radical Reactions. I. Reactions with H2, CH4, C2H6, and C3H8 at 295 K

Abstract: A fast flash photolysis kinetic spectrophotometer capable of measuring rates of up to 105 s−1 is described. The rates of hydrogen abstraction from H2, CH4, C2H6, and C3H8 by OH radicals at 295 ± 2 ...

Rhodopsin flash photolysis in man.

Abstract: 1. Human rhodopsin in vivo was flash bleached by a 600 musec xenon flash which could deliver to the retina up to 15 rod-equivalent quanta per rhodopsin molecule, and the fraction bleached measured by fundus reflexion densitometry. 2. The curve relating fraction rhodopsin bleached to intensity of flash saturates at 0.5 to 0.6. Thus, 40-50% of the rhodopsin is left photo-regenerated by the brightest flashes. 3. Three types of densitometry experiments confirm the saturation of the bleaching curve. 4. The kinetic constants required to account for the observed photo-regeneration were somewhat discrepant with in vitro and in situ estimates from infrahuman species. Specifically, (i) the quantum efficiency of the back reaction, metarhodopsin I hv leads to rhodopsin, was inferred to be nearly as high as that of the forward reaction; and (ii) the rate of the metarhodopsin I leads to metarhodopsin II dark reaction was inferred to be less than 500 sec(-1).

On the Kinetics of Polymer Degradation in Solution. II. Laser Flash Photolysis Studies on Poly(phenyl vinyl ketone) and Butyrophenone

Abstract: Using the nanosecond flash photolysis technique transient absorption spectra of poly(pheny1 vinyl ketone) (PPVK) and butyrophenone (BPI in benzene solution at room temperature were recorded. Samples were irradiated with 347.1-nm light flashes. With both substances the transient spectra decayed by a first-order process with k 1 = 1.0 f 0.2 X lo7 sec-'. In PPVK and BP solutions containing naphthalene or biphenyl the established triplet absorption of these compounds is sensitized. It is concluded that the observed transient absorptions are due to triplet states of PPVK and BP, respectively. The quencher studies yielded as an estimate a triplet quantum yield d3PPVK*) equal to 0.1 to 0.3. kq (bimolecular rate constant of energy transfer) is about two times smaller for bi- phenyl than for naphthalene. For BP kq is about two and a half times higher than for PPVK (in the case of naph- thalene: ~Q(BP) = 5.0 f 1.5 X lo9 M-' sec-' and kQ(PPVK) = 2 i 1 X lo9 M-' sec-'). This result reflects the in- fluence of the diffusion constant of the polymer on ks as expected for diffusion-controlled reaction and agrees with earlier findings'O concerning rate constants of the reaction of scavengers with macroradicals.

Nucleophilic Aromatic Photosubstitution

Abstract: Publisher Summary This chapter outlines the recent developments in the field of nucleophilic aromatic photosubstitution The chapter addresses fundamental questions and discusses the suggested lines of investigation, illustrating the pathways and the intermediates of nucleophilic aromatic photosubstitution, the mechanism of the reaction steps, and the factors governing product composition and quantum yields The photohydrolyses are often very clean reactions with good quantum yields The reaction may follow a different course with different nucleophiles Experiments with 18 O-labelling have shown that m- nitrophenyl phosphate, upon illumination, undergoes hydrolytic attack by water at the phosphorus atom, whereas with hydroxide ion and methylamine genuine aromatic substitution through attack at the ring carbon atom takes place The reaction is first order in light intensity and zero order in the light absorbing aromatic reaction partner The results obtained by conventional flash photolysis and with laser photolysis are discussed in the chapter

Photochemische Erzeugung und Reaktionen des Benzonitril‐benzylids. 42. Mitteilung über Photoreaktionen

Abstract: Photochemical Generation and Reactions of Benzonitrile-benzylide The low temperature irradiation of 2,3-diphenyl-2H-azirine (1) in DMBP-glass at −196° has been reinvestigated. It was possible to convert 1 nearly quantitatively into the dipolar species benzonitrile-benzylide (3, Φ3 = 0,78), which exhibits UV.-absorptions at 344 (ϵ = 48000) and 244 nm (ϵ = 28500) (Fig. 1, Tab. 1). Irradiation of 3 with 345 nm light at −196° resulted in almost complete reconversion to the azirine 1 (Φ = 0,15; Fig. 2). When the solution of 3 in the DMBP-glass was warmed up to about −160° a quantitative dimerization to 1,3,4, 6-tetraphenyl-2,5-diaza-1,3,5-hexatriene (8) occurred. This proves that 8 is not only formed by the indirect route 3 + 1 711 8 known before (Scheme 1), but also by dimerization of 3 either by direct head to head coupling or via the intermediate e (p. 2675), followed by a fast thermal hydrogen transfer reaction. The occurrence of the dipolar intermediate 11 in the photochemical conversion of the bicyclic compound 7 to 8 could also be demonstrated by low temperature experiments: On irradiation at −196° 7 gave the cherry red dipolar intermediate 11 (λmax = 520 nm), which at −120° isomerizes to 8. It should be noted, that neither 7 nor 11 are formed by dimerization reactions of 3. Experiments carried out at room temperature demonstrate, that both processes for the formation of 8 may compete: Irradiation of a solution of 1 (DMBP, c = 8 × 10−4 to 5 × 10−3M) with 350 nm light of high intensity (which does not excite the bicyclic compound 7) leads to a relative high photostationary concentration of the dipolar species 3. Under these conditions the formation of 8 is due to dimerization of 3 (Φ8 = 0,19). With low light intensity only a very low stationary concentration of 3 can be obtained. Therefore the reaction of 3 with 1, leading to the bicyclic intermediate 7, becomes now predominant (Φ−1 = 1,55, which corresponds with the expected value of 2 × 0,8). Irradiation of 1 at −130° with 350 nm light of high intensity gives 8 with a quantum yield of 0,44. This is in agreement with the theoretical value Φ8 = 0,4 for an exclusive formation of 8 by dimerization of 3. The lower quantum yield for the formation of 8 at room temperature makes probable that under these conditions 3 not only dimerizes to 8, but also to another, so far unidentified dimer, e.g. 2,3,5,6-Tetraphenyl-2,5-dihydropyrazine. By flash photolysis of a solution of 1 (cyclohexane, c = 10−4M, 25°) the disappearance of 3 could directly be measured by UV.-spectroscopy: At relative high concentrations (c ≥ 10−7M) 3 disappeared according to a second order reaction with the rate constant k = 5 × 107M−1S−1. At lower concentrations (c ≤ 10−7M) the rate of disappearance of 3 follows first order kinetics. The rate constant of this pseudo first order reaction (3 + 1 7) has been determined to be 1 104M−1S−1. Using Padwa's table of relative rates for the cycloaddition of the dipolar species 3 to various dipolarophiles, including the azirine 1, an absolute rate constant of k ≈ 8 × 108M−1S−1 for the addition of 3 to the most active dipolarophile fumaronitrile could be estimated. In cyclohexane at room temperature, the diffusion controlled rate constant equals 6,6 × 109M−1S−1. In Table 1 the UV.-maxima of several nitrile-ylides, among them a purely aliphatic one, are given.

Rate constants of the combination of methyl radicals with nitric oxide and oxygen

Abstract: Rate constants for the combination of methyl radicals with NO and O2 have been measured by flash photolysis of azomethane coupled with product analysis by gas chromatography. Values of the rate constants have been obtained over the pressure region from 50 to 700 torr with He, N2, and Ar as quenching molecules. The high-pressure limits were obtained through an RRKM model calculation and were found to be The rate constants were measured relative to the methyl combination reaction k1 with k1 = 9.5 × 10−11 cm3/molec · sec. The RRKM model suggests D0(CH3O2) = 32 ± 3 kcal/mole.

Photochemische Erzeugung und Reaktionen des Benzonitril-benzylids

Abstract: The low temperature irradiation of 2,3-diphenyl-2H-azirine (1) in DMBP-glass at -196° has been reinvestigated. It was possible to convert 1 nearly quantitatively into the dipolar species benzonitrile-benzylide (3, psi3 = 0,78), which exhibits UV.-absorptions at 344 (epsilon = 48000) and 244 nm (epsilon = 28500) (Fig. 1,Tab. 1). Irradiation of 3 with 345 nm light at -196° resulted in aImost complete reconversion to the azirine 1 (psi = 0,15; Fig. 2). When the solution of 3 in the DMBP-glass was warmed up to about -160° a quantitative dimerization to 1,3,4,6-tetraphenyl-2,5-diaza-1,3,5-hexatriene (8) occurred. This proves that 8 is not only formed by the indirect route 3 + 1 to 7 to 11to 8 known before (Scheme 1), but also by dimerization of 3 either by direct head to head coupling or via the intermediate e (p. 2675), followed by a fast thermal hydrogen transfer reaction. The occurrence of the dipolar intermediate 11 in the photochemical conversion of the bicyclic compound 7 to 8 could also be demonstrated by low temperature experiments: On irradiation at -196° 7 gave the cherry red dipolar intermediate 11 (lambda max = 520 nm), which at -120° isomerizes to 8. It should be noted, that neither 7 nor 11 are formed by dimerization reactions of 3. Experiments carried out at room temperature demonstrate, that both processes for the formation of 8 may compete: Irradiation of a solution of 1 (DMBP, c = 8x10-4 to 5x10-3 M) with 350 nm light of high intensity (which does not excite the bicyclic compound 7) leads to a relative high photostationary concentration of the dipolar species 3. Under these conditions the formation of 8 is due to dimerization of 3 (psi8 = 0,19). With low light intensity only a very low stationary concentration of 3 can be obtained. Therefore the reaction of 3 with 1, Ieading to the bicyclic intermediate 7, becomes now predominant (psi-1 = 1,55, which corresponds with the expected value of 2 x 0,8). Irradiation of 1 at - 130° with 350 nm light of high intensity gives 8 with a quantum yield of 0,44. This is in agreement with the theoretical vaIue psi8 = 0,4 for an exclusive formation of 8 by dimerization of 3. The lower quantum yield for the formation of 8 at room temperature makes probable that under these conditions 3 not only dimerizes to 8, but also to another, so far unidentified dimer, e.g. 2,3,5,6-tetraphenyl-2,5-dihydropyrazine. By flash photolysis of a solution of 1 (cyclohcxane, c = 10-4 M, 25°) the disappearance of 3 could directly be measured by UV.-spectroscopy: At relative high concentrations (c larger than 10-7 M) 3 disappeared according to a second order reaction with the rate constant k = 5 x 10-7 M-1 s-1. At lower concentrations (c < 10-7 M) the rate of disappearance of 3 follows first order kinetics. The rate constant of this pseudo first order reaction (3+1 to 7) has been determined to be 1 x 104 M-1 s-1. Using Padwa's table of relative rates for the cycloaddition of the dipolar species 3 to various dipolarophiles, including the azirine 1, an absolute rate constant of k ca. 8 x 108 M-1s-1 for the addition of 3 to the most active dipolarophile fumaronitrile could be estimated. In cyclohexane at room temperature, the diffusion controlled rate constant equals 6,6 x 109 M-1 s-1. In Table 1 the UV.-maxima of several nitrile-ylides, among them a purely aliphatic one, are given.

Mechanism and rate constant of the reaction between methylene and methyl radicals

Abstract: The chemistry of the reaction between methyl and triplet methylene radicals has been examined by means of flash photolysis of azomethane and ketene followed by gas chromatographic analysis of the hydrocarbon products. Using the combination rate constant of triplet CH2 and the combination of CH3 a value for the reaction 3CH2 + CH3 to C2H5* to C2H4 + H has been determined.

Photophysics of exciplexes: some kinetic aspects

Abstract: The characteristics of systems where fluorescence quenching does not follow simple kinetics are discussed and illustrated. The complicating effects discussed include ground-state complexing, transient effects in diffusion-controlled kinetics and feedback from an excited state complex or exciplex. Methods for differentiating these complicating effects are presented as well as methods for extracting rate parameters from systems where feedback is operating. When the interaction between an excited molecule and some other molecule results in the disappearance of the excited state, quenching is said to have occurred. When this phenomenon involves short-lived species (nanosecond or sub-nanosecond) then the kinetics of disappearance of the excited molecule are generally followed by fluorescence quenching if fluorescence is present, or by the sensitized fluorescence of the quencher, if generated. Laser flash photolysis is also used. In the typical fluorescent quenching experiment, one measures quantitatively the diminution of either the fluorescent intensity or the fluorescent lifetime of the fluorophor as a function of the quencher concentration. Ideally, both are measured. Frequently, one observes the same second-order rate constant for both the steady-state and transient measurements and is inclined to regard the quenching mechanism as simple. Not-so-simple systems may deviate from this norm in a number of subtle and fascinating ways which can yield insights into the kinetics, mechanism of quenching, and to some extent the nature of the interactions involved. Of special interest is the case where fluorescent quenching is accompanied by spectral changes in the absorption and/or emission spectrum of the system. Unfortunately, the study of these three aspects, namely the spectroscopic behaviour of the system, the transient kinetics of fluorescence quenching and the steady-state kinetics, is seldom carried out in the course of an investigation of a fluorescence quenching mechanism. It is the purpose of this paper to illustrate some of the departures from simple t Contribution No. 107 from the Photochemistry Unit, Depanment of Chemistry, University of Western Ontario, London, Canada.

Nanosecond flash photolysis of rhodopsin.

Abstract: THIS note presents results from a neodymium laser flash spectroscopy study of bovine rod outer segments (ROS), the only chromophore of which is rhodopsin. Previously unreported transients are interpreted in terms of a charge-transfer excited state which gives rise to radicals and/or triplets.

Vacuum ultra-violet photodissociation of alkyl iodides. Kinetics of the reaction H + ICH3→ HI + CH3

Abstract: The vacuum u.v. photodissociation of alkyl iodides has been investigated using flash photolysis techniques. At wavelengths between 140 and 170 nm, the primary process is selective and results in C—H rather than C—I scission: it is also sensitive to deuterium isotope substitution. At wavelengths > 170 nm, the quantum yield of C—H scission is 170 nm, has been observed. These, and related observations, can be understood if, in photodissociation or bimolecular iodine atom transfer, the methyl radical attains a near planar conformation before recoiling from the neighbouring iodine atom.

Light‐induced fluorescence of short‐lived species produced by a pulse of radiation: The benzophenone ketyl radical

Abstract: The optical absorption spectra, kinetic behavior and other properties of numerous short‐lived species have been studied by flash photolysis and pulse radiolysis. By a simple modification of the technique we have now been able to measure the emission produced when a short‐lived species, in this case the benzophenone ketyl radical, is itself excited by absorption of light, a property which is normally observable only when transient species are stabilized in a rigid medium. (AIP)