Showing papers on "Flash photolysis published in 1994"

Artificial Photosynthesis. 2. Investigations on the Mechanism of Photosensitization of Nanocrystalline TiO2 Solar Cells by Chlorophyll Derivatives

Abstract: The mechanism of photosensitization of colloidal TiO 2 electrodes with chlorin e 6 and copper chlorophyllin is deduced from static and time-resolved fluorescence quenching as well as laser flash photolysis and photocurrent/ voltage transients in combination with cyclic voltammetry and spectroelectrochemistry. The fluorescence of chlorin e 6 on TiO 2 decays to 80% within 0.4 ns, indicating efficient electron injection from the singlet excited state with k et =2.2×10 9 s -1

Temperature dependence of fluorescence and photoisomerization in symmetric carbocyanines. Influence of medium viscosity and molecular structure

Abstract: The temperature dependence of fluorescence emission and photoisomerization of five symmetric carbocyanines was studied in a group of primary n-alcohols from 0 to 70 o C. The trans-cis isomerization on the excited state potential surface was studied by steady-state fluorescence emission and flash photolysis. The back isomerization rate constant on the ground-state potential surface from the photoisomer to the normal form was also studied by flash photolysis. In all cases fluorescence quantum yields were found to diminish with temperature and to increase with viscosity (η), while isomerization quantum yields showed the opposite behavior

Inclusion Complex of .gamma.-Cyclodextrin-C60: Formation, Characterization, and Photophysical Properties in Aqueous Solutions

Abstract: A simple method of preparing water-soluble C[sub 60] at room temperature has been developed by complexing it with [gamma]-cyclodextrin. By this method concentrations as high as 1.0 x 10[sup [minus]4] mol dm[sup [minus]3] of C[sub 60] can be solubilized in water. The complex is characterized by thermogravimetric, optical absorption, and powder X-ray diffraction analysis. The optical absorption spectrum, extinction coefficient, and equilibrium constant for a 2:1 (CD:C[sub 60]) complex of C[sub 60] with [gamma]-cyclodextrin are reported. With the help of picosecond and nanosecond laser photolysis techniques, the triplet characteristics of the CD-C[sub 60] complex are determined. 21 refs., 5 figs., 1 tab.

Photochemical generation of radical cations from thiophene oligomers

Abstract: Thc photoexcitation process for a series of thiophene oligomers nT, from terthiophene to sexithiophene, has been analyzed in dichloromethane solution using laser flash photolysis. The immediately formed excited triplet states 3 nT * , which have been characterized by their visible absorption spectra, show a lifetime on the order of a few tens of microseconds. The corresponding radical cations nT .+ have been obtained either by electron transfer from 3 nT * to an electron acceptor or by direct photogeneration from nT using high excitation energy

Absolute rate constants for some reactions of perfluoro-n-alkyl radicals in solution

Abstract: Absolute rate constants have been measured at 298±2 K for some reactions of C 2 F 5 . , n-C 3 F 7 . , n-C 7 F 15 . and n-C 8 F 17 . by time-resolved (laser flash photolysis) and competition kinetics. The last three of these radicals exhibit essentially equal reactivities, but C 2 F 5 . is somewhat more reactive

Kinetics and Mechanism of the CIO + CIO Reaction: Pressure and Temperature Dependences of the Bimolecular and Termolecular Channels andThermal Decomposition of Chlorine Peroxide, CIOOCI

Abstract: The kinetics and mechanism of the CIO + CIO reaction and the thermal decomposition of CIOOCI were studied using the flash photolysis/long path ultraviolet absorption technique. Pressure and temperature dependences were determined for the rate coefficients for the bimolecular and termolecular reaction channels, and for the thermal decompositon of CIOOCI.

Photoinduced Electron Transfer between C60 and Polysilane Studied by Laser Flash Photolysis in the Near-IR Region

Abstract: The photoinduced electron transfer between C[sub 60] and a [sigma]-conjugated polysilane was investigated by laser flash photolysis in the near-IR region. The transient absorption spectra of C[sub 60] triplet ([sup 3]C[sub 60]*), C[sub 60] radical anion, and the poly(methylphenylsilylene) radical cation were observed in the region 600-1600 nm in a polar solvent. The quenching rate constant k[sub q] of [sup 3]C[sub 60]* corresponded well with the formation rate constant k[sub f] of the poly(methylphenylsilylene) radical cation. Direct electron transfer from poly(methylphenylsilylene) to [sup 3]C[sub 60]* is suggested. The electron-transfer rate constants between C[sub 60] and poly(methylphenylsilylene) showed the polar solvent effect and the local concentration effect of the polymer chain. 33 refs., 9 figs.

Interfacial Electron Transfer in Colloidal SnO2 Hydrosols Photosensitized by Electrostatically and Covalently Attached Ruthenium(II) Polypyridine Complexes

Abstract: The complex [(H 2 Nphen)Ru(bpy) 2 ] 2+ (H 2 Nphen=5-amino-1,10-phenanthroline; bpy≡2,2'-bipyridine) was covalently attached to the surfaces of colloidal SnO 2 particles (ca. 4-nm diameter) by using cyanuric chloride and (3-aminopropyl)triethoxysilane as coupling agents. Electron injection from the MLCT excited state of the complex, * RuL 3 2+ , into the conduction band of SnO 2 and recombination of RuL 3 3+ with the conduction band electron, e cb - (SnO 2 ), were investigated by nanosecond laser flash photolysis (532-nm excitation) and steady-state luminescence techniques

Photochemistry of textile azo dyes. spectral characterization of excited state, reduced and oxidized forms of acid orange 7

Abstract: The spectral characterization of the singlet and triplet excited states of an azo dye (Acid Orange 7 (AO)) was performed using picosecond and nanosecond laser flash photolysis. The excited singlet state lifetime, estimated from the bleaching recovery, is around 135 ps. The triplet excited state generated by triplet—triplet (TT) energy transfer shows a difference absorption maximum at 650 nm with a lifetime of μs. The oxidized and reduced forms of the dye were generated by reacting AO with pulse radiolytically generated azide (N3) and eaq radicals. These radicals were also detected as the primary intermediates in visible-laser-induced photoionization. The photoelectrochemical reduction of the dye in UV-irradiated TiO2 colloidal suspension results in the irreversible decolorization of the dye.

Spin—orbit-coupling-induced triplet formation of triphenylpyrylium ion: a flash photolysis study

Abstract: The excited singlet of the 2,4,6-triphenylpyrylium ion (TPP + ) reacts with iodobenzene (PhI) by electron transfer leading to radicals and the triplet state of TPP + . The induced formation of 3 TPP + by spin—orbit coupling has been observed. An encounter complex intermediate is proposed for the quenching of 3 TPP +* by PhI. Homogeneous recombination of radicals is also observed in the millisecond time domain. Flash photolysis has been used to study the excited state reactions of TPP + .

Internal electron transfer in cytochrome c oxidase is coupled to the protonation of a group close to the bimetallic site.

Abstract: Absorbance changes following CO dissociation by flash photolysis from mixed-valence cytochrome oxidase have been followed in the Soret and alpha regions. Apart from CO dissociation and recombination, three kinetic phases with rate constants in the range 10(5)-10(3) s-1 at pH 7.5 can be resolved in both spectral regions. The slowest one of these phases, which had earlier only been observed in the alpha region, has now been detected in the Soret region by the use of a low CO concentration to slow down the recombination reaction. This phase had been assigned to a structural change, but a kinetic difference spectrum demonstrates that it represents electron transfer from cytochrome a3 to cytochrome a. A kinetic deuterium isotope effect of 2-3 at pH 7.5 suggests that it involves proton transfer as well. The temperature dependence of the reaction gives an Arrhenius activation energy of 42 kJ.mol-1. The reaction is faster at low pH, and the equilibrium is shifted toward cytochrome a as the pH is raised. The rate and equilibrium changes can be described as involving acid-base groups with pKa values of approximately 7.7 and 8.7, respectively. The kinetic results can be simulated on the basis of a model in which one acid-base group interacts with cytochrome a3, so that its pKa drops on oxidation of this center. The group is in proton equilibrium with the solvent via a proton pathway, suggested to be a proton channel. The rate of a shift in the redox equilibrium between the two cytochromes reaches a high limit at low pH, where the channel is saturated with protons.(ABSTRACT TRUNCATED AT 250 WORDS)

Laser Flash Photolysis Study of the Photoinitiator System Safranine T-Aliphatic Amines for Vinyl Polymerization

Abstract: The polymerization of 2-hydroxyethyl methacrylate, in a methanol solution, proceeds efficiently using the photoinitiator safranine T in the presence of tertiary aliphatic amines. Polymerization rates were measured at several amine concentrations. These values increase with the amine concentration, reaching a maximum, and further amine addition slowly reduces the polymerization rate. The quenching of the dye excited states by amines was analyzed by static fluorescence and laser flash photolysis. The excited singlet quenching involves a charge-transfer process with rate constants near the diffusional limit. Flash photolysis studies of safranine in the presence of amines, in a methanol solution, yield the triplet species 3 SH + and 3 S + and the unprotonated semireduced radical

Charge-Transfer Excitation of Electron Donor-Acceptor Complexes in Zeolite Matrixes: Direct Observation of Radical ion Pairs from Picoseconds to Milliseconds by Diffuse-Reflectance Laser Flash Photolysis

Abstract: Laser excitation of charge-transfer complexes between pyridinium-analogous acceptors and arene donors incorporated into zeolite-Y and zeolite-L generates transient radical ion pairs that can be monitored using time-resolved diffuse-reflectance spectroscopy techniques. At least three types of radical ion pairs have been distinguished kinetically with lifetimes from picoseconds to milliseconds, depending on donor and acceptor strength and on the zeolite type. As cause for the complex decay kinetics, the interactions between the radical ion pairs and the zeolite framework leading to spatial separation are discussed. Furthermore, the unique effects of coadsorbed water are described

Reaction of chlorine atoms with methylperoxy and ethylperoxy radicals

Abstract: Flash photolysis combined with time-resolved UV spectroscopy and transient IR absorption measurements shows that chlorine atoms react rapidly with CH 3 O 2 and C 2 H 5 O 2 , producing ClO and CO among other products. The room temperature rate constants are k 2 =(1.5±0.2)×10 -10 cm 3 s -1 for both reactions. The distribution of products is interpreted to indicate that the reaction with methylperoxy radicals proceeds via two channels to yield CH 3 O+ClO with a branching ratio of 0.51±0.05 and CH 2 O 2 +HCl with a branching ratio of 0.49 ±0.05

A Kinetic and Mechanistic Study of the Self-Reaction and Reaction with HO2 of the Benzylperoxy Radical

Abstract: The kinetics and mechanism of the reactions C 6 H 5 CH 2 O 2 +C 6 H 5 CH 2 O 2 →2C 6 H 5 CH 2 O+O 2 (3a), C 6 H 5 -CH 2 O 2 +C 6 H 5 CH 2 O 2 →C 6 H 5 CHO+C 6 H 5 CH 2 OH+O 2 (3b), and C 6 H 5 CH 2 O 2 +HO 2 →C 6 H 5 CH 2 OOH+O 2 (4) have been investigated using two complementary techniques: flash photolysis/UV absorption for kinetic measurements and continuous photolysis/FTIR spectroscopy for end-product analyses and branching ratio determinations. The reaction of chlorine atoms with toluene was found to yield benzyl radicals exclusively and was used to generate benzylperoxy radicals in excess oxygen

Elementary reactions of NH(a1Δ) and NH(X3Σ) with N, O and NO

Abstract: The elementary reactions of NH(a1Δ) with N, O, and NO and the reactions of NH(X3Σ−) with N, O and NO were studied in a quasi static laser flash photolysis cell at room temperature (T=298T) and low pressures (15≤p/mbar≤20). NH(a) was produced by HN3 laser photolysis at λ = 248 nm. NH(X) was obtained from the fast quenching of NH(a) with Xe. The O atom source was the NO + N reaction ([N]0>>[NO]0), with N atoms formed in a microwave discharge of N2/He. NH(a), NO(X) and the primary products NH(X), OH(X) and NO (X) were detected by pulsed laser induced fluorescence (LIF). The absolute N atom concentrations were determined via the reaction of N with NO. Time resolution was achieved by time delay between photolysis and probe laser pulses. For overall depletion: the following rate constants were obtained: k1, = (6.0±1.5)·1011 cm3/(mol·s); k2 = (5.6±2)·1012cm3/(mol·s); and k3 = (1.8±0.2)·1013 cm3(mol·s). By fitting the NH(X) concentration profiles, determined experimentally, with computer simulations, the following rate constants for quenching channels were obtained: k1q = (6.0±1.2)·1011 cm3/(mol·s) and k2q = (1.2±0.3)·1012cm3/(mol·s). For the reaction pathways of reaction (2):

Solvent effect on the decarbonylation of acyl radicals studied by laser flash photolysis

Abstract: Absorption spectra of transient radicals generated from dibenzyl and di-tert-butyl ketones and rate constants for their fragmentation and termination reactions were obtained by laser flash photolysis in various solvents. Whereas the pivaloyl radical absorbs only for λ⩽ 240 nm, the first absorption band of phenylacetyl is red shifted which is ascribed to a phenyl–carbonyl conjugation. The rate constant for the decarbonylation of the acyl radicals decreases markedly with increasing solvent polarity. This is explained by the dipole moment which decreases during the scission process.

4-Carboxybenzophenone-sensitized photooxidation of sulfur-containing amino acids in alkaline aqueous solutions. Secondary photoreactions kinetics

Abstract: Sulfur-containing amino acids and alanine were oxidized via photosensitization by 4-carboxybenzophenone (CB) in alkaline aqueous solutions. The mechanism of this reaction was examined using steady-state and laser flash photolysis techniques. The rate constants were determined for the quenching of the CB triplet state by five sulfur-containing amino acids and alanine and were found to be ∼10 9 and 1.8×10 8 M 8 s -1 , respectively. The observation of the (S..S) + radical cations of some of the amino acids showed that the quenching process involves an electron transfer from the sulfur atom to the triplet state of CB

Unusually long lifetimes of the singlet nitrenes derived from 4-azido-2,3,5,6-tetrafluorobenzamides

Abstract: Laser flash photolysis (308 nm, 20 ns, 150 mJ) of 4-azido-2,3,5,6-tetrafluorobenzamides, esters, and thioesters generates singlet nitrenes which can be intercepted with pyridine to produce strongly absorbing ylides. It was possible to resolve the rate of formation of the ylides as a function of pyridine concentration. This has led to direct measurements of the absolute rate constants of (a) the reaction of the nitrene with pyridine, (b) ring expansion of the nitrene to a ketenimine, and (c) singlet to triplet nitrene intersystem crossing

Photoinduced electron transfer from the triplet state of zinc cytochrome c to ferricytochrome b5 is gated by configurational fluctuations of the diprotein complex.

Abstract: The intracomplex electron-transfer reaction 3Zncyt/b5(III)-->Zncyt+/b5(II) within electrostatic and covalent complexes of zinc(II) cytochrome c and ferricytochrome b5 is studied by laser flash photolysis. Kinetic effects of protein cross-linking and of solution viscosity are interpreted in terms of dynamic mobility of the associated proteins with respect to each other. The rate constant for the monoexponential reaction in the electrostatic complex is 3.5 x 10(5) s-1 in aqueous solution; this value is independent of protein concentration and ionic strength, but it decreases markedly as viscosity is raised by addition of glycerol or sucrose. The multiexponential reaction in the covalent complex was analyzed also in terms of the stretched exponential, exp[-(kt)n]. The best fit requires k = 6.8 x 10(4) s-1 and n = 0.56 in aqueous solution; this rate constant is independent of protein concentration and ionic strength, but it decreases slightly as viscosity is raised. Fitting of the viscosity dependence to a simple two-state kinetic model yields a rate constant of 3.0 x 10(5) s-1 for rearrangement of the electrostatic complex 3Zncyt/b5(III) from the initial docking configuration to a different, more reactive, configuration. The corresponding rate constant for rearrangement of the electrostatic complex 3Zncyt/pc(II) containing plastocyanin, determined previously, is 2.5 x 10(5) s-1. Evidently, the intracomplex reaction in both electrostatic complexes is gated by a rearrangement process.(ABSTRACT TRUNCATED AT 250 WORDS)

Study of the kinetics and equilibrium of the benzyl‐radical association reaction with molecular oxygen

Abstract: The forward rate constant, k1, and the equilibrium constant, Kp, for the association reaction of the benzyl radical with oxygen have been determined. The rate constant k1 was measured as a function of temperature (between 298 and 398 K) and pressure (at 20 and 760 torr of N2) by two different techniques, argon-lamp flash photolysis and excimer-laser flash photolysis, both of which employed UV absorption spectroscopy (at 253 nm and 305 nm, respectively) to monitor the benzyl radical concentration. Over the range of conditions studied, we find that the reaction is independent of pressure and is almost independent of temperature, which is in accord with two early studies of the reaction but in apparent disagreement with more recent work. For our results in 760 torr of N2 and for 298 < T/K < 398, a linear least-squares fitting of the data yield the expression: k1 = (7.6 ± 2.4) × 10−13 exp[(190 ± 160)K/T] cm3 molecule−1 s−1. With the flash-photolysis technique, we determined Kp over the temperature range 398–525 K. Experimental values were analyzed alone and combined with theoretically determined entropy values of the benzyl and benzylperoxy radicals to determine the enthalpy of reaction: ΔH = (−91.4 ± 4) kJ mol−1. Previous work on the benzyl radical enthalpy of formation allows us to calculate ΔH°f 298 (Benzylperoxy) = (117 ± 6) kJ mol−1. In addition, we carried out an RRKM calculation of k1 using as constraints the thermodynamic information gained by the study of Kp. We find that all the studies of the association reaction are in good agreement once a fall-off effect is taken into account for the most recent work conducted at pressures near 1 torr of helium. © 1994 John Wiley & Sons, Inc.

Merocyanine dyes: effect of structural modifications on photophysical properties and biological activity.

Abstract: Merocyanine derivatives were prepared by structural alterations at the barbituric acid or chalcogenazole moieties. The photophysical properties of the dyes were markedly influenced by the presence of selenium rather than sulfur as a substituent at position 2 of the barbiturate. In methanol, quantum yields of both triplet state (phi T) and singlet oxygen sensitization (phi delta) were increased by over an order of magnitude, with a concomitant decrease in fluorescence, when selenium was present in the molecule. Photoisomerization, one of the dominant deactivation pathways in the sulfur- or oxygen-containing analogues, was completely absent in the selenium-containing derivatives. Efficient triplet state formation was observed for selenium-containing derivatives incorporated into L1210 cells by diffuse reflectance laser flash photolysis. Cytotoxicity studies, carried out using clonogenic assays on L1210 leukemia cells, showed a good correlation with phi T and phi delta, measured in solution. Experimental evidence provided by this paper supports a triplet state-, and probably singlet oxygen-, mediated phototoxic mechanism. Photoisomerization or singlet state mechanisms can be discounted.