Showing papers on "Flash photolysis published in 1996"

Environmental Photochemistry on Semiconductor Surfaces: Photosensitized Degradation of a Textile Azo Dye, Acid Orange 7, on TiO2 Particles Using Visible Light

Abstract: Photosensitized degradation of a textile azo dye, Acid Orange 7, has been carried out on TiO2 particles using visible light Mechanistic details of the dye degradation have been elucidated using diffuse reflectance absorption and FTIR techniques Degradation does not occur on Al2O3 surface or in the absence of oxygen The dependence of the dye degradation rate on the surface coverage shows the participation of excited dye and TiO2 semiconductor in the surface photochemical process Diffuse reflectance laser flash photolysis confirms the charge injection from the excited dye molecule into the conduction band of the semiconductor as the primary mechanism for producing oxidized dye radical The surface-adsorbed oxygen plays an important role in scavenging photogenerated electrons, thus preventing the recombination between the oxidized dye radical and the photoinjected electrons Diffuse reflectance FTIR was used to make a tentative identification of reaction intermediates and end products of dye degradation

Environmental Photochemistry on Semiconductor Surfaces. Visible Light Induced Degradation of a Textile Diazo Dye, Naphthol Blue Black, on TiO2 Nanoparticles

Abstract: Visible light induced degradation of the textile diazo dye Naphthol Blue Black (NBB) has been carried out on TiO2 semiconductor nanoparticles. Diffuse reflectance transient absorption and FTIR techniques have been used to elucidate the mechanistic details of the dye degradation. The failure of the dye to degrade on insulator surfaces such as Al2O3 or in the absence of oxygen further highlights the importance of semiconducting properties of support material in controlling the surface photochemical processes. The primary event following visible light excitation is the charge injection from the excited dye molecule into the conduction band of the semiconductor TiO2, producing the dye cation radical. This was confirmed by diffuse reflectance laser flash photolysis. The surface-adsorbed oxygen plays an important role in scavenging photogenerated electrons, thus preventing the recombination between the dye cation radical and photoinjected electrons. Diffuse reflectance FTIR study facilitated identification of r...

Photosensitization of Nanocrystalline Semiconductor Films. Modulation of Electron Transfer between Excited Ruthenium Complex and SnO2 Nanocrystallites with an Externally Applied Bias

Abstract: In situ spectroelectrochemical measurements have been carried out to probe the charge injection from excited Ru(bpy)2(dcbpy)2+, Ru(II), into the SnO2 nanocrystallites. The dependence of luminescence yield and lifetime at various applied potentials suggests that the heterogeneous electron transfer from excited sensitizer into the semiconductor can be controlled by the externally applied electrochemical bias. The maximum quenching is seen at positive potentials while an increase in the luminescence yield and lifetime is seen at negative potentials. Laser flash photolysis of Ru(II)-modified SnO2 nanocrystalline film has been carried out to record the transient absorption spectra at different applied potentials. The yield of electron transfer product, Ru(III), decreases as the applied bias is switched to negative potentials. At an applied bias of −0.7 V the only observable transient is the excited Ru(II) complex (Ru(II)*). The maximum apparent electron transfer rate constant, ket (∼4 × 108 s-1), observed at p...

Time-resolved epr study of photoexcited triplet-state formation in electron-donor-substituted acridinium ions

Abstract: The 10-methylacridinium ion displays an emission associated with a charge-shifted (CSH) species when substituted in the 9-position with a substituent having a relatively low ionization potential (naphthyl, biphenyl). Flash photolysis and time-resolved EPR (TREPR) measurements show that photoexcitation of these donor−acceptor systems generates an acridinium-localized excited (LE) triplet state. While values of zero-field splitting parameters are virtually unaffected by the nature of the substituent, spin polarization patterns observed in the TREPR spectra display a striking dependence on substituent as well as orientation of donor ring system. Flash photolysis and TREPR data show that the LE triplet state is formed from the CSH singlet state. In these directly linked donor−acceptor molecules, in which the aromatic rings are near perpendicular because of steric hindrance, CSH singlet → LE triplet intersystem crossing (isc) is driven by spin−orbit coupling. This mechanism generates a unique dependence of isc...

Dye-Capped Semiconductor Nanoclusters. Excited State and Photosensitization Aspects of Rhodamine 6G H-Aggregates Bound to SiO2 and SnO2 Colloids

Abstract: SiO2 and SnO2 colloids are capped with a cationic dye, rhodamine 6G, by electrostatic interaction. The close packing of these dye molecules on the negatively charged SiO2 or SnO2 colloid results in the formation of H-aggregates. These aggregates are nonfluorescent but can inject electrons from the excited state into SnO2 colloids. The photophysical and photochemical properties of rhodamine-6G-aggregate on SiO2 and SnO2 colloids have been investigated using picosecond laser flash photolysis. Charge injection from the excited dye aggregate into SnO2 nanocrystallites occurs with a rate constant of 5.5 × 109 s-1. The application of these dye aggregates in extending the photoresponse of nanocrystalline SnO2 film has been demonstrated by constructing a photoelectrochemical cell. A maximum incident photon-to-photocurrent efficiency of ∼1% was observed for the photosensitized current generation. Fast reverse electron transfer between the injected electron and the cation radical of the dye aggregate is a limiting ...

Photoinduced Hydrogen Abstraction from Phenols by Aromatic Ketones. A New Mechanism for Hydrogen Abstraction by Carbonyl n,π* and π,π* Triplets

Abstract: Nanosecond laser flash photolysis studies have been carried out of the kinetics of inter- and intramolecular phenolic hydrogen abstraction by alkoxyacetophenone, 5-alkoxyindanone, and 4-alkoxybenzophenone triplets in acetonitrile and benzene solution. Information on the geometric requirements for abstraction by carbonyl n,π* and π,π* triplets is derived from the results for a series of ketones which contain a para-phenolic moiety attached via a para-oxyethyl linkage. For all of these compounds, the deuterium kinetic isotope effect on the triplet lifetime in acetonitrile solution indicates that triplet decay is determined by the rate of intramolecular abstraction of the remote phenolic hydrogen, which yields the corresponding phenoxyl−hemipinacol biradical. The biradicals have also been detected, and are about an order of magnitude longer-lived than the triplet in each case. For three of the compounds, the rates of the intramolecular process follow the same trend as that observed in the rates of bimolecula...

Acid−Base Properties of Arylnitrenium Ions

Abstract: This study uses a combination of laser flash photolysis (LFP) and product analysis to show that singlet nitrenes from the irradiation of phenyl, 4-biphenylyl, and 2-fluorenyl azide can be trapped by protonation in aqueous solutions forming nitrenium ions. With phenyl azide, the phenylnitrenium ion is indicated by the formation of ring-substituted anilines in yields of up to 50% in 1 M acids. The acidity dependence furnishes the ratio kH:kexp = 1.1, where kH refers to H+-trapping of singlet phenylnitrene and kexp to ring expansion of this species. With kH expected to be 2−4 × 1010 M-1 s-1, kexp is therefore estimated as 2−4 × 1010 s-1. Protonation by solvent water also occurs, but even though the rate constant is of the order of 109 s-1, it constitutes a minor pathway in competition with the ring expansion. LFP studies in acids reveal a transient that is assigned the structure of N-protonated 4-hydroxy-2,5-cyclohexadienone imine, the intermediate formed by water addition to the para position of the phenyln...

Kinetic Solvent Effects on Hydrogen Atom Abstraction from Phenol, Aniline, and Diphenylamine. The Importance of Hydrogen Bonding on Their Radical-Trapping (Antioxidant) Activities1

Abstract: Absolute rate constants for hydrogen atom abstraction by alkoxyl radicals from phenol, aniline, and diphenylamine have been measured in 14 solvents at room temperature by laser flash photolysis. For all three substrates the rate constants decline as the solvent becomes a stronger hydrogen-bond acceptor (HBA). Thus, on changing the solvent from CCl4 to CH3CN the rate constants decline by factors of 148, 7.1, and 4.7 for PhOH, PhNH2, and Ph2NH, respectively. The kinetic solvent effect for phenol correlates rather well with Abraham's scale of relative HBA activities of the solvents we have employed as measured as solutes in CCl4 solvent. This correlation is not quite so good with aniline and it is almost nonexistent for diphenylamine. With all three substrates the “deviant” solvents produce higher rate constants than would be expected from the value of the solvent and, generally, these are the solvents in which steric hindrance to hydrogen-bond formation would appear probable. The kinetic data for the three ...

Alcohol Effect on Equilibrium Constants and Dissociation Dynamics of Xanthone−Cyclodextrin Complexes

Abstract: The effect of alcohol addition on the ground state complexation of xanthone with cyclodextrins (CDs) and on the dissociation rate constants of triplet xanthone from these complexes was studied by fluorescence and laser flash photolysis experiments. In the case of β- and Hp-β-CD, the addition of alcohol led to the formation of weaker ternary complexes when compared to the xanthone CD binary complexes. In contrast, for γ-CD a slight increase of the complexation strength was observed for the ternary complexes. Addition of alcohols decreased the dissociation rate constant of triplet xanthone from β- and γ-CD by at least a factor of 5. The fact that the dissociation processes was slowed down for both CDs suggests that the effect of ternary complexation agents on the dynamics of complexation was not related to the strength of the ternary complexes formed.

Photochemical investigations of functionalised 1,4,7,10-tetraazacyclododecane ligands incorporating naphthyl chromophores

Abstract: The fluorescence properties of two macrocyclic ligands incorporating bis-(β-naphthyl amide) groups, –CH2CON(CH2C10H7)2, as fluorophores, have been studied in water and acetonitrile. The ligands exhibit distinctive luminescence behaviour in the presence of quenching (e.g. Pb2+, Ni2+, Cu2+ and Eu3+) and non-quenching ions (e.g. Zn2+ and Cd2+). Excimer-to-monomer intensity ratios are affected to differing extents, as are the total integrated fluorescence intensities. The behaviour is contrasted to that reported previously for related ligands containing ‘isolated’ naphthyl units. The luminescence properties of the terbium complex of one of these earlier ligands, containing just one naphthalene group, have been studied in detail. Naphthyl-sensitised metal emission is observed but a nanosecond laser flash photolysis study reveals that back energy transfer to the naphthyl triplet state provides a facile deactivation mechanism for the metal excited state.

9,10-Dehydroanthracene: p-Benzyne-Type Biradicals Abstract Hydrogen Unusually Slowly

Abstract: The 9,10-dehydroanthracene biradical, a model for the p-benzyne-type biradicals implicated in DNA cleavage by the enediyne antitumor antibiotics, was prepared by photodissociation of a propellane in solution. Trapping products characteristic of biradicals, e.g. anthracene-d2, are found. The rates of hydrogen abstraction by the biradical from acetonitrile and isopropyl alcohol are measured directly by flash photolysis/transient absorption spectroscopy, giving second-order rate constants of kMeCN,abstr = (1.1 ± 0.2) × 103 M-1 s-1 and ki-PrOH,abstr = (6.5 ± 0.6) × 103 M-1 s-1 at room temperature, which are 100−200 times lower than the corresponding rate constants for phenyl or 9-anthryl radical. A second decay route for the biradical is found, and assigned, based on thermochemical, kinetic, and trapping arguments, to a retro-Bergman reaction that converts the 9,10-dehydroanthracene biradical into the ring-opened 3,4-benzocyclodeca-3,7,9-triene-1,5-diyne. Although the retro-Bergman reaction is relatively fast...

Laser Flash Photolysis and Time-Resolved ESR Study of Phosphinoyl Radical † Structure and Reactivity §

Abstract: The photochemistry of two phosphine oxides and the rate constants of reaction of their daughter radicals with several alkenes, halocarbons, and oxygen have been determined. Photolysis of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (1) and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (4) in each case affords a phosphinoyl and a benzoyl radical. The phosphinoyl radicals are readily detected by laser flash photolysis and exhibit absorption maxima at 325 and 450 nm for the diphenylphosphinoyl (3) and 2,6-dimethoxybenzoyl-2,4,4-trimethylpentylphosphinoyl (6) radicals, respectively. The rate constants for reaction of the phosphinoyl radicals with alkyl halides, alkenes, and oxygen range from 104 to 109 M-1 s-1. Radical 3 is 2−6 times more reactive than radical 6. For example, 3 adds to methyl methacrylate with a rate constant of (11 ± 2) × 107 M-1 s-1 whereas 6 has an addition rate constant for the same reaction of (2.3 ± 0.3) × 107 M-1 s-1. The rate constants for reaction with alkyl halid...

C60 as a Photocatalyst of Electron-Transfer Processes: Reactions of Triplet C60 with Chloranil, Perylene, and Tritolylamine Studied by Flash Photolysis and FT-EPR

Abstract: Photoprocesses in benzonitrile solutions of C60 and chloranil (CA) have been studied by complementary techniques of nanosecond laser photolysis and Fourier transform EPR Direct oxidation of 3C60 by CA is slow (k = (20 ± 03) × 107 M-1 s-1), consistent with the high oxidation potential of 3C60 However, the formation rate and yield of CA- are much increased by addition of perylene (Pe) or tritolylamine (TTA) via the fast reactions 3C60 + Pe → C60 + 3Pe, followed by 3Pe + CA → Pe+ + CA-, or 3C60 + TTA → C60- + TTA+, followed by C60- + CA → C60 + CA- These reactions utilize the broad absorption and initial high triplet yield of C60, as well as the low oxidation potential of 3Pe or high reduction potential of 3C60, to catalyze efficient formation of CA- and enhance separation of radicals Triplet C60 also reacts with Pe by electron transfer, forming Pe+ and C60- with rate one-third that of energy transfer However, the CA- formed in the Pe-catalyzed reaction is strongly spin-polarized, indicating that it i

Kinetics and Mechanisms of the Self-Reactions of CCl3O2 and CHCl2O2 Radicals and Their Reactions with HO2

Abstract: The kinetics and mechanism of the reactions CCl3O2 + CCl3O2 → 2CCl3O + O2 (1), CHCl2O2 + CHCl2O2 → 2CHCl2O + O2 (2a), CHCl2O2 + CHCl2O2 → CHCl2OH + CCl2O + O2 (2b), CCl3O2 + HO2 → products (3), and CHCl2O2 + HO2 → products (4) have been investigated as a function of temperature at total pressures of 700−760 Torr. Two complementary techniques were used: flash photolysis/UV absorption for kinetic measurements and continuous photolysis/FTIR spectroscopy for end-product analyses. The UV absorption spectra of CHCl2O2 and CCl3O2 were determined between 220 and 280 nm; they have shapes similar to those of other alkyl peroxy radicals, but with broader and less intense bands. The rate constant k1 was determined between 273 and 460 K from the formation rate of CCl2O in the Cl atom initiated oxidation chain of chloroform, where reaction 1 was the rate-limiting step; k1 = (3.3 ± 0.6) × 10-13 exp[(745 ± 58)K/T] cm3 molecule-1 s-1, where quoted (1σ) errors represent only statistical uncertainties. Reaction 2 proceeds ...

Photoinduced oxidation of microperoxidase-8 : generation of ferryl and cation-radical porphyrins

Abstract: The electron-transfer reactivity of microperoxidase-8 (MP8), the heme octapeptide derived from enzymatic cleavage of cytochrome c, has been studied by nanosecond flash photolysis methods. Ferric MP8 is rapidly oxidized by photogenerated Ru(bpy)33+ in acidic solutions to a ferric cation-radical porphyrin (k ∼ 5.6 × 109 M-1 s-1); the oxidation product in alkaline solutions is ferryl MP8 (k ∼ 2.2 × 109 M-1 s-1). Numerical simulations of the kinetics for the direct oxidation of ferric to ferryl MP8 predict a marked pH effect on the rate of reaction in alkaline solutions; however, only a very weak pH dependence is observed in the range 7−8.5, indicating that the ferryl species is generated by intramolecular electron transfer within a ferric cation-radical porphyrin. Transient spectra taken between pH 6 and 8.5 show increasing ferryl absorption as the pH is increased, demonstrating a pH-dependent equilibrium between the two oxidized forms of MP8.

Investigation of carotenoid radical cations and triplet states by laser flash photolysis and time-resolved resonance raman spectroscopy : observation of competitive energy and electron transfer

Abstract: The first nanosecond time-resolved resonance Raman study of carotenoid radical cations is reported for the polyenes septapreno-β-carotene and 7,7‘-dihydro-β-carotene. In addition, previously unreported resonance Raman spectra of the ground and triplet states of these molecules are reported. The radical cations were generated following electron transfer quenching of triplet 1-nitronaphthalene in methanol and Triton X-100 micelles. The quenching of triplet 1-nitronaphthalene by these carotenoids involves solvent-dependent competition between energy transfer and electron transfer, and for both carotenoids, estimates are given for the efficiencies of these two processes in methanol and hexane. The resonance Raman spectra of septapreno-β-carotene ground and triplet states are consistent with spectra reported previously for other carotenoids. However, the resonance Raman spectrum of the triplet state of 7,7‘-dihydro-β-carotene displays an intensity profile not found in the triplet spectra of other carotenoids. ...

Singlet versus Triplet Reactivity in Photodegradation of C40 Carotenoids

Abstract: Direct observations of triplet excited states of astaxanthin (I), β-carotene (II), canthaxanthin (III), and zeaxanthin (IV) in toluene at 25 °C following unsensitized laser flash photolysis and using transient absorption spectroscopy have yielded the singlet−triplet intersystem crossing yields 3.7 × 10-3 (I), 5.4 × 10-4 (II), 9.7 × 10-3 (III), and 1.8 × 10-3 (IV), based on triplet−triplet extinction coefficients obtained in anthracene-sensitized experiments. A carbonyl rather than a hydroxy group distinguished the carotenoids from each other, as further evidenced by rate constants for oxygen quenching of triplet carotenoids, 2.1 × 108 (I), 1.1 × 108 (II), 2.5 × 108 (III), and 0.95 × 108 M-1 s-1 (IV), determined under similar conditions and by the fluorescence quantum yields which depend on excitation wavelength (355 versus 430 nm) more significantly for II (factor of 13) and IV (factor of 21) than for I and III (common factor of 2). For II and III, using continuous-wave photolysis, competing oxygen-depend...

Kinetics of photobleaching of beta-carotene in chloroform and formation of transient carotenoid species absorbing in the near infrared.

Abstract: Upon laser flash photolysis of β–carotene in chloroform instantaneous bleaching of β–carotene and concomitant formation of near infrared absorbing species are observed. One species, absorbing with maximum at 920 nm, is formed during the laser pulse (10 ns) and is practically gone in one millisecond, the decay showing a bi-exponential behaviour. The second species, absorbing with maximum at 1000 nm, is formed from the species absorbing at 920 nm by first order kinetics with a rate constant of 4.9·104 s−1 at 20°C. This second species decays by second order kinetics and is gone within a few milliseconds. An additional slow bleaching of β–carotene and formation of the species absorbing at 920 nm is observed. This slow bleaching/formation of transient absorption is probably due to processes involving free radicals generated during the instantaneous bleaching. The species absorbing at 920 nm is suggested to be either (i) a free radical adduct formed from β–carotene and chloroform or (ii) β–carotene after abstra...

Lifetimes of Radical Anions of Dicyanoanthracene, Phenazine, and Anthraquinone in the Excited State from the Selective Electron-Transfer Quenching

Abstract: The selective eletron-transfer quenching of the radical anions of dicyanoanthracene, phenazine, and anthraquinones in the excited state by a quencher such as fumaronitrile or dicyanobenzene is investigated in N,N-dimethylformamide solution at room temperature using the pulse radiolysis−laser flash photolysis combined method. The radical anions generated by pulse radiolysis do not change upon irradiation with a laser flash at 532 nm. The radical anions in the excited state decay into the ground state within the laser flash (5 ns). Lifetimes of approximately 4 ns are estimated for three radical anions in the excited state assuming a diffusion-controlled rate constant for the electron-transfer quenching. The shorter lifetimes of 1.0−1.4 ns for methyl and chloro substituents on anthraquinone are discussed in terms of internal conversion from the excited to the ground state of the radical anions accelerated by rotation of the substituents. The energy gap between the excited and ground states of the radical ani...

Stilbene radical anions in the excited doublet state

Abstract: Stilbene radical anions (St•-) in the second excited doublet state generated by a pulse radiolysis−laser flash photolysis combined method undergo unimolecular isomerization from c-St•- to t-St•-, photoelectron ejection, and electron transfer to biphenyl added as a quencher. From the selective electron transfer from the St•- D2 states to biphenyl (Bp), kBpτ = 10.7 and 17.8 M-1 were obtained where kBp and τ denote the rate constants of the electron transfer and the lifetimes of the c-St•- and t-St•- D2 states, respectively. The τ values are estimated to be approximately 1.5 ± 0.4 ns and 2.5 ± 0.7 ns for the c-St•- and t-St•- D2 states, respectively, from kBpτ assuming the diffusion-controlled rate constant for kBp. The shorter τ of the c-St•- D2 state is attributed to isomerization via twisting about the central CC double bond. The τ values of the c-St•- and t-St•- D2 states are 1 order of magnitude longer than those of c-St•+ and t-St•+ D2 states. The transient phenomena of the D2 states of c-St•- and t-St...

Photochemistry of the Nonspecific Hydroxyl Radical Generator, N-Hydroxypyridine-2(1H)-thione

Abstract: The photochemistry of N-hydroxypyridine-2(1H)-thione (N-HPT) has been investigated in aqueous and organic solvents using laser flash photolysis (λexc = 308 or 355 nm). Independent of the environment, UV excitation of N-HPT causes homolytic N−O bond cleavage, which leads to formation of the 2-pyridylthiyl (PyS•) and hydroxyl (•OH) radicals. In aqueous media, this process occurs efficiently from both the anionic and neutral forms (ΦN-O ≈ 0.20−0.30). In addition to N−O bond scission, N-HPT undergoes other primary photoprocesses which are pH-dependent. At pH = 7, photoionization (Φe− = 0.09 (λexc = 308 nm) and 0.05 (λexc = 355 nm)) of the anionic form generates the hydrated electron as well as the semioxidized radical of N-HPT. Fast rearrangement of the latter species produces the N-oxy-2-pyridylthiyl radical. At pH = 2, where the uncharged structure predominates, formation of an excited triplet state (ET ≥ 59.5 kcal mol-1) is observed (ΦT ≥ 0.05 using λexc = 355 nm) but photoionization does not take place. T...

A Novel Flash Photolysis/UV Absorption System Employing Charge-Coupled Device (CCD) Detection: A Study of the BrO + BrO Reaction at 298 K

Abstract: A novel flash photolysis/kinetic absorption spectroscopy system has been constructed for the study of gas phase reactions The experiment incorporates a charge-coupled device (CCD) detector and represents the first application of such devices to the study of gas phase kinetics The CCD enables the recording of rapid sequential time-resolved UV/visible absorption spectra before, during, and after photolysis of a gas mixture The unequivocal identification and monitoring of several absorbing components in the reacting mixture is therefore possible, thereby maximizing the amount of information gathered from a single flash photolysis experiment The experimental system is described in full here Results from a preliminary kinetic study of the BrO self-reaction at 298 K are also described: BrO + BrO → 2Br + O2 (1a); BrO + BrO → Br2 + O2 (1b) Experiments were performed to independently determine the rates of the overall reaction, k1, defined by (−d[BrO]/dt = 2k1[BrO]2) and both individual reaction channels (1