Showing papers on "Flash photolysis published in 1995"

Photolysis of caged Ca2+ facilitates and inactivates but does not directly excite light-sensitive channels in Drosophila photoreceptors

Abstract: Substantial evidence implicates the phosphoinositide cascade in invertebrate phototransduction, but the final pathway of excitation remains obscure. In order to test the hypothesis that Ca2+ is the excitatory messenger rapid concentration jumps of cytosolic Ca2+ were achieved in dissociated Drosophila photoreceptors by flash photolysis of the caged Ca2+ compounds DM-nitrophen and nitr-5. Both compounds were introduced via patch pipettes used to record whole-cell currents. Calibrations using INDO-1 and Mag-INDO-1 indicated that photolysis of DM-nitrophen (5 mM loaded with 4 mM Ca2+), raised Ca, to ca. 20-50 microM, and nitr-5 (same loading) to ca. 1-2 microM. In mutants lacking light responses (ora, lacking rhodopsin; norpA, lacking phospholipase C; trp, which is inactivated by conditioning lights), the only current evoked by photolysis of DM-nitrophen was a small inward current with no detectable latency. This current did not reverse at +80 mV and was blocked by substitution of external Na+ for Li+, suggesting it represents activation of an electrogenic Na+/Ca2+ exchanger. A similar current was also the only current elicited by caged Ca2+ during the 5 msec latent period in wild type (WT) photoreceptors. To investigate possible modulatory effects of caged Ca2+ on the light-activated conductance, cells were first stimulated with a saturating light stimulus, itself incapable of releasing significant Ca2+, and then the photolytic flash was discharged during the response. During the rising phase of the response, photolysis of DM-nitrophen (but not nitr-5) induced a pronounced facilitation in WT photoreceptors. When photolysed during the plateau phase both DM-nitrophen and nitr-5 induced a rapid inactivation of the light-induced current. By contrast, in trp photoreceptors, which lack one class of Ca2+ permeable light-sensitive channel, photolysis of DM-nitrophen induced a significant facilitation during the falling phase of the response, but during the rising phase photolysis significantly depressed the overall response. In conclusion, caged Ca2+ failed to activate any channels in Drosophila photoreceptors but profoundly affected the light-dependent channels once they have been activated.

Exploratory laser flash photolysis study of free radical reactions and magnetic field effects in melatonin chemistry

Abstract: Melatonin is an excellent free radical scavenger, reacting with tert-butoxyl and cumyloxyl radicals with rate constants of 3.4 x 10 7 and 6.7 x 10 7 M -1 s -1 , respectively. Reaction with benzophenone triplet occurs with a near-diffusion-controlled rate constant of 7.6 x 10 9 M -1 s -1 in acetonitrile and probably involves charge transfer. When the radical pair formed by reaction of benzophenone triplet and melatonin is sequestered in a micelle, it is subject to extensive magnetic field effects that can be readily interpreted by the radical pair model.

Internal electron transfer in cytochrome c oxidase from Rhodobacter sphaeroides.

Abstract: Absorbance changes following CO dissociation by flash photolysis from mixed-valence aa3 cytochrome oxidase from Rhodobacter sphaeroides have been followed in the Soret and alpha regions. They reflect internal electron transfer in the partially reduced enzyme, and the kinetics of the reactions has been determined. As with the bovine enzyme, three kinetic phases are found with relaxation time constants at neutral pH of about 3 microseconds, 35 microseconds, and 1 ms. The first reaction phase represents electron transfer from cytochrome a3 to cytochrome a, and the extent of this reaction is about 3 times larger compared to the bovine enzyme. The energetics of the reaction has been analyzed on the basis of measurements of its temperature dependence. The reorganization energy is close to 120 kJ mol-1, and it is suggested that this rather high value is the result of changes in solvation at the cytochrome a3-CuB site. The subsequent electron transfer between cytochrome a and CuA, with a time constant of 35 microseconds, is almost activationless and has a very low reorganization energy. The final phase, with a time constant close to 1 ms at neutral pH, represents a further shift in the equilibrium between cytochrome a3 and cytochrome a, and it is limited by proton-transfer reactions. The pKa values of the groups involved are significantly shifted in the bacterial oxidase compared to the bovine one. The total extent of electron transfer in the three backflow reactions has also been determined by a comparison of the CO-recombination rates in the mixed-valence and fully reduced enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Prototype Si—H insertion reaction of silylene with silane. Absolute rate constants, temperature dependence, RRKM modelling and the potential-energy surface

Abstract: Time-resolved studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with monosilane, SiH4. The reaction was studied in the gas phase over the pressure range 1–100 Torr, with both Ar and SF6 as bath gases, at six temperatures in the range 298–665 K. The reaction of SiH2 with SiH4 to form disilane, Si2H6, is pressure dependent, consistent with a third-body assisted association reaction. The high-pressure rate constants, obtained by extrapolation, gave the Arrhenius equation: log(k∞/cm3 molecule–1 s–1)=(–9.91 ± 0.04)+(3.3 ± 0.3 kJ mol–1)/RT In 10. These Arrhenius parameters are consistent with a fast, nearly collision-controlled, process. RRKM modelling, based on a variational transition state, used in combination with a weak collisional deactivation model, gave good fits to the pressure-dependent curves. The step sizes (energies removed in a down collision) corresponded to collisional efficiencies (βc) of ca. 0.5 for SF6 and ca. 0.2 for Ar.The rate constants for the insertion and reverse decomposition (of Si2H6) have been combined to obtain a precise value of the equilibrium constant Kp at 552 K. Using the third-law method, a value for ΔfH°(SiH2)= 273 ± 2 kJ mol–1 is derived which represents the most precise experimental value for this quantity yet obtained. Ab initio calculations at the correlated level, reveal the presence of two weak complexes (local-energy minima) on the potential-energy surface corresponding to either direct or inverted geometry of the inserting silylene fragment. Surprisingly, the latter is the lower in energy, lying 51.5 kJ mol–1 below the unassociated reactants. These complexes rearrange to disilane with very low barriers. The implications of these findings and the nature of the insertion process are discussed.

Kinetics of reactions of primary, secondary and tertiary β-hydroxy peroxyl radicals

Abstract: The UV absorption spectra and kinetics of reactions of the primary, secondary and tertiary β-hydroxy peroxyl radicals HOCH2CH2O2, CH3CH(OH)CH(O2)CH3 and (CH3)2C(OH)C(O2)(CH3)2 at or near 298 K have been studied using both the molecular modulation (MM) and the laser flash photolysis (LFP) techniques. The radicals were produced by the UV photolysis (254 or 248 nm) of mixtures containing H2O2 and one of the symmetric monoalkenes ethene, (E)-but-2-ene and 2,3-dimethylbut-2-ene, diluted in air. The spectra of CH3CH(OH)CH(O2)CH3 and (CH3)2C(OH)C(O2)(CH3)2 were found to be very similar to that measured previously for HOCH2CH2O2, with maximum cross-sections of ca. 4 × 10–18 cm2 molecule–1 at 245 nm. At high monoalkene concentrations, the observed second-order removal of the peroxyl radicals (RO2) provided values of k3obs(in units of 10–12 cm3 molecule–1 s–1) of 3.1 ± 0.4, 0.84 ± 0.06 and 0.0114 ± 0.0017 for HOCH2CH2O2, CH3CH(OH)CH(O2)CH3 and (CH3)2C(OH)C(O2)(CH3)2, respectively, the quoted values being the mean of the MM and LFP determinations: RO2+ RO2→ products (3), The values of k3obs are greater than the values for the elementary rate coefficients, k3, owing to secondary removal of the RO2 radicals. The details of the secondary chemistry and the probable magnitude of the additional removal are discussed. At low monoalkene concentrations, simultaneous production of HO2 in the systems allowed the reactions of RO2 with HO2 to be investigated: RO2+ HO2→ ROOH + O2(2), Values of k2(in units of 10–11 cm3 molecule–1 s–1) of 1.5 ± 0.3, 1.5 ± 0.4 and ca. 2 were determined for HOCH2CH2O2, CH3CH(OH)CH(O2)CH3 and (CH3)2C(OH)C(O2)(CH3)2, respectively, using the LFP technique.The kinetic data obtained in the present work, and those obtained previously for the allyl peroxyl radical, are used to infer rate coefficients for some of the reactions of the complex peroxyl radicals formed from the OH-initiated oxidation of isoprene. A mechanism describing the degradation of isoprene to first generation products is constructed using the derived coefficients, and key parameters obtained or inferred from other sources, which gives a reasonable description of product yields measured in the laboratory in both the presence and absence of NOx.

Solvent Polarity Effect on Photoinduced Electron Transfer between C60 and Tetramethylbenzidine Studied by Laser Flash Photolysis

Abstract: The photoinduced electron transfer between C{sub 60} and N,N,N{prime},N{prime}-tetramethylbenzidine (NTMB) in polar and nonpolar solvents and their mixtures has been investigated by nanosecond laser flash photolysis/transient absorption spectroscopy in the visible and near-IR regions. The transient absorption bands of the C{sub 60} triplet state ({sup T}C{sub 60}*) and the NTMB radical cation were observed in both nonpolar and polar solvents. In polar solvents such as benzonitrile, the reaction rate constants were determined from the decay of {sup T}C{sub 60}* at 740 nm, which were consistent with the rate constants evaluated from the growth of the NTMB radical cation, suggesting that direct electron transfer occurs from ground-state NTMB to {sup T}C{sub 60}*. In nonpolar solvents such as benzene, the NTMB radical cation and C{sub 60} radical anion were produced immediately after the nanosecond laser pulse, simultaneous with the formation of {sup T}C{sub 60}*, indicating that a different electron transfer mechanism exists in nonpolar solvents. In the solvent mixtures, two simultaneous reaction routes are present for both forward and back electron transfer reactions. Pronounced solvent effects found for electron transfer reaction kinetics and rates are characteristic of the photoinduced electron transfer reactions between C{sub 60} and some kinds of aromatic amines. 25more » refs., 8 figs., 2 tabs.« less

Critical Review of Rate Constants for Reactions of Transients from Metal Ions and Metal Complexes in Aqueous Solution

Abstract: Kinetic data for transient metal species in aqueous solution have been critically reviewed. The compilation covers over 2000 measurements of rate constants involving 660 metal ions and metal complexes from Groups 4‐15; lanthanides and actinides are not included. Most of the data have been obtained by the methods of pulse radiolysis or flash photolysis. Data have been collected from 500 publications through 1993.

Kinetic and Spectroscopic Studies on Acyl Radicals in Solution by Time-Resolved Infrared Spectroscopy

Abstract: A number of acyl radicals, RĊ=O, have been generated in hexane or di -t-butyl peroxide as solvent at room temperature by 308 nm laser flash photolysis, and their spectroscopic and kinetic properties have been examined by time-resolved infrared spectroscopy. The C=O stretching frequencies for the RĊ=O radicals are found to be higher than those of the corresponding aldehydes, RCHO, by between 108 and 128 cm-1, an effect attributed to a higher C=O bond order in the radicals. For the RĊ=O radicals some typical values of vC =O are: CH3Ċ=O, 1864 cm-1; (CH3)3CĊ=O, 1848 cm-1; and C6H5Ċ=O, 1828 cm-1, while the corresponding acylperoxyl radicals, RC(O)OO, formed by reaction with oxygen have vC =O values of 1838, 1840 and 1820 cm-1, respectively. The acyl radicals exhibit a reactivity towards a variety of substrates that is roughly comparable to that of simple alkyl radicals. For reactions of the benzoyl radical some typical rate constants/M-1 s-1 are: CCl4, 6.0x104; C6H5SH, 4.8×107; CCl3Br, 2.2×108; Tempo, 1.1×109; and oxygen, 1.8×109. Alkanoyl radicals have a rather similar reactivity to benzoyl. The propanoyl radical reacts with tributyltin deuteride with a rate constant of 3×105 M-1s-1. The hex-5-enoyl radical undergoes a 5-exo-trig cyclization to form the 2-oxocyclopentylmethyl radical with a rate constant of 2.2×105 s-1, a value which is almost identical to that for cyclization of the hex-5-enyl radical. It is hoped that our kinetic data will prove useful in the planning of organic synthetic strategies which involve acyl radical chemistry.

Photochemistry on Surfaces: Matrix Isolation Mechanisms Study of Interactions of Benzophenone Adsorbed on Microcrystalline Cellulose Investigated by Diffuse Reflectance and Luminescence Techniques

Abstract: The swelling of microcrystalline cellulose by the use of polar protic solvents such as ethanol or methanol enables the penetration of benzophenone into submicroscopic pores of the natural polymer, while solvents such as benzene or dichloromethane do not open the polymer chains, thus not producing any entrapped benzophenone. Ground-state diffuse reflectance studies revealed a dramatic blue shift in the 350-nm absorption of benzophenone in the former case, in accordance with a strong interaction of the hydroxyl groups of cellulose with the ketone. Diffuse reflectance laser flash photolysis studies of benzophenone adsorbed on microcrystalline cellulose showed, in cases where benzophenone is entrapped in the polymer chain, the formation of a transient which decays nonexponentially and exhibits a maximum absorption at about 530 nm, assigned to triplet benzophenone. After ca. 25 μs, this transient generates another species with an absorption maximum at 545 nm. We assigned this new species to the diphenylketyl radical. In all cases where the solvent does not swell cellulose, a different behavior was observed typical forbenzophenone microcrystals triplet decay. The ketyl radical formation is greatly reduced in this case. Triplet benzophenone decays by complex kinetics and lives about 10 μs when adsorbed onto microcrystalline cellulose, while the ketyl radical, when formed, lives 1 order of magnitude longer than the triplet. Samples which exhibit a high yield of ketyl radical formation also have a smaller phosphorescence emission in accordance with the fact that large amount of triplet molecules were consumed in the process of hydrogen abstraction from the matrix, involving hydrogens linked to carbons bearing a hydroxyl group

Myoglobin-no at low ph : free four-coordinated heme in the protein pocket

Abstract: In either sperm whale or horse heart myoglobin, binding of NO and lowering of solution pH work together to weaken, and ultimately break, the bond between iron and the proximal histidine. This is reminiscent of the reaction observed at neutral pH in the case of guanylate cyclase, the heme enzyme that catalyzes the conversion of GTP to cGMP. Bond breaking is characterized by a spectral change from a nine-line to a three-line ESR signal and accompanied by a shift from 420 to 387 nm in the UV-vis spectrum of the Soret band maximum. Analysis of the pH-dependent spectral changes shows that they are reversible, at least within a few hours, that the transition is cooperative, involving six protons during pH lowering but only two as it is raised, and that the pK is about 4.7. Different proteins exhibit different pK values, which are generally lower than that for "chelated" protoheme. The pK differences reflect the extra bond stability afforded by the protein structure. Investigations of thermal and photochemical NO displacement by CO suggest that the local pocket around the ligand, although significantly altered (according to circular dichroism investigations), nonetheless still imposes a barrier against the outward diffusion of ligand into the solvent. Nanosecond and picosecond flash photolysis shows that in proteins at low pH there is an extremely efficient geminate recombination of the ligand with the four-coordinated species through a single-exponential process. This occurs to a significantly larger extent than for the case of NO-"chelated" protoheme (where no distal barrier for ligand is present). At neutral pH, when the proximal histidine bond is intact, the geminate recombination for NO takes longer and displays multiexponential kinetics. Altogether, these results suggest that, even though distal effects probably also play a role, proximal effects make an important contribution in modulating ligand-iron bond formation.

Photochemistry. A Direct Time-Resolved Spectroscopic Study of Both Radical Fragments

Abstract: The photochemistry of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (1) has been investigated using time-resolved infrared (TRIR) and time-resolved UV spectroscopy (laser flash photolysis (LFP)), in addition to conventional photochemical steady-state techniques and product analysis. Direct photolysis of 1 in dichloromethane (CH2C12) or n-heptane solution causes a-cleavage to afford the corresponding 2,4,6-trimethylbenzoyl (2) and diphenylphosphonyl(3) radicals. The 2,4,6-trimethylbenzoyl radical (2) is readily detected using TRIR spectroscopy, exhibiting carbonyl IR absorption maxima at 1805 and 1797 cm-I in n-heptane and CH2C12 solution, respectively. The diphenylphosphonyl radical (3) is readily detected by LIT, exhibiting a strong UV absorption in the 300-350 nm region. Rate constants for the reactions of radicals 2 and 3 with bromotrichloromethane (BrCCk), thiophenol (PhSH), and benzhydrol (Ph2CHOH) have been determined. Both radicals react with BrCC13 and PhSH via atom abstraction with rate constants in the 107-108 M-' s-l range. Photolysis of 1 in the absence of a radical scavenger affords diphenyl((2,4,6-trimethylbenzoyl)oxy)phosphine (9) in addition to the expected free radical-derived products. Compound 9 is also produced upon photolysis of 1 in the presence of 0.05 M BrCC13, indicating that it is formed mainly via cage recombination of radicals 2 and 3. The reactivity of radicals 2 and 3 is discussed.

Laser flash photolysis studies on hydrogen atom abstraction from phenol by triplet naphthoquinones in acetonitrile

Abstract: By means of ns laser flash photolysis, the absorption spectra and molar absorption coefficients (Iµ) of the 1,4-naphthosemiquinone radical (NQH˙) and 2,3-dimethyl-1,4-naphthosemiquinone radical (DMNQH˙) have been determined to be 8200 dm3 mol–1 cm–1 at 365 nm and 7100 dm3 mol–1 cm–1 at 368 nm in acetonitrile (MeCN) and MeCN–H2O (4 : 1 v/v) at 295 K. On the basis of the determined absorption spectra and Iµ values, hydrogen atom abstraction (HA) by triplet 1,4-naphthoquinone (NQ) and 2,3-dimethyl-1,4-naphthoquinone (DMNQ) from phenol (PhOH) in MeCN was studied by laser photolysis techniques. It was found that HA by triplet NQ and DMNQ (3NQ* and 3DMNQ*) from PhOH occurred in a collision process with quenching rate constants, kq= 8.6 × 109 and 5.5 × 108 dm3 mol–1 s–1, respectively. On the basis of the obtained values of kq and quantum yields (ΦHA), the efficiencies (ψHA) for HA by both 3NQ* and 3DMNQ* from PhOH were obtained to be unity. The rate constants (kHA) for HA of 3NQ* and 3DMNA* were determined to be 8.6 × 109 and 5.5 × 108 dm3 mol–1 s–1, respectively. The difference in kHA may be derived from (1) the steric hindrance by the methyl groups of DMNQ and (2) the degree of 3(π,π*) character mixed in with the 3(n,π*) of NA and DMNQ. The triplet–triplet absorption spectra of NQ and DMNQ were identified, and their absolute molar absorption coefficients (IµT–Tλ) were determined to be 8200 dm3 mol–1 cm–1 at 365 nm and 5200 dm3 mol–1 cm–1 at 368 nm, respectively, in MeCN on the basis of the quantum yields of intersystem crossing (Φisc; 0.74 for NQ, 0.98 for DMNQ) evaluated by thermal lensing techniques. The deactivation mechanism of 3NQ* and 3DMNQ* in the absence of PhOH was found to be self-quenching by NQ and DMNQ in MeCN with the rate constants, kSQ= 9.7 × 108 and 1.2 × 108 dm3 mol–1 s–1, respectively.

Time Resolved Electron Spin Resonance and Laser Flash Spectroscopy Investigation of the Photoreduction of Anthraquinone-2,6-disulfonic Acid, Disodium Salt by Sodium Sulfite in Aerosol OT Reverse Micelles

Abstract: Spin-correlated radical pairs (SCRP) of radical anions formed by photoreduction of anthraquinone-9,lOdisulfonic acid, disodium salt (AQ) by sodium sulfite in solution of Aerosol OT (AOT) reverse micelles in isooctane have been studied by time resolved electron spin resonance (TR ESR) and nanosecond laser flash photolysis employing optical absorption. The molar ratio of water to AOT w was varied from 5 to 50, and the spectra of SCRP were observed in micelles with w 2 20. The value of exchange interaction in such micelles was determined and found to be on the order of a few tenths of a gauss. The photoreduction of AQ is accompanied by strong emissive ESR spectra of the radicals AQ'- and S03'-. The hyperfine coupling in AQ'- is fully resolved at the relatively long time of several microseconds after the laser flash. The initial SCRP spectra are succeeded in time by an WA ESR pattern due to a radical pair mechanism. The rate constants of radical recombination within micelles and radical pair exit from the micelles were determined by laser flash photolysis. The results are consistent with the formation of associates such as AQ'-/Na+/ SO3'- triads in reverse micelles; Le., the radical anions are held together by a bridging sodium ion.

Mechanism of 4-carboxybenzophenone-sensitized photooxidation of methionine-containing dipeptides and tripeptides in aqueous solution

Abstract: The mechanism of 4-carboxybenzophenone (CB)-sensitized photooxidation of methionine-containing dipeptides (Met-Gly and Gly-Met) and tripeptides (Met-Gly-Gly, Gly-Met-Gly, and Gly-Gly-Met) was investigated using nanosecond flash photolysis and steady-state photolysis. The rate constants for quenching of the CB triplet by sulfur-containing peptides were determined to be in the range (1.8-2.3) x 10{sup 9} M{sup -1} S{sup -1} for neutral and alkaline solutions. The presence of the various electron-transfer intermediates accompanying the CB triplet quenching events was identified through the use of a multiple-regression procedure that was used to resolve the experimental transient spectra into components. The intermediates identified were the CB ketyl radical anion, the CB ketyl radical, intermolecularly (S.{sup {center_dot}}.S)-bonded radical cations, and intramolecularly (S.{sup {center_dot}}.N)-bonded radical cations derived from peptides. The types of intermediates were found to depend on the pH of the solution and on the location of the methionine unit with respect to the terminal functions. The quantum yields of all the transients and the kinetics of their formation and decay were measured by flash photolysis, and quantum yields of CO{sub 2} formation were measured by steady-state photolysis. 50 refs., 6 figs., 3 tabs.

Rate coefficients for quenching of Cl(2P1/2) by various atmospheric gases

Abstract: Rate coefficients for quenching of the upper spin–orbit state of chlorine Cl(2P1/2) by a variety of collision partners have been measured at room temperature. The method used was laser flash photolysis of NOCl or ClONO2 with resonance fluorescence detection of chlorine atoms using a strongly reversed resonance lamp for preferential detection of Cl(2P1/2). The rate coefficients were (units of cm3 molecule–1 s–1): CF4, (2.3 ± 0.3)× 10–11; CF2Cl2, (3.3 ± 0.5)× 10–10; CH4, (2.2 ± 0.3)× 10–11; ClONO2, (1.7 ± 0.3)× 10–10; O2, (3.5 ± 0.5)× 10–13; N2, (5.0 ± 1.5)× 10–15; He, (6.0 ± 1.0)× 10–14; Ar, (3.0 ± 1.0)× 10–16. These values are in good overall agreement with recent determinations which used IR laser detection of Cl(2P1/2). Quenching by helium was found to be unusually fast, explaining in part discrepancies with earlier studies which used resonance absorption detection. The reaction with CH4 was shown to proceed exclusively by quenching, and not to involve enhanced reactivity of Cl(2P1/2) relative to the ground-state species. Additionally, the yield of Cl(2P1/2) from the photolysis of chlorine nitrate was found to be 30% of the overall chlorine atom yield at 308 nm. The rate coefficient for the reaction of Cl(2P3/2) with NOCl was measured to be (9.6 ± 1.5)× 10–11 cm3 molecule–1 s–1.

Examination of the Exchange Interaction through Micellar Size. 3. Stimulated Nuclear Polarization and Time Resolved Electron Spin Resonance Spectra from the Photolysis of Methyldeoxybenzoin in Alkyl Sulfate Micelles of Different Sizes

Abstract: Stimulated nuclear polarization (SNP) and time resolved electron spin resonance (TR ESR) spectra were recorded during the laser flash photolysis of [sup 13]C carbonyl labeled [alpha]-methyldeoxybenzoin solubilized in a series of alkyl sulfate micelles of different sizes. While the SNP spectra show a decrease in the splitting of the two hyperfine lines with decreasing micelle size, this decrease in hyperfine splitting is not seen in the experimental TR ESR spectra The qualitatively different variations between the SNP and TR ESR spectra, as a function of micelle size were interpreted in terms of the stochastic Liouville equation as applied to the model of the microreactor. 28 refs., 6 figs., 2 tabs.

Photochemistry on Surfaces. Intermolecular Energy and Electron Transfer Processes between Excited Ru(bpy)32+ and H-Aggregates of Cresyl Violet on SiO2 and SnO2 Colloids

Abstract: Cresyl violet, a cationic dye (CV{sup +}), forms H-aggregates on the negatively charged SiO{sub 2} and SnO{sub 2} colloids. These aggregates exhibit broad absorbance around 520 nm. By coadsorbing a sensitizer, Ru(bpy){sub 3}{sup 2+}, we are able to characterize the triplet excited state and reduced form of dye-aggregates on the colloidal SiO{sub 2} and SnO{sub 2} suspensions. On SiO{sub 2} surfaces, the excited state quenching of Ru(bpy){sub 3}{sup 2+} by dye-aggregates proceeds via an energy transfer mechanism. Picosecond laser flash photolysis experiments indicate that such a surface-promoted energy transfer is completed within 20 ps. On the other hand dye-aggregates adsorbed onto SnO{sub 2} colloids undergo photosensitized reduction since the excited sensitizer, Ru(bpy){sub 3} {sup 2+}, is efficiently quenched by the semiconductor support. The role of support material in promoting energy and electron transfer processes is described. 87 refs., 11 figs.

Recovery mechanism of the reaction intermediate produced by photoinduced cleavage of the intramolecular hydrogen bond of dibenzoylmethane

Abstract: The recovery mechanism of the reaction intermediate (non-chelated enol form) produced by photoinduced cleavage of the intramolecular hydrogen bond of dibenzoylmethane was studied in various solvents by nanosecond laser flash photolysis. The recovery rate and mechanism depend strongly on the nature of the solvent. Unimolecular recovery of the intermediate to the chelated enol form takes place in acetonitrile, diethyl ether and dimethylsulphoxide with extremely small rate constants (1.1, 1.5 and 6.6 s −1 respectively) despite the small activation energy (3.6 kcal mol −1 in 3-methylpentane). The slow unimolecular recovery rate can be ascribed to the small frequency factor (7.0 × 10 5 s −1 ), i.e. the large negative entropy change for the formation of the chelated enol form. In non-polar aliphatic hydrocarbon solvents, a bimolecular recovery process via hydrogen-bond interactions between two intermediate molecules is included in addition to unimolecular recovery. In alcohols, a solvent-assisted recovery process by mutual hydrogen exchange between the intermediate and alcohol molecule(s) accelerates the recovery rate. Basic catalysts, e.g. KOH in ethanol and triethylamine in acetonitrile, increase the recovery rate considerably by an additional process through the enolate anion.

Migration of photoinduced oxidative damage in models for DNA

Abstract: Laser flash photolysis with 193 nm light of single stranded oligodeoxynucleotides in aqueous solution indicates that the photoinduced radical cation of adenine or cytosine migrates to guanine, whereas the oxidative damage is not transferred from adenine to either cytosine or guanine via a cytosine linker