Laser flash photolysis study on the retinol radical cation in polar solvents

Laser flash photolysis (LFP) of retinol in argon-saturated methanol gives rise to a transient at 580 nm (transient A). Formation of transient A is accompanied by a transient growth at 370 nm. The rate of this growth is retinol concentration-dependent. The transient growth at 370 nm was removed in the presence of N(2)O, which is known to scavenge solvated electrons. These results can be interpreted by formation of retinol˙(+) (λ(max) = 580 nm) and solvated electrons following LFP of retinol. Subsequently, the solvated electrons are rapidly scavenged by retinol to form retinol˙(-) (λ(max) = 370 nm in methanol). On the other hand, transient A is not ascribed to the retinyl cation, as was previously proposed, because the retinyl cation, generated from LFP of retinyl acetate, and transient A show different reactivities towards halide ions (e.g. k(Br) = 1.7 × 10(9) and 1.51 × 10(10) M(-1) s(-1) respectively, in acetonitrile). After demonstrating the identity of transient A as retinol˙(+), its reactions with carotenoids were examined in air-saturated polar solvents. In the presence of carotenoids, an enhancement in the decay of retinol˙(+) was observed and was accompanied by formation of the corresponding carotenoid radical cations via electron transfer from carotenoids to retinol˙(+). Furthermore, the reactivity of retinol˙(+) towards pyridine derivatives was investigated in air-saturated polar solvents. It was found that the decay of retinol˙(+) was accelerated with concomitant formation, with the same rate, of a transient at 370 nm. Similar observations were obtained with increasing pH of air-saturated aqueous 2% Triton X-100 of retinol˙(+). The 370 nm (or 380 nm in the case of Triton X-100) transient is attributed to the base adducts or deprotonated neutral radicals. On the basis of these results, the reactivities of the retinyl cation and retinol˙(+) are compared and the consequences of retinol˙(+) formation within biological environments are discussed.     

SET-induced photorearrangement of 2-phenylallyl phosphites. Stereochemistry at phosphorus. Application to cyclic nucleotide derivatives

The stereochemistry at phosphorus of the SET-induced photorearrangement of diastereomeric 4-tert-butyl-2-phenylallyl-1,3,2-dioxaphosphorinanes (8) to the corresponding 2-phenylallylphosphonates (9), which involves exicted singlet 1,4-dicyanonaphthalene (1DCN*) as one-electron oxidant, was investigated. The rearrangement occurs with close to complete retention of configuration at phosphorus. The previously postulated mechanism for this photorearrangement is shown to be consistent with the stereochemical finding. Thus, one-electron reduction by DCN.- of the presumably stereospecifically formed distonic cyclic 1,3-cation radical intermediate 15, generated from cis-8 (Scheme 2), yields the thermodynamically stable diradical 16. beta scission of 16 forms phosphonate cis-9. An alternative mechanism involving beta scission of 15 to a styryl cation radical, prior to one-electron reduction to 15, is discounted on the basis of unpublished trapping studies using MeOH. The direct, kinetically controlled formation of diradical 16 rather than the thermodynamically less stable 21 with CH2 bonded apically to phosphorus is argued to be consistent with the essentially equal values of the quantum yield for phosphonate formation (phi P) on SET-induced rearrangement of the acyclic 2-phenylallyl phosphite 1 and phosphite 7 with phosphorus incorporated in a six-membered (1,3,2-dioxaphosphorinane) ring. This mechanism is contrasted to that for the previously reported triplet-sensitized photorearrangements of phosphites 1 and 7, which have greatly different phi P values. For these reactions, kinetic formation of the triplet analogue of 21, but without the tert-butyl substituent, requires a permutation of substituents for conversion to diradical 16 prior to intersystem crossing and beta scission to form the phosphonate corresponding to 7. The preparative-scale SET-induced photorearrangement of the thymidine-based 2-phenylallyl 3',5'-phosphite 10 gave both diastereomers of phosphonate 11 that were separated by HPLC. The 2-phenylallyl functionality provides an opportunity for further functionalization. As reported elsewhere, 11 was not formed in useful amounts via triplet-sensitized reaction of 10.     

Laser flash photolysis study of methanol addition to the C60 radical cation

Stable alcohol adducts of buckminsterfullerene (C₆₀) can be created via addition to C₆₀ radical cations. The radical cations were generated by photosensitized electron transfer from C₆₀ in a solution of N-methylacridinium hexafluorophosphate and biphenyl. Growth and decay of the C₆₀ radical cation population was monitored by transient absorption spectroscopy at 980 nm. The lifetime of the transient decreases in the presence of methanol, supporting trapping of the radical cation.     

Photodissociation of naphthalene dimer radical cation during the two-color two-laser flash photolysis and pulse radiolysis-laser flash photolysis

Photodissociation of naphthalene (Np) dimer radical cation (Np2*+) to give naphthalene radical cation (Np*+) and Np and the subsequent regeneration of Np2*+ by the dimerization of Np*+ and Np were directly observed during the two-color two-laser flash photolysis in solution at room temperature. When Np2*+ was excited at the charge-resonance (CR) band with the 1064-nm laser, the bleaching and recovery of the transient absorption at 570 and 1000 nm, assigned to the local excitation (LE) and CR bands of Np2*+, respectively, were observed together with the growth and decay of the transient absorption at 685 nm, assigned to Np*+. The dissociation of Np2*+ proceeds via a one-photon process within the 5-ns laser flash to give Np*+ and Np in the quantum yield of 3.2 x 10(-3) and in the chemical yield of 100%. The recovery time profiles of Np2*+ at 570 and 1000 nm were equivalent to the decay time profile of Np*+ at 685 nm, suggesting that the dimerization of Np*+ and Np occurs to regenerate Np2*+ in 100% yield. Similar experimental results of the photodissociation and regeneration of Np2*+ were observed during the pulse radiolysis-laser flash photolysis of Np in 1,2-dichloroethane. The photodissociation mechanism can be explained based on the crossing between two potential surfaces of the excited-state Np2*+ and ground-state Np*+.     

Laser flash photolysis studies of alkoxyl radical kinetics using 4-nitrobenzenesulfenate esters as radical precursors

4-Nitrobenzenesulfenate esters were used as precursors for the generation of alkoxyl radicals under laser flash photolysis conditions. The esters were efficiently cleaved using the Nd:YAG third harmonic (355 nm) to produce alkoxyl radicals and the 4-nitrobenzenethiyl radical. Rate constants for beta-scission and 1, 5-hydrogen abstraction reactions of alkoxyl radicals were measured.     

Laser flash photolysis of benzophenone in polymer films

With a nanosecond laser we studied flash photolysis of benzophenone (BP) dissolved in four different polymer films. We measured kinetics of decay of a triplet state of benzophenone (3)BP as well as kinetics of decay of benzophenone ketyl free radicals BPH(?). Polymer matrices have plenty of reactive C-H bonds, and the hydrogen abstraction by (3)BP leads to a formation of geminate pair which either recombines into molecular products or dissociates. Decay kinetics of (3)BP is well described by dispersive kinetics and in particular by the kinetic law suggested in Albery, W. J.; et al. J. Am. Chem. Soc. 1985, 107, 1854. We observed a broader distribution of rate constants in hard films. It was observed that the decay kinetics of transients radicals in the "hard" polymers is quite satisfactory described by the same law for dispersive kinetics. Kinetics of radicals decay in "soft" polymers is satisfactorily described as a diffusion-enhanced reaction. Effect of a hardness of polymer matrix on the measured kinetic parameters is discussed.     

One-electron oxidation of alcohols by the 1,3,5-trimethoxybenzene radical cation in the excited state during two-color two-laser flash photolysis

One-electron oxidation of alcohols such as methanol, ethanol, and 2-propanol by 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) was observed during the two-color two-laser flash photolysis. TMB*+ was formed by the photoinduced bimolecular electron-transfer reaction from TMB to 2,3,5,6-tetrachlorobenzoquinone (TCQ) in the triplet excited-state during the first 355-nm laser flash photolysis. Then, TMB*+* was generated from the selective excitation of TMB*+ during the second 532 nm laser flash photolysis. Hole transfer rate constants from TMB*+* to methanol, ethanol, and 2-propanol were calculated to be (5.2 +/- 0.5) x 10(10), (1.4 +/- 0.3) x 10(11), and (3.2 +/- 0.6) x 10(11) M-1 s-1, respectively. The order of the hole transfer rate constants is consistent with oxidation potentials of alcohol. Formation of TCQH radical (TCQH*) with a characteristic absorption peak at 435 nm was observed in the microsecond time scale, suggesting that deprotonation of the alcohol radical cation occurs after the hole transfer and that TCQ radical anion (TCQ*-), generated together with TMB*+ by the photoinduced electron-transfer reaction, reacts with H+ to give TCQH*.     

Laser flash photolysis of carboxymethylcellulose in an aqueous solution

Laser flash photolysis with excitation at 248 nm was used to study photochemically derived changes of carboxymethylcellulose (CMC) in aqueous solutions. Transient absorption spectra of solutions after photolysis revealed a broad band with a maximum of approximately 720 nm, which could be ascribed to the signal of the hydrated electron. The interaction of the hydrated electron with CMC was slow (<10⁷ dm³ mol^?1 s^?1), but the OH radical, formed by the decomposition of H₂O₂, reacted with CMC at a high rate constant (9.5–11.0 × 10⁸ dm³ mol^?1 s^?1). The rate constant of the reactions of CMC with hydroxyl radicals depended on the conformation of the macromolecules, which was determined by the pH of the solution. Transient absorption was recorded at a wavelength shorter than 370 nm for CMC solutions photolyzed in the presence of H₂O₂. As a result of OH attack, long‐lived radicals were formed on CMC. The recombination of macroradicals led to the formation of crosslinking bonds between side‐chain groups, and as a result of it an insoluble gel arose in low‐pH solutions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 505–518, 2005     

Photochemical C2-C6 cyclization of enyne-allenes: detection of a fulvene triplet diradical in the laser flash photolysis

A series of enyne-allenes, with and without benzannulation at the ene moiety and equipped with aromatic and carbonyl groups as internal triplet sensitizer units at the allene terminus, was synthesized. Both sets, the cyclohexenyne-allenes and benzenyne-allenes, underwent thermal C(2)-C(6) cyclization exclusively to formal ene products. In contrast, the photochemical C(2)-C(6) cyclization of enyne-allenes provided formal Diels-Alder and/or ene products, with higher yields for the benzannulated systems. A raise of the temperature in the photochemical cyclization of enyne-allene 1b' led to increasing amounts of the ene product in relation to that of the formal Diels-Alder product. Laser flash photolysis at 266 and 355 nm as well as triplet quenching studies for 1b,b' indicated that the C(2)-C(6) cyclization proceeds via the triplet manifold. On the basis of a density functional theory (DFT) study, a short-lived transient (tau = 30 ns) was assigned as a triplet allene, while a long-lived transient (tau = 33 micros) insensitive to oxygen was assigned as fulvene triplet diradical. An elucidation of the reaction mechanism using extensive DFT computations allowed rationalization of the experimental product ratio and its temperature dependence.     

Mass spectral evidence for the hydroxymethylene radical cation

Collisionally-activated decompositions show that ·CHO⁺H ions are stable, confirming theoretical predictions.     

On the electrophilicity of hydroxyl radical: a laser flash photolysis and computational study

The rate coefficients for reactions of hydroxyl radical with aromatic hydrocarbons were measured in acetonitrile using a novel laser flash photolysis method. Comparison of kinetic data obtained in acetonitrile with those obtained in aqueous solution demonstrates an unexpected solvent effect on the reactivity of hydroxyl radical. In particular, reactions of hydroxyl radical with benzene were faster in water than in acetonitrile, and by a significant factor of 65. Computational studies, at the B3LYP and CBS-QB3 levels, have confirmed the rate enhancement of hydroxyl radical addition to benzene via calculation of the transition states in the presence of explicit solvent molecules as well as a continuum dielectric field. The origin of the rate enhancement lies entirely in the structures of the transition states and not in the pre-reactive complexes. The calculations reveal that the hydroxyl radical moiety becomes more anionic in the transition state and, therefore, looks more like hydroxide anion. In the transition states, solvation of the incipient hydroxide anion is more effective with water than with acetonitrile and provides the strong energetic advantage for a polar solvent capable of hydrogen bonding. At the same time, the aromatic unit looks more like the radical cation in the transition state. The commonly held view that hydroxyl radical is electrophilic in its reactions with DNA bases is, therefore, strongly dependent on the ability of the organic substrate to stabilize the resulting radical cation.     

Laser flash photolysis studies of elementary reactions of significance in combustion

Recent measurements of radical-radical and radical-atom rate coefficients and of the heats of formation of ethyl and t-butyl radicals are discussed, in the context of the provision of rate data for elementary reactions of importance in combustion and pyrolysis.     

Laser flash photolysis of diphenylsulfonyldiazomethane: detection of the sulfene and a sulfene-pyridine ylide

The photochemistry of diphenylsulfonyldiazomethane (DSD) was studied by means of nanosecond laser flash photolysis. The photochemical behavior of this molecule upon UV irradiation is characterized by sulfene formation, presumed to arise via Wolff rearrangement of a carbene. We were able to detect the sulfene and the sulfene ylide formed upon sulfene trapping by pyridine. Sulfene quenching by nucleophiles was also examined.     

Laser flash photolysis of spiropyran in the presence of metal ions in solution

Conclusions Intermediate trans isomers of spiropyran with different stability participate in the reaction of complexing of spiropyran with metal ions. Several types of complexes are formed. A scheme of the reaction of complexing of spiropyran with metal ions is proposed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2243–2250, October, 1988.     

Kinetic studies of a fast, reversible alkene radical cation cyclization reaction

The radical cation formed by mesylate heterolysis from the 1,1-dimethyl-7,7-diphenyl-2-mesyloxy-6-heptenyl radical was studied in several solvents. Computational results suggest that the initially formed acyclic radical cation is a resonance hybrid with partial positive charge in both double bonds of 1,1-diphenyl-7-methyl-1,6-octadiene (10). Thiophenol trapping was used as the competing reaction for kinetic determinations. The acyclic radical cation rapidly equilibrates with a cyclic distonic radical cation, and thiophenol trapping gives acyclic product 10 and cyclic products, mainly trans-1-(diphenylmethyl)-2-(1-methylethenyl)cyclopentane (11). The rate constants for cyclization at ambient temperature were k = (0.5-2) x 10(10)(s-1), and those for ring opening were k = (1.5-9) x 10(10)(s-1). Laser flash photolysis studies in several solvents show relatively slow processes (k = (2.5-260) x 10(5)(s-1) that involve rate-limiting trapping reactions for the equilibrating radical cations. In mixtures of fluoroalcohols RfCH2OH in trifluoromethylbenzene, variable-temperature studies display small, and in one case a negative, activation energies, requiring equilibration reactions prior to the rate-limiting processes. Fast equilibration of acyclic and cyclic radical cations implies that product ratios can be controlled by the populations of the acyclic and cyclic species and relative rate constants for trapping each.     

Laser flash photolysis kinetic studies of enol ether radical cations. Rate constants for heterolysis of alpha-methoxy-beta-phosphatoxyalkyl radicals and for cyclizations of enol ether radical cations

Two series of enol ether radical cations were studied by laser flash photolysis methods. The radical cations were produced by heterolyses of the phosphate groups from the corresponding alpha-methoxy-beta-diethylphosphatoxy or beta-diphenylphosphatoxy radicals that were produced by 355 nm photolysis of N-hydroxypryidine-2-thione (PTOC) ester radical precursors. Syntheses of the radical precursors are described. Cyclizations of enol ether radical cations 1 gave distonic radical cations containing the diphenylalkyl radical, whereas cyclizations of enol ether radical cations 2 gave distonic radical cation products containing a diphenylcyclopropylcarbinyl radical moiety that rapidly ring-opened to a diphenylalkyl radical product. For 5-exo cyclizations, the heterolysis reactions were rate limiting, whereas for 6-exo and 7-exo cyclizations, the heterolyses were fast and the cyclizations were rate limiting. Rate constants were measured in acetonitrile and in acetonitrile solutions containing 2,2,2-trifluoroethanol, and several Arrhenius functions were determined. The heterolysis reactions showed a strong solvent polarity effect, whereas the cyclization reactions that gave distonic radical cation products did not. Recombination reactions or deprotonations of the radical cation within the first-formed ion pair compete with diffusive escape of the ions, and the yields of distonic radical cation products were a function of solvent polarity and increased in more polar solvent mixtures. The 5-exo cyclizations were fast enough to compete efficiently with other reactions within the ion pair (k approximately 2 x 10(9) s(-1) at 20 degrees C). The 6-exo cyclization reactions of the enol ether radical cations are 100 times faster (radical cations 1) and 10 000 times faster (radical cations 2) than cyclizations of the corresponding radicals (k approximately 4 x 10(7) s(-1) at 20 degrees C). Second-order rate constants were determined for reactions of one enol ether radical cation with water and with methanol; the rate constants at ambient temperature are 1.1 x 10(6) and 1.4 x 10(6) M(-1) s(-1), respectively.     

Photochemistry of a crown ether styryl dye adsorbed on silica gel and in acetonitrile solution: a comparative flash photolysis study

The photochemical behaviour of a photochromic crown ether styryl dye (BOB) adsorbed on silica gel has been found by a steady-state technique to be similar to that observed in a fluid solution of high polarity. The intermediate spectra obtained by diffuse reflectance laser flash photolysis of BOB on silica gel in a nitrogen atmosphere have been preliminarily attributed to the triplet-triplet (T-T) absorption of BOB and the absorption of BOB cation radicals. For comparison, the absorption spectrum of BOB triplets with a lifetime of ∼0.8 μs in acetonitrile solution has been obtained using biphenyl as a sensitizer.