Properties of molecular switch triad compounds for photoinduced intramolecular energy transfer

This paper reports the synthesis and transient absorption decay kinetics of several triad compounds, in which a sulfur aryl section is used as the spacer for intramolecular energy transfer (IET). After flash photolysis the producing sulfur radicals will provide an ‘energy trap’ to stop the IET process, after stopping flash photolysis the sulfur radicals reversibly recombine and the IET process recovers, and then a rapid photoinduced IET switchable function is realized.     

PHOTOPHYSICAL AND PHOTOCHEMICAL PROPERTIES OF TRITON X-165 IN AQUEOUS AND METHANOLIC SOLUTIONS

Abstract Laser flash photolysis of the nonionic surfactant Triton X-165 was carried out at 248 nm in methanolic and aqueous solutions at different pHs. Cation radical, triplet, phenoxy radical and solvated electron were observed as the transient absorbing species. Various characteristics of these transient species and their decay constants are reported. Photoionization was found to be a major process and it was monophotonic. The results obtained are compared with the photolysis of a small molecule p -methoxytoluene (PMT). Fluorescence and phosphorescence properties of the surfactants Triton X-100, Triton X-165 and PMT are also studied. A suitable reaction scheme is proposed to account for the observed results.     

Photochemical and cidep studies of the reactions of naphthalene and 2,6-Di-tert-butyl-p-cresol in solution

The photoreaction of the solution system of naphthalene and 2,6-di-tert-butyl-p-cresol was investigated by a combination of optical spectroscopy, ESR and time-resolved CIDEP techniques. ESR signal of 2,6-di-tert-butyl-4-methylphenoxy radical was detected during 308 nm irradiation. Three different types of polarization mechanisms contribute to the CIDEP of the phenoxy radicals. In the initial stage RPM with singlet precursor predominates, whereas RPM with F-pairtnplet precursor persists at later stage. The contribution from TM is superimposed during all the time regions but to a small extent. Both the CIDEP intensity at the initial stage as well as the depletion rate of naphthalene during photolysis were found to be proportional to the square of the light intensity, which indicates that most phenoxy radicals were produced via a biphotonic process.     

Magnetic Field Effects on the Dynamics of Radical Pairs in Micelles: A New Approach to Understanding the "Cage Effect"

The effect of temperature on the photolysis of dibenzyl ketone and 4-methyldibenzyl ketone in sodium dodecyl sulfate micelles was studied by laser flash photolysis and product distributions derived from steady-state photolysis. At high temperatures, the product distribution and radical decay kinetics are primarily due to random encounters of radicals, and the "cage effect" cannot be rationalized by geminate recombination reactions that occur before the radicals escape from the micelles. A mechanism is proposed in which the enhancement of the crosstermination product derived from random encounters is due to the different partitioning of each radical species between the micelles and the aqueous phase, thereby leading to different rates for the self-termination reactions.     

Evidence for a lack of reactivity of carotenoid addition radicals towards oxygen: a laser flash photolysis study of the reactions of carotenoids with acylperoxyl radicals in polar and non-polar solvents

In this paper, we report the results of a laser flash photolysis study of the reactions of a range of carotenoids with acylperoxyl radicals in polar and nonpolar solvents. The results show, for the first time, that carotenoid addition radicals do not react with oxygen to form carotenoid peroxyl radicals; an observation which is of significance in relation to antioxidant/pro-oxidant properties of carotenoids. Acylperoxyl radicals, generated by photolysis of ketone precursors in oxygenated solvents, display high reactivity toward carotenoids in both polar and nonpolar solvents, but the nature of the carotenoid radicals formed is dependent on solvent polarity. In hexane, acylperoxyl radicals react with carotenoids with rate constants in the region of 10(9) M(-1) s(-1) and give rise to transient absorption changes in the visible region that are attributed to the formation of addition radicals. All of the carotenoids show bleaching in the region of ground-state absorption and, with the exception of 7,7'-dihydro-beta-carotene (77DH), no distinct absorption features due to addition radicals are observed beyond the ground state absorption region. For 77DH, the addition radical displays an absorption band that is spectrally resolved from the parent carotenoid absorption. The rate of decay of the 77DH addition radical is unaffected by oxygen in the concentration range 10(-4)-10(-2) M, suggesting that these resonance-stabilized carbon-centered radicals are not scavenged by oxygen. At low incident laser intensities, the 77DH addition radical decay kinetics are 1st order with k(1) approximately 4 x 10(3) s(-1) at room temperature. The 1st order decay is attributed to an intramolecular cyclization process, which is supported by the substantial negative entropies of activation obtained from measurements of the decay rate constants for different 77DH addition radicals as a function of temperature. No transient absorption features are observed in the red or near-infrared regions in hexane for any of the carotenoids studied. In polar solvents such as methanol, acylperoxyl radicals also react with carotenoids with rate constants in the region of 10(9) M(-1) s(-1), but give rise to transient absorption changes in both the visible and the red/near-infrared regions, where it is evident that there are two distinct species. For 77DH, the addition radical absorption around 450 nm is still evident, although its kinetic behavior differs from its behavior in hexane. For 77DH and zeta-carotene (zeta-CAR) the spectral and kinetic resolution of the various absorption bands simplifies kinetic analysis. The kinetic evidence suggests that addition radical formation precedes formation of the two near-infrared absorbing species, and that the kinetics of the addition radical decay match the kinetics of formation of the first of these species (NIR1, absorbing at shorter wavelengths). The decay of NIR1 leads to NIR2, which is attributed to the carotenoid radical cation. The solvent dielectric constant dependence of the relative amounts of NIR1 and NIR2 formed leads us to speculate that NIR1 is an ion-pair. However, an alternative assignment for NIR1 is an isomer of the radical cation. The results, in terms of the pattern of reactivity the carotenoids display and of the properties of the carotenoid radicals formed, are discussed in relation to the antioxidant/pro-oxidant properties of carotenoids.     

Decay kinetics of aryloxy and semiquinone radicals in the presence of copper ions

The decay kinetics of aryloxy and semiquinone radicals in the presence of copper ions in aqueous solutions has been studied by means of the flash photolysis technique. The radicals are involved in electron transfer reactions and those leading to the formation of intermediate complexes with copper ions. The complexes of p-benzosemiquinone anion radicals and 2-hydroxyphenoxy radicals with cupric ions decay in bimolecular self-reactions at a much slower rate than the original radicals. The increased stability of the complexes compared with the initial radicals is attributed to partial delocalization of the unpaired electron over the electron shell of copper and to steric hindrances in the self-reactions of complexes.     

Rate of reaction of N with CN (υ = 0,1)

CN radicals were generated by 193 nm ArF laser flash photolysis of C₂N₂ in a fast-flow system, and their decay was monitored by dye-laser-induc     

On the reaction of the NH2 radical with oxygen

The decay kinetics of NH₂ radicals formed by flash photolysis of NH₃ + O₂ and NH₃ + N₂ mixtures at total pressures below 30 Torr we     

Photo-stabilising action of metal chelate stabilisers in polypropylene—Part VI: Importance of singlet (1Δg) oxygen quenching and UV screening,and flash photolysis studies

The importance of singlet (¹Δ_g) oxygen quenching in the stabilisation mechanism of polypropylene by metal chelates has been investigated. Methylene blue was used as the sensitiser and 9,10-dimethyl anthracene was used as a singlet oxygen chemical trap. The results show that singlet oxygen quenching has no important rôle in the stabilising mechanism of metal chelates, in a polypropylene matrix.The importance of ultraviolet screening by the chelates as a stabilising mechanism is also considered. Various concentration levels, ranging from 0·001% to 1·0% w/w, were used. The results show that, as in the case of singlet oxygen quenching, ultraviolet screening is unimportant as an effective photo-stabilising mechanism.The photo-stability of four nickel chelates in both polar and non-polar solvents has been examined using conventional flash photolysis. It is found that their activity correlates with their ability to protect the polymer at concentrations below 0·2% w/w. The radicals formed during flash photolysis are tentatively assigned to be phenoxy radicals.     

Decay kinetics of the triplet and anthrasemiquinone radicals in aqueous organic media

Electron transfer involving triplet molecules of anthraquinone sulfonates has been studied by flash photolysis. It was revealed that semiquinone radicals were formed in one-electron oxidation of carbonate anions. Absorption spectra and the kinetics of decay transients in aqueous and aqueous-alcoholic solutions have been studied.     

Kinetics of the sulfate radical‐mediated photo‐oxidation of humic substances

The kinetics of the aqueous phase reaction of sulfate radicals with commercial humic acids and with organic matter extracted from vermicompost (VC) was studied by flash‐photolysis. The results can be interpreted by a mechanism that in a first step considers the reversible binding of the sulfate radicals by the humic substances. Both the bound and free sulfate radicals decay to oxidized products. From experiments performed with Aldrich humic acids in the temperature range from 283 to 303 K, the enthalpy change associated with the binding process was estimated to be ?(36 ± 11) kJ mol^?1. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 40: 19–24, 2008     

Chain-breaking antioxidant activity of phosphite esters

The inhibition of the autoxidation of hydrocarbons and polypropylene by aliphatic, aromatic, sterically hindered and cyclic phosphites has been studied by means of volumetric and ³¹P-NMR techniques. The antioxidant activity of phosphites depends on the rate of their reactions with peroxyl radicals and on the way they react with alkoxyl radicals. Only those phosphites which react by substitution to give free aryloxyl radicals are effective as chain-breaking antioxidants.The reaction modes of various phosphites with various peroxyl and alkoxyl radicals have been studied in some model reactions and the relationship between structure, reaction mechanism and antioxidant activity has been elucidated.     

Photochemical studies on a phenolic phenylcoumarone lignin model molecule in relation to the photodegradation of lignocellulosic materials. Part 2. Photophysical and photochemical studies

The structure of the photoproducts obtained by irradiation of a phenylcoumarone molecule PCO, model of the photodegradation of lignin, being established, the mechanism of their formation was approached by fluorescence and flash photolysis techniques, low temperature absorption spectroscopy in rigid matrix, and continuous irradiation in solution and in the solid state. Some evidence was given for the formation of a phenoxy radical leading either by oxidoreductive processes to the formation of a catechol IR3, or by coupling with the starting phenol to the generation of a cyclobutane radical dimer, prone to be transformed into stilbene phenoxy radical by retro “2π+2π” reaction. It was proposed that the oxidation of the latter could give the very coloured quinone methide IR1 or the α-carbonylstilbenol IR2. The singlet manifold appears to be the main pathway of the production of IR1, IR2 and IR3 which are formed in the presence of ground state oxygen.     

MNDO calculation of reaction heats determininig the mechanism of antioxidant action of the phenols in polyolefins

The quantum chemical substantiation of antioxidant action of the substituted phenols in polyolefins is presented. The electronic structure of the phenols, phenoxy radicals, and model products of polyolefin destruction was calculated by the MNDO method. On the basis of the presumed chemical reactions, the electronic structure criteria determining the antioxidant activity are selected. These are the O—H bond dissociation energy in phenols and the charge on the atoms of π-system in phenoxy radicals. The competition between these reactions takes place, and the calculation of the reaction heats shows that the reactions with the formation of the substituted quinones are in preference to the reactions in which the peroxycyclo-hexadienones are formed.     

Laser flash spectroscopy of iodine and iodoform

The absorption spectra and decay times of transient species produced by laser flash photolysis of iodine and iodoform solutions are reported. The results show that the primary process in the photolysis of iodoform is the formation of atomic iodine.     

Benzo[d]-1,2-oxaphospholes as precursors of stabilized C-centered radicals

[reaction: see text] Several benzo[d]-1,2-oxaphosphole 2-oxides were examined as potential precursors of stabilized C-centered radicals. The transient absorption spectra obtained after laser flash photolysis in the presence of di-tert-butyl peroxide showed the features of benzylic radicals with formation and decay kinetics not significantly influenced by the presence of oxygen. In the case of compounds with two possible diastereomeric forms, the C-H bond of the trans-isomers [corrected] is more reactive toward hydrogen abstraction.     

A PHYTOCHROME PHOTOTRANSFORMATION STUDY USING TWO-LASER/TWO-COLOR FLASH PHOTOLYSIS: ANALYSIS OF THE DECAY MECHANISM OF I700*

The mechanism of I₇₀₀ decay, representing an early event in the phytochrome P_r→ P_fr phototransformation, was reanalyzed in the microsecond range by conventional laser flash photolysis as well as by two-laser/two-color flash photolysis. Three kinetic models that might describe the I₇₀₀ decay mechanism following P_r excitation were considered: a parallel, a sequential, and an equilibrium model. These models were used to mathematically simulate both the one- and two-laser flash experiments in an effort to select the model best describing the I₇₀₀ decay. The sequential model could be excluded already on the basis of the one-laser flash photolysis results alone. Discussion of the two-laser/two-color flash rcsults in the context of the equilibrium and the parallel models is presented.     

Rapid cleavage of the naphthylmethyl-oxygen bond in higher triplet excited states

Rapid cleavage of the naphthylmethyl-oxygen bond of 1- and 2-[(4-benzoylphenoxy)methyl]naphthalenes in higher triplet excited states occurred within a laser flash of 5 ns to give 1- and 2-naphthylmethyl radicals with formation quantum yields of 0.042 +/- 0.004 and 0.020 +/- 0.002, respectively, during two-colour two-laser flash photolysis.     

Photolysis of 1,2-dihydroquinolines in micellar solutions of anionic and cationic surfactants

The kinetics of the photolysis of substituted 1,2-dihydroquinolines (DHQ) in micellar solutions was studied by steady-state and flash photolysis. The photolysis mechanism depends dramatically on the location of DHQ molecules in micelles, which is governed by the surfactant nature. In micellar solutions of the anionic surfactant sodium dodecyl sulfate (SDS), where the DHQ molecules are located in the Stern layer, the intermediate species decay kinetics follows a first-order law. When DHQ is in neutral form (pH 4–12), the rate constant of the intermediate carbocation decay increases from 25 to 198 s^?1 with an increasing concentration of DHQ in micelles. The positive micellar catalysis is caused by the acceleration of the final product formation with the DHQ molecule via proton abstraction from the intermediate cation. The formation of several types of intermediate species—carbocations in the aqueous phase and aminyl radicals in micelles—is observed in micellar solutions of the cationic surfactant cetyltrimethylammonium bromide (CTAB) due to the preferential location of DHQ molecules in the micellar core. The carbocation decays via a pseudofirst-order reaction with a rate constant close to that in the aqueous solution. The lifetime of the DHQ aminyl radicals in the micellar solutions is longer by several orders of magnitude than the lifetime observed for homogeneous solutions of hydrocarbons and alcohols.     

Activation-controlled hydrogen abstraction by benzophenone triplet

Through nanosecond flash photolysis measurements, activation energies for hydrogen abstraction by benzophenone triplet have been shown to correlate with solvent molecular structure. We demonstrate that two successive processes result in photoproduct generation. One, triplet quenching, is independent of macroscopic viscosity, and is not simply reflected in the observed quantum yield. The second process involves separation of two caged radicals, and does not occur in rigid media.