Ketyl-phenoxyl triplet radical pair in glycerol: Magnetic field effect and cage kinetics in terms of the two-state model.

The geminate recombination kinetics of the triplet pairs of neutral radicals formed as a result of hydrogen atom transfer from 4-methylphenol to 4,4'-dimethylbenzophenone in the triplet excited state was studied by laser flash photolysis in the temperature range of 263-323 K. The pair included aromatic ketyl and 4-methylphenoxyl radicals produced in a viscous (∼10 P) solution of the initial reagents in glycerol. The geminate recombination kinetics demonstrate a "tail" c(t) ∼ 1/t, with contribution of magnitude An, which increases with temperature. The two-state (TS) model, which takes into account the potential well caused by the interaction between radicals at short distances and the radicals that escaped from a well but returned to it after free diffusion outside the well, was used to fit the data. The application of an external magnetic field (MF) (0.2 T) gave rise to the retardation of the geminate recombination and to the increase of fraction of radicals in the solvent bulk. The bell-shaped dependence of the magnetic field effect (MFE) value upon temperature (with a maximum near 300 K) was found. This dependence correlates with An and its dependence on the MF. At temperatures lower than 270 K, both the contribution of the nonexponential part of geminate recombination kinetics and the MFE were found to be small. The TS model can quantitatively describe the effects of solvent viscosity and MF on the kinetics of geminate recombination of triplet RP.

Photoinduced Reactions of Benzophenone in Biaxially Oriented Polypropylene.

The photoinduced reactions of benzophenone (B) in biaxially oriented polypropylene (BOPP) were studied with nanosecond laser photolysis (N2 laser, λ337.1 nm). The first observed transient was a triplet state 3B*. Decay of 3B* led to formation of a radical pair (RP) of BH• and R•, where R• is a radical formed by hydrogen abstraction from BOPP (RH) by 3B*. We studied BOPP after the preheating for a short time in a temperature range 298-423 K, which is essentially lower than its melting point of 453 K. All measurements with not-heated and with preheated (annealed) BOPP were made at 298 K. A radical pair (RP) apparently decays as a contact pair 3[BH•, R•] in nonheated BOPP. A critical phenomenon takes place: dissociation of RP with a formation of free radicals in the polymer bulk is observed at preheating temperature Tcrit ≈ 403 K and at a higher T. The physical process of heating and cooling of BOPP apparently resulted in the restructuring of crystallites, their agglomeration, shrinking of the distribution of crystallites according to their sizes in BOPP. Overall BOPP becomes softer which manifests itself in the radical kinetics. The decay kinetics of 3B* and RP in the cage fits well the first-order law. Rate constants were obtained. Radicals BH•, which exit into the polymer bulk at temperatures of preheating T ≥ 403 K, decay by cross-termination according to the second-order law. A relatively high rate constant ∼108 M-1·s-1 for this reaction was obtained due to diffusion of BH• enclosed in the soft amorphous phase of BOPP. Properties of BOPP containing B were studied with ESR, DSC, IR, and WAXD.

Reactivity of Benzophenone Ketyl Free Radicals in an Elongated Elastomer Film.

The effect of polymer film elongation leading to the thinning of the film up to three times on the decay of transients was studied. Kinetics of benzophenone (B) triplet state (3)B* and ketyl free radical BH• in soft rubber poly(ethylene-co-butylene) (abbreviated as E) was investigated by nanosecond laser flash photolysis. We monitored decay kinetics of the triplet state of (3)B* and of BH• decay in the polymer cage and decay of BH• in the polymer bulk. The fast exponential decay of (3)B*(lifetime τT ≈ 200 ns) is accompanied by hydrogen atom abstraction from E with the formation of BH• and a polymer free radical R•. The decay of BH• in the polymer cage occurs during τc ≈ 1 µs. Cage recombination, in turn, was followed by a cross-termination of BH• in the polymer bulk (τb ≈ 100 µs under our conditions) and is characterized by a rate constant kb ≈ 10(8) M(-1) s(-1). We studied changes of rates of transients decay upon elongation (thinning) of E. Decay of (3)B* is practically independent of elongation of the film. Recombination of BH• in the solvent bulk occurs with a two times lower kb than in a nonelongated E. The decrease in kb is ascribed mainly to a lower fractional polymer free volume Vf in elongated E compared with that in nonelongated E. Dependencies of log10 kb versus [Formula: see text], where [Formula: see text] is the thickness of the film, turned out to be linear with a negative slope. At the same [Formula: see text] recombination proceeds slower in the elongated elastomer compared with the nonelongated elastomer. Cage effect increases twice as well due to a lower rate of radicals escape from the polymer cage in the elongated film. We observed relatively large effects of external magnetic field (B = 0.2T) on the kinetics of cage recombination and recombination in the polymer bulk. Magnetic field effect on recombination rates in the cage and in the solvent bulk does not depend on elongation.

Kinetics of benzophenone ketyl free radicals recombination in a polymer: reactivity in the polymer cage vs. reactivity in the polymer bulk.

The decay kinetics of intermediates produced under photolysis of benzophenone (B) dissolved in soft rubber poly(ethylene-co-butylene) films (abbreviated as E) was studied by ns laser flash photolysis in the temperature range of 263-313 K. We monitored decay kinetics of the triplet state of (3)B* and of benzophenone ketyl free radical BH˙. The fast exponential decay of (3)B* (life-time τT ≈ 200 ns) is accompanied by hydrogen atom abstraction from E with a formation of BH˙ and a polymer free radical R˙. Decay of (3)B* was followed by decay of BH˙ in the polymer cage (geminate recombination) with τc ≈ 1 µs. Cage recombination in turn was followed by a decay of BH˙ in the polymer bulk (τb ≈ 100 µs). Fortunately, all three processes are separated in time. Both cage and bulk reactions were decelerated by the application of magnetic field (MF) of 0.2 T by approximately 20%. Geminate recombination was fit to the first-order kinetic law, and recombination in the solvent bulk fits well to the second-order law. Both geminate recombination and recombination in the solvent bulk are predominantly a reaction between BH˙ and R˙. It was assumed that the reaction radius ρ12 and a mutual diffusion coefficient D12 of BH˙ and R˙ are the same for the cage and bulk recombination, respectively. This led to an estimation of ρ12 = 3.3 nm and D12 = 1 × 10(-7) cm(2) s(-1). These values are discussed. We obtained activation energy, Eact, equal to 6 kcal mol(-1) and 7 kcal mol(-1) for cage decay and for recombination in the polymer bulk, respectively. These Eact coincide with each other within experimental error of their determination (±0.5 kcal mol(-1)). This indicates the same diffusion character in the cage and in the polymer bulk. It was demonstrated that an exponential model of cage effect sufficiently describes the obtained experimental data in rubber.

Decay kinetics of benzophenone triplets and corresponding free radicals in soft and rigid polymers studied by laser flash photolysis.

The kinetics of transients formed under photoexcitation of benzophenone (B) dissolved in three different polymers was studied by ns laser flash photolysis. These polymers were the soft rubbers poly (ethylene-co-butylene) (EB), polystyrene block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) and hard polystyrene (PS). We monitored the decay kinetics of triplet state (3)B(*) and of ketyl radicals BH(●). We observed exponential decay of (3)B(*) and two-stage decay kinetics of BH(●) in EB. The first stage is a fast cage recombination of a radical pair (BH(●), radical of polymer R(●)). The second slow stage of BH(●) decay follows the second-order law with a relatively high rate constant, which corresponds to recombination of BH(●) in a homogeneous liquid with a viscosity of only ~0.1 P (about five times of 2-propanol viscosity). Application of a magnetic field (MF) of 0.2 T leads to deceleration of both stages of BH(●) decay in EB by approximately 20%. Decay kinetics of both transients were observed in SEBS. There was no MF effect on BH(●) decay in SEBS. We only observed (3)B(*) in PS. Decay kinetics of (3)B(*) in this case were described as polychromatic dispersive first-order kinetics. We discuss the effects of polymer structure on transient kinetics and the MF effect.

Photolysis of endoperoxides in the presence of nitroxides: a laser flash photolysis study with optical and ESR detection.

Time-resolved electron paramagnetic resonance spectroscopy, transient absorption, and phosphorescence spectroscopy were used to investigate the spin polarization of a nitroxide free radical induced by interaction with singlet oxygen ((1)O2). The latter was generated by photolysis of endoperoxides of two anthracene derivatives. Although both anthracene endoperoxides are structurally similar, opposite spin polarization of the nitroxide was observed. Photolysis of one endoperoxide leads to absorptive nitroxide spin polarization due to interaction with the generated (1)O2. Photolysis of the other endoperoxide generated emissive nitroxide spin polarization, probably due to interaction of the endoperoxide triplet states with nitroxides.

Time-resolved EPR study of singlet oxygen in the gas phase.

X-band EPR spectra of singlet O2((1)Δg) and triplet O2((3)Σg(-)) were observed in the gas phase under low molecular-oxygen pressures PO2 = 0.175-0.625 Torr, T = 293-323 K. O2((1)Δg) was produced by quenching of photogenerated triplet sensitizers naphthalene C8H10, perdeuterated naphthalene, and perfluoronaphthalene in the gas phase. The EPR spectrum of O2((1)Δg) was also observed under microwave discharge. Integrated intensities and line widths of individual components of the EPR spectrum of O2((3)Σg(-)) were used as internal standards for estimating the concentration of O2 species and PO2 in the EPR cavity. Time-resolved (TR) EPR experiments of C8H10 were the main focus of this Article. Pulsed irradiation of C8H10 in the presence of O2((3)Σg(-)) allowed us to determine the kinetics of formation and decay for each of the four components of the O2((1)Δg) EPR signal, which lasted for only a few seconds. We found that the kinetics of EPR-component decay fit nicely to a biexponential kinetics law. The TR EPR 2D spectrum of the third component of the O2((1)Δg) EPR spectrum was examined in experiments using C8H10. This spectrum vividly presents the time evolution of an EPR component. The largest EPR signal and the longest lifetime of O2((1)Δg), τ = 0.4 s, were observed at medium pressure PO2 = 0.4 Torr, T = 293 K. The mechanism of O2((1)Δg) decay in the presence of photosensitizers is discussed. EPR spectra of O2((1)Δg) evidence that the spin-rotational states of O2((1)Δg) are populated according to Boltzmann distribution in the studied time range of 10-100 ms. We believe that this is the first report dealing with the dependence of O2((1)Δg) EPR line width on PO2 and T.

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.

Cage effect dynamics.

The kinetics of recombination/dissociation of photogenerated radical pairs (RPs) is described with a generalized model (GM), which combines exponential models (EMs) and contact models (CMs) of cage effect dynamics. The main assumption of EM is the irreversible dissociation of RP as a first-order reaction. CM takes into account repetitive contacts of radicals in the pair or reversible diffusional separation of radicals. The present GM accounts for both the recombination of RP within the cage of outcome and their diffusional separation in a free space. GM predicts that a short initial time RP decay is described by EM, and RP kinetics at longer times of observation is described by CM. This theory successfully describes available experimental data. Solvent viscosity effect on the cage effect values is also analyzed.

Kinetics of geminate recombination of organic free radicals in viscous solvents.

The kinetics of geminate recombination of multiatomic organic free radicals in glycerol solutions have been investigated. Radicals were produced by ns laser flash photolysis under photoreduction of carbonyl compounds. An external moderate magnetic field decelerates geminate recombination leading to smaller cage effect values. The duration of geminate recombination is discussed.

A time-resolved electron paramagnetic resonance investigation of the spin exchange and chemical interactions of reactive free radicals with isotopically symmetric (14N-X-14N) and isotopically asymmetric (14N-X-15N) nitroxyl biradicals.

Interactions between reactive free radicals (r) with stable mononitroxyl radicals (N) and bisnitroxyl radicals (N-X-N) were studied by time-resolved electron paramagnetic resonance (TR-EPR). Reactive spin-polarized free radicals (r#), with non-Boltzmann population of spin states were produced by laser flash photolysis of benzil dimethyl monoketal or of (2,4,6-trimethylbenzoyl)diphenyl phosphine oxide (the superscript # symbol indicates electron spin polarization). Both isotopically symmetric nitroxyl biradicals (14N-X-14N) and isotopically asymmetric nitroxyl biradicals, with one nitroxyl bearing 15N and the other nitroxyl bearing 14N (14N-X-15N), were employed as probes of the spin exchange and chemical interactions between r and the nitroxyl biradicals. The interaction of r# with the asymmetric ortho-nitroxyl biradical (14N-O-15N), which exists in a condition of strong spin exchange, proved to be particularly informative. In this case, spin polarized (14N-O-15N)# (product of spin exchange with r#) and two polarized monoradicals (r14N-O-15N)# and (14N-O-15Nr)# (products of chemical reaction with r#) were observed. The latter three species possess three distinct TR-EPR spectra with different line splittings. The relative cross sections for spin exchange (Rex) and chemical reaction (Rrxn) were achieved through computer simulation of the TR-EPR spectra. The cross section for spin exchange, Rex, between r# and (N-X-N) biradical is estimated to be 4-6 times larger than the cross section of chemical reaction, Rrxn, between r# and (N-X-N). The para-nitroxyl biradical (14N-P-15N) exists in weak spin exchange, and behaves as an equimolar mixture of 14N and 15N mononitroxyls.

Deactivation processes of the lowest excited state of [UO2(H2O)5]2+ in aqueous solution.

A detailed analysis of the photophysical behaviour of uranyl ion in aqueous solutions at room temperature is given using literature data, together with results of new experimental and theoretical studies to see whether the decay mechanism of the lowest excited state involves physical deactivation by energy transfer or a chemical process through hydrogen atom abstraction. Comparison of the radiative lifetimes determined from quantum yield and lifetime data with that obtained from the Einstein relationship strongly suggests that the emitting state is identical to that observed in the lowest energy absorption band. From study of the experimental rate and that calculated theoretically, from deuterium isotope effects and the activation energy for decay support is given to a deactivation mechanism of hydrogen abstraction involving water clusters to give uranium(v) and hydroxyl radicals. Support for hydroxyl radical formation comes from electron spin resonance spectra observed in the presence of the spin traps 5,5-dimethyl-1-pyrroline N-oxide and tert-butyl-N-phenylnitrone and from literature results on photoinduced uranyl oxygen exchange and photoconductivity. It has previously been suggested that the uranyl emission above pH 1.5 may involve an exciplex between excited uranyl ion and uranium(v). Evidence against this mechanism is given on the basis of quenching of uranyl luminescence by uranium(v), together with other kinetic reasoning. No overall photochemical reaction is observed on excitation of aqueous uranyl solutions, and it is suggested that this is mainly due to reoxidation of UO2+ by hydroxyl radicals in a radical pair. An alternative process involving oxidation by molecular oxygen is analysed experimentally and theoretically, and is suggested to be too slow to be a major reoxidation pathway.