P-aminophenylthiyl Radical Research Articles

Magnetic field effects on the reaction of a triplet-born radical pair consisting of two equivalent sulphur-centre radicals under ultrahigh fields of up to 28 T

A water-cooled pulsed magnet having a room temperature bore of diameter 20 mm was constructed. Magnetic field effects on a triplet sensitization reaction of p-aminophenyl disulfide (APDS) with xanthone (Xn) were studied in a sodium dodecylsulfate (SDS) micellar solution by a nanosecond laser flash photolysis technique under ultrahigh magnetic fields of up to 28 T. Upon irradiation of the SDS micellar solution containing APDS and Xn, triplet radical pairs of p-aminophenylthiyl radicals were generated. The yield of the escaped p-aminophenylthiyl radical was found to decrease by 6 ± 2% at 28 T compared with that at 0 T.

Anomalous Magnetic Field Effects Interpreted by the Relaxation Mechanism under Ultrahigh Fields of up to 28 T

Abstract Magnetic field effects on the triplet sensitization reaction of p-aminophenyl disulfide were studied in a sodium dodecylsulfate micellar solution by a nanosecond laser flash photolysis technique under ultrahigh magnetic fields of up to 28 T. The yield of the escaped p-aminophenyl thiyl radical decreased by 6 ± 2% at 28 T compared with that at 0 T.

Molecular dynamics simulation of the geminate radical recombination in solution: effects of the large dipole moment of the radicals

Molecular dynamics simulation of a simple model system of the geminate radical pair in solution has been performed to elucidate the dynamic behavior of radicals. The diffusion process of radicals in the microscopic region was simulated and the effect of the dipole of the radicals was investigated in both the nonpolar and polar solvents. We found that the dipole-dipole interaction stabilized the radical pair with a small separation. The dipole pair can be the precursor of the experimentally observed sandwich radical dimer. The conformation of the dipole pair may not be favorable for the recombination of the radicals, which can be the reason of the high escape probability observed for the p-aminophenylthiyl radical.

Geminate Recombination of the p-Aminophenylthiyl Radical Pair Produced by the Photodissociation of p-Aminophenyl Disulfide in Nonpolar Solvents

Photodissociation of p-aminophenyl disulfide and the following geminate recombination of the p-aminophenylthiyl radical pair as well as radical dimer formation in nonpolar solvents have been investigated by using picosecond transient absorption spectral measurement techniques. Temperature effects of the geminate recombination and the sandwich dimer formation were measured and compared with the results of the molecular orbital calculation. The spectral broadening of the radical band observed in the short delay times was considered to be an indication of the radical−radical interaction in the geminate pair and was analyzed to obtain the time dependence of the separation distance between the radicals. The exponential narrowing in time was found for the p-aminophenylthiyl radical band in n-hexane, which should carry information about the diffusion process in the solvent cage and is compared with the results of the simulation. The speed of the increase in the radical−radical separation distance shows a larger temperature effect than for the geminate radical recombination to form the parent molecule. The characteristic feature of the dynamic behavior of the p-aminophenylthiyl radical pair, the formation of the sandwich dimer, and the slow recombination of the linear dimer should be due to the large dipole moment of the radical, which not only prevents the recombination but keeps the radicals in close proximity with each other.

Competition between Geminate Recombination and Solvation of Polar Radicals following Ultrafast Photodissociation of Bis(p-aminophenyl) Disulfide

Supercontinuum probe pulses are used for studying the ultrafast photodissociation of bis(p-aminophenyl) disulfide into two p-aminophenylthiyl radicals and their subsequent geminate recombination in polar solvents. The kinetic investigations are complicated by a spectral absorption shift of the photolytically generated radicals due to solvation. The dissociation and recombination dynamics are separated from the solvation dynamics by a moment analysis of the transient spectra. Geminate recombination in polar solvents is observed only if the time scale for dielectric relaxation of the solvent is comparable to or slower than the time scale for recombination. The latter depends on the initial distance distribution of the radical pairs.

Photodissociation and geminate dynamics in solution phase: Picosecond transient absorption studies of tetraphenylhydrazines and diphenyl disulfides

Photodissociation of aromatic molecules and the following geminate dynamics of the photochemically produced fragment radical pair in liquid phase are discussed in relation to the results of ultrafast transient absorption measurements. Photodissociation of tetraphenylhydrazine occurs from the unequilibrated excited singlet state competing with the relaxation to the fluorescence state. In addition to the rapid fragmentation, the N−N bond rupture is clearly demonstrated to take place in the fluorescence state of tetraphenyl-and tetratolyl-hydrazine in tens of picoseconds time scale. In the case of di-p-aminophenyl disulfide in nonpolar solvents, geminate recombination of the p-aminophenylthiyl radical and the formation of the radical dimer were observed in tens of picoseconds. The observed slow geminate recombination can be explained by the repulsion between the large dipole of the fragment radical preventing the reformation of the S−S bond and by the restricted conformation required for the dimer formation at the encounter of radicals.

Photodissociation of aminophenyldisulfide and aminophenylthiol in the liquid phase. Geminate recombination and the dimer formation of p-aminophenylthiyl radical

Measuring picosecond time-resolved absorption spectra in the visible region, the photodissociation of p-aminophenyldisulfide and p-aminophenylthiol in various solutions has been investigated. In tens of picoseconds delay times a decay of p-aminophenylthiyl radical ascribed to the geminate recombination was observed for p-aminophenyldisulfide in nonpolar solvents, while the decay of the radical was not detected for p-aminophenylthiol in the same time region. In addition to a slight change in the spectral shape of p-aminophenylthiyl radical, the rise of the broad absorption band, which should be assigned to the radical dimer, follows the geminate recombination in nonpolar solvents.

Magnetic field effects on hydrogen abstraction of triplet xanthone with p-aminothiophenol in an SDS micellar solution

The photoreduction of xanthone with p-aminothiophenol in a sodium dodecylsulfate solution was investigated with the aid of a nanosecond laser flash photolysis technique at magnetic fields of 0–10 T. The lifetime of the radical pair generated by hydrogen abstraction of triplet xanthone and the yields of the escaped xanthone ketyl and p-aminophenylthiyl radicals increased steeply with increasing magnetic field strength (B) in a low-field region (B⩽0.05 T) and decreased more slightly with increasing B in a high-field region (0.05 T <B ⩽ 10 T). The observed magnetic field effects can be interpreted by a combination of the hyperfine coupling and the Δg mechanisms in the case of a triplet radical pair

Magnetic field effect on the photodecomposition of p-aminophenyl disulfide in a micellar solution

Upon laser flash photolysis of p-aminophenyl disulfide at room temperature, the radical pair of two p-aminophenylthiyl radicals was found to be generated by direct excitation and triplet sensitization in the magnetic field range 0-1.35 T. Only in the case of triplet sensitization did the yield of the escaped radical increase with increasing magnetic field strength. The yield at 1.35 T was 1.14±0.2 times as large as that at 0 T. This is the first report of a magnetic field effect on a reaction of two sulfur-centered radicals

Solvation of photolytically generated p-aminophenylthiyl radicals studied by sub-picosecond transient absorption

By pump-and-probe spectroscopy, the photolysis of bis-( p-aminophenyl) disulfide in diethyl ether has been followed with sub-picosecond time resolution. The visible absorption band of generated p-aminophenylthiyl radicals rises in two stages: an unresolved fast initial rise is followed by a second, exponential rise with a time constant of 5.9 ps. During the second process, the peak absorption of the band shifts from its initial position at 557.9 to 572.0 nm. A rate-equation model quantitatively accounts for the observations. The first stage is assigned to the formation of the radical pair in a common solvent cage, and the second stage to the separation and solvation of the radicals.

Resonance cars spectrum of p-aminophenylthiyl radical and Raman spectrum of bis-( p-aminophenyl) disulfide

The resonance CARS spectrum of p-aminophenylthiyl radical, produced by the UV photolysis of bis-( p-aminophenyl) disulfide, was observed in solution at room temperature. The Raman spectrum of the parent molecule was also recorded for the first time. The single bond nature of the CS bond in the radical molecule was suggested from a comparison of the observed Raman frequencies of the two species.

The extinction coefficient for the p-aminophenylthiyl radical as determined by reaction with galvinoxyl

The reaction of the p-aminophenylthiyl radical (RS) with galvinoxyl has been used to determine the extinction coefficient of RS in several solvents. Values of ϵ RS range from 11 000 M −1 cm −1 in cyclohexane to 19 000 M −1 cm −1 in acetonitrile. The addition of RS to galvinoxyl and the second-order recombination of RS both proceed at nearly diffusion-controlled rates with rate constants of (1.1 − 2) × 10 9 M −1 s −1. Rates of addition of RS to styrene have been determined to elucidate ground state radical solvation effects. In cyclohexane RS adds to styrene with a rate constant of 1.7 × 10 5 M −1 s −1. Increasing the polarity of the solvent lowers this rate constant to a limiting value of 4.8 X 10 3 M −1 s −1 in acetone and more polar solvents.

Kinetics of the addition of p-aminophenylthiyl radicals to vinyl monomers

The kinetics of the addition of p-aminophenylthiyl radicals to vinyl monomers were studied in cyclohexane. Thiyl radicals, generated by flash photolysis of the p-aminophenyldisulfide, react with monomers in a reversible fashion. These addition rates show a good correlation with the electron density of the monomer double bond and with stability parameters of the resulting adduct.

Spectral shifts of the p-aminophenylthyl radical absorption and emission in solution

Abstract The absorption and fluorescence spectra of the reactive p-aminophenylthiyl radical were observed in several solvents. Analysis of the spectral shifts yielded dipole moments for the ground and excited states of 4.3 D and 7.3 D, respectively. These values suggest a significant contribution of ionic resonance forms to both states.