P. P. Levin

Transfer of a hydrogen atom to acridine in the triplet state

ConclusionsThe quenching of triplet acridine by substituted phenols, thiophenols, and anilines is accomplished with high rate constants by hydrogen atom transfer with the quantitative formation of the corresponding neutral radicals.The solvation of the reagents by ethanol molecules or a decrease in the electron donor capacity of the quencher is accompanied by a decrease in the reaction rate constants.

Laser photolysis study of the triplet states of sterically hindered ortho-quinones

Conclusions1.Laser photolysis was used to determine the spectral kinetic characteristics of the triplet states, semiquinones and radical-anions of 3,5-di-tert-butyl-1,2-benzoquinone and 3,6-di-tert-butyl-1,2-benzoquinone in liquid solutions. The quantum yields for the formation of the triplet states are close to 100%.2.The triplet states of 3,5-di-tert-butyl-1,2-benzoquinone and 3,6-di-tert-butyl-1,2-benzoquinone are quenched by the corresponding hydroquinones with high rate constants and the formation of semiquinone radicals. Semiquinone radicals are formed in the case of N,N′-diethylaniline in nonpolar media while radical-ions are formed in polar media.

Effect of solvent and substituents on electron and hydrogen atom transfer in quenching of quinone triplets by secondary aromatic amines

Conclusions1.Hydrogen-atom transfer in quenching of triplet states of quinones by secondary aromatic amines takes place by establishment of prototropic equilibrium in the radical pair due to electron transfer. Electron transfer is the first event in quenching.2.The characteristics of the effect of the substituents and the medium on the position of prototropic equilibrium between radical pairs of neutral and charged radicals were established. A V-shaped dependence of the equilibrium constant on dielectric permeability in nonsolvating solvents was found. The specific solvation of hydroxyl-containing solvents results in a significant shift in the equilibrium toward formation of ion-radicals.

EPR spectral and laser photolysis study of radical pairs and their annihilation in quinone-doped pyrocatechol monocrystals

ConclusionsThe possibility of studying radical pairs in monocrystals by laser photolysis has been demonstrated. Comparison of the optical absorption spectra and EPR signals indicated that the radical pairs in 3,6-di-tert-butylpyrocatechol monocrystals with added 3,6-di-tertbutyl-o-benzoquinone at 77 K are comprised from the corresponding semiquinone radicals.

Kinetic isotope effect for hydrogen atom transfer from substituted anilines and phenols to the triplet state of anthraquinone

Conclusions1.The rates of triplet quenching of anthraquinone by substituted anilines and phenols and their OH- and NH2-deuterated analogs have been measured by pulse photolysis in toluene and acetonitrile solutions.2.In the case of anilines the kinetics of hydrogen atom transfer are determined by charge transfer in the transition state or by the formation of charge transfer complexes as intermediates in these reactions. The kinetic isotope effect for these reactions is increased up to five as the anthraquinone triplet quenching rate constants are decreased as a function of reduced electron-donating ability of substituted anilines.3.The kinetic isotope effect for hydrogen atom transfer reactions from substituted phenols to the triplet state of anthraquinone reaches a maximum of two and is determined not only by the degree of charge transfer in the transition state but also by the kinetics of formation of an intermediate hydrogen-bonded complex.

Kinetics of the reduction of iron porphyrin complexes by the methylviologen radical-cation

Conclusions1.Flash photolysis was used to measure the rate constant for the reduction of Fe(III)-and Fe(II)O2-porphyrin complexes by methylviologen radical cations in acetonitrile.2.The rate of electron transfer from MV+. to ΠFe(III) is virtually entirely determined by diffusion processes.

Quenching of triplet states of quinones by phenols

Conclusions1.The kinetics of quenching the triplet states of quinones by phenols in liquid solutions are determined by the process of forming a hydrogen-bond complex between the triplet and the quenching agent and by transfer of a hydrogen atom in this complex.2.The isotope effect for the rate constant in quenching triplet states of quinones by phenols first increases with increasing acidity of the phenol, and then drops off.3.The quantum yields of phenoxyl and semiquinone radicals formed by quenching triplet states of quinones by phenols are usually close to 100%, In quenching by phenols with strong acidic properties, the quantum yield of the radicals is lower if the triplet state of the quinone has a low energy.

Effect of the viscosity of the solvent on the kinetics of quenching of the triplet state of anthanthrone by aromatic electron and hydrogen atom donors

Conclusions1.The dependences of the rate constants of quenching of the triplet state of anthanthrone by aromatic electron and hydrogen atom donors on the viscosity of the solvent were obtained. The diffusion and kinetic elementary stages of the quenching reaction were distinguished, and the rate constants of these stages were determined.2.The reaction of formation of a complex with a hydrogen bond, which is an intermediate stage in quenching of the triplet state of anthanthrone by phenols, is controlled by diffusion but is characterized by relatively low rate constants, since rigorous mutual orientation of the reagents is required in this reaction.3.The rate constant of formation of an intermediate complex with a hydrogen bond between the triplet of anthanthrone and phenols decreases with an increase in the strength of the hydrogen bond, which requires a more rigorous mutual orientation of the reagents.4.The rate constant of transfer of a hydrogen atom from phenol to the triplet of anthanthrone in a complex with a hydrogen bond decreases when electronegative substituants are introduced in the phenol molecule.

Mechanism of quenching of triplet exciplexes by aromatic hydrogen-atom donors

Conclusions1.In the quenching of triplet exciplexes of quinones by phenols and secondary aromatic amines, hydrogen atom transfer is accomplished with the formation of phenoxyl or aminyl radicals. The radical yields depend on the prototropic equilibrium in the complexes. The complex consisting of neutral particles dissociates into radicals; however, the main channel for the destruction of the complex containing the charged particles is intercombinational conversion to the ground state.2.The addition of an alcohol to the nonpolar solvent lowers the yield of radicals in quenching triplet exciplexes of quinones by phenols and secondary aromatic amines. Specific solvation of the complex formed after hydrogen atom transfer shifts the prototropic equilibrium in this complex toward the complex with charged particles. Here, the main role is played by solvation of the electron donor introduced into the composition of the triplet exciplex.

Quenching of the triplet states of carbonyl compounds by nitroxyl radicals

Conclusions1.Quenching of triplet states of ketones by nitroxyl radicals occurs by charge transfer and by energy transfer from the triplet molecule to the radical. The relative contributions of these mechanisms depends on the triplet energy of the ketone and the electron donor ability of the radical.2.Quenching of the triplet state of anthanthrone by nitroxyl radicals occurs by charge transfer from the radical to the triplet molecule. Products of electron or hydrogen atom transfer in this case are not formed as a rule. Their formation is possible under conditions where the radical contains a fragment which is a stronger quencher than the radical center.