M. Ya. Mel'nikov

Stabilization and photochemical reactions of thiyl radicals in glassy matrices at 77 k

Abstract The possibility of stabilization of thiyl radicals formed during the photolysis of thiol solutions at a wavelength λ = 253.7 nm in glassy matrices at 77 K was shown by means of electron spin resonance and UV spectroscopy. The main alternative process is the formation of adducts RSS(H)R (R  CH3, C2H5, i-C3H7, tert-C4H9) in thiol self-associate. Stabilized alkyl-thiyl radicals are able to enter into photochemical reactions leading to C—C or C—H bond rupture in the position β to the sulphur atom. The ability of stabilized thiyl radicals to enter into photochemical reactions is suggested as the reason for their low concentrations in dilute thiol solutions in alcohol and hydrocarbon matrices subjected to photolysis at λ = 253.7 nm at 77 K.

Spectral Characteristics and Transformations of Intermediates in Irradiated Freon 11, Freon 113, and Freon 113a

Optical absorption bands observable in Freon 11, Freon 113, and Freon 113a irradiated at 77 K were assigned to various intermediates (radical cations, radical ion pairs, and complexes of radicals with ions). The transformations of these species in thermal and photochemical reactions occurring at 77 K were studied. On the basis of experimental results, it was suggested that the radical anions of Freon 11 and Freon 113 are unstable at 77 K and the spatial distribution of the intermediates produced is inhomogeneous.

Photophysical properties of mixed-ligand complexes (ArS)2ZnPhen with interligand charge-transfer excited states: influence of substituents in the arylthiolate ligand

Abstract Complexes (4-X-C6H4S)2 ZnPhen (XH, Me, t-Bu) where Phen  1,10-phenanthroline, Memethyl and t-Bu  tertbutyl in glassy isobutironitrile have been found to emit both from the lowest interligand charge-transfer (ILCT) excited state and the upper-lying triplet excited state of coordinated phenanthroline ligand. MeO-substituted complex in the same conditions emits only from the ILCT state and the Me2N-substituted one does not give luminescence at all. It was shown that variations in the emission quantum yields from the 3Phen state are determined by differences in the population efficiencies of this state but not in its relaxation rates to the lowest ILCT state. The latter process was found to be absent. Two models of the phenomenon have been considered, one taking into account the possibility of emission from two non-interacting excited states, and the other based on the existence of several emitting forms of complexes in solid solution. The second model seems to be more appropriate because of a better description of temperature variations in the emission lifetimes and quantum yields. Non-radiative relaxation in singlet and triplet ILCT excited states was found to be thermally activated with a tendency to lowering the corresponding activation energies with growth of p-substituent donor ability in the arylthiolate ring. Substituent effects are connected with the possibility of interligand three-electron bonding between thiolate ligands in the ILCT state.

MECHANISM AND KINETICS OF PHOTORADICAL REACTIONS IN POLYACRYLIC-ACID

Abstract Photochemical conversion of paramagnetic centres formed under γ-irradiation of polyacrylic acid (PAA) at 77 K has been studied. Middle macroradicals of PAA are involved in the photodissociation reaction to form hydroxyl radicals. They are also found to be capable of recombining. A scheme for the photoradical processes in PAA at 77 K is proposed.

The kinetics and mechanism of photo-ageing of polyvinylbutyral

Abstract The mechanism of photo-ageing of polyvinylbutyral has been investigated. The quantum yields of photochemical reactions of the radicals are determined. It is shown that the cross-linking of the polymer is a result of thermal reactions of the radicals, whereas its decomposition is due to photodissociation of radicals ∼ CH(O·CO·C 3 H 7 )·CH 2 ·C H ∼ The relative contributions of decomposition and cross-linking depend on the spectral composition of the light and temperature of radiation.

The study of photochemical reactions in polyoximurethane

SummaryThe irradiation of polyoximurethane at 77K yields only iminoxy radicals. The gaseous products are CO and CO2. The mechanism of photodegradation, including 2 initial processes of N-O and C-O bond scission is discussed. The ratio between the initial processes depends on the wavelength of the light used. Only the C-O bond cleavage results in polymer degradation.

Photochemical reactions of radical cations of dimethylformamide in freon matrices at 77 K

Spectral characteristics of the radical cations (RC) of DMF (λmax = 415 nm, εmax = (2.6±0.8)·103 L mol−2 cm−2) stabilized in an irradiated glassy freon mixture (CFCl3 and CF2BrCF2Br) at 77 K were determined. Amide type radicals and RC of the matrix were shown to be formed by irradiation (λ=365–436 nm) of the radical cations of DMF in freon matrices using ESR and UV spectroscopy. The quantum yields of photoconversion of the DMF radical cations are independent of the wavelength of exciting light. It was found that the matrix structure affects the processes stabilizing the products of photoconversion of the DMF radical cations.

Effectiveness of the photodissociation of acyl radicals at −196°C

Abstract Quantum yields of the photodissociation of some acyl radicals in solid glass matrices have been measured at −196°C. The effectiveness of the photodissociation process was found to be satisfactorily descried in terms of the free volume model.

CYCLOPROPANE RING OPENING IN THE REACTION OF METHYLENECYCLOPROPANE WITH SILYL RADICALS STABILIZED ON AN ACTIVATED AEROSIL SURFACE

Alkyl type radicals stable at room temperature and incorporating a double bond not conjugated with the free valence, ≡Si−C(=CH2)−CH2−CH2, are formed in the reaction of methylenecyclopropane with silyl radicals (≡SiO)3Si on an activated Aerosil surface.

The photochemistry of mixed-ligand complexes (ArS)2ZnPhen with the lowest ligand—ligand charge transfer excited states

Abstract Flash photolysis and spin-trapping experiments were performed to investigate the photochemistry of the mixed-ligand zinc complexes (4—XC 6 H 4 S) 2 Zn(Phen) (XH, MeO or Me 2 N) in liquid solutions. The lowest ligand—ligand charge transfer excited state of these complexes was shown to dissociate, yielding arylthiyl radicals and a reduced zinc-phenanthroline moiety. Back electron transfer with nearly diffusion-controlled rates was found to be a predominant process in the case of MeO- and Me 2 N-substituted arylthiolate complexes while the recombination of unsubstituted phenylthiyl radicals is also significant. The reductive dissociation product may be trapped by oxygen or methyl viologen (MV 2+ ) cations, leading to the photo-oxidation of complexes in aerated solutions or production of MV + radical cations. The latter decays via diffusion-controlled electron transfer to arylthiyl radicals. Steady state illumination produces MV + only in solutions of phenylthiolate complex where its oxidation by phenylthiyl radicals competes with recombination of the latter.