A. Kh. Vorobiev

Temperature dependence of amplitudes of libration motion of guest spin-probe molecules in organic glasses

Electron paramagnetic resonance (EPR) lineshapes of nitroxide spin probes in a variety of organic supercooled liquids and glasses were analyzed within the framework of the fast librational motion model. Computer simulation confirmed that the EPR lineshape may be fitted well by the librational model. For all systems studied, a mean-squared amplitude of this motion, 〈α2〉, was found to depend linearly on temperature in the low-temperature region, which is typical of harmonic solids. The slope of the linear dependence varies from glass to glass and does not change for different nitroxides in the same glass. This slope seems to correlate with the strength of the intermolecular bonds and with a degree of the fragility of the glass. Steep rises in 〈α2〉 were observed at high temperature.

Solid phase photoinitiated ligand exchange reactions in chromium(III) complexes

Summary It is shown that the photolysis of glassy frozen solutions of [Cr(NH,),] 3*, [Cr(NH,),H,0]3’, [Cr(NH,),OH]‘+ and [Cr(NH,),(NCS)J- in 5 M and 10 M NaOH, 5 M and 10 M LiCl and 95% ethanol involves photochemical ligand exchange reactions. In the cases investigated, which included aqueous alkali and aqueous salt matrices, the substituting ligands were chloride and hydroxyl ions. No photoaquation was observed. The direction of the solid phase photochemical reactions was the same as that in the liquid solutions. The initial quantum yield is not dependent on temperature, increases with the concentration of the substituting ligand and is of the order of 1W2. The quantum yield in the long wave d-d absorption band is generally higher than that in the short wave band. In the discussion of the results it is assumed that the photoinitiated ligand exchange reaction takes place in ion pairs or in more complex ion associates composed of the substituting ligand and the complex ion in the ground state. The reaction kinetics are controlled by the distribution of reactivities (quantum yields) of the complex molecules which is due to the different structures of their environments. The dissipation of absorbed photon energy in the matrix may result in restructuring of the immediate environment of the complex molecule and a change in its reactivity. A model based on these experimental observations is proposed to explain the reaction kinetics.

Photoisomerization of 9-tert-butylanthracene in polystyrene films under photoselection conditions. Consideration of molecular thermal rotation

Abstract Photoselection in thermoreversible photochemical reactions was theoretically and experimentally considered using the photoisomerization of 9-tert-butylanthracene in polystyrene as a model system. The influence of the thermal rotational mobility of the reactive molecules on the photoselection is discussed. A satisfactory agreement between the theoretically calculated kinetics of photoisomerization and the accumulation of dichroism and the experimental results is obtained. It is demonstrated that photo-oxidation in a polymer matrix corrupts the photoselection process.

Phenothiazine derivatives as enhancers of peroxidase-dependent chemiluminescence

Some N-alkyl phenothiazines with different ionic groups were studied as enhancers of chemiluminescence catalyzed by soybean peroxidase. It was shown that under experimental conditions, the compounds with positively charged groups do not exhibit enhancing ability, while the addition of phenothiazines with negatively charged groups to a substrate mixture significantly increased the chemiluminescence intensity. The relationship between the enhancing activity of phenothiazines and their capacity for enzymatic oxidation by hydrogen peroxide was found. The enhancers discovered new opportunities for increasing the sensitivity of determination of analytes by chemiluminescent enzyme immunoassay.

The Rotation Mobility of the Guest Molecules and its Reactivity in the Thermal Reaction in the Solid Matrix

Abstract Characteristic times of the thermal reaction and the molecular rotation for the guest molecules have been measured in polystyrene films. Rotation mobility of molecules was determined from the relaxation of the light induced optical anisotropy of the photoselected samples. Close agreement of the activation energies and characteristic times of thermal reaction and molecular rotation was observed. By the comparison of the induced optical anisotropy of the initial and photoconverted molecules the angles of the rotation of the molecular dipole transition moment in the instant of the elementary act of the photochemical conversion were determined.

Free radicals in photolysis of mixed phosphonium–iodonium ylides and in their reactions with acetylenes

UV irradiation of mixed phosphonium–iodonium ylide in CH2Cl2 leads to formation of free radicals with lifetimes of a few minutes detected by EPR. In mixtures of ylides with acetylenes, the structure of radicals changes, and their concentration and stability increase. In the presence of acetylenes, the radicals contain ylide and acetylene residues, and their EPR spectra have hyperfine coupling constants typical for 31P nuclei in C-radicals and for 1H nuclei, depending on the acetylene structure. It has been demonstrated that the observed radical products are formed from short-lived primary radicals.

Photochemical bistability and oscillations in isothermal closed systems. Substances in dimer-monomer equilibrium

New reaction schemes showing photochemical instabilities in closed isothermal conditions are proposed. Bistability and oscillations are predicted for a system in which one reactant is in rapid dimer-monomer equilibrium. The necessary feedback originates from the light absorption law at high total absorption of a stirred solution. For the experimental observation of instabilities, the rates of thermal and photochemical reactions must be comparable. The necessary relations of the parameters are obtained from the presented equations.

Investigation of hole mobility by photoorientation. X−2 ions in glasses

Abstract The mobility of the hole defects in the dark and at irradiation by light has been investigated in halide-containing glassy matrices. The UV and ESR spectra of the holes are identical to the ones for X − 2 in liquid solution. Defects move in the solid by an electron-transfer mechanism. The UV spectra of X − 2 are shifted in the course of the irradiation. The reversible shift is similar to spectral diffusion in hole-burning experiments. Photoorientation of X − 2 manifests itself as light-induced anisotropy of the samples. The quantum yield of the photoinduced jump of X − 2 has been estimated from the kinetics of photoorientation. The energy distribution of traps has been obtained by studying the temperature dependence. Theoretical models of hole mobility are discussed. The potential surface is concluded to be characterized by both site and bond disorder. The percolation model is most adequate to rationalize the experiments.

The influence of photoselection on the kinetics of photochemical reactions in strongly absorbing media

Abstract The theoretical analysis of the influence of photoselection on the photochemically induced distribution of reagent in the strongly absorbing media is presented for the first time. The treatment of the problem was based on the consideration of the reagents orientation distribution functions and performed for a media free of translation and rotation diffusion. The procedures for theoretical prediction of time and space evolution of the photoinduced wavelike concentration and dichroism profiles are obtained. It is theoretically shown that the polarized irradiation (photoselection conditions) can cause deeper penetration of photochemical reaction in the strongly absorbing samples in comparison with the case without photoselection. The magnitude of this effect is determined by the ability of molecule for anisotropic light absorption. It is found that the irradiated sample with high optical density contains the space recorded ‘image’ of the kinetics of the photoselection and the photochemical reaction carried out in the optically thin sample.