Andrey Kh. Vorobiev

Rotational mobility of guest molecules studied by method of oriented spin probe

Oriented spin probe method was used for study of rotational mobility of di-p-anisylnitroxide in low-temperature ethanol, toluene and isopropylbenzene glasses as well as in glassy polystyrene, poly-(N-vinylpyrrolidone) and polyvinyl butyral matrixes. Alignment of probe molecules in initially isotropic samples was induced by polarized light irradiation (photoselection phenomenon). The temperature dependencies of relaxation of alignment were measured. The rotational relaxation of light-induced alignment in low-temperature glasses was found to complete near the glass transition temperature. A step-type relaxation of alignment on a time scale of minutes has been found in the temperature range 150–300 K for polystyrene, poly-(N-vinylpyrrolidone) and polyvinyl butyral. The results are in dramatic contradiction with the commonly known rotational correlation times of probe molecules in glassy polymers obtained by the traditional spin probe method. The nonlinear least-squares fitting of experimental EPR spectra has been used for treatment of this contradiction. Quantitative agreement of simulated and experimental EPR spectra in the temperature range 77–300 K has been achieved simply by using the model of quasi-libration. By this term we mean that the probe molecules are in stochastic fast angular motion in the anisotropic cage of the matrix. The average angular displacement at ∽300 K has been estimated as 5–15°.

ESR and optical study of photo-orientation in azobenzene-containing liquid-crystalline polymer.

We report a comprehensive experimental study of photoinduced processes in azobenzene-containing liquid-crystalline polymer. The kinetic and optical characteristics of the photoisomerization reaction and photo-orientation process have been determined. The spin probe technique has been used to elucidate the details of orientation order and rotational mobility of molecular fragments within the liquid-crystalline polymer. The evolution of orientation order parameters , , and in the course of photoinduced ordering of the material as well as the change of liquid-crystalline domain sizes have been studied quantitatively. The obtained experimental results are compared with the predictions of the existing models of photo-orientation.

Photo-Orientation of Azobenzene-Containing Liquid-Crystalline Materials by Means of Domain Structure Rearrangement

A novel mechanism of photo-orientation of azobenzene-containing liquid-crystalline materials is proposed. This mechanism is based on the notion of photochemically induced domain rearrangement driven by destabilization of liquid-crystalline phase in light absorbing domains due to photochemical formation of non-mesogenic cis-azobenzene moieties. The experimental evidence of photoinduced movement of a domain boundary is presented, and the velocity of this movement is measured. A mathematical model for photo-orientation of a polydomain azobenzene-containing material is formulated. The values of model parameters for a liquid-crystalline azopolymer have been measured in separate experiments. Theoretical predictions demonstrate quantitative agreement with the experimental observations.

Orientational order of guest molecules in aligned liquid crystal as measured by EPR and UV–vis techniques

Orientational order of guest molecules in aligned liquid crystal 4-cyano-4′-pentylbiphenyl (5CB) is studied via optical dichroism and electron paramagnetic resonance (EPR) spectra measurements. The guest molecules used are bifunctional molecules bearing paramagnetic nitroxide group and photochromic azobenzene moiety. The bifunctional probe with rigidly bonded nitroxide and azobenzene moieties was found to align as a whole, while flexible long spacer between the moieties provides independent alignment for the nitroxide and azobenzene parts. Intermolecular interactions responsible for the alignment of azobenzene and nitroxide moieties of the probe molecules are discussed. The molecules with cis-configuration of azobenzene moiety are able to align in the liquid-crystalline medium, but to a lesser extent than the molecules with trans-configuration. Directions of orientational axes and characteristics of rotational mobility of spin probes are determined. Second, fourth and, in some cases, sixth rank order parameter values are found.

Simulation of Rigid-Limit and Slow-Motion EPR Spectra for Extraction of Quantitative Dynamic and Orientational Information

© 2012 Vorobiev and Chumakova, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Simulation of Rigid-Limit and Slow-Motion EPR Spectra for Extraction of Quantitative Dynamic and Orientational Information

The Novel Stable Nitroxide Radicals as Perspective Spin Probes for Study of Orientation Order of Liquid Crystals and Polymers

Stable nitroxide radicals with rigid core were used as spin probes for investigation of orientational order of nematic liquid crystals MBBA and 5CB, uniaxially stretched polyethylene and porous polyethylene. The orientation distribution functions of probe molecules were determined. The order parameters of the true orientation axes were estimated. It was demonstrated that the novel nitroxide radicals show orientational alignment of both liquid crystals and polymers more effectively than ordinary spin probes.

Fullerene as Photocatalyst: Visible-Light Induced Reaction of Perfluorinated α,ω-Diiodoalkanes with C60

Solution phase photochemical reaction of fullerene with perfluorinated alkyldiiodides I-RF-I can be efficiently initiated by visible range irradiation that targets solely the fullerene component. Photoinduced electron transfer from fullerene onto the diiodide component effects dissociative formation of alkyl radicals RFI• subsequently consumed by C60 to give the principal detectable radical intermediate C60RFI•. Experimentally established second-order kinetics with respect to the fullerene concentration evidence that fullerene plays its two roles of photocatalyst and reactant in a decoupled fashion, which suggests its catalytic ability to be of potential use in more complex photochemical systems. The main final product of the photochemical transformation observed is the singly linked dimer of the intermediates, I-RF-C60-C60-RF-I. Side reactions of C60RFI• with the environment lead to quenching of the unpaired electron density by ortho- or para- attachment of hydrogen or iodine. The outlined kinetic findings are discussed in detail.

Orientation order and rotation mobility of nitroxide biradicals determined by quantitative simulation of EPR spectra

The problem of quantitative numerical simulation of electron paramagnetic resonance (EPR) spectra of biradical probes in both isotropic and aligned media was solved for the first time. The models suitable for the description of the spectra of the probes, both in the rigid limit and in the presence of rotational motions, were developed and successfully applied to model systems. The simulation of EPR spectra allows obtaining the following information about the molecular structure and dynamics: the values of orientation order parameters, the type of rotation mobility and its quantitative characteristics, and the sign and value of the spin exchange constant of the biradical. Model systems used in this work include solutions of nitroxide biradicals in a viscous solvent (squalane) in the range of temperatures 100-370 K and in the aligned liquid crystal n-octylcyanobiphenyl (8CB, 100-298.5 K). Unexpectedly, it was found that in 8CB the main orientation axis of the biradical molecule is perpendicular to the longest molecular axis.

Direct Microscopic Observation of Domain Rearrangement Mechanism of Photo-Orientation Process in Azobenzene-Containing Materials

The evolution of the domain structure of two azobenzene-containing materials during irradiation with polarized light was monitored using polarized optical microscopy with additional λ-waveplate for discerning the domains oriented parallel and perpendicular to irradiating light polarization. It is shown that the process of photo-orientation consists in growth of perpendicularly oriented domains and diminishing of parallel domains via the movement of domain boundaries. These data confirm the mechanism of photo-orientation via domain structure rearrangement.