Optimization of the photo-orientation rate of an azobenzene-containing polymer based on a kinetic model of photoinduced ordering.

The explicit dependence of the rate of photoinduced ordering (photo-orientation) of an azobenzene-containing liquid-crystalline polymer on the photostationary concentration of cis-azobenzene has been predicted theoretically and found experimentally. The employed kinetic model based on the photoinduced rearrangement of the domain structure of a liquid-crystalline material predicts the maximum rate of photo-orientation at ca. 50% content of the cis-isomer in the photostationary state of irradiation. For experimental fine tuning of the photostationary trans-cis ratio, the simultaneous irradiation of material with two beams of light with different wavelengths was employed. The excellent agreement of theory and experiment indicates that the difference of photostationary fractions of cis-azobenzene fragments in adjacent domains of different orientations is the driving force of photoinduced ordering.

Mobility of liquids intercalated into the interplane space of graphite oxide as revealed by a combination of 19F NMR, 1H NMR and EPR spin probe methods.

A combination of 19F and 1H NMR with the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence was used to examine the mobility of liquids in the interplane space of graphite oxide (GO) for the first time. The proposed approach allows for the reduction of NMR signals from immobile hydrogen-containing groups on the surface of GO and for monitoring of the molecular mobility of intercalated liquids. The mobile fractions of H2O, CH3CN and CF3CH2OH were detected inside the corresponding swollen GO samples. For H2O, the amount of mobile liquid showed a peculiar temperature dependence where a certain portion survived well below 273 K. The sensitivity of the proposed 1H NMR + CPMG procedure is also compared to the sensitivity of the EPR nitroxide spin probe method.

Size-tunable MRI-visible nitroxide-based magnetic mixed micelles: preparation, stability, and theranostic application.

Metal-free magnetic mixed micelles (mean diameter: 16 nm) composed of biocompatible surfactant Tween 80 and hydrophobic pyrrolidine-N-oxyl radical were prepared by mixing them in phosphate-buffered saline. The magnetic mixed micelles were characterized by dynamic light scattering and small angle neutron scattering measurements. The stability of the micelles is found to depend on the length of alkyl side chain in the nitroxide compounds and degree of unsaturation in the hydrophobic chain in the surfactant. The size of the mixed micelle can be tuned by changing the molar ratio of Tween 80 and nitroxyl radical. In view of theranostic application of the micelle, the cytotoxicity and stability in a physiological environment was investigated; the mixed micelle exhibited no cytotoxicity, high colloidal stability and high resistance towards reduction by large excess ascorbic acid. The in vitro and in vivo magnetic resonance imaging (MRI) revealed sufficient contrast enhancement in the proton longitudinal relaxation time (T 1) weighted images. In addition, hydrophobic fluorophores and an anticancer drug are stably encapsulated in the mixed micelles and showed fluorescence (FL) upon reduction by ascorbic acid and cytotoxicity to cancer cells, respectively. For example, the paclitaxel-loaded mixed micelles efficiently suppressed cancer cell growth. Furthermore, they were found to give higher MRI contrast (higher r 1 value) in vitro than the micelles without paclitaxel. The magnetic mixed micelles presented here are promising theranostic agents in nanomedicine due to their high biocompatibility and high resistivity towards reduction as well as functioning as a drug carrier in therapy and MR or FL imaging probe in diagnosis.

Magnetic Mixed Micelles Composed of a Non-Ionic Surfactant and Nitroxide Radicals Containing a D-Glucosamine Unit: Preparation, Stability, and Biomedical Application.

Metal-free magnetic mixed micelles (mean diameter: < 20 nm) were prepared by mixing the biocompatible non-ionic surfactant Tween 80 and the non-toxic, hydrophobic pyrrolidine-N-oxyl radicals bearing a D-glucosamine unit in pH 7.4 phosphate-buffered saline (PBS). The time-course stability and in vitro magnetic resonance imaging (MRI) contrast ability of the mixed micelles was found to depend on the length of the alkyl chain in the nitroxide radicals. It was also confirmed that the mixed micelles exhibited no toxicity in vivo and in vitro and high stability in the presence of a large excess of ascorbic acid. The in vivo MRI experiment revealed that one of these mixed micelles showed much higher contrast enhancement in the proton longitudinal relaxation time (T1) weighted images than other magnetic mixed micelles that we have reported previously. Thus, the magnetic mixed micelles presented here are expected to serve as a promising contrast agent for theranostic nanomedicines, such as MRI-visible targeted drug delivery carriers.

Magnetic field effects in nematic and smectic liquid crystals probed by time resolved observation of orientation relaxation of the spin probe.

The kinetics of reorientation of the liquid crystal HOPOOB in nematic and smectic phases in the magnetic field was studied by the spin probe technique. The time evolution of the EPR spectrum shape in the course of reorientation in the nematic phase indicates that the mechanism of alignment comprises redistribution between domains of different orientations rather than the turn of the orientation distribution function. A microscopic model of domain structure rearrangement is provided to qualitatively describe the experimental data. In the smectic C phase, partial reorientation is observed upon the action of the magnetic field. A possible mechanism of this process is the reorientation of tilt direction within the smectic layers. It is shown that this process is constrained, and the value of the corresponding elastic constant is estimated.

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.

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.

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.

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.

Electrochemical, ESR and theoretical studies of [6,6]-opened C(60)(CF(2)), cis-2-C(60)(CF(2))(2) and their anions.

Electrochemical behavior of C(60)(CF(2))(n), n = 1, 2 and C(60)(CCl(2)) has been investigated. [6,6]-Opened C(60)(CF(2)) and cis-2-C(60)(CF(2))(2) exhibit reversible reductions at potentials 150 and 145 mV more positive than C(60), unlike the related [6,6]-closed C(60)(CCl(2)), which shows reduction at the potential close to that for C(60). The structures, electron localization and lifetimes of the observed radical anions C(60)(CF(2))(-*) and cis-2-C(60)(CF(2))(2)(-*) have been elucidated by the ESR study and DFT calculations. The protonation pathway of decay of C(60)(CF(2))(-*) yields [6,6]-opened 1,9-dihydro-(1a,1a-difluoro-1aH-1(9)a-homo(C(60)-I(h))fullerene, C(60)(CF(2))H(2), which becomes the first characterized derivative of [6,6]-opened C(60)(CF(2)). DFT calculations of the structure and electron affinity of a number of homofullerene structures have been carried out to rationalize the experimental findings.

Determination of orientation distribution function of anisotropic paramagnetic species by analysis of ESR spectra angular dependence.

A method has been developed to determine orientation distribution function (ODF) of anisotropic paramagnetic species by analysis of the angular dependence of the ESR spectra. The method is based on computational spectra simulation. The ODF is represented as an expansion in terms of orthonormal functions. The expansion coefficients are determined through minimization of discrepancies between simulated spectra and experimental ones. By means of the suggested method we have determined the orientation distribution functions for radical probe 2,2,6,6-tetramethyl-4-ol-piperidinooxyl in 4-n-amyl-4'-cyanobiphenyl aligned by magnetic field and 2-septadecyl-2,3,4,5,5-pentamethylimidazolidine in polyethylene stretched films. In each case, thermal evolution of the ODF has been investigated.

Synthesis of conducting polyelectrolyte complexes of polyaniline and poly(2-acrylamido-3-methyl-1-propanesulfonic acid) catalyzed by pH-stable palm tree peroxidase.

Comparison of the stability of five plant peroxidases (horseradish, royal palm tree leaf, soybean, and cationic and anionic peanut peroxidases) was carried out under acidic conditions favorable for synthesis of polyelectrolyte complexes of polyaniline (PANI). It demonstrates that palm tree peroxidase has the highest stability. Using this peroxidase as a catalyst, the enzymatic synthesis of polyelectrolyte complexes of PANI and poly(2-acrylamido-3-methyl-1-propanesulfonic acid) (PAMPS) was developed. The template polymerization of aniline was carried out in aqueous buffer at pH 2.8. Varying the concentrations of aniline, PAMPS, and hydrogen peroxide as reagents, favorable conditions for production of PANI were determined. UV-vis-NIR absorption and EPR demonstrated that PAMPS and PANI formed the electroactive complex similar to PANI doped traditionally using low molecular weight sulfonic acids. The effect of pH on conformational variability of the complex was evaluated by UV-vis spectroscopy. Atomic force microscopy showed that a size of the particles of the PANI-PAMPS complexes varied between 10 and 25 nm, depending on a concentration of PAMPS in the complex. The dc conductivity of the complexes depends also on the content of PAMPS, the higher conductivity being for the complexes containing the lower content of the polymeric template.