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Article pubs.acs.org/JPCB

Molecular Orientational Order of Nitroxide Radicals in Liquid Crystalline Media
N. A. Chumakova, T. S. Yankova, K. E. Fairfull-Smith, S. E. Bottle, and A. Kh. Vorobiev*,




Department of Chemistry, M. V. Lomonosov Moscow State University, Moscow 119991, Russian Federation School of Chemistry, Physics and Mechanical Engineering, Faculty of Science and Engineering, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia
S * Supporting Information

ABSTRACT: The orientational distribution of a set of stable nitroxide radicals in aligned liquid crystals 5CB (nematic) and 8CB (smectic A) was studied in detail by numerical simulation of EPR spectra. The order parameters up to the 10th rank were measured. The directions of the principal orientation axes of the radicals were determined. It was shown that the ordering of the probe molecules is controlled by their interaction with the matrix molecules more than the inherent geometry of the probes themselves. The rigid fused phenanthrene-based (A5) and 2-azaphenalene (A4) nitroxides as well as the rigid core elongated C11 and 5-cholestane (CLS) nitroxides were found to be most sensitive to the orientation of the liquid crystal matrixes.

I. INTRODUCTION Admixtures to liquid crystalline (LC) media are widely used for the development of new materials such as polymer-dispersed liquid crystals (PDLS)1-3 or polymer-stabilized mesophases.4,5 In addition, admixture molecules can be introduced into liquid crystals to investigate the properties of the medium. The research use of admixture molecules is based on the molecular probe technique, where properties of specially introduced guest molecules are measured to characterize the medium. In particular, light-absorbing and light-emitting probe molecules (dyes) are used, mainly for determination of the second rank order parameters and rarely for estimation of fourth rank order parameters of liquid crystals by means of optical measurements.6-14 More detailed information concerning the molecular orientational order in liquid crystals and characteristics of molecular motions in the studied media, however, can be extracted by using stable paramagnetic molecular additives (spin probes) in combination with EPR spectroscopy.15-19 To date, there is only a limited set of substances that have been applied as optical and paramagnetic probes. These probes have been chosen by empirical, trial-and-error experiments wi th th e impl icit a ssumption t hat the main cause of orientational alignment in the ordered medium arises from the elongated shape of the molecules. Consequently, a number of elongated probe molecules have traditionally been employed for the investigation of the liquid crystals. On the other hand, it is known that compact piperidine spin probes (TEMPONE, TEMPOL, etc.) also demonstrate considerable orientational ordering in LC media in spite of an almost circular, nonelongated molecular shape.20-22 Therefore, the shape of the dopant molecules is not the only factor that governs molecular orientation of the probes doped into liquid crystals. Thus, it is necessary to ascertain real orientation axis of the guest
© 2014 American Chemical Society

molecule, i.e., the molecular axis that is ordered in the aligned liquid crystal to the maximum extent. Another assumption that is commonly made when using probe additives in LC media is the concept of equivalence for all of the guest molecules. Indeed, all probe molecules are ideally considered to be completely identical particles, all exposed to the same surroundings. In actual fact, LC media are orientationally ordered in the case of nematic mesophases or orientationally and spatially ordered in the case of smectic mesophases. It is logical to expect that the same probe molecules occupy different host-guest orientations in the differing locations within LC medium. Thus, the aim of this work is to investigate the factors that control the orientation of guest molecules in anisotropic LC media. Deeper understanding of such factors would be of value for the selection of the most effective probe substances and for the development of LC composition materials with desired properties. For this purpose, standard nitroxide spin probes as well as novel stable nitroxide radicals with different molecular shapes were applied. The nematic 4-n-pentyl-4-cyanobiphenyl (5CB) and smectic 4-n-octyl-4-cyanobiphenyl (8CB) were used as model liquid crystalline media. The orientation distributions of the radicals in the aligned samples were determined in detail by means of EPR spectroscopy. Before the spectra were recorded, the aligned samples were snap-frozen to prevent the reorientation of nematic phases along the magnetic field of EPR spectrometer during the angular dependence acquisition. The second advantage of frozen samples is the more simple simulation of rigid limit spectra EPR in comparison with simulation of spectra for rotating radicals.
Received: November 29, 2013 Revised: April 30, 2014 Published: April 30, 2014
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dx.doi.org/10.1021/jp411729j | J. Phys. Chem. B 2014, 118, 5589-5599


The Journal of Physical Chemistry B

Article

Figure 1. Nitroxide radicals used in the present work.

Liquid crystals often form a supercooled glassy state with conservation of molecular order under rapid cooling.23-27 This technique was shown previously to produce reliable and detailed orientational molecular characteristics such as order parameters and the angles which define the orientation axes of molecules.13,18,28

II. EXPERIMENTAL AND COMPUTATIONAL METHODS Materials and Reagents. Liquid crystals 5CB and 8CB were obtained from Merck and used without further purification. The phase transition temperatures for these materials were measured using a polarizing optical microscope MIN-8 equipped with a temperature control unit. The temperature range for the formation of the nematic phase was 293-320 K which is in accordance with literature reports.29,30 8CB showed nematic and smectic phases within the ranges of 306-314 K and 294-306 K correspondingly, which are close to the known properties of these materials.29,31 The structures of the nitroxide radicals examined in this study are presented in Figure 1. The radicals 2,2,6,6-tetramethyl4-ol-piperidinoxyl (TEMPOL) and 3-DOXYL-5-cholestane (CLS) were obtained from Aldrich and used without further purification. The synthetic procedures used to prepare radicals C11,32,33 A1 and A2,34 A3,35 A4,36 and A537 have been described previously. Radical A6 was prepared from 3,4,9,10-perylenetetracarboxylic dianhydride in a manner similar to that previously reported.38 Preparation of the Samples. Quartz ampules for EPR measurements (3 mm diameter) were filled with liquid crystals containing the spin probes, evacuated at pressure 10-3 Torr, and sealed. The spin probe concentration was 1 в 10-3 mol/L. Two types of samples were studied. The unordered samples were prepared by heating above the clearing temperature followed by rapid cooling down to 77 K. Samples with orientational order were prepared by heating the material to the temperature of the
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mesophase, exposing for 5 min to a 0.5 T magnetic field of an EPR spectrometer at that temperature, and then rapidly cooling to 77 K while keeping within the magnetic field. The freezing was r