EPR Studies on Molecular Orientation in a Surface-Stabilized Paramagnetic Liquid Crystal Cell

Abstract

By EPR spectroscopy, we have developed a new method for determining the molecular orientation in a surface-stabilized liquid crystal (LC) cell, which includes a paramagnetic LC, (2S,5S)-2,5-dimethyl-2-heptyloxyphenyl-5-[4-(4-octyloxybenzenecarbonyloxy)phenyl]pyrrolidine-1-oxy (1), whose spin source is fixed in the rigid core. For each phase of racemic [(+/-)] and enantiomerically enriched [(S,S)] 1 in a surface-stabilized LC cell (4 microm thickness), the observed g-value profiles depending on the angle between the applied magnetic field and the cell plane were successfully simulated by the orientation models: (i) the LC molecule in the nematic (N) phase of (+/-)-1 freely rotates around the long axis, which is always parallel to the rubbing direction; (ii) the long axis of the freely rotating LC molecule in the chiral nematic (N*) phase of (S,S)-1 is always parallel to the cell plane but rotates in the plane to form a helical superstructure; and (iii) in the crystalline phase of (S,S)-1, the molecular long axis forms a helical superstructure similar to that of the N* phase, but the molecule is fixed around the long axis so that the NO bond lies in the cell plane. Fitting the temperature profile of the g-value in the N phase of (+/-)-1 by use of the Haller equation, we determined the molecular g-values along the molecular long axis (g(parallelM)) and short axis (g(perpendicularM)), which were successfully reproduced by the use of the set of principal g-values of a similar nitroxide with consideration of the structure of the LC molecule optimized by Molecular Mechanics 3 (MM3).

Magnetic-field-induced molecular alignment in an achiral liquid crystal spin-labeled by a nitroxyl group in the mesogen core

“…By using the principal g-values (g x ¼ 2.00990, g y ¼ 2.00639, g z ¼ 2.00266) of the nitroxide 2 (Fig. 1c), 9,10 which has a similar nitroxide structure to that of 1, the g iso and g k of 1 are calculated to be 2.00632 and 2.00540, respectively.…”

EPR characterization of diamagnetic and magnetic organic soft materials using nitroxide spin probe techniques

“…Another approach was carried out to obtain g// and g⊥ by conducting a coordinate transformation of a set of principal g-values of a similar nitroxide molecule into the molecular long axis frame [27]. These calculated g values (g// = 2.00477, g ⊥ = 2.00744) are in good agreement with the g values obtained by the Haller fitting of the temperature-dependence experiment.…”

“…The present authors reported for the first time the molecular orientation in the N and N* phase of 18b confined in surface-stabilized LC cells ( Figure 10) [27]. First, the angular dependence of g-value for the N and N* phases of (±)-18b and (S,S)-18b, respectively, was measured by EPR spectroscopy.…”

“…In this case, orientational order and intermolecular magnetic interactions can be evaluated by analysing the g-values and H change, respectively. In this section, the following four current findings by the present authors are described: (1) The principal g-values (g// and g⊥) and molecular orientation of the LC nitroxide radical molecule 18b (m=8, n=7) confined in a surface-stabilized LC cell were determined by EPR spectroscopy [27]. 2The magnetic-field-induced molecular orientation in the bulk nematic and SmC phases of (±)-18c (m=n=13) was determined by EPR spectroscopy [28].…”

Preparation and Characterization of Inclusion Compounds Using TEMPOL and an Organic 1-D Nanochannel as a Template

“…The structure of TEMPO radicals in the TPP nanochannels could not be confirmed by single-crystal X-ray diffraction (XRD), even at 100 K. Instead, we obtained information about the molecular orientation of TEMPO in the TPP nanochannels by the spin probe technique using electron spin resonance (ESR) [18][19][20][21]. For the co-inclusion compound of TEMPO and 2,2,6,6-tetramethylpiperidine [TEMP, Scheme 1(c)], ([TPP=(TEMPO) 0.017 À (TEMP) 0.983 ] IC ), it was shown that the TEMPO molecule undergoes uniaxial reorientation around the channel axis of TPP [22].…”