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

Rui Tamura, Katsuaki Suzuki, Yoshiaki Uchida and Yohei Noda

DOI: 10.1039/9781849734837-00001

This review article summarizes the recent advances in the EPR studies of the molecular orientation and magnetic properties in the various rod-like liquid crystalline (LC) phases of the second-generation of organic nitroxide radical materials in the bulk state or in a surface-stabilized LC cell, compared with the conventional EPR studies using classical organic nitroxide spin probes. Noteworthy is the first observation and characterization of a sort of spin glass-like ferromagnetic interactions (J > 0) induced by weak magnetic fields in the various LC phases of the second-generation of organic nitroxide radical materials. This unique magnetic property has been referred to as positive "magneto-LC effects". The utility of such novel LC nitroxide radical materials as the EPR spin probes is also presented.

1 Introduction

Electron paramagnetic resonance (EPR) spectroscopy using spin probes has been widely recognized as a convenient and powerful means to obtain direct information on the molecular dynamics and microenvironment in various diamagnetic host materials. Particularly, the use of organic nitroxide radical compounds as spin probes has proved to be invaluable for obtaining the information concerning molecular local structure, mobility, micropolarity, acidity, and redox status in the various condensed phases of host organic materials, because of the existence of the established analytical methodologies using EPR spectroscopy. For example, the EPR spectroscopy using nitroxide spin probes is a useful technique for investigating the physical properties of organic liquid crystalline (LC) materials as the representative organic soft materials, such as the orientational order, anisotropic interactions between non-LC solutes and LC solvents, fluctuation of director, anisotropy of molecular rotation, elastic behavior, and biaxiality. In this context, a number of reports have been documented for the EPR studies using easily available non-LC nitroxide spin probes dissolved in diamagnetic host LC materials. In contrast, very few LC nitroxide spin probes were developed, due most likely to the difficulty in the molecular design and synthesis which must satisfy the molecular linearity or planarity necessary for the existence of LC phases and the radical stabilization at the same time, although LC spin probes are anticipated to be more compatible with host LC materials than non-LC ones.

In this chapter, first we briefly survey both non-LC and LC nitroxide spin probes which have been used for the EPR studies of diamagnetic host LC materials. Then the molecular alignment studies and the observation of unique magnetic interactions by EPR spectroscopy in the various LC phases of second-generation of rod-like nitroxide radical materials, with and without using a surface-stabilized LC cell, are presented. From these studies, it would be well understood that EPR spectroscopy is an excellent tool for analyzing the magnetic properties of LC nitroxide radical soft materials at high temperatures, for which SQUID magnetization measurement is not suitable. In this article, we do not use the term 'spin-labelled LC materials', because the nitroxide radical unit in the second generation of LC nitroxide radical materials plays an important role in determining the molecular dielectric anisotropy as well as serving as the spin source.

2 Non-LC nitroxide spin probes in diamagnetic LC hosts

Nitroxide radical materials such as 1–5 have been employed as spin probes used in diamagnetic host LC materials (Fig. 1).

The use of nitroxide spin probes provides three important parameters characterizing EPR spectra; the g -value which determines the resonance magnetic field (H0), the nitrogen hyperfine coupling constant (A), and the line-width (ΔH). For instance, an EPR spectrum of a nitroxide radical molecule dissolved in a diamagnetic host LC material is schematically shown in Fig. 2. Since any motion of the nitroxide radical molecule is greatly influenced by the molecular dynamics of the surrounding host molecules, the orientational order parameters (S) and rotational correlation times (τR), which characterize the molecular motion of the spin probe, are generally derived from these parameters according to the following theoretical equations [2].

[MATHEMATICAL EXPRESSION OMITTED] (1)

where the subscripts x, y, and z refer to the principal axes of the a- and g-tensors determined in the rigid limit and and are their respective averages.

[MATHEMATICAL EXPRESSION OMITTED] (2)