Unique Superparamagnetic‐like Behavior Observed in Non‐π‐delocalized Nitroxide Diradical Compounds Showing Discotic Liquid Crystalline Phase

Abstract

A unique superparamagnetic‐like behavior and a large “positive magneto‐LC effect” were observed in the solid phases and the hexagonal columnar (Colh) liquid crystalline (LC) phase, respectively, of novel achiral non‐π‐delocalized nitroxide diradical compounds (R,S)‐1, which showed polymorphism in the solid phases (solids I and II). The SQUID magnetization measurement revealed that (1) (R,S)‐1 containing a small amount of racemic diastereomers (R*,R*)‐1 possessed an unusual and large temperature‐independent magnetic susceptibility (χTIM>0) component in the original nanocrystalline solid I that was responsible for the observed superparamagnetic‐like behavior under low magnetic fields and did not arise from the contamination by extrinsic magnetic metal or metal ion impurities, besides ordinary temperature‐dependent paramagnetic susceptibility (χpara>0) and temperature‐independent diamagnetic susceptibility (χdia<0) components, (2) a large increase in molar magnetic susceptibility (χM) (positive magneto‐LC effect) that occurred at the solid I‐to‐liquid crystal transition upon heating was preserved as an additional χTIM increase in the resulting polymorphic nanocrystalline solid II by cooling, and (3) such unique magnetic phenomena were induced by thermal processing for (R,S)‐1 or by adding a small amount of (R*,R*)‐1 to (R,S)‐1 as the impurity.

Spin Symmetry Breaking: Superparamagnetic and Spin Glass-Like Behavior Observed in Rod-Like Liquid Crystalline Organic Compounds Contacting Nitroxide Radical Spins

“…Here, it should be emphasized that the positive magneto-LC effect (χM increase) (i) proved to be independent of the magnetic field-induced molecular reorientation because of the too small molecular magnetic anisotropy (∆χ) [45] and (ii) did not result from the contamination by extrinsic magnetic metal or metal ion impurities, as confirmed by the quantitative analysis using inductively coupled plasma atomic emission spectroscopy (ICP-AES) as well as the observation of thermal and impurity effects [42].…”

“…It is also important to examine the possibility of contamination by ferromagnetically-ordered metal or metal ion spins for such nonlinear magnetization curves [54]. Consequently, we could exclude this possibility by the quantitative analysis using inductively coupled plasma-atomic emission spectroscopy (ICP-AES), verifying 0.5~1.3 ppm for the iron content and 0.3~0.6 ppm for the nickel content for LC nitroxide radicals [42]. These values are less than one-tenth of the observed χ M increase for the individual compounds.…”

“…Thus, the observed magnetic behavior reminds us of a spin glass state. In fact, for example, considerable enhancement of a positive magneto-LC effect or superparamagnetic behavior was noted by adding a small amount (5 to 20 mol%) of racemic cis-diastereomers (R*,R*)-3 [a 1:1 mixture of (R,R)-3 and (S,S)-3] to meso (R,S)-3 as an organic impurity [42]. Such thermal and impurity effects are known to be commonly observed for metallic spin glass materials.…”

“…With this situation in mind, since 2004, we have designed and synthesized a series of chiral and achiral all-organic LC nitroxide radicals such as 1-5 having one or two PROXYL (2,2,5,5-tetrassubstituted-1-pyrrolidinyloxyl) units in the core position and, thereby, with a negative dielectric anisotropy (∆ε < 0) ( Figure 3) [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44]. Their LC phases and magnetic properties have been fully characterized.…”

“…These experimental results suggest that the interactions between the electric polarization and the magnetic moment in the magnetic LC phase are likely to Representative PROXYL-based organic radicals synthesized since 2004 and their LC transition temperatures. Rod-like monoradicals trans-1 [28][29][30][31][32][33][34][35][36][37][38][39][40] and biradical (S,S,S,S)-2 [41], discotic diradical meso (R,S)-3, and the non-LC racemic diastereomers (R*,R*)-3 [42], discotic monoradical trans-4 [44], and rod-like hydrogen-bonded monoradical trans-5 [43]. The LC temperatures refer to the heating process, except for monotropic racemice trans-5.…”