Electric Field Dependence of Molecular Orientation and Anisotropic Magnetic Interactions in the Ferroelectric Liquid Crystalline Phase of an Organic Radical Compound by EPR Spectroscopy

Abstract

To confirm whether chiral organic radical compound 1, which showed the generation of a sort of spin glass-like inhomogeneous ferromagnetic interactions (the average spin-spin interaction constant J > 0) in the bulk liquid crystalline state under weak magnetic fields, has a spin easy axis or exhibits anisotropic magnetic interactions in the SmC* or SmC phase, we have measured the temperature dependence of g-value and relative paramagnetic susceptibility (χrel) for the ferroelectric SmC* phase of (2S,5S)-1 confined in a thin sandwich cell by EPR spectroscopy.

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

“…Although the detailed mechanism is still under investigation, this large χ M increase was proved to be not due to the molecular reorientation effect caused by the molecular magnetic anisotropy (Δ χ ) . Moreover, the first observation of “magnetoelectric effect (or coupling)” at as high as 75 °C was achieved for the LC monoradical compound showing both the ferroelectricity and the positive magneto‐LC effect in a surface‐stabilized liquid crystal cell …”

“…[16,20] Moreover,t he first observation of "magnetoelectric effect (or coupling)" at as high as 75 8Cwas achieved for the LC monoradical compound showing both the ferroelectricity and the positive magneto-LC effect in as urface-stabilized liquid crystal cell. [21][22][23] Meanwhile, it was anticipated that i) the positive magneto-LC effect could be induced under low magnetic fields in the columnar LC phases of organic radical compounds as well, by aj udiciousm olecular design so as to avoid the strong p-p dimer formation that resultsi nt he antiferromagnetic interactions (or negative magneto-LC effect), and thereby ii)a magnetic-field-controlled photoinduced hole transport in the semiconducting columnar phase may be feasible due to the relatively high-lying singly occupied molecular orbital( SOMO)a nd its delocalization on the aromatic core in comparison to their closed-shell analogue. In 2014 an achiral p-delocalized hexaperi-hexabenzocoronened erivative carrying ac onjugated tertbutyl nitroxide moiety wasr eported to show the positive magneto-LCe ffect in the hexagonal columnar LC phase, [24] whereas disc-liket riphenylenes carrying an on-conjugatedc yclic nitroxide monoradical group [25] and other three types of disc-like compounds with p-delocalized photoluminescentt ris(2,4,6-trichlorophenyl)methyl, [26] and semiconducting 6-oxoverdazyl [27,28] or benzo[e] [1,2,4]triazinyl monoradicalm oieties [29] as the spin source did not exhibit the positive magneto-LC effect in their achiral columnar LC state.…”

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

“…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.…”

“…In addition, it is noteworthy that the nonlinear magneto-electric (ME) effect was, for the first time, detected for organic LC materials, i.e., (S,S)-1 (m = n = 13) showing a ferroelectric LC [chiral SmC (SmC*)] phase [37][38][39]. By applying an external DC electric field (between +25 V and −25 V) to the (S,S)-1 confined in a rubbed surface-stabilized liquid-crystal thin (4 µm thick) sandwich cell, the hysteresis loops were observed for the experimental EPR parameters such as the g-value, ∆H pp , and χ rel (relative paramagnetic susceptibility).…”