Towards Magnetic Liquid Crystals
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M. K. PATEL, JIANPING HUANG, AND PIOTR KASZYNSKI* OrganicMaterialsResearch Group, Department of Chemistry, Vanderbilt University, Nashville, TN 37235 USA
ABSTRACT A series of new sulfur-nitrogen ring systems with a neutral radical center were designed for incorporation into liquid crystal molecules and formation of discotic and calamitic phases. It is expected that these materials will display unusual magnetic properties associated with partially oriented bulk liquid crystal structures. Synthetic progress toward new radicals and the first discogen with nr-delocalized neutral radical is described. INTRODUCTION The thioaminyl group1 (1, Figure 1) has been featured as a structural element in many chemically and thermally stable organic free radicals. The structural versatility of 1 has led to the formation of many acyclic 2-5 and cyclic derivatives. 6-11 Some of these radicals melt above 150 'C without decomposition, 2 some exist as liquids, 9 and some have been explored as possible unidimensional conductors 12-15 and molecular magnetic materials.13
N-S-**
..
-4--
e----,e** *,
1 FIGURE 1 Resonance structure of thioaminyl radical fragment (1). The two-coordinate nature of the thioaminyl fragment facilitates its introduction into a wide variety of aromatic ring systems, some of which are illustrated in Figure 2. These ring systems with neutral n-delocalized radical centers could, in turn, be introduced into the rigid core of liquid crystals (LC), provided the ring design allows appropriate substitutions generic to liquid crystal molecules. Ring systems A-E with appropriate substitutions would result in molecules with near linear geometries of rigid cores and flexible tails, a design typical 16 for calamitic liquid crystal molecules (Figure 3). A molecular geometry with a disc-like rigid core and radially distributed flexible tails, typical of a discotic liquid crystal molecule (Figure 3), would result from ring systems F and G (Figure 2). Since magnetic behavior and other physical properties of a molecular material strongly depend on contact between molecules and orbital-orbital interactions, it is expected that the long range molecular order characteristic of LC will play an important role. 17 ,18 In multiphase LC incorporating such a n-radical system, the anticipated extended lateral spin-spin interactions should result in significant changes in magnetic behavior associated with different molecular arrangements. Recently, we have reported a more detailed discussion of the design and first experimental results for persistent radical 2.19 In this communication, we present our progress toward formation of neutral it-radical systems incorporated into discotic liquid crystal molecular structures.
349 Mat. Res. Soc. Symp. Proc. Vol. 374 0 1995 Materials Research Society
Ra
YI
R' R
OR
R"
RD
R"
B
A
C Calamitic (nematic, smectic) R
>)R"
R
0
N-S
E
R
R
R
R
X=-X
R
y--< Y~Y
F
N-S
R
G )iscotic
X, Y = N, CH; R= 4-alkylphenyl, alkoxyphenyl;
R',R" = alkyl, cycloalkyl, aryl
FIGURE 2 Schematic repres
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