FERROELECTRIC BEHAVIOR IN METAL-CONTAINING LIQUID-CRYSTALS - A STRUCTURE-ACTIVITY STUDY

The synthesis and physical characterization (thermotropic and ferroele ctric behavior) of eight ortho-palladated dimers [Pd-2(mu-Cl)(2)L(2)( )] is reported. They are derived from chiral imines HL() (HL* = p-R]O C6H4CH=NC(6)H(4)OR(2)-p, with R(1), R(2), Or both equal to (R)- or (S) -1-methylheptyl). The chiral imines, which contain one or two stereoge nic centers, are not liquid crystals themselves. However, the dimeric complexes, which contain two or four stereogenic centers in their stru cture, exhibit both SmA and SmC mesophases and enable the first compa rative structure-activity study on metal-containing ferroelectric liqu id crystals to be made. The ferroelectric properties obtained are bett er when there are chiral chains on the ortho-metalated aromatic rings, because the fixed conformation of these rings diminishes the chiral t ail freedom, thus allowing a better coupling of molecular dipoles. The complexes with two chiral tails show important Ps(max) values in the range 44-131 nC.cm(-2). The complex with four chiral chains (R(1)=R(2) =(R)-methylheptyl) is the first such compound ever reported containing four stereogenic centers; furthermore, it shows a remarkably high Ps( max) value of 206 nC.cm(-2), the highest value reported so far for a m etal-containing liquid crystal.