Chiral oxovanadium(IV), copper(II), and palladium(II) β-diketonates show a room-temperature columnar mesophase which undergoes ferroelectric switching. All the compounds were obtained as liquid crystals at room temperature, and crystallization or melting processes were not detected by differential scanning calorimetry carried out to −20 °C. The mesophase was investigated by optical microscopy, DSC and X-ray diffraction, and identified as a rectangular columnar (P21). The flowerlike texture observed for all the compounds led us to deduce a high tilt angle (ca. 40°) of the molecules with respect to the column axis. Circular dichroism has confirmed the existence of a helical arrangement within the column. This result is in accordance with the so-called columnar mode found at low frequencies (ca. 10-3 Hz) in dielectric spectroscopy studies. The electrooptical response of these materials has been examined by means of a photomultiplier. The results obtained can be explained by considering a strong influence of the high tilt angle found in the mesophase.

The synthesis and physical characterization (thermotropic and ferroelectric behavior) of eight ortho-palladated dimers [Pd2(ji-C1)2L*2] is reported. They are derived from chiral imines HL* (HL* = />-R1OC6H,(CH=NQH4-OR* 12-p, with R1, R2, or both equal to (R)or (5)-l-methylheptyl). The chiral imines, which contain one or two stereogenic centers, are not liquid crystals themselves. However, the dimeric complexes, which contain two or four stereogenic centers in their structure, exhibit both SmA and SmC* mesophases and enable the first comparative structure-activity study on metal-containing ferroelectric liquid crystals to be made. The ferroelectric properties obtained are better when there are chiral chains on the ortho-metalated aromatic rings, because the fixed conformation of these rings diminishes the chiral tail freedom, thus allowing a better coupling of molecular dipoles. The complexes with two chiral tails show important Psmax values in the range 44-131 nC*cm-2. The complex with four chiral chains (R1 = R2 = (R)-methylheptyl) is the first such compound ever reported containing four stereogenic centers; furthermore, it shows a remarkably high Psmax value of 206 nC-cnr2, the highest value reported so far for a metal-containing liquid crystal.

It is known that mesomorphism results in many cases from a spontaneous self-organization of molecules, commonly achieved by hydrogen bonding."] Recently, we reported that a series of pyrazoles and isoxazoles substituted with two 4-alkoxyphenyl groups show smectic A and C mesophases. ['] In the same paper, we discussed the possibility of obtaining disk-like mesogens by dimerization of the pyrazoles through intermolecular hydrogen bonding. However, although hydrogen bonding was detected by spectroscopic studies even at high temperatures (near 200 "C), none of the pyrazoles studied substituted with two alkoxy groups in the 3,4 or 3.5 positions of each phenyl ring was found to exhibit mesomorphism.[21 This suggests that hydrogen bonding in the pyrazoles is not efficient enough to generate discotic mesogens.In this paper we wish to describe a novel approach, in which discotic pyrazole derivatives are formed by dimerization of half-disk-shaped promesogenic pyrazoles through BH, bridges. The pyrazaboles thus generated are known to be a very stable, resonance-stabilized, kind of ring.[31 We prepared and investigated the mesogenic properties of three pyrazaboles appended with four phenyl groups with two or three decyloxy groups each (series 3, Scheme 1). At the same time we isolated the precursor P-diketones (series 1) and pyrazoles (series 2) and explored their potential mesomorphism. Two of the pyrazaboles prepared form hexagonal columnar mesophases, identified by microscopy, DSC, miscibility and X-ray diffraction. We here report our results on this series of compounds for which we investigated the influence of the number and the position of peripheral chains on mesomorphism.When designing discotic liquid crystals, molecules with an approximately planar central core are preferred. The X-ray diffraction studies of differently substituted pyrazaboIesr4] show that the central B,N, ring can adopt either a chair, boat or planar conformation, depending on the substituents of the boron atom. Unfortunately, data with regard to the unsubstituted compound (BH, bridges) have not been reported. If the B,N, ring is in the chair or planar form, the pyrazole (C,N,) rings should be coplanar and the whole pyrazabole system should be approximately planar. On the other hand, if the B,N, ring adopts the boat conformation, the pyrazabole is no longer planar. However. the absence of coplanarity of the central core is not incompatible with the appearance of discotic mesophases, as there are a number of reports of non-planar (pyramidic, conical, bowl-like) discogens in the l i t e r a t~r e .~~] The pyrazaboles described in this paper (Scheme 1) possess a rigid central core consisting of the pyrazabole ring substituted in the 1, 3, 5 and 7 positions by benzene rings, each carrying two or three alkoxy chains (eight or ten chains in total) which constitute the flexible part of the molecule. We prepared the compounds in which each phenyl ring carries two decyloxy groups in the 3 and 4 positions (compound 321) and in the 3 and 5 positions (compo...