The development of polar discotic metallomesogens Vanadyl 1,3-diketonate complexes

Abstract

The dependence of mesomorphism of 1,3-diketonate vanadium-oxo (vanadyl) complexes on the number of side chains was investigated. These complexes have a large dipole normal to the disc plane and are under investigation in an effort to generate discotic phases with polar order. Relatively complex phase behaviour is observed when the vanadyl 1,3-diketonate complexes are appended with four phenyl groups with two octyloxy or decyloxy side chains each for a total ofeight side chains. These complexities are most likely related to the order/disorder associated with the polar vanadyl group. Vanadyl 1,3-diketonate complexes with four side chains were prepared by replacement of two of the phenyl groups with methyl or trifluoromethyl groups, and these complexes displayed only crystal phases. Complexes with two trialkyloxy phenyls and two dialkyloxy phenyls were synthesized and found to display a very stable D,, phase. The presence of additional alkyloxy groups was found also to promote a linear chain structure,in the crystal phase. IntroductionNew liquid crystals with polar mesogens are of interest as a means of generating materials which can interact strongly with electric fields. Polar mesogens may also be used to produce new materials with ferroelectric behaviour, given that bulk dipolar order can be established. In the quest for new achiral ferroelectric liquid crystals, columnar phases of pyramid shaped macrocyclic organic mesogens have been investigated [l]. The pyramid shape of the mesogen in these systems promotes a headto-tail organization and hence can create polar order within a given column. There have been a number of elegant studies on these organic pyramidal liquid crystals [l], but ferroelectric behaviour has not been demonstrated. This fact is probably due to small dipoles and large activation energies for redirecting the dipoles, since recent theoretical studies indicate that ferroelectric phases should exist [2].As a result of these difficulties associated with prospective ferroelectric organic discotic systems, we have been focusing on inorganic vanadium-oxo (vanadyl) complexes. The vanadyl group is attractive since it often exhibits a square pyrimidal structure and has a large directional dipole associated with the V=O bond [3]. In addition, work in this laboratory has recently shown that liquid crystalline vanadyl complexes can arrange in a linear chain structure (i.e. --V=O--V=O--), thereby creating local polar order between mesogens [4]. We report herein our results on a series of discotic 1,3-diketonate vanadyl complexes for which we explore the effects of the number of side chains on the mesomorphic behaviour.

Star­like substituted hexaarylbenzenes: synthesis and mesomorphic properties

“…The small reflection angles at 2h between 5-17 ‡ should indicate the coexistence of unidentified crystal phase and columnar mesophase. 17 Therefore, the peaks at 56.5 and 66.6 ‡C in the DSC trace on the first heating scan are attributed to melting and clearing point, respectively.…”

“…4, which is similar to some textures exhibited by columnar mesophases reported earlier. 17,18 The powder Xray diffraction (XD) pattern of 1 recorded at room temperature prior to heating is that of a typical crystalline phase. XD patterns measured at 60 ‡C on heating, 60 ‡C on cooling and subsequently at room temperature are similar and exhibit a sharp reflection at small angles corresponding to a d-value of 38.4 A ˚(100) and several reflections in the wide angle region, the most prominent one corresponding to 4.8 and 4.3 A ˚, as shown in Fig.…”

Stabilization of Nondiscoid Columnar Liquid Crystals: Studies of Unsymmetrical Copper Bis-β-diketonates

“…We note that other columnar phases based upon nondiscoid molecules have been reported to be stabilized by the organization of dipoles 2b,d We have also investigated analogous metal bis(β-diketonate) complexes 1 and 2 (type 1 and 2 systems, respectively) and find that although 1 displays well-behaved liquid crystallinity with good thermodynamic stability,2c 2 did not appear to be liquid crystalline . In this paper we describe complexes related to 2 wherein an extension of the mesogenic core and the addition of dipoles have been used to stabilize liquid crystallinity.…”

Liquid Crystal Polymers Containing Ni(II), Pd(II), or VO(II) in the Main Chain

“…In particular, for Ni(II)-containing LC polymers, no data concerning linear polymers ar available, while some cross-linked polymers have been reported by Hanabusa et al , As to Pd(II)-containing LC polymers, a few cases have been reported concerning liotropic polymers, side chain orthopalladate polysiloxanes, and one single example of linear polymer . Finally, notwithstanding the good magnetic properties of oxovanadium complexes (a magnetic moment at room temperature of the order of 1.6 μ B has been reported for salicyladiminate oxovanadium complexes) and the possibility, experimentally assessed for low-molecular-weight complexes, − of building molecular structures compatible with the onset of liquid crystallinity, a single example of a liquid-crystal polymer containing coordinated oxovanadium groups has been reported by Serrano and co-workers …”