LIQUID-CRYSTAL PROPERTIES OF METAL-SALICYLALDIMINE COMPLEXES - CHEMICAL MODIFICATIONS TOWARDS LOWER SYMMETRY

A body of work involving liquid crystalline phases in the family of tr ansition metal complexes of salicylaldimines has been reviewed. An acc ount is given as to how the mesophases can be derived from archetypica l coordination compounds in the liquid state by an approach of lowerin g symmetry. Attention is confined to reported examples of well-establi shed metal-salicylaldimine mesogens. Firstly, it is shown that the nem atic behavior can be related to the molecular shape anisotropy as a pr imary factor, which works advantageously to generate the orientational order. A group of 4-alkoxy)benzoyIoxy-N-alkylsalicylaldiminato]metal complexes represents the nematics. Smectic fluctuations in, and eventu al ordering of, the nematic phases are treated together, and several o ther effects conducive to this tailoring of phase property are pointed out. Pure smectic phases are found in bis[4-alkoxy-N-arylsalicylaldim inato]metal complexes. The change in the rigid core shape and the pack ing considerations for the terminal alkyl chain substituents are discu ssed in relation to the positional ordering. The literature is cited t o shed light on the increased biaxial character of the molecular struc ture possibly leading to unusual local ordering. Finally, columnar pha ses in complexes of[M(salen)] type with heavily incorporated long alky l chains are introduced and contrasted to simpler dialkylated derivati ves which are smectogenic. (C) 1998 Elsevier Science S.A. All rights r eserved.