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Acta Cryst. (1997). B53, 323-324

Metallomesogens - synthesis, properties and applications

Edited by J. L. Serrano

Pp. xix + 498. Weinheim: VCH Verlagsgesellschaft, 1996
Price DM 298. ISBN 3-527-29296-9

Metal-containing liquid crystals have been known for some 80 years, and longer if the lyotropic metal soaps are included. However, the topic has been recognized as a distinct area with an intrinsic interest of its own for less than 10 years. Indeed, the term `metallomesogen' was introduced by our group in Sheffield only in 1990.

Metallomesogens reflect the idea that scientific advances often take place at the interfaces between disciplines, in this case that between liquid crystals and inorganic chemistry. The coordination templates of the 60 or so participating metals offer a wide variety of unusual molecular shapes (described e.g. as `open books', `bricks' and `shish-kebabs'), which give rise to liquid crystalline properties and allow a large range of potential `advanced materials' with interesting and unusual properties to be constructed. Liquid crystalline materials containing a metal atom in the organic molecular skeleton have properties derived from both the organic ligand and the metal. Thus, for example, the metal atom arranges the ligands around it in characteristic shapes and also offers a large and very polarizable electron density, which may give rise to new types of macroscopic (e.g. optical) properties.

The first review on the topic was published in 1991 and contained some 150 references. It reflects the rate of progress and the interest in the area, that its first book, a 500 page monograph, has now appeared. Its editor, J. L. Serrano, a liquid crystal chemist who specializes in the synthesis and characterization of these compounds, has gathered a group of Spanish colleagues and former students to put together an excellent and comprehensive account. The book begins with a general introduction to (organic) liquid crystals and describes the essential features of the main classes (thermotropic, which change phase with temperature; lyotropic, which involve a solvent, usually water) and of the main subclasses of each [e.g. rod-like (calamitic) and disc-like (discotic) thermotropics]. The actual metallomesogens are arranged, in Part A, by both ligand and metal, making it easy to see which systems have already been investigated.

A further chapter, in Part B of the book, describes the principles behind the design and synthesis of the ligand systems which give rise to metallomesogens. Both low molecular weight and polymeric metallomesogens are discussed in full, though no experimental details of the syntheses are offered. In the low molecular weight sections, some macroscopic properties are rationalized, in terms of the molecular structures present, by adapting the general ideas developed for organic liquid crystals. These interpretations are very useful, since the appearance or absence of liquid crystal properties in any given molecule depends on subtle geometric and electronic factors.

Part C deals with methods for structural characterization, including X-ray diffraction and EPR spectroscopy; magnetic properties are explained, and there is also a section on the various optical properties exhibited by metallomesogens. Some comments on possible uses of these molecules are included.

The text is well organized and well illustrated with clear diagrams showing the essential features, both of the metallomesogenic molecules and the various liquid crystalline phases; as many of the concepts and molecular shapes may be unfamiliar, this is particularly helpful to the non-specialist. The text offers a good introduction to the inorganic chemist interested in new materials and new structures, and to the liquid crystal scientist who is interested in the new properties offered by these novel systems. If there is a caveat it is that since the novice entering a new research area will always make some mistakes before getting going, some of the known problems and pitfalls might have been more clearly marked out.

Peter Maitlis

Department of Chemistry
The University of Sheffield
Sheffield S3 7HF

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