Carbosilane Liquid Crystalline Dendrimers: From Molecular Architecture to Supramolecular Nanostructures Sergey A. Ponomarenko, † Natalia I. Boiko, † Valery P. Shibaev,* ,† Robert M. Richardson, ‡ Iain J. Whitehouse, ‡ Eugenii A. Rebrov, § and Aziz M. Muzafarov § Chemistry Department, Moscow State University, Vorobyevy Gory, Moscow, 119899 Russia; School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, U.K.; Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, 70 Profsoyuznaya st., Moscow, 117393 Russia Received January 21, 2000; Revised Manuscript Received April 18, 2000 ABSTRACT: A systematic study of the influence of generation number on the phase behavior of LC dendrimers is presented. For this purpose, phase behaviors and structures of first to fifth generations of liquid crystalline (LC) carbosilane dendrimers with 8, 16, 32, 64, and 128 terminal cyanobiphenyl groups were investigated. Investigation of thermal behavior of the LC dendrimers by means of polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction experiments reveals smectic-type mesophases over a wide temperature region. It is shown that with increasing generation number the isotropization temperature increases whereas the enthalpy of this phase transition decreases. The strongest influence of spherical molecular architecture on the phase behavior of the LC dendrimers appears at high generations. In the case of LC dendrimer of the fifth generation, it leads to the formation of two levels of a structural organization. This dendrimer forms different supramolecular nanostructures of columnar type in addition to smectic-like arrangement of mesogenic groups. Possible structures of all mesophases formed are discussed. Introduction Currently, there is a growing interest in synthesis and investigation of systems with unusual molecular struc- ture showing mesomorphic properties as evidenced by the growth in papers published in this field. Liquid crystalline (LC) dendrimers are a clear example of such systems. LC dendrimers are a new class of highly branched LC compounds. Their molecules combine structural units capable of LC mesophase formation (mesogenic groups) 1 with dendritic or “cascade” archi- tecture. 2 Up to now LC dendrimers of different chemistries have been synthesized: polyorganosiloxane, 3 carbosi- lane, 4 poly(propyleneimine), 5 and poly(amidoamine). 6 Recently the first ferrocene-containing LC dendrimer 7 as well as a ferroelectric LC dendrimer 8 has been re- ported. All of them were shown to form different smectic mesophases. Dendrimers consisting of mesogenic groups incorporated in each branching point form smectic and nematic mesophases. 9 Some hyperbranched polymers were also shown to be liquid crystalline, 10 but they should be considered as another class of LC compounds. These polymers differ from dendrimers because they have disorder in the arrangement of branching points and do not possess some of the peculiarities of dendrim- ers. 11 Several dendrimers without mesogenic groups were also shown to be liquid crystalline. 12 However, they have a different origin of the LC mesophase formation. Depending on the location of mesogenic units in the molecule, two classes of LC dendrimers may be consid- ered. 13 These are LC dendrimers with mesogen-contain- ing branching units 9 and LC dendrimers with terminal mesogenic groups. 3-8 Both LC dendrimers with terminal mesogenic groups and side chain liquid crystalline polymers consist of molecules built up from the following general structural units: polymeric chain, spacer, and terminal (or side chain) mesogenic groups. The main difference between these LC compounds is the different topology of the polymer chain. It is linear in the case of side chain LCPs, while in the case of LC dendrimers it has superbranched, “dendritic” or “cascade” architec- ture. It should be noted that LC dendrimers are mono- disperse compounds. This is another distinction from side-chain LCPs, which are usually polydisperse, as are most synthetic polymers. It is well-known that all of the structural units forming side chain LCPs influence on the phase behav- ior of LCPs. 1 For instance, changing the spacer length could give rise not only to the oscillation of the transition temperature (odd/even effect) 14 but also to a change in the mesophase type. 15 An increase of the main-chain length (degree of polymerization) in LCPs leads to an increase of the clearing temperature (T Cl ) in the case of low molar weight LC oligomers. However, the effect vanishes in high molar weight LCPs. 16 There is very little information in the literature about the influence of the same structural units on the phase behavior of LC dendrimers with terminal mesogenic groups. From this point of view, it is very important to establish the main features of their phase behavior including (i) the influence of generation number (re- sponsible for dimensions and molecular weight of den- drimers), (ii) the spacer length between the mesogenic units and the dendritic matrix, and (iii) the chemical nature of the terminal mesogenic groups. The effect of mesogen type on the properties of first-generation LC dendrimers in bulk and in dilute solutions was pre- sented in previous works. 4a,17 Influence of spacer length * Author for correspondence. Telephone: 7 095 939 11 89; Fax: 7 095 939 01 74. E-mail: lcp@genebee.msu.su; shival@online.ru. † Moscow State University. ‡ University of Bristol. § Russian Academy of Sciences. 5549 Macromolecules 2000, 33, 5549-5558 10.1021/ma0001032 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/08/2000