Organized Monolayers of Carbosilane Dendrimers with Mesogenic Terminal Groups Kirsten L. Genson, † Jason Holzmueller, † Ignaty Leshchiner, ‡ Elena Agina, ‡ Natalia Boiko, ‡ Valery P. Shibaev, ‡ and Vladimir V. Tsukruk* ,† Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, and Faculty of Chemistry, Moscow State University, Leninskie gory, Moscow, 119899 Russia Received February 11, 2005; Revised Manuscript Received July 11, 2005 ABSTRACT: The structural ordering of mesogenic-terminated carbosilane dendrimers within surface monolayers was studied. We have shown that despite their symmetrical spherical shape layering with average spacing close to 6 nm is observed for dendrimers with butoxyphenylbenzoate terminal groups deposited on hydrophilic silicon substrates. The molecular layered packing is formed by flattened dendritic molecules with terminal groups stacking into bilayers. This layered ordering is controlled by strong interactions between the polar mesogenic groups and the hydrophilic surface and by microphase separation of the dendritic core and terminal groups. The dilution of the outer mesogenic shell with a nonmesogenic component partially disturbs the layered packing within the monolayer. Complete disruption of the regular lamellar ordering is observed if butoxyphenylbenzoate terminal groups are replaced with shorter and more polar cyanbiphenyl groups. For all dendrimers studied here the replacement of the hydrophilic support with hydrophobic one prevents the formation of dense surface layers due to antagonistic interactions between polar terminal groups and a methyl-terminated surface. However, the presence of the nonmesogenic hydrophobic terminal groups stimulates the formation of two-dimensional circular molecular structures which are characteristic of columnar LC dendrimer phases. Introduction The assembly and ordering of branched molecules is dependent on the molecular architecture with major factors being the degree of branching, the core shape, the flexibility of the branches, the generation number, and the terminal functionality. 1 Tailoring the chemical and physical properties of dendritic polymers, facilitated by the stepwise synthesis methodologies, has been the driving force in the exploration of different molecular architectures and chemical composition of dendrimers. 2,3 Although the vast majority of dendrimers synthesized to date are carbon-based materials, several examples of inorganic-based molecules (such as silicon or phos- phorus-containing dendrimers) have been reported. 4,5 The exploration of heteroatom dendrimer architecture offers greater possibilities in catalysis and organic- inorganic hybrid material applications, especially for the stable dispersion of metal nanoparticles. Additionally, the inclusion of silicon in the inner repeat units of the dendritic core offers favorable synthetic routes for ideal dendrimers as well as thermodynamically and kineti- cally stable molecules. 6-8 Carbosilane dendrimers are favorable for further functionalization because the low polarity and high energy of the Si-C bonds creates a chemically stable core. The exceptional flexibility of the carbosilane dendrimers also makes them easily adapt- able with an overall shape dictated by interactions among terminal groups and terminal groups with sup- porting surfaces in cases of their adsorption on solid substrates. 9 Depending upon interfacial interactions and molecular flexibility, a variety of diverse organized nanostructures (compressed nanoparticles, uniform monolayers, nanofibers, individual cylinders, and lay- ered, rectangular, and hexagonal lattices) have been observed for functional dendritic molecules. 10,11 The incorporation of liquid crystalline (LC) fragments in dendritic architecture is an intriguing design ap- proach which can result in fabrication of hybrid struc- tures combining LC properties and dendritic function- alities and shapes. 12 Competing trends of mesogenic fragments to arrange in highly ordered structures and dendritic cores to form symmetrical shapes can fuse into a multitude of amalgam structures. Indeed, the alter- nating mesogenic fragments and aliphatic spacers within the dendritic architecture compel the molecules to form an onion-type column largely different from the meso- phases typical for end functionalized LC dendrimers. 13 The controlled synthetic methodology allows for the preparation of dendrimers with alternating mesogenic fragments for each generation. 14-16 The ellipsoidal shape of the dendritic core has been shown to adversely affect the order of the mesogenic fragments attached to higher generation dendrimers. 17 Similarly, properties such as dielectric relaxation have been influenced by the shape and flexibility of the dendritic core affecting the ordering and mobility of the mesogenic fragments. 18 The functionality of the terminal groups might exert ultimate control on the molecular packing of dendrimer cores. 19-21 The combination of flexible dendritic cores and mesogenic groups creates novel LC behavior. 22 The attachment of mesogenic groups to PAMAM dendritic cores has been observed to force the molecules to adopt a cylindrical shape, forming lamellae structures with influence of the generation number on the ordering. 23 The inclusion of mesogenic groups at the periphery forced a transition to a disk shape facilitating a colum- nar ordering. 23c Similarly, the grafting of mesogenic groups to a PAMAM core produced a transition from smectic ordering to columnar ordering as the molecular cross-section increased. 24 A fifth generation carbosilane * To whom correspondence should be addressed. E-mail: Vladimir@iastate.edu. † Iowa State University. ‡ Moscow State University. 8028 Macromolecules 2005, 38, 8028-8035 10.1021/ma050304b CCC: $30.25 © 2005 American Chemical Society Published on Web 08/18/2005