Chirally Homogeneous and Heterogeneous Dendritic Liquid Crystals A. Belaissaoui,* I. M. Saez, S. J. Cowling, and J. W. Goodby Department of Chemistry, University of York, Heslington Road, York YO10 5DD, U.K. * S Supporting Information ABSTRACT: We have coupled methyl α-D-glucoside (G) and methyl α-D-mannoside (M) as the chiral structural variant moieties in the core and at the branching sites within dendritic scaffolds surrounded by 12 cyanobiphenyl mesogens (CB). Systematic studies of the influence of the chiral moieties on thermal and mesomorphic properties were carried out by positional permutation approach of the pyranose units G and M within the core and branching points. The mesomorphic properties of the chirally homogeneous dendrimers GG 4 /MM 4 and the corresponding chirally heterogeneous dendritic homologues GM 4 /MG 4 were investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Remarkably, the thermal profile and the mesophase structure of the four dendrimers appear to be significantly independent of the nature of the central chiral core. The outer chirality at the periphery significantly dominates the liquid crystalline properties. ■ INTRODUCTION Although remarkable progress has been made in nanoscience, there are still huge challenges in arranging molecular functional moieties into structures where they are organized in a controllable and preprogrammed fashion to perform specific and predefined tasks at macroscopic scale. We have focused on functional liquid crystals as they combine the functional components, relevant to perform chemical, biological, and physical tasks, with the high self-organization, remarkable control, and large variety of mesophase morphologies present in liquid crystals. Dendritic systems 1-3 are useful models since their size, shape, constitution, branching, functionality, 4 and morphology can be readily controlled at the molecular level. Their defined singular functional and structural features are essential ingredients in the molecular engineering of liquid crystals that can lead to unique materials with fine-tunable functions controlling the mesophase properties. Following our studies on supermolecular 5-7 liquid-crystalline chiral tripedes and tetrapedes, 8,9 we have developed novel functional liquid crystalline dendritic 10-12 analogues that combine mesogenic units with chiral functional moieties in a single molecule. In this article, we describe the architectures of a homologous series of chiral dendrimers 13-18 that we have prepared with the aim to investigate the impact of the chirality in dendritic systems and how the chiral information encoded at molecular level is transferred and expressed at various levels of organization, from the molecular through to the macroscopic level into the mesophase. In this study, we have coupled methyl α-D-glucoside (G) and methyl α-D-mannoside (M) as the chiral structural variant moieties in the core and at the branching points within dendritic scaffolds surrounded by 12 cyanobi- phenyl mesogens (CB). The two pyranose cores differ only at the C2 position with R configuration for the glucoside and S for the mannoside. We report the influence of the stereogenic centers on the mesomorphic properties of the chirally homogeneous dendrimers GG 4 /MM 4 (the core and branching points having identical pyranose units) and compare it with the corresponding chirally heterogeneous dendritic homologues GM 4 /MG 4 (the core and branching points having different pyranose units) (Figure 1). ■ EXPERIMENTAL SECTION The C2 epimers methyl α-D-glucoside (G) and methyl α-D-mannoside (M) were used as starting materials for the synthesis of dendrimers building blocks, carbohydrate core molecules G 0 and M 0 as well as dendrons G 1 and M 1 , as shown in Scheme 1. Allylation reactions of glucoside G and mannoside M were carried out using allyl bromide and sodium hydride NaH in DMF to yield quantitatively their per-allylated corresponding derivatives G a and M a , followed by hydroboration reaction with 9-BBN and oxidation, to afford the tetra-O-(3-hydroxypropyl)pyranose homologues G 0 and M 0 in excellent yields. The carboxylic acid tripedes G 1 and M 1 , which we have previously reported the synthesis, 9 were coupled with the tetraol glucoside and mannoside derivatives G 0 and M 0 by esterification reaction under DCC/DMAP coupling conditions. The reaction was monitored by GPC until completion to yield the monodisperse dendrimers GG 4 , MG 4 , GM 4 , and MM 4 . ■ RESULTS AND DISCUSSION The mesomorphic properties of the dendrimers were investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). When viewed through cross polarizers, upon cooling from the isotropic state to 90 °C, GG 4 displayed a planar texture showing left-handed helicity, identifying clearly the mesophase as a chiral nematic. At 89.7 °C, a transition to a smectic A mesophase was observed where Received: December 24, 2012 Revised: January 22, 2013 Article pubs.acs.org/Macromolecules © XXXX American Chemical Society A dx.doi.org/10.1021/ma3026332 | Macromolecules XXXX, XXX, XXX-XXX