Self-assembled supra-structures from liquid crystalline carbo-benzenes – Supracarbo_Ukraine

The project addresses the study of mesophases constituted of molecules, hereafter called "carbo-mesogens", built around a C18 carbo-benzene core. This new type of core includes the following features: all-carbon composition, hexagonal symmetry, extended flatness, high magnetic aromaticity, "acetylenic" flexibility, hollow discotic character (diameter ˜ 0.8 nm). Seeking for new types of thermotropic liquid crystals (LCs), the study will focus on carbo-mesogens 1,10-D2-C18Ar4 or 1,7,13-D3-C18Ar3, bearing 2 or 3 dispersion groups in quadrupolar or octupolar symmetry, respectively, where D = 3,4,5-(RO)3-C6H2-C=C-, R is a more or less fluorinated aliphatic chain of length n = 8, and Ar = 4-X-C6H4, X = H, CF3, CmH2m+1, 5 = m = 8. Beyond its discotic character anticipated to favor columnar phases, the quadrupolar version could also lead to other less ordered mesophases, expected to be more smectic than nematic in nature because of the much larger extent of the core as compared to that of the two dispersion groups. The more general objective is the study of supramolecular assemblies under various conditions, using appropriate analysis tools: crystalline phases (XRD), 3D LC mesophases (POM, DSC, TGA, synchrotron beam PXRD), 2D layer on HOPG surface (STM), in solution (NMR), or in silico (DFT calculations). The work will be systematically conducted in two stages by resorting to the specific expertise of each partner: synthesis and characterization of carbo-mesogens in Toulouse, studies of 3D and 2D mesophases in Taipei. In each series, quadrupolar or octupolar, the effects of the variation of the R and Ar substituents of the carbo-benzene core will be scrutinized. The C18 macrocycle of the latter being more circular than discoid (inner hollow van der Waals diameter = 3.5 Å), inclusion compounds are also naturally envisaged: guest species being supposed to provide specific optical, electric or magnetic properties, metal ions will be more specifically considered.
The relevance of both the project and partnership is based on first results recently published reporting the synthesis, thermotropic properties and 2D supramolecular assemblies of the carbo-mesogen Ar = Ph, R = n-C12H25, and evidencing a rectangular columnar discotic LC behavior at 115°C.
Preliminary theoretical results also suggest the stability and structural features of carbo-benzene-transition metal cation host-guest associations. While addressing poorly explored fundamental aspects ("carbo-supramolecular" chemistry), the project relies on a rational use of analytical tools that have not been applied in the carbo-mer series yet. After non-linear optics and electrical conductivity (ANR project funded in 2011), a new field of application of functional carbo-meric materials is explored: the field of liquid crystals, which, beyond a use in many commercial devices (e.g. LCDs), must more than ever face specific challenges by the design of novel types of mesogenic structures. The progress of the project is directed toward 4 general objectives (synthesis of carbo-mesogens, delineation of structure-property relationships for 3D and 2D supramolecular assemblies, uncovering of novel types of LCs, appraisal of cooperative effects between carbo-mesogen inclusion complexes), and based on four particular tasks (optimization of quadrupolar carbo-mesogens by variation of the Ar and R groups, exemplification of octupolar carbo-mesogens, study of carbo-mesogen-transition metal inclusion complexes).