Encoding calamitic mesomorphism in thermotropic lanthanidomesogens

Abstract

Peripheral cyanobiphenyl dendrimers impose a microphase organization compatible with smectic mesomorphism, in which the bulky nine-coordinate lanthanide core is located between the decoupled mesogenic sublayers made up of parallel cyanobiphenyl groups.The design of thermotropic metallomesogens (metal-containing liquid crystals) 1 follows similar rules as those applied for organic compounds, and a common strategy consists of designing anisometric molecules possessing at least two incompatible parts, which are separated by a molecular interface. The incompatible parts generally correspond to polarizable rigid cores and poorly polarizable flexible chains, the latter being located at the periphery of the molecule. 2 The existence of these two spatially separated portions is responsible for the generation of two successive phase transitions corresponding (i) to the melting of the aliphatic chains, and the concomitant formation of the liquid-crystalline phase, followed by (ii) the ultimate melting of residual packed rigid cores, which leads to the nematic phase or to the isotropic liquid. 2 For trivalent lanthanide metal ions, Ln III , the considerable size of their high-coordination spheres limits the efficiency of the microsegregation processes operating between the polarizable bulky metalloorganic core and the appended flexible paraffinic chains, and the occurrence of thermotropic lanthanide-containing liquid crystals (lanthanidomesogens) remains rare. 3 Two main strategies have been considered for restoring the minimal electronic and structural incompatibilities required for an efficient microsegregation: (1) the Ln III ions are embedded into aromatic polarizable cavities produced by several wrapped ligands, which globally display rodlike or disklike shapes (Fig. S1, ESI{), [3][4][5] or (2) the coordination of Ln III to pro-mesogenic polycatenar ligands possessing curved molecular interfaces and large melting entropies helps to induce thermotropic mesomorphism at 'low' temperature, as observed for [Ln(L1)(NO 3 ) 3 ] (Fig. 1). 6 An alternative strategy was reported for the design of [60]fullerene-containing liquid crystals, in which the bulky C 60 unit is embedded within a flexible part connecting two peripheral mesogenic dendritic cyanobiphenyl cores in L2 (Fig. 1). 7 Since the intermolecular interactions responsible for the residual organization of the rigid cores of L2 in the mesophase mainly result from the parallel arrangement of the mesogenic cyanobiphenyl groups, a smectic A phase was obtained. Based on this study, one can envisage replacing C 60 with other bulky edifices, such as lanthanide cores. Following this reasoning, we aim at forcing lanthanidomesogens to form fluid thermotropic smectic and nematic phases by using the lowgeneration dendritic complexes [Ln(Li)(NO 3 ) 3 ] (i = 3, 4), in which a central tridentate 2,6-bis(benzimidazole)pyridine unit replaces the malonate spacer found in L2 (Fig. 1). The recent report of nematic mesomorphism around 85-90 uC for the related, but unsymmetrical complexes [...

Thermodynamics of dimerization in solution as a rational tool for inducing nematic vs. smectic organizations in lanthanidomesogens

“…6b,c Consequently, the low melting temperatures T m evidenced for polycatenar and dendrimeric units can be assigned to the large entropic gain accompanying the decorrelation of the divergent flexible chains. 7 This concept is well illustrated by the connection of the dendrimeric synthon R 0 to bulky fullerene 8 or of synthon R 1 to lanthanide complexes (Ln is a trivalent 4f-block ion), 9 which induces the formation of stable thermotropic smectic A mesophases (Scheme 1). The layered organization in the mesophase of these dendrimers has been attributed to attractive enthalpic intermolecular interactions operating between the terminal cyanobiphenyl groups, which are optimized for smectic arrangements.…”

Implementing Liquid‐Crystalline Properties in Single‐Stranded Dinuclear Lanthanide Helicates

“…The diffractogram collected at 120 °C for [Y 2 ( L6 )(hfac) 6 ], taken as a representative example, showed two sharp small‐angle reflections in the ratio 1:2, which was characteristic of 1D lamellar ordering (the reflections were indexed with the 001 and 002 Miller indices), and the position of which gave a weakly temperature‐dependent periodicity of d = 105 Å. This periodicity in [Y 2 ( L6 )(hfac) 6 ] is shorter (i) than the total length of L6 in its extended conformation (estimated to be about 130 Å; see Figure S26 in the Supporting Information) and (ii) than the periodicity previously reported for the SmA phases ( d ≈ 120 Å) produced by the related mononuclear dendritic complexes [Ln 2 ( L9 )(NO 3 ) 6 ] and [Ln 2 ( L10 )(NO 3 ) 6 ] (see Figure S26) 15i–15j …”

“…Alkylation of intermediate 11 with 15 gave the target ligand L5 , whereas esterification with 16 15i yielded L6 (Scheme and Appendix 1 in the Supporting Information).…”

Enthalpy/Entropy Compensation in the Melting of Thermotropic Nitrogen‐Containing Chelating Ligands and Their Lanthanide Complexes: Successes and Failures

“…This observation has considerable (and negative) consequences for the development of potential correlations between solution‐based thermodynamics and our original interpretation of intermolecular interactions occurring in the mesophases, whereby no solvent molecule is present. Based on the latter experiments, dichloromethane should be preferred over chloroform for recording parameters that are amenable to being used in potentially rationalizing cohesion in thermotropic lanthanidomesogens,6d,6e but there is no fundamental justification for considering dichloromethane itself as an innocent solvent. The crucial effect of solvation in the formation of lanthanide complexes and related aggregates in chloroform is further confirmed by the enthalpic and entropic contributions measured for Equilibria (5) and (6) (Table 2), which imply entropically driven processes only compatible with the two‐step desolvation model first introduced by Choppin 26…”