Columnar Mesophases and Organogels Formed by H-Bound Dimers Based on 3,6-Terminally Difunctionalized Triphenylenes

Abstract

A series of triphenylene (TP) compounds-denoted 3,6-THTP-DiCnOH-bearing four hexyloxy ancillary chains and two variable-length alkoxy chains terminally functionalized with hydroxyl groups have been synthesized and characterized. The shorter homologs revealed mesogenic characteristics, giving rise to thermotropic mesophases in which π-stacked columns of H-bound dimers self-organize yielding superstructures. Molecular-scale models are proposed to account for their structural features. The three studied compounds yielded supramolecular gels in methanol; their ability to gelify higher alcohols was found to be enhanced by the presence of water. The intermediate homolog also gelled n-hexane. Compared to their isomeric 2,7-THTP-DiCnOH analogs, the 3,6-derivatives showed a higher tendency to give rise to LC phases (wider thermal ranges) and a lower organogelling ability (variety of gelled solvents, lower gels stabilities). The overall results are analyzed in terms of different kinds of competing H-bonds: intramolecular, face-to-face dimeric, lateral polymeric, and solvent-TP interactions.

Keywords: H-bonded dimers; columnar liquid crystals; organogel; triphenylene functionalized triphenylenes.

Figure 1

General description of triphenylenes based on a three-block molecular architecture, terminally functionalized at different positions ((left): 2,7; (right): 3,6). Pink: functional group; violet: functional chains (alkoxy chain bearing FG at the free end); green: ancillary chains. The schematic representation emphasizes the position of the FG relative to the “crown” (grey circle) formed by the disordered ancillary chains, (center): numbering scheme.

Figure 2

Compounds studied in this work.

Scheme 1

Synthetic strategy used to obtain the 3,6-THT-DiCnOH; n = 4, 6 and 10.

Figure 3

DSC traces of the second heating–cooling cycles for 3,6-THTP-DiCnOH (top: n = 4, middle: n = 6, bottom: n = 10).

Figure 4

POM pictures taken on cooling from the isotropic state for the studied 3,6-THTP-DiCnOH compounds: (a): n = 4 at 78 °C, (b): n = 6 at 62 °C, (c): n = 10 at room temperature.

Figure 5

XRD patterns for 3,6-THTP-DiC4OH (left) and 3,6-THTP-DiC6OH (right) at selected temperatures.

Figure 6

Proposed models for the organization of the triphenylene molecules in the columnar mesophase of 3,6-THTP-DiC4OH (left) and 3,6-THTP-DiC6OH (right), explaining their main structural features including the lamellar superstructure. Top: schematic representation of the suggested H-bond dimers.

Figure 7

(Left) Plot of Tgel as a function of 3,6-THTP-DCnOH concentration in (a) methanol, (b) ethanol, and (c) n-hexane. (Right) Plot of Tgel as a function of %V/V water added in mixtures of (d) methanol, (e), ethanol and (f) 2-propanol.

Figure 8

Photograph of inverted tube with gel formed from 3,6THTP-DIC6OH in water–hexane at a ratio of 80:20.

Figure 9

SEM images of 3,6-THT-DiCnOH xerogels obtained from methanol. (a) n = 10, (b) n = 6, and (c) n = 4, and 3,6-THT-DiC6OH in hexane (d).

Figure 10

Analysis of the structural features expected for supramolecular dimers of 3,6-THTP-DiC4OH and 3,6-THTP-DiC6OH under the assumption of extended zig-zag aliphatic chains.

Figure 11

Two possible interaction schemes involving multiple H-bonds (green doted lines) between 3,6-THTP-DiCnOH molecules giving rise to supramolecular dimers.

Figure 12

Schematic representation of a model for the LC phase that includes the multiple H-bonds schemes, a higher concentration of the functionalized chains in the inner region of the supramolecular dimers, and “molten” ancillary chains, which agrees with all the experimental facts.

Figure 13

Schematic model depicting the increasing feasibility of intramolecular H-bonds (green doted lines) as the functionalized chains increase in length, thus destabilizing the supramolecular gels formed by 3,6-THT-DiCnOH and alcohols. The red ovals represent OH groups.

Figure 14

Schematic representation of the hydrogen bond interactions in the 2,7- and 3,6-functionalized isomers (extended lateral vs. localized). Red ovals represent OH groups, dotted green lines represent hydrogen bonds.