Symmetrical Diacetylenes Outfitted with Ionic Liquid-like Groups: Structural, Polymerization, and Carbonization Studies

Abstract

Three symmetrical diacetylenes (DAs) bearing tetraalkylammonium substituents have been prepared, namely, 1,6-bis(triethylammonium)hexa-2,4-diyne diiodide (2), dinitrate (3), and bis[bis(trifluoromethylsulfonyl)imide] (4). For these three salts, the duality between polymerization and carbonization has been investigated, and the results have been rationalized in terms of solid-state organization and molecular structure. These DAs have been irradiated at 254 nm with concomitant annealing at 80 °C (4) or 110 °C (2 and 3), and the lack of polydiacetylene (PDA) formation is in agreement with the fact that the CC–CC rods do not have a suitable orientation for 1,4-addition. Compound 4 is an ionic liquid. This DA starts melting at 88 °C with a maximum peak value of 104 °C, as ascertained by differential scanning calorimetry and thermogravimetric analyses. It is stable in the liquid state at 120 °C for several hours and remains unchanged at 170 °C for a few minutes without any sign of PDA formation, which means that if some kind of organization exists in the liquid phase, it is not helpful for 1,4-polymerization. Thermolyses of 2–4 have been conducted under a nitrogen flow up to 220 °C (3) and 1200 °C (2 and 4). In all three cases, graphite-like carbon materials were obtained. The graphite-like structures start to form around 200 °C, which is the temperature at which cycloaromatization of the triple bonds takes place. The residues from the pyrolyses of 2 and 4 exhibit nitrogen contents of 1.75 and 1.40 wt %, respectively, and powder X-ray diffraction and Raman analyses indicate that these materials have coherently scattering domain sizes in the range of 1–3 nm depending on the crystallographic direction. The Brunauer, Emmett, and Teller specific surface area of 2@1200 derived from dinitrogen sorption experiments is 88 m2 g–1 and that of 4@1200 is 33 m2 g–1. These values are much higher than those measured in previous works for carbon residues prepared at 1100 °C from imidazolium- and benzimidazolium-appended diacetylenes, thereby highlighting the pivotal influence of the size of the cation on the microstructure of the resulting carbon material. In addition, 2@1200 appears to be mostly microporous and 4@1200 mesoporous, which suggests that the anion also plays a central part in the structuring of the final solid. Last, X-ray photoelectron spectroscopy analysis of 4@1200 indicates that, besides nitrogen, this residue also contains small amounts of fluorine and sulfur, thus making carbonization of ionic diacetylenes an alternative method to introduce doping elements in a graphite structure.