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

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 degrees C (4) or 110 degrees 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 degrees C with a maximum peak value of 104 degrees C, as ascertained by differential scanning calorimetry and thermogravimetric analyses. It is stable in the liquid state at 120 degrees C for several hours and remains unchanged at 170 degrees 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 degrees C (3) and 1200 degrees C (2 and 4). In all three cases, graphite-like carbon materials were obtained. The graphite-like structures start to form around 200 degrees 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 m(2) g(-1) and that of 4@1200 is 33 m(2) g(-1). These values are much higher than those measured in previous works for carbon residues prepared at 1100 degrees C from imidazoliumand 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.