Double Core Carbon Nanothreads by High-Pressure Mechanochemical Synthesys

Nanothreads are the last finding concerning carbon based nanomaterials, synthesized compressing aromatic molecules at high pressure (tens of GPa). The great interest they have aroused in the scientific community is due to their extraordinary mechanical properties and potential technological applications. Indeed, the nanothreads structure closely recalls the one of the diamond since they are composed by a fully saturated (sp3) carbon backbone, having more than one bond along their cross section and thus differentiating these materials from conventional polymers. These compounds are expected to combine flexibility with excellent resilience, tensile strength and a stiffness only slightly lower than the one of diamond, making them the perfect candidates for the engineering of fibers which potential technological applications affects an entire plethora of scientific areas, ranging from material science, aerospace industry, biomedicine and many others. Thanks to versatile approach provided by high-pressure methods enhancing the reconstruction of chemical bonds in condensed matter, the first experimental synthesis of nanothreads was achieved compressing benzene up to 20 GPa and showing a crystalline material, organized in densely packed bundles characterized by a long range order of tens of nanometers. This finding opened the way to the discovery of a very rich variety of nanothreads and showing their synthesis was a common feature resulting from the compression of aromatic compounds. In particular, nanothreads properties were modified tailoring these materials with different functional groups, obtaining compounds with new mechanical, optical and electronic properties, spreading the potential applicability range of these compounds. The next challenge was the tunability of aforementioned properties, modulating at will the type and amount of functional groups, issue of not trivial solution considering the small volume of the recovered products obtainable exploiting high-pressure techniques, making the post-syntheses manipulation of these materials difficult and time consuming. On the other hand, lowering as possible the pressure required for the synthesis, is the other main challenge regarding these materials. During the course of this work a new class of nanomaterials, belonging to the nanothreads class, were successfully synthesized under high-pressure/high-temperature conditions and they were characterized in detail by several experimental techniques. The synthesis was achieved both employing diamond anvil cell devices and large volume multi anvil apparatus. In particular, the recovered products were analyzed by Fourier transform infrared spectroscopy, single crystal and powder X-ray diffraction and high resolution transmission microscopy, as well as the reaction mechanism was analyzed studying both the reaction kinetics and the structure/reactivity relationship. Complementary density functional theory calculations had shown a perfect agreement with experimental data, providing other useful information concerning the backbone structure assumed by the threads. The syntheses were achieved compressing trans-azobenzene, trans-stilbene, diphenylacetylene and substitutional mixed crystal of the previous monomers. These isostructural monomers are composed by a pair of aromatic rings covalently bonded to each other by different functional groups (azo-, ethyl- and acethyl- groups). The aromatic rings reacted forming two threads, while the functional groups in their between acted as a bridge "gluing" the two cores. These new materials were thereby called: "double core nanothreads". The structure modifications occurred compressing these compounds were studied in detail, shading light on the mechanism of formation of these materials whose topochemical or stress-induced nature is actually debated. In particular, the role of the uniaxial stress provided by diamond anvil cells (with no pressure transmitting medium) inducing the reactivity, was deeply explored. In fact, being this set of materials isostructural, they were found to be the perfect probes to compare the effect of the non-hydrostatic and quasi-hydrostatic compression about triggering the reactivity of these systems into nanothreads. In particular, it was found the reaction of diphenylacetylene into nanothreads was topochemical, being insensible to the geometry of compression but driven by the polymerization of the acetylene portion. Stilbene and azobenzene was instead found to react through a non-topochemical reaction pathway, strongly dependent on the anisotropy of the compression. It was even found that the reactivity of the latter systems into nanothreads was improvable mixing these two monomers in equal proportions. This kind of studies provided paramount information since only the deep knowledge of the structure/reactivity relationship may lead to lower as possible the required pressure for the synthesis of nanothreads, permitting to know in advance which qualities the reagent should posses to produce high-quality nanothreads with a good yield. Furthermore, a new set of functionalized materials were presented in this work: the unique characteristics of azobenzene-derived nanothreads are indeed due to the azo group belonging to the monomer which is maintained untouched, thus potentially preserving the features characterizing this chromophore that is notoriously known for its efficient photoisomerization and for its tunable emission and photochemical properties, achievable by changing the ring substituents. On the other hand, the acetylene moiety in diphenylacetylene-derived nanothreads were found to polymerize in a polyacetylene chain embedded between the two thread cores, making this compound an organic semiconductor since it showed to be a low band-gap material, adding to nanothreads the unexplored before electron transfer property. Finally, compressing substitutional mixed crystal of azobenzene/stilbene and diphenylacetylene/stilbene, the modulation of the aforementioned properties was achieved for the first time, since composite nanothreads were synthesized, tuning at will the amount the azo and acetylene moieties in the crystal of the relative nanothreads, obtained upon compression. This last finding have opened the way to the synthesis of engineered nanothreads which properties are imposed by the reagents composition, thus overcoming the issues related to the difficult post-syntheses manipulation of these materials. In particular, this feature was never achieved before from the experimental point of view.