Structure-reactivity relationship in the high-pressure formation of double-core carbon nanothreads from azobenzene crystal

Saturated carbon nanothreads are one of the most attractive new materials produced under high pressure in the last years. Nanothreads can be considered as a monodimensional diamond; in fact, they preserve some of the mechanical properties of the diamond itself, like stiffness, but their intrinsic flexibility makes them excellent nanowires. Since their discovery, many advancements have been made, and nowadays, they can be obtained from the compression of several aromatic molecular crystals. However, it is often not clear why certain starting crystals give high-quality nanothreads while others do not or which are the best conditions for the synthesis in terms of pressure, temperature, compression rate, and reaction time. In other words, the mechanisms that allow their formation with respect to other byproducts are often unclear. This is an important piece of information that can be used for the design of a synthetic strategy for the production of functional materials with targeted characteristics, like conductivity and electro-optical properties, while preserving the mechanical ones. Here, we report an X-ray diffraction study in which we followed the transformation induced by the pressure of trans-azobenzene using polycrystalline samples compressed with and without a pressure-transmitting medium. With this approach, we were able to highlight the structural relations along the reactive path leading to double-core saturated carbon nanothreads. The features that we discovered could be common to all pseudo-stilbene crystals, a class of compounds isostructural to azobenzene and characterized by two phenyl rings connected by a variety of different linkers, thus representing excellent starting materials for the synthesis of functional nanothreads.