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.