Insights into topochemical stress-induced high-pressure reactivity of azobenzene by single crystal X-ray diffraction.

This study addresses azobenzene's structural compression and reactivity under hydrostatic high-pressure conditions. Synchrotron X-ray diffraction data of single crystals compressed with neon as the pressure-transmitting medium allowed the refinement of the crystal structure up to 28 GPa, at which the onset of the reaction was observed. Analysis of the pressure-dependent lattice parameters reveals a first-order isostructural phase transition at 13 GPa. We have solved the crystal structure of the high-pressure phase of azobenzene offering a key insight into the strong contribution of stress on the structural compression mechanism and crystal's reaction chemistry at elevated pressures. While the collapse of the cell parameter, previously observed under non-hydrostatic conditions, was identified as the crucial step toward the formation of azobenzene-derived double-core nanothreads, under quasi-hydrostatic conditions the compression of the cell parameters up to 33 GPa followed a different route. The evolution of the cell parameters and the refinement of the crystal structure close to the onset of the reaction identified a topochemical polymerization path, corroborated by reaction kinetics data by infrared spectroscopy and by computed polymer structures, suggesting a complex growth process, resulting in a distinctly different material compared to that formed upon non-hydrostatic compression. These findings underscore the pivotal role of compression conditions in determining the reaction pathways of azobenzene, providing novel insights for its application in nanomaterial synthesis.