Insights into topochemical <i>versus</i> 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 <i>b</i> 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.