Pressure-Induced Diversity of π-Stacking Motifs and Amorphous Polymerization in Pyrrole

Abstract

The behavior of pyrrole under high pressure has been investigated by in situ high-pressure synchrotron X-ray diffraction (XRD) and Raman scattering up to 34 GPa. A solid-to-solid transition at about 6.2 GPa with a large collapse of volume (∼40%) from Pnma to P21/c has been found after a liquid-to-solid transition at 0.6 GPa, which is caused by the molecular rotational repacking of π-stacking. This new phase P21/c plays a central role in the following pressure-induced polymerization due to the formation of a closed dimer, a very important precursor. The threshold of C···C distance with steric hindrance in dimer is about 1.62 Å at ∼10.2 GPa. After this steric hindrance is overcome, a crystal–amorphous transformation starts at ∼14.3 GPa. When completely released from 34 GPa, the recovered solid product with single bond is identified by in situ Raman measurement.