High-Pressure Behavior of C₂I₂ and Polymerization to a Conductive Polymer

Crystalline diiodoacetylene (C₂I₂) was synthesized and then studied under high-pressure conditions using synchrotron X-ray diffraction, Raman/infrared spectroscopies, and first-principles calculations. At ∼0.3 GPa, the starting tetragonal (P4₂/n) phase, which is stabilized by donor-acceptor interactions, transforms into a new orthorhombic structure (Cmca) that is more densely packed and analogous to the low-temperature phase of acetylene. Above approximately 4 GPa, compressed C₂I₂ molecules in the Cmca structure begin to polymerize to form a predominantly sp² amorphous carbon network that maintains a significant fraction of C-I bonds. Transport measurements reveal that the polymeric material is electrically conducting. The magnitude of the electrical conductivity is similar to Br-doped polyacetylene and undoped trans-polyacetylene at 8 GPa and 1 atm, respectively. Elemental analyses performed on recovered samples show that the iodine concentration varies with specific processing conditions. Optimization of the pressure-induced polymerization pathway could allow for enhanced electrical properties to be realized, in addition to postpolymerization functionalization using the weak C-I bonds.