Chemical dimerization of crystalline anthracene produced by transient high pressure

We report the production of a chemically bound dimer of anthracene produced by the shock compression of crystalline anthracene. The experimental probe used to detect the dimer structure was time-of-flight (TOF) mass spectrometry. The principal method used to produce the shock compression was the impact of electrically accelerated flyers (‘‘slappers’’) with the surface of the anthracene crystals. Our work correlates well with earlier experimental and theoretical work concerning chemical processes that can occur in pressurized anthracene and other aromatic materials. We briefly review the earlier work. The anthracene crystals were shocked to various pressures in the interval 9 to 22 GPa. Also various crystal thicknesses were employed; this allowed us to control the time interval over which various anthracene ‘‘particles’’ in the crystal were held at high pressure and temperature. At a pressure of ∼22.1 GPa we observe dimer formation in 20 ns or less. For a shock pressure of 9 GPa no dimer is produced, whereas for shock pressures of 18.4 GPa or higher dimer production is always observed. Under some conditions significant conversion of monomer to dimer is seen (up to a conversion of over 50%). A remarkably simple experimental observation is that the only new chemical species observed in the experiments at 18.4 GPa and higher is the dimer species. We present evidence that the dimerization is the result of a pressure-driven Diels–Alder reaction and that such reactions are characteristic of materials containing aromatic rings when they are shocked to high pressure.