Pentacene at high pressure

We report a high-pressure optical study of pentacene by Raman, absorption, and reflectance spectroscopy. Two modifications, denoted C and H, were used as starting material. Raman spectra indicate a sluggish phase transition of the polymorph C to the H-phase with a very low onset pressure of 0.2 GPa. The H-phase remains stable up to at least I I GPa. The vibrational properties of the H phase in this pressure range were modeled by lattice dynamics calculations. The optical absorption edge of pentacene exhibits a initial red shift of about -80 meV/GPa. At pressures near 10 GPa the pipi* reflection bands disappears gradually, indicating a loss of aromaticity due to the formation of saturated C-C bonds.