High Pressure Photoinduced Ring Opening of Benzene

The chemical transformation of crystalline benzene into an amorphous solid $(a-\mathrm{C}:\mathrm{H})$ was induced at high pressure by employing laser light of suitable wavelengths. The reaction was forced to occur at 16 GPa, well below the pressure value (23 GPa) where the reaction normally occurs. Different laser sources were used to tune the pumping wavelength into the red wing of the first excited singlet state $S_1{(}^1{B}_{2u})$ absorption edge. Here the benzene ring is distorted, presenting a greater flexibility which makes the molecule unstable at high pressure. The selective pumping of the $S_1$ level, in addition to structural considerations, was of paramount importance to clarify the mechanism of the reaction.