Effect of pressure on the Raman and electronic absorption spectra of naphthalene‐ and perylenetetracarboxylic dianhydrides

The effect of pressure on the optical absorption and Raman spectra of 1,4,5,8‐naphthalenetetracarboxylic dianhydride (NTCDA) and on the optical absorption edge in 3,4,9,10‐perylenetetracarboxylic dianhydride (PTCDA) was investigated, using a diamond anvil cell. In NTCDA the absorption edge shifts red by about 8000 cm⁻¹ in the range 0–180 kbar and in PTCDA by about 3000 cm⁻¹ in the range 0–60 kbar pressure. At the upper limit of pressure the materials turn black, but on release of pressure the shifts were perfectly reversible. In the case of NTCDA, where both absorption and Raman data (up to 70 kbar) were obtained, the shift in the electronic absorption has been related to a parameter proportional to the intermolecular force constant. The latter was obtained from the pressure shift of an external mode Raman frequency, which showed the largest pressure dependence. The shift, which is nonlinear with pressure, becomes linear when plotted against the square of this frequency, normalized to its ambient pressure value [ν_(p)/ν₍₀₎]². To our knowledge, this is the first time a combined absorption and Raman study has been performed on an organic system to establish such a direct relationship between electronic absorption and intermolecular interaction. No pressure‐induced phase transition was encountered in NTCDA up to 180 kbar and in PTCDA up to 60 kbar, the limits of pressure in the present study. Earlier explanations for the pressure‐induced red shifts in aromatic systems are briefly discussed.