Calculations of inter- and intramolecular vibrations of a perylene derivative to interpret observed absorption spectra of the crystal

Perylene 3,4,9,10-tetracarboxylic dianhydride (PTCDA), one of the perylene derivatives, has many absorption peaks in the THz region. Density functional theory (DFT) analysis of the crystal geometry elucidated that intermolecular vibrations of relative rotation and translation, and intramolecular vibrations of torsion and bending dominate below and above 2 THz, respectively. The frequency shifts relative to the frequencies obtained in isolated-molecule calculations and the frequency splitting of in- and out-of-phase vibrations of the two molecules in the crystal unit cell were caused by crystal-packing interactions. The higher absorption peaks of the PTCDA crystal originate from frequencies shifted and split from the IR-active intramolecular vibration mode in the isolated-molecule.