The Electronic Spectra of Aromatic Molecular Crystals. II. The Crystal Structure and Spectra of Perylene

Abstract A new crystalline form of perylene has been found and its crystal structure has been determined by the two-dimensional X-ray diffraction method. The new form is monomeric, while the previously known form has a dimeric structure. The electronic absorption and fluorescence spectra of both forms have been measured ; on the basis of these data, an assignment is proposed for the observed transitions. Although the fluorescence spectrum of the monomer form shows a small shift, that of the dimer form shows a larger shift than would be expected from the absorption spectrum. This is explained as a stabilization of the lowest excited singlet state by a charge-transfer resonance interaction between the molecules in a pair. Because this stabilization increases as the distance between two molecules decreases, it is suggested that the molecules in a pair approach each other after excitation. At the liquid helium temperature the motion of molecules in the crystal will be frozen out, the larger stabilization of the excited level cannot occur, and the green emission which is characteristic of the monomer form is observed from the dimer form. The rate process of the molecular motion at lower temperatures has been analyzed.