Absorption and Emission Spectra of the Perylene Dimer

A method for obtaining perylene dimers is given and their absorption and emission spectra are reported. In absorption the long-axis-polarized band near 4200 Å and the short-axis-polarized band near 2500 Å have been measured and the effects of vibronic coupling discussed for the former. The resonance splittings of the two excited states corresponding to these bands have been estimated and the dipole—dipole term of the point multipole expansion has been shown to be grossly in error. Both excited dimer and excimer emission from the perylene dimer have been found with a very small interconversion barrier to the latter and the results prove that in the excimer state the intermolecular distance is smaller than in the ground state. The absorption spectrum of the dimer has been compared with the crystal spectrum and it has been shown that the neglect of molecular vibrations in the simple Davydov theory leads to misleading assignments of the crystal spectrum.