The absorption, fluorescence, and reflectivity spectra of anthracene-PMDA (pyromellitic acid dianhydride) single crystals are investigated at 2 °K. From the intensity of the zero-phonon transition and from the width of the phonon sidebands, one obtains information about the strength of the electron-phonon coupling. Within the framework of a configuration coordinate model one can show that about six phonons of frequency 62 cm-1 are emitted after a vertical Franck-Condon excitation from the ground state to the lowest excited charge transfer state. This rather strong electron-phonon coupling is supported by a model calculation on a localized charge transfer complex which assumes that the excited state relaxation arises primarily from the Coulombic attraction of the ionic donor-acceptor states. Copyright © 1977 American Institute of Physics.