Zero‐phonon lines in the singlet spectrum of the charge transfer crystal anthracene‐PMDA: Experimental evidence and model calculations

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.