Excimer formation in pyrene crystals

The fluorescence of pyrene crystals shows a short-time component, called B-fluorescence, and a long-time component which is similar to the excimer fluorescence of pyrene solutions. Our model for the excimer creation is based on the pairwise arrangement of the molecules in the crystal. In the limiting case of vanishing intra-pair distance the electronic excitations of the crystal are symmetric and antisymmetric combinations of molecular excitations. The symmetric state that can be excited optically is identified with the B-state. The antisymmetric state is strongly localized and gives rise to the excimer fluorescence. The temperature dependence of the transition rate between the stales is calculated analytically and is in good agreement with the experiment. The fluorescence spectra of the B-state and the excimer state are discussed.