PICOSECOND STUDY ON EXCIMER FORMATION IN PYRENE SINGLE-CRYSTALS .2. THE EXCIMER PRECURSOR STATE IN THE HIGH-TEMPERATURE PHASE

Temperature dependent measurements of transient fluorescence spectra and fluorescence rise and decay times were performed in the high temperature phase of pyrene (155-300 K) with experimental response down to 15 ps. These prove the two-step excimer formation process via the precursor B-state as suggested in part 1. The residual vibronic substructure in the B-fluorescence spectra indicates moderate electron-phonon coupling and partial self-trapping in the B-state. The B-state can be excited directly. Experiments with variable excess excitation energy show that this is not needed for excimer formation. There are good reasons that the B-state is the S//1-state of pyrene. A kientic scheme for the coupled energy relaxation processes has been applied providing kinetic rates and activation energies.