Investigation of the excimer dynamics in 9,10-dichloroanthracene crystals using picosecond spectroscopy

Abstract

The dynamics of excimer formation, excimer migration, and excimer dissociation in β-9,10-dichloroanthracene crystals were investigated by means of picosecond time-resolved fluorescence spectroscopy. In the temperature range from T = 20 K to T = 40 K we were able to temporally resolve the relaxation into the excimer state. The excimer formation rates were determined to be k ≈ 1.8 × 10 11 s −1 at T = 40 K and k ≈ 2.0 × 10 10 s −1 at T<30 K. The excimer migration was investigated by measuring the excimer annihilation rate as a function of temperature. At room temperature 20% of the excimers are dissociated. The excimer binding energy is estimated to be B = 1360 cm −1. The experimental results are explained in terms of a kinetic scheme comprising the population and depopulation of exciton, trap, and excimer states. The nature of the trap state is identified and it is shown that thermal activation of a 25 cm −1 librational mode induces the relaxation of the trap into the excimer state.