Fluorescence Lifetimes and Site-selective Photochemistry of Anthracene Crystals Doped with 2-hydroxyanthracene

The rise and decay of the prompt fluorescence of 2-hydroxyanthracene (2-OHA) doped anthracene crystals have been investigated at 8 K following excitation with frequency-doubled pico-second pulses from a synchronously pumped dye laser. By studying fresh crystals and crystals irradiated with u-v light at 298 K so as to introduce the photodimer, the roles played by the two different sites (O₈) and (O₉) occupied by the 2-OHA, by the X-traps and by other structural defects in fluorescence and energy transfer have been elucidated. Energy transfer between impurity sites (O₈ and O₉) is slow whereas that between guest associated X-traps and impurity is rapid. For the O₉ site, both before and after photodimerization, a simple time-independent energy transfer occurs from the anthracene host to the 2-OHA guest. The transfer to O₈ sites is associated with the presence of an extended region of disorder associated with the impurity. Such regions are excited directly.