Temperature dependent intersystem crossing and triplet-triplet absorption of rubrene in solid solution

Relative fluorescence quantum yields and lifetimes of the first excited singlet state (S1) of rubrene in sucrose octa-acetate glass have been measured as a function of temperature. From the thermal activation of intersystem crossing, it was found that the triplet state to which S1 intersystem crosses lies 4600±400 cm-1 above S1. System dynamics were measured with picosecond pump-probe experiments as a function of temperature. The data yield the temperature dependent intersystem crossing rate constants and the triplet-triplet extinction coefficient, which was found to be 8800±600 L mol-1 cm-1 at 532 nm. The triplet-triplet extinction coefficient provides absolute calibration of the previously reported triplet-triplet absorption spectrum. Inclusion of stimulated emission quantitatively in the analysis of pump-probe experiments involving more than one pair of states is shown to be necessary for a proper analysis of system dynamics.