Abstract
We study the temperature dependence of the triplet and singlet exciton dynamics in the archetype small molecule fluorescent guest-host system tris(8-hydroxyquinolinato) aluminum $({\mathrm{Alq}}_{3})$ doped with 4-(dicyanomethylene)-2-methyl-6-julolidyl-9-enyl-4H-pyran (DCM2). We develop a comprehensive model of the exciton dynamics, and use it to fit the transient photoluminescence under different pulsed optical pumping in the temperature range of $80<T<295\phantom{\rule{4pt}{0ex}}\mathrm{K}$. The triplet decay has a significantly different temperature dependence than that of singlets. From 295 to 80 K, the triplet-triplet annihilation (TTA) rate decreases by two orders of magnitude, whereas the singlet-triplet annihilation rate decreases by $<50%$, primarily a result of the different energy transfer mechanisms of singlets and triplets. The temperature dependence of the TTA rate reveals two regimes separated by a transition at 180 K from Marcus to Miller-Abrahams transfer. This work deepens our understanding of exciton dynamics and energy transfer in small molecule organic materials.
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