The temperature dependence of the Eu3+ 5D0 relaxation rate in solid europium tris(2,2,6,6-tetramethyl-3,5-heptanedionato) is presented. Eu3+ emission is strongly quenched in the region of room temperature. It is proposed that thermal quenching of 5D0 emission is via crossover from the 5D0 to a ligand-to-lanthanide charge transfer state. Activation energies for the crossover process were determined using the models of Mott and of Englman and Jortner, yielding values of 4120 ± 20 and 5700 ± 300 cm-1, respectively. The crossover process is less efficient in media in which Eu(thd)3 exists as a monomer.

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