Temperature Dependence of Singlet—Triplet Intersystem Crossing in Anthracene Crystals

The formation of triplet excitons from singlet excitons via a radiationless transition (intersystem crossing) is studied in anthracene crystals. Using blue exciting light with 430 mμ>λ>350 mμ the temperature dependence of the intersystem crossing is found to follow an exponential law with an activation energy of 800±50 cm−1. It is concluded that this transition starts from a vibrationally excited single state and involves the second triplet state. The total energy of this vibrational state is about 26 100 cm−1 in good agreement with that suggested by Kellogg, 26 050 cm−1, for an excited triplet level. No emission from the triplet—triplet transition could be observed.