Yttrium-89 magic angle spinning NMR study of rare-earth pyrochlores

---

4670

J . Am. Chem. SOC.1990, 112,4670-4675

rather than decrease as is observed. A T1-TI annihilation mechanism also provides a basis for describing the two distributions in the TI carotenoid population. For T,-TI annihilation to be an effective TI decay route, the TI molecules must be at or near van der Waals distances to each other. This distance restriction would seem to prohibit TI-Tl annihilation with a TI carotenoid concentration of only 1 pM or 1% of the total carotenoid population. However, it must be emphasized that the T, carotenoids are formed by singlet fission as adjacent triplet pairs, at van der Waals distances, and annihilation should simply be considered as the reverse reaction. Triplet formation would then be described with a