Quenching of image image emission by image and image

Energy transfer between and is studied by luminescence spectroscopy and time-resolved measurements. In earlier experiments, Zachau et al. (Proceedings—Electrochemical Society, 97 (29) (Luminescent Materials, pp. 314–324, 1998)) observed no energy transfer from the level of to the level of for , Eu, in spite of the favorable spectral overlap for energy transfer. This unexpected result was not explained. To resolve this issue, we first calculate the critical distance for energy transfer using the Förster theory and show that efficient energy transfer between and nearest neighbors is predicted. Nevertheless, luminescence experiments show no emission upon excitation in the level of in , Eu confirming the results in (Proceedings—Electrochemical Society, 97 (29) (Luminescent Materials, pp. 314–324, 1998)). The experiments do show a strong quenching of the emission by . The quenching is explained by a low-energy metal-to-metal charge transfer state (–) for Pr–Eu pairs. Additional support for this quenching mechanism is provided by the observation that addition of also quenches the emission.