Ligand-to-metal charge-transfer quenching of the Eu3+(5D1) state in europium-doped tris(2,2,6,6-tetramethyl-3,5-heptanedionato)gadolinium (III)

Rate constants for the decay of the Eu3+(5D1) state in dilute Eu3+-doped tris(2,2,6,6-tetramethyl-3,5-heptanedionato)gadolinium(III) are reported for a temperature range 50–315 K. A temperature-independent contribution of 1.3×106 s−1 is attributed to carbonyl mediated 5D1→5D0 relaxation. A temperature-dependent contribution, that has been parameterized as kCT=Ae−Ea/kT with Ea=2540±140 cm−1 and A=1.3×1012 s−1 is attributed to quenching of 5D1 by a low-lying ligand-to-metal charge-transfer state (CTS). The height of the activation barrier, Ea, relative to the previously reported height for charge-transfer quenching of the Eu3+(5D0) state in the pure europium analog (J. Phys. Chem. 100 (1996) 9216) is discussed in terms of mechanistic implications. Specifically, these data strongly suggest that quenching of 5D1 by the charge-transfer state is mediated by the 5D0 state with a novel, indirect mechanism.