Ligand-to-metal charge-transfer quenching of the Eu(D1) 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 k_CT=Ae^−E_a/kT with E_a=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, E_a, 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.