Tunable radiationless energy transfer in Eu[Au(CN)₂]₃ 3H₂O by high pressure

Abstract

The title compound consists of two-dimensional layers of [Au(CN)₂]- complexes alternating with layers of Eu³⁺ ions. Due to this structure type, the lowest electronic transitions of the dicyanoaurates(I) exhibit an extreme red
shift of Δν(max)/Δp = -130 ± 10 cm⁻¹/kbar under high-pressure application at least up to ca. 60 kbar (T = 20 K), while the shifts of the different Eu³⁺ transitions lie between -0.70 and -0.94 cm⁻¹/kbar. At ambient pressure,
the usually very intense emission of the dicyanoaurates(I) is completely quenched due to radiationless energy
transfer to the Eu³⁺ acceptors. As a consequence, one observes a strong emission from Eu³⁺, which is assigned
to stem mainly from ⁵D0 but also weakly from ⁵D₁. At T = 20 K, ⁵D₃ seems to be the dominant acceptor term. It is a highlight of this investigation that, with increasing pressure, the emission from the dicyanoaurate(I) donor
states can continuously be tuned in by tuning off the resonance condition (spectral overlap) for radiationless
energy transfer to ⁵D₃. With further increase of pressure, successively, ⁵D₂ and ⁵D₁ become acceptor terms, however, being less efficient. Interestingly, ⁵D0 does not act as an acceptor term even with maximum spectral
overlap. Between 30 and 60 kbar, when only the ⁷F0 ―› ⁵D₁ acceptor absorption overlaps with the donor emission,
one finds a linear dependence of the (integrated) ⁵D0 emission intensity on the spectral overlap integral, as is
expected for resonance energy transfer. As the dominant transfer mechanism, the Dexter exchange mechanism is proposed. Besides the high-pressure studies of the Eu3+ line structure at T = 20 K, the Eu³⁺ emission is also
investigated at T = 1.2 K (p = 0 kbar) by time-resolved emission spectroscopy, which strongly facilitates the
assignments of the emitting terms.