Tunable Radiationless Energy Transfer in Eu[Au(CN)2]3·3H2O by High Pressure

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 Δν̄max/Δp = -130 ± 10 cm-1/kbar under high-pressure application at least up to ≈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.