Solid-State Photoluminescence Sensitization of Tb3+ by Novel Au2Pt2 and Au2Pt4 Cyanide Clusters

The syntheses are reported for two novel Tb3+ heterotrimetallic cyanometallates, K2[Tb(H2O)4(Pt(CN)4)2]Au(CN)2·2H2O (1) and [Tb(C10N2H8)(H2O)4(Pt(CN)4)(Au(CN)2)]·1.5C10N2H8·2H2O (2) (C10N2H8 = 2,2′-bipyridine). Both compounds have been isolated as colorless crystals, and single-crystal X-ray diffraction has been used to investigate their structural features. Crystallographic data (MoKα, λ = 0.71073 Å, T = 290 K): 1, tetragonal, space group P42/nnm, a = 11.9706(2) Å, c = 17.8224(3) Å, V = 2553.85(7) Å3, Z = 4; 2, triclinic, space group P1̅, a = 10.0646(2) Å, b = 10.7649(2) Å, c = 17.6655(3) Å, α = 101.410(2)°, β = 92.067(2)°, γ = 91.196(2)°, V = 1874.14(6) Å3, Z = 2. For the case of 1, the structure contains Au2Pt4 hexameric noble metal clusters, while 2 includes Au2Pt2 tetrameric clusters. The clusters are alike in that they contain Au−Au and Au−Pt, but not Pt−Pt, metallophilic interactions. Also, the discrete clusters are directly coordinated to Tb3+ and sensitize its emission in both solid-state compounds, 1 and 2. The Photoluminescence (PL) spectra of 1 show broad excitation bands corresponding to donor groups when monitored at the Tb3+ ion f-f transitions, which is typical of donor/acceptor energy transfer (ET) behavior in the system. The compound also displays a broad emission band at ∼445 nm, assignable to a donor metal centered (MC) emission of the Au2Pt4 clusters. The PL properties of 2 show a similar Tb3+ emission in the visible region and a lack of donor-based emission at room temperature; however, at 77 K a weak, broad emission occurs at 400 nm, indicative of uncoordinated 2,2′-bipyridine, along with strong Tb3+ transitions. The absolute quantum yield (QY) for the Tb3+ emission (5D4 → 7FJ (J = 6−3)) in 1 is 16.3% with a lifetime of 616 μs when excited at 325 nm. In contrast the weak MC emission at 445 nm has a quantum yield of 0.9% with a significantly shorter lifetime of 0.61 μs. For 2 the QY value decreases to 9.3% with a slightly shorter lifetime of 562 μs. The reduced QY in 2 is considered to be a consequence of (1) the slightly increased donor−acceptor excited energy gap relative to the optimal gap suggested for Tb3+ and (2) Tb3+ emission quenching via a bpy ligand-to-metal charge transfer (LMCT) excited state.