Solid-state photoluminescence sensitization of Tb3+ by novel Au2Pt2 and Au2Pt4 cyanide clusters

The syntheses are reported for two novel Tb(3+) heterotrimetallic cyanometallates, K(2)[Tb(H(2)O)(4)(Pt(CN)(4))(2)]Au(CN)(2)·2H(2)O (1) and [Tb(C(10)N(2)H(8))(H(2)O)(4)(Pt(CN)(4))(Au(CN)(2))]·1.5C(10)N(2)H(8)·2H(2)O (2) (C(10)N(2)H(8) = 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 P4(2)/nnm, a = 11.9706(2) Å, c = 17.8224(3) Å, V = 2553.85(7) Å(3), Z = 4; 2, triclinic, space group P1, 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) Å(3), Z = 2. For the case of 1, the structure contains Au(2)Pt(4) hexameric noble metal clusters, while 2 includes Au(2)Pt(2) 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 Tb(3+) 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 Tb(3+) 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 Au(2)Pt(4) clusters. The PL properties of 2 show a similar Tb(3+) 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 Tb(3+) transitions. The absolute quantum yield (QY) for the Tb(3+) emission ((5)D(4) → (7)F(J (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 Tb(3+) and (2) Tb(3+) emission quenching via a bpy ligand-to-metal charge transfer (LMCT) excited state.