Emission Enhancement through Dual Donor Sensitization: Modulation of Structural and Spectroscopic Properties in a Series of Europium Tetracyanoplatinates

The synthesis of three different europium tetracyanoplatinates all incorporating 2,2′:6′,2′′-terpyridine (terpy) have been carried out in acetonitrile/water mixtures by reaction of Eu3+ salts with terpy and potassium tetracyanoplatinate. The use of different Eu3+ sources results in the isolation of Eu(C15H11N3)(H2O)2(NO3)(Pt(CN)4)·CH3CN (1), {Eu(C15H11N3)(H2O)3}2(Pt(CN)4)3·2H2O (2), or [Eu(C15H11N3)(H2O)2(CH3COO)2]2Pt(CN)4·3H2O (3) for the nitrate, triflate, or acetate salts, respectively. All three compounds have been prepared 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, monoclinic, space group P21/c, a = 12.835(1), b = 15.239(1), c = 13.751(2) Å, β = 105.594(9)°, V = 2590.8(5) Å3, Z = 4; 2, triclinic, space group P1̅, a = 9.1802(8) Å, b = 10.8008(9) Å, c = 13.5437(9) Å, α = 84.491(6)°, β = 75.063(7)°, γ = 79.055(7)°, V = 1272.4(2) Å3, Z = 1; 3, triclinic, space group P1̅, a = 12.110(3) Å, b = 12.7273(11) Å, c = 18.7054(16) Å, α = 92.859(7)°, β = 92.200(11)°, γ = 118.057(10)°, V = 2534.8(7) Å3, Z = 2. Variation of the counteranions in these systems provides the opportunity to modify the structures and coordination environment of Eu3+ for 1−3. Compounds 1 and 2 are both one-dimensional, polymeric compounds that contain Eu3+ ions chelated by terpy and bridged by tetracyanoplatinate anions. 3 is a zero-dimensional complex salt in which Eu3+ is coordinated by terpy, acetate, and water, but not tetracyanoplatinate. The structural differences result in varying sensitization phenomena for the three compounds. Compounds 1 and 2 display efficient donor−acceptor intramolecular energy transfer (IET) where dual donor species, terpyridine and tetracyanoplatinate, simultaneously enhance the acceptor Eu3+ emission. In both compounds the donor species are directly coordinated to the acceptor ion, and hence a highly efficient dual-donor effect is exhibited for the IET mechanisms. In 3 where only the terpy ligand is directly coordinated to Eu3+, the sensitization involves only one donor species. The Pt(CN)42− unit in 3, which lacks direct bonding to Eu3+, exhibits strong emission indicating the lack of cooperative enhancement of the lanthanide emission.