Rapid energy transfer in non-porous metal-organic frameworks with caged Ru(bpy)32+ chromophores: oxygen trapping and luminescence quenching

Two non-porous metal-organic frameworks (MOFs) with caged Ru(bpy)(3)(2+) chromophores, [Ru(bpy)(3)][-Zn-2(C2O4)(3)] (1) and [Ru(bpy)(3)][NaAl(C2O4)(3)] (2), were synthesized and characterized. Their emission properties were studied by both steady-state and time-resolved luminescence measurements. Air-free microcrystals of 1 and 2 exhibit long-lived triplet metal-to-ligand charge transfer ((MLCT)-M-3) excited states with lifetimes of 760 and 1305 ns, respectively. Lifetimes are significantly shortened (to 92 ns for 1 and 144 ns for 2) by trapping of trace amounts of oxygen in the non-porous MOFs, presumably due to amplified luminescence quenching of Ru(bpy)(3)(2+)*. Following MLCT excitation, Ru(bpy)(3)(2+)*/Ru(bpy)(3)(2+) energy transfer migration in 1 and 2 results in efficient quenching of Ru(bpy)(3)(2+)* by Os(bpy)(3)(2+) added as an energy transfer trap at doping levels of 0.2-1.0%. A kinetic analysis indicates that the three-dimensional chromophore connectivity in 1 and 2 provides a network for rapid excited state energy transfer migration among Ru(bpy)(3)(2+) units, ultimately, finding an Os(bpy)(3)(2+) trap site. These crystalline frameworks with caged chromophores have proven to be ideal systems for studying light harvesting processes in artificial supramolecular systems.