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

Abstract

Two non-porous metal–organic frameworks (MOFs) with caged Ru(bpy)₃²⁺ chromophores, [Ru(bpy)₃][Zn₂(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 (³MLCT) 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)₃²⁺*. Following MLCT excitation, Ru(bpy)₃²⁺*/Ru(bpy)₃²⁺ energy transfer migration in 1 and 2 results in efficient quenching of Ru(bpy)₃²⁺* by Os(bpy)₃²⁺ 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)₃²⁺ units, ultimately, finding an Os(bpy)₃²⁺ trap site. These crystalline frameworks with caged chromophores have proven to be ideal systems for studying light harvesting processes in artificial supramolecular systems.