Excitation-Dependent Photoluminescence Color Tuning in Lanthanide-Organic Hybrid Materials

A series of lanthanide organic hybrid materials was synthesized via hydrothermal methods and structurally characterized using single-crystal X-ray diffraction. Four phases were obtained from reactions of La, Eu, and Tb ions with 1,10-phenanthroline (phen) and 2-thiophenecarboxylate (TC): [La-2(phen)(2) (k(1)-TC)(2) (mu(2)-TC)(2) (eta(3)-TC)(2) (H2O)(2)] (La-1/La-2), [Eu-2 (phen)(2)(k(2)-TC)(2) (mu(2)-TC)(2) (eta(3)-TC)(2)]center dot 2(H2O) (Eu-3), and [Tb-2(phen)(2) (k(2)-TC)(2) (mu(2)-TC)(4)]center dot 2(H2O) (Tb-4). Although each of the structures consists of homometallic ligand bridged dimers, the four distinct phases arise from subtle differences in ligand binding modes and supramolecular interactions. Ln doping was explored and resulted in analogous heterometallic systems, [Eu2-xLax(phen)(2)(k(2)-TC)(2) (mu(2)-TC)(2) (eta(3)-TC)(2)]center dot 2(H2O) (Eu2-xLax-3; x = 0.47-1.51) and [Tb2-yLay(phen)(2)(k(2)-TC)(2) (mu(2)-TC)(4)]center dot 2(H2O) (Tb2-yLay-4; y = 0.42, 0.67). The photoluminescent properties of the solid-state materials were assessed using steady-state and time-dependent techniques to obtain excitation and emission profiles, transition energies, and lifetimes. The La phase exhibited ligand-based emission, whereas both Eu and Tb phases produced characteristic red and green metal-centered emission, respectively. By comparison, the heterometallic compounds exhibited both Ln- and ligand-based emission and photoluminescent color tuning of emission chromaticity. Further examination revealed that the color tuning was dependent on the relative La/Eu or La/Tb ratios as well as the excitation wavelength. These compounds are a rare example of single-phase Ln hybrid materials built from molecular units that exhibit excitation-dependent photoluminescent color tuning in the solid state.