Developing Luminescent Lanthanide Coordination Polymers and Metal-Organic Frameworks for Bioimaging Applications

This study focuses on the solvothermal synthesis of two lanthanide-based coordination polymer/metal-organic framework systems assembled from 1,3,5-benzenetricarboxylic acid (BTC) in the nano-sized regime for use as bioimaging agents. These materials were synthesized using two different lanthanide ions, a luminescent center (Eu, Tb) for optical imaging purposes and Gd, whose magnetic properties are particularly beneficial in magnetic resonance imaging (MRI) asa contrast agent. Together, these two features allow for multimodal imaging, useful in the study and diagnosis of disease. Under identical reaction conditions, two different compounds were formed upon changing the identity of the optically active lanthanide metal ion. Compound 1 ([EuGd(BTC)2(H2O)12]) emerged as a one dimensional coordination polymer, increasing in size with reaction time; while compound 2 ([TbGd(BTC)2(H2O)2]n•2DMF) emerged as a three dimensionalframework, decreasing in size with time. Both compounds displayed vibrant luminescence upon UV excitation, indicating potential as bioimaging agents

Structural, Spectroscopic, and Computational Studies of [2,2′-bithiophene]-5-carboxylic Acid

The crystal structure of [2,2′-bithiophene]-5-carboxylic acid was obtained from in-situ decarboxylation of [2,2′-bithiophene]-5,5′-dicarboxylic acid during solvothermal treatment. UV–Vis absorption and fluorescence spectroscopies were conducted in solution and solid-state on these two molecules as well as the precursor, 2,2′-bithiophene. These molecules were modeled using DFT level of theory to explain the observed structural features and spectroscopy

Reversible Solvent-Induced Transformation of a One-Dimensional Uranyl Coordination Polymer Using 4,4′-Oxybis(benzoate)

The solvothermal synthesis of uranyl nitrate with 4,4′-oxybis(benzoic acid) produces a coordination polymer [(UO2)(C15H8O5)(DMF)]n with coordinated DMF residing in the void space. This compound consists of one-dimensional chains that extend down [0 1 0] from UO7 pentagonal bipyramidal monomers. Thermogravimetric analysis indicates that the material loses the coordinated DMF ligand at 200 °C. No sensitized uranyl emission is seen within this system and the only other uranyl compound reported with the same target linker, but weak emission from the organic linker is observed and justified through molecular modeling studies. This system is able to convert between the DMF coordinated and previously reported water coordinated systems upon treatment with water or DMF, respectfully. The title compound showed marked resistance to transformation and/or degradation in a host of other organic solvents

Nitroaromatic Sensing with a New Lanthanide Coordination Polymer [Er2(C10H4O4S2)3(H2O)6]N Assembled by 2,2′-Bithiophene-5,5′-Dicarboxylate

A new three dimensional lanthanide-containing coordination polymer composed of 2,2′-bithiophene-5,5′-dicarboxylic acid ([Er2(C10H4O4S2)3(H2O)6]n) was solvothermally synthesized and characterized using single crystal X-ray diffraction, PXRD, FTIR, TGA, and luminescence measurements. The emission of this compound dispersed in ethanol is linker based and is quenched upon addition of nitroaromatic species due to a charge transfer from the MOF to the nitroaromatic species. This compound is able to sense a variety of nitroaromatic species, but is particularly sensitive to nitrophenols and nitroaniline

Rethinking Sensitized Luminescence in Lanthanide Coordination Polymers and MOFs: Band Sensitization and Water Enhanced Eu Luminescence in [Ln(C15H9O5)3(H2O)3]n (Ln = Eu, Tb)

A coordination polymer [Ln(C15H9O9)3(H2O)3]n (1-Ln = Eu(III), Tb(III)) assembled from benzophenonedicarboxylate was synthesized and characterized. The organic component is shown to sensitize lanthanide-based emission in both compounds, with quantum yields of 36% (Eu) and 6% (Tb). Luminescence of lanthanide coordination polymers is currently described from a molecular approach. This methodology fails to explain the luminescence of this system. It was found that the band structure of the organic component rather than the molecular triplet state was able to explain the observed luminescence. Deuterated (Ln(C15H9O9)3(D2O)3) and dehydrated (Ln(C15H9O9)3) analogues were also studied. When bound H2O was replaced by D2O, lifetime and emission increased as expected. Upon dehydration, lifetimes increased again, but emission of 1-Eu unexpectedly decreased. This reduction is reasoned through an unprecedented enhancement effect of the compound’s luminescence by the OH/OD oscillators in the organic-to-Eu(III) energy transfer process

Enhancing Luminescence in Lanthanide Coordination Polymers Through Dilution of Emissive Centers

A Ln-adipate metal-organic framework templated with 4,4′-bipyridine (GWMOF-6) was synthesized with Eu(III) or Tb(III) ions with various concentrations of optically inert Gd(III) ion in order to evaluate the effects of diluting the emissive metal center on the efficiency of luminescence. The doping ratios of Ln:Gd (Ln = Eu or Tb) showed that of the ratios studied, the 50:50 system had among the highest quantum yields (Eu = 3.9%, Tb = 7.6%) with increasing lifetimes as Gd(III) concentration increased. A 50:50 Eu:Tb system was also synthesized and showed to be more efficient than the Eu:Gd systems (Φ = 15.3%), with an exceptionally high energy transfer efficiency (80%) from the 5D4 state of the Tb(III) ion to the 5D1 and 5D0 energy levels of the Eu(III) ion

A General Model of Sensitized Luminescence in Lanthanide-Based Coordination Polymers and Metal–Organic Framework Materials

Luminescent lanthanides containing coordination polymers and metal–organic frameworks hold great potential in many applications due to their distinctive spectroscopic properties. While the ability to design coordination polymers for specific functions is often mentioned as a major benefit bestowed on these compounds, the lack of a meaningful understanding of the luminescence in lanthanide coordination polymers remains a significant challenge toward functional design. Currently, the study of these compounds is based on the antenna effect as derived from molecular systems, where organic antennae are used to facilitate lanthanide-centered luminescence. This molecular-based approach does not take into account the unique features of extended network solids, particularly the formation of band structure. While guidelines for the antenna effect are well established, they require modification before being applied to coordination polymers. A series of nine coordination polymers with varying topologies and organic linkers were studied to investigate the accuracy of the antenna effect in coordination polymer systems. By comparing a molecular-based approach to a band-based one, it was determined that the band structure that occurs in aggregated organic solids needs to be considered when evaluating the luminescence of lanthanide coordination polymers