Emission Dependence on Guest Molecule Structure in Luminescent Metal-Organic Frameworks Doped with Polypyridyl Osmium Complexes

Metal-organic frameworks (MOFs) are compounds made of a metal center coordinated with organic molecules. They form porous, 3-D, lattice-like structures that may store small molecules and can be used in analytical sensing through luminescence. Transition metal complexes (TMCs) can be incorporated into the MOF scaffold to create luminescent MOFs (LMOFs). The Zn3(bpdc)6(bpy)2 MOF, (bpdc = biphenyldicarboxylic acid, bpy = 4,4- bipyridine), has been doped previously with [Os(CO)2Cl2(dcphen)]. The produced LMOF exhibited unique fluorescence response in alcohols. A metal to ligand charge transfer (MLCT) peak from the osmium TMC in the 550-600 nm range which is observed in the activated MOF and in nonpolar solvents, is not observed when the LMOF is soaked in polar solvents. Reproducibility tests were performed for the Zn3(bpdc)6(bpy)2 MOF and LMOF. It was found through these tests and by ICP-OES analysis that variability between samples in the intensity of the MLCT peak is caused by variability in the amount of TMC incorporated into the MOF scaffold. Tests in ethanol and toluene mixtures also suggest that the shutdown of the MLCT peak occurs because ethanol destabilizes the excited states of the TMC. The unique response in polar solvent was also believed to relate to pore size, and to ethanol interacting with itself within the pores and not the osmium complex. This hypothesis was investigated by replacing bpy in the MOF scaffold with linkers of varying lengths to achieve differing pore sizes: pyrazine, DABCO, and bipyridylethylene. Of these, only the Zn LMOF with DABCO linkers showed the same response in polar solvent. This supports the dependence of the response in polar solvents on pore size. Doping of the analogous [Co3(bpdc)3(bpy)]·4 DMF·H2O MOF with [Os(CO)2Cl2(dcphen)] was attempted to gain insight into the effect of the metal center, but doping proved unsuccessful