Concentration Dependent Dimensionality of Resonance Energy Transfer in a Postsynthetically Doped Morphologically Homologous Analogue of UiO-67 MOF with a Ruthenium(II) Polypyridyl Complex

A method is described here by which to dope ruthenium(II) bis(2,2'-bipyridine) (2,2'-bipyridyl-5,5'-dicarboxylic acid), RuDCBPY, into a UiO-67 metal-organic framework (MOF) derivative in which 2,2'-bipyridyl-5,5'-dicarboxylic acid, UiO-67-DCBPY, is used in place of 4,4'-biphenyldicarboxylic acid. Emission lifetime measurements of the RuDCBPY triplet metal-to-ligand charge transfer, (3)MLCT, excited state as a function of RuDCBPY doping concentration in UiO-67-DCBPY are discussed in light of previous results for RuDCBPY-UiO-67 doped powders in which quenching of the (3)MLCT was said to be due to dipole-dipole homogeneous resonance energy transfer, RET. The bulk distribution of RuDCBPY centers within MOF crystallites are also estimated with the use of confocal fluorescence microscopy. In the present case, it is assumed that the rate of RET between RuDCBPY centers has an r(-6) separation distance dependence characteristic of Förster RET. The results suggest (1) the dimensionality in which RET occurs is dependent on the RuDCBPY concentration ranging from one-dimensional at very low concentrations up to three-dimensional at high concentration, (2) the occupancy of RuDCBPY within UiO-67-DCBPY is not uniform throughout the crystallites such that RuDCBPY densely populates the outer layers of the MOF at low concentrations, and (3) the average separation distance between RuDCBPY centers is ∼21 Å.