Photophysical Characterization of a Ruthenium(II) Tris(2,2′bipyridine)-Doped Zirconium UiO-67 Metal Organic Framework

The photophysical properties of ruthenium(II) tris(5,5'dicarboxy-2,2'-bipyridine), RuDCBPY, doped into the metal organic framework of Zr6O4(OH)(4)(BPDC)(6), RuDCBPY-UiO67 (where BPDC is L I ng High Loading 4,4'-biphenyldicarboxylic acid), are presented as a function of the degree of RuDCBPY doping. Steady-state diffuse reflectance of RuDCBPYUiO67 powder shows an absorption maxima at 4S5 nm, which is effectively insensitive to doping concentration. The energy of the emission maxima and excited state lifetimes are, however, quite sensitive to concentration of RuDCBPY in UiO-67. At low doping concentrations, the emission maxima lies at 630 nm. The emission decay can be adequately modeled using a single discrete exponential decay function with an observed lifetime of 1.4 mu s. The emission lifetime and the energy of the emission maxima are found to decrease with increased RuDCBPY concentration. However, at higher doping the emission decay becomes nonexponential. Equally adequate fits to the data were obtained using stretched exponential and biexponential functions. A two-state model is presented in which the nonexponential behavior observed at higher RuDCBPY doping concentrations is due to two separate solvation environments within UiO-67. It is proposed that a single RuDCBPY preferentially occupies the larger octahedral cages of UiO-67 by incorporation into the backbone of the cage and experiences a dimethylformamide (DMF)-like solvation environment. At higher doping concentrations, in addition to incorporation of RuDCBPY into the backbone of the octahedral cavities, populations of encapsulated RuDCBPY can also be found in separate octahedral UiO-67 cavities. Encapsulation is assumed to restrict the solvent occupancy within the pore; thereby limiting the solvation of RuDCBPY by DMF. This leads to the disparity in lifetimes between the slow (>100 ns) and fast (similar to 20-30 ns) components of the lifetime decay, as well as the lack of doping concentration dependence on the fast decay component. The decreased lifetime of the slow phase with increased doping concentration is attributed to intermolecular energy transfer between neighboring RuDCBPY molecules incorporated into the backbone of the octahedral cages.