The effect of cavity size on ruthenium (II) tris-(2,2-bipyridine) photophysics encapsulated within zirconium based metal organic frameworks

Abstract

Metal organic frameworks (MOFs) are well known for their exceptionally large surface areas and high porosity making them ideal candidates for gas storage and adsorption, drug delivery systems, and light harvesting platforms. A class of MOF’s that have gained much attention for their exceptional hydrothermal stability are the zirconium oxide MOF’s. One in particular is Uio-66 which is composed of Zr6O8 clusters connected by benzene dicarboxylate linkers. One important characteristic of the Uio-66 series of MOFs is that extension of organic linkers results in a systematic increase in cavity size while retaining the same overall framework topology. This property affords the opportunity to examine the effects of cavity size on the photophysics of encapsulated guests. Here a comparison of the photophysics of ruthenium (II) tris-(2,2′-bipyridine) (RuBpy) encapsulated within three Zr-based MOF’s Uio-66, Uio-67, and Uio-68 is presented. All three RuBpy@Uio-xx (xx = 66, 67 or 68) materials display a bathochromic shift in the steady state spectra and an emission lifetimes that fit to a biexponential decay function. Both RuBpy@Uio-66 and RuBpy@Uio-67 exhibit extended emission lifetimes which is characteristic of deactivation of the higher energy triplet ligand field state (3LF) and depopulation from a higher energy metal-to-ligand charge transfer state (3MLCT). Interestingly, RuBpy@Uio-68 exhibits a solution like emission lifetime that would be consistent with the larger cavity providing sufficient volume for access to the 3LF state. However, closer examination of the photophysics reveals that even the larger cavity imposes a significant level of confinement although lesser than that observed for the RuBpy@Uio-66 and RuBpy@Uio-67 systems.