Photophysical Study of the Heteroleptic Ruthenium(II) Bis(2,2′-bipyridine) (1,10-Phenanthroline)-Templated Cadmium–Benzene Dicarboxylate Metal–Organic Framework, RWLC-7

Metal–organic frameworks (MOFs) have emerged as an important class of porous materials that exhibit exceptional porosity and functionality. The highly porous nature of these materials allows for the encapsulation of a wide variety of molecules including photoactive guests. It has now been demonstrated that ruthenium(II) tris(2,2′-bipyridine) (RuBpy) can serve as a templating agent facilitating the formation of new MOF topologies containing noncovalent RuBpy guests that exhibit distinct photophysical properties relative to the complex in solution. One such templated MOF is RWLC-5 formed solvothermally from RuBpy, Cd(NO3)2, and benzene 1,4-dicarboxylate with crystallographically resolved RuBpy encapsulated within an anionic framework constructed from Cd2Cl2(COO)4 clusters linked through the benzene part of the 1,4-BDC giving a Cd2Cl2(COO)4(RuBpy) formula. Using the heteroleptic complex, Ruthenium(II) bis(2,2′-bipyridine)(1,10-phenanthroline) (RuBpy2Phen), in place of RuBpy under RWLC-5 synthesis conditions, a new framework, RWLC-7, is formed. The new framework can be reduced to a distorted 6-connected uninodal net of the pcu topological type containing three structural channels with RuBpy2Phen guests along the main crystallographic directions. The RuBpy2Phen exhibits a hypsochromic shift in the steady-state emission spectra relative to RuBpy2Phen in ethanol (599 for RWLC-7 vs 604 nm for RuBpy2Phen), which is attributed to both low-solvent reorganization within the framework and possibly dipole interaction between the cavity and the complex. Analysis of the materials emission lifetime reveals a biphasic decay consistent with two distinct populations of encapsulated RuBpy2Phen, one population with an emission decay lifetime that is faster than that observed for the complex in solution (268 ns vs 430 ns in solution) attributed to RuBpy2Phen complexes located within nonperiodic regions that undergoes self-quenching with a closely spaced neighboring complex. The second population exhibits an extended emission lifetime relative to solution (1135 ns vs 430 ns in solution) consistent with RuBpy2Phen tightly confined within the cavities of RWLC-7.