Light Harvesting in Photoactive Guest-Based Metal-Organic Frameworks

Harvesting sunlight as a clean and renewable energy source has long been a goal of the scientific community. One potential route for developing artificial light harvesters is using a bioinspired approach which utilizes the critical features found in these natural systems. Such features include a way in which to capture a broad range of the solar spectrum, fixed distance and orientation of redox pairs to facilitate efficient directional transfer of electrons, transfer of electrons to a terminal acceptor, and recovery of the systems for repeated use. Metal organic frameworks (MOFs) are attractive candidates for developing light harvesting systems due their ability to effectively encapsulate a variety of photoactive catalysts including transition metal polyimines and metalloporphyrins. Of particular interest is the MOFs ability to modulate guest photophysics particularly those of Ru2+ polyimines. It has now been demonstrated that for certain MOFs the lifetime of the triplet metal to ligand charge transfer excited state (3MLCT) for Ru polyimines encapsulated in a MOF was nearly double or triple that of the complexes lifetime in solution. Extending the lifetimes of the chromophores directly increases their efficiency for artificial photosynthesis by minimizing the energy loss through nonradiative mechanisms. In the process of developing light harvesting systems, ruthenium tris(2,2’-bipyridine) (RuBpy) also demonstrated an ability to template new MOF topologies when introduced during the synthetic of previously reported materials. This discovery led to a series of RuBpy templated MOFs, the RWLC series, which exhibit biphasic emission decays consistent which two populations of RuBpy. The photophysics of the long-lived population are consistent with RuBpy encapsulated within a restrictive environment which inhibits access to a rapidly decaying triplet ligand field (3LF) state. Controlling the population of the 3LF state could provide one potential avenue for tuning the excited state lifetimes in order to increase the efficiency of MOF based light harvesting systems. To that end, the photophysics of the RuBpy guest were further tuned by modifying the coordinated ligands. In addition, the preliminary results reported herein demonstrates the potential of select Ru@MOFs in light harvesting application such a singlet oxygen generation or photoinduced electron transfer (PET). Overall, the results presented here establish a foundation on which future developments in MOF-based photocatalysts can be applied to light harvesting applications