"Antenna-like" Light-Harvesting in Partially Metalated Porphyrin Hydrogen-Bonded Organic Frameworks

Arrays of light harvesting molecules with long-range order hold promise as materials capable of efficient collection of photons as well as rapid transport of captured solar energy to chemical catalysts. Hydrogen-bonded organic frameworks (HOFs) could be promising artificial light-harvesting platforms, due to their crystalline nature, their molecular-scale porosity, and their comparative ease of synthesis. In this study, HOFs containing both zinc-metalated tetrakis(4-carboxyphenyl)porphyrin (Zn-TCPP; energy donor) and corresponding free-base porphyrin (FB-TCPP; energy acceptor) units were examined and found to display "antenna-like" light harvesting behavior. Using steady-state and time-resolved emission spectroscopy, energy transfer from zinc porphyrin to free-base porphyrin was assessed. Based on changes in the emission peak profile as the ratio of zinc porphyrin to free-base porphyrin is varied, we find that, in the limit of high dilution of the acceptor, energy can be transferred from any of ∼70 donor chromophores. Application of Förster resonance energy transfer (FRET) theory and time-dependent density functional theory to a simplified model of the HOF, suggests that Coulomb coupling can account for energy transfer from ∼30 Zn-TCPPs. Closer examination of FRET modeling indicates a strong bias for transfer along the direction of porphyrin stacking. We speculate, by analogy to the behavior of related MOFs (metal-organic frameworks), that rapid transport of molecular excitons (via donor-to-donor FRET) accounts for participation of Zn-TCPP units beyond the ca. 30 expected from simplified FRET modeling. Measurements at comparatively high acceptor concentrations indicate a limiting energy-transfer efficiency of ∼95%, with a limiting time of roughly 20-50 ps. These findings suggest that stacked chromophores within HOFs can act as light harvesting "antennae" and may offer prospective application as photochemical energy conversion systems.