Resonance Raman Intensity Analysis of Photoactive Metal-Organic Frameworks

Plant photosystems are highly efficient light-harvesting constructs and their properties have inspired scientists to mimic their design for photochemistry and energy storage. Here we present a biomimetic light harvesting and energy storage system based on scaffolded metal-organic frameworks. To optimize the light-harvesting process it is important to understand the dynamics of the materials when they interact with light, in order to reduce any energy loss pathways. To investigate these dynamics, we have studied four (NH₂)₂ UiO-68 MOFs with aldehyde modifier groups attached at one of the amino positions, which results in varying photophysical properties such as electronic transition energy (475-535 nm) and fluorescent quantum yield (0.16% - 0.41%). To probe these samples, we use resonance Raman intensity analysis (RRIA), which quantifies how the MOF linkers distort in the excited state and identifies the vibrational pathways responsible for energy dissipation. We found that the C-C stretches of the aromatic rings furthest from the modifier are most distorted upon excitation relative to the other moieties, including those closer to the modifier such as the C-C stretches of the rings attached to the functional group. This reveals that the vibrations of the rings furthest from the functional group are responsible for a disproportionate amount of energy loss relative to the rest of the MOF and in future designs should be constrained to increase energy collection.