Light-Harvesting Metal-Organic Frameworks (MOFs): Efficient Strut-to-Strut Energy Transfer in Bodipy and Porphyrin-Based MOFs

Title: Light-Harvesting Metal-Organic Frameworks (MOFs): Efficient Strut-to-Strut Energy Transfer in Bodipy and Porphyrin-Based MOFs

Authors: Chang Yeon Lee, Omar K. Farha, Bong Jin Hong, Amy A. Sarjeant, SonBinh T. Nguyen, and Joseph T. Hupp

Journal: Journal of the American Chemical Society

Affiliation: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States

Take Home Importance: This work demonstrates an architectural-esque means of arranging chromophores in solid materials to permit energy transfer.

Figure 1. Portions of the extended crystal structures for a) BOB and b) BOP (boy, chemists sure do think of some funny acronyms) with inset images showing confocal laser microscopy images of the crystals under various excitation energies (black = no observed emission). Yellow = Zn, red = O, green = Br,  blue = N, sky blue = F, gray = C.

Summary: Upon absorption of photons in the periphery of their light harvesting complexes, plants and other phototrophs quickly funnel that energy with nearly 100% efficiency to reaction centers where it is utilized to drive the synthesis of chemical fuels. It is believed that the precise structural arrangement of donor and acceptor chromophores in the light harvesting complexes (i.e. the structure of Photosystem II) permits this rapid energy transfer. Chemists have sought to mimic this strategy of structurally arranging chromophores to permit efficient energy transfer for applications in solar energy, and the group of Joseph Hupp (Northwestern University) has recently reported a new means of achieving structurally-controlled energy transfer in metal-organic frameworks (MOFs). MOFs are extended crystalline materials made from organic ligands (typically multidentate) attached to metal nodes or metal clusters.

In contrast to prior studies in MOFs where only a portion of the visible spectrum could be efficiently collected, the authors prepared two MOFs, BOB and BOP, each of which contain two types of chromophores which in theory would permit absorption across the visible spectrum. The idea is that if one chromophore was good at absorbing higher energy light (say 400-550 nm), and a second chromophore was good at absorbing lower energy light (say 500-700 nm), then if both could be combined into one material you could collect more of the solar spectrum.

Here, two distinct chromophore types were chosen: 1) chromophores with four carboxylate binding sites (L1 and L2 in Figure 2), and 2) a linear chromophore (L3 in Figure 2) terminated in pyridyl nitrogens which can bind metal ions via nitrogen lone-pair donation. These two types of chromophores are distinguished by how they bind metals in the exteneded MOF structure: the planar ligands L1 and L2 can form two-dimensional sheets when bound with metal ions, while the linear ligand L3 can connect these individual 2D sheets to create a three-dimensional structure. In the case of the porphyrin ligand L2, the central nitrogen atoms provide an additional zinc binding site during the synthesis of BOP.

Figure 2. Preparation of MOFs BOB and BOP, shown as digital pictures of the resulting crystals. See Figure 1 for crystal structures of BOB and BOP.

Using these multidentate ligands the authors have prepared two extended crystalline lattices which contain precisely and regularly ordered sets of chromophoric ligands (L1 and L3 in BOB and L2 and L2 in BOP). Additionally, as the fluorescence of L3 overlaps with the Q-band absorption of the porphyrin L2, it could be expected that energy transfer could occur from excited-state donor L3 to acceptor L2 – and this is what was observed in the BOP MOF! In Figure 1b above, excitation of only ligand L3 resulted in emission not from L3 but from L2, indicating that energy was transferred in a donor-acceptor fashion. Energy transfer was not observed in the BOB MOF as ligand L1 does not have absorbance at the correct region to accept energy from the donor L2. As further evidence energy transfer occurred in BOP, the authors replaced ligand L3 in crystals of BOP with pyridine via soaking. These pyridine-exchanged crystals were found to no longer display energy transfer, underlining the importance of having the correct two chromophores in the framework for energy transfer to be observed.

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