Ultrafast Energy Migration in Porphyrin-based Metal Organic Frameworks (MOFs)
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Ultrafast Energy Migration in Porphyrin-based Metal Organic Frameworks (MOFs) Sameer Patwardhan1, Shengye Jin2, Ho-Jin Son3, George C. Schatz1 1 Argonne-Northwestern Solar Energy Research Center, Northwestern University, 2145 Sheridan Road, Evanston, IL 20208-3113, U.S.A. 2 Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, U.S.A. 3 Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 202083113, U.S.A. ABSTRACT In this paper, we have studied the energy transport properties of two porphyrincontaining metal organic frameworks (MOFs) for light-harvesting applications. The photoinduced singlet exciton migration is investigated using fluorescence quenching experiments, whereas details on exciton transport anisotropy and net displacements are obtained using a Förster theory analysis. The striking difference in the energy-transport properties for the two MOFs, albeit for similar molecular organization, is attributed to dissimilar spatial expanse and difference in the electronic structure of their porphyrin struts. The observed exciton displacements, of up to 60 nm, provides motivation to explore new MOF materials. Several new linkers are considered, leading to predictions of MOF structures, which provide both broadwavelength harvesting and unidirectional energy transporting MOFs with selected examples.
Figure 1: Supramolecular assembly of DA-MOF and F-MOF depicting its constituents, namely porphyrin struts (DA-ZnP, F-ZnP), TCPB linkers and zinc ions (photographs of crystals provided). The adjacent neighbors (direction of exciton hops) are depicted, i.e. AB, AC, AD, AE. This figure is adapted from Ref. 4.
INTRODUCTION Evolution has lead to optimum design of functional biomaterials in nature, for example, DNA as a carrier of genetic information, proteins for carrying out a variety of enzymatic functions, and light harvesting antenna systems for photosynthesis. Interestingly, examples of truly 'biomimetic' systems that borrow building blocks and design principles from nature are rare in material science.1-2 Considering the requirements on stability, cost and toxicity for commercial light harvesting systems, the use of robust material designs, such as metal-organic frameworks (MOFs),3 holds great promise but has so far been little explored. We have utilized photosensitive porphyrin-based struts, DA-ZnP and F-ZnP, to construct two MOFs, namely DA-MOF and F-MOF (Figure 1).4 To study energy transport experimentally, we incorporated ferrocene-based quenchers into the bulk in varying concentrations by solution techniques. The enhanced quenching of fluorescence, which is a result of exciton diffusion through porphyrin struts, provides the average number of distinct struts visited by the excitons during their lifetimes: 45 struts in DA-MOF (2025 hops),
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