Light Harvesting and Energy Transfer in Metal-Organic Frameworks

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Virginia Tech


A key component of natural photosynthesis are the antenna chromophores (chlorophylls and carotenoids) that capture solar energy and direct it towards the reaction centers of photosystems I and II. Highlighted by highly-ordered crystal structures and synthetic tunability via crystal engineering, metal–organic frameworks (MOFs) have the potential to mimic the natural photosynthetic systems in terms of the efficiency and directionality of energy transfer. Owing to their larger surface areas, MOFs have large absorption cross sections, which amplifies the rate of photon collection. Furthermore, MOFs can be constructed using analogues of chlorophyll and carotenoids that can participate in long-range energy transfer. Herein, we aimed to design photoactive MOFs that can execute one of the critical steps involved in photosynthesis - photon collection and subsequent energy transfer.

The influence of spatial arrangement of chromophores on the efficiency and directionality of excitation energy transfer (EET) was investigated in a series of mixed-ligand pyrene- and porphyrin-based MOFs. Due to the significant overlap between the emission spectrum of 1,3,6,8-tetrakis(p-benzoic acid)pyrene (TBAPy) and the absorption spectrum of meso-tetrakis(4-carboxyphenyl)porphyrin (TCPP), the co-assembly of these two ligands in a MOF should enable facile energy transfer. Bearing this in mind, three TBAPy-based MOFs with markedly different network topologies (ROD-7, NU-901, and NU-1000) were chosen and a small number of TCPP units were incorporated in their backbone. To gain insight into the photophysical properties of mixed-ligand MOFs, we conducted time-resolved and steady-state fluorescence measurements on them. Stern-Volmer analysis was performed on the fluorescence lifetime data of mixed-ligand MOFs to determine the quenching constants. The quenching constant values for ROD-7, NU-901, NU-1000, and TBAPy solution were found to be 15.03 ± 0.82 M-1, 10.25 ± 0.99 M-1, 8.16 ± 0.41 M-1, and 3.35 ± 0.30 respectively. In addition, the ratio of the fluorescence intensities of TCPP and TBAPy was used to calculate the EET efficiencies in each of the three MOFs. EET efficiencies were in the following order: ROD-7 > NU-901 > NU-1000 > TBAPy-solution. Based on the trends observed for quenching constants and EET efficiencies, two conclusions were drawn: (1) the ligand-to-ligand energy transfer mechanism in MOFs outperforms the diffusion-controlled mechanism in solution phase, (2) energy transfer in MOFs is influenced by their structural parameters and spectral overlap integrals. The enhanced EET efficiency in ROD-7 is attributed to shorter interchromophoric distance, larger orientation factor, and larger spectral overlap integral. Directionality of energy transfer in these MOFs was assessed by calculating excitonic couplings between neighboring TBAPy linkers using the atomic transition charges approach. Rate constants of EET (kEET) along different directions were determined from the excitonic couplings. Based on the kEET values, ROD-7 is expected to demonstrate highly anisotropic EET along the stacking direction.

In order to explore the mechanistic aspects of EET in porphyrin-based MOFs, we studied the energy transfer characteristics of PCN-223, a zirconium-based MOF containing TCPP ligands. After performing structural characterization, the photophysical properties of PCN-223 and free TCPP were investigated using steady state and time-resolved spectroscopy. pH-dependent fluorescence quenching experiments were performed on both the MOF and ligand. Stern-Volmer analysis of quenching data revealed that the quenching rate constants for PCN-223 and TCPP were 8.06 ± 1011 M-1s-1 and 2.71 ± 1010 M-1s-1 respectively. The quenching rate constant for PCN-223 is, therefore, an order of magnitude larger than that for TCPP. Additionally, PCN-223 demonstrated a substantially higher extent of quenching (q = 93%) as compared to free TCPP solution (q = 51%), at similar concentrations of quencher. The higher extent of quenching in MOF is attributed to energy transfer from neutral TCPP linkers to N-protonated TCPP linkers. Using the Förster energy transfer model, the rate constant of EET in PCN-223 was calculated. The magnitude of rate constant was in good agreement with the kEET values reported for other porphyrin-based MOFs. Nanosecond transient absorption measurements on PCN-223 revealed the presence of a long-lived triplet state (extending beyond 200 μs) that exhibits the characteristic features of a TCPP-based triplet state. The lifetime of MOF is shorter than that of free ligand, which may be attributed to triplet-triplet energy transfer in the MOF. Lastly, femtosecond transient absorption spectroscopy was employed to study the ultrafast photophysical processes taking place in TCPP and PCN-223. Kinetic analysis of the femtosecond transient absorption data of TCPP and PCN-223 showed the presence of three distinct time components that correspond to: (a) solvent-induced vibrational reorganization of excitation energy, (b) vibrational cooling, and (c) fluorescence.

Materials that allow control over the directionality of energy transfer are highly desirable. Core-shell nanocomposites have recently emerged as promising candidates for achieving long-distance, directional energy transfer. For our project, we aim to employ UiO-67-on-PCN‐222 composites as model systems to explore the possibility of achieving directional energy transfer in MOF-based core-shell structures. The core–shell composites were synthesized by following a previously published procedure. Appropriate amounts of Ruthenium(II) tris(5,5′-dicarboxy-2,2′-bipyridine), RuDCBPY, were doped in the shell layer to produce a series of Ru-UiO-67-on-PCN‐222 composites with varying RuDCBPY loadings (CS-1, CS-2, and CS-3). The RuDCBPY-doped core–shell composites were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) imaging, Nitrogen adsorption-desorption isotherms, and diffuse reflectance spectroscopy. Efforts are currently underway to quantify RuDCBPY loadings in CS-1, CS-2, and CS-3. After completing structural characterization, the photophysical properties of CS-1, CS-2, and CS-3 will be investigated with the help of time-resolved and steady-state fluorescence spectroscopy.



Metal-Organic Frameworks, pyrenes, porphyrins, excitation energy transfer, interchromophoric distance, orientation factor, spectral overlap integral