Role of a 3D Structure in Energy Transfer in Mixed-Ligand Metal–Organic Frameworks

Abstract

We present a detailed investigation of the photophysical properties of mixed-ligand pyrene-and porphyrin-based metal−organic frameworks (MOFs) as a function of their 3D structure. Solvothermal reactions between metal salts (InCl 3 , Zr(acac) 4 , and ZrCl 4 ) and suitable ratios of 1,3,6,8-tetrakis(p-benzoic acid)pyrene (TBAPy) and meso-tetrakis-(4-carboxyphenyl)porphyrin (TCPP) were performed to prepare a series of mixed-ligand ROD-7, NU-901, and NU-1000 MOFs. Time-resolved and steady-state fluorescence measurements were conducted on the mixed-ligand MOFs to study their photophysics. Based on the results, we concluded that upon excitation of TBAPy linkers in the MOFs, singlet excitation energy migrates across TBAPy linkers until it finds a TCPP unit. TCPP acts as an energy trap and quenches the excitation. The efficiency of TBAPy-to-TBAPy energy transfer was found to be sensitive to the structural parameters of MOFs. Analysis of steady-state and time-resolved fluorescence data revealed that excitation energy transfer (EET) is most efficient in ROD-7, followed by NU-901 and NU-1000. We propose that topology that invokes the shorter interchromophoric distances between TBAPy linkers in ROD-7 is responsible for its higher EET efficiency. The distance dependence of the EET rate constant (k EET ) was investigated to gain insight into the mechanistic aspects of energy transfer in MOFs. This study revealed that (a) energy transfer in MOFs deviates from the classical Forster model and (b) the geometrical arrangement of linkers influences the mechanism of EET in MOFs. A theoretical investigation was also performed to determine energy-transfer rate constants along different directions and assess the directionality of energy transfer in these MOFs. The magnitude of rate constants indicated that energy transfer in ROD-7 should be highly anisotropic along the stacking direction. These findings suggest that ROD-7 is a promising candidate to play the role of the light-harvesting and energy-transfer component in solar energy conversion devices, where directional energy transfer is required.

Dipole-mediated exciton management strategy enabled by reticular chemistry

“…Owing to their rigid structures that place organic ligands at welldefined distances and angles from each other, metal-organic frameworks (MOFs) have recently gained traction as platforms for controlling excitons. [12][13][14][15][16][17][18][19][20][21][22][23] One MOF topology that allows perpendicular arrangement of donor and acceptor moieties at close distance is a square lattice defined by dinuclear metal paddlewheel tetracarboxylates with rectangular ligands, pillared by a second type of ligand, typically linear bipyridyl molecules (Fig. 2).…”

Topology- and wavelength-governed CO2 reduction photocatalysis in molecular catalyst-metal–organic framework assemblies

“…Besides mass transport, transferring conclusions from recent reports on MOF-based exciton migration suggest that charge separation (CS) probability is significantly favoured for Re-222 over Re-224, due to topology-induced higher exciton hopping rates and a lower Förster radius R between chromophores (Förster resonance energy transfer scaling with R −6 ). 20,49 Here, the translational hopping distance along the c-axis is ~17 Å for 222, and ~39 Å for 224 (Figures 4c, 4f and non-translational distances in Figure S21). Additionally, 222's triangular motif with cofacial conformation translating along the lateral direction of the crystallographic c-axis further improves excited state delocalisation and hopping rates, 50 due to head-to-tail coupling.…”

Visible-Light-Driven Production of Solar Fuels Catalyzed by Nanosized Porphyrin-Based Metal–Organic Frameworks and Covalent–Organic Frameworks: A Review

“…Their often high emission intensity has been used in materials or applications such as sensors, − bioimaging, , and efficient upconversion materials. − The dynamic properties of the lowest singlet excited state have also been evaluated, and it is sufficiently long-lived [for instance, 1.88 ns for Zn­(TCPP 4 – )] to allow for efficient energy- and electron-transfer processes. Indeed, singlet–singlet energy-transfer processes in MOFs and COFs are well documented, − as is energy migration. − In parallel, the presence of heavy atoms facilitates the population of the lowest energy triplet state, with many MOFs − and COFs showing intense phosphorescence with typical emission lifetimes in the microsecond scale. Triplet energy transfer , and energy migration (Figure ) throughout these materials have also been reported.…”