Improving Energy Transfer within Metal–Organic Frameworks by Aligning Linker Transition Dipoles along the Framework Axis

Abstract

Crystalline metal–organic frameworks (MOFs) can assemble chromophoric molecules into a wide range of spatial arrangements, which are controlled by the MOF topology. Like natural light-harvesting complexes (LHCs), the precise arrangement modulates interchromophoric interactions, in turn determining excitonic behavior and migration dynamics. To unveil the key factors that control efficient exciton displacements within MOFs, we first developed linkers with low electronic symmetry (as defined by large transition dipoles) and then assembled them into MOFs. These linkers possess extended conjugation along one molecular axis, engendering low optical bandgaps and improved oscillator strength for their lowest-energy transition (S0 → S1). This enhances absorption–emission spectral overlap and boosts the efficiency of Förster resonance energy transfer, which was observed experimentally by a sizable decrease in emission quantum yield (QY), accompanied by a faster population decay profile. We find that MOFs that orient these elongated linkers along their asymmetric pore channel, e.g., the hexagonal pores in an xly network, manifested >50% decrease in their emission QY with faster decay profiles relative to their corresponding solution dissolved linkers. This is due to an efficient migration of photogenerated excitons at the crystallite peripheral sites to internal sites, which was facilitated by polarized absorption–emission overlap among the parallelly aligned linkers. In contrast, symmetric MOFs, such as those with sqc-a topological net, orient elongated linkers along two perpendicular crystal axes, which hinders efficient exciton migration. The present study underscores that MOFs are promising to develop artificial LHCs, but that to achieve an efficient exciton displacement, appropriate topology-guided assembly is required to fully realize the true potential of linkers with low electronic symmetry.

Excited State Energy Transfer in Metal‐Organic Frameworks

“…Energy transfer in MOFs can be improved by adjusting orientations of transition dipoles between chromophores. [18] Deria's group chose ligands with high S 0 -S 1 oscillator strength and aligned them in parallel orientation in the MOF structure to increase electronic coupling, which leads to enhanced energy transfer as compared to the MOF with perpendicularly installed linkers (Figure 2a,b). Wiederrecht and co-workers performed a series of studies on energy transport through oriented MOF thin films, which were deposited with a thin layer of dye molecules on surface of the film as reporters of transported excited states.…”

“…Directional migration of excited states in MOF is suggested to play an important role. [18] Copyright 2020, American Chemical Society. c) (left) A schematic of the fabricated Zn-SURMOF-2 structure and direction of anisotropic energy transfer; (top-right) ADB linker; (bottom-right) DPP linker.…”

“…b) Stern-Volmer plot of BUT-14 (olive), SIU-50 (blue), SIU-75 (orange) and SIU-100 (pink) with node-installed ferrocenecarboxylate as the redox quencher. a,b) Reproduced with permission [18]. Copyright 2020, American Chemical Society.…”

Metal‐Organic Framework Hybrids Aid Metabolic Profiling for Colorectal Cancer

“…[ 14 ] In particular, laser desorption/ionization (LDI) MS techniques are appealing for clinical use especially in cancer diagnosis, due to minimal sample preparation requirements and simplified experimental workflows. [ 15 ] Notably, the LDI process is matrix dependent, among which metal‐organic frameworks (MOFs) have been considered as ideal matrices, owing to the π–π structure from the organic linker for light absorption and energy transfer, [ 16 ] electron transport by π‐conjugated networks for photon‐induced desorption, [ 17 ] and high surface area and abundant benzene rings skeleton for selective extraction of metabolites from complex biofluids. [ 18 ] Especially Fe‐MIL‐88A MOFs (MIL stands for Materials from Institute Lavoisier) can be easily synthesized with controlled size, good water solubility, wide ultraviolet‐visible (UV–vis) absorption band, and Fe (III) metal active center for charge transfer.…”

Highly luminescent scintillating hetero-ligand MOF nanocrystals with engineered Stokes shift for photonic applications

“…( 4 ) adapted to the DPT-DPA pair). Considering the structure and size of MOF 47 , the theoretical energy transfer rate k ET and efficiency ϕ ET are therefore calculated as a function of the nanocrystal composition under the assumption of an exciton diffusion-mediated energy transfer process in the rapid diffusion limit ( Supplementary Information , Section 5) 41 , 48 . The solid line in Fig.…”