Directional Exciton Migration in Benzoimidazole-Based Metal–Organic Frameworks

Abstract

Highly luminescent metal–organic frameworks (MOFs) have recently received great attention due to their potential applications as sensors and light-emitting devices. In these MOFs, the highly ordered fluorescent organic linkers positioning prevents excited-state self-quenching and rotational motion, enhancing their light-harvesting properties. Here, the exciton migration between the organic linkers with the same chemical structure but different protonation degrees in Zr-based MOFs was explored and deciphered using ultrafast laser spectroscopy and density functional theory calculations. First, we clearly demonstrate how hydrogen-bonding interactions between free linkers and solvents affect the twisting changes, internal conversion processes, and luminescent behavior of a benzoimidazole-based linker. Second, we provide clear evidence of an ultrafast energy transfer between well-aligned adjacent linkers with different protonation states inside the MOF. These findings provide a new fundamental photophysical insight into the exciton migration dynamics between linkers with different protonation states coexisting at different locations in MOFs and serve as a benchmark for improving light-harvesting MOF architectures.

Energy Transfer in Metal–Organic Frameworks for Fluorescence Sensing

“…Metal–organic frameworks (MOFs) are porous-crystalline materials constructed from metal ions or metal clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures, with an outstanding level of structural and compositional control. 1 − 8 Introducing emissive organic linkers or metal clusters may lead to the formation of MOF porous structures with interesting luminescent properties. These luminescent metal–organic frameworks (LMOFs) 9 − 15 have attracted tremendous attention over the past several years because of their abundant photophysical properties and enormous potential in sensing applications.…”

Symmetry-Guided Synthesis of N,N′-Bicarbazole and Porphyrin-Based Mixed-Ligand Metal–Organic Frameworks: Light Harvesting and Energy Transfer

“…containing MOFs that can further increase chemical stabilities of EnT mimics is desirable yet challenging since the robust metal–ligand bonds in these systems typically limit the coordinative reversibility needed to form ordered crystalline structures. Several recent works have opted for Zr-based MOFs to build EnT systems with increased chemical stability; − however the precise placement of donor and acceptor in geometrically distinct positions with atomistic precision is extremely challenging to be realized. One example was just reported by Li and co-workers involving a stepwise linker insertion strategy to install the energy acceptor into a framework composed of an energy donor ligand (and vice versa).…”

Impact of nanosizing a host matrix based on a metal–organic framework on solid-state fluorescence emission and energy transfer

“…These effects were mitigated by the nanosizing of the MOF crystals, affecting the fraction of the rate constants of fluorescence transition, nonradiative deactivation (QY: φ FL = k FL /( k FL + Σ k nr )). 9,14,19,25,26 Although it is difficult to prove the mechanism without directly observing the luminescence phenomenon from ca. 10 nm-sized crystals and their dynamics, it is believed that a combination of several factors caused the strong luminescence enhancement from nanosized host crystals.…”

“…Because an organic chromophore can be introduced in the framework and/or mesopores of the materials, host–guest interactions between dyes in the framework and/or mesopores have been investigated. 16–19 Furthermore, the emissive dye-encapsulation strategy using mesoporous materials has also been applied to the hybridization of a photocatalytic system in mesoporous silica, which has resulted in the enhancement of photocatalytic performance and the durability. 20 Although there have been numerous reports using combinations of host porous materials and guest emissive and/or photoactive molecules, they are not sufficient to understand the relationship between the porous structure of the host matrix and the properties of the guests.…”