Abstract
Aggregation-induced emission (AIE) dyes have been shown to be a potential ligand class for multiphoton absorbing metal–organic frameworks (MPA-MOFs); however, the influence of framework flexibility on the local ligand conformation and its ramifications on the nonlinear absorption properties of this material class have sparsely been understood. In this study, we systematically investigate the two-photon absorption properties of two pillar-layered MOFs comprising tetraphenylethylene AIE ligands and compare the results to the organic ligand in crystal form, using a combination of linear and nonlinear optical characterization methods and electronic-structure calculations. We demonstrate that self-confining the AIE ligand is key to enhance the nonlinear optical absorption properties, as a structure transformation to contracted frameworks strongly increases the two-photon absorption response, which can be addressed by specific ligand substitution. Our results have important implications on the design of MPA-MOFs and provide synthetic guidelines not only from a fundamental point of view but also application-wise.
Highlights
Aggregation of molecules into dense phases can change their optical properties in manifold ways
The metal−organic framework (MOF) crystallize in the triclinic space group P1̅ and consist of zinc-based secondary building units (SBUs) in a paddlewheel geometry, which are connected by the TCPE (TCPE-F) ligand to form rectangular grids [Zn2(X)]∞ (X = TCPE/TCPE-F)
We demonstrated that the porosity of a coordination polymer can be a double-edged sword when it comes to the crystal engineering of MOFs with maximized 2PA and emission properties based on aggregation-induced emission (AIE)-ligand
Summary
Aggregation of molecules into dense phases can change their optical properties in manifold ways. Most organic and organometallic compounds feature high fluorescence quantum yields only in solutions of low concentration, whereas their emission properties deteriorate in solutions of higher concentration and/or almost completely disappear in the solid state. This phenomenon, referred to as concentration quenching or aggregation-caused quenching (ACQ), is the result of the formation of nonemissive states that may emerge for certain aggregation modes of aromatic chromophores. The fundamental mechanism that lies behind increased quantum yields of aggregated molecules was found to originate from the reduction of nonradiative relaxation pathways such as the restriction of intramolecular vibrational and rotational motions.[6,7]
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