Tunable Charge Transfer in Europium Metal-Organic Frameworks for Ratiometric Sensing of a Sarin Simulant

Precise regulation of ligand-to-Eu energy-transfer pathways and the antenna effect is essential for constructing robust ratiometric luminescent sensors based on lanthanide metal-organic frameworks (Ln-MOFs). Here we report an isoreticular series of Eu-MOFs (SNNU-624-626) featuring stepwise amino-functionalized biphenyldicarboxylate linkers and demonstrate their use for detecting diethyl chlorophosphate (DCP), a sarin simulant. Progressive amino substitution reshapes the excited-state distribution and the balance between ligand-centered emission and Eu sensitization, giving rise to distinct photophysical signatures and sensing modes across the series. Notably, the monoamino framework SNNU-625 exhibits a clear ratiometric response (I₄₆₈/I₆₁₂): the ligand-centered emission at 468 nm is selectively amplified, while the Eu³⁺ emission at 612 nm serves as an internal reference, producing an easily visible color shift from pink to blue (limit of detection, 27 ppb; response time, <10 s). In contrast, the diamino analogue SNNU-626 shows a ligand-dominated turn-on response (LOD = 32 ppb), whereas the amino-free SNNU-624 remains essentially inactive. Mechanistic studies indicate that DCP preferentially engages pore-confined amino sites and initiates a hydrolysis-assisted protonation process; this reaction preserves the framework integrity yet attenuates ligand-to-Eu³⁺ energy transfer, thereby unleashing ligand-centered emission. These results establish a direct structure-energy-transfer-sensing relationship and provide a useful design principle for designing Ln-MOF-based ratiometric sensors for organophosphorus nerve-agent simulants.