Interligand Energy Level Regulation for Fluorescence Modulation in Dual-Ligand Lanthanide Metal–Organic Frameworks

Lanthanide metal-organic frameworks (Ln-MOFs) have attracted great attention due to their unique antenna effect emission, while the introduction of dual ligands in Ln-MOFs could improve their performance significantly. Correspondingly, revealing the energy transfer mechanisms in dual-ligand Ln-MOFs becomes critical for understanding the effect of energy level matching, optimizing the optical behavior, and improving the application potential. Here, we study the luminescence performance of Ln-MOFs through the energy level modulation with dual-ligand systems. We first constructed the database of triplet excited state (T1) energies for 40 phthalic acid derivatives using density functional theory calculations. The T1 energy gap between the ligands in dual-ligand Ln-MOFs was carefully studied. Utilizing Eu3+ as the metal node, the luminescent properties were studied for a series of single- and dual-ligand Eu-MOFs. Comparative studies revealed that, compared to the corresponding single-ligand Eu-MOFs, twelve dual-ligand Eu-MOFs exhibited significant fluorescence enhancement, with intensities increased by 5- to 30-fold. By combining theoretical calculations with experimental results, we also revealed that the optimal T1 energy gap between the ligands (ΔE3) was 900-2400 cm-1. Furthermore, the dual-emissive Eu-4-TFMPA/Phen was successfully applied to the ratiometric fluorescence detection of fleroxacin (FLO), achieving a low limit of detection of 0.21 μM. Thus, we provide the theoretical basis for the rational combination of dual ligands for achieving the high-performance Ln-MOFs.