Unveiling the unique mechanism of UV-sensitive phosphonate-based metal–organic frameworks

Fluorescent metal–organic frameworks (MOFs) exhibit distinguished applications in sensing, especially in UV dosimeters. Although the design strategy focuses on choosing metal ions and ligands, utilizing the host–guest interactions of phosphonate-based MOFs to tune sensitivity and reliability is relatively rare, and the cases for understanding the mechanism are even fewer. Based on the crystal engineering method, a series of guest molecules were prepared to design host–guest interactions with variable π systems and three fluorescent MOFs were successfully constructed based on uranyl cations and phosphonate ligands (uranyl phosphonate frameworks, UPFs), namely, [4,4-bpyH]_1.5(UO₂)(TppmH_4.5)(H₂O)·2H₂O (UPF-112), [2,2-bpyH](UO₂)₂(TppmH₃)(H₂O)·H₂O (UPF-113) and [phenH](UO₂)_1.5(TppmH₄) (UPF-114), for UV detection via fluorescence quenching, originating from UV-excited radicals. Their sensitivity, reliability, and repeatability depend on the concentration and lifetime of radicals, which are tuned by host–guest interactions (π system mainly). The fluorescence quenching of UPF-114 could reach 95.56% at a UV dose of 2.701 × 10⁻⁴ mJ in 120 min, and the quenching of UPF-112 and UPF-113 was 75.42% and 90.29%, respectively, in 180 min at a UV dose of 4.053 × 10⁻⁴ mJ. Based on the collected data, the detection limits of UPF-112, UPF-113, and UPF-114 were calculated to be 9.54 × 10⁻¹² J, 5.98 × 10⁻¹² J and 2.96 × 10⁻¹² J, respectively. Such a low detection limit is one of the best among fluorescence-based UV detectors that are currently available. The results guide the preparation of highly sensitive and reliable UV dose detectors through the rational design of host–guest interactions.