A D–A–D-type di-lophine derivative-based photoactive metal–organic framework: fluorescence sensing of UO22+ and photochromic behavior

Abstract

Cadmium(II)-based metal–organic frameworks (MOFs) have been widely investigated for fluorescence sensing due to their chemical stability and tunable optical properties. However, how to precisely improve their selectivity and sensitivity to the target analytes is a key issue to be addressed. In this work, a photoactive MOF (PMOF) exhibiting specific cation fluorescence sensing and photochromism was constructed using Cd²⁺ and di-lophine based tetracarboxylic acid with D–A–D configuration. The effective combination of two components can form a stable porous framework with multiple electron transfer paths such as short-range electron transport in ligands and metal-to-ligand charge transfer (MLCT). The Cd-MOF exhibits strong absorption in the visible light range of 400–450 nm and strong fluorescence emission at 475 nm. Theoretical calculations show that the absorption and emission mainly originate from charge transfer between the metal and the ligand. In addition, due to the stable and anionic porous framework, the Cd-MOF demonstrated fluorescence quenching sensing to a variety of substrates, such as UO₂²⁺, Fe³⁺, Cr₂O₇²⁻, MnO₄⁻, nitrofuran antibiotics, TNP explosives, etc. In particular, the fluorescence detection constants (K_sv) of the Cd-MOF for UO₂²⁺ could reach 2.0 × 10⁵ M⁻¹, which mainly originated from strong electrostatic interaction. The adsorption of UO₂²⁺ ions could effectively lower the energy level of the LUMO, which promotes the charge transfer from the [Cd(COO)₄]⁻ node to UO₂²⁺ ions, leading to the fluorescence quenching of the Cd-MOF. In addition, the Cd-MOF can be photochromically responsive to visible light, generating a new strong absorption characteristic peak at 670 nm and a color change from yellow to green. The sensitive photochromic behavior should be assigned to the generation of imidazole radicals stabilized by the spatial confinement effect of the Cd-MOF, which lowers the energy level of π* → π transition within the ligand. These findings suggest that the strategic synthesis of PMOFs utilizing D–A–D-type dilophine ligands can significantly modulate both the light absorption and fluorescence sensing capabilities, which paves a new way for the development of advanced, high-performance photoactive materials.