Central metal ion of MOFs modulates the fluorescence detection of Fe3+

Abstract

A new π-conjugated bridging-type dicarboxylic acid, 3′,5′-bis(2-methyl-1H-imidazol-1-yl)[1,1′-biphenyl]-3,5-dicarboxylic acid (H₂L), was designed and synthesized to construct fluorescent metal–organic frameworks (MOFs). The two MOFs, named [ZnL]·1.5H₂O (1) and [SrL]_n (2), were successfully exploited as fluorescent sensors for the detection of Fe³⁺ in an aqueous medium. With Zn²⁺ as the central ion, compound 1 exhibited (4, 4)-connected topological networks with part of the uncoordinated carboxyl oxygen atoms. After replacing Zn²⁺ with a larger Sr²⁺ ion, all carboxyl oxygen atoms took part in the coordination in the three-dimensional compound 2. Fluorescence studies revealed that 1 and 2 could effectively detect Fe³⁺ in H₂O via luminescence quenching (K_sv = 3.18 × 10⁴ M⁻¹ and LOD = 6.48 × 10⁻⁴ mM for 1; K_sv = 1.75 × 10⁴ M⁻¹ and LOD = 6.14 × 10⁻⁴ mM for 2). The synergistic competitive absorption and coordination interaction mechanism explained the detection of Fe³⁺. In addition, the high electron density of the uncoordinated carboxyl O atoms in 1 was advantageous for the detection of electron-deficient Fe³⁺.