Central metal ion of MOFs modulated 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 detection of Fe3+ in aqueous medium. Using Zn2+ as central ion, compound 1 exhibited (4, 4)-connected topological networks with part of the carboxyl oxygen atoms uncoordinated. Replacing Zn2+ with larger Sr2+ ion, all the carboxyl oxygen atoms took part in coordination in three-dimensional compound 2. Fluorescence studies displayed that both 1 and 2 could effectively detect Fe3+ in H2O through luminescence quenching (Ksv = 3.18 × 104 M-1 and LOD = 6.48 × 10-4 mM for 1; Ksv = 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 were advantageous for detection of electron-deficient Fe3+. Key words: Fe3+ detection; Metal-organic framework; effect of central metal ion; fluorescence property