Cd²⁺-selective fluorescence response of TQEN (N,N,N',N'-tetrakis(2-quinolylmethyl)ethylenediamine) derivatives bearing ether oxygen-binding sites

Moderate Zn²⁺ selectivity over Cd²⁺ (I_Zn/I_Cd = 1.6) in the fluorescence enhancement of TQEN (N,N,N',N'-tetrakis(2-quinolylmethyl)ethylenediamine) was changed to Cd²⁺ preference via the introduction of a methoxymethyloxy (MOMO) substituent at the 8-position of one of the four quinoline rings (I_Zn/I_Cd = 0.2). Thus, 8-MOMOTQEN (N-(8-methoxymethyloxy-2-quinolylmethyl)-N,N',N'-tris(2-quinolylmethyl)ethylenediamine) showed not only high Cd²⁺-selectivity but also an enhanced fluorescence quantum yield upon Cd²⁺ binding and high sensitivity for Cd²⁺ detection as shown by ϕ_Cd = 0.065 and LOD (limit of detection) = 19 nM. The two oxygen atoms of the MOMO group in 8-MOMOTQEN play a crucial role in the fluorescent metal-ion selectivity because the corresponding hydroxy (8-OHTQEN) and methoxy (8-MeOTQEN) derivatives resulted in a poor fluorescent response and metal selectivity, respectively. Another N6O2 ligand, N,N'-bis(8-methoxy-2-quinolylethyl)-N,N'-bis(2-quinolylmethyl)ethylenediamine ((8-MeO)₂TQEN) exhibited a Zn²⁺-selective fluorescence enhancement (I_Zn/I_Cd = 2.2), indicating the superiority of the MOMO group for the selective sensing of Cd²⁺.