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

Moderate Zn 2+ selectivity over Cd 2+ ( I Zn / I Cd = 1.6) in the fluorescence enhancement of TQEN ( N , N , N ', N '-tetrakis(2-quinolylmethyl)ethylenediamine) was changed to Cd 2+ 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 Cd 2+ -selectivity but also an enhanced fluorescence quantum yield upon Cd 2+ binding and high sensitivity for Cd 2+ 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) 2 TQEN) exhibited a Zn 2+ -selective fluorescence enhancement ( I Zn / I Cd = 2.2), indicating the superiority of the MOMO group for the selective sensing of Cd 2+ .