8-TQEN (N,N,N',N'-tetrakis(8-quinolylmethyl)ethylenediamin e) analogs as fluorescent cadmium sensors: strategies to enhance Cd super(2+)-induced fluorescence and Cd super(2+)/Zn super(2+) selectivity

In order to exploit the untapped sensing potential of the TQEN (N,N,N',N'-tetrakis(2-quinolylmethyl)ethylenediamine) family of fluorescent probes, 8-TQEN (N,N,N',N'-tetrakis(8-quinolylmethyl)ethylenediamine) analogs were designed and characterized. Although 8-TQEN lacks practicality owing to poor solubility in both aqueous media and organic solvents, 6-MeO-8-TQEN (N,N,N',N'-tetrakis(6-methoxy-8-quinolylmethyl) ethylenediamine, 1b) exhibits Cd super(2+)-selective fluorescence enhancement at 395 nm in DMF-HEPES buffer (1 : 1), (I sub(Cd)/I sub(0) = 25, lambda sub(ex) = 332 nm, phi sub(Cd) = 0.025). Zn super(2+) induces weaker fluorescence (I sub(Zn)/I sub(0) = 9, I sub(Zn)/I sub(Cd) = 0.35). In contrast, both the parent probes TQEN (I sub(Zn)/I sub(0) = 23, I sub(Cd)/I sub(Zn) = 0.64) and 6-MeOTQEN (I sub(Zn)/I sub(0) = 11, I sub(Cd)/I sub(Zn) = 1.29 exhibit higher sensitivity toward Zn super(2+). When the quinoline groups were replaced with 8-hydroxyquinoline different responses were observed. The propanediamine derivative, 8-TQOEPN (N,N,N',N'-tetrakis(8-quinolyloxyethylene)propanedi am ine, 2c) exhibits significant fluorescence enhancement at 423 nm upon Cd super(2+) binding ( lambda sub(ex) = 325 nm, I sub(Cd)/I sub(0) = 19, phi sub(Cd) = 0.31). Fluorescence enhancement is Cd super(2+)-specific as Zn super(2+) induces only more modest emission increases (I sub(Zn)/I sub(0) = 8.8, I sub(Zn)/I sub(Cd) = 0.45).