Cd²⁺-Specific Fluorescence Response of Methoxy-Substituted N,N-Bis(2-quinolylmethyl)-2-methoxyaniline Derivatives

The N3O1 tetradentate ligand, TriMeOBQMOA (N,N-bis(5,6,7-trimethoxy-2-quinolylmethyl)-2-methoxyaniline), was developed as a Cd²⁺-specific fluorescent sensor. The structure of TriMeOBQMOA is half of TriMeOBAPTQ (N,N,N',N'-tetrakis(5,6,7-trimethoxy-2-quinolylmethyl)-1,2-bis(2-aminophenoxy)ethane), which is a tetrakisquinoline derivative of the well-known calcium chelator BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid). The fluorescent Cd²⁺ selectivity of TriMeOBAPTQ (I_Zn/I_Cd = 5.3% in the presence of 3 equiv of metal ions in MeOH-HEPES buffer (9:1)) comes from the formation of fluorescent dinuclear cadmium (M₂L) and nonfluorescent OH-bridged dizinc ((μ-OH)M₂L) complexes. TriMeOBQMOA also exhibits excellent Cd²⁺ specificity in fluorescence enhancement (I_Zn/I_Cd = 2.3% in the presence of 5 equiv of metal ions in DMF-HEPES buffer (1:1, HEPES 50 mM, KCl 0.1 M, pH = 7.5)) via substantial formation of a highly fluorescent bis(μ-chloro)dinuclear cadmium complex ([Cd₂(μ-Cl)₂L₂]²⁺), which is in equilibrium with the mononuclear Cd²⁺ complex ([CdLCl]⁺), and extremely poor stability of the TriMeOBQMOA-Zn²⁺ complex. The all-nitrogen derivatives of BQMOA and BAPTQ, namely, N,N-BQDMPHEN (N,N-bis(2-quinolylmethyl)-N',N'-dimethyl-1,2-phenylenediamine) and BPDTQ (N,N,N',N'-tetrakis(2-quinolylmethyl)-2,2'-(N,N'-dimethylethylenediamino)dianiline), respectively, and their methoxy-substituted derivatives were also prepared, and the fluorescent metal ion sensing properties are discussed.