The pH‐Triggered Improvement and Reversal of Fluorescent Cd²⁺/Zn²⁺ Selectivity in Bisquinoline Derivatives

The TriMeOBQDBEN ( N,N’ ‐bis(5,6,7‐trimethoxy‐2‐quinolylmethyl)‐ N,N’ ‐dibenzylethylenediamine, L3 ) exhibits Cd 2+ preference over Zn 2+ in fluorescence enhancement ( I Zn / I Cd = 0.20 in the presence of 3 equiv. of metal ion) in DMF‐H 2 O (1:1). This moderate Cd 2+ selectivity was significantly improved ( I Zn / I Cd = 0.03 in the presence of 3 equiv. of metal ion) by changing the pH using 10 mM KOH in the presence of 0.1 M KCl (pH = 11). Moreover, the fluorescent Zn 2+ sensor, 6‐MeOBQDMEN ( N,N’ ‐bis(6‐methoxy‐2‐quinolylmethyl)‐ N,N’ ‐dimethylethylenediamine, L4 ), turned to Cd 2+ sensor at pH = 13 under similar conditions (3 M KOH in the presence of 0.1 M KCl). In part, this improvement/reversal of the fluorescence response results from the difference in binding affinity of the sensors with Cd 2+ and Zn 2+ . The preferred formation of Zn(OH) n over Cd(OH) n ( n = 1–4) at high pH is another basal mechanism for this manipulation. The addition of chloride ion effectively stabilizes the bis(μ‐chloro) dinuclear cadmium complex [(μ‐Cl) 2 Cd 2 (L) 2 ] 2+ , which has been elucidated by X‐ray crystallography.