Off–On, Ratiometric, and On–Off Fluorescence Responses of Thioether‐Linked Bisquinolines toward Hg 2+ and Fe 3+ Ions

Fifteen bisquinoline derivatives with a thioether linker have been prepared, and their Hg 2+ ‐ and Fe 3+ ‐specific fluorescence responses have been investigated. Upon the addition of Hg 2+ ions, unsubstituted 2‐quinolylmethyl derivatives 1a – 1c exhibit a fluorescence enhancement (off–on) response. On the other hand, ratiometric and fluorescence quenching (on–off) responses are accomplished for derivatives with monomethoxy ( 2a – 2c ) and trimethoxy ( 3a – 3c ) substituents on the aromatic ring, respectively. The 8‐quinolylmethyl derivatives 4a – 4c and 5a – 5c exhibit a similar response pattern to those of 1 and 2 with enhanced Hg 2+ ion selectivity. Increasing the number of sulfur atoms through the extension of the linker chain also improves the Hg 2+ ion selectivity.