Off-On, Ratiometric, and On-Off Fluorescence Responses of Thioether-Linked Bisquinolines toward Hg2+ and Fe3+ Ions

Fifteen bisquinoline derivatives with a thioether linker have been prepared, and their Hg2+- and Fe3+-specific fluorescence responses have been investigated. Upon the addition of Hg2+ 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 Hg2+ ion selectivity. Increasing the number of sulfur atoms through the extension of the linker chain also improves the Hg2+ ion selectivity.