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. The selectivity of fluorescent bisquinoline derivatives with thioether linkers for Hg2+ ions is improved by 8‐quinolylmethyl tethering and increasing the number of sulfur atoms through the extension of the linker.