Differential Sensing of Zn(II) and Cu(II) via Two Independent Mechanisms

Selective reduction of an anthracenone–quinoline
imine derivative, 2, using 1.0 equiv of NaBH4 in 95% ethanol affords
the corresponding anthracen-9-ol derivative, 3, as confirmed
by 1H NMR, 13C NMR, ESI-MS, FTIR, and elemental
analysis results. UV–vis and fluorescence data reveal dramatic
spectroscopic changes in the presence of Zn­(II) and Cu­(II). Zinc­(II)
coordination induces a 1,5-prototropic shift resulting in anthracene
fluorophore formation via an imine–enamine tautomerization
pathway. Copper­(II) induces a colorimetric change from pale yellow
to orange-red and results in imine hydrolysis in the presence of water.
Spectroscopic investigations of metal ion response, selectivity, stoichiometry,
and competition studies all suggest the proposed mechanisms. ESI-MS
analysis, FTIR, and single-crystal XRD further support the hydrolysis
phenomenon. This is a rare case of a single sensor that can be used
either as a chemosensor (reversibly in the case of Zn­(II)) or as a
chemodosimeter (irreversibly in the case of Cu­(II)); however, the
imine must contain a coordinating Lewis base, such as quinoline, to
be active for Cu­(II).