Mechanism for Different Fluorescence Response of a Coumarin–Amide–Dipicolylamine Linkage to Zn(II) and Cd(II) in Water

A coumarin-amide-dipicolylamine linkage (L) was synthesized and used as a fluorescent receptor for metal cations in water. The receptor dissolved in water with neutral pH shows almost no fluorescence due to the photoinduced electron transfer (PET) from the amide and amine nitrogens to the excited state coumarin moiety. Coordination of Zn(2+) or Cd(2+) with L creates strong fluorescence at 437 or 386 nm, respectively, due to the suppression of PET. In contrast, other metal cations scarcely show fluorescence enhancement. IR, NMR, and potentiometric analysis revealed that both Zn(2+) and Cd(2+) are coordinated with two pyridine N, amine N, and amide O; however, the Zn(2+) center is also coordinated with a hydroxide anion (OH(-)). The structure difference for Zn and Cd complexes results in longer- and shorter-wavelength fluorescence. Ab initio calculations revealed that π electrons on the excited state Cd complex are delocalized over the molecules and the Cd complex shows shorter-wavelength emission. In contrast, π electrons of OH(-)-coordinated Zn complex are localized on the coumarin moiety. This increases the electron density of coumarin moiety and shows longer-wavelength fluorescence.