A smart single molecular probe for Cu2+/Fe3+/Mg2+ by three-emission behaviors

An azacrown[NS2O2]-azastyryl-modified fluorene probe (AC-AS-F) has been designed and synthesized. Systematic optical studies show that AC-AS-F displays a distinct two-step fluorescence response to the increasing quantity of Cu2+: The fluorescent emission of this compound centered at 490 nm is first blue-shifted to 450 nm with a concomitant increase in fluorescence intensity and then is only enhanced along with successive increase in Cu2+ amount from 0 to 1 and 1-2 equiv. In contrast, addition of Fe3+ leads to just an opposite manner in the fluorescence spectrum: the corresponding fluorescence emission is red-shift to 540 nm simultaneously accompanied by a obvious decrease in fluorescence intensity. After adding Mg2+, the maximum fluorescence emission of AC-AS-F is red-shifted to 510 nm synchronously but with a little decrease in the fluorescence intensity. Meanwhile AC-AS-F also displays three obviously distinct absorption behaviors with diverse absorption-ratiometric signals upon respective addition of Cu2+, Fe3+ or Mg2+, which induce an obvious change in the solution color from pale yellow to yellowish-brown and then to dark yellow upon successive addition of Mg2+, Cu2+ and Fe3+. These, together with the dual optical responses to the metal displacement of AC-AS-F-M(M = Cu2+/ Mg2+) by Fe3+/Cu2+, endow AC-AS-F the multi-stimuli-channel sensing nature for these metal ions, in particular for Cu2+ and Fe3+ with fast-response function.