The compounds also exhibit obvious fluorescence spectra response
to copper ions. As is shown in Figs. 2b and S1b, both compounds 1 and
2 are fluorescence turn-off response to copper ions. With the increase
of copper ion concentration, the fluorescence intensity of 1 at 481 nm
and 2 at 475 nm gradually decreases. The decrease of fluorescence
may be derived fromthe intramolecular photoinduced electron transfer
(PET) fromthe excited fluorophore to the central copper ion in the complex
[25,26].