Fluorescence turn on of coumarin derivatives by metal cations: A new signaling mechanism based on C=N isomerization

A new sensing mechanism based on C = N isomerization, which shows a very significant fluorescence enhancement to the metal cations in a simple and efficient way, is demonstrated. A coumarin derivative (L) containing a C = N group was designed as an example for illustration. The free ligand L is almost nonfluorescent due to the isomerization of CN double bond in the excited state. However, the solution of ligand shows about a 200-fold increase of fluorescence quantum yield (about 30%) upon addition of Zn(ClO4)(2).