Fluorescence Quenching of a Europium Coordination Compound for the Detection of Trace Amounts of Water: Uncovering the Response Mechanism by Structural Confirmation

The coordination compound Eu(DAF)(2)(NO3)(3) (1, DAF = 4,5-diazafluorene), which shows strong red fluorescence, has been synthesized and characterized. The luminescence-quenching effect of 1 has been applied to the detection of water in CH3CN. For the first time, the response mechanism has been uncovered through the confirmation of the structure of the resultant nonluminous compound [Eu(NO3)(2)(H2O)(5)](NO3)(DAF)(2)H2O (2). The replacement of the coordinated DAF ligands by the added water led to quenching of the emission of the Eu-III ion and the simultaneous enhancement of the ligand emission. X-ray powder diffraction and thermogravimetric analyses also confirmed this mechanism. Theoretical calculations by the semi-empirical method revealed the origin of the absorption spectra of 1. This fluorometric detection of trace amounts of water in CH3CN by 1 is a direct, rapid, and sensitive method.