A coordinatively unsaturated mononuclear europium( <scp>iii</scp> ) complex: a luminescent sensor for adenosine monophosphate in aqueous media

Luminescent probes for sensing adenosine monophosphate (AMP), particularly in the presence of ADP and ATP, are rare. Herein, we report a novel Eu(III) complex, [(H₂O)(L)₂Eu(SO₃CF₃)₂](SO₃CF₃) (EuC), for this purpose, which is synthesized through the reaction of a urea and acyl hydrazone-based ligand L and Eu(III) triflate salt. Thorough characterization, including single-crystal X-ray diffraction analysis, reveals the coordinatively unsaturated structure of EuC. By taking advantage of this unsaturation, the said EuC complex can sense AMP in aqueous medium with a commendable LOD value of 0.412 μM, even in the presence of other anions, including several competitive phosphate anions. Only in the presence of AMP, the characteristic Eu(III)-centered emission peaks for the ⁵D₀ → ⁷F_<i>J</i> (<i>J</i> = 0-4) transitions, obtained upon irradiating the ligand center, are found to be significantly intensified, which further leads to the enhancement in the relative quantum yield and Eu(III)-centered lifetime values. Attempts are also made to shed light on the plausible mechanism of the AMP-sensing efficacy of EuC. The UV-vis absorption and fluorescence titration results suggest a probe-analyte adduct formation between EuC and AMP. The formation of such an adduct is further validated by ³¹P and ¹⁹F NMR spectroscopy, as well as DFT calculations (RI-BP86-D4/desf-TZVP level of theory). Interestingly, the existence of some non-covalent interactions between the ligand and the adenine base of AMP is found to be instrumental in the selective sensing mechanism by the said EuC complex, which is quite unprecedented in the literature.