A selective emissive chromogenic and fluorogenic seleno-coumarin probe for Cu²⁺ detection in aprotic media

A new selenium containing coumarin (compound 7) was designed and synthesized from the amide linkage between coumarin-519 (6) and 2-(butylselanyl)ethanamine (5). The molecular structure of 7 was accurately characterized, and its photophysical properties in acetonitrile, ethanol and chloroform solutions were studied by absorption, stationary and time-resolved fluorescence spectroscopies. Changes in the solvent polarity affected the Stokes shift, quantum yields and lifetime of the excited states. The spectroscopic behavior of compound 7 was evaluated in the presence of different monovalent, divalent and trivalent metallic cations (Na⁺, K⁺, Ca²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺, Hg²⁺, Hg⁺, Ag⁺, Al³⁺, Fe³⁺, Ga³⁺ and Cr³⁺) in acetonitrile solution. Among the tested cations, 7 exhibited high selective interaction with Cu²⁺, which was evidenced by the not expected absorption hypsocromic shift (usually coumarin-519 gives red-shifted complexes) and intense chelation-enhanced fluorescence quenching (CHEQ). We performed spectrophotometric and spectrofluorimetric titrations of 7 upon addition of Cu²⁺. From these data, the minimal detectable and quantifiable amounts were calculated and found to be 0.2 and 0.4 μmol L^-1 by absorption and 0.6 and 1.0 μmol L^-1 by emission, respectively. The 7-Cu²⁺ compound presented the 1 : 1 stoichiometry and the stability constant values of absorption and emission were found to be log β = 5.78 and log β = 6.32 respectively. Taking into account the high selectivity of the 7-Cu²⁺ compound in organic solvent systems, and considering the role of copper in organic transformations, it can be regarded as a promising fluorescent sensor for studies concerning the determination of oxidation-dependent transient entities in organic reactions like those involving cuprates. Additionally, it can be used for the detection and quantification of this metal cation in vitro in aprotic biological systems.