Synthesis, Characterization And Properties Study Of Two Coumarin-based Molecular Fluorescent Probes

As the second most abundant transition metal after iron in human body, zinc plays multiple roles in biological functions, on the other hand, zinc is a metal pollutant of the environment. Superabundant zinc may lower the soil microbial activity that can lead to phytotoxic effects. Due to the importance of zinc, simple, rapid, and highly sensitive analytical methods for monitoring trace amounts of zinc are in great demand. Up to now, many detection methods are widely applied to determination of zinc, amond these methods, spectrofluorometric techniques, following changes of fluorescence are viable alternatives to traditional methods because of its high sensitivity, simple operation. Fluorescence probe is a kind of molecular which can effectively convey the recognition events through fluorescent signals. Due to their difference from fluorophore, fluorescent probes can be divided into rhodamine derivatives, fluorescein derivatives and coumarin derivatives and so on. Coumarin become a new hotspot because of its advantages of high fluoerscence quantum yields; large Stokes shift and good photostability.In this paper, we report two coumarin salen-based fluorescent sensors L1and L2. Compound L1which have potentially tetradentate N2O2donor are known to form stable complexes with transition metals, synthesized by the reaction of8-formyl-7-hydroxy-4-methylcoumarin and (1S,2S)-1,2-diphenylethane-1,2-diamine. Compound L2which synthesized by the reaction of8-formyl-7-hydroxy-4-methylcoumarin and2,2’-(piperazine-1,4-diyl)diethanamine possess favourable flex-ibility and good coordination space. The structures of compound L1and L2were characterized by IR,1H NMR,13C NMR, MS.Next, we studied fluorescence and UV-vis properties of Compound L1and L2.Compound L1exhibits weak fluorescence in CH3CN-HEPES buffer (20.0mM, pH=7.4)(9:1, v/v), but the fluorescence is significantly enhanced (20.5-fold) upon the addition of Zn2+. Other cations including Cu2+, Fe3+, Cr3+, Cd2+, Co2+, Ni2+, Ag+, alkali and alkaline earth metal cations show little spectral changes in L1solution. Fluorescence titration experiment, the Job plot experiment, UV-vis titration experiment, indecate a1:1ratio for L1-Zn2+complex. The competition experiments shows that compound L1has high selectivity over most competing metal ions. The bonding constant and detection limit of L1was deduced to be3.98×105M-1and1.25μmol/L. We optimized the pH to increase the efficiency of the fluorescence, and found that L1exhibited satisfactory Zn2+sensor abilities when the pH range is5.0～11.0. The calculated energy-minimized structure of L1-Zn2+complex reveals that L1can effectively create suitable space to match Zn2+ion. The bond lengths of the Zn2+-N and Zn2+-O are within the normal range. Base on above results, Compound L1can serve as an effective fluorescent probe for detection Zn2+ Compound L2which has a weak fluorescence response shows fluorescence enhancement (3-fold) upon adding Zn2+in CH3CN-HEPES buffer (9:1, v/v). The competition experiment shows that the fluorescence intensity of L1-Zn2+system decreased upon adding other cations. We speculate that may be the flexible of L2is better, other cations such as Cu2+, Fe3+，Cd2+produce competition effect toward Zn2+, so ligands L2need further improvement, in order to improve its selectivity to Zn2+...