| Coumarin fluorescent compounds have many excellent optical properties and become a newresearch hotspot in recent years. Fluoride detection plays an important role in the field of anionicdetection because fluorine is an indispensable trace element in human body and is also associatedwith various damage to humanity and environment. So we choose the synthesis of the coumarin-based fluorescent probe for fluoride as the research subject, which mainly includes the followingcontents:(1) The recognition sites of coumarin-based fluorescent probes for fluoride mainly includetwo categories. One is the Si–O bond, which could be broken by addition of fluoride due to thestrong affinity of fluorine to silicon (the very high bond energy of Si–F bond). The other is activehydrogen (as NH2, NH or OH group), based on the strong electronegativity of fluorine to bringabout deprotonation on the polar groups and thereby change in the fluorescence intensity of theanalyte. But fluorescent probes for fluoride with double recognition sites has not been reported sofar. We designed and synthesized coumarin-based fluorescent probes for fluoride with single anddouble recognition site(s). The structures of the synthesized compounds were characterized by1HNMR,13C NMR,19F NMR, MS and IR spectra.(2) Four coumarin-based fluorescent probes, compound5,6,9and10, containing Si–O bondas the recognition site for fluoride were synthesized in32%,46%,46%and75%yield,respectively, of which compound5,6and9are new compounds. Considering the reactivity andeasiness of desorption, we selected t-butyldimethylsilyl chloride (TBDMSCl) as the best silylationreagent for introduction of Si–O bond into coumarin. The base needed in the reaction was alsoexplored and NaH was chosen instead of many tertiary amines. The optimum molar ratio of thereagents (coumarin: TBDMSCl: NaH) was found to be1:2:1.2.(3) Three coumarin-based fluorescent probes La1, La4and Lb1with two recognition sitesfor fluoride were synthesized in45%,29%and36%yield, respectively. Two different syntheticroutes were explored to obtain the target new compounds. At first we tried to introduce a silylgroup and then introduced polar group NH by reacting with hydrazine compounds. However, thisroute did not work because Si-O bond could be easily broken in the second step. Then we triedthe condensation reaction between courmain and hydrazine compounds. After that a silyl groupwas indroduced. And we achieved our aim by this way finally.(4) The recognition behavior of probe La1towards fluoride was studied. It was found thatLa1could be used to detect fluoride in both acetonitrile and aqueous solution with remarkablesensitivity. Measurements on UV-Vis and fluorescence emission spectrum of La1at different pHshowed that neutral and weakly acidic conditions were favorable to the detection. Upon additionof fluoride to the solution of La1in acetonitrile solution, La1showed almost100nm red-shift ofλmaxand distinct color change from faint yellow to purple. Addition of other halide did not causeany color change and addition of AcO–, HSO4–or H2PO4–caused slight color change from faint yellow to yellow. In aqueous solution of La1neither the color nor the fluorescence emissionspectrum changed markedly upon addition of fluoride. Instead, addition of iodide to an aqueoussolution of La1caused fluorescence quenching of La1. |
