Abstract
Stable zwitterionic spirocyclic Meisenheimer compounds were synthesized using a one-step reaction between picric acid and diisopropyl (ZW1) or dicyclohexyl (ZW3) carbodiimide. A solution of these compounds displays intense orange fluorescence upon UV or visible light excitation, which can be quenched or “turned-off” by adding a mole equivalent amount of F− or CN− ions in acetonitrile. Fluorescence is not quenched in the presence of other ions such as Cl−, Br−, I−, NO2−, NO3−, or H2PO4−. These compounds can therefore be utilized as practical colorimetric and fluorescent probes for monitoring the presence of F− or CN− anions.
Highlights
Spirocyclic compounds have found use in the fields of organic optoelectronics [1,2,3], photochromism [4,5], and medicinal chemistry [6,7,8]
We have explored the use of zwitterionic spirocyclic Meisenheimer complexes ZW1 and ZW3 as colorimetric and fluorescent chemosensors of fluoride and cyanide anions in solution
Based on previous studies on the pH-dependence of the optical properties of ZW1 [11], the behavior observed should result from the interaction of the iminium proton of ZW1 and ZW3 with fluoride and cyanide ions, which must: (i) alter the absorption spectrum of the nearby 1,3,5-trinitrocyclohexadiene anion chromophore, as already observed upon deprotonation; and (ii) increase the electron density around the nitrogen atoms of the triazine ring so as to enable fluorescence quenching of the chromophore by photoinduced electron transfer
Summary
Spirocyclic compounds have found use in the fields of organic optoelectronics [1,2,3], photochromism [4,5], and medicinal chemistry [6,7,8]. We demonstrated the reversible fluorescence (on/off) switching of ZW1 upon deprotonation using tetrabutylammonium hydroxide (TBAOH) as a base in acetonitrile (ACN) [11] This behavior resulted from emission quenching in the Meisenheimer complex (MC2, Scheme 2) formed upon base addition, which was ascribed to photoinduced electron transfer from the deprotonated imine group to the photoexcited 1,3,5-trinitrocyclohexadiene anion fluorophore. This process was inhibited upon protonation with perchloric acid to regenerate the zwitterionic state of the system, demonstrating that ZW1 acts as a fluorescent pH-sensor (Scheme 2) [11]
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