The interaction of fluoride with fluorogenic ureas: An ON 1-OFF-ON2 response

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Abstract

The anion binding tendencies of the two fluorogenic ureas L1H and L2H, containing the 2-anthracenyl and 1-pyrenyl moieties as signaling units, respectively, have been investigated in MeCN and DMSO by absorption, emission, and 1H NMR spectroscopies. The formation of stable 1:1 receptor:anion H-bond complexes has been confirmed by structural studies on the crystalline [Bu4N][L 1···Cl] and [Bu4N][L 2H···CH3COO] salts. Complexation induces significant variations of the emission properties of L1H and L2H according to a multifaceted behavior, which depends upon the fluorogenic substituent, the solvent, and the basicity of the anion. Poorly basic anions (Cl-, Br-) cause a red shift of the emission band(s). Carboxylates (CH3COO-, C6H 5COO-) induce fluorescence quenching due to the occurrence of an electron-transfer process taking place in the locally excited complex [L-H···X]-. However, this excited complex may undergo an intracomplex proton transfer from one urea N-H fragment to the anion, to give the tautomeric excited complex [L···H-X] -*, which emits at higher wavelength. F- displays a unique behavior: It forms with L1H a stable [L- H···F]- complex which in the excited state undergoes intracomplex proton transfer, to give the poorly emissive excited tautomer [L···H-F]-*. With L2H, on moderate addition of F-, the 1:1 H-bond complex forms, and the blue fluorescence of pyrene is quenched. Large excess addition of F- promotes deprotonation of the ground-state complex, according to the equilibrium [L2H···F]- + F- ⇆ [L2]- + HF2-. The deprotonated receptor [L2]- is distinctly emissive (yellow fluorescence), which generates the fluorimetric response ON1-OFF-ON2 of receptor L2H with respect to F-. © 2013 American Chemical Society.

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Amendola, V., Bergamaschi, G., Boiocchi, M., Fabbrizzi, L., & Mosca, L. (2013). The interaction of fluoride with fluorogenic ureas: An ON 1-OFF-ON2 response. Journal of the American Chemical Society, 135(16), 6345–6355. https://doi.org/10.1021/ja4019786

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