The Interaction of Fluoride with Fluorogenic Ureas: An ON1–OFF–ON2 Response

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 c...

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Published in:Journal of the American Chemical Society 2013-04, Vol.135 (16), p.6345-6355
Main Authors: Amendola, Valeria, Bergamaschi, Greta, Boiocchi, Massimo, Fabbrizzi, Luigi, Mosca, Lorenzo
Format: Article
Language:English
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Summary: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][L1···Cl] and [Bu4N][L2H···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–, C6H5COO–) 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–.
ISSN:0002-7863
1520-5126
DOI:10.1021/ja4019786