The Interaction of Fluoride with Fluorogenic Ureas: An ON1–OFF–ON2Response

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–.