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

The anion binding tendencies of the two fluorogenic ureas (LH)-H-1 and (LH)-H-2, containing the 2-anthracenyl and 1-pyrenyl moieties as signaling units, respectively, have been investigated in MeCN and DMSO by absorption, emission, and H-1 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 center dot center dot center dot Cl] and [Bu4N][(LH)-H-2 center dot center dot center dot CH3COO] salts. Complexation induces significant variations of the emission properties of (LH)-H-1 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 electrontransfer process taking place in the locally excited complex [*L-H center dot center dot center dot 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 center dot center dot center dot H-X](-)*, which emits at higher wavelength. F- displays a unique behavior: It forms with (LH)-H-1 a stable [L-H...F]- complex which in the excited state undergoes intracomplex proton transfer, to give the poorly emissive excited tautomer [L center dot center dot center dot H-F](-)*. With (LH)-H-2, 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 [(LH)-H-2 center dot center dot center dot F](-) + F- reversible arrow HF2-. The deprotonated receptor [L-2](-) is distinctly emissive (yellow fluorescence), which generates the fluorimetric response ON1-OFF-ON2 of receptor (LH)-H-2 with respect to F-.