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 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<sup>–