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–.本研究针对两种分别以2-蒽基（2-anthracenyl）和1-芘基（1-pyrenyl）作为信号单元（signaling unit）的荧光生色脲（fluorogenic urea）类受体L1H与L2H，通过吸收光谱（absorption spectroscopy）、发射光谱（emission spectroscopy）及1H核磁共振光谱（1H nuclear magnetic resonance spectroscopy, 1H NMR）技术，在乙腈（MeCN）与二甲基亚砜（DMSO）中探究了其阴离子结合特性。通过对结晶态四丁基铵（tetrabutylammonium, Bu4N）盐[Bu4N][L1···Cl]与[Bu4N][L2H···CH3COO]的结构表征，证实了受体与阴离子以1:1比例形成稳定的氢键（hydrogen bond, H-bond）配合物。配合物的形成会显著改变L1H与L2H的发射特性，且该变化呈现多维度特征，具体表现取决于荧光生色取代基、溶剂体系以及阴离子的碱性强弱。弱碱性阴离子（Cl⁻、Br⁻）会引发发射峰发生红移。羧酸根阴离子（carboxylate，CH3COO⁻、C6H5COO⁻）可诱导荧光猝灭，这源于局域激发态复合物[*L-H···X]⁻中发生的电子转移过程。不过，该激发态复合物可发生分子内质子转移：脲基上的一个N-H质子转移至阴离子，进而生成互变异构激发态复合物[L···H–X]⁻*，该产物会在更长波长处发射荧光。氟离子（fluoride anion, F⁻）则表现出独特的行为：它可与L1H形成稳定的[L–H···F]⁻配合物，该配合物在激发态下会发生分子内质子转移，生成荧光量子产率极低的互变异构激发态产物[L···H–F]⁻*。当向L2H中加入适量F⁻时，会形成1:1氢键配合物，此时芘的蓝色荧光发生猝灭；当加入过量F⁻时，会促使基态配合物发生去质子化反应，对应的平衡为：[L2H···F]⁻ + F⁻ ⇌ [L2]⁻ + HF2⁻。去质子化后的受体[L2]⁻具有显著的荧光发射（黄色荧光），这使得受体L2H针对F⁻的荧光响应呈现出ON1–OFF–ON2的变化模式。