Highly colored oxidized 3,3^/-di(indolyl)arylmethanes (DIAM) based probes have been synthesized in good yield using modified single-step synthesis. The internal charge transfer state of the molecules as well as protonation equilibrium of indolyl units are dominated by the electronic nature of substituents (R=−CH₃, −NMe₂, −NO₂ etc) present on the central aryl moiety. However, irrespective to the nature of the substituents, all compounds (HL) show selective chromogenic response towards basic anions, like F⁻ and H₂PO₄⁻ etc in CH₃CN. At low concentration, both analytes induce hypsochromic shifts in absorption maximum owing to the formation of H-bonded complex (HL^….A). However, only F⁻ ion at high concentration results in a large bathochromic shift (▵λ∼90 nm) at λ_max, probably due to the formation of deprotonated receptor (L⁻), attributed by higher stability of H-bonded [HF₂⁻] dimer. Thus, one can easily discriminate between these ions via color-changing response. On the other hand, HSO₄⁻ being an acidic anion results in protonation of indolyl nitrogen center (H₂L⁺) even at low concentration in CH₃CN-H₂O (1 : 1) mixture medium, leading to bathochromic shift of absorption maxima.

中文翻译：

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对 F− 和 H2PO4− 的差异显色响应：氢键与去质子化

已经使用改进的单步合成法以良好的产率合成了基于高度着色的氧化 3,3 ^/-二（吲哚基）芳基甲烷 (DIAM) 的探针。分子的内部电荷转移状态以及吲哚基单元的质子化平衡受中心芳基部分上存在的取代基（R=-CH ₃、-NMe ₂、-NO ₂等）的电子性质支配。然而，无论取代基的性质如何，所有化合物 (HL) 都对CH _{3 中的}碱性阴离子（如 F ^-和 H ₂ PO ₄ ^-等）显示出选择性的显色响应CN. 在低浓度下，由于形成氢键复合物 (HL ^.... A) ，两种分析物都会引起吸收最大值的低色偏移。然而，只有高浓度的F ^-离子会导致λ _max处的大红移（▵λ∼90 nm），这可能是由于形成去质子化受体（L ^-），归因于 H 键 [HF ₂ ^- ] 二聚体。因此，人们可以通过颜色变化响应轻松区分这些离子。另一方面，作为酸性阴离子的HSO ₄ ^-即使在 CH ₃ CN-H 中的低浓度下也会导致吲哚氮中心 (H ₂ L ⁺ ) 的质子化₂ O (1 : 1) 混合介质，导致吸收最大值的红移。