Visible colorimetric fluoride and hydroxide sensing by asymmetric tris-urea receptors: combined experimental and theoretical studies
A series of asymmetric tris-urea receptors with electron withdrawing group (EWG) or electron donating group (EDG), L1–L4, were synthesized and characterized by high resolution mass spectroscopy (HRMS), nuclear magnetic resonance spectroscopy (NMR) and single-crystal X-ray diffraction (XRD) techniques. Receptors with EWG substituent showed different visible colorimetric response to fluoride and hydroxide anions. Binding studies toward various monovalent anions were performed by UV-Vis and NMR titration experiments. The experimental results revealed that these tris-urea receptors underwent stepwise deprotonation of the three N–H protons in the presence of varying excess of fluoride and hydroxide anions. This phenomenon was signalled by the development of vivid colours. These findings were also supported by density functional theory (DFT) and time-dependent DFT (TDDFT) theoretical calculations. DFT studies proved that the deprotonation of receptors with EWG substituent was energetically favourable in comparison with the receptor containing EDG substituent. TDDFT calculations for molecular orbital distribution, oscillator strength, and the electron transition process between the ground state and excited state of receptors and their corresponding deprotonated receptors were performed to elucidate the different absorption properties.