Functionalized guanidinium chloride based colourimetric sensors for fluoride and acetate: single crystal X-ray structural evidence of -NH deprotonation and complexation

Abstract

A series of new symmetrically functionalized guanidinium chlorides (S1–S10) are synthesized in good yields and their sensing ability toward anions is studied in MeCN–DMF (24 : 1) (v/v). The absorption bands of these molecules in the presence of anions are tuned by varying the functional groups attached to the guanidinium moiety (which resembles urea) with respect to (i) aromaticity (S1–S4), (ii) electron induction effect (S1, S5–S9), (iii) positional isomeric effect (S7–S9), (iv) indole functionality (S10) of the conjugated aryl units. Anions that are above Cl⁻ in the Hofmeister series (F⁻, AcO⁻, H₂PO₄⁻) are eligible as an analyte in this series of molecules whereas less basic anions than Cl⁻ do not cause any interference. Thus, this series of molecules are suitable for the detection of anions in the narrow window of the Hofmeister series. Out of all the anions, only fluoride causes vivid colour changes from yellow to red to reddish orange and finally to blue, irrespective of the increasing aromaticity, induction and positional isomeric effect of the substituent that is attached to the guanidinium moiety. Interestingly, S9 has shown the ability to sense distinctly both F⁻ and AcO⁻ colourimetrically. Further S10, a sensor attached with indole functionality shows selective sensing of F⁻ colourimetrically with a NIR signature at ∼930 nm though both these outputs are very unstable in nature. Stability constants for complex formation of S1–S10 (except S5) with F⁻, AcO⁻ are calculated by UV–vis titration experiments. Finally single crystal X-ray structural studies on the species 1 formed upon treating S6 with sodium fluoride confirms -NH deprotonation, whereas the reaction of S6 and S2 with sodium benzoate shows 1 : 1 host : guest binding that results in complexes 2 and 3 respectively.