Colorimetric ‘naked-eye’ and fluorescent sensors for anions based on amidourea functionalised 1,8-naphthalimide structures: anion recognition via either deprotonation or hydrogen bonding in DMSO

Abstract

The synthesis and the spectroscopic studies of three new amidourea-based sensors for anions, 1–3, are described. These are based on the use of 4-hydrazine-1,8-naphthalimides which upon reaction with isocyanates give rise to the formation of the desired amidoureas. 1–3 absorb strongly in the visible region, due to the internal charge transfer excited state character of the naphthalimide moieties. Large colorimetric changes were observed upon the addition of various anions such as acetate, dihydrogenphosphate and fluoride to 1–3 in DMSO, and are brought about through either hydrogen bonding to, or deprotonation of, the amidourea. These changes were clearly visible to the naked eye, changing from yellow/green to purple, and were reversed upon addition of protic solvents. Moreover, each of the three anions gave rise to unique changes in the structure of the absorption spectra which can be considered as being a ‘fingerprint’ identity for each of them. The fluorescence emission spectra were also affected upon anion binding, being significantly red shifted upon excitation. Non-linear regression analysis of the ground and excited state changes showed the anions were recognized in either 1 : 1 or 2 : 1 stoichiometry, and that the aryl-urea substituents govern the sensitivity of the binding; which in the case of acetate was in the order of 3 > 1 > 2. The anion recognition was also monitored by ¹H NMR spectroscopy in DMSO-d₆.