Abstract
The environmental presence of anions of natural origin or anthropogenic origin is gradually increasing. As a tool to tackle this problem, carbazole derivatives are an attractive gateway to the development of luminescent chemosensors. Considering the different mechanisms proposed for anion recognition, the fluorescence properties and anion-binding response of several newly synthesised carbazole derivatives were studied. Potential anion sensors were designed so that they combined the native fluorescence of carbazole with the presence of hydrogen bonding donor groups in critical positions for anion recognition. These compounds were synthesised by a feasible and non-expensive procedure using palladium-promoted cyclodehydrogenation of suitable diarylamine under microwave irradiation. In comparison to the other carbazole derivatives studied, 1-hydroxycarbazole proved to be useful as a fluorescent sensor for anions, as it was able to sensitively recognise fluoride and chloride anions by establishing hydrogen bond interactions through the hydrogen atoms on the pyrrolic nitrogen and the hydroxy group. Solvent effects and excited-state proton transfer (ESPT) of the carbazole derivatives are described to discard the role of the anions as Brönsted bases on the observed fluorescence behaviour of the sensors. The anion–sensor interaction was confirmed by 1H-NMR. Molecular modelling was employed to propose a mode of recognition of the sensor in terms of complex stability and interatomic distances. 1-hydroxycarbazole was employed for the quantitation of fluoride and chloride anions in commercially available medicinal spring water and mouthwash samples.
Highlights
Dysregulation of chloride anion levels is related with neurodegenerative diseases, it is decreased in the case of amyotrophic lateral sclerosis [7] and it is increased in the case of Parkinson’s disease [8]
Reversibility is an essential property of chemosensors, and anion recognition proceeds through the formation of non-covalent bonds, mainly hydrogen bonds
In order to check the relevance of the hydrogen bonds in the anion recognition, the behaviour of these compounds was compared with that of corresponding methoxy derivatives
Summary
Dysregulation of chloride anion levels is related with neurodegenerative diseases, it is decreased in the case of amyotrophic lateral sclerosis [7] and it is increased in the case of Parkinson’s disease [8]. The control of chloride anion in drinking water is a mandatory analytical determination in the official methods of analysis. The determination of this anion by means of fluorescent sensors is an alternative to the official methods of the water quality control agencies. The fluoride anion has considerable toxicological importance due to its ability to induce alterations of synapsis-related proteins [10] and, various abnormalities in blood cells [11], among other toxic effects [12–17]. Causes oxidative stress due to decreased levels of reduced glutathione, an effect that has been associated with chloride anions [19]. Anion sensing generally involves molecular recognition of analytes via hydrogen bonding or hydrophobic interactions [29–33]. The distance between the NH and OH groups is crucial for the charge transfer process to be effective in enhancing fluorescence
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