Abstract
A new urea functionalised 4-amino-1,8-naphthalimide based fluorescent anion sensor was synthesised in 64% yield over three steps. Fluorescence and 1H NMR titrations showed that the sensor complexes strongly with acetate and dihydrogen phosphate and to a lesser extent bromide. The corresponding binding stoichiometries were examined using 1H NMR titrations. Results show that the sensor molecule initially forms 1:1 complexes through hydrogen bonding to the urea moiety, followed by secondary complexation to form higher order host:guest stoichiometries. Specifically, oxyanions complex to the sensor via hydrogen bonding through synergistic aryl C-H and N-H anion interactions in a 1:2 sensor:oxyanion arrangement. Furthermore, 2:1 sensor:oxyanion complexes are formed through an oxyanion linkage between two urea functionalities on different host molecules. This contrasts the majority of previous reports for similar hosts, which indicate 1:1 binding stoichiometry.
Highlights
It has been well established that molecules based on the 4-amino-1,8-naphthalimide fluorophore in combination with urea and thiourea anion recognition groups, are potent “on-off ” sensors for the dihydrogen phosphate, acetate and fluoride anions[1,2,3,4,5,6,7,8]
We show that the new sensor (4, Fig. 1) forms both 1:1 and higher order complexes with acetate and dihydrogen phosphate oxyanions in hydrated (0.5% water) DMSO solution
The imide 2 was synthesised as described by Niu et al.[14]
Summary
It has been well established that molecules based on the 4-amino-1,8-naphthalimide fluorophore in combination with urea and thiourea anion recognition groups, are potent “on-off ” sensors for the dihydrogen phosphate, acetate and fluoride anions[1,2,3,4,5,6,7,8]. Similar to previous reports examining the affinity of naphthalimide sensors for anions[6], the addition of acetate to 4 resulted in significant quenching of the fluorescence emission, with approximately 27% of the original signal remaining after the addition of 20 equivalents (Fig. 2). As shown, (Fig. 3) a downfield shift of approximately 3.3 ppm was evident for both urea N-H proton resonances, consistent with strong H-bonding between the urea N-H and the acetate anion.
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