Abstract
The interactions between chemosensors, 3-amino-5-(4,5,6,7-tetrahydro-1H-indol-2-yl)isoxazole-4-carboxamide (AIC) derivatives, and different anions (F− Cl−, Br−, AcO−, and H2PO4−) have been theoretically investigated using DFT approaches. It turned out that the unique selectivity of AIC derivatives for F− is ascribed to their ability of deprotonating the host sensors. Frontier molecular orbital (FMO) analyses have shown that the vertical electronic transitions of absorption and emission for the sensing signals are characterized as intramolecular charge transfer (ICT). The study of substituent effects suggests that all the substituted derivatives are expected to be promising candidates for fluoride chemosensors both in UV-vis and fluorescence spectra except for derivative with benzo[d]thieno[3,2-b]thiophene fragment that can serve as ratiometric fluorescent fluoride chemosensor only.
Highlights
The design and synthesis of receptors capable of binding and sensing anions selectively have received significant attention in recent years because anions play an important role in a wide range of biological, environmental, and chemical processes [1−6]
Different signaling mechanisms have been suggested for F−, such as photoinduced electron transfer (PET) [15], excited state proton transfer (ESPT) [16], intramolecular charge transfer (ICT) [17,18], excimer and exciplex formation [19], and metal-ligand charge transfer (MLCT) [20], etc
One can conclude that the host chemosensors have a much stronger affinity to F− than to Cl−, Br−, AcO−, and H2PO4− through intermolecular proton transfer, which leads to the formation of chemosensor anions by F−
Summary
The design and synthesis of receptors capable of binding and sensing anions selectively have received significant attention in recent years because anions play an important role in a wide range of biological, environmental, and chemical processes [1−6]. Colorimetric chemosensors provide an immediate qualitative signal, which allows direct naked-eye detection of anions because of a specific color change of solution upon anion complexation [10,11]. A fluoride chemosensor made of a derivative of pyrrole-isoxazole, 3-amino-5-(4,5,6, 7-tetrahydro-1H-indol-2-yl)isoxazole-4-carboxamide (AIC, Scheme I), has been reported [21]. To the best of our knowledge, neither calculations of the host-guest interactions with the basis set superposition error (BSSE) corrections nor sophisticated level optimizations for the neutral, anion, and complexes forms of AIC derivatives in ground states (S0) and first excited singlet states (S1) have been reported so far. To investigate the substituent effect, several derivatives (1–5), as shown in Scheme I, have been designed by introducing functional groups to pyrrole-isoxazole to provide a demonstration for the rational design of new fluorescent and/or chromogenic chemosensors for fluoride anion
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