Auto-reusable receptors for selective colorimetric recognition of fluoride

Abstract

Two auto-reusable colorimetric receptors for selective recognition of fluoride in polar medium are reported.

Influence of aromatic nitro-substituents on auto-reusability of oxime-based fluoride receptors

“…24 However, the presence of H-bond acceptor functionality within the receptor facilitates dissociation of the fluoride-receptor species. 25 A probable mechanism for automatic regeneration of fluoride-free receptor was proposed earlier and we anticipate interplay of a similar mechanism ( Figure 6) in case of 1 and 2 as well. 25 Even though the acidity of oxime protons present in the receptors reported herein are comparable to earlier reported oxime receptors, the receptor-fluoride complexes formed by 1-2 are considerably labile.…”

“…25 A probable mechanism for automatic regeneration of fluoride-free receptor was proposed earlier and we anticipate interplay of a similar mechanism ( Figure 6) in case of 1 and 2 as well. 25 Even though the acidity of oxime protons present in the receptors reported herein are comparable to earlier reported oxime receptors, the receptor-fluoride complexes formed by 1-2 are considerably labile. In order to form H-bonded host-guest complex in solution, fluoride ion has to compete with H-bond acceptor moiety present within the receptors.…”

“…The above behaviour is typical of oxime-based receptors containing hydrogen bond acceptor functionality. 25 The rate constant of decomposition of the receptor-fluoride complexes were determined from the decrease in absorbance with time at wavelength 498 and 437 nm for 1 and 2 respectively. A straight line is obtained when in (A t /A o ) is plotted against time where A t represents absorbance at the above-specified wavelengths after time t and A o is the initial absorbance instantly after addition of anion (Figure 3).…”

“…However, both UV-visible and 1 H NMR spectroscopic studies corroborate that the receptor-fluoride complexes formed by 1-2 are considerably labile as compared to those formed by earlier reported oxime based fluoride receptors. 24,25 It has been observed that the presence of phenolic -OH, an additional H-bond donor moiety within the oxime-based receptor facilitates fluoride binding and this leads to the formation of relatively inert receptor-fluoride complexes. 24 However, the presence of H-bond acceptor functionality within the receptor facilitates dissociation of the fluoride-receptor species.…”

“…In such cases, the free receptor species is automatically regenerated from the receptor-fluoride complex after a certain time interval. 25 Indeed, the regenerated free receptor species thus obtained can be further employed for colorimetric detection of fluoride up to three cycles. Herein, we have investigated structure-property relationship to impart auto-reusability in oxime-based fluoride receptors by varying the position of the nitro substituent on the aromatic ring present in the receptor.…”