Subrata Patra

Cation-induced fluorescent excimer emission in calix[4]arene-chemosensors bearing quinoline as a fluorogenic unit: experimental, molecular modeling and crystallographic studies

A number of calix[4]arene-based fluorescent chemosensors containing amide as a binding site and quinoline as a fluorogenic unit have been synthesised and characterized by a single crystal X-ray diffraction study. These compounds have been designed with variations in conformation and steric crowding in the calix moiety to investigate its effect on ion selectivity and thereby on excimer emission. The ion-binding property of these fluoroionophores has been investigated with a large number of cations and anions and the ion-recognition event was monitored by luminescence, UV-Vis and 1H NMR (for anions) spectral changes. Out of a large number of cations, Hg2+, Pb2+, Fe3+ and Cu2+ exhibited strong complexation with all of the ionophores, as evident from luminescence and UV-vis spectroscopy. In the case of anions, F− and HSO4− exhibited strong complexation with two of the ionophores, as shown by fluorescence and NMR spectroscopy. Interestingly, complexation with metal ions resulted in a new band at lower energy due to excimer emission, which was not observed for anions. The binding constants for all of the three fluoroionophores with strongly interacting ions have been determined from fluorescence, UV-vis and NMR titration data. The DFT calculations for all of the three compounds were performed and the results show that the modulation of frontier molecular orbital energies upon complexation of metal ions causes the fluorescent excimer emission.

Calixarenes: Versatile molecules as molecular sensors for ion recognition study

AbstractThis article presents a brief account on designing of calixarene-based molecular sensor for recognition of various metal ions and anions and also different analytical techniques to monitor the recognition event. This review focuses only on calix[4]arene derivatives, in which mainly the lower rim is modified incorporating either crown moiety to make calix–crown hybrid ionophore to encapsulate metal ions or some fluoregenic inorganic and organic moieties to use it as signalling unit. In order to investigate effect of conformation of the calixarene unit and steric crowding on ion selectivity, designing of these molecules have been made using both the cone and 1,3-alternate conformations of the calixarene unit and also incorporating bulky ter-butyl group in few cases to impose controlled steric crowding. Among various ions, here focuses are mainly on biologically and commercially important alkali metal ion such as K + , toxic metal ions such as Hg2 + , Pb2 + , Cd2 + , important transition metal ion such as Cu2 +  and toxic anion like F − . The techniques used to monitor the recognition event and also to determine binding constants with strongly interacting ions are fluorescence, UV-vis and 1H NMR spectroscopy. Most of the ionophores reported in this review have been characterized crystallographically, however no structural information (except one case) are incorporated in this article, as it will occupy space without significant enhancement of chemistry part. Different factors such as size of the ionophore cavity, size of metal ion, coordination sites/donor atoms, steric crowding and solvents, which determine selectivity have been discussed. Response of ion recognition process to different analytical techniques is another interesting factor discussed in this article. Graphical AbstractIon-recognition property of a large variety of calix[4]arenes incorporating crown moiety to make hybrid ionophore, substituents to impose steric crowding and fluoregenic moieties as signalling unit has been reported. This report demonstrates how factors such as sizes of the ionophore and metal ions, donor atoms, steric crowding and solvents, influence ion-selectivity.

Effect of steric crowding on ion selectivity for calix-crown hybrid ionophores: experimental, molecular modeling and crystallographic studies

A number of calix[4]arene-azacrowns with variation in ring size and substituents at the upper and lower rims have been synthesized to investigate the effect of steric crowding towards ion selectivity. The structural elucidation of these ionophores has been carried out mainly by 1H NMR and ES-MS in solution and by single crystal X-ray study in the solid state. Interaction of these ionophores with a large number of cations has been investigated by NMR studies. The ionophore with tert-butyl at the upper rim (3) exhibits selectivity towards Na+ only whereas an ionophore of the same size but without tert-butyl at the upper rim (1) shows selectivity towards both Na+ and K+. An ionophore of the same size but with three tosyl substituents at the lower rim (4) exhibits no complexation with any cation. The ionophore with the larger crown ring and without tert-butyl at the upper rim (2) exhibits complexation with K+, Rb+, Ba2+ and weak interaction with Na+. Binding constants with these metal ions have been determined by NMR titration. Molecular modeling studies performed by a molecular mechanics force field (MMFF94) using the Monte Carlo search method and DFT calculations predicted the observed higher selectivity for sterically crowded receptor.