[2]Pseudorotaxane Formation with FRET Based Luminescence Response: Demonstration of Boolean Operations through Self-Sorting on Solid Surface

Binary pseudorotaxane formation between an aza crown derivative as host (<b>H</b>) and two different imidazolium derivatives as guests (<b>G</b>_<b>1</b> and <b>G</b>_<b>2</b>) have been studied in detail by NMR (¹H NMR, 2D NOESY), optical (steady state electronic and emission spectroscopy), and mass spectroscopy. Binding stoichiometry (1:1), association constant for the respective [2]­pseudorotaxane formation (<i>K</i>_a^{<b>H.G</b>_<b>1</b>} = (2.61 ± 0.015) × 10³ M^–1 and <i>K</i>_a^{<b>H.G₂</b>} = (1.27 ± 0.16) × 10³ M^–1), and associated thermodynamic parameters are also evaluated based on isothermal titration calorimetric (ITC) studies. FRET based <i>luminescence ON</i> responses are observed on formation of the binary pseudorotaxane (<b>H.G</b>_<b>1</b> and <b>H.G</b>_<b>2</b>) in a nonpolar medium like dichloromethane. The thermodynamic feasibility of such an energy transfer process is also examined. The higher affinity of <b>H</b> and 18-crown-6 toward K⁺, as compared to those toward <b>G</b>_<b>1</b> or <b>G</b>_<b>2</b>, and the reversibility in the host–guest binding process are utilized in demonstrating the self-sorting phenomena with associated changes in luminescence responses that could be correlated for Boolean operators like YES, INHIBIT, OR, and AND gates

Counteranion Driven Homochiral Assembly of a Cationic <i>C</i>₃‑Symmetric Gelator through Ion-Pair Assisted Hydrogen Bond

The helical handedness in achiral self-assemblies is mostly complex due to spontaneous symmetry breaking or kinetically controlled random assembly formation. Here an attempt has been made to address this issue through chiral anion exchange. A new class of cationic achiral <i>C</i>₃-symmetric gelator devoid of any conventional gelation assisting functional units is found to form both right- and left-handed helical structures. A chiral counteranion exchange-assisted approach is successfully introduced to control the chirality sign and thereby to obtain preferred homochiral assemblies. Formation of anion-assisted chiral assembly was confirmed by circular dichroism (CD) spectroscopy, microscopic images, and crystal structure. The X-ray crystal structure reveals the construction of helical assemblies with opposite handedness for (+)- and (−)-chiral anion reformed gelators. The appropriate counteranion driven ion-pair-assisted hydrogen-bonding interactions are found responsible for the helical bias control in this <i>C</i>₃-symmetric gelator

Cucurbit[7]uril Induced Formation of FRET-Enabled Unilamellar Lipid Vesicles

A unique fluorescence resonance energy transfer (FRET) process is found to be operational in a unilamellar lipid self-assembly in the aqueous phase. A newly synthesized naphthyl based long chain lipid derivative [<i>N</i>-(naphthalene-1-ylmethyl)­tetradecane-1-ammonium chloride, 14NA⁺] forms various self-assembled architectures in the aqueous phase. Controlled changes in lipid concentration lead to a transition of the self-assemblies from micelles to vesicles to rods. In the presence of cucurbit[7]­uril (CB7), 14NA⁺ forms a host–guest [2]­pseudorotaxane complex (CB7∋14NA⁺) and secondary interactions lead to the formation of a lipid bilayer with hydrophobic pockets situated in between the layers. The change in the structure of 14NA⁺ assemblies, interaction with CB7 and formation of supramolecular assemblies of CB7∋14NA⁺ were examined using light scattering, spectroscopic, and microscopic techniques. Entrapment of a luminescent dye, anthracene within the hydrophobic bilayer of the supramolecular assembly CB7∋14NA⁺ favors a modified luminescent response due to an efficient FRET process. Further, the FRET process could be controlled by thermal and chemical stimuli that induce transformation of unilamellar vesicles

Hydrogen Bonding Interaction between Active Methylene Hydrogen Atoms and an Anion as a Binding Motif for Anion Recognition: Experimental Studies and Theoretical Rationalization

Two new reagents, having similar spatial arrangements for hydrogen atoms of the active methylene functionalities, were synthesized and interactions of such reagents with different anionic analytes were studied using electronic spectroscopy as well as by using ¹H and ³¹P NMR spectroscopic methods. Experimental studies revealed that these two reagents showed preference for binding to F^– and OAc^–. Detailed theoretical studies along with the above-mentioned spectroscopic studies were carried out to understand the contribution of the positively charged phosphonium ion, along with methylene functionality, in achieving the observed preference of these two receptors for binding to F^– and OAc^–. Observed differences in the binding affinities of these two reagents toward fluoride and acetate ions also reflected the role of acidity of such methylene hydrogen atoms in controlling the efficiencies of the hydrogen bonding in anion–H_methylene interactions. Hydrogen bonding interactions at lower concentrations of these two anionic analytes and deprotonation equilibrium at higher concentration were observed with associated electronic spectral changes as well as visually detectable change in solution color, an observation that is generally common for other strong hydrogen bond donor functionalities like urea and thiourea. DFT calculations performed with the M06/6-31+G**//M05-2X/6-31G* level of theory showed that F^– binds more strongly than OAc^– with the reagent molecules. The deprotonation of methylene hydrogen atom of receptors with F^– ion was observed computationally. The metal complex as reagent showed even stronger binding energies with these analytes, which corroborated the experimental results

Specific Reagent for Cr(III): Imaging Cellular Uptake of Cr(III) in Hct116 Cells and Theoretical Rationalization

A new rhodamine-based reagent (<b>L</b>_<b>1</b>), trapped inside the micellar structure of biologically benign Triton-X 100, could be used for specific recognition of Cr­(III) in aqueous buffer medium having physiological pH. This visible light excitable reagent on selective binding to Cr­(III) resulted in a strong fluorescence <i>turn-on</i> response with a maximum at ∼583 nm and tail of that luminescence band extended until 650 nm, an optical response that is desired for avoiding the cellular autofluorescence. Interference studies confirm that other metal ions do not interfere with the detection process of Cr­(III) in aqueous buffer medium having pH 7.2. To examine the nature of binding of Cr­(III) to <b>L</b>_<b>1</b>, various spectroscopic studies are performed with the model reagent <b>L</b>_<b>2</b>, which tend to support Cr­(III)-η²-olefin π-interactions involving two olefin bonds in molecular probe <b>L</b>_<b>1</b>. Computational studies are also performed with another model reagent <b>L</b>_<b>M</b> to examine the possibility of such Cr­(III)-η²-olefin π-interactions. Presumably, polar functional groups of the model reagent <b>L</b>_<b>M</b> upon coordination to the Cr­(III) center effectively reduce the formal charge on the metal ion and this is further substantiated by results of the theoretical studies. This assembly is found to be cell membrane permeable and shows insignificant toxicity toward live colon cancer cells (Hct116). Confocal laser scanning microscopic studies further revealed that the reagent <b>L</b>_<b>1</b> could be used as an imaging reagent for detection of cellular uptake of Cr­(III) in pure aqueous buffer medium by Hct116 cells. Examples of a specific reagent for paramagnetic Cr­(III) with luminescence <i>ON</i> response are scanty in the contemporary literature. This ligand design helped us in achieving the turn on response by utilizing the conversion from spirolactam to an acyclic xanthene form on coordination to Cr­(III)