Fluorescent Bis-Calix[4]arene-Carbazole Conjugates: Synthesis and Inclusion Complexation Studies with Fullerenes C60 and C70

Supramolecular chemistry has become a central theme in chemical and biological sciences over the last decades. Supramolecular structures are being increasingly used in biomedical applications, particularly in devices requiring specific stimuli-responsiveness. Fullerenes, and supramolecular assemblies thereof, have gained great visibility in biomedical sciences and engineering. Sensitive and selective methods are required for the study of their inclusion in complexes in various application fields. With this in mind, two new fluorescent bis-calix[4]arene-carbazole conjugates (4 and 5) have been designed. Herein, their synthesis and ability to behave as specific hosts for fullerenes C₆₀ and C₇₀ is described. The optical properties of the novel compounds and their complexes with C₆₀ and C₇₀ were thoroughly studied by UV-Vis and steady-state and time-resolved fluorescence spectroscopies. The association constants (K_a) for the complexation of C₆₀ and C₇₀ by 4 and 5 were determined by fluorescence techniques. A higher stability was found for the C₇₀@4 supramolecule (K_a = 5.6 × 10⁴ M⁻¹; ΔG = −6.48 kcal/mol). Evidence for the formation of true inclusion complexes between the host 4 and C₆₀/C₇₀ was obtained from NMR spectroscopy performed at low temperatures. The experimental findings were fully corroborated by density functional theory (DFT) models performed on the host–guest assemblies (C₆₀@4 and C₇₀@4).

Enhanced Characterization of Pyrene Binding in Mixed Cyclodextrin Systems via Fluorescence Spectroscopy

Although significant effort has been expended to analyze the binding of pyrene in β-cyclodextrin and γ-cyclodextrin, little has been published on the binding of this guest in β-cyclodextrin derivatives (methyl-β-cyclodextrin and 2-hydroxypropyl-β-cyclodextrin) or in mixtures of such derivatives, despite the fact that these derivatives are known to have different supramolecular properties that facilitate unique modes of complexation. Reported herein is a detailed spectroscopic investigation of the binding of pyrene in β-cyclodextrin derivatives and in binary mixtures of cyclodextrins. Py values, defined as the ratio of representative vibronic bands in the fluorescence emission of pyrene, were used to measure changes in the pyrene microenvironment in the presence of the cyclodextrin hosts, and indicated that unmodified β-cyclodextrin is unique in providing a fully hydrophobic environment for pyrene through the use of two cyclodextrins to bind a single pyrene guest. By comparison, both γ-cyclodextrin and modified β-cyclodextrin analogues bind pyrene in a less hydrophobic environment through 1:1 binding stoichiometries that allow for continued interactions between the incompletely encapsulated pyrene guest and the aqueous solvent system. Binary mixtures of cyclodextrins were also explored and reinforce the unique properties of the unmodified β-cyclodextrin host. Graphical Abstract The unique binding geometries of pyrene in beta-cyclodextrin and its derivatives leads to measurable fluorescence emission signals, whose information can be used to elucidate the highly structurally dependent binding geometries and stoichiometries.

Theoretical study of the absolute inner-shell photoionization cross sections of the formic acid and some of its hydrogen-bonded clusters

Inner-shell absolute photoabsorption and photoionization cross sections of the formic acid, HCOOH, and its small hydrogen-bonded clusters, i.e., (HCOOH)₂, HCOOH₂ ⁺, HCOHOH⁺, and HCOOH·H₃O⁺, were calculated at the time-dependent density functional theory (TDDFT) level, and the results were used to analyze the effect of the formic acid clustering on the carbon and oxygen K-edge photoionization cross sections. The discrete electronic pseudospectra obtained with square-integrable (L²) basis set calculations were used in an analytic continuation procedure based on continued fraction functions to obtain the photoabsorption cross sections. Symmetry adapted cluster configuration interaction calculations on the small formic acid clusters have also been performed at the oxygen K-edge to assign the discrete transitions and ionization potentials in support to the TDDFT results.

Solubilization of Pyridone-Based Fluorescent Tag by Complexation in Cucurbit[7]uril

Aimed at further exploring the hosting properties of cucurbit[7]uril (CB7), we have exploited the spectroscopic and photophysical properties of a known fluorescent label as the guest molecule, namely, 3-cyano-6-(2-thienyl)-4-trifluoromethyl pyridine (TFP), in neat solvents. The formation of an inclusion host-guest complex with CB7 was checked by UV-vis absorption spectroscopy, and the value of binding constant (9.7 × 10⁵ M^-1) was extracted from the spectrophotometric data. The modulation of keto-enol equilibrium in TFP by the local environment is governed by the interplay between dimerization through intermolecular hydrogen bonding between individual solute molecules, favoring the enol form, and intermolecular hydrogen bonding between TFP and the surrounding solvents, favoring the keto form. Time-resolved fluorescence results established that the macromolecular CB7 host stabilizes preferentially the neutral enol form over the keto form of TFP. Unprecedentedly, our results reveal a linear dependence of the amplitudes of the extracted decay-associated spectra from the time-resolved fluorescence spectra of TFP on the sum of polarity/polarizability and hydrogen bonding parameters of the local environment, confirming that TFP at micromolar concentration in the CB7 complexes is experiencing a methanol-like environment. The results rationalized the 42-fold enhancement in the solubility of TFP in water media by complexation in CB7.

Theoretical insights into excited-state intramolecular and multiple intermolecular hydrogen bonds in 2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone

The photophysical properties and photochemistry reactions of 2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone (HPQ) system in different solutions are studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Our theoretical investigation explores that an ultrafast barrier-free excited state intramolecular proton transfer (ESIPT) process occurs and the configuration twisting is found in the electronic excited state. In the polar protic methanol solution, the hydrogen-bonded complex composed by HPQ and two methanol molecules (HPQ-2M) could exist stably in the ground state. Upon photoexcitation the isolated HPQ is initially excited to the first excited state, while the HPQ-2M system is firstly excited to the S₃ state and undergoes internal conversion (IC) to the S₁ state. The intermolecular hydrogen bonds are strengthened in the excited state. The simulated electronic spectra agree well with the experimental results. The strengthening of the intermolecular hydrogen bonds is also confirmed by the calculated vibrational spectra. In addition, the intramolecular charge transfer happens in both HPQ and HPQ-2M systems from the frontier molecular orbital analysis.

Donor–π–acceptor (D–π–A) dyad for ratiometric detection of Hg2+ and PPi

A donor–π–acceptor (D–π–A) dyad 1 has been successfully synthesized by linking phenanthrenequinone as an electron donor unit and anthraquinone as an electron acceptor unit through a phenyl ring.

19F NMR Characterization of the Encapsulation of Emerging Perfluoroethercarboxylic Acids by Cyclodextrins

Legacy perfluoroalkyl substances (PFASs) are known environmental pollutants with serious adverse health effects. Perfluoroethercarboxylic acids (PFECAs), emerging PFASs now being substituted for legacy PFASs, have recently been detected in the environment. Cyclodextrins (CDs) have been proposed as agents for the remediation of problematic pollutants, including legacy PFASs. The current study uses ¹⁹F NMR spectroscopy to measure the complexation of mono-, di-, and triether PFECAs by CDs for eventual environmental applications. Eight PFECAs were characterized by ¹⁹F and ¹³C NMR. The change in chemical shift of individual fluorines upon complexation of CDs at various stoichiometric ratios was used to determine the host-guest association constants. All studied PFECAs were most strongly encapsulated by β-CD, with association constants from 10²-10⁵ M^-1 depending on chain length and number of ether functionalities. ¹⁹F-¹H heteronuclear Overhauser effect spectroscopy (HOESY) NMR experiments were performed for the β-CD complexes of two branched monoethers, PFPrOPrA ("GenX") and PFDMMOBA, to elucidate the structural details of the complexes, determine the specific orientation, and position of β-CD along the PFAS chain, and assess the roles of hydrogen-bonding and PFECA branching on the host-guest interactions. The results give new understanding into the fundamental nature of the host-guest complex between cyclodextrins and perfluorinated surfactants.

A supramolecular self-assembly host–guest system from cyclodextrin as an absolute water-soluble fluorescence sensor for aluminium ions: synthesis, characterization and sensing activity

In the study, an organic molecule derived from paeonol and anthraniloyl hydrazine (L) was synthesized via a Schiff-base reaction.

Enhanced fluorescence of aqueous BODIPY by interaction with cavitand cucurbit[7]uril

Host–guest interactions of the host cucurbit[7]uril (CB[7]) with a water soluble cationic BODIPY dye 2 are scarcely reported.

β-Cyclodextrin induced hierarchical self-assembly of a cationic surfactant bearing an adamantane end group in aqueous solution

A cationic surfactant with adamantane as the end group, 1-[11-((adamantane-1-carbonyl)oxy)-undecyl]pyridinium bromide (AP), has been synthesized. Its β-cyclodextrin (β-CD) induced hierarchical self-assembling behaviors in aqueous solution were investigated using transmission or scanning electron microscopy methods and small-angle X-ray scattering measurements. Like conventional single chain surfactants, micelles could be formed by AP itself in dilute solutions. However, the dramatic phase transitions of these micelles occurred when host-guest inclusions between AP and β-CD were sequentially produced at different host/guest molar ratios (R), corresponding to the supramolecules with different chemical structures. The AP micelles could be changed into spherical unilamellar vesicles by adding β-CD to reach an R value of 1 : 1. Such vesicles then evolved into multi-wall nanotubes or hydrogels when the β-CD amount was further increased to obtain an R value of 2 : 1. The unique structural characteristics of these supramolecular aggregates come from their "monolayer-like" walls, which have rarely been reported in the past for CD/surfactant inclusion complexes. The interesting results obtained here not only enrich the β-CD/surfactant aggregation systems, but also provide a novel and facile strategy to tune the morphology and structure of aggregates.

Surfactants induced release of a red emitting dye from the nanocavity of a molecular container: A spectroscopic and calorimetric study

Supramolecular interaction of a red emitting dye Nile blue A (NBA) with Cucurbit[7]uril (CB7) in aqueous solution was studied and the release of the dye from the hydrophobic cavity of CB7 was reported. To investigate the supramolecular host-guest complex formation and release of dye, we have used the steady state absorption, fluorescence and time resolved fluorescence emission spectroscopy, (1)H NMR spectroscopy and isothermal titration calorimetry (ITC). The spectral properties of NBA were changed in the presence of CB7. The change in spectral features of NBA in presence of CB7 indicates the formation of supramolecular host-guest complexes. By using the SED equation the diameter of the complex was estimated. The complex formation further affirmed by the (1)H NMR study. Upfield and downfield shifts of the protons of NBA was observed in both the aliphatic and aromatic region. From the ITC measurement, we have drawn up the forces involved for the complexation of NBA with CB7. We have studied the release of NBA from the hydrophobic cavity of CB7 by using ionic, neutral surfactants and ionic liquid with the help of spectroscopic and calorimetric techniques. It is observed that on addition of SDS and ionic liquid (<cmc) ion-pair formation takes place between NBA and surfactant monomer whereas, it was not observed for neutral and cationic surfactant. Above cmc of the surfactants, complex is formed between NBA and micelle.

Supramolecular interaction of a cancer cell photosensitizer in the nanocavity of cucurbit[7]uril: A spectroscopic and calorimetric study

The interaction of small biologically active molecules in the nanocavity of supramolecular host is very interesting and thriving research area. In the presence of supramolecular host the absorption and emission properties of small biologically active molecules were modulated several folds compared to bulk solution. In this study we have investigated the supramolecular interaction of a cancer cell photosensitizer molecule harmane in the presence of cucurbit[7]uril (CB7) as host in aqueous buffer solution (pH∼7.2). We have used steady state absorption, emission and time resolved fluorescence spectroscopy techniques. The thermodynamics of the binding between harmane in the nanochannel of CB7 were studied by using isothermal titration calorimetry (ITC) method. The emission properties of harmane are modulated several fold in the presence of CB7. ITC study indicates that the complexation between harmane and CB7 are enthalpically favourable.

Complexes of fluconazole with sodium p-sulfonatocalix[n]arenes: characterization, solubility and antifungal activity

Aiming at providing new formulations capable of improving the biopharmaceutical properties of fluconazole, we studied the formation of host–guest complexes of this antifungal agent with water-soluble sodium p-sulfonatocalix[n]arenes.

Supramolecular interactions of nonsteroidal anti-inflammatory drug in nanochannels of molecular containers: a spectroscopic, thermogravimetric and microscopic investigation

Supramolecular host-guest complexation between the nonsteroidal anti-inflammatory drug indomethacin (IMC) and molecular containers were investigated. The weakly fluorescent drug molecule becomes highly fluorescent on complexation with different molecular containers, and time-resolved fluorescence emission spectroscopy reveals that the lifetime components of IMC significantly increase in the presence of molecular containers, compared with the lifetimes in neat water. The respective solid host-guest complexes were synthesised and characterised by Fourier transform infrared and (1) H nuclear magnetic resonance spectroscopic analysis. Microscopy techniques were used to analyse modifications of the surface morphology, owing to the formation of supramolecular complexes. The effect of the molecular container on the optical properties of IMC has also been investigated to determine the effect of nanochannels of different size and structure.

Supramolecular interaction between a hydrophilic coumarin dye and macrocyclic hosts: spectroscopic and calorimetric study

The photophysics of a hydrophilic molecule, 7-(diethylamino)-coumarin-3-carboxylic acid (7-DCCA), was studied in the presence of two macrocycles, (2-hydroxypropyl)-γ-cyclodextrin and cucurbit[7]uril. We have used steady-state absorption, fluorescence, and time-resolved fluorescence emission spectroscopy; Fourier transform infrared (FTIR) spectroscopy; (1)H NMR spectroscopy; and isothermal titration calorimetry (ITC) to confirm the supramolecular host-guest complex formation. The spectral properties of 7-DCCA were modulated in the presence of both macrocycles. It was assigned that 7-DCCA forms a 1:2 complex with (2-hydroxypropyl)-γ-cyclodextrin and cucurbit[7]uril. The large modulation of the emission properties of 7-DCCA in the presence of the macrocycles indicates the formation of supramolecular complexes. A significant shift in the bond vibration frequencies in the FTIR studies showed encapsulation of the dyes in the hydrophobic cavity of the macrocycles. This is further substantiated by the (1)H NMR studies, in which the upfield and downfield shifts of the protons were observed in both the aliphatic and aromatic region in the presence of macrocycles. The time-resolved anisotropy measurements further reinforce the conception of host-guest supramolecular complex formation because, in both cases, the rotational relaxation time increases significantly compared to that in water. A deeper understanding between the differences in interaction of an anionic molecule with cucurbit[7]uril and (2-hydroxypropyl)-γ-cyclodextrin will be achieved through this work. From the ITC measurement, we have formulated the forces due to complex formation.

Intermolecular hydrogen bond and twisted intramolecular charge transfer in excited state of fast violet B (FVB) in methanol solution

The excited state hydrogen bonding dynamics and corresponding photophysical processes of fast violet B (FVB) in hydrogen-donating methanol (MeOH) solution are investigated by using time-dependent density functional theory (TDDFT) method. In the FVB molecule, there are -C=O, -N-H groups which could act as hydrogen acceptor and donor. It is demonstrated that both the intramolecular hydrogen bond O⋯H-N in FVB and intermolecular hydrogen bond C=O⋯H-O between FVB and MeOH are formed in the ground state S0 and strengthened in the excited state S1. The absorption spectra are obviously red shifted for the hydrogen-bonded complex in comparison with FVB monomer in the low energy range. The theoretical investigation demonstrates that the twisted intramolecular charge transfer takes place in the excited states for both isolated FVB and hydrogen-bonded complex, and the dominant twisting is along N2-C3 bond. The potential energy curve is investigated to understand the photophysics process of FVB and hydrogen-bonded complex.