Recognition-mediated light-up of thiazole orange with cucurbit[8]uril: exchange and release by chemical stimuli

N-Methyl- and N-Phenylpiperazine Functionalized Styryl Dyes Inside Cucurbiturils: Theoretical Assessment of the Factors Governing the Host-Guest Recognition

The family of cucurbiturils (CBs), the unique pumpkin-shaped macrocycles, has received great attention over the past four decades owing to their remarkable recognition properties. They have found diverse applications including biosensing and drug delivery technologies. The cucurbituril complexation of guest molecules can modulate their pKas, improve their solubility in aqueous solution, and reduce the adverse effects of the drugs, as well as enhance the stability and/or enable targeted delivery of the drug molecule. Employing twelve cationic styryl dyes with N-methyl- and N-phenylpiperazine functionality as probes, we attempted to understand the factors that govern the host-guest complexation of such molecules within CB[7] and CB[8] host systems. Various key factors determining the process were recognized, such as the pH and dielectric constant of the medium, the cavity size of the host, the chemical characteristics of the substituents in the guest entity, and the presence/absence of metal cations. The presented results add to our understanding (at the molecular level) of the mechanism of encapsulation of styryl dyes by cucurbiturils, thus shedding new light on various aspects of the intriguing complexation chemistry and the underlying recognition processes.

Metal-Assisted Complexation of Fluorogenic Dyes by Cucurbit[7]uril and Cucurbit[8]uril: A DFT Evaluation of the Key Factors Governing the Host-Guest Recognition

With the emergence of host-guest systems, a novel branch of complexation chemistry has found wide application in industries such as food, pharmacy, medicine, environmental protection and cosmetics. Along with the extensively studied cyclodextrins and calixarenes, the innovative cucurbiturils (CB) have enjoyed increased popularity among the scientific community as they possess even better qualities as cavitands as compared to the former molecules. Moreover, their complexation abilities could further be enhanced with the assistance of metal cations, which can interestingly exert a dual effect on the complexation process: either by competitively binding to the host entity or cooperatively associating with the CB@guest structures. In our previous work, two metal species (Mg²⁺ and Ga³⁺) have been found to bind to CB molecules in the strongest fashion upon the formation of host-guest complexes. The current study focuses on their role in the complex formation with three dye molecules: thiazole orange, neutral red, and thioflavin T. Various key factors influencing the process have been recognized, such as pH and the dielectric constant of the medium, the cavity size of the host, Mⁿ⁺ charge, and the presence/absence of hydration shell around the metal cation. A well-calibrated DFT methodology, solidly based and validated and presented in the literature experimental data, is applied. The obtained results shed new light on several aspects of the cucurbituril complexation chemistry.

Interplay of conformational relaxation and hydrogen bond dynamics in the excited states of fluorescent Schiff base anions

Time resolved fluorescence spectroscopic investigation of four Schiff base anions has established that their excited state dynamics is governed by several solvent properties: polarity, viscosity and hydrogen bond donating ability. With viscous protic solvents like glycerol, fluorescence lifetimes of anions have been found to be markedly longer than those in ethanol, implying that conformational relaxation of molecules plays a key role in their nonradiative relaxation. Surprisingly, the lifetimes in less viscous aprotic solvents, like acetonitrile, are found to be even longer. The only plausible rationalization of this observation is in the light of hydrogen bond-assisted nonradiative phenomena that are operative in protic solvents. This contention draws support from a time evolution of the emission in the red end of the spectrum in low to moderately hydrogen bond donating protic solvents, with regard to an absence of such a rise time in aprotic solvents and strongly hydrogen bond donating solvents, viz., 2,2,2-trifluoroethanol. Rudimentary quantum chemical calculations provide a preliminary idea about the nature of excited state hydrogen bond redistribution involved in the process.

Molecular Engineering of Noncovalent Dimerization

Dimers are probably the simplest model to facilitate the understanding of fundamental physical and chemical processes that take place in much-expanded systems like aggregates, crystals, and other solid states. The molecular interplay within a dimer differentiates it from the corresponding monomeric state and determines its features. Molecular engineering of noncovalent dimerization through applied supramolecular restrictions enables additional control over molecular interplay, particularly over its dynamic aspect. This Perspective introduces the recent effort that has been made in the molecular engineering of noncovalent dimerization, including supramolecular dimers, folda-dimers, and macrocyclic dimers. It showcases how the variation in supramolecular restrictions endows molecular-based materials with improved performance and/or functions like enhanced emission, room-temperature phosphorescence, and effective catalysis. We particularly discuss pseudostatic dimers that can sustain molecular interplay for a long period of time, yet are still flexible enough to adapt to variations. The pseudostatic feature allows for active species to decay along an alternate pathway, thereby spinning off emerging features that are not readily accessible from conventional dynamic systems.

Cucurbituril-Based Supramolecular Assemblies: Prospective on Drug Delivery, Sensing, Separation, and Catalytic Applications

Precise control over the stimuli-responsive noncovalent interactions operative in a complex molecular system has emerged as a convenient way to realize applications in the detection and sensing of trace analytes, metal ion separation, uptake-release, in situ nanoparticle synthesis, and catalytic activity. This feature article focuses on the attributes and advantages of noncovalent host-guest interactions involving cucurbituril homologues (CBs) with a wide range of organic and inorganic guests, starting from organic dyes to drugs, proteins, surfactants, metal ions, and polyoxometalates. The unique structural features of CBs provide interaction sites for cations at the portals, polyanions at the periphery, and hydrophobic groups in its cavity. The facile complexation and consequent compositional and geometrical arrangements of guests such as naphthalenediimides, coumarins, porphyrins, and triphenylpyrylium ions with the host CBs led to remarkable changes in many molecular properties, especially aggregation, the proton binding and release affinity, and novel emissive dimers, and each of such spectroscopic signatures have been appropriately channeled to drug delivery and activation to improve the antibacterial efficacy and shelf life of drugs by increasing their photostability. Several technological advantages have also been extracted from the interaction of CBs with inorganic guests as well. The interaction of CB7 with the heptamolybdate anion resulted in the precipitation of a hybrid complex material which enabled a convenient separation methodology for the use of clinically pure radioactive ^99mTc in diagnostic applications. Certain cucurbituril-based hybrid materials have been developed for enhanced SO₂ adsorption at low pressures, high-efficiency hydrogen production, and reversible catalytic systems. Thus, this feature article provides a glimpse of the vast potential of cucurbituril homologues with organic and inorganic guests and calls for a dedicated effort to explore supramolecular strategies for better sensors, therapeutics, smart drug delivery modules, and facile devices.

Photophysics of thiazole orange in deep eutectic solvents

Photophysics and torsional dynamics of thiazole orange (TO) as a function of temperature have been studied in two deep eutectic solvents (DESs) using spectroscopic techniques. Two DESs are used as a solvent namely DES-I (choline chloride + urea, mole ratio 1: 2) and DES-II (N,N diethyl ethanol ammonium chloride + urea, mole ratio 1: 2). We explore the influence of DESs on the photophysical properties of TO. The fluorescence quantum yield and fluorescence lifetime of TO decreases with increasing temperature due to thermal deactivation. At higher temperature, fluorescence quantum yield of TO decreases in DESs may be due to the molecular rotor nature of TO, with the benzothiazole and quinoline ring of this dye being able to be rotated relative to each other in the excited state. In these solvents, the free volume idea was found to provide a truthful report of the solvent viscosity-temperature behavior, and the probe torsional dynamics. Fluorescence lifetime imaging microscopy (FLIM) was used to insight and observed the distribution of lifetime of TO in the surface of both DESs. The contact angle was determined to show the hygroscopic nature of the DESs.

Molecular structure, DNA binding mode, photophysical properties and recommendations for use of SYBR Gold

SYBR Gold is a commonly used and particularly bright fluorescent DNA stain, however, its chemical structure is unknown and its binding mode to DNA remains controversial. Here, we solve the structure of SYBR Gold by NMR and mass spectrometry to be [2-[N-(3-dimethylaminopropyl)-N-propylamino]-4-[2,3-dihydro-3-methyl-(benzo-1,3-thiazol-2-yl)-methylidene]-1-phenyl-quinolinium] and determine its extinction coefficient. We quantitate SYBR Gold binding to DNA using two complementary approaches. First, we use single-molecule magnetic tweezers (MT) to determine the effects of SYBR Gold binding on DNA length and twist. The MT assay reveals systematic lengthening and unwinding of DNA by 19.1° ± 0.7° per molecule upon binding, consistent with intercalation, similar to the related dye SYBR Green I. We complement the MT data with spectroscopic characterization of SYBR Gold. The data are well described by a global binding model for dye concentrations ≤2.5 μM, with parameters that quantitatively agree with the MT results. The fluorescence increases linearly with the number of intercalated SYBR Gold molecules up to dye concentrations of ∼2.5 μM, where quenching and inner filter effects become relevant. In summary, we provide a mechanistic understanding of DNA-SYBR Gold interactions and present practical guidelines for optimal DNA detection and quantitative DNA sensing applications using SYBR Gold.

Self-assembled, optically-active {naphthalene diimide}U{cucurbit[8]uril} ensembles in an aqueous environment

Naphthalene diimides (NDIs) are shown to arrange spontaneously co-facially with cucurbit[8]uril (CB[8]) in an aqueous environment through purely non-covalent interactions. The resultant 2 : 2 supramolecular complex of NDI and CB[8] is highly fluorescent (>30 times more than the constituent NDIs) due to the formation of NDI-NDI excimers within the supramolecular complex.

Broad Applications of Thiazole Orange in Fluorescent Sensing of Biomolecules and Ions

Fluorescent sensing of biomolecules has served as a revolutionary tool for studying and better understanding various biological systems. Therefore, it has become increasingly important to identify fluorescent building blocks that can be easily converted into sensing probes, which can detect specific targets with increasing sensitivity and accuracy. Over the past 30 years, thiazole orange (TO) has garnered great attention due to its low fluorescence background signal and remarkable 'turn-on' fluorescence response, being controlled only by its intramolecular torsional movement. These features have led to the development of numerous molecular probes that apply TO in order to sense a variety of biomolecules and metal ions. Here, we highlight the tremendous progress made in the field of TO-based sensors and demonstrate the different strategies that have enabled TO to evolve into a versatile dye for monitoring a collection of biomolecules.

Supramolecular and suprabiomolecular photochemistry: a perspective overview

In this perspective review article, we have attempted to bring out the important current trends of research in the areas of supramolecular and suprabiomolecular photochemistry. Since the spans of the subject areas are very vast, it is impossible to cover all the aspects within the limited space of this review article. Nevertheless, efforts have been made to assimilate the basic understanding of how supramolecular interactions can significantly change the photophysical and other related physiochemical properties of chromophoric dyes and drugs, which have enormous academic and practical implications. We have discussed with reference to relevant chemical systems where supramolecularly assisted modulations in the properties of chromophoric dyes and drugs can be used or have already been used in different areas like sensing, dye/drug stabilization, drug delivery, functional materials, and aqueous dye laser systems. In supramolecular assemblies, along with their conventional photophysical properties, the acid-base properties of prototropic dyes, as well as the excited state prototautomerization and related proton transfer behavior of proton donor/acceptor dye molecules, are also largely modulated due to supramolecular interactions, which are often reflected very explicitly through changes in their absorption and fluorescence characteristics, providing us many useful insights into these chemical systems and bringing out intriguing applications of such changes in different applied areas. Another interesting research area in supramolecular photochemistry is the excitation energy transfer from the donor to acceptor moieties in self-assembled systems which have immense importance in light harvesting applications, mimicking natural photosynthetic systems. In this review article, we have discussed varieties of these aspects, highlighting their academic and applied implications. We have tried to emphasize the progress made so far and thus to bring out future research perspectives in the subject areas concerned, which are anticipated to find many useful applications in areas like sensors, catalysis, electronic devices, pharmaceuticals, drug formulations, nanomedicine, light harvesting, and smart materials.

Oligomeric Cucurbituril Complexes: from Peculiar Assemblies to Emerging Applications

Proteins are an endless source of inspiration. By carefully tuning the amino-acid sequence of proteins, nature made them evolve from primary to quaternary structures, a property specific to protein oligomers and often crucial to accomplish their function. On the other hand, the synthetic macrocycles cucurbiturils (CBs) have shown outstanding recognition properties in water, and a growing number of (host)_n :(guest)_n supramolecular polymers involving CBs have been reported. However, the burgeoning field of discrete (n:n) host:guest oligomers has just started to attract attention. While 2:2 complexes are the major oligomers, 3:3 and up to 6:6 oligomers have been described, some associated with emerging applications, specific to the (n:n) arrangements. Design rules to target (n:n) host:guest oligomers are proposed toward new advanced host:guest systems.

Conditional Copper-Catalyzed Azide-Alkyne Cycloaddition by Catalyst Encapsulation

Supramolecular encapsulation is known to alter chemical properties of guest molecules. We have applied this strategy of molecular encapsulation to temporally control the catalytic activity of a stable copper(I)–carbene catalyst. Encapsulation of the copper(I)–carbene catalyst by the supramolecular host cucurbit[7]uril (CB[7]) resulted in the complete inactivation of a copper‐catalyzed alkyne–azide cycloaddition (CuAAC) reaction. The addition of a chemical signal achieved the near instantaneous activation of the catalyst, by releasing the catalyst from the inhibited CB[7] catalyst complex. To broaden the scope of our on‐demand CuAAC reaction, we demonstrated the protein labeling of vinculin with the copper(I)–carbene catalyst, to inhibit its activity by encapsulation with CB[7] and to initiate labeling at any moment by adding a specific signal molecule. Ultimately, this strategy allows for temporal control over copper‐catalyzed click chemistry, on small molecules as well as protein targets.

Controlling the structure and photophysics of fluorophore dimers using multiple cucurbit[8]uril clampings

A modular strategy has been employed to develop a new class of fluorescent molecules, which generates discrete, dimeric stacked fluorophores upon complexation with multiple cucurbit[8]uril macrocycles. The multiple constraints result in a “static” complex (remaining as a single entity for more than 30 ms) and facilitate fluorophore coupling in the ground state, showing a significant bathochromic shift in absorption and emission. This modular design is surprisingly applicable and flexible and has been validated through an investigation of nine different fluorophore cores ranging in size, shape, and geometric variation of their clamping modules. All fluorescent dimers evaluated can be photo-excited to atypical excimer-like states with elongated excited lifetimes (up to 37 ns) and substantially high quantum yields (up to 1). This strategy offers a straightforward preparation of discrete fluorophore dimers, providing promising model systems with explicitly stable dimeric structures and tunable photophysical features, which can be utilized to study various intermolecular processes.

Dimerisation of a wide range of fluorophores through multiple CB[8] clampings leads to constrained intracomplex motion and distinct photophysical properties.

Supramolecular Assembly Induced Emission of Thiazole Orange with Sulfobutylether β-cyclodextrin: A Stimuli-Responsive Fluorescence Sensor for Tyramine

Modulation and control of stimuli responsive features of molecular assemblies in organized assemblies/cavitand macrocycles have received immense attention in many areas. In this study, we have established the formation of a discrete molecular assembly of thiazole orange (TO) dyes at the portals of the sulfobutylether β-cyclodextrin (SBE₇ βCD) macrocycle leading to the evolution of a strong and distinct emission band from aggregated TO. The supramolecular assembly promoted portal aggregation of TO in its 1 : 4 (SBE₇ βCD : TO) composition, characterized by absorption, fluorescence, circular dichroism, ITC and ¹ H NMR measurements, was probed to be selectively responsive to tyramine among other biogenic amines/neurotransmitters. For the first time, the different extent of emission quenching of SBE₇ βCD : TO assembly in the presence of biogenic amines/neurotransmitters is translated to achieve a selective on-off fluorescence sensor for the detection of tyramine against other neurotransmitters with a limit-of-detection (LOD) as low as ∼575 nM (79 ppb). The emission features of the assembly with changes in temperature is found to be highly reproducible even after several temperature cycles and is promising to design an optical supramolecular thermometer in the ambient temperature range.

A Cucurbit[8]uril 2:2 Complex with a Negative pKa Shift

A viologen derivative carrying a benzimidazole group (V-P-I ²⁺ ; viologen-phenylene-imidazole V-P-I) can be dimerized in water using cucurbit[8]uril (CB[8]) in the form of a 2:2 complex resulting in a negative shift of the guest pK_a , by more than 1 pH unit, contrasting with the positive pK_a shift usually observed for CB-based complexes. Whereas 2:2 complex protonation is unclear by NMR, silver cations have been used for probing the accessibility of the imidazole groups of the 2:2 complexes. The protonation capacity of the buried imidazole groups is reduced, suggesting that CB[8] could trigger proton release upon 2:2 complex formation. The addition of CB[8] to a solution containing V-P- I³⁺ indeed released protons as monitored by pH-metry and visualized by a coloured indicator. This property was used to induce a host/guest swapping, accompanied by a proton transfer, between V-P-I ³⁺ ⋅CB[7] and a CB[8] complex of 1-methyl-4-(4-pyridyl)pyridinium. The origin of this negative pK_a shift is proposed to stand in an ideal charge state, and in the position of the two pH-responsive fragments inside the two CB[8] which, alike residues engulfed in proteins, favour the deprotonated form of the guest molecules. Such proton release triggered by a recognition event is reminiscent of several biological processes and may open new avenues toward bioinspired enzyme mimics catalyzing proton transfer or chemical reactions.

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.

Ultra-Bright Rhodamines with Sulfobutylether-β-Cyclodextrin: A Viable Supramolecular Dye Laser in Aqueous Medium

Although aqueous dye lasers are much sought after, they have been of no practical use, as laser dyes show a strong tendency for aggregation in water, thus diminishing their optical output. Contributing towards this shortcoming, we studied the noncovalent interactions of two prominent laser dyes, namely, rhodamine 6G and rhodamine B, with a water soluble macrocyclic host, sulfobutylether-β-cyclodextrin (SBE₇ βCD). Spectral changes in the absorption and fluorescence behavior of dyes in presence of the SBE₇ βCD host indicated adequate complex formation between dye and host (K∼10⁴  M^-1 ). A combination of various photophysical parameters evaluated from measurements such as Job plot, changes in the fluorescence lifetime/anisotropy values, and favorable thermodynamic parameters from isothermal titration calorimetric measurements adjudicated a 1 : 1 stoichiometric complex formation between dye and SBE₇ βCD host. Consequently, SBE₇ βCD prevents dye aggregation/adsorption and present rhodamine dyes in their monomeric forms with enhanced fluorescence yield and brightness. These vital parameters were utilized to optimize and demonstrate cost-effective supramolecular broad-band and narrow-band aqueous dye laser systems with improved lasing efficiencies (∼25 % higher for the SBE₇ βCD : RhB system and ∼10 % higher for SBE₇ βCD : Rh6G system), better beam profile, and enhanced durability compared to the respective dyes in optically matched ethanol solutions.

A stimuli-responsive supramolecular assembly between inverted cucurbit[7]uril and hemicyanine dye

The effect of inverted curcurbit[7]uril (iQ[7]) on the binding mode of 2-(4-(dimethylamino)styryl)-1-methylpyridinium (DASPMI) was determined in this study.

Modulating Thiazole Orange Aggregation in Giant Lipid Vesicles: Photophysical Study Associated with FLIM and FCS

Thiazole orange (TO) exists mainly as a monomer in aqueous medium, where its fluorescence is negligibly small due to intramolecular movements. In the present study, it has been shown that in presence of giant unilamellar vesicles, produced from anionic lipid molecules, TO prefers to form H-dimer and H-aggregates at low lipid concentrations. The nonfluorescent form of TO (monomer) starts fluorescing in the aggregated or dimeric forms. At higher 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) concentration, the TO aggregates disintegrate to the monomeric variants. This is principally due to generation of more surface of residence for the TO molecules. The dye molecules/aggregates reside on the outer surface as well as percolate inside the lipid vesicles toward the inner water pool due to the presence of anionic charges at the interface. We adopted fluorescence lifetime imaging to find out the heterogeneity in photophysics of the different forms of TO inside the lipid vesicles supported by fluorescence correlation spectroscopy to characterize the formation or disintegration of the TO aggregates.

Modulation in the acidity constant of acridine dye with cucurbiturils: stimuli-responsive pKa tuning and dye relocation into live cells

Cucurbit[7/8]uril–acridine complexes result in striking modulation of the excited state properties of acridine and allowed a tunable upward pK_a shift, which is applied for controlled relocation of the dye from the host to live cells.

Metal ion-induced supramolecular pKa tuning and fluorescence regeneration of a p-sulfonatocalixarene encapsulated neutral red dye

Supramolecular pK_a shift and fluorescence quenching in a neutral red dye in the presence of p-sulfonatocalix[4/6]arenes have been demonstrated, which are relevant for the off–on switch, ion sensitive electrodes and drug delivery vehicles.

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.

Fluorescent supramolecular nanoparticles signal the loading of electrostatically charged cargo

Supramolecular nanoparticles (SNPs) become responsive to the loading of cargo by attaching a fluorescent dye to one of the building blocks. The SNPs shrink upon loading them with a positively charged cargo polymer. When using a dye-labeled cargo, FRET occurs between the SNP components and the cargo.

Metal-ion-mediated assemblies of thiazole orange with cucurbit[7]uril: a photophysical study

The formation of molecular superstructures by metal-ion-mediated noncovalent self-assembly has been demonstrated using the macrocycle, cucurbit[7]uril (CB7), and the dye, thiazole orange (TO), as building blocks. Interestingly, the association of these molecular building blocks can be tuned by the chemical environment, leading to self-assembled structures of different stoichiometries, which is supported by absorption, fluorescence, (1)H NMR, and AFM measurements. Most importantly, the self-assembly process of the CB7/TO/metal ion system is observed to be remarkably different for alkali (Na(+)) and alkaline earth (Ca(2+)) metal ions. Fluorescence enhancement is observed in the presence of Ca(2+) ions, which is attributed to the formation of short dimeric structures composed of two 1:1 CB7-TO complexes. Solution turbidity is detected in the presence of Na(+) ions, which is proposed to be due to the formation of extended structures by the assembly of many 1:1 CB7-TO complexes.

CB[8] gated photochromism of a diarylethene derivative containing thiazole orange groups

Photochromism in a diarylethene derivative (1O) can be gated by a host–guest interaction where the thiazole orange groups are bound into the hydrophobic cavity of CB[8] in water.

Targeting G-quadruplex structures with extrinsic fluorogenic dyes: promising fluorescence sensors

This article provides a brief account of the recent reports on the fluorescence properties of some of the fluorogenic dyes towards G-quadruplex DNAs, which have been turned into promising bio-analytical methods.

Triple emission from p-dimethylaminobenzonitrile-cucurbit[8]uril triggers the elusive excimer emission

The intriguing dual-emission behavior of p- dimethylaminobenzonitrile (DMABN) and the identity of the associated excited states is, arguably, the most extensively investigated and also controversially discussed molecule- specific phenomenon of modern photochemistry. We have now found a new, third fluorescence band when DMABN is encapsulated within the water-soluble molecular container cucurbit[8]uril (CB8). It is centered between the previously observed emissions and assigned to the elusive excimer emission from DMABN through 1:2 CB8:DMABN complex formation. Heating of the CB8⋅(DMABN)2 complex from 0 to 100 °C results in the dissociation of the ternary complex and restoration of the dual-emission properties of the monomer. Alternatively, monomer emission can be obtained by selecting cucurbit[7]uril (CB7), a host homologue that is too small to accommodate two DMABN molecules, or by introducing ethyl instead of methyl groups at the amino terminus of the aminobenzonitrile guest.

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.

A simple assay for quality binders to cucurbiturils

A new approach towards the rapid identification of quality binders to cucurbiturils--those that combine high affinity with high selectivity for a particular homologue--was developed. The assay exploits macrocycle-specific optical fingerprints (colorimetric or fluorimetric) of carefully selected indicators dyes. The screening of a guest library revealed known (e.g., adamantane derivatives) and new (e.g., terpenes) quality binders. The predictive power of the assay was underpinned by the modeling of the involved thermodynamic equilibria.

Stimuli-responsive supramolecular micellar assemblies of cetylpyridinium chloride with cucurbit[5/7]urils

This article demonstrates, for the first time, construction of novel cucurbituril (CB)-adorned supramolecular micellar assemblies of a cationic surfactant, cetylpyridinium chloride (CPC), through noncovalent host-guest interactions. The distinct cation receptor features and cavity dimensions of the CB5 and CB7 homologues assert that the macrocyclic hosts remain complexed with the CPC monomers and take part in the micelle formation, a unique observation in contrast to that of the classical host, β-cyclodextrin. The cooperative contributions of the CB macrocycles in the micelle formation have been documented by the photochemical, surface tension, conductivity, DOSY NMR, and SANS measurements. The contrasting downward and upward shifts in the cmc of the CPC surfactant, respectively, with CB5 and CB7 hosts provide a unique opportunity for the controlled tuning of the micellization region for CPC from 0.57 to 1.6 mM, by using a combination of the macrocyclic hosts. The article also establishes the reversible response of these soft supramolecular micellar structures to thermal-stimuli, which projects their utility for on-demand smart drug-delivery vehicles.