Photophysical and Quantum Chemical Studies on the Interactions of Oxazine-1 Dye with Cucurbituril Macrocycles

In house synthesized novel distyryl-BODIPY dye and polymer assembly as deep-red emitting probe for protamine detection

In this contribution, we designed and synthesized a deep-red emitting distyryl-BODIPY dye (dye 3) which is non-fluorescent in aqueous solution due to the formation of non-emissive aggregates. However, in presence of an amphiphilic polymer (polystyrene sulfonate, PSS), the aggregated dye molecules de-aggregate and form dye 3-PSS complex, which significantly modulates the optical features of the bound dye. Interestingly, the dye 3-PSS complex shows turn-on fluorescence response in deep-red region in presence of protamine (Pr) due to the formation of dye 3-PSS-Pr ternary complex. Such enhancement follows a linear trend in the dynamic range of 0-8.75 μM of Pr which has been utilized to determine Pr with limit of detection (LOD) of 15.04(±0.5) nM in phosphate buffer. Furthermore, excellent selectivity of the dye 3-PSS system towards Pr allows us to determine Pr even in complex biological matrix like 1% human serum. Thus, dye 3-PSS system can be applied as a very effective tool for the detection and quantification of Pr in deep-red region, overcoming several limitations encountered with the probes in the shorter wavelength region. This is the first report on BODIPY dye based supramolecular assembly for sensing and quantification of protamine.

An overview from simple host-guest systems to progressively complex supramolecular assemblies

Supramolecular chemistry involving macrocyclic hosts is a highly interdisciplinary and fast-growing research field in chemistry, biochemistry, and materials science. Host-guest based supramolecular assemblies, as constructed through non-covalent interactions, are highly dynamic in nature, and can be tuned easily using their responses to various external stimuli, providing a convenient approach to achieve excellent functional materials. Macrocyclic hosts, particularly cyclodextrins, cucurbit[n]urils, and calix[n]arenes, which have unique features like possessing hydrophobic cavities of different sizes, along with hydrophilic external surfaces, which are also amenable towards easy derivatizations, are versatile cavitands or host molecules to encapsulate diverse guest molecules to form stable host-guest complexes with many unique structures and properties. Interestingly, host-guest complexes possessing amphiphilic properties can easily lead to the formation of various advanced supramolecular assemblies, like pseudorotaxanes, rotaxanes, polyrotaxanes, supramolecular polymers, micelles, vesicles, supramolecular nanostructures, and so on. Moreover, these supramolecular assemblies, with varied morphologies and responsiveness towards external stimuli, have immense potential for applications in nanotechnology, materials science, biosensors, drug delivery, analytical chemistry and biomedical sciences. In this perspective, we present a stimulating overview, discussing simple host-guest systems to complex supramolecular assemblies in a systematic manner, aiming to encourage future researchers in this fascinating area of supramolecular chemistry to develop advanced supramolecular materials with superior functionalities, for their deployment in diverse applied areas.

Stimuli Responsive Confinement of a Molecular Rotor Based BODIPY Dye inside a Cucurbit[7]uril Nanocavity

Present study reports the interaction of a molecular rotor based BODIPY dye, 8-anilino-BODIPY (ABP), with a versatile macrocyclic molecule, cucurbit[7]uril (CB7), investigated through various techniques such as ground-state absorption, steady-state fluorescence, time-resolve emission, proton NMR, and quantum chemical studies. Although BODIPY dyes have widespread applications due to their intriguing photochemical properties, studies on their noncovalent interactions with different macrocyclic hosts, especially regarding their supramolecularly induced modulations in photophysical properties are very limited. The investigated BODIPY dye, especially its protonated ABPH⁺ form (pH ∼ 1), shows a large fluorescence enhancement on its interaction with the CB7 host, due to large reduction in the structural flexibility for the bound dye, causing a suppression in its nonradiative de-excitation process in the excited state. Unlike ABPH⁺, the neutral ABP form (pH ∼ 7) shows considerably weaker interaction with CB7. For ABPH⁺-CB7 system, observed photophysical results indicate formation of both 1:1 and 1:2 dye-to-host complexes. Plausible geometries of these complexes are obtained from quantum chemical studies which are substantiated nicely from ¹H NMR results. The response of the ABPH⁺-CB7 system toward changing temperature of the solution have also been investigated elaborately to understand the potential of the system in different stimuli-responsive sensor applications.

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.

Does the degree of substitution on the cyclodextrin hosts impact their affinity towards guest binding?

The degree of substitution on βCD rims by sulfobutylether groups significantly modulates the binding affinity of the SBE_nβCD hosts for the studied cationic guest molecule.

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.

Contrasting interactions of DNA-intercalating dye acridine orange with hydroxypropyl derivatives of β-cyclodextrin and γ-cyclodextrin hosts

The present study demonstrates contrasting binding interactions of acridine orange dye with HPβCD and HPγCD hosts, always illustrating fluoresence “turn on” in the case of HPβCD and showing an interesting fluorescence “off/on switching” in the case of the HPγCD host.

A styryl based fluorogenic probe with high affinity for a cyclodextrin derivative

A styryl-based fluorogenic near-IR probe registers a very high association constant with sulfobutylether substituted β-cyclodextrin host, having prospects as biological marker and improved pH and temperature sensor.

Unraveling multiple binding modes of acridine orange to DNA using a multispectroscopic approach

The interaction of acridine orange (AOH(+)) with calf thymus DNA (ct-DNA) under different dye-DNA conditions has been investigated in detail using multispectroscopic techniques, unraveling a number of hitherto unexplored intricacies of dye-DNA binding. The observed results intriguingly show contrasting binding features when low (2.4 μM) and significantly high (23 μM) dye concentrations are used. It is conclusively inferred from absorption, steady-state fluorescence, circular dichroism, fluorescence decay and anisotropy decay studies that at low [DNA] to [dye] ratio, especially with higher dye concentration, dimeric AOH(+) predominantly binds externally to DNA surfaces through electrostatic interactions. At sufficiently high [DNA] to [dye] ratios, however, the interaction intriguingly changes to monomeric AOH(+) bound to DNA, predominantly in the intercalative mode between DNA base pairs, with partly an electrostatic binding on DNA surfaces. With very low initial dye concentration, monomeric (AOH(+)) mostly binds to DNA through intercalative and electrostatic modes for most DNA to dye ratios. The present study demonstrates a systematic correlation of the striking changes in the photophysical properties of the dye upon multimode binding with DNA. The observed results are of great significance in understanding the fundamental insights of dye/drug binding to DNA hosts, of use in the design of effective therapeutic agents.

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.