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G-Quadruplex DNAs represent out-of-the-way nucleic acid conformations, frequently formed by guanine-rich sequences. They have emanated as cancer-associated targets for designed small molecules. The variation in the binding affinity of the synthesized compounds to duplex and quadruplex structures is an intriguing quest, solved by spectroscopic analysis. In this paper, we report the synthesis of a porphyrin-cyclodextrin conjugate, characterized by utilizing FT-IR, NMR, and mass spectrometry. Further, two benzimidazolylguanidines are synthesized which form host: guest complexes with the porphyrin-cyclodextrin conjugate. The structure of the complexes is optimized by analyzing their 2D ROESY spectra. The interactions of the host, guest, and the host: guest complexes with the duplex (calf thymus DNA) and quadruplex (kit22) nucleic acids are investigated employing UV-vis, fluorescence, circular dichroism, and DNA melting experiments. The calculated strengths of the compounds' binding with kit22 are in the order of 10

Although cyclodextrins have been extensively utilized in various branches of supramolecular chemistry due to their numerous attractive attributes, however, to achieve even advanced applications, they often need structural modification through substitutions of suitable functional groups at their rims. A systematic investigation on how the degree of substitution on the cyclodextrin rims affects the binding affinity for a given guest molecule has however rarely been reported, especially from the perspective of photophysical studies. Herein, we report the non-covalent interaction of a styryl based dye, LDS-798, with three different sulfobutylether beta cyclodextrin (SBEnβCD) derivatives bearing varying degrees of substitution (n), using ground state absorption, steady-state emission, excited-state lifetime and time-resolved fluorescence anisotropy measurements. The dye–host binding constant values indicate that the strength of the interaction between LDS-798 and SBEnβCD derivatives follows an increasing trend with an increasing number of tethered sulfobutylether substituents on the cyclodextrin rims, which is attributed to the gradual increase of the electrostatic interaction between the negatively charged sulfobutylether groups and the positively charged LDS-798. Excited state lifetime measurements and ionic strength dependent studies on the dye–SBEnβCD complexes further support the increased affinity between the dye and the host in the supramolecular complexes, with an increasing number of sulfobutylether substituents on the βCD rims. The obtained results suggest that the molecular recognition of LDS-798 with SBEnβCD derivatives can be tuned very effectively by varying the number of sulfobutylether substituents on the cyclodextrin rims. Considering that SBE7βCD is one of the FDA approved agents for drug formulations, the obtained results with other SBEnβCD hosts may be useful in designing selective drug delivery applications, drug formulations, and effective fluorescence on–off switches.

Interaction of acridine orange (AOH+) dye with water soluble anionic p-sulfonatocalix[n]arene (SCXn) hosts, SCX4 and SCX6, having different cavity dimensions, has been investigated using multispectroscopic techniques. Intriguing modulation in the photophysical properties of AOH+ upon interaction with SCXn hosts indicate the formation of different host-guest complexes at different regions of the host concentrations. At lower host concentrations, AOH+ undergoes SCXn assisted aggregation, causing a drastic reduction in fluorescence intensity. At higher host concentrations, aggregated-AOH+-SCXn complexes disintegrate and monomeric-AOH+-SCXn exo and inclusion complexes are eventually formed, leading to a huge fluorescence enhancement finally. Observed effects are more pronounced with SCX6 as compared to SCX4 host. Time-resolved fluorescence studies indicate that at very high host concentrations, there is also a diffusion-controlled dynamic quenching for both monomeric-AOH+-SCXn exo and inclusion complexes, caused by the free SCXn present in the solution, a phenomenon not reported before for such host-guest systems. The aggregated-AOH+-SCXn complexes at lower host concentration were employed to investigate displacement study using an antiviral drug, 1-adamantanamine (AD) and a neurotransmitter, acetylcholine (AcCh), as the competitive binders cum external stimuli, which resulted in a drastic recovery of the fluorescence reduced initially due to aggregation process. Though both the AOH+-SCXn systems act as efficient supramolecular assemblies in sensing AD and AcCh as the analytes through fluorescence "OFF-ON" mechanism, the effect is more pronounced for AOH+-SCX4 system as compared to AOH+-SCX6. SCXn induced interesting modulation in the photophysical properties of AOH+ and the stimulus responsive dye displacement observed for aggregated-AOH+-SCXn systems can expectedly find applications in fluorescence OFF-ON sensing, supramolecular functional materials and similar others.

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.

Understanding the kinetics and energetics of photoinduced electron transfer (PET) reactions in constrained media has attracted considerable research interest, as constrained media provide a handle to tune the microenvironments and consequently the mechanisms of PET reactions. In this study, PET reactions between excited 7-aminocoumarin acceptors and ground-state

Non-covalent interactions of different dyes/drugs with various biological agents, which lead to large modulations in photophysical properties, have attracted attention of many scientists over the decades due to their versatile applicability in different fields. Herein, we report the spectroscopic investigation of non-covalent interaction of a synthesized BODIPY-benzimidazole conjugate (BDZ) dye with bovine serum albumin (BSA), which results in distinctive modulations in the photophysical properties of the dye. The structural immobilization, and reduced propensity of forming the weakly emissive intramolecular charge transfer (ICT) state of the dye, inside the confined BSA cavity, results into a significant increase in fluorescence intensity, which is also supported by the increase in the excited-state lifetime and fluorescence anisotropy decay time of the dye. Molecular docking calculations and competitive binding experiments reveal that the BDZ dye binds selectively to site IB with a reasonably high binding constant Kb ˜ 1.38 × 104 M−1. The BDZ-BSA complex shows a notable decrease in fluorescence intensity in the presence of guanidinium hydrochloride, indicating the selective binding of the dye with the native form of the BSA protein as compared to its denatured form. Formation of BDZ-BSA complex, which emits in the biologically favorable red region, makes the studied system a probable choice to monitor the denatured form of the BSA protein, biological sensing study, and to understand the transport mechanism of similar kind of drugs and other metabolites with serum albumin protein.

During the past decade, carbon dots have emerged as a fascinating class of luminescent nanomaterials with versatile application potentials in bioimaging, labeling, photocatalysis and optoelectronics. Currently, intensive research is concentrated on understanding the intriguing optical properties of these promising materials and their utility as luminescence sensors. In this article, we describe the photoluminescence of carbon dots obtained from a bioresource (lemon juice) and from a small molecule precursor (glycerol), especially the quenching of their emission by nitrobenzene and Hg2+ ions, as representative cases. Stern-Volmer analysis using steady-state and time-resolved emission measurements, suggests the involvement of both transient quenching and dynamic quenching mechanisms in the interaction of the carbon dots with nitrobenzene. The radius of the quenching sphere is estimated to be slightly greater than the contact distances between the respective carbon dots and nitrobenzene, which is in reasonable agreement with the "sphere of action" model for transient quenching. In the interaction with Hg2+ ions, electrostatic attraction plays a major role, and the quenching mechanism involves predominantly static and dynamic quenching. The static quenching constant matches well with the binding constant of the carbon dots with the metal ion.

Acridine orange, a well known DNA intercalating dye, exists in two prototropic forms, the cationic form AOH+ and neutral form AO. Systematic investigations on the interactions of AO and AOH+ with two homologous cyclodextrin derivatives, hydroxypropyl-β-cyclodextrin (HPβCD) and hydroxypropyl-γ-cyclodextrin (HPγCD), having different cavity sizes, reveal interesting modulations in the photophysical and acid–base properties of the dye. Ground state absorption, steady-state fluorescence, circular dichroism and time-resolved fluorescence studies have documented quite contrasting changes in the photophysical properties of both AO and AOH+ in their interaction with HPγCD as compared to HPβCD. Results indicate that while 1 : 1 inclusion complexes are only formed using HPβCD, leading to significant fluorescence enhancement for both AO and AOH+, the use of HPγCD on the other hand assists dimerization of both AO and AOH+ at lower HPγCD concentration, resulting in significant fluorescence quenching initially, and subsequently there is a clear conversion of the dimeric dye–host complexes to 1 : 1 dye to host inclusion complexes at higher HPγCD concentration, causing a significant enhancement in fluorescence intensity. To the best of our knowledge this is the first report to document sequential fluorescence quenching and enhancement for both neutral and protonated forms of a dye with gradually increasing host concentration. Both HPβCD and HPγCD hosts cause a significant downward pKa shift for acridine orange dye and the results are in accordance with the stronger binding of the neutral AO form with the studied hosts as compared to the cationic AOH+ form of the dye.

This study explores the intriguing modulations in the excitation wavelength dependence of carbon dot photoluminescence (PL), induced by the solvent medium. Our results indicate that different emissive states of carbon dots are stabilized to different extents by the surrounding solvent environment. Consequently, in some solvents, such as ethyl acetate and acetonitrile, the PL of the carbon dots is strongly dependent on the excitation wavelength, while in other solvents, like water, the PL of the same carbon dot becomes independent of the excitation wavelength. These observations contribute to the enhancement of our understanding of the photophysics and PL mechanisms of this important class of luminescent materials, especially to discriminate between the PL arising from the "molecular state" and the "surface state".

The interaction of cinnamic acid (CA) and its analogues viz. p-coumaric acid (PCA) and caffeic acid (CFA) with core-shell micelles of a moderately hydrophobic Pluronic® P123 has been investigated using cloud point (CP), viscosity, dynamic light scattering (DLS), small-angle neutron scattering (SANS) and steady-state fluorescence measurements. These solubilizates alter micellar behaviour of copolymer solution dependant on their hydrophobicity. P123 micelles exhibit time-dependent restructuring and responsive growth processes at different rates, depending on pH and in the presence of additives. Thus, restructuring and growth of P123 micelles can be easily tuned up to a substantial extent just by changing the concentration of the additives and the pH of the solution. Considering the medical applications of cinnamic acid and its analogues and considering use of Pluronics® in drug delivery systems, the present study can provide important insight of the possible time-dependant delivery mechanism of drugs.

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.

Although cyclodextrins have been extensively utilized in various branches of supramolecular chemistry due to their numerous attractive attributes, however, to achieve even advanced applications, they often need structural modification through substitutions of suitable functional groups at their rims. A systematic investigation on how the degree of substitution on the cyclodextrin rims affects the binding affinity for a given guest molecule has however rarely been reported, especially from the perspective of photophysical studies. Herein, we report the non-covalent interaction of a styryl based dye, LDS-798, with three different sulfobutylether beta cyclodextrin (SBE

This study investigates the role of varying alkyl chain lengths of a series of surface-active 1-alkyl-3-methylimidazolium tetrafluoroborate ([CnMIm][BF4], n = 4, 6, and 10) ionic liquids (ILs) as cosurfactants in modifying the micellar characteristics of a tetronic star-block copolymer, T1304, and the consequent effects on bimolecular photoinduced electron transfer (PET) reactions carried out in these T1304–IL mixed micellar systems. Using coumarin 153 as the probe dye and following ground-state absorption, steady-state fluorescence, and time-resolved emission measurements, the micropolarity, microviscosity, and solvent relaxation dynamics in the micellar palisade layer have been revealed both in pure T1304 and in T1304–IL systems. With increasing alkyl chain length of the ILs, the palisade layer of the micelles gradually becomes more polar and less viscous, suggesting better incorporation of the longer alkyl chain length ILs as cosurfactants into the T1304 micelles. The bimolecular PET reactions, involvi...

Micellar aggregates formed in aqueous solution of Gemini surfactant: N, N′-didodecyl-N,N, N′,N′ tetramethyl-N,N′-ethanediyldiammonium dibromide (12-2-12), in presence of organic additive: Phenyl Propiolic acid (PPA), has been systematically characterized by rheology, 1H-NMR, fluorescence spectroscopy, dynamic light scattering (DLS), small angle neutron scattering (SANS), transmission electron microscopy (TEM), and contact angle (CA) techniques as a function of temperature. 12-2-12 undergo micellar transition from elongated ellipsoidal micelles to unilamellar vesicles (ULV) upon addition of varying amount of PPA as a function of temperature which was inferred by scattering and microscopy results. The quantification of the geometrical parameters of 12-2-12 micelles was achieved by a complete fit of the obtained SANS data using prolate ellipsoidal form factor and the structure factor model thereby confirming the presence of elongated ellipsoidal micelles and ULV. Variation of the viscosity with the shear rate for the ratio 50 mM 12-2-12: 10 mM PPA exhibited the typical viscoelastic characteristics which are further investigated by dynamic rheological measurements. 1H-NMR study indicated the substantial change in the chemical shifts of the protons for 12-2-12 with the addition of PPA, thereby inferring the significant interaction between the two components leading to such micellar transition. Furthermore, DLS and Fluorescence study suggested the formation, aggregation and collapse procedure of the network structures of the 12-2-12+PPA system, which either resulted in increase or decrease of viscosity as a function of PPA concentration and temperature. Such observation is evidently complimented by TEM findings and CA study. The structural dynamics of these aggregates corresponding to the interaction between them are well correlated with quantum simulation approach using Density Functional Theory (DFT)/B3LYP with the basis set 3-21G method.

This study explores the excited state prototropic behavior of the fluorophore, 7-hydroxy-4-methylcoumarin (7H4MC), in the [Cnmim][BF4] (n = 2, 4, 6, 8, 10) series of ionic liquid (IL)–water mixtures at low water contents. In pure IL media, 7H4MC exists in the neutral form in both ground and excited states. However, on addition of water to the ILs, the excited neutral form of the dye is gradually converted to the anionic and the tautomeric species, leading to characteristic changes in the emission spectra. The similarity in the spectral features of 7H4MC in the IL–water system with that in a conventional reverse micelle system rather than with organic solvent–water mixtures, suggests that in the presence of water, the ILs are organized into reverse micelle-like structures with the consequent formation of confined water pockets. The results further suggest that formation of water nanoclusters and the ensuing changes in excited state prototropic behavior of the dye, is facilitated by increase in the alkyl chain length of the IL cation. These propositions are supported by time-resolved fluorescence studies. To the best of our knowledge this is the first report on proton transfer reaction in IL–water mixtures at low water contents. Considering that ILs are useful as solvents and surfactants, and IL–water mixtures in particular have applications in chemical extractions and biocatalysis, an understanding of the structural organization and water pool formation in these systems is quite important. The insights obtained from the prototropic transformations of 7H4MC are significant not only for fundamental self-assembly studies, but also for the development of ILs as chemical reaction media.

The unidirectional proton coupled electron transfer (PCET) from the excited state of Ru(II) imidazole phenanthroline complex [Ru(bpy)2 ipH]2+ to 1,4-benzoquinone, was studied by steady-state (SS) and time-resolved (TR) fluorescence and transient absorption (TA) measurements. The pKa (9.7) and pKa * (8.6) values of the complex suggest that it behaves as a photoacid on excitation. The difference in the quenching rates obtained from SS and TR fluorescence studies indicate participation of both dynamic quenching and static quenching involving the hydrogen bonded ipH ligand of [Ru(bpy)2 ipH]2+ with the 1,4-benzoquinone quencher, formed in the ground state. Within the hydrogen bonded complex, the ruthenium centre acts as the electron donor, while the ipH ligand acts as the proton donor to the hydrogen bonded 1,4-benzoquinone that acts simultaneously both as the electron and proton acceptor. It is proposed that the static quenching in the hydrogen bonded [Ru(bpy)2 ipH]2+ -1,4-benzoquinone pairs occurs involving the PCET mechanism, while the dynamic quenching occurs through the simple ET mechanism, on diffusional encounter of the isolated 1,4-benzoquinone with the excited [Ru(bpy)2 ipH]2+ complex. The occurrence of broad TA bands around 420-430 nm suggests formation of both 1,4-benzoquinone radical anion as well as the 1,4-benzosemiquinone radical by the interaction of excited [Ru(bpy)2 ipH]2+ with 1,4-benzoquinone, thus supporting the ET process in the studied system.

Supramolecularly assisted modulations in the photophysical properties of the well known and practised antibiotic drug ciprofloxacin (Cpf) with versatile macrocyclic cavitand molecule cucurbit[7]uril (CB7) has been explored under different pH conditions. Both the cationic (Cpf-H+) and neutral (Cpf) forms of the drug exhibits strong binding interaction with CB7. Ground state absorption, steady-state fluorescence, time-resolved emission and fluorescence anisotropy decay measurements documented the host assisted intriguing changes in the photophysical properties of the drug, occurred through the successive formations of the 1:1 and 1:2 (drug to host) inclusion complexes, and thereby causing the intriguing small initial enhancement in fluorescence intensity at lower host concentrations and subsequently leading to a strong fluorescence quenching at higher CB7 concentration. Inferences drawn from the photophysical studies are nicely supported by the results obtained from the 1H NMR measurements. Investigations on the photodegradation studies involving the Cpf-H+ and Cpf forms of the drug, both in the absence and in the presence of CB7 host, revealed that there is a large enhancement in the photostability for both the prototropic forms of the drug, by about 3 times, on their inclusion complex formation with the CB7 host. Observed results from the supramolecular interactions of the ciprofloxacin drug with CB7 macrocycle are suggested to have implications in improving drug stability and in the possible deployments in drug formulations and drug delivery systems.

Excited state intramolecular proton transfer (ESIPT) in 18-dihydroxy-9,10-anthraquinone (18DHAQ) dye has been investigated in two series of 1-alkyl-3-methylimidazolium ([C n mIm] + ) based ionic liquid (IL) solvents, [C n mIm][NTf 2 ] and [C n mIm][BF 4 ], with n = 2, 4, 6, 8 and 10, using steady-state (SS) and time-resolved (TR) fluorescence studies. In both the IL series, fluorescence intensity for tautomer (T*) form gradually decreases relative to normal (N*) form with increasing n value for [C n mIm] + cations. Observed results suggest microstructure formation and its consequent effect on the ESIPT process of the dye in these IL solvents. Since more extensive microstructures are likely with larger n values of [C n mIm] + cations, the dye is expectedly solubilized more in the polar microdomains of the solvent microstructures, resulting better stabilization for the more dipolar N* state and hence a higher emission intensity from this state. This proposition is clearly supported by ultrafast (sub-picosecond) fluorescence kinetics for both N* and T* states, though sub-nanosecond TR results indicate very similar fluoresce decays for both N* and T*, suggesting an eventual kinetic equilibrium between two states subsequent to the initial ultrafast and fast forward and backward ESIPT processes. Small differences between the results in the [C n mIm][NTf 2 ] and [C n mIm][BF 4 ] series of ILs are attributed to dissimilar size, shape and basicity of [NTf 2 ]− and [BF 4 ]− anions, responsible for some characteristic changes in the microstructures formed in the respective solvent series. To the best of our knowledge, present study is the only report demonstrating modulations in the ESIPT process through microstructure formations in neat [C n mIm] + based IL solvents.

The presence of excess of denaturants like urea unfolds proteins and disfavours micellization of surfactants; the mode of interaction of urea has always been a matter of debate. Inspired by this need, the effect of urea on the monolayer and micellar characteristics of three polyethylene oxide/propylene oxide (PEO/PPO) starblock copolymers Tetronics ® viz. T1301, T1304 and T1307 (with almost same molecular weight of PPO but varying% PEO as 10, 40 and 70%, respectively) in aqueous urea solutions was investigated through scattering/spectral technique and Langmuir film balance. The three copolymers showed markedly different behaviour. In the absence of urea, the most hydrophobic T1301 forms vesicular structures (few hundred nm), T1304 with moderate hydrophobicity forms spherical micelles (∼10–20 nm) with low polydispersity while very hydrophilic T1307 exists as a highly polydispersed system with unimers (∼3–5 nm), loose aggregates (∼10–20 nm) and clusters (with few hundred nm size). However, the addition of urea (0–8 M) to these copolymer solutions leads to the disruption of any loose aggregates formed which is probably due to its binding with the copolymer chains that enhances the solubility of Tetronics ® and overall polydispersity of the copolymer solutions. In support to our hypothesis, the binding of urea was validated using fluorescence measurements/Langmuir film balance and it was established that urea follows the direct mechanism (as proposed in earlier reports) and the intercalation of urea between the PEO chains leads to the extended conformations as observed by Langmuir film balance. Although, urea had a common denaturing effect on all the three copolymers, the hydrating effect of urea was most evident for copolymers with more hydrophilic character.

This study explores the interesting effect of p-sulfonatocalix[n]arene hosts (SCXn) on the excited-state tautomeric equilibrium of Chrysazine (CZ), a model antitumour drug molecule. Detailed photophysical investigations reveal that conversion of CZ from its more dipolar, excited normal form (N*) to the less dipolar, tautomeric form (T*) is hindered in SCXn-CZ host-guest complexes, which is quite unexpected considering the nonpolar cavity of the hosts. The atypical effect of SCXn is proposed to arise due to the partial inclusion or external binding of CZ with the hosts, which facilitates H-bonding interactions between CZ and the sulfonate groups present at the portals of the hosts. The intermolecular H-bonding subsequently leads to weakening of the pre-existing intramolecular H-bond network within CZ, and thus hinders the tautomerizaion process. Our results suggest that rather than the binding affinity, it is the orientation of CZ in the SCXn-CZ complexes, and its proximity to the portals of the host that plays a predominant role in influencing the tautomeric equilibrium. These observations are supported by quantum chemical calculations. Thermodynamic studies validate that SCXn-CZ interaction is essentially enthalpy driven and accompanied by small entropy loss, which is consistent with the binding mechanisms.

The photophysics of a newly synthesized donor-acceptor substituted chromophore, 9-cyano-10-(1H-pyrrol-1-yl)anthracene (CPAN), reveals interesting features that suggest the involvement of two types of charge transfer configurations in its excited state. Dual emission bands are observed in polar and nonpolar solvents due to fluorescence from both locally excited (LE) and intramolecular charge transfer (ICT) states. From detailed analysis of the time-resolved fluorescence data, it is proposed that by increasing the polarity of the medium the nature of the emissive ICT state is switched from a molecular conformation that is more planar (ICT1) to one that is more twisted (ICT2), compared to the ground state. Preliminary geometry optimizationsand calculated molecular parameters of CPAN in the ground and excited states, lend support to the experimental results. Qualitative potential energy diagrams are presented to substantiate the proposed model and rationalize the observed photophysics.

Intriguing changes in photophysical and acid-base properties of fluorogenic antimalarial drug, quinine (QN), are presented on its interaction with macrocyclic cucurbit[7]uril (CB7)host. While dicationic QNH22+ form shows large enhancement in both steady state and time-resolved fluorescence, monocationic QNH+ and neutral QN forms show significant reductions. The pKa(1) and pKa(2) values of the drug undergo ∼1 and ∼1.7 units of upward shifts, respectively, on its binding to CB7 host, substantiating the differential binding strengths of various prototropic forms. Whereas the QNH22+ and QNH+ forms undergo 1:2 (dye to CB7) inclusion complex formation, the neutral QN is suggested to form 1:1 inclusion complex with CB7.Interestingly, the observed results in the present study are substantially different and in some cases quite contrasting to the results reported in a recently published paper (Phys. Chem. Chem. Phys., 2016, 18, 30520). In the present study we could unequivocally establish that there were many serious inconsistencies in the above paper, which we could now convincingly rectify, designing appropriate experimental and sample conditions. With the intriguing modulations observed in the photophysical and acid-base properties, the quinine-CB7 system is suggested to act as an improved fluorimetric standard than the free quinine system. Additionally, the modulations have implications in drug stabilization, drug delivery, sensors and other applications.

Remarkable and systematic pH-dependent changes are observed in the absorption and emission spectra of carbon dots derived for the first time from lemon juice, a natural bioresource. Detailed photophysical studies of these novel carbon dots (henceforth termed LD), in conjunction with Fourier transform infrared spectra, reveal that among the two possible prototropic equilibria, phenol ↔ phenolate and carboxylic ↔ carboxylate, that occur at the surface of LD, it is the former that is actually coupled with the emissive moiety and directly involved in determining the nature of the electronic energy levels and the associated optical transitions. Apart from providing valuable mechanistic insights on the photoluminescence (PL) of carbon dots, the pH dependence of LD is also demonstrated to yield variable PL signals and perform elementary Boolean logic operations in response to chemical stimulants. The pH effect can therefore complement the optoelectronic functionalities of these promising luminescent nanomaterials and help in the future development of molecular devices and intelligent multianalyte detection systems.

Ethylene oxide-propylene oxide (EO-PO) block copolymer micelles are useful potential nanoreservoirs for the delivery of hydrophobic drugs. Considering that glycine is an excipient and can favorably affect the surface/micellar behavior and thus improve solubilization power/dispersion stability/wetting characteristics we have reported here studies on aqueous solution behavior of two commercially available branched block copolymers (Tetronics®) with differing hydrophobicities namely Tetronics® 1307 and 1304, hereafter referred as T1307 and T1304, in the presence of glycine. Steady state fluorescence studies using pyrene as a probe, equilibrium and dynamic surface tension measurements, wetting and dispersion stability studies of Teflon (polytetrafluoroethylene; PTFE) and solubilization studies of a hydrophobic antioxidant/anticancer drug quercetin (QN) have been examined. The cloud point (CP) and critical micelle temperature (CMT) decrease while micelle hydrodynamic size (Dh) increases with the addition of glycine as well as on loading of the drug in the micelles. Water penetration through packed PTFE powder and dynamic surface tension confirm the enhanced micellization process for aqueous Tetronic® solutions in presence of glycine and accordingly restricted diffusion for the surfactant molecules towards air-water and PTFE-water interface. The contact angles for Tetronic® solutions in presence of glycine indicate moderate decrease. The pressure-area curves of the copolymers in water and glycine solutions were also constructed. Surface and micellar properties of copolymers are markedly altered in the presence of glycine and can be tuned for use of these nanocarriers in delivery systems.

Nanoparticles (NPs) have promising applications in catalysis, sensing, theranostics, plasmonics and other areas. Since bare NPs undergo spontaneous agglomeration, which diminishes their activity, they need to be stabilized by suitable capping agents to maintain their dispersion in water. Capping agents that bind with NPs involving reasonably mild interactions are preferred because they allow target molecules to access NP surfaces quite easily, resulting in the most desired effects. Cucurbit[n]uril (CB[n]) macrocycles have been identified as soft capping agents, as they bind to NP surfaces using simple noncovalent interactions involving their polarizable carbonyl portals. Moreover, CB[n] hosts help in bringing out additional functionalities of the NPs. Extensive studies have been reported on various CB[n]-assisted NP systems, exploring their potential in diverse applications. In this chapter, our effort has been to summarize some of the important results reported in the literature, discussing mainly CB[n]-assisted metal nanoparticles and some inorganic nanomaterial systems.

Supramolecular host–guest pairs, with a very high association constant, represent excellent molecular recognition motifs, and serve as useful building blocks for numerous exciting supramolecular functional materials. Cyclodextrins, an important class of glucopyranose-based hosts, generally, suffer from low affinity for most guest molecules (102–104 M−1). Herein, we report a styryl-based fluorogenic probe which registers a very high association constant of 7.84 × 105 M−1 with sulfobutylether substituted β-cyclodextrin in contrast to a low association constant of 1.89 × 102 M−1 with unmodified β-cyclodextrin (β-CD). This exceptionally high binding affinity of the fluorogenic probe with the sulfobutylether substituted β-CD has been attributed to the strong electrostatic interactions between the cationic guest and the polyanionic host along with improved hydrophobic interactions due to the extended butylether groups present on the rims of the cyclodextrin cavity. The significant modulations in the photophysical properties of guest molecule, upon interaction with host molecule, have been investigated, in detail, using ground-state absorption, steady-state fluorescence and time-resolved emission measurements. The effect of temperature and ionic strength of the medium have been employed to investigate the nature of the underlying interactions in the present host–guest system. The possibility of indicator displacement assay involving the present host–guest system has also been demonstrated using a competitive binder.

Interaction of an anionic diazo dye, Congo red (CR), with conventional surfactants: cetyltrimethylammonium bromide (CTAB) and Gemini surfactant: N,N'-dihexadecyl-N,N,N',N'-tetramethyl-N,N'-ethanediyl-diammonium dibromide (16-2-16), in their pre-micellar and post-micellar concentration regions has been investigated using conductometry, surface tensiometry, UV-visible spectroscopy, fluorescence spectroscopy, dynamic light scattering (DLS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV) techniques. Various interfacial, micellar, band gap and electrochemical parameters are estimated at 303.15 K. The observed results are explained in terms of critical micelle concentration (CMC) of the individual surfactants and their ability to undergo aggregation in the presence of dianionic CR. It is found that the CMC of both the surfactants decreases in the presence of CR, which is attributed to the electrostatic attraction, short-range π-π stacking of CR molecules, and the hydrophobic forces operating between CR and the surfactant molecules. The spectral results for CR showed a hypsochromic (blue) shift below the CMC for both the surfactants, which indicates the resultant CR-surfactant complexation through ion-pair formation, which further inferred H-aggregation of CR in the pre-micellar region. Above the CMC, CR reverts back to its monomeric state and gets bound to the micelle. Job's method is employed to determine the stoichiometric ratio between CR and the individual surfactant. The size distribution of surfactants in the presence of CR is determined by DLS. The CV and LSV measurements were performed in the absence and presence of a varying concentration of surfactants to support the spectral findings. Diffusion coefficient (D) calculated by LSV confirmed the successive interaction between CR and surfactants. Furthermore, the CR-surfactant interaction is also elucidated by the computational simulation using the Gauss View 5.0.9 package to validate the obtained experimental findings.

Common milk allergens, such as, β-lactoglobulin (β-LG), poses a major health concern for public, particularly in children. Thus, sensitive and selective methods for detection of common milk allergens are highly warranted. To the best of our knowledge, detection and quantification of β-lactoglobulin, a common milk allergen, by fluorescence based technique has not been reported till date. Herein, we present the first ever fluorescence based report for the selective detection and quantification of milk allergen, β-LG, using LDS-798 as a fluorogenic probe, which registers an exceptionally large fluorescence enhancement of ∼2900 fold in the far-red region upon binding with β-LG with LOD of ∼6.3 (±0.3) nM for β-LG in aqueous buffer solution and 0.67 (±0.04) μM in 5% bovine milk matrix respectively. High sensitivity and selectivity, fast response, high repeatability and reproducibility, easy operation, along with emission in the technologically advantageous far-red region make the fluorometric quantification of β-LG, using LDS-798 as a fluorogenic probe, far more advantageous than the other techniques, generally used for β-LG quantification. Importantly, we have demonstrated the applicability of this probe for quantification of β-LG in real milk samples which has been validated by an established method.

Stimuli-responsive supramolecular host assisted guest aggregation has attracted tremendous attention, of late, because of its various technological applications. However, a large majority of them involve anionic supramolecular hosts such as p-sulfonato calixarenes, sulfated cyclodextrin and so on, in combination with cationic guests. On the other hand, the phenomenon of host-assisted aggregation of anionic guests with cationic supramolecular hosts, which can significantly broaden the scope of this exciting field, has rarely been reported in the literature. In the present work, we report, host assisted aggregation of an anionic organic probe molecule, pyrene tetrasulfonate (PTS), in the presence of a cationic cyclodextrin derivative, amino-β-CD (AβCD), which has been studied, in detail, using various spectroscopic techniques including UV–Vis spectroscopy, fluorescence spectroscopy, time-resolved spectroscopy, and molecular docking calculations. The effect of ionic strength of the medium on the photophysical properties of PTS-AβCD complex indicates that electrostatic interaction is the predominant force in assisting the formation of PTS-AβCD complex. The monomer-aggregate equilibrium of PTS in the present system is also found to be extremely sensitive towards external stimuli like temperature, and pH of the medium. Finally, the PTS-AβCD assembly has been used for sensing an important charged bio-analyte, adenosine triphosphate (ATP), demonstrating the applicability of the present host-guest system.

In this article our aim is to review the supramolecularly assisted modulations in the properties of the organic chromophoric dyes, on their interactions with the macrocyclic host molecules, focusing on the possible uses of such modulated dye properties in different applications. We restrict ourselves on the modulations of two important properties of the chromophoric dyes, namely the fluorescence characteristics and the prototropic behavior, the properties that usually undergo large changes through host–guest inclusion complex formation of the dyes with the macrocyclic host molecules and show great prospects for their applications in diverse areas like sensors, catalysis, functional materials, electronic devices, pharmaceuticals, drug formulations, drug delivery, nanomedicines, and many others. To restrain the length of the article, our discussion has also been restricted to only two types of macrocyclic molecules, namely cyclodextrin (CD) and cucurbit[n]uril (CBn) hosts, that are realized to be most promising cavitand molecules with regard to their influence in modulating the dye properties leading to their applications in the aforementioned areas. We have considered suitable examples from the dye–CD and dye–CBn systems as reported in the literature by different research groups including ours to substantiate the discussions on different aspects of the macrocyclic hosts and those on the modulations of the fluorescence and acid–base properties of the organic chromophoric dyes, bringing out their possible applications in different areas, as have been the main objectives of the present review. We strongly feel that this review article will showcase the titled theme, i.e. the supramolecularly assisted modulations in the properties of the chromophoric dyes, which can lead to different useful applications of the supramolecular host–guest systems, highlighting the immense prospects of such studies to explore the versatility of the supramolecular host–guest assemblies for their applications for the benefit of the mankind.

This study reveals the intriguing photophysics of lumichrome (LC) in two types of organized assemblies, namely, normal micelles and reverse micelles, formed by the cationic surfactant, benzyldimethylhexadecylammonium chloride (BHDC). Although LC exists in alloxazine form at around neutral pH in aqueous solutions, in BHDC micelles and reverse micelles under similar pH conditions, LC undergoes selective deprotonation at its N-1 position to generate LC anion in the tautomerically stabilized isoalloxazine form (N10 anion). Considering that deprotonation at both N-1 and N-3 positions of LC are equally feasible (similar pKa) in water at high pH, the preferential formation of N10 anion rather than N3 anion is a unique phenomenon realized in BHDC organized assemblies. This selective prototropism occurs due to combined electrostatic effects of the cationic head groups of BHDC and steric constraints that determine the orientation of LC at the micellar interface. The study also reveals that despite being formed by the same surfactant, BHDC micelles and reverse micelles possess different interfacial structures and hydration characteristics that induce distinct photophysical changes of LC in the two assemblies.

Here we report comprehensive analysis of the influence of ionic liquids (ILs) on the physicochemical properties of an ethylene oxide–propylene oxide (EO–PO) star shaped block copolymer, Tetronic®1304 (total mol. wt = 10 500 and %PEO = 40%) hereafter referred to as T1304, by employing cloud point (CP), viscosity, dynamic light scattering (DLS), small-angle neutron scattering (SANS), high sensitivity differential scanning calorimetry (HSDSC) and fluorescence measurements and 1H NMR spectroscopy. 1-Alkyl-3-methylimidazolium based ILs varying in alkyl chain length (C4–C10) and anion, namely chloride, tetrafluoroborate, trifluoromethanesulfonate and hexafluorophosphate, were used. At lower concentrations, the ILs with different anions showed identical effects on the T1304 micelles but a significant change was noticed at higher concentrations. ILs with longer alkyl chains formed smaller mixed micelles while those with shorter chains remained in the bulk. Moreover, an increase in the alkyl chain length significantly increased the CP while the apparent hydrodynamic diameter (Dh) of the micelles decreased. In line with this, the variation in alkyl chain length has no significant effect on the CMT up to C8mimCl but decreases efficiently for C10mimCl. On the other hand, the variation in counter-ions seldom influenced the CP/Dh/CMT of the T1304 solutions at lower concentrations of the IL, but showed sufficient change at higher concentrations. The presence of NaCl suppressed the effect of the IL on the T1304 micelles. The microstructural changes as observed for the T1304 micelles in the presence of different ILs are explained using SANS data. Fluorescence studies with coumarin-481 as a probe showed shifts in the emission spectra and enhancement in the fluorescence decay, suggesting incorporation of the ILs in the micelles. The penetration of ILs with longer alkyl chains in the T1304 micelles was further confirmed using 1H NMR spectroscopy. The present study provides valuable information on tuning the solution behaviour of T1304 with ILs that may prove to be beneficial for different industrial applications.

The micellar characteristics of an aqueous solution of the polyoxyethylene based alkyl aryl type non-ionic surfactant Triton X-100 (TX-100) in the presence of several 1-alkyl-3-methylimidazolium (Cnmim+) based non-amphiphilic, amphiphilic and biamphiphilic ionic liquids (ILs) varying in the alkyl chain length of the cation (C4–C10) and different anions viz. chloride [Cl−], tetrafluoroborate [BF4−], trifluoromethanesulfonate [CF3SO3−], hexaflurophosphate [PF6−] and octylsuphate [C8SO4−] have been systematically investigated by cloud point (CP), viscosity, dynamic light scattering (DLS), nuclear magnetic resonance (NMR), fluorescence and small-angle neutron scattering (SANS) measurements. These ILs interact with TX-100 micelles on account of their hydrophobicity; ILs with a hydrophilic counter ion (Cl−) favour demicellization, increase CP, decrease the solution viscosity and apparent hydrodynamic diameter (Dh) of micelle, whereas ILs with a hydrophobic counter ion (PF6−) increase the solution viscosity and exhibit micellar growth. SANS data on the size/shape of TX-100 micelles in the presence of different ILs are reported to provide information on microstructural changes in the micelles. The location of ILs, which has a dominating effect on the structure of the micelles, was evaluated using fluorescence and 1H NMR studies, and these results are discussed in terms of the insertion of the ILs in TX-100 micelles. IL molecules with long alkyl chains penetrate deeper in the TX-100 micelles. The present study provides useful information on tuning the solution characteristics of TX-100 in the presence of ILs that may have industrial applications.

Understanding the kinetics and energetics of photoinduced electron transfer (PET) reactions is of considerable research interest in photochemical sciences. In this study, interactions between trivalent lanthanide ions (Ln(III)) as electron acceptors and coumarin dyes as electron donors have been investigated in aqueous media using ground state absorption, steady-state (SS) emission, time-resolved (TR) fluorescence and laser flash photolysis (LFP) techniques. Among different Ln(III) ions, only Eu(III), Yb(III) and Sm(III) are found to display substantial interaction with the excited singlet (S1) state of the coumarin dyes, which correspond well with the favorable free energy changes (negative ΔG0) associated with the PET reactions in these coumarin-Ln(III) systems. For other Ln(III) ions, as ΔG0 is not favorable, they do not show any interaction with the excited coumarin dyes. Among the interacting coumarin-Ln(III) systems, bimolecular quenching rate constant (kq) increases consistently with the increasing exergonicity (−ΔG0), when each of the Ln(III) ions are considered independently, except for Eu(III), for which the kq values have already reached the diffusion controlled limit. However, considering all the Ln(III) ions together, the kq values do not show any common correlation with the changing ΔG0 values. The kq values are in general much lower for Yb(III) quencher as compared to Sm(III), even though the exergonicity values for coumarin-Yb(III) systems are more favorable than the coumarin-Sm(III) systems. Correlating the observed results based on the diffusion mediated bimolecular reaction model, we infer that depending on the ΔG0 values, some Ln(III) quenchers possibly also engage their upper electronic energy states as well to assist multichannel ET with the excited coumarin donors, and thereby modulating the kq values very substantially for different coumarin-Ln(III) systems. Apparently, the involvement of multichannel ET causes the total reorganization energy (λ) for ET reactions to differ largely for different Ln(III) ions, which would otherwise be very similar for all the coumarin-Ln(III) pairs. Observed results suggest that the intramolecular reorganization energy (λi) associated with lower electronic states of the hydrated Ln(III) ions is much larger than that associated with higher electronic states of these ions. While the involvement of multichannel ET is itself an intriguing inference, modulation in the ET kinetics and total λ values for different Ln(III) quenchers through the differential participation of the multichannel PET is also expected to find potential uses in the controlled utilization of such PET reactions in many practical areas.

The 1′-hydroxy-2′-acetonaphthone (HAN) is a well-known excited-state intramolecular proton-transfer (ESIPT) dye that has been used as a useful fluorescence probe to understand various homogenous and microheterogenous media. However, interaction of HAN with proteins is still largely less explored. The present study is aimed to investigate the interaction of HAN with the most abundant animal circulatory protein, bovine serum albumin (BSA), using multi-spectroscopic techniques and molecular docking studies. The photophysical properties of HAN undergo significant modulation upon its interaction with BSA protein. The confinement and reduced polarity experienced by bound HAN into BSA binding pocket causes a large fluorescence enhancement. The binding constant for HAN-BSA complex is evaluated to be ∼4.3 × 104 M−1. Time-resolved fluorescence measurements reveal the exclusive presence of only keto (K*) form and twisted keto rotamer (KR*) form of the dye in the excited state inside the BSA binding pocket, though in the ground state the dye exists entirely in its enol (E) form. Results suggest a quantitative ESIPT reaction for HAN even inside the BSA pocket. Time resolved fluorescence anisotropy studies indicate very tight binding of HAN into the BSA pocket such that independent rotational diffusion of the dye is completely ceased. Molecular docking studies provide best docking conformation and involved interaction forces, suggesting the preferred binding of HAN to the site IIA (Sudlow's site I) of the BSA protein. Observed photophysical results provide valuable insights of HAN–BSA binding interaction, revealing for the first time that the intriguing excited-state equilibrium between K* and KR* forms of HAN is largely shifted towards KR* form, as the dye binds to the BSA pocket. To the best of our knowledge such an intriguing modulation in the excited state prototautomeric conversion process has not been reported before for the studied dye in the presence of any other host. Results of the present study can be useful in understanding the pharmacology of structurally similar dyes/drugs, having relevance to drug delivery, pharmaceutical sciences, sensors, and so on.

Developing sensitive and selective near-infrared fluorescent bio-probes for serum albumin detection is an ambitious and highly rewarding task. Herein, we report a styryl based fluorophore for serum albumin detection, which displays an exceptional turn-on emission enhancement of ∼500 fold, the highest reported so far in the near-infrared region, and more importantly enables quantification of albumin in the complex serum matrix.

Detailed photophysical investigations have been carried out using a probe dye, coumarin-153 (C153), to understand the microenvironments of micelles formed by the newly introduced Tetronic star block copolymers, T1304 and T1307, having the same poly(propylene oxide) (PPO) block size but different poly(ethylene oxide) (PEO) block sizes. Ground state absorption, steady-state fluorescence, and time-resolved fluorescence measurements have been used to estimate the micropolarity, microviscosity, and solvation dynamics within the two micelles. To the best of our knowledge, this is the first report on these important physicochemical parameters for this new class of the star block copolymer micelles. Our results indicate that T1307 micelle offers a relatively more polar and less viscous microenvironment in the corona region, compared to T1304. The effect of the two micellar systems has subsequently been investigated on the bimolecular photoinduced electron transfer (ET) reactions between coumarin dyes (electron acceptors) and aromatic amines (electron donors). On correlating the energetics and kinetics of the ET reactions, clear Marcus inversion (MI) behavior is observed in both of the micellar media. Interestingly, the ET rates for all of the donor-acceptor pairs are much higher in T1307 than in T1304, and the onset of MI also appears at a relatively higher exergenocity (-Δ G

Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai-400 085, India E-mail : hpal@barc.gov.in Manuscript received 27 October 2017, accepted 01 November 2017 Understanding the kinetic and mechanistic details of the electron transfer (ET) reactions in constrained media is an area of intense research in chemical sciences. The well known Marcus outer-sphere ET theory predicted an interesting inversion behavior for the ET rates with reaction exergonicity (–ΔG0 ), which remains debatable for last more than fifty years for bimolecular ET reactions, though clear Marcus Inversion (MI) behaviors have been well documented for many intramolecular ET systems. For bimolecular ET reactions in conventional solvents, the observed reaction rate at the higher exergonicities usually reach a saturation limit equal to the bimolecular diffusional rate (kd), displaying the typical Rehm-Weller behavior than the theoretically predict MI behavior. Though many explanations have been put forward to justify the observed Rehm-Weller behavior for the bimolecular ET rates with reaction exergonicity, research had been continuing to find out the means that can overcome the limitations associated with the bimolecular ET reactions and can lead to appearance of the theoretically predicted MI behavior even for the bimolecular ET reactions. With this perspective and considering that the ET reactions are the most fundamental reactions occurring ubiquitously in chemistry and biology, extensive studies have been pursed on bimolecular ET reactions for last many decades to understand all the intricate details that can modulate the kinetics of such fundamental reactions. For last about one and half decades, our extensive efforts to understand bimolecular photoinduced ET (PET) reactions under various experimental conditions have made us to explore the requisite situations for the reaction environments that can favorably assist in realizing the MI behavior for bimolecular PET reactions. It has been comprehended from our studies that using constrained reaction media like microherogeneous surfactant assemblies like micelles, reverse micelles, etc., or high viscosity and slow relaxing solvents like ionic liquids, the MI behavior is observed very easily for bimolecular PET reactions, assisted jointly by ultraslow reactant diffusion and exceedingly slow solvent relaxation dynamics compared to the ET rates. In this article we provide an overview of various aspects of bimolecular ET reactions bringing out how the constrained reaction media help in realizing MI behavior even for such bimolecular ET reactions