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.

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

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 (SBEnßCD) 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 SBEnßCD 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-SBEnßCD 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 ßCD rims. The obtained results suggest that the molecular recognition of LDS-798 with SBEnßCD derivatives can be tuned very effectively by varying the number of sulfobutylether substituents on the cyclodextrin rims. Considering that SBE7ßCD is one of the FDA approved agents for drug formulations, the obtained results with other SBEnßCD hosts may be useful in designing selective drug delivery applications, drug formulations, and effective fluorescence on-off switches.

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.

Multimode binding and stimuli responsive displacement of acridine orange dye complexed with p-sulfonatocalix[4/6]arene macrocycles.

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.

An insight into the molecular and surface state photoluminescence of carbon dots revealed through solvent-induced modulations in their excitation wavelength dependent emission properties.

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".

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

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 ß-cyclodextrin in contrast to a low association constant of 1.89 × 102 M-1 with unmodified ß-cyclodextrin (ß-CD). This exceptionally high binding affinity of the fluorogenic probe with the sulfobutylether substituted ß-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.

A comprehensive insight on H-type aggregation in Congo red-surfactant systems revealed through spectroscopic and electrochemical study unified with a simulation framework.

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.

Excited State Interaction of Ruthenium (II) Imidazole Phenanthroline Complex [Ru(bpy)2 ipH]2+ with 1,4-Benzoquinone: Simple Electron Transfer or Proton-Coupled Electron Transfer?

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.

Excited-state prototropism of 7-hydroxy-4-methylcoumarin in [Cnmim][BF4] series of ionic liquid-water mixtures: insights on reverse micelle-like water nanocluster formation.

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.

Inhibition of the prototropic tautomerism in chrysazine by p-sulfonatocalixarene hosts.

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.

Complexation induced aggregation and deaggregation of acridine orange with sulfobutylether-ß-cyclodextrin.

The present study reports a contrasting interaction behaviour of a biologically important dye, acridine orange (AOH+), with a highly water soluble anionic host, based on a ß-cyclodextrin (ßCD) scaffold, i.e. sulfobutylether-ß-cyclodextrin (SBEßCD), in comparison to native ßCD. AOH+ shows striking modulation in its photophysical properties, representing sequential changes in the modes of interaction with increasing SBEßCD concentration. At lower SBEßCD concentrations, AOH+ preferentially binds in dimeric forms at the negatively charged SBEßCD portals, leading to strong fluorescence quenching. At higher SBEßCD concentrations, the dimeric dyes convert to monomeric forms and subsequently undergo both inclusion and exo complex formation with 1 : 1 stoichiometry, resulting in a large fluorescence enhancement. The intriguing observation of sequential fluorescence switch off and switch on for an AOH+-SBEßCD system is clearly facilitated by the presence of butylether chains with SO3- end groups at the portals of SBEßCD, providing an additional ion-ion interaction and much enhanced hydrophobic interaction for cationic AOH+ compared to the native ßCD host. To the best of our knowledge, such fluorescence off/on switching through multistep host-guest binding has not been reported so far in the literature. The present study not only provides a detailed insight into the unique binding interactions of AOH+ with the SBEßCD host, but the findings of this study are also expected to be useful in designing supramolecular based drug formulations, drug delivery systems, sensors, and so on.

Contrasting tunability of quinizarin fluorescence with p-sulfonatocalix[4,6]arene hosts.

This study reveals the intriguing modulations in the photophysics of quinizarin (QZ) on its interaction with p-sulfonatocalix[4]arene (SCX4) and p-sulfonatocalix[6]arene (SCX6) hosts. While the SCX6-QZ system shows the usual reduction in both fluorescence intensity and lifetime, the SCX4-QZ system shows a contrasting effect of enhancement in the fluorescence intensity and reduction in the fluorescence lifetime. Such a contrasting effect is not only unusual but also observed for the first time for any host-guest system. The observed results are justifiably correlated with the changes in both radiative and nonradiative decay rate constants for the SCX4-QZ system.

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.

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.

Tuning of electron transfer reactions in pluronic-surfactant supramolecular assemblies.

Photoinduced electron transfer (ET) reaction between an anionic acceptor, coumarin-343 (C343), and a neutral donor, N,N-dimethylaniline (DMAN), has been investigated in composite supramolecular assemblies (mixed micelles) comprised of a pluronic copolymer (P123: EO20-PO70-EO20 or F88: EO103-PO39-EO103 where EO: ethylene oxide and PO: propylene oxide) and a cationic surfactant (CTAC: cetyltrimethylammonium chloride), following fluorescence quenching studies. Systematic increase in the quenching rates for the studied donor-acceptor system with the increasing CTAC to pluronic molar ratio in the mixed micelles demonstrates a large modulation in the ET rates. The mixed micellar systems in the present cases are formed through the incorporation of the hydrocarbon chains of CTAC into the poly-PO core of the pluronic micelles whereby the cationic head groups of CTAC are placed at the periphery of the micellar core, protruded into the hydrated poly-EO corona region, leading to the formation of a positively charged layer deep inside these mixed micelles. Thus, the anionic C343 dye, initially dissolved at the micelle-water interface, experiences a gradually increasing electrostatic attraction and is therefore systematically dragged deeper inside the micellar corona, as the CTAC composition is increased in the mixed micellar systems. Consequently, the ET rate of the C343-DMAN pair undergoes a large enhancement in the studied mixed micellar systems with the increasing CTAC to pluronic molar ratio. The present strategy of modulating ET reactions using such composite supramolecular assemblies can find applications in areas where bimolecular ET is an integral reaction step.

pH-Assisted control over the binding and relocation of an acridine guest between a macrocyclic nanocarrier and natural DNA.

The differential binding affinity of the hydroxypropyl-ß-cyclodextrin (HPßCD) macrocycle, a drug delivery vehicle, towards the protonated and deprotonated forms of the well-known DNA binder and model anticancer drug acridine has been exploited as a strategy for dye-drug transportation and pH-responsive delivery to a natural DNA target. From pH-sensitive changes in the ground state absorption and steady-state fluorescence characteristics of the studied acridine dye-HPßCD-DNA ternary system and strongly supported by fluorescence lifetime, fluorescence anisotropy, Job's plots, (1)H NMR and circular dichroism results, it is revealed that in a moderately alkaline solution (pH ∼ 8.5), the dye can be predominantly bound to the HPßCD macrocycle and when the pH is lowered to a moderately acidic region (pH ∼ 4), the dye efficiently detaches from the HPßCD cavity and almost exclusively binds to DNA. In the present study we are thus able to construct a pH-sensitive supramolecular assembly where pH acts as a simple stimulus for controlled uptake and targeted release of the dye-drug. As pH is an essential and sensitive factor in various biological processes, a simple yet reliable pH-sensitive model such as is demonstrated here can have promising applications in the host-assisted delivery of prodrug to the target sites, such as cancer or tumour microenvironments, with an enhanced stability, bioavailability and activity, and also in the design of new fluorescent probes, sensors and smart materials for applications in nano-science.

Intriguing multichannel photoinduced electron transfer in lanthanide(iii)-diphenylamine systems.

Photoinduced electron transfer (PET) and charge transfer (CT) interactions of trivalent lanthanide ions (Ln(3+)) with a fluorogenic diphenylamine (DPA) donor in aqueous solution have been investigated. Present donor-acceptor systems have a relevance to the leaching out and mitigation of lanthanide ions in the geoenvironment where DPA finds its presence as industrial and agricultural waste. The formation of weak ground state CT complexes in the present systems is confirmed from ground state absorption studies. There is no emissive exciplex formation as evidenced from steady-state (SS) fluorescence measurements. SS fluorescence quenching results, however, indicate substantial static quenching, attributed to ground state complex formation. Time-resolved (TR) fluorescence quenching results show a quite efficient dynamic quenching process. It is established that the TR fluorescence quenching of DPA by lanthanide ions is not due to energy transfer but due to PET from excited DPA to Ln(3+). Direct evidence for the PET mechanism is obtained from laser flash photolysis measurements where a transient absorption band at around 670 nm for the DPA cation is clearly evident. The time constant for DPA cation growth confirms that PET occurs from the excited S1 state of DPA to lanthanide ions. No correlation is, however, observed for the estimated quenching constants with the free energy changes of the PET reactions, due to the participation of multiple PET channels involving vacant electronic states of lanthanide ions. As realized, lanthanide ions are complex electron acceptors in the PET reactions and many extensive follow up studies are expected to understand the details of the multichannel PET in these geologically important redox systems.

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, ß-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.

A study on the photophysics of 9-amino-10-cyanoanthracene: probing its dual absorption and emission behavior.

The photophysics of a donor-acceptor substituted chromophore, 9-amino-10-cyanoanthracene (ACAN), has been investigated in polar and nonpolar solvents to understand its intriguing dual absorption and emission behavior. Steady-state and time-resolved fluorescence studies clearly indicate that the short wavelength emission band of ACAN arises from the higher excited singlet state, S2, while the longer wavelength emission band arises from the intramolecular charge transfer (ICT) state, S1. Interestingly, both these states can be populated by direct excitation from the ground state. Temperature dependent studies reveal a pronounced activation controlled nonradiative decay channel for the ICT state of ACAN. It is proposed that this activation controlled nonradiative de-excitation arises because of a large relative displacement and a cross-over of the potential energy (PE) surfaces of ACAN in the ground and the ICT states, as a result of different twist angles of the amino group in these two states. Qualitative PE diagrams have accordingly been presented to correlate and rationalize the observed results. The present study also brings to light the interesting excited state prototropic behavior of ACAN and the consequent modulation of the ICT emission that has not been reported in the literature so far.

Aggregation studies of dipolar coumarin-153 dye in polar solvents: a photophysical study.

Photophysical studies have been carried out to explore the aggregation behavior of coumarin-153 (C153) dye in polar organic solvents of both aprotic and protic nature, namely acetonitrile (ACN) and ethanol (EtOH). No unusual behavior is observed in aprotic ACN solvent, suggesting only the monomers of the dye as the single emitting species in the solution. In protic EtOH solvent, however, the dye shows the presence of multiple emitting species in the solution. The concentration-, temperature- and wavelength-dependent changes in the fluorescence decays, and the time-resolved emission spectra (TRES) and area-normalized emission spectra (TRANES) suggest the coexistence of dye aggregates along with the dye monomers in the EtOH solution. Observed results indicate that the emission spectra of the aggregates are substantially blue-shifted compared to the spectra of the monomers, suggesting the H-aggregation of the dye in the present cases. Time-resolved fluorescence anisotropy, ultrafast fluorescence up-conversion measurements and scanning electron microscopy studies support the aggregation of the dye in EtOH solution. Strong dipole-dipole interaction is supposedly responsible for the aggregation of C153 dye (dipole moment ∼6.4 D) and the polar protic solvent EtOH apparently stabilizes the aggregates through solute-solvent hydrogen bonding interaction, which is not possible in aprotic ACN solvent. This is further supported by the time-resolved fluorescence results in a strongly hydrogen bond donating solvent, 2,2,2-trifluoroethanol. Aggregation of C153 dye observed in the present study in polar protic organic solvent is an intriguing finding, as the dye is widely used as a fluorescent probe for various photochemical studies, where overlooking such aggregation will definitely mislead the observed results.