Study of the Binding Interactions of a Hemicyanine Dye with

Article pubs.acs.org/JPCC

Study of the Binding Interactions of a Hemicyanine Dye with Nanotubes of β‑Cyclodextrin and Effect of a Hofmeister Series of Potassium Salts M. Sowmiya,† Amit K. Tiwari,† Sonu,† G. Eranna,‡ Ashok K. Sharma,‡ and Subit K. Saha*,† †

Department of Chemistry, Birla Institute of Technology & Science (BITS), Pilani, Rajasthan 333 031, India Sensors & Nanotechnology Group, Central Electronics Engineering Research Institute (CEERI) Pilani, Rajasthan 333 031, India

‡

S Supporting Information *

ABSTRACT: The photophysical properties of a hemicyanine dye, 4-[4-(dimethylamino)styryl]-1-docosylpyridinium bromide (DASPC22), have been studied in homogeneous media of pure solvents and mixed solvents using UV−vis absorption and fluorescence spectroscopies. These properties were explored to study the binding interactions between DASPC22 and nanotubes of β-cyclodextrin (β-CD) using UV−vis absorption, steady-state fluorescence and fluorescence anisotropy, and time-correlated single-photon-counting (TCSPC) fluorescence measurements of the dye. DASPC22 molecules form H-aggregates in pure water. β-CD forms a simple inclusion complex (1:2 stoichiometry) below its critical aggregation concentration (cac) by encapsulating the chromophoric part of the dye. The Haggregate dissociates significantly to the monomeric form of the dye only when the nanotubes of β-CD molecules start to form above its cac. The dye molecule exists in its monomeric form upon inclusion of its chromophoric part along with the aliphatic tail inside the hydrophobic nanotubular cavity of β-CD. A 350-fold increase in fluorescence intensity of DASPC22 inside the nanotubular cavities formed by an 8 mM concentration of β-CD compared to the fluorescence intensity in the form of a simple inclusion complex was observed. The effects of a Hofmeister series of potassium salts, namely, KClO4, KI, KCl, and KF, in both low and high concentration ranges on the binding strength between host and guest molecules were studied. Salts in their very low concentration ranges enhanced the stability of the host−guest complexes, resulting in a further increase in fluorescence intensity. The fluorescence properties can be tuned by the selective addition of potassium salts with various anions. The tuning of the optical properties of the dye in β-CD nanochannels could help materials scientists to develop novel supramolecular materials.

1. INTRODUCTION Hemicyanines are a class of organic compounds with one quarternary nitrogen within a heterocycle at one end and one acyclic tertiary nitrogen at the other end.1 They are also called aminostyryl dyes.1 Substituted hemicyanine dyes are used to monitor the sol−gel transition process to determine the microviscosity in living cells and also as a voltage-sensitive fluorescent probe of living cell membranes because of their sensitivity toward the polarity and viscosity of the environment.2−4 Hemicyanines have also been applied in the detection of very low concentrations of DNA and proteins, as well as the fluorescence staining of DNA and proteins in electrophoresis gels.5 In addition, these dyes are applied in a large array of systems ranging from neuronal activity detectors6 to lasers.7 The photophysical studies of mono- and dichromophoric hemicyanine dyes in solvents and in Langmuir−Blodgett (LB) films have also been reported.8,9 Hemicyanine dyes are useful for their ability to form stable LB monolayers and multilayers at the air/water interface, which permits their control at the molecular level. These phenomena of hemicyanine dyes result in a large second-order hyperpolarizability and make them useful in second harmonic generation (SHG).10 To obtain such © XXXX American Chemical Society

second-order effects, the primary requirement is the achievement of noncentrosymmetry. However, it is more difficult to induce noncentrosymmetry at the molecular and macroscopic levels. LB deposition and self-assembly are well-known techniques for creating noncentrosymmetric arrangements in the desired hemicyanine dyes.11,12 Hemicyanines are push−pull systems with electron-donor and -acceptor moieties in a single molecule. A hemicyanine molecule has the electron-donating −N(CH3)2 group on one end and the electron-accepting alkyl pyridinium group on the other end. This donor−acceptor system is expected to give dual fluorescence in polar solvents by creating twisted intramolecular charge-transfer (TICT) state upon excitation.13−20 However, unlike one of the extensively studied molecules with TICT fluorescence, namely, (dimethylamino)benzonitrile (DMABN),13−20 the absence of dual fluorescence of studied hemicyanines suggested a nonfluorescent TICT state. The present study was performed with the hemicyanine dye 4-[4Received: July 2, 2013 Revised: January 2, 2014

A

dx.doi.org/10.1021/jp406533u | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

probes.32 However, there is a limitation with these dyes, especially those absorbing at longer wavelengths, because of their susceptibility to photochemical degradation. Another drawback with organic chromophores is their tendency to aggregate, which induces multichromophoric interactions. As a result, the color quality is altered, and the photoluminescence is quenched. Supramolecular encapsulation strategies can solve these issues by isolating the individual dye molecules and preventing self-aggregation or similar interactions with the chemical environment.33,34 These strategies can also be adopted for the drug delivery system in human body.35 Passier et al.36 recently reported the temperature dependence of a thiacarbocyanine dye in its various aggregation states. The thermal stability of the present dye, DASPC22, has been shown to be improved upon inclusion inside the cavity of a host amylase helix and analyzed using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and thermal desorption mass spectrometry (TDMS).37 This inclusion of hemicyanine dye in a host molecule induced emission properties, which were further used to determine the first hyperpolarizability, β, of a free dye molecule and a supramolecular complex of the dye with the host molecule using Rayleigh scattering with fluorescence suppression.12 Kajikawa et al. prepared self-assembled monolayers (SAMs) of DASPC1 and DASPC22 by deposition on a silica surface. They concluded that the polar orientation of the DASPC22 SAM is strongly influenced by the presence of a long alkyl chain in the chemical structure.38 Hemicyanine dyes with long alkyl chains attached to the pyridine nitrogen are well-explored, as they show noncentrosymmetric arrangements and are active for second harmonic generation.10 In the present study, photophysical characterization of the hemicyanine dye DASPC22 was carried out in selected pure solvents and in mixtures of solvents to analyze the interactions of DASPC22 in different homogeneous environments. As discussed above, the self-assembly of the dye also induces noncentrosymmetry in the molecule. However, DASPC22 exists as an H-aggregate in aqueous solution39 and is nonfluorescent. Hence, the inclusion of DASPC22 with β-CD was explored to control the dye aggregation and improve the fluorescence emission of DASPC22.40 Recently, the supramolecular interactions between some hemicyanine homologues and cucurbit[n]urils were investigated, and enhancement of the fluorescence intensity was reported.41−44 The fluorescentprobe- (guest-) molecule-induced formation of nanotubes of cyclodextrin and their secondary assembly45−47 kindled our interest in studying the formation of an inclusion complex of DASPC22 with nanotubes of β-CD in aqueous medium. Hence, the changes in the photophysical properties of DASPC22 are utilized to study this complex. Our aim was to see whether the inclusion of the dye in the nanotubular cavities gives better enhancement of fluorescence intensity than reported earlier for other supramolecular systems.41,42 We also tried to determine the possibility of tuning the fluorescence intensity having the dye inside the nanotubular cavity. Dehydration of the solute upon the formation of an inclusion complex is a known phenomenon.48 Enhancement of the binding strength between hydrophobic host and guest molecules in the presence of a salting-out agent is a fundamental concept.49 Hydrophobic interaction is the main driving force for the formation of inclusion complexes between CD and dye molecules that can be perturbed by the alteration of their solubility in water. It has been widely reported that the

(dimethylamino)styryl]-1-docosylpyridinium bromide (DASPC22) (Scheme 1). Scheme 1. Molecular Structure of 4-[4(Dimethylamino)styryl]-1-docosylpyridinium Bromide (DASPC22) in its Trans Conformation, Where ϕ1−ϕ4 indicate Four Possible Rotations Inside the Moleculea

a

Possible rotamerism process for DASPC22 adopted from that shown by Cao et al.21

Cao et al.21 performed a theoretical study of solvent effects on the intramolecular charge transfer of the hemicyanine dye 4[4-(dimethylamino)styryl]-1-methylpyridinium (DASPC1; Scheme S1, Supporting Information) with the methylpyridinium group as the acceptor instead of the long alkyl chain attached to the pyridinium nitrogen in the present dye, DASPC22. Upon photoexcitation, DASPC1 undergoes internal rotation that leads to the TICT state. The possible rotamerism process for DASPC22 shown in Scheme 1 was adopted from that shown by Cao et al.21 The rotamerism process in DASPC1 can take place through several pathways around one double bond and three single bonds, resulting in a very small fluorescence quantum yield in solution.21,22 The fluorescence quantum yield of DASPC1 depends strongly on the polarity and viscosity of the solvent. With increasing polarity of the solvents, the absorption peak maximum of DASPC1 shifts to the blue, and the fluorescence quantum yield decreases with a red shift in the band maximum. This is in contrast to the spectroscopic properties of most of the compounds showing TICT fluorescence, for which both the absorption and fluorescence peak maxima shift to the red in polar solvents. This observation suggests that the state from which the fluorescence originates is a non-TICT state and that the formation of the TICT state lowers the fluorescence quantum yield. Recently, Cao et al. and McHale21,23,24 have confirmed the nonradiative feature of TICT state of hemicyanine dyes by Raman spectroscopy and theoretical calculations. Recent experimental and theoretical studies21,22,25−28 on DASPC1 revealed that (i) the internal rotational motion can lead to several TICT states, although fluorescence occurs from the non-TICT state, and (ii) the TICT state is mostly obtained by twisting of the aniline moiety through ϕ2, which involves a comparatively much lower potential energy barrier. This energy barrier is further lowered in the presence of polar solvents, giving almost zero energy barrier in water. Our expectation is that the emission properties of DASPC22 should be similar to those of DASPC1, as there is not much difference in their structures except for the length of alkyl group attached to quarternary nitrogen atom. It is pertinent to note that, so far, there has been no report on the counterion-induced polarization of the excited states of such kinds of hemicyanine dyes as has been observed for some distyrylbenzene derivative chromophores.29 Dyes are widely used as pigments in many commercial products such as paints,30 textile products,31 and biological B



Article

respective solvents/mixtures of solvents to make a final volume of 10 mL. The relative fluorescence quantum yields were determined with respect to Rhodamine 6G in ethanol as the standard (ϕf = 0.9458), calculating the area under the corrected fluorescence bands of both DASPC22 and the standard. For the preparation of aqueous solutions with different concentrations of β-CD and a constant concentration of DASPC22, 0.1 mL of DASPC22 solution in methanol was added to the required volume of aqueous solution of β-CD, and the mixture was kept in a sonicator (Branson, model 1510) for 30 min to ensure complete miscibility. The final volume of the solution was then adjusted to 10 mL. Methanol was added to ensure that the dye was completely soluble in the aqueous solution of β-CD. However, only 1% methanol was present in each of the solutions. Freshly prepared solutions were used for all spectroscopic measurements. The absorption spectra were recorded using a Hitachi U2900 UV−vis spectrophotometer. Fluorescence measurements were performed using a Fluoromax-4 (Horiba Jobin Yvon) scanning spectrofluorimeter. The excitation and emission slit widths used for the fluorescence measurements were 3 nm each. The emission spectra were corrected for instrument sensitivity. A quartz cuvette was used for all spectroscopic measurements. Steady-state fluorescence anisotropy measurements were performed with the same steady-state spectrofluorimeter fitted with a polarizer attachment and the steadystate anisotropy, r, can be represented as58,59

presence of a salting-in or salting-out agent affects the hydrocarbon solubility.49 The presence of anions (e.g., Cl−, ClO4−) or cations (e.g., Na+, Li+) can alter the solubility of hydrocarbons in water depending on their polarizabilities and sizes. Small ions such as Li+ and C1− decrease hydrocarbon solubility in water, whereas large ions such as Gn + (guanidinium ion), C1O4−, and I− increase solubility.50 The effect of such salts was first proposed by Hofmeister in the year 1888. Hofmeister proposed the qualitative ordering of ions based originally on their propensity to salt-out proteins from aqueous proteins.51 Heuvingh et al.52 reported the effects of a series of salts (Hofmeister series) on the morphology and mechanical properties of hollow polyelectrolyte multilayer capsules and proposed that these salts can act as plasticizers in the multilayers and induce annealing effects. Lima et al.53 analyzed the effects of a series of inorganic anions on the inclusion of hexanoic and decanoic acids in β-CD. They concluded that the anions of salts such as NaCl, NaBr, NaClO4, and NaNO3 have little effect on inclusion complexation using the variation in NMR chemical shifts. Turshatov et al.54 reported the effects of salts on monolayers of amphiphilic 4(3′,4′-dimethoxystyryl)-N-octadecylpyridinium perchlorate and bromide using the isotherm measurements, reflection spectroscopy, and Brewster angle microscopy. They observed that the phase transition in the isotherm in the presence of 10 mM of each KF, KCl, KI, and KClO4 varies according to the Hofmeister series. In the present study, it is shown that the supramolecular assembly of β-CD is initiated by the hemicyanine dye DASPC22 and the fluorescence intensity of the dye is enhanced dramatically in β-CD nanotubular cavities. Further enhancement of the fluorescence intensity and also tuning of the fluorescence intensity are demonstrated by the use of a Hofmeister series of potassium salts. We studied the effects of potassium salts on the extent of binding interactions between DASPC22 and nanotubes of β-CD by changing the salts according to the Hofmeister series. The promising future scope of this study is that further work on this molecule involving tuning of the nonlinear optical properties of the dye in β-CD nanochannels will help materials scientists to develop novel supramolecular materials. The present system could possibly be useful in constructing potential logic gates by simple host− guest chemistry, avoiding the requirement for complicated synthetic processes in most of the reported studies.55−57

r=

IVV − GIVH IVV + 2GIVH

(1)

where IVH and IVV are the intensities obtained from the excitation polarizer oriented vertically and the emission polarizer oriented in the horizontal and vertical positions, repectively. The factor G is defined as G=

IHV IHH

(2)

The excited singlet-state lifetimes were determined from intensity decays using a Horiba Jobin Yvon Fluorocube-01-NL picosecond time-correlated single-photon-counting (TCSPC) experimental setup. A picosecond diode laser at 375 nm (NanoLED 375L, IBH, Glasgow, U.K.) was used as a light source. The fluorescence signals were detected at the magicangle (54.7°) polarization using a TBX photon detection module (TBX-07C). The instrument response function of this laser system is ∼165 ps. The decays were analyzed using IBH DAS-6 decay analysis software. The goodness of fit was analyzed by the χ2 criterion and visual inspection of the residuals of the fitted function to the data. An EUTECH PC 510 pH meter was used to measure the pH of the solutions. All experimental solutions were adjusted to a pH value of 7.4. Atomic force microscopy (AFM) experiments were performed using a Nanoscope II instrument (Digital Instruments Inc., Buffalo, NY) with a lateral resolution of 1 Å and a vertical resolution of 0.1 Å. The substrate used for AFM was highly oriented pyrolytic graphite (HOPG). All measurements were carried out at room temperature (25 ± 1 °C).

2. MATERIALS AND METHODS DASPC22 and β-CD were obtained from Aldrich Chemical Co. (Milwaukee, WI) and used as received. KI and KClO4 were obtained from Merck, Mumbai, India. KCl and KF were supplied from Qualigens, Mumbai, India. All of the solvents used were of spectroscopic grade and were obtained from Spectrochem Chemical Company, Mumbai, India. Triply distilled water was used for the preparation of aqueous solutions. Rhodamine 6G was obtained from Sigma-Aldrich (St. Louis, MO) and used as received. Dilute sulfuric acid and sodium hydroxide solutions were used to adjust the pH values of the prepared solutions. NaOH and H2SO4 were obtained from Merck, Mumbai, India. A stock solution of DASPC22 (5 × 10−4 M) was prepared in pure methanol to record the UV− vis absorption and fluorescence spectra of DASPC22 in pure solvents; 0.1 mL of this solution was poured into a 10 mL volumetric flask and left for few hours for complete evaporation of methanol, and then the compound was dissolved in the

3. RESULTS AND DISCUSSIONS 3.1. UV−Vis Absorption Study of DASPC22 in Homogeneous Media. In pure water, the absorption band C



Article

of DASPC22 appears with a peak maximum at ∼420 nm (Figure 1). This band is reported to be predominantly for H-

significantly red-shifted in less polar solvents. DASPC22 is sparingly soluble in most nonpolar solvents; hence, nonpolar solvents other than chloroform are avoided. It is ensured that the dye is completely soluble in chloroform. However, the decrease in extinction coefficient of the dye in chloroform as compared to the other solvents used could be because of static quenching forming a ground-state complex between dye and chloroform molecules, as the latter is known to be a good fluorescence quencher.61 Also, to avoid any possibility of solubility problems in less polar solvents, the present study was carried out with solutions of only up to 80% dioxane, ensuring the complete solubilization of the dye. As mentioned above, hemicyanine dyes with long alkyl chains are especially prone to forming H-aggregates in water as the solvent, resulting in deviations from Beer’s law. Such deviations lead to a blue shift in the absorption peak maximum, as well as fluorescence quenching.62,63 The dye molecules with long hydrophobic wings attached to them try to avoid the hydrogen-bonded water structure, which results in the formation of H-aggregates. However, in comparatively less polar solvents such as methanol, acetonitrile, and dioxane/ water mixtures, there are interactions between these solvents and the dye molecules, and the latter exist only in the form of solvated monomeric species in these solvents. Kim and Choi60 also reported that the absorption band of DASPC22 with λmax ≈ 475 nm is due to the monomeric form of the dye. Therefore, with decreasing solvent polarity, the H-aggregates of the dye molecules are dissociated into the monomeric form. The blue shift in the absorption peak maxima with increasing solvent polarity, except in the case of ethylene glycol (discussed later), shows the negative solvatochromic behavior of the dye (Table 1), indicating that the ground state has a larger dipole moment than the excited state (Franck−Condon state).9 In agreement with the study by Kim and Choi60 on the absorption spectra of DASPC22 in DMSO/water mixtures, we also did not notice any clear isosbestic point in the absorption spectra of the dye in dioxane/water mixtures (Figure 1). In the present study with dioxane/water mixtures, a sharp transition from H-aggregates to the monomeric state of DASPC22 was noticed at 23% dioxane (Figure S1, Supporting Information). 3.2. Fluorescence Study of DASPC22 in Homogeneous Media. DASPC22 does not give any fluorescence emission in pure water as H-aggregates are nonfluorescent.63−65 However, it gives a broad emission band in other pure solvents, as shown in Figure 3. As mentioned above, DASPC22 is not soluble in most nonpolar solvents. The emission band maxima

Figure 1. Absorption spectra of DASPC22 (5 μM) in dioxane/water mixtures with different dioxane percentages.

aggregates of the dye molecule.9,60 For the systematic monitoring of the effect of solvent polarity on the aggregation of DASPC22 molecules, UV−vis absorption spectra were recorded in dioxane/water mixtures of various compositions, as also shown in Figure 1. All absorption bands are broad and structureless. On increasing the percentage of dioxane in the dioxane/water mixture, the absorption band with λmax ≈ 420 nm due to H-aggregates gradually disappears as a new band appears with a progressive increase in absorbance at ∼475 nm. The UV−vis absorption spectra of DASPC22 were also recorded in selected pure solvents, as shown in Figure 2. The

Figure 2. Absorption spectra of DASPC22 in pure solvents. [DASPC22] = 5 μM.

absorption peak maxima are listed in Table 1. As compared to those in pure water, the absorption bands of DASPC22 are Table 1. Absorption Peak Maxima (λmax ab ), Fluorescence Peak Maxima (λmax fl ), and Fluorescence Quantum Yields (ϕf) of DASPC22a in Pure Solvents

a

solvent

εb

λmax ab (nm)

c λmax (nm) fl

ϕfc

water ethylene glycol acetonitrile methanol ethanol chloroform

78.36 37.30 35.94 32.66 24.55 2.24

420 481 474 478 482 501

− 610 614 607 604 574

0 0.081 0.007 0.014 0.035 0.140

Figure 3. Fluorescence spectra of DASPC22 (5 μM) in selected pure solvents (λexc = 470 nm).

[DASPC22] = 5 μM. bε = solvent dielectric constant. cλexc = 470 nm. D



Article

environment and can be monitored using the absorption/ fluorescence characteristics of DASPC22. Based on these experimental results, further studies of the interaction of the dye with different concentrations of β-CD were carried out and are discussed in the following section. 3.3. Absorption Characteristics of DASPC22 in β-CD. The absorption spectra of DASPC22 with varying concentration of β-CD are shown in Figure 4. It can be seen in this

of DASPC22 in selected pure solvents are also listed in Table 1. With increasing solvent polarity (e.g., from chloroform to acetonitrile), there is a red shift in the emission spectrum of DASPC22 with a decrease in fluorescence quantum yield. Positive solvatochromism in the emission (red shift in the emission spectra with increasing polarity of the solvent) and negative solvatochromism in the absorption (blue shift in the absorption spectra with increasing polarity of the solvent) are well-known phenomena for hemicyanine dyes.25,66 As mentioned above, the fluorescence quantum yield gradually decreases with increasing solvent polarity. The higher possibility of the creation of a TICT state in a polar solvent leads to the quenching of fluorescence. The absorption and fluorescence spectral properties of DASPC22 are in accordance with the fact that the TICT state is nonfluorescent. Even though there is a possibility of fluorescence quenching in chloroform (mentioned above), the lower fluorescence quantum yield in other solvents used as compared to chloroform is because of the higher possibility of the creation of the TICT state in the other solvents. In correlation with the results of a theoretical study on the hemicyanine dye DASPC1 by Cao et al.,21 it could be possible to predict that, in the case of DASPC22, the TICT state is also obtained by twisting of the aniline moiety through ϕ2 (Scheme 1), as this pathway is expected to involve a comparatively much lower potential energy barrier. Detailed theoretical calculation to confirms the actual path are beyond the scope of the present work. The quantum yield of DASPC22 is much higher (0.081) in ethylene glycol as the solvent than in other solvents such as methanol (0.014) and acetonitrile (0.007), even though their polarities are not much different.9 The fact that the formation of the TICT state is more difficult in a viscous medium because of inhibited twisting motion is supported by the higher fluorescence quantum yield value in a solvent of high viscosity. This phenomenon explains why the greater possibility of the formation of the TICT state would result in a lower fluorescence quantum yield. Fluorescence quenching upon the formation of the TICT state has also been observed in case of azobenzene derivatives.67 It has been reported that cyanine dyes that form Haggregates in aqueous solutions exhibit very low quantum yields because of their enhanced triplet-state yields.63,68 Our assumption is that fluorescence quenching of H-aggregates of DASPC22 molecules in water is also because of the enhanced rate of formation of triplet states. The photobleaching of cyanine dyes is due to the production of singlet oxygen as a result of energy transfer from an excited-state dye molecule.64 It might be due to fluorescence self-quenching caused by the collisions of aminostyryl pyridinium fluorophores.69 To systematically monitor the effect of solvent polarity on the intensity of fluorescence, emission spectra of DASPC22 were also recorded in dioxane/water mixtures of various compositions, similar to the absorption spectra, and are shown in Figure S2 (Supporting Information). This figure shows that, with increasing amount of dioxane in the dioxane/water mixtures, the fluorescence intensity increases. Because H-aggregates are nonfluorescent in pure water, the monomeric form alone is responsible for the entire fluorescence in a comparatively less polar medium. The increase in fluorescence intensity is because of the progressive increase in the concentration of the monomeric component with decreasing polarity of the solvent. From these observations, it is evident that the presence of the monomeric or aggregate form of DASPC22 is highly dependent on the solvent

Figure 4. Absorption spectra of DASPC22 (5 μM) as a function of the concentration of β-CD.

figure that the absorption bands appearing with a peak maximum at ∼420 nm at low concentrations of β-CD (up to 2 mM) are predominantly for H-aggregates of DASPC22 molecules. The prominent absorption bands seen at high concentrations of β-CD with a peak maximum at ∼462 nm are characteristic of the monomeric form of the dye. Rao et al. reported the dissociation of dimers of some cyanine dyes by complexation with 2,6-dimethyl-β-cyclodextrin (Me-β-CD) with a concomitant increase in absorbance, as well as the appearance of a red-shifted absorption band.63 In the present study, in the presence of β-CD, initially, an absorption band predominantly for H-aggregates of the dye molecules appears (peak maximum ≈ 420 nm). However, a further increase in the concentration of β-CD results in the appearance of a new absorption band initially with a peak maximum at ∼456 nm. This band is then progressively red-shifted to ∼462 nm with an increase in absorbance upon further addition of β-CD. The appearance of bands in this wavelength range (456−462 nm) indicates the dissociation of H-aggregates into the monomeric form of the dye. The increase in width of the band with increasing concentration of β-CD even at a low concentration range suggests the mixing of bands with characteristics of the monomeric form with those of the aggregated form. Above a βCD concentration of 2 mM, the fact that monomer formation is favored could be due to initial stage of formation of nanotubes, as discussed later. The progressive increase in absorbance in the wavelength range of 456−462 nm with increasing concentration of β-CD reflects an increased concentration of the monomeric form of the dye.70 It is noteworthy that DASPC22 has a long alkyl chain attached to the pyridinic nitrogen atom. With increasing concentration of β-CD, increasing numbers of CD molecules are threaded on to the alkyl chain of the dye, resulting in the formation of the monomeric species. It is seen that no further red shift of the band occurs after a certain concentration of βCD, suggesting the saturation of the formation of monomeric species. E



Article

concentrations of β-CD, the absorbance ratio increases very slowly, whereas at higher concentrations, it increases abruptly. From this figure, the critical concentration of β-CD required for the dissociation of aggregates to take place is found to be 2.7 mM. Beyond this concentration, an abrupt increase in the absorbance ratio indicates the pronounced formation of inclusion complexes between β-CD and monomer species. It was reported earlier47 in the case of β-CD and the guest molecule trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) that, above 2.5 mM concentration, which is the critical aggregation concentration (cac), β-CD molecules start to form extended nanotubes. However, the formation of simple 1:1 inclusion complexes between β-CD and DMASBT molecules occurs below the cac. Thus, the sudden increase in the absorption ratio at 2.7 mM β-CD (Figure 5) suggests that the dissociation of H-aggregates becomes significant only when the formation of nanotubes of β-CD is pronounced. To our knowledge, this kind of study on the effect of nanotubes of βCD on the formation of monomeric species of hemicyanine dye and its photophysical properties has not previously been reported. 3.4. Steady-State Fluorescence Characteristics of DASPC22 in β-CD. DASPC22, being in the form of Haggregates in aqueous solution in the absence of β-CD, is nonfluorescent. However, the dye exhibits fluorescence in the presence of β-CD. Fluorescence is observed even in the presence of 1 mM β-CD. It is known that the dye is fluorescent in the monomeric form. The intensity of the fluorescence band with a peak maximum at ∼590 nm increases with increasing concentration of β-CD (Figure 6). Fluorescence spectra with

Because hemicyanine dyes exhibit negative solvatochromism and charge transfer occurs upon excitation, the ground state is more polar than the excited state. Therefore, the pyridinium cation has a greater influence on the spectral shift in a polar medium at the ground state relative to the neutral dimethylamino group in the excited state.71 In the case of a hemicyanine dye, a red shift in the absorption band occurs when the pyridinium moiety resides within the hydrophobic cavity, which is nonpolar in nature. Therefore, the excited state will have stronger influence on the intramolecular chargetransfer (ICT) process. At this state, there would be a shift of the resonance balance toward the quinoid form, which has more delocalized π-electrons (B, Scheme 2) than the benzenoid Scheme 2. Molecular Structures of A and B Representing Two Proposed Mesomeric Forms of the DASPC22 Ion

form (A, Scheme 2).72 However, a blue shift in the absorption band should occur when the pyridinium cation is exposed to an environment that is polar in nature. Thus, the ground state will have more influence on the ICT process. In fact, there is no further shift of the absorption band of monomeric species upon addition of β-CD, once the dissociation of H-aggregates becomes saturated. Both donor and acceptor moieties of DASPC22 are expected to be included in the hydrophobic cavities, experiencing a similar environment. It is obvious that, in this situation, there would be very little influence on the ICT process in the excited state, leading to a negligible shift in the absorption spectrum.71 The long alkyl chain is attached to the pyridinium nitrogen in such a way that the charged pyridinium moiety is included completely within the nonpolar host cavity in the inclusion state. At the same time, the aniline moiety would also be included inside the hydrophobic cavity of β-CD. The ratio of the absorbance of the monomeric to the aggregate species of DASPC22 is plotted against the concentration of β-CD in Figure 5. At low

Figure 6. Variation in the fluorescence intensity of DASPC22 as a function of the concentration of β-CD. Inset: Fluorescence spectra of DASPC22 (5 μM) as a function of the concentration of β-CD (λexc = 423 nm).

peak maxima at ∼590 nm in various concentrations of β-CD are shown in the inset of Figure 6. The dramatic increase in the fluorescence intensity of DASPC22 occurs above 2.7 mM β-CD (Figure 6). This is the critical concentration of β-CD above which significant dissociation of H-aggregates into the monomeric form of the dye takes place. This critical concentration agrees well with that found from the absorption data (Figure 5), which further supports our suggestion that the H-aggregates dissociate significantly only when β-CD molecules start to form nanotubes. It is noteworthy that the cac of β-CD in the present case is 2.7 mM. It can also be noted from the absorption and fluorescence data that the process of formation of monomers reaches saturation at ∼8 mM β-CD. It is worth noting that, unlike for DASPC1 as a guest molecule present inside a host cavity such

Figure 5. Absorbance ratio of DASPC22 (5 μM) as a function of the concentration of β-CD. F



Article

hydrophobic cavities of two β-CD molecules, forming an inclusion complex with a stoichiometric ratio of 1:2. However, our expectation is that these dye molecules with the chromophoric part encapsulated by β-CD molecules will still be in aggregated form with the dye molecules whose chromophoric part is not encapsulated as a result of the aggregation of their tails. Although an absorption spectrum (Figure 4) at a low concentration of β-CD shows absorption due to H-aggregates as well as the monomeric form, there might not be any equilibrium between pure H-aggregates and pure monomeric form because of their aggregation. Scheme 3 shows pictorial representations of probable binding processes that occur in DASPC22−β-CD systems in various β-CD concentration ranges. The association constant for the formation of the simple inclusion complex was calculated from the slope and intercept of the Benesi−Hildebrand plot and found to be 2.53 × 106 M−2. We did not accept a 1:1 stoichiometry, as the intercept of the corresponding Benesi−Hildebrand plot was found to be negative (Figure 7, inset). With this observation, our proposition is that both the donor and acceptor moieties of the dye are also included in the hydrophobic cavity of a nanotube formed by β-CD molecules at high concentration. This is because the simple inclusion complex with a stoichiometric ratio of 1:2 formed at low concentrations of βCD is followed by the formation of extended nanotubes by the successive threading of β-CD molecules onto the tail of the dye at high concentrations of β-CD (Scheme 3). Enhancement of the emission intensity of hemicyanine dyes by complexation with various surfactants as well as CDs and modified CDs has been widely reported. Park and Park73 reported a 6.4-fold increase in the fluorescence intensity of DASPC1 dye in β-CD compared to that in water. In addition, they also synthesized modified β-CDs such as β-CD-NS (sulfonaphthyl) and β-CD-Py (pyrenyl) by substitution on the primary face of β-CD. The inclusions of DASPC1 dye with βCD-NS and β-CD-Py increase the emission intensities 14-fold and 56-fold, respectively, as compared to that in water. Mishra et al.75,76 reported a 40-fold increase in fluorescence intensity for DASPC14 dye in 20 mM SDS and a 90-fold increase for C18 dye in 10 mM CTAB. Recently, Li et al. reported a 270fold increase in the fluorescence intensity of DASPC1 upon binding with cucurbit[6]uril.41 In contrast, in the present study with DASPC22 dye, a 350-fold increase in the fluorescence intensity of the dye in nanotubular cavities of 8 mM β-CD with respect to that in the form of a simple inclusion complex with β-CD was observed (Figure 6). This can be explained by the fact that the high concentration range of β-CD chosen in this study gives nanotubes that protect the dye molecules well from their contact with the polar environment, giving a large extent of the monomeric form of the dye, which is highly fluorescent. 3.5. Fluorescence Anisotropy of DASPC22 in β-CD. The steady-state fluorescence anisotropy of DASPC22 with varying concentration of β-CD were determined and are shown in Figure S3 (Supporting Information). The fluorescence anisotropy rises very sharply even at very low concentrations of β-CD and shows a value of ∼0.29 at 2 mM β-CD. The increase in anisotropy is continued with further increasing amounts of βCD until a saturated anisotropy value of ∼0.32 is reached at a high concentration of β-CD. This increase in anisotropy with increasing concentration of β-CD and the high value of fluorescence anisotropy even at a low concentration of β-CD indicate the formation of nanotubes of β-CD induced by the

as cucurbir[6]uril,41 no change in the emission maximum of DASPC22 was observed with increasing concentration of βCD. This result indicates that the percentage of the monomeric form of the dye increases upon addition of β-CD to the solution. It is very unlikely that the increase in fluorescence intensity is because of blocking of TICT state formation of DASPC22 in the nonpolar cavity of β-CD. Otherwise, there would have been a blue shift in the fluorescence band, as was observed in the case of the fluorescence spectra of the dye with solvents of decreasing polarity (Figure 3). Moreover, the aggregated form of the dye is nonfluorescent. No change in the fluorescence band of the monomeric dye with increasing concentration of β-CD further supports our suggestion based on the absorption data that both the pyridine and aniline moieties are included in the hydrophobic cavities of β-CD. Park and Park73 reported the inclusion complex formation of DASPC1 dye with β-CD. They also did not observe any pronounced blue shift of the emission maxima; however, enhancement of the fluorescence intensity was observed with increasing amount of β-CD. They suggested that the dimethylamino group of the dimethylaminostyryl moiety interacts with β-CD to give enhanced stability to the complexes.73 In this study, in the case of DASPC22, we report that both the acceptor and donor parts of the dye are included in the cavities of β-CD; further evidence in favor of this conclusion is given in the next paragraph. To determine the stoichiometry of the simple inclusion complex formed between the dye and β-CD molecules below the cac of β-CD, we employed a Benesi−Hildebrand plot74 using fluorescence intensity data. The concentration range of βCD chosen for this study was only up to 2.5 mM to ensure that the β-CD molecules were unable to form nanotubes in this range, because once nanotubes are formed, the Benesi− Hildebrand equation is not valid.47 The Benesi−Hildebrand plot47 for the simple inclusion complex of the DASPC22−βCD system is shown in Figure 7 (regression coefficient, 0.987)

Figure 7. Benesi−Hildebrand plot of DASPC22 (5 μM) in β-CD for a 1:2 stoichiometry Inset: Benesi−Hildebrand plot for a 1:1 stoichiometry.

and indicates a 1:2 (DASPC22/β-CD) stoichiometric ratio of the complex. Unless the chromophoric part of the dye is included inside the β-CD cavity, no change in fluorescence intensity is expected. It is very unlikely that just threading two β-CD molecules onto the tail of the dye molecule will make any significant change in fluorescence intensity. Thus, the Benesi− Hildebrand plot supports the fact that both the donor and acceptor moieties of the dye molecule are present inside the G



Article

Scheme 3. Pictorial Representations of Probable Binding Processes Occurring in DASPC22−β-CD Systems in Various Concentration Ranges of β-CD and Potassium Salt: (a) H-Aggregate of Dyes in Pure Water, (b) Aggregation of Dyes and β-CD Encapsulated Dyes with 1:2 Stoichiometry, (c) Monomeric Forms of Dyes Inside Partially Formed Nanotubes and HAggregated Dyes, (d) Monomeric Dyes Inside Nanotubes and Secondary Aggregation of Nanotubes, (e) More Stronger Nanotubes and Their Secondary Aggregation, and (f) H-Aggregates of Dyes and β-CD Moleculesa

a

To save space, secondary aggregation of only four nanotubes is shown.

guest molecule, DASPC22. This is in agreement with the reports by many groups, where the formation of nanotubes of cyclodextrins is supported by the large degrees of fluorescence anisotropies of guest molecules.45,46,77−80 The increase in fluorescence anisotropy in the presence of β-CD implies an increase in the rigidity of the microenvironment around the fluorophore.58 These results further suggest that both the donor and acceptor parts of the dye molecule are present inside the rigid cavity of the β-CD nanotube (Scheme 3). It is very unlikely that inclusion of only the tail part of the dye molecule inside the cavity of a nanotube would produce such a high fluorescence anisotropy. 3.6. Effect of Inorganic Salts on the Binding of Dye with β-CD Nanotubes. 3.6.1. UV−Vis Absorption and Fluorescence Measurements. The formation of an inclusion complex between CD and a dye molecule is mainly due to the hydrophobic interaction between them.47,49 Any factor that increases the solubility of the dye or of CD in water will perturb the binding interaction between them as a result of reduced

hydrophobic interaction. In this study, the effect of a series of salts (Hofmeister series) on the β-CD−DASPC22 complex was studied by monitoring the changes in the absorption and emission spectra of the guest molecule, DASPC22. The concentration of β-CD was taken to be 8 mM at which the emission intensity of DASPC22 was found to reach saturation. The concentration of the dye in all of these experiments was 5 μM. The inorganic salts KClO4, KI, KCl, and KF were selected, in which the cationic part was common and the size of the anions varied. The absorption of DASPC22 in 8 mM β-CD shows a broad spectrum corresponding to the monomeric species. At low concentrations of the salt in the presence of βCD, the absorption of the monomeric form of DASPC22 increases, and at high concentrations of salt, the absorption band appears in the high-energy region, which is characteristic of H-aggregates of the dye (Figure 8). The changes in absorption spectra of DASPC22 in 8 mM β-CD with varying concentration of KClO4 and KF are shown in Figure 8 and H



Article

Figure 8. Variation in the absorption spectra of DASPC22 (5 μM) in β-CD (8 mM) with varying concentrations of KClO4.

Figure 9. Variation in the fluorescence intensity ratio of DASPC22 in β-CD with varying concentrations of inorganic salts of the Hofmeister series. [DASPC22] = 5 μM, [β-CD] = 8 mM, λex = 423 nm.

Figure S4 (Supporting Information), respectively. Similar trends were observed with the other two salts as well. The ClO4− ion is a salting-in agent.49,81 However, the increase in the absorbance of the monomeric form of the dye at a low concentration of salt indicates that the salt is apparently acting as a salting-out agent, enhancing the binding interactions between the host and the guest. Yet, at a higher concentration of the salt, the appearance of an absorption band corresponding to H-aggregates at the expense of the monomeric band of DASPC22 shows the salting-in behavior of the salt, reducing the binding interactions between the host and the guest (Scheme 3). The concentration-dependent dual behavior of the ClO4− ion was reported by Dey et al.81 and later by Saha and Kanchanamala49 in fluorescence studies and is discussed later. Interestingly, similar trends were noticed for all of the salts studied with the β-CD−DASPC22 system. Figure S5 (Supporting Information) presents some absorption spectra of aqueous solutions of DASPC22 with varying concentrations of KClO4 in the absence of β-CD. A spectral band of this kind is characteristic of H-aggregates of the dye molecules. The absorption due to H-aggregates increases with increasing concentration of KClO4, except at some high concentrations, demonstrating the salting-in effect of the salt. To further confirm the difference in behavior of the salts from high to low concentration, the emission spectra of DASPC22 were recorded in 8 mM β-CD with varying concentrations of KClO4, KI, KCl, and KF (Figure S6, Supporting Information). Similar to absorbance, the fluorescence intensity also increases in the low concentration ranges of the salts. The emission intensity ratios of DASPC22 in 8 mM β-CD with varying concentrations of salts are plotted in Figure 9, where F0 and F are the fluorescence intensities in the absence and presence of salts, respectively. Each plot passes through a maximum. Even though the series of salts chosen for this study have anions of varying size, all of the salts at low concentrations help in the nanotubular assembly, thereby increasing the host− guest binding interaction, whereas at higher concentrations, they increase the solubilities of both β-CD and DASPC22 in water and lead to the annihilation of the nanotubular assembly of the DASPC22−β-CD system. Hence, in the presence of high concentrations of salts, the DASPC22 molecules come out of the β-CD cavities and exist as H-aggregates in aqueous medium (Scheme 3), resulting in the decrease in the emission intensity of the dye (Figure S6, Supporting Information). As expected, ClO4−, because of its large size, shows strongest salting-in effect, whereas the F− ion, with its small size, exhibits a saltingin effect only at very high concentration. The salting-in

efficiency occurs according to the Hofmeister series. It can be seen in Figure 9 that there is a concentration range in which ClO4− ions act as a salting-in agent, whereas I− ions behave as a salting-out agent. It can be inferred from Figure 9 that the emission intensity of DASPC22 increases until the KClO4 concentration reaches 8.1 × 10−8 M and then decreases continuously. A similar trend is also noticed in the presence of the other salts KI, KCl, and KF. Dey et al.81 and later Saha and Kanchanamala49 also noticed concentration-dependent dual behavior of NaClO4, showing an increase in the association constant of a guest molecule containing a −NH2 group with β-CD at a low concentration of salt. However, they observed a continuous decrease in association constant with increasing concentration of NaClO4 in the case of a guest molecule containing a −N(CH3)2 group. Dey et al.81 proposed that a ternary complex is formed by the −NH2 group of the guest molecule, a ClO4− ion, and a secondary hydroxyl groups on the larger rim of β-CD through specific interactions. The ClO4− ions apparently act as a saltingout agent at low concentration, increasing the binding interaction between the host and guest. However, in the case of a guest molecule containing a −N(CH3)2 group, no ternary complex is formed, and ClO4− ions show a salting-in effect throughout the whole concentration range.81 It is noteworthy that Dey et al.81 and Saha and Kanchanamala49 did not realize the formation of any nanotubes in their works. Contrary to this observation, in the case of DASPC22 as a guest molecule, an increase in binding interaction was observed at a low concentration range of salts even though the dye has a less polar −N(CH3)2 group attached to it. This could be because of the fact that, instead of ternary complex formation, a ClO4− ion provides an anchor site to two neighboring β-CDs in a nanotube through hydrogen bonds.47 However, at a higher concentration of perchlorate ions, binding becomes difficult because of the dominating salting-in effect as well as competition between ClO4− ions and DASPC22 molecules for binding with CD molecules.49,81,82 However, in the case of F− and Cl− ions, it is expected that the decrease in fluorescence intensity at high concentrations could be due to only a saltingin effect rather than competition with the dye. It can be seen in Figure 9 that the F/F0 value at a maximum is highest with KClO4 but lowest with KCl and KF. These results suggest that, in the low concentration range of salts, the binding interactions between the host and guest are strongest with KClO4 and weakest with KF. I



Article

corresponding peak maxima. Because we did not obtain good χ2 values for KCl and KF, we do not report lifetime data in the presence of these two salts here. The difficulties could be because of the low lifetime values in these cases, as the instrument response function of the present laser system is ∼165 ps, which provides lifetime values correctly if they are greater than ∼80 ps. However, in the presence of the other two salts (KClO4, KI), we obtained very good χ2 values. The decays were found to be biexponential. Values of the average lifetime (⟨τ⟩) were calculated using the equation

Figure 10 represents a bar chart showing a concentration of inorganic salts at which the fluorescence intensity ratio (F/F0)

⟨τ ⟩ = ∑ aiτi

(3)

where ai is the pre-exponential factor of the ith component and τi is the lifetime of the ith component. The average lifetimes of the dye at three different concentrations of a salt with χ2 values are reported in Table 2. These three concentrations of salt

Figure 10. Bar chart showing the concentration of salts required to achieve maximum binding interactions between DASPC22 and β-CD nanotubes. [DASPC22] = 5 μM, [β-CD] = 8 mM.

Table 2. Excited State Lifetimesa (τ1 and τ2), PreExponential Factorsb (a1 and a2), and Average Lifetimes (⟨τ⟩) of DASPC22 in Nanotubes in the Presence of KClO4 and KI

is a maximum in Figure 9. The concentrations of salts at which the maximum stability of the complex between the dye molecules and nanotubes occurs are 8.1× 10−8, 3.0 × 10−6, 4.0 × 10−4, and 1.0 × 10−3 M for KClO4, KI, KCl, and KF, respectively. These results indicate that the concentration of salt required to achieve the maximum binding interaction is lowest for KClO4 and highest for KF. In the presence of 8.1 × 10−8 M of KClO4, the intensity ratio increases by a factor of 2.5 compared to that at zero concentration of KClO4, which is much higher than the increases observed with the other salts. Jungwirth and Tobias83 reported, based on molecular dynamics simulation data of 1.5 M solutions of sodium halogenides, that ions such as Br−, I−, and presumably ClO4− are more surfaceactive than Cl− and F−. The interfacial concentrations of the first three ions were found to be higher than those of the last two ions. In fact, F− ions are repelled from the interface. Recently, Turshatov et al.54 showed that the anions ClO4− and I− interact more strongly than other ions with the cationic chromophore of hemicyanine dyes. Based on these reports, our proposal is that, even though Cl− and F− ions are good saltingout agents, the higher efficiency of ClO4− and I− ions toward strengthening of host−guest complex at low concentrations of salts is because of closer contact with the cationic chromophore of the latter two ions as compared to the former two ions. As a consequence of large attractive interactions between the highly polarizable ions (I−, ClO4−) and the cationic chromophore, the binding efficiency of the dye with the nonpolar β-CD cavity is higher in the presence of these ions than in the presence of the other two. As a result, significant increases in absorbance and fluorescence intensity are observed in the low concentration range of salts containing I− and ClO4−. Between ClO4− and I− ions, the higher binding efficiency in the presence of the former is because of its extra anchoring effect. However, at high concentrations of salts, the salting-in effect predominates over the salting-out effect. The decreasing order of contribution toward strengthening the binding interactions between the host and guest and also salting-in efficiency is KClO4 > KI > KCl > KF, following a Hofmeister series. 3.6.2. Fluorescence Lifetime Measurements. To support the effect of potassium salts at low concentrations on the binding of the dye with nanotubes, the excited-state lifetimes of DASPC22 (5 μM) were calculated in the presence of 8 mM βCD and salts from the fluorescence collected at the

a

[salt] (M)

a1

τ1 (ps)

5.0 × 10−9 8.1 × 10−8 9.0 × 10−7

0.97 0.98 0.97

89 109 96

5.0 × 10−7 3.5 × 10−6 2.0 × 10−5

0.96 0.97 0.96

a2

KClO4 0.03 0.02 0.03 KI 78 0.04 92 0.03 85 0.04

τ2 (ps)

⟨τ⟩ (ps)

χ2

489 577 519

102 121 109

1.2 1.1 1.0

430 512 445

91 104 98

1.1 1.1 1.2

λexc = 375 nm. bAll pre-exponential factors (a) are normalized

correspond to fluorescence intensities of the dye before a maximum, at a maximum, and after a maximum (Figure 9). It can be seen that the lifetime increases and then decreases with increasing concentration of salt, supporting the steady-state fluorescence data. The maximum lifetime values were found to be 121 and 104 ps in the presence of KClO4 and KI, respectively. With these data, we can expect a trend following the Hofmeister series. Salts at low concentration enhance host−guest binding interactions. As a result, the dye molecules experience a less polar environment, and thus, the emitting state is destabilized. That is why the fluorescence lifetime increases with increasing fluorescence intensity. However, at high salt concentrations, because of the annihilation of the nanotubular assembly, the dye molecules are transferred from a nonpolar to a polar environment. The emitting state is stabilized in such a polar medium resulting in a decrease in the lifetime with quenching of fluorescence. It was observed that the fluorescence intensity of DASPC22 in nanotubular cavities of 8 mM concentration of β-CD was enhanced by 350-fold. The experimental results with potassium salts indicate that the fluorescence intensity can be further enhanced in the presence of very low concentrations of potassium salts. Moreover, the change in fluorescence intensity can be tuned by selective addition of potassium salts with various anions according to a Hofmeister series. Therefore, this simple host−guest supramolecular system induced enhanced fluorescence property of a dye, and tuning of the fluorescence just by the addition of small amounts of salts could possibly be useful in constructing potential logic gates41,55−57 and also J



Article

KClO4 or a low concentration of KClO4 is seen in the micrograph. At a higher concentration, KClO4 acts as a saltingin agent and increases the solubilities of both CD and DASPC22 in water by breaking the water structure, which results in a decrease in hydrophobic interactions between them. This leads to the destruction of the nanotubes of the β-CD− DASPC22 system, giving shorter nanotubes and a lesser extent of secondary aggregation. Although AFM images taken in the solid phase might not exactly match the actual situation in the solution phase, they provide an idea about molecular aggregation that might not be entirely different from that in the solution phase.

could help materials scientists develop novel supramolecular materials. 3.7. AFM Images. The top views of AFM images of secondary aggregations45−47,79,84 of nanotubes of the β-CD− DASPC22 system are shown in Figure 11. These images were

4. CONCLUSIONS DASPC22 shows negative solvatochromism in absorption and positive solvatochromism in emission. It is nonfluorescent in water, being in the form of H-aggregates. The fluorescence intensity of the dye decreases with increasing polarity of the solvent. The gradual disaggregation with increasing amount of dioxane in dioxane/water mixtures correlates with reduced fluorescence quenching. The higher fluorescence quantum yield in a solvent of high viscosity, such as ethylene glycol, as compared to other solvents of similar polarity supports the fact that the formation of a TICT state is difficult in a viscous medium due to inhibited twisting motion. Therefore, any reason in favor of the formation of TICT state reduces the fluorescence intensity. Below the cac of β-CD (∼2.7 mM), a simple 1:2 (chromophoric part/β-CD) inclusion complex is formed between DASPC22 and β-CD, encapsulating both the donor and acceptor moieties inside the cavities of β-CD. Above the cac, the β-CD molecules are threaded onto the remaining part of the dye molecule, that is, the alkyl chain, forming nanotubes. This process is followed by the dissociation of Haggregates into the monomeric form of the dye. There is a 350fold increase in fluorescence intensity of DASPC22 in nanotubular cavities formed by 8 mM β-CD with respect to the fluorescence intensity of the dye in the form of a simple inclusion complex with β-CD. In addition, the effect of potassium salts on the binding of DASPC22 with β-CD nanotubes has been demonstrated. Potassium salts at high concentrations act as salting-in agents and reduce the binding strength between host and guest molecules according to the Hofmeister series, giving H-aggregates of the dye. However, all of the salts at very low concentrations show a further increase in fluorescence intensity because of the enhanced stability of binding between the host and guest molecules. A 2.5-fold increase in fluorescence intensity was observed upon the addition of ClO4− ions. The change in fluorescence intensity can be tuned by selective addition of potassium salts with various anions according to a Hofmeister series. Even though F− and Cl− ions are good salting-out reagents, strong host− guest binding interactions were observed with low concentration of ClO4− and I− ions as well. The presence of ClO4− and I− ions at low concentrations offers stronger binding interactions between host and guest molecules. This is possibly because of stronger Coulombic interactions between these ions and the cationic chromophore of the dye as compared to the other two ions. The exceptionally high host−guest binding interaction in the presence of low concentrations of ClO4− could be because of its additional anchoring effect. Enhanced emission of the dye DASPC22 in the presence of nanotubular assemblies of β-CD might warrant the development of highly ordered nanomaterials with sensitive fluorescence properties in

Figure 11. AFM micrographs of the β-CD−DASPC22 system with [DASPC22] = 5 μM and [β-CD] = 8 mM: [KClO4] = (a) 0, (b) 8.1 × 10−8, and (c) [KClO4] = 1 × 10−4 M.

taken with 8 mM β-CD in the presence of 5 μM DASPC22. The DASPC22 molecule has an alkyl chain with 22 methylene groups. Hence, on increasing the concentration of β-CD above its cac of 2.7 mM, the CD molecules are threaded onto the alkyl chain of the dye, in addition to the pyridine and aniline moieties being included within the hydrophobic cavity forming a nanotube. The nanotubes then undergo secondary aggregation, forming rodlike structures as a result of internanotubular hydrogen-bonding interactions.45−47,79,84 The well-ordered aggregates shown in Figure 11a possibly suggest the threading of CDs on the alkyl chain followed by secondary aggregation. The lengths of rodlike structures of secondary aggregates lie in the range of 150−200 nm, and the widths of the rods (i.e., the lengths of the nanotubes) lie in the range of 20−27 nm. To see the effect of a low concentration of KClO4 on the binding interactions between β-CD and DASPC22, an AFM micrograph was taken with 8.1 × 10−8 M of KClO4, as shown in Figure 11b. At this concentration of KClO4, the lengths of the rods vary from 150 to 300 nm, and the lengths of the nanotubes are in the range of 21−29 nm. Thus, the extents of both nanotube formation and secondary aggregation seem to be comparatively higher than they were without KClO4. This is because of the fact that the binding strength is a maximum at this concentration of KClO4. As discussed above, up to this low concentration, KClO4 supports the formation of nanotubular assemblies, providing an anchor site to two neighboring β-CDs through hydrogen bonds.47 Figure 11c represents an AFM micrograph of the β-CD− DASPC22 system with a high concentration (1 × 10−4 M) of KClO4. In this high concentration range, a lesser extent of secondary aggregation as compared to a solution with no K



Article

(8) Huang, Y.; Cheng, T.; Li, F.; Huang, C.-H.; Wang, S.; Huang, W.; Gong, Q. Photophysical Studies on the Mono- and Dichromophoric Hemicyanine Dyes III. Ultrafast Fluorescence Up-Conversion in Methanol: Twisting Intramolecular Charge Transfer and “Two-State Three-Mode” Model. J. Phys. Chem. B 2002, 106, 10041−10050. (9) Huang, Y.; Cheng, T.; Li, F.; Luo, C.; Huang, C.-H.; Cai, Z.; Zeng, X.; Zhou, J. Photophysical Studies on the Mono- and Dichromophoric Hemicyanine Dyes II. Solvent Effects and Dynamic Fluorescence Spectra Study in Chloroform and in LB Films. J. Phys. Chem. B 2002, 106, 10031−10040. (10) Ashwell, G. J. Langmuir−Blodgett Films: Molecular Engineering of Non-Centrosymmetric Structures for Second-Order Nonlinear Optical Applications. J. Mater. Chem. 1999, 9, 1991−2003. (11) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (12) Clays, K.; Olbrechts, G.; Munters, T.; Persoons, A.; Kim, O.-K.; Choi, L.-S. Enhancement of the Molecular Hyperpolarizability by a Supramolecular Amylose−Dye Inclusion Complex, Studied by HyperRayleigh Scattering with Fluorescence Suppression. Chem. Phys. Lett. 1998, 293, 337−342. (13) Gorse, A.-D.; Pesquer, M. Intramolecular Charge Transfer Excited State Relaxation Processes in Para-Substituted N,N-Dimethylaniline: A Theoretical Study Including Solvent Effects. J. Phys. Chem. 1995, 99, 4039−4049. (14) Hayashi, S.; Ando, K.; Kato, S. Reaction Dynamics of ChargeTransfer State Formation of 4-(N,N-Dimethylamino)benzonitrile in a Methanol Solution: Theoretical Analyses. J. Phys. Chem. 1995, 99, 955−964. (15) LaFemina, J. P.; Duke, C. B.; Paton, A. Electronic Structure and Twisted Intramolecular Charge Transfer in Dimethylanilines. J. Chem. Phys. 1987, 87, 2151−2157. (16) Rotkiewicz, K.; Grellmann, K. H.; Grabowski, Z. R. Reinterpretation of the Anomalous Fluorescense of p-N,N-dimethylamino benzonitrile. Chem. Phys. Lett. 1973, 19, 315−318. (17) Kosower, E. M.; Dodiuk, H. Multiple Fluorescences. II. A New Scheme for 4-(N,N-Dimethylamino)benzonitrile Including Proton Transfer. J. Am. Chem. Soc. 1976, 98, 924−929. (18) Majumdar, D.; Sen, R.; Bhattacharyya, K.; Bhattacharyya, S. P. Twisted Intramolecular Charge Transfer of p-(N,N-Dimethylamino)benzonitrile: An Approximate Quantum Mechanical Study Including Solvation Effects. J. Phys. Chem. 1991, 95, 4324−4329. (19) Marguet, S.; Mialocq, J. C.; Millie, P.; Berthier, G.; Momicchioli, F. Intramolecular Charge Transfer and Trans−Cis Isomerization of the DCM Styrene Dye in Polar Solvents. A CS INDO MRCI Study. Chem. Phys. 1992, 160, 265−279. (20) Schenter, G. K.; Duke, C. B. Theory of Photoinduced Twisting Dynamics in Polar Solvents: Application to Dimethylaminobenzonitrile in Propanol at Low Temperatures. Chem. Phys. Lett. 1991, 176, 563−570. (21) Cao, X.; Tolbert, R. W.; McHale, J. L.; Edwards, W. D. Theoretical Study of Solvent Effects on the Intramolecular Charge Transfer of a Hemicyanine Dye. J. Phys. Chem. A 1998, 102, 2739− 2748. (22) Strehmel, B.; Seifert, H.; Rettig, W. Photophysical Properties of Fluorescence Probes. 2. A Model of Multiple Fluorescence for Stilbazolium Dyes Studied by Global Analysis and Quantum Chemical Calculations. J. Phys. Chem. B 1997, 101, 2232−2243. (23) McHale, J. L. Subpicosecond Solvent Dynamics in ChargeTransfer Transitions: Challenges and Opportunities in Resonance Raman Spectroscopy. Acc. Chem. Res. 2001, 34, 265−272. (24) Cao, X.; McHale, J. L. Resonance Raman Study of Solvent Dynamics on the Spectral Broadening and Intramolecular Charge Transfer of a Hemicyanine Dye in Aqueous Solution. J. Chem. Phys. 1998, 109, 1901−1911. (25) Kim, J.; Lee, M. Excited-State Photophysics and Dynamics of a Hemicyanine Dye in AOT Reverse Micelles. J. Phys. Chem. A 1999, 103, 3378−3382.

the future. In addition, the dramatic enhancement of fluorescence properties of the dye in a simple host−guest supramolecular system and its tuning through the use of potassium salts might help in the development of potential logic gates.

■

ASSOCIATED CONTENT

S Supporting Information *

Absorption ratio, absorption peak maxima, and emission spectra of DASPC22 in dioxane/water mixtures with different percentages of dioxane; steady-state fluorescence anisotropy of DASPC22 with varying concentrations of β-CD; absorption spectra of DASPC22 in the presence of β-CD with varying concentrations of KF; emission spectra of DASPC22 in the presence of β-CD with varying concentrations of KClO4, KI, KCl, and KF; absorption spectra of aqueous solutions of DASPC22 with varying concentrations of KClO4 in the absence of β-CD; and molecular structure of DASPC1. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-1596 515279. Fax: +91-1596 244183. E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS S.K.S. acknowledges the University Grants Commission for financial support under Major Research Project 33-257/ 2007(SR) and Special Assistance Programme F.540/14/DRS/ 2007 (SAP-I); the Council of Scientific and Industrial Research (CSIR), Government of India, for financial support under Major Research Project 01(2213)/08/EMR-11; and also the Department of Science and Technology (DST) FIST program, Government of India, for financial support. M.S. and A.K.T. acknowledge CSIR for financial support under senior research fellowships, and S. acknowledges UGC for financial support under a junior research fellowship.

■

REFERENCES

(1) Yan, P.; Xie, A.; Wei, M.; Loew, L. M. Amino(Oligo)ThiopheneBased Environmentally Sensitive Biomembrane Chromophores. J. Org. Chem. 2008, 73, 6587−6594. (2) Wandelt, B.; Mielniczak, A.; Turkewitsch, P.; Darling, G. D.; Stranix, B. R. Substituted 4-[4-(Dimethylamino)styryl]pyridinium Salt as a Fluorescent Probe for Cell Microviscosity. Biosens. Bioelectron. 2003, 18, 465−471. (3) Miller, E.; Wandelt, B.; Wysocki, S.; Józwik, D.; Mielniczak, A. The Fluorescence Studies of the Sol−Gel Transition by Styrylpyridine Derivative. Biosens. Bioelectron. 2004, 20, 1196−1202. (4) Rokitskaya, T. I.; Antonenko, Y. N.; Kotova, E. A. Effect of the Dipole Potential of a Bilayer Lipid Membrane on Gramicidin Channel Dissociation Kinetics. Biophys. J. 1997, 73, 850−854. (5) Turkewitsch, P.; Darling, G. D.; Powell, W. S. Enhanced Fluorescence of 4-(p-Dimethylaminostyryl)pyridinium Salts in the Presence of Biological Macromolecules. J. Chem. Soc., Faraday Trans. 1998, 94, 2083−2087. (6) Kuhn, B.; Fromherz, P. Anellated Hemicyanine Dyes in a Neuron Membrane: Molecular Stark Effect and Optical Voltage Recording. J. Phys. Chem. B 2003, 107, 7903−7913. (7) Zhao, C. F.; Gvishi, R.; Narang, U.; Ruland, G.; Prasad, P. N. Structures, Spectra, and Lasing Properties of New (Aminostyryl)Pyridinium Laser Dyes. J. Phys. Chem. 1996, 100, 4526−4532. L



Article

Nanotubes of β-Cyclodextrin and Their Secondary Assembly: Study with trans-2-[4(Dimethylamino)styryl]benzothiazole, a TICT-Fluorescence Probe. J. Photochem. Photobiol. A: Chem. 2009, 205, 186−196. (48) Zakharov, M.; Masunov, A. E.; Dreuw, A. Water Deficient Environment Accelerates Proton Exchange: Acetone−Water Reaction Catalyzed by Calix[4]hydroquinone Nanotubes. J. Phys. Chem. C 2009, 113, 10395−10401. (49) Saha, S. K.; Kanchanamala, A. K. Complexations of 2Aminofluorene and 2-N,N′-Dimethylaminofluorene to β-Cyclodextrin in Presence of Sodium Perchlorate in Aqueous Solution. J. Surf. Sci. Technol. 2006, 22, 35−54. (50) Breslow, R. Hydrophobic Effects on Simple Organic Reactions in Water. Acc. Chem. Res. 1991, 24, 159−164. (51) Gurau, M. C.; Lim, S.-M.; Castellana, E. T.; Albertorio, F.; Kataoka, S.; Cremer, P. S. On the Mechanism of the Hofmeister Effect. J. Am. Chem. Soc. 2004, 126, 10522−10523. (52) Heuvingh, J.; Zappa, M.; Fery, A. Salt Softening of Polyelectrolyte Multilayer Capsules. Langmuir 2005, 21, 3165−3171. (53) Lima, S.; Goodfellow, B. J.; Teixeira-Dias, J. J. C. How Inorganic Anions Affect the Inclusion of Hexanoic and Decanoic Acid in βCyclodextrin. J. Phys. Chem. A 2004, 108, 10044−10049. (54) Turshatov, A. A.; Möbius, D.; Bossi, M. L.; Hell, S. W.; Vedernikov, A. I.; Lobova, N. A.; Gromov, S. P.; Alfimov, M. V.; Zaitsev, S. Y. Molecular Organization of an Amphiphilic Styryl Pyridinium Dye in Monolayers at the Air/Water Interface in the Presence of Various Anions. Langmuir 2006, 22, 1571−1579. (55) Pischel, U. Chemical Approaches to Molecular Logic Elements for Addition and Subtraction. Angew. Chem., Int. Ed. 2007, 46, 4026− 4040. (56) Szaciłowski, K. Digital Information Processing in Molecular Systems. Chem. Rev. 2008, 108, 3481−3548. (57) Andreasson, J.; Pischel, U. Smart Molecules at Work Mimicking Advanced Logic Operations. Chem. Soc. Rev. 2010, 39, 174−188. (58) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Kluwer Academic: New York, 1999. (59) Sowmiya, M.; Tiwari, A. K.; Saha, S. K. Fluorescent Probe Studies of Micropolarity, Premicellar and Micellar Aggregation of Non-Ionic Brij Surfactants. J. Colloid Interface Sci. 2010, 344, 97−104. (60) Kim, O.-K.; Choi, L. S. Supramolecular Inclusion Complexation of Amylose with Photoreactive Dyes. Langmuir 1994, 10, 2842−2846. (61) Saha, S. K.; Dogra, S. K. Fluorescence Quenching of Aromatic Amines by Chloromethanes. J. Lumin. 1997, 75, 117−125. (62) West, W.; Pearce, S. The Dimeric State of Cyanine Dyes. J. Phys. Chem. 1965, 69, 1894−1903. (63) Rao, T. V. S.; Huff, J. B.; Bieniarz, C. Supramolecular Control of Photophysical Properties of Cyanine Dyes. Tetrahedron 1998, 54, 10627−10634. (64) Byers, G. W.; Gross, S.; Henrichs, P. M. Direct and Sensitized Photooxidation of Cyanine Dyes. Photochem. Photobiol. 1976, 23, 37− 43. (65) Busch, W.; Martin, R.; Herrmann, R. G. Sensitivity Enhancement of Fluorescence in Situ Hybridization on Plant Chromosomes. Chromosome Res. 1994, 2, 15−20. (66) Narang, U.; Zhao, C. F.; Bhawalkar, J. D.; Bright, F. V.; Prasad, P. N. Characterization of a New Solvent-Sensitive Two-PhotonInduced Fluorescent (Aminostyryl)Pyridinium Salt Dye. J. Phys. Chem. 1996, 100, 4521−4525. (67) Toro, C.; Thibert, A.; De Boni, L.; Masunov, A. E.; Hernandez, F. E. Fluorescence Emission of Disperse Red 1 in Solution at Room Temperature. J. Phys. Chem. B 2008, 112, 929−937. (68) Pant, D. D.; Bhagchandani, C. L.; Pant, K. C.; Verma, S. P. Aggregation in Xanthene Dyes, Exciton Emission and Phosphorescence Enhancement. Chem. Phys. Lett. 1971, 9, 546−547. (69) Peng, L.; Domanov, Y.; Loffe, V.; Finashin, A.; Gorbenko, G. Interaction of a Series of Fluorescent Probes with Glucose Oxidase. J. Biol. Phys. Chem. 2006, 6, 77−82. (70) Kim, O.-K.; Choi, L.-S.; Zhang, H.-Y.; He, X.-H.; Shih, Y.-H. Second-Harmonic Generation by Spontaneous Self-Poling of Supra-

(26) Görner, H.; Gruen, H. Photophysical Properties of Quaternary Salts of 4-Dialkylamino-4′-azastilbenes and Their Quinolinium Analogues in Solution: IX. J. Photochem. 1985, 28, 329−350. (27) Kumar, C. V.; Turner, R. S.; Asuncion, E. H. Groove Binding of a Styrylcyanine Dye to the DNA Double Helix: The Salt Effect. J. Photochem. Photobiol. A: Chem. 1993, 74, 231−238. (28) Fromherz, P.; Heilemann, A. Twisted Internal Charge Transfer in (Aminophenyl)pyridinium. J. Phys. Chem. 1992, 96, 6864−6866. (29) Masunov, A.; Tretiak, S.; Hong, J. W.; Liu, B.; Bazan, G. C. Theoretical Study of the Effects of Solvent Environment on Photophysical Properties and Electronic Structure of Paracyclophane Chromophores. J. Chem. Phys. 2005, 122, 224505−224514. (30) Saito, K.; Yokoyama, H. Transient Photocurrent Response in Merocyanine Langmuir−Blodgett Films. Thin Solid Films 1994, 243, 526−529. (31) Ozdemir, O.; Armagan, B.; Turan, M.; Ç elik, M. S. Comparison of the Adsorption Characteristics of Azo-Reactive Dyes on Mezoporous Minerals. Dyes Pigm. 2004, 62, 49−60. (32) Soper, S. A.; Mattingly, Q. L. Steady-State and Picosecond Laser Fluorescence Studies of Nonradiative Pathways in Tricarbocyanine Dyes: Implications to the Design of Near-IR Fluorochromes with High Fluorescence Efficiencies. J. Am. Chem. Soc. 1994, 116, 3744−3752. (33) Arunkumar, E.; Forbes, C. C.; Smith, B. D. Improving the Properties of Organic Dyes by Molecular Encapsulation. Eur. J. Org. Chem. 2005, 2005, 4051−4059. (34) Saenger, W. Cyclodextrin Inclusion Compounds in Research and Industry. Angew. Chem., Int. Ed. 1980, 19, 344−362. (35) Manivannan, C.; Vijay Solomon, R.; Venuvanalingam, P.; Renganathan, R. Studies on the Inclusion Behavior of 9-Aminoacridine into Cyclodextrins: Spectroscopic and Theoretical Evidences. Spectrochim. Acta A 2013, 103, 18−24. (36) Passier, R.; Ritchie, J. P.; Toro, C.; Diaz, C.; Masunov, A. E.; Belfield, K. D.; Hernandez, F. E. Thermally Controlled Preferential Molecular Aggregation State in a Thiacarbocyanine Dye. J. Chem. Phys. 2010, 133, 134508−134514. (37) Lau, S. F.; Sosnowik, A. J.; Choi, L. S.; Callahan, J. H.; Kim, O. K. Thermal Properties of a Molecule in a Constrained State. J. Therm. Anal. Calorim. 1996, 46, 1081−1092. (38) Kajikawa, K.; Yoshida, I.; Seki, K.; Ouchi, Y. Orientational Structure in Hemicyanine Self-Assembled Films Studied by Absorption Spectroscopy and Optical Second-Harmonic Generation. Chem. Phys. Lett. 1999, 308, 310−316. (39) Das, P. K.; Banerjee, D.; Bagchi, S. Spectroscopic Study of Association of a Hemicyanine Dye in Mixed Aqueous Binary Solvents. Spectrochim. Acta A 2007, 67, 225−229. (40) Herkstroeter, W. G.; Martic, P. A.; Farid, S. Inclusion by Cyclodextrins to Control Dye Aggregation Equilibria in Aqueous Solution. J. Am. Chem. Soc. 1990, 112, 3583−3589. (41) Li, Z.; Sun, S.; Liu, F.; Pang, Y.; Fan, J.; Song, F.; Peng, X. Large Fluorescence Enhancement of a Hemicyanine by Supramolecular Interaction with Cucurbit[6]uril and Its Application as Resettable Logic Gates. Dyes Pigm. 2012, 93, 1401−1407. (42) Pischel, U.; Uzunova, V. D.; Remon, P.; Nau, W. M. Supramolecular Logic with Macrocyclic Input and Competitive Reset. Chem. Commun. 2010, 46, 2635−2637. (43) Fedorova, O. A.; Chernikova, E. Y.; Fedorov, Y. V.; Gulakova, E. N.; Peregudov, A. S.; Lyssenko, K. A.; Jonusauskas, G.; Isaacs, L. Cucurbit[7]uril Complexes of Crown-Ether Derived Styryl and (Bis)styryl Dyes. J. Phys. Chem. B 2009, 113, 10149−10158. (44) Zhang, H.; Wang, Q.; Liu, M.; Ma, X.; Tian, H. Switchable VType [2]Pseudorotaxanes. Org. Lett. 2009, 11, 3234−3237. (45) Wu, A.; Shen, X.; He, Y. Investigation on γ-Cyclodextrin Nanotube Induced by N,N′-Diphenylbenzidine Molecule. J. Colloid Interface Sci. 2006, 297, 525−533. (46) Wu, A.; Shen, X.; He, Y. Micrometer-Sized Rodlike Structure Formed by the Secondary Assembly of Cyclodextrin Nanotube. J. Colloid Interface Sci. 2006, 302, 87−94. (47) Sowmiya, M.; Purkayastha, P.; Tiwari, A. K.; Jaffer, S. S.; Saha, S. K. Characterization of Guest Molecule Concentration Dependent M



Article

molecular Thin Films of an Amylose−Dye Inclusion Complex. J. Am. Chem. Soc. 1996, 118, 12220−12221. (71) Heuer, W. B.; Lee, H. S.; Kim, O.-K. Influence of Hemicyanine Dye Structures on Spectral Properties of Their Supramolecular Complexes with Amylose. Chem. Commun. 1998, 2649−2650. (72) Muniz-Miranda, M.; Puggelli, M.; Ricceri, R.; Gabrielli, G. Sers and Molecular Orientation in Langmuir−Blodgett Films Deposited onto Smooth Copper Surfaces. Langmuir 1996, 12, 4417−4420. (73) Park, J. W.; Park, K. H. Inclusion of (Aminostyryl)-1Methylpyridinium Dyes by β-Cyclodextrin and Its Use for Fluorescent-Probe Studies on Association of Cationic and Neutral Molecules with β-Cyclodextrin. J. Inclusion Phenom. Macrocyclic Chem. 1994, 17, 277−290. (74) Benesi, H. A.; Hildebrand, J. H. A Spectrophotometric Investigation of the Interaction of Iodine with Aromatic Hydrocarbons. J. Am. Chem. Soc. 1949, 71, 2703−2707. (75) Mishra, A.; Patel, S.; Behera, R. K.; Mishra, B. K.; Behera, G. B. Dye−Surfactant Interaction: Role of an Alkyl Chain in the Localization of Styrylpyridinium Dyes in a Hydrophobic Force Field of a Cationic Surfactant (CTAB). Bull. Chem. Soc. Jpn. 1997, 70, 2913−2918. (76) Mishra, A.; Behera, R. K.; Mishra, B. K.; Behera, G. B. Dye− Surfactant Interaction: Chain Folding During Solubilization of Styryl Pyridinium Dyes in Sodium Dodecyl Sulfate Aggregates. J. Photochem. Photobiol. A: Chem. 1999, 121, 63−73. (77) Li, G.; McGown, L. B. Molecular Nanotube Aggregates of βand γ-Cyclodextrins Linked by Diphenylhexatrienes. Science 1994, 264, 249−251. (78) Pistolis, G.; Malliaris, A. Nanotube Formation between Cyclodextrins and 1,6-Diphenyl-1,3,5-hexatriene. J. Phys. Chem. 1996, 100, 15562−15568. (79) Jaffer, S. S.; Saha, S. K.; Eranna, G.; Sharma, A. K.; Purkayastha, P. Intramolecular Charge Transfer Probe Induced Formation of αCyclodextrin Nanotubular Suprastructures: A Concentration Dependent Process. J. Phys. Chem. C 2008, 112, 11199−11204. (80) Jaffer, S. S.; Saha, S. K.; Purkayastha, P. Fragmentation of Molecule-Induced γ-Cyclodextrin Nanotubular Suprastructures Due to Drug Dosage. J. Colloid Interface Sci. 2009, 337, 294−299. (81) Dey, J.; Roberts, E. L.; Warner, I. M. Effect of Sodium Perchlorate on the Binding of 2-(4′-Aminophenyl)- and 2-(4′-(N,N′Dimethylamino)phenyl)benzothiazole with β-Cyclodextrin in Aqueous Solution. J. Phys. Chem. A 1998, 102, 301−305. (82) Rohrbach, R. P.; Rodriguez, L. J.; Eyring, E. M.; Wojcik, J. F. An Equilibrium and Kinetic Investigation of Salt−Cycloamylose Complexes. J. Phys. Chem. 1977, 81, 944−948. (83) Jungwirth, P.; Tobias, D. J. Ions at the Air/Water Interface. J. Phys. Chem. B 2002, 106, 6361−6373. (84) Muthusubramanian, S.; Tiwari, A. K.; Sonu; Saha, S. K. A Cationic Surfactant-Concentration Dependent Binding of a Guest Molecule with the Nanotubes of β-Cyclodextrin and Its Release from the Nanotubular Cavities. Soft Matter 2012, 8, 11072−11084.

N


Study of the Binding Interactions of a Hemicyanine Dye with

Recommend Documents