Metal-Ion-Mediated Assemblies of Thiazole Orange with Cucurbit[7

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Metal Ion-Mediated Assemblies of Thiazole Orange with Cucurbit[7]uril: A Photophysical Study Meenakshi N. Shinde, Sharmistha Dutta Choudhury, Nilotpal Barooah, Haridas Pal, Achikanath C. Bhasikuttan, and Jyotirmayee Mohanty J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp512802u • Publication Date (Web): 06 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Metal Ion-Mediated Assemblies of Thiazole Orange with Cucurbit[7]uril: A Photophysical Study Meenakshi N. Shinde†, Sharmistha Dutta Choudhury*, Nilotpal Barooah, Haridas Pal, Achikanath C. Bhasikuttan and Jyotirmayee Mohanty* Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India

KEYWORDS. Supramolecular, self-assembly, cucurbit[7]uril, thiazole orange, noncovalent.

ABSTRACT. The formation of molecular superstructures by metal ion-mediated noncovalent self-assembly has been demonstrated, using the macrocycle, cucurbit[7]uril (CB7) and the dye, thiazole orange (TO) as building blocks. Interestingly, the association of these molecular building blocks can be tuned by the chemical environment, leading to self-assembled structures of different stoichiometries, which is supported by absorption, fluorescence, 1H NMR and AFM measurements. Most importantly, the self-assembly process of the CB7/TO/metal ion system is observed to be remarkably different for alkali (Na+) and alkaline earth (Ca2+) metal ions. Fluorescence enhancement is observed in the presence of Ca2+ ions, which is attributed to the formation of short dimeric structures composed of two 1:1 CB7-TO complexes. On the other hand, solution turbidity is detected in the presence of Na+ ions, which is proposed to be due to formation of extended structures by the assembly of many 1:1 CB7-TO complexes.

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INTRODUCTION Molecular assemblies held together by noncovalent interactions between selected monomer building blocks are not only structurally attractive but also have great potential as advanced materials with novel functionalities.1-3 In contrast to their covalent counterparts, noncovalent supramolecular structures are relatively easy to fabricate and offer several advantages. Most importantly, the dynamic and relatively weaker noncovalent interactions impart reversibility and flexibility to the prepared superstructures. On exposure to external stimulants, the assemblies can spontaneously respond and re-adjust to the new environment by changing the connectivity between the building blocks. This adaptability is very useful in altering the nature of the molecular assemblies and provides the basis for constructing smart functional materials.1-3 In recent years, the specificity and selectivity of host-guest interactions is being increasingly exploited for designing a number of fascinating supramolecular structures. Cucurbit[n]urils (CBn) are a particularly interesting family of macrocyclic hosts that serve as effective building blocks for such architectures.4-7 Depending on the number of glycoluril units, the cucurbituril homologues of different cavity sizes are known, most prominently, cucurbit[7]uril (CB7, Scheme 1) and cucurbit[8]uril (CB8). The barrel shaped CBn molecular containers can encapsulate a wide variety of positively charged guest molecules by virtue of both hydrophobic interactions with the inner cavity and ion-dipole interactions with the carbonyl laced portals.4-6 Among the different CBn homologues, CB8 has a suitably sized cavity that can simultaneously encapsulate two guest molecules. This dimeric inclusion property of CB8 provides a strong binding motif for creating supramolecular structures.8 Host-guest complexes with CB8 have been widely investigated for constructing a variety of dynamic and stimuli responsive structures such as, vesicles, block copolymers, dendrimers, molecular necklaces and molecular machines.8-12


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Another strategy for generating host-guest architectures is to screen for suitable guest molecules that can link and arrange the participating building blocks into desired superstructures. In this regard, molecules that have inherent self-assembling properties can be a good choice. Thiazole Orange (TO) is one such interesting molecule that is quite prone to self-aggregate in aqueous solutions (Scheme 1).13 The cationic dye, TO, is widely used as a fluorogenic probe for the detection of nucleic acids.14,15 Depending on its concentration in solution, TO can exist in monomeric, dimeric or higher aggregated forms, each having characteristic absorption and fluorescence spectral signatures. We have previously studied surfactant induced aggregation patterns of TO.16 Recently, specific ion effects have been found to control the organization of this dye.17 Lau and Heyne have reported the formation of fluorescent H-aggregates of TO in the presence of the host, calix[4]arene sulfonate.18 The interaction of TO with cucurbit[n]uril hosts has also been studied. We have observed the formation of highly fluorescent dimeric complexes of TO with CB8, which can further exhibit controlled release and exchange with chemical stimuli.19 Pang and coworkers have shown that CB8-TO interaction at higher TO concentrations can lead to formation of linear supramolecular polymers although no such polymer formation was observed with CB7, which has a smaller cavity size than CB8.20 In the present study, we describe an interesting new finding on the interaction of CB7 with TO, especially in the presence of metal ions. Metal ion connectors play important roles in constructing supramolecular architectures. Infinite metal-organic backbones can be engineered by proper ligand design and metal ion selection, for achieving specific morphologies. Very recently, Forgan et al. have reported cyclodextrin based metal-organic frameworks by the interaction of β-cyclodextrin with alkali and alkaline earth metal cations.21 The CBn macrocycles are also known to have considerable affinity toward metal ions due to the ion-dipole interactions


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with the carbonyl portals.22-24 We have earlier reported the formation of fluorescent supramolecular capsules by the cooperative interaction of another fluorescent probe, Thioflavin T, with CB7 host and Na+ ions.25 The construction of coordination polymers and supramolecular network structures based on the interaction of CBn with metal ions or their complexes and various organic molecules that act as structure inducers, is an active area of research.26-29 In this paper, we demonstrate, through photophysical studies, a novel switchover in the host-guest assemblies of CB7-TO, achieved simply by changing the chemical environment, specifically the concentrations of the building blocks and the nature of the metal cation (alkali or alkaline earth metal). Present observation is not only significant in view of creating supramolecular architectures but can also be useful for sensing or distinguishing between different metal ion species.

O N

N

N

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O N

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O N

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O N N O

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N N ON N N O N N N O N

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O O

Thiazole Orange (TO)

Cucurbit[7]uril (CB7)

Scheme 1. Chemical structures of the dye, Thiazole Orange (TO) and the macrocyclic host, cucurbit[7]uril (CB7)

EXPERIMENTAL SECTION Thiazole orange {1-methyl-4-[(3-methyl-2(3H)-benzothiazolyli-dene) methyl]quinolinium ptosylate; TO}, sodium chloride and calcium chloride were obtained from Sigma-Aldrich and were used as received. Cucurbit[7]uril (CB7) was synthesized and purified following the


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reported procedures in the literatures.30-32 The purity of CB7 was checked using 1H-NMR spectroscopy. Nanopure water (Millipore Gradient A10 System; conductivity of 0.06 μS cm-1) was used to prepare the sample solutions in Tris buffer (pH 7). Absorption spectra were recorded with a Jasco UV-vis spectrophotometer (model V-650), and steady-state fluorescence spectra were recorded with a Hitachi spectrofluorimeter (model F-4500). The fluorescence spectra were obtained by excitation at 465 nm to maintain minimum change in the absorbance, and changes, if any, have been normalized. Time-resolved fluorescence measurements were carried out using a time-correlated single photon counting (TCSPC) spectrometer (Horiba Jobin Yvon, UK). The samples were excited by light pulses from a nano-LED source (445 nm, repetition rate of 1 MHz) and the fluorescence was detected using a PMT based detection module (model TBX4). A reconvolution procedure was used to analyze the observed decays using a proper instrument response function obtained by substituting the sample cell with a light scatterer (turbid solution of TiO2 nanoparticles in water).33,34 With the present setup, the instrument time resolution is adjudged to be about 40 ps. The fluorescence decays were analyzed as a sum of exponentials as,

I( t ) =

∑ B exp(− t τ ) i

i

i

(1)

where I(t) is the time-dependent fluorescence intensity and Bi and τi are the pre-exponential factor and the fluorescence lifetime for the ith component of the fluorescence decay, respectively. The quality of the fits and consequently the mono-/multi-exponential natures of the decays were judged by the reduced chi-square (χ2) values and the distribution of the weighted residuals among the data channels. For a good fit, the χ

2

value was close to unity and the weighted

residuals were distributed randomly among the data channels. For anisotropy measurements, samples were excited with a vertically polarized excitation beam, and the vertically and


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horizontally polarized fluorescence decays were collected with a large spatial bandwidth of ~32 nm. Using these polarized fluorescence decays, the anisotropy decay function, r(t), was constructed as follows:33

r(t) =

I V ( t ) − GI H ( t ) I V ( t ) + 2GI H ( t )

(2)

IV(t) and IH(t) are the vertically and horizontally polarized decays, respectively, and G is the correction factor for the polarization bias of the detection setup. The G factor was determined independently by using a horizontally polarized excitation beam and measuring the two perpendicularly polarized fluorescence decays. All anisotropy measurements were repeated twice. The AFM images were recorded on a NT-MDT solver model P47 instrument (Russia) with 50μm scanner head and silicon nitride tip, in semi contact mode. The sample for AFM measurement was prepared by drop casting a dilute solution on a mica sheet followed by drying. 1

H NMR spectra were recorded on a Bruker Avance WB 500 MHz spectrometer at TIFR,

Mumbai, India.

RESULTS AND DISCUSSION The absorption spectrum of TO (3 μM) in aqueous solution shows maximum around 500 nm, corresponding to the monomer form of the dye. On addition of the macrocyclic host, CB7, significant spectral changes are observed as shown in Figure 1. At lower concentrations of CB7 (up to ~50 μM), the absorbance (or optical density, OD) for the 500 nm peak gradually decreases


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and new bands develop with maximum at 483 nm and a shoulder around 510 nm. The new spectrum matches well with that of TO H-dimer.16,19 An apparent isosbestic point can be discerned around 487 nm, supporting conversion of TO monomers to dimers. With further increase in the CB7 concentration, the 483 nm absorption band starts decreasing and the spectral characteristics revert to the monomeric form of TO, with an isosbestic point around 490 nm. However, the absorbance does not match completely with that of the monomer form in pure water.

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Wavelength (nm) Figure 1. Absorption spectra of TO (3 μM) with varying concentrations of CB7/μM: 0, 2, 5, 10, 15, 20, 25, 50 (1-8, solid lines), 100, 200, 300, 400, 600, 800 (9-14, dashed lines). Inset shows the variation in OD at 505 nm with increasing CB7 concentrations, the solid line is given as a guide for viewing.

Figure 2 shows the fluorescence spectra of TO in the presence of different concentrations of CB7. The fluorescence intensity of TO in aqueous solutions is very weak (φf = 0.0002)19,35 and shows significant Raman scattering. The weak fluorescence of TO is due to the rapid nonradiative deactivation by photoisomerization as well as torsional motion between the benzothiazole and quinoline groups around the monomethine bond (Scheme 1).35,36 According to literature reports, the emission maximum for TO monomer may be expected at 528 nm.35,36 With


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increasing CB7 concentrations, a gradual increase in the fluorescence intensity is observed with maximum emission around 600 nm. This corresponds to the emission from TO H-dimers.16,19 Beyond ~100 μM CB7 concentration, the fluorescence intensity starts decreasing again and a significant blue shift in the fluorescence spectra can be observed. Although the maximum in the final fluorescence spectrum is not clear due to associated Raman scattering, it appears that the spectral characteristics resemble to a considerable extent to that of TO monomers (maximum ~528 nm).

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Wavelength (nm) Figure 2. Emission spectra of TO (3μM) with varying concentrations of CB7/μM: 0, 5, 10, 15, 20, 30, 40, 50 (1-8, solid lines), 100, 200, 300, 400, 600 (9-13, dashed lines); λex = 465 nm, the band R represents the Raman scattering. Inset shows the variation in the fluorescence intensity at 600 nm with increasing CB7 concentrations, the solid line is given as a guide for viewing.

Both the absorption and fluorescence spectral changes of TO in the presence of CB7 indicate that at lower concentrations the host induces formation of TO H-dimers in the host-guest complex whereas at higher concentrations the host stabilizes the monomer form of TO in the host-guest complex. Since the CB7 host is laced with carbonyl portals, ion-dipole interaction of the cationic TO dye with the CB7 portals can be envisaged. The inherent self-aggregating nature


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of TO coupled with the ion-dipole interaction between TO and CB7 may thus lead to the accumulation of TO dimers at both the portals of the CB7 macrocycle. To examine this possibility, the binding stoichiometry for CB7-TO complex was determined by monitoring absorption changes (at 500 nm) for different mole fractions of TO, keeping the total host-guest concentration constant (5 μM). The resulting Job plot shows two inflection points, one at about 0.33 (which is discussed later) and another at 0.8 mole fraction of TO (Figure S1, Supporting Information). The inflection point at 0.8 mole fraction of TO clearly indicates that at low CB7 concentrations, four molecules of TO are bound to one CB7 moiety. This supports our proposition of TO dimer formation at each of the two portals of CB7 at low host concentrations (Scheme 2, I). Considering a 1:4 host-guest stoichiometry at lower CB7 concentrations, the binding constant for the interaction of each individual TO molecule with CB7 is found to be ~2.8x105 M-1 (Figure S2, Note S1, Supporting Information). The aggregation of TO at the CB7 portals is quite intriguing and is in contrast to the general de-aggregating effect of CB7.37,38 In a recent study, we have shown that cetylpyridinium chloride (CPC) surfactant undergoes a premicellar aggregation at the carbonyl portals in the presence of CB7 (or CB5), before forming the mixed micelle.39 Moreover, the aggregation propensity of TO is also showcased in another study with p-sulfonatocalix[4]arene where the negatively charged sulfonate portals induce the aggregation of TO dyes at the portals.18 With increasing CB7 concentrations, as the availability of host molecules in the solution increases, the H-dimer form (1:4 complex) gradually dissociates and gives way to the host encapsulated monomer form of TO. Since this phenomenon occurs only at higher host concentrations, logically a 2:1 host-guest complex can be envisaged where both the benzothiazole and the quinoline moieties of TO are encapsulated by CB7 (Scheme 2, III), thus


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preventing dimerization of TO. The formation of 2:1 CB7-TO complexes at higher host concentrations is supported by the observation of an inflection point at about 0.33 mole fraction of TO, in the Job plot (Figure S1, Supporting Information). Complex formation between CB7 and TO with 2:1 host-guest stoichiometry has also been suggested earlier by Pang and coworkers.20 From the present study, the binding constant for the 2:1 CB7-TO complex is estimated to be ~8x106 M-2 (Figure S3, Note S2, Supporting Information). The interaction between TO and CB7 was further investigated by 1H NMR studies (Figure S4, Note S3, Supporting Information). In the presence of low CB7 concentrations, the 1H NMR spectra of TO shows significant broadening of the signals, which is in agreement with the formation of TO H-dimers at the CB7 portals. In addition to the broadening, an up-field shift of the N-CH3 protons in the benzothiazole moiety of TO, is also observed in the 1H NMR spectra. This suggests the co-existence of some 1:1 CB7-TO inclusion complexes along with the TO Hdimer form at low host concentrations (Scheme 2, II). Considering the relative dimensions of TO and the CB7 cavity, the formation of 1:1 CB7-TO complexes at low CB7 concentrations seems quite reasonable. However, no separate spectral signature for the 1:1 complexes is, observed in the absorption or emission spectra. This is possibly because the monomer form is masked by the strong overlapping absorption/emission of the dimer form (also discussed later). In the presence of high concentrations of CB7, the 1H NMR spectra of TO were found to show significant upfield shifts for the protons on both the benzothiazole as well as the quinoline moieties and a down-field shift for the methine proton of TO. These results are in accordance with the proposition that both benzothiazole and quinoline moieties are bound to CB7, thus placing the monomethine group between the carbonyl portals of the two CB7 macrocycles in a 2:1 stoichiometry (Scheme 2, III). Geometry optimised structures of CB7-TO complexes having


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different stoichiometries, along with their stabilization energies are presented in Figure S5, Supporting Information. Time-resolved fluorescence studies were carried out to further distinguish between the 1:4 CB7-TO complexes (bearing the spectral signature of TO H-dimers) that are predominantly formed at lower CB7 concentrations and the 2:1 CB7-TO complexes (bearing the spectral signature of TO monomer) that are formed at higher CB7 concentrations. The fluorescence decay time of TO in aqueous solutions is very short, of the order of few picoseconds,36 which is beyond the time resolution of the present TCSPC instrument. In the presence of CB7, the decays are significantly slowed down to become measurable in the present instrument and are found to be quite different at lower (20 μM) and higher (600 μM) host concentrations (Figure 3A). (A)

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IRF

0.1 10 1

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Figure 3. (A) Fluorescence intensity decay traces and (B) fluorescence anisotropy decay traces recorded at 610 nm for (1) CB7(20μM)-TO (3μM) and (2) CB7(600μM)-TO (3μM). λex = 450 nm, IRF is the instrument response function.

The fluorescence decay traces follow triexponential decay kinetics (fitted parameters presented in Table 1). The shortest lifetime component is fixed at 40 ps (time resolution of TCSPC setup) and is ascribed to free TO molecules that are present in the solution. The intermediate lifetime component, in the range of 1.1-1.4 ns, is assigned to the TO H-dimers (1:4 CB7-TO complexes). A clear reduction in the contribution of this lifetime component can be


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observed on increasing the CB7 concentration from 20 μM to 600 μM, which is in accordance with our proposition that the 1:4 complexes dissociate and are converted to 2:1 CB7-TO complexes at higher CB7 concentrations. The ~3.5 ns lifetime component that is observed at low CB7 concentrations is assigned to CB7-TO complexes with 1:1 stoichiometry that are also indicated to be present in the solution to some extent, in addition to the 1:4 CB7-TO complexes that are predominantly formed under these experimental conditions (Scheme 2, I and II). The long lifetime component ~6 ns, which has the highest contribution (68%) in the intensity decay Table 1. Fluorescence decay times of TO under different chemical environments monitored at 610 nm with excitation wavelength of 450 nm. System[a] CB7(20μM)+TO

Fluorescence decay times τ1[a] τ2 40 ps (5%) 1.4 ns (59%)

τ 3.5 ns (36%)

CB7(20μM)+TO+Na+(1M)

40 ps (16%)

1.1 ns (37%)

3.2 ns (47%)

CB7(20μM)+TO+Ca2+(1M)

40 ps (20%)

1.0 ns (28%)

3.0 ns (52%)

CB7(600μM)+TO

40 ps (15%)

1.1 ns (17%)

6.0 ns (68%)

1.3 ns (61%)

5.8 ns (32%)

CB7(600μM)+TO+Ca2+(1M) 40 ps (7%) [a]

These values are within the time resolution of the TCSPC instrument used in the present study. Values in parenthesis indicate the relative contributions of each decay time.

observed at 600 μM CB7 concentration, is attributed to the 2:1 CB7-TO complex. Quite understandably, this large increase in the fluorescence lifetime of TO is brought about due to the restriction in the torsional motion around the monomethine bond of TO by the binding of two CB7 hosts to both the benzothiazole and quinoline ends of the dye. Concurrent with the distinct fluorescence decay traces, the fluorescence anisotropy decays of TO were also quite different at low and high CB7 concentrations (Figure 3B). Analysis of the


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anisotropy decays (single exponential fitting) revealed a rotational correlation time (τr) of 460 ps for TO in the presence of 20 μM CB7 and 770 ps in the presence of 600 μM CB7. This difference in τr bears testimony to the different natures of the host-guest complexes formed under the two different concentrations of the host. From the τr values, the average radii of the hostguest complexes formed at low and high CB7 concentrations, are estimated to be about 8 Å and 9.5 Å, respectively (Note S4, Supporting Information). The lower rotational correlation time and hence lower average radius for the CB7-TO complexes in the presence of 20 μM CB7 is qualitatively in accordance with the loose external binding of TO and hence the more dynamic nature of the 1:4 host-guest complex, as well as the co-existence of 1:1 CB7-TO complexes at low CB7 concentrations (depicted in Scheme 2, I). Similarly, the higher rotational correlation time and higher average radius for the CB7-TO complexes in the presence of 600 μM CB7, is concurrent with the formation of host-guest complexes with 2:1 stoichiometry (Scheme 2, III). Considering the favorable binding affinity of CB7 towards metal ions and its relevance as external stimuli, we further carried out measurements to screen a series of alkali, alkaline earth and few transition metal ions. The results are compiled in the Table 2. Though the solution containing lower concentration of CB7 (~ 20 μM) was clear in presence of these metal ions, turbidity appeared in the CB7(1 mM)-TO system in the presence of Li+, Na+ or K+ ions but not for Cs+ ion. However, the solution remained clear for alkaline earth metals (Mg2+, Ca2+, Sr2+, Ba2+), with Ca2+ also showing ~ 2.5 fold enhancement in the emission. Unfortunately, due to the optical interference or due to pH conditions, we could not carry out similar measurements with most of the transition metal ions. Only in the presence of Cd2+ ion, the solution remained colorless, clear and displayed ~ 1.4 times enhancement in the emission intensity. For other transition metal ions, it was observed that the colored solutions too remained transparent,


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indicating no precipitation. From Table 2, it is concluded that, barring the large sized Cs+ ion, while alkali metal ions resulted in solution turbidity/precipitation, the alkaline earth metal ions, in general, provided clear solutions. Therefore, we narrowed down our measurements on the effect of two representative ions, Na+ and Ca2+ on the TO-CB7 complex. These metal ions have similar ionic radii (Na+ ~1.18 Å and Ca2+ ~1.12 Å), however, the higher charge of Ca2+ compared to Na+ leads to a greater binding affinity of the former with the CB7 portals (KCB7+ Na ‫׽‬100

M-1, KCB7-Ca2+‫׽‬700 M-1).25

Table 2: Physical appearance and fluorescence changes in the solution containing TO (~3μM) and CB7 (1mM) with the addition of different metal ions.

Metal ion (Ionic radii Å)#

Solution appearance

Fluorescence enhancement

Li+ (0.92) Na+ (1.18)

Turbid Turbid

-----

K+ (1.51)

Turbid

---

Cs+ (1.74)

Clear

No significant change

Mg2+(0.89)

Clear


Ca2+ (1.12)

Clear

~ 2.5 fold enhancement

Sr2+ (1.36)

Clear


Ba2+ (1.42)

Clear


Cd2+(1.1)

Clear

~1.4 fold enhancement

(# Taken from CRC Hand Book) Figure 4 shows the absorption and fluorescence spectral changes of CB7 (20 μM)-TO(3 μM) system with increasing concentrations of Na+ or Ca2+. The nature of the spectral changes is similar both in the presence of Na+ or Ca2+. Notably, with increasing concentrations of the metal


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ions, the absorption spectra of TO changes from that of the H-dimer form (maximum ~483 nm) toward the monomer form (maximum ~500 nm). This is accompanied by a concurrent decrease in the fluorescence intensity of TO. Both these changes suggest that the TO H-dimer form (1:4 CB7-TO complex) is disrupted to release free TO molecules in solution and/or to expose the 1:1 TO-CB7 complexes that are otherwise masked by the dimer form. A Job plot analysis for the CB7-TO system (total host-guest concentration of 5 μM) in the presence of NaCl or CaCl2 (1 M) shows an inflection point at 0.5 mole fraction of TO (Figure S6, Supporting Information), indicating the predominance of 1:1 CB7-TO complexes under these experimental conditions. A better picture emerges on examining the fluorescence decay traces of CB7 (20 μM)-TO (3 μM) system and the changes in the relative contributions of the three lifetime components, in the presence of the metal ions (Table 1 and Figure S7, Supporting Information). It is seen that the metal ions lead to an increase in the contribution of the ~40 ps lifetime component, which implies an increase in population of free TO monomers in solution. The longer lifetime component (~3.0-3.5 ns) assigned to 1:1 CB7-TO complexes also increases in the presence of the metal ions. On the other hand, a decrease in the contribution of the intermediate lifetime component (~1.1-1.4 ns, corresponding to TO H-dimers) is observed on addition of Na+ or Ca2+


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If @ 600 nm


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400 800 CaCl2 (mM)

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8 600

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Figure 4. Absorption (A) and emission (B) spectra of TO(3 μM)-CB7(20 μM) with varying concentrations of NaCl/mM: 0, 25, 50, 100, 192, 285, 370, 450, 750, 1000 (1-10); inset shows variation in the intensity at 600 nm with increasing NaCl concentration, the solid line is given as a guide for viewing. λex = 465 nm, TO represents the fluorescence intensity of the free dye. Absorption (C) and emission (D) spectra of TO(3 μM)-CB7(20 μM) with varying concentrations of CaCl2/mM: 0, 30, 50, 100, 195, 340, 650, 1000 (1-8); inset shows variation in the intensity at 600 nm with increasing CaCl2 concentration, the solid line is given as a guide for viewing. λex = 465 nm. TO represents the fluorescence intensity of the free dye.

(more prominent in the case of Ca2+). This indicates a clear decrease in the population of the Hdimer form of TO in the solution. The observed changes in the decay components in the presence of Na+ or Ca2+ can be explained by the competitive binding of metal ions to the carbonyl laced portals of CB7. The relatively larger changes observed in the presence of Ca2+ in comparison to Na+, is in accordance with the stronger ion-dipole interaction and the stronger binding affinity of Ca2+ with CB7. Essentially, the metal ion binding displaces the accumulated TO H-dimers from


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the portals and thus releases free TO monomers into the solution (Scheme 2). The disappearance of the TO H-dimers consequently leads to an increased contribution of the ~3.0 ns lifetime component, corresponding to the 1:1 CB7-TO inclusion complexes that are masked in the presence of the latter. As mentioned earlier, CB7-TO inclusion complexes with 1:1 stoichiometry exist at low CB7 concentrations, even in the absence of the metal ions (Table 1), along with the 1:4 exclusion complexes that are predominantly formed under these conditions. Qualitatively, it can be envisaged that the competitive binding of metal ions at the CB7 portals can dissociate the 1:4 CB7-TO exclusion complexes more easily compared to the 1:1 CB7-TO inclusion complexes. Accordingly, the 1:1 CB7-TO complexes are mostly undisturbed in the presence of metal ions and remain in the solution. In comparison to the behavior of the CB7(20 μM)-TO(3 μM) system, the CB7(600 μM)TO(3 μM) system displays completely different and more intriguing changes in the presence of Na+ or Ca2+ ions. In this case (i.e. higher CB7 concentration) the effects of the two metal ions are also remarkably different. Interestingly, on addition of Na+, the CB7(600 μM)-TO(3 μM) solution appears to become turbid. This precludes us from carrying out any reliable spectroscopic studies (Figure S8, Supporting Information). Figure 5 shows photographs of the CB7(600 μM)TO(3 μM) solution before and after the addition of Na+ and the AFM image obtained by drop casting a solution of CB7(600 μM)-TO(3 μM) after addition of 0.2 M Na+. The presence of particulates is clearly indicated in the AFM image. The line scan shows an average height of ~20-60 nm for the particles. Such nanoscale structures are not observed in the control sample, that is CB7(600 μM)-TO(3 μM) solution in the absence of Na+.


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60 40 20 0

0

5

10

15

μm Figure 5. AFM image obtained by drop casting a solution of CB7(600 μM)-TO(3 μM)-Na+(0.2 M). The right panel shows a line scan from the AFM image and photographs of the CB7(600 μM)-TO solution before (A) and after the addition of Na+ (B).

Contrary to the above observation, no turbidity is detected in the CB7(600 μM)-TO(3 μM) solution on addition of Ca2+. Figure 6 depicts the absorption and emission spectral changes of CB7(600 μM)-TO(3 μM) in the presence of increasing concentrations of Ca2+. A gradual decrease in the absorbance of the monomer form of TO (maximum ~500 nm) is accompanied by the appearance of a shoulder band around 483 nm (Figure 6A). These changes suggest the gradual formation of the TO H-dimer form in solution. In the fluorescence spectra, an increase in fluorescence intensity is observed on addition of Ca2+. The intensity enhancement is in marked contrast to the decrease in intensity observed in the CB7(20 μM)-TO(3 μM) system (lower CB7 concentration) in the presence of either Na+ or Ca2+. The increased emission intensity of the CB7(600 μM)-TO(3 μM) system in the presence of Ca2+ is also reflected in the fluorescence decay trace (Figure S9, Table 1). The contribution of the intermediate lifetime component (~1.1 ns, corresponding to the H-dimer form of TO) shows a significant increase from 17% for the CB7(600 μM)-TO(3 μM) system in the absence of Ca2+ to 61% on addition of Ca2+ to the same solution. This is in accordance with the appearance of the 483 nm shoulder band, corresponding


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to TO H-dimers, in the absorption spectra. The increase in the ~1.1 ns lifetime component is accompanied by a decrease in the contribution of the long lifetime component (~5.9 ns). This indicates the disruption of the 2:1 CB7-TO complex in the presence of Ca2+ and corroborates well with the decrease in absorbance of the monomer form of TO with increasing Ca2+ concentrations (Figure 6A).

Fl. Intensity

6 0.04

0.00

450

6

(B) 1000

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If @ 600nm

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650

700

750

Wavelength (nm)

Figure 6 Absorption (A) and emission (B) spectra of TO(3 μM)-CB7(600 μM) with varying concentrations of CaCl2/mM: 0, 100, 200, 600, 900, 1000 (1-6). Inset shows variation in intensity at 600 nm with increasing CaCl2 concentration. λex = 465 nm.

To interpret the interesting features that are observed on addition of metal cations (Na+ or Ca2+) to the CB7(600 μM)-TO(3 μM) system (higher CB7 concentrations) we propose a plausible extended assembly formation mechanism that is demonstrated pictorially in Figure S10, Supporting Information. Essentially, the competitive binding effect of the metal ions (both alkali and alkaline earth metal) to the carbonyl portals of CB7 leads to the dissociation of the 2:1 CB7-TO complex and its conversion to the lower stoichiometry 1:1 CB7-TO complex. It is proposed that the 1:1 CB7-TO complexes thus formed, subsequently undergo further noncovalent associations among themselves to yield different supramolecular assemblies depending on the tendency for self-aggregation of TO and the relative affinities of TO and the metal cations (Na+ or Ca2+) for the CB7 portals. In the case of Na+, it is likely that because of the


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weak binding affinity toward CB7, Na+ does not displace the TO molecules from the CB7 portals completely. The accumulation of both Na+ and TO at the CB7 portals, favors formation of molecular superstructures comprising of TO, CB7 and Na+ as building blocks. Basically, an extended assembly can be formed due to self aggregation of TO molecules that are accumulated at the CB7 portals and this is assisted by the presence of Na+ ions (depicted in Figure S10, IV, Supporting Information). The formation of such elongated and complex supramolecular structures possibly leads to the appearance of turbidity in the CB7(600 μM)-TO(3 μM) system on addition of Na+. FT-IR spectra of the particulates showed characteristic vibrations of CB7 and TO, thus confirming the participation of both these units in the self-assembled structure (Figure S11, Supporting Information).

Scheme 2. Schematic representation of various CB7-TO structures that are proposed to be formed under different chemical environments (also see Figure S10) It is important to emphasize that, all the building block components namely, TO, Na+ and high CB7 concentrations are essential for the formation of these superstructures. At low CB7 concentrations even in the presence of similar amounts of Na+, such superstructures are not favored. Under these conditions (that is CB7(20 μM)-TO(3 μM) system in the presence of Na+) the predominant supramolecular structure that exists in solution is the 1:1 CB7-TO complex. The


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insufficient population of the 1:1 CB7-TO complexes at low host concentrations, probably cannot support the formation of extended and complex superstructures by the further self assembly of the individual 1:1 host-guest assemblies. Quite surprisingly, the extended assembly formation is highly selective toward the metal ion. The extended superstructures are not formed in the presence of Ca2+, either at low or high CB7 concentrations. We attribute this interesting feature to the higher charge and hence higher binding affinity of Ca2+ toward the CB7 portals compared to Na+. The strong competitive binding of Ca2+ to CB7, inhibits the external binding and aggregation of TO molecules at the CB7 portals. In the presence of higher CB7 concentrations, only short, compact and discrete supramolecular structures formed by the association of two 1:1 host-guest building blocks (as depicted Figure S10, V, Supporting Information) are plausible, which are further stabilized by the binding of Ca2+ at the free portal ends of CB7. The appearance of the 483 nm shoulder band, corresponding to TO H-dimers, in the absorption spectra (Figure 6A), is in accordance with the self assembly of two 1:1 CB7-TO complexes to form such a proposed structure. The fluorescence enhancement observed for the CB7(600 μM)-TO(3 μM) system in the presence of Ca2+ is also in support of the formation of such compact structures. The self association of two 1:1 CB7-TO complexes is reflected in the increased contribution of the 1.3 ns lifetime component corresponding to TO H-dimers. For a better illustration of the entire phenomenon that occurs in the CB7/TO/metal ion system under different chemical environments, the interesting changes in the fluorescence characteristics are collectively presented in Figure 7 (corresponding absorption spectra are presented in Figure S12, Supporting Information). The difference in the spectral characteristics


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+

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Figure 7 (A) Emission spectra of TO (3 μM) under different solution conditions: (1) TO in water, (2) 1 in the presence of 20 μM CB7. (3) 1 in the presence of 600 μM CB7, (4) 2 in the presence of 1M NaCl, (5) 2 in the presence of 1M CaCl2 and (6) 3 in the presence of 1M CaCl2. (B) Variation in the fluorescence intensity at 600 nm for (I) TO with increasing CB7 concentrations, (II) CB7 (20 μM) -TO(3 μM) with increasing NaCl/CaCl2 concentration and (III) CB7(600 μM)-TO(3 μM) with increasing CaCl2 concentration.

of TO in the presence of low and high CB7 concentrations, and the subsequent changes that are observed on addition of metal ions to the CB7(20 μM)-TO(3 μM) and CB7(600 μM)-TO(3 μM) systems, are clearly visible from this figure. The most interesting aspect that emerges from this study is the profoundly different effect exerted by the alkali and alkaline earth metal ions on the self-assembly process of the CB7/TO/metal ion system, at high CB7 concentrations. This remarkable difference, that is, fluorescence enhancement in the CB7(600 μM)-TO(3 μM) system in the presence of Ca2+ ions and appearance of turbidity and particulate formation in the presence of Na+ ions, provides us with a novel supramolecular approach to distinguish between Ca2+ or Na+ ions (Scheme 2). Moreover, the rupture of the 1:4 host-guest complex in the CB7(20 μM)TO(3 μM) system, by Ca2+ or Na+ ions is another important feature exhibited by the CB7/TO/metal ion system at low CB7 concentrations. This behavior is a classic representation of stimulus responsive adaptability of molecular superstructures with projected applications in drug


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binding and release mechanisms. The building up of elaborate supramolecular structures by hierarchical self-assembly is an important step toward mimicking biological systems. In this regard, the metal ion-assisted, tunable and extendable superstructure formation in the multicomponent CB7/TO/metal ion system is an interesting strategy for construction of a wide variety of self-assembled structures having many potential applications.

CONCLUSIONS In summary, this study demonstrates the feasibility of creating a variety of molecular structures and extended assemblies by careful choice of suitable noncovalent building blocks. In this case, the multi-component building blocks include, the macrocyclic host, CB7, the self-aggregating cationic dye, TO and the alkali/alkaline earth metal ions, Na+ or Ca2+. Depending on the chemical environment, different self-assembled structures are realized. At low host concentrations, the predominant species generated in solution are the 1:4 CB7-TO complexes, consisting of TO dimers accumulated at each of the two CB7 portals. In the presence of Na+ or Ca2+, this structure is ruptured, leaving free TO molecules and 1:1 CB7-TO complexes in solution. At higher CB7 concentrations, 2:1 CB7-TO complexes are formed that consist of two CB7 moieties encapsulating TO from opposite ends. In the presence of Na+ or Ca2+, this structure is again dismantled to yield 1:1 CB7-TO complexes. The subsequent self-assembly of these 1:1 complexes at higher host concentrations is found to be sensitive to the nature of the metal ions. It is proposed that short dimeric structures composed of two 1:1 CB7-TO complexes are generated in the presence of Ca2+ ions leading to fluorescence enhancement whereas extended superstructures are formed by the assembly of many such 1:1 CB7-TO complexes in the presence of Na+ ions, leading to solution turbidity. This remarkable distinction between the


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supramolecular structures by changing the nature of the metal ions can have potential analytical applications. ASSOCIATED CONTENT Supporting Information. Determination of binding constants, stoichiometry, geometry optimised structures, NMR, FT-IR, Figures S1-S12 . This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. Present Addresses †

Student under BARC-SPPU PhD Program, Department of Chemistry, Savitribhai Phule Pune

University, Pune, India ACKNOWLEDGMENT We thank Dr V. Sudarsan, Chemistry Division, BARC, for the AFM measurements. We also gratefully acknowledge the support and encouragement from our host institute.

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Metal Ion-Mediated Assemblies of Thiazole Orange with Cucurbit[7]uril: A Photophysical Study M. N. Shinde,, S. Dutta Choudhury*,, N. Barooah, H. Pal, A. C. Bhasikuttan and J. Mohanty*


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Metal-Ion-Mediated Assemblies of Thiazole Orange with Cucurbit[7

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