A bio-inspired design strategy: Organization of tryptophan-appended naphthalenediimide into well-defined architectures induced by molecular interactions

Abstract

The chemistry of molecular assemblies involves weak yet complex non-covalent interactions, and the molecular organization of the π-conjugated material is crucial in determining the performance of an organic electronic device. Herein we demonstrate a bioinspired design strategy to tune the self-assembly of naphthalenediimides (NDIs) by minute structural variations, π–π stacking, hydrophobic interactions and metal interactions. We address some of the limitations associated with current design strategies, such as restriction to a specific molecular interaction or the difficulty in controlling the assembly due to several complicated intermolecular interactions. Hydrophobic-effect-induced J-type aggregation and sodium-interaction-induced H-type aggregation of tryptophan-appended NDIs have been illustrated. ¹H NMR spectra further reveal sodium cation–π interactions in tryptophan-appended NDIs, while NMR and IR spectroscopic studies confirm the structural variations associated with the molecular assembly. In summary, the molecular organization has been successfully transformed from nanospheres to particles, nanobelts, fibers and fractals. Such drastic changes in the morphology are clear and striking evidence of the importance of non-trivial weak non-covalent forces.

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Introduction

Electronic devices based on organic semiconductors rely on the organization of π-conjugated materials induced by non-covalent interactions.^1–4 To realize the extensive applications of organic electronic devices, both p- and n-type organic semiconductors are essential. The p-type organic semiconductors have been thoroughly investigated over recent decades,^5,6 but the performance of n-type organic semiconductors has lagged behind that of p-type semiconductors. Naphthalenediimides (NDIs) are among the most promising n-type semiconductors for electronic devices based on organic materials,^7,8 and they have potential applications in organic field effect transistors, supramolecular switches, fluorescent chemosensors, electron and energy transfer systems.^9–12 NDIs possess excellent characteristics for the construction of artificial photosystems.^13,14 The planarity and high π-acidity of the NDI system is ideal for face-to-face π-stacking,¹⁵ while their enhanced solubility offers better processability than other aromatic imides. However, in spite of these benefits, morphology tuning of NDI nanostructures is largely unexplored.^16–18 For the potential applications of organic semiconductors in electronics, there is a need to be able to tune the molecular interactions, and hence construct desired architectures.

Molecular interactions and their organization are the basis for various biological and non-biological systems. The molecular recognition process is complicated as it is influenced by several factors. Nature provides an exhaustive illustration of the elegance with which non-covalent interactions can be employed to cater for various requirements. To mimic Nature's versatility in controlling molecular interactions, we employ supramolecular design principles. Despite significant advances in supramolecular chemistry, direct control of molecular organization remains a daunting task, and although we have improved our ability to tune the selectivity and specificity of weak, complicated molecular interactions, the field is still in its infancy. Controlling the non-covalent assembly of aromatic moieties with variable functionality into well-defined architectures remains a challenging task for chemists.

The main challenge in organizing aromatic moieties lies in controlling and optimizing the relatively strong π–π interactions by adjusting the substituents. The most commonly employed strategy to functionalize arylene diimides is with a long-chain alkyl, alkoxy or phenyl substituent.^19,20 The solubility is crucial for the solution processing of individual molecules, and this requires appropriate side-chain modification. Substitution with long or branched alkyl groups can aid solubility in organic non-polar solvents, while their hydrophobic effect in polar solvents can facilitate aromatic stacking. Generally, linear alkyl chains lead to 1D architectures while branched chains lead to 0D agglomerates.¹⁹ Increasing the size of the aromatic core can aid in molecular packing, but planarity and solubility can be affected.

Solvent–molecule interactions are yet another factor affecting the morphology of molecular self-assembly. Moreover, aggregation can vary from being spontaneous to being a rate-controlled process, depending on the alkyl chain length. In contrast to alkyl chains, alkoxy imide substituents (bolaamphiphiles), being hydrophilic, engenders solubility in polar solvents, although they can still be assembled in non-polar solvents. Amphiphiles with alkyl and alkoxy imide substituents on either side are also employed in molecular designs, while phenyl substituents offer rigidity and additional aromatic interactions. However, all these substituents are somewhat restricted to simple, specific interactions such as hydrophobic, hydrophilic or aromatic interactions, and lack a combination of non-covalent forces that can act in a cooperative manner. This limitation led us to look for better alternatives, and we realized that a solution might lie with amino acids.

Amino acids are important structural and signaling biomolecules due to their molecular recognition and distinctive sequence-specific self-assembly properties.^21,22 Herein we report on an exclusive investigation of the molecular organization of NDI appended with two tryptophan moieties (Scheme 1). The choice of tryptophan as an imide substituent is unique due to its polar carboxylic acid group and an indole aromatic heterocycle offering both hydrophilic and hydrophobic properties in a single moiety. In addition, tryptophan provides interaction sites for metal binding, flexibility with regard to functionalization, and biocompatibility. Our work demonstrates the possible ways in which these properties can be exploited. Di-tryptophan-appended NDI has excellent solubility in polar solvents. A small structural modification such as using the methyl ester of tryptophan improves solubility in non-polar solvents. J-type aggregation was induced by hydrophobic forces in the tryptophan-appended NDIs. We also show that NDIs can be self-assembled as H-type aggregates by the addition of sodium hydroxide, which involves sodium cation–π interactions. All these features bring about drastic changes in the morphology of NDIs by transforming them into well-defined architectures. The molecular organization can be changed from nanospheres to particles, nanobelts, fibers and fractals, the latter having a broad range of applications due to their wide range of length scales.²³ In our work, the well-defined architectures produced have been thoroughly characterized by photophysical, spectroscopic and morphological studies. Such drastic changes in the morphology of NDIs architectures are clear and striking evidence of the importance of these weak, complicated non-covalent forces. Thus, our bioinspired design strategy provides opportunities to adjust the molecular organization for various potential applications.

Scheme 1 Chemical structures of tryptophan (NDI 1) and tryptophan methyl ester (NDI 2) appended naphthalenediimides.

Results and discussion

J-type aggregation: Hydrophobic effect

A modified procedure of Sanders and coworkers²⁴ was employed to synthesize NDI 1. A typical UV-vis absorption spectra of NDI 1 in acetonitrile (100 μM) shows absorption bands at 340 nm, 358 nm and 378 nm due to characteristic π–π* transitions (Fig. 1a). Increasing the solvent polarity by means of water (which has the highest solvophobic effect) induces stacking interactions between the aromatic molecules. Water molecules solvating the aromatic surface have a higher energy than bulk water, and aromatic stacking reduces the total surface area exposed to the solvent. The bathochromic shift of the absorption band with solvent polarity indicates the J-type aggregation of NDI 1. A bathochromic shift of 2 nm, 4 nm and 6 nm for the 340 nm, 358 nm and 378 nm absorption bands was observed respectively. The absorption spectrum of NDI 2 in acetonitrile also possesses absorption bands at the same wavelength, as it is unaffected by methyl ester substitution (Fig. 1b). The hydrophobic effect on NDI 2 in terms of bathochromicity was found to be minimal, as can be seen from the absorption spectra. A bathochromic shift of 1 nm and 2 nm for 358 nm and 378 nm bands respectively was observed, while 340 nm band was unaffected. Aromatic interactions are intriguing molecular recognition elements because they are expected to be strong in water due to their hydrophobic interactions. At the same time, the aromatic interactions should be selective if the electrostatic component is significant. Here the aqueous medium displays the best features of both hydrophobic and hydrogen-bonding interactions.

Fig. 1 UV-vis spectra of NDI 1 (a) and NDI 2 (b) in acetonitrile (100 μM) with increasing percentage of added water. UV-vis spectra of NDI 1 (c) and NDI 2 (d) in 10% aqueous acetonitrile (100 μM) with the addition of NaOH (in equiv).

H-type aggregation: Sodium cation–π interaction

Solvent-induced J-type aggregation was found to be affected by protecting the free carboxylic acid groups of NDI 1 with methyl esters, as in NDI 2. In contrast, deprotonation of the carboxylic acid protons of NDI 1 with an alkali such as sodium hydroxide shows surprisingly different results. To a 10% aqueous acetonitrile solution of NDI 1 (100 μM) NaOH was added portionwise. The absorption bands for NDI 1 were at 341 nm, 358 nm and 379 nm respectively in the absence of NaOH (Fig. 1c). Successive addition resulted in a marginal bathochromic shift up to a total of 4 equiv of NaOH. With additional NaOH (5, 10, 20, 50 and 100 equiv) the absorption bands show a strong hypsochromic shift. The absorption spectrum of NDI 1 with 10 equiv of NaOH contains bands at 331 nm, 349 nm and 364 nm. With the help of intermediate spectral data, we attribute these bands to the hypsochromic shift of the 341 nm, 358 nm and 379 nm absorption bands respectively. Thus an H-type aggregation mode was induced in NDI 1.²⁵ Further increase in the NaOH content resulted in a very broad band. The anion of NDI 1 produced on treatment with NaOH should exert strong electrostatic repulsion and hence hinder agglomeration. We believe that the Na⁺-mediated interaction with the anion of NDI 1 can reduce electrostatic repulsion. The strong hypsochromic shift observed is thus attributed to the interaction of Na⁺ with NDI 1 anion. However, NDI 2 on NaOH treatment resulted in spectral features as shown in Fig. 1d. It is not surprising that NDI 2 shows absorption spectra similar to that of NDI 1 (except for the initial slight bathochromic shift). Hydrolysis of methyl ester in NDI 2 by added NaOH generates the sodium salt, as in the case of NDI 1. The difference between the spectral features of NDI 1 and NDI 2 for the initial equiv of NaOH can be explained by means of the resulting side-products water and methanol from NDI 1 and NDI 2 respectively. The hydrophobic effect, which has been shown to cause the bathochromic shift (Fig. 1a, 1b), is held responsible for the initial bathochromic shift observed in the case of NDI 1, as water is the byproduct. However, the weaker solvophobic interactions of methanol compared to water results in minimal bathochromic shifts, as observed in the case of NDI 2.

Circular dichroism (CD) studies

To gain further insight into the mode of aggregation, we carried out CD studies on NDI 1 and NDI 2. The CD spectrum of NDI 1 in acetonitrile solution (100 μM) shows intense negative bands at 221 nm with a shoulder at 202 nm, and less intense bands at 276 nm, 335 nm, 352 nm and 372 nm (Fig. 2a). The negative bands at 221 nm and 202 nm are attributed to n–π* and π–π* transitions of the imide chromophore respectively. The intense 221 nm band suggests possible additional contributions. The close proximity of n–π* (221 nm) and π–π* (202 nm) transitions can facilitate mixing in a phenomenon known as the one-electron effect, which can enhance n–π* bands. However, the specific interactions with aromatic chromophores could also be responsible for the enhanced 221 nm band.^26,27 The band centered at 276 nm is due to the π–π* transitions of tryptophan, while those at 335 nm, 352 nm and 372 nm are due to that of NDI imposed by a chiral chemical environment.

Solvent-dependent CD studies on the aggregation mode of NDI 1 are shown in Fig. 2a. The negative band at 221 nm for NDI 1 in acetonitrile was shifted to 213 nm (Δλ = 8 nm) in 90% aqueous acetonitrile. The n–π* transition in amides is dependent on solvent, and increasing the solvent polarity by adding water shifts the n–π* transition to lower wavelengths. With increasing solvent polarity, the 221 nm (n–π* transition) band is quenched preferentially, rather than the 202 nm (π–π* transition) band. The spectral band features of NDI 1 almost disappeared in 90% aqueous acetonitrile. The broad band centered at 276 nm was also found to quench with increasing solvent polarity, without any shift in band position. The π–π* transition bands of NDI at 335 nm, 352 nm and 372 nm were found to have a bathochromic shift to 337 nm, 357 nm and 378 nm respectively, similar to the bathochromic shift observed during absorption spectroscopic studies (Fig. 1a). Thus, the bathochromic shift of the NDI CD bands induced by hydrophobic effect can also be attributed to J-type aggregation. The CD study of hydrophobic effect on NDI 2 is shown in Fig. 2b. NDI 2 exhibits a bathochromic shift with respect to NDI (π–π*) bands, but a hypsochromic shift with respect to the imide (221 nm) band, similar to NDI 1. In 60% aqueous acetonitrile, NDI 2 shows a 1 nm hypsochromic shift of the 221 nm band, and a ∼1 nm bathochromic shift for the 352 nm and 372 nm bands. The hypsochromic shift of the 221 nm band and the bathochromic shift of NDI bands observed in CD measurements are due to the different mechanisms involved, as already discussed.

Fig. 2 CD spectra of NDI 1 (a) and NDI 2 (b) in acetonitrile (100 μM) with increasing percentage of added water. CD spectra of NDI 1 (c) and NDI 2 (d) in 10% aqueous acetonitrile (100 μM) with added NaOH (in equiv). Concentration-dependent CD spectra of NDI 1 (e) and NDI 2 (f) in acetonitrile.

Surprisingly, NDI 1 with NaOH was found to display a bathochromic shift of the 221 nm negative band (Fig. 2c). A bathochromic shift of 2 nm was observed with 10 equiv of NaOH. The bands above 240 nm showed minimal changes for various equiv of NaOH. At 100 equiv of NaOH the bands above 240 nm completely disappeared. This is in agreement with the disappearance of the characteristic spectral features as observed in the absorption spectroscopic studies (Fig. 1c). Interestingly, the 202 nm band due to the π–π* transition of imide was found to have a hypsochromic shift. The NaOH-mediated interaction of NDI 1 is believed to shift the 202 nm band to below 200 nm. In addition, positive band-like features were observed at 210 nm and 240 nm. CD studies on NDI 2 with NaOH reveal spectral features similar to NDI 1 (Fig. 2d). A bathochromic shift of only 1 nm was observed for the 221 nm band in the case of NDI 2.

Concentration-dependent CD studies of NDI 1 and NDI 2 are shown in Fig. 2e and Fig. 2f respectively. The chiral-field-induced π–π* transitions of tryptophan result in a very broad negative band centered at 276 nm. However, the chiral-field-induced π–π* transitions of NDI result in negative bands at 335 nm, 352 nm and 372 nm. For low concentrations of NDI 1 (1 μM and 10 μM) in acetonitrile these characteristic bands above 240 nm were not observed. Unlike NDI 1, the CD spectrum of 500 μM NDI 2 shows bands at 282 nm, 356 nm and 375 nm. The concentration-dependent bathochromic shifts of bands at >240 nm are attributed to J-type aggregation of NDI 2. The reduced solubility of NDI 2 is believed to aid in their molecular organization.

Vibrational spectroscopic studies

Vibrational spectroscopic studies of NDI 1 and NDI 2 further revealed the changes in their vibrational modes. NDI 1 exhibits vibrational absorption frequencies at 1580 cm⁻¹, 1670 cm⁻¹ and 1706 cm⁻¹, among others (see ESI†). The vibrational frequency at 1706 cm⁻¹ is assigned to ν_C=O of carboxylic acid groups, 1580 cm⁻¹ to aromatic ν_C=C and the 1670 cm⁻¹ peak is attributed to the imide ν_C=O vibrational frequency, commonly known as the amide I band. The chemical environment constrained by conformation alters the amide I band, but the lack of conclusive structure–spectra correlations in the literature along with their several categories complicates the unambiguous assignment of amide I bands with the corresponding secondary structure. On the basis of the observed vibrational absorption frequency at 1670 cm⁻¹, we attribute NDI 1 to a class of β-turn conformations.²⁸ NDI 1 in 90% aqueous acetonitrile shows peaks at 1580 cm⁻¹, 1666 cm⁻¹ and 1704 cm⁻¹ corresponding to aromatic ν_C=C, imide ν_C=O and ν_C=O of carboxylic acid functional groups respectively. These changes in the vibrational frequencies are attributed to hydrophobic-effect-induced structural variations. However, in the case of NDI 2 the peak at 1719 cm⁻¹ is attributed to ester ν_C=O, and the 1580 cm⁻¹ peak to aromatic ν_C=C, while the 1705 cm⁻¹ and 1678 cm⁻¹ peaks are assigned to the imide ν_C=O vibrational frequencies (see ESI†). NDI 2 showed slight changes in the vibrational modes in 60% aqueous acetonitrile. In another experiment NaOH was added portionwise to an acetonitrile solution of NDI 1; the corresponding spectral changes are shown in Fig. 3. With successive addition of NaOH, the carboxylic acid ν_C=O and imide carbonyl ν_C=O were found to shift towards lower frequencies. The carboxylic acid ν_C=O was found to be at 1700 cm⁻¹, while the imide carbonyl ν_C=O at 1660 cm⁻¹ with 2 equiv of NaOH (deprotonation of carboxylic acid protons). For 100 equiv of NaOH, very broad vibrational modes at 620 cm⁻¹, 800 cm⁻¹, 1429 cm⁻¹, 1574 cm⁻¹, 1598 cm⁻¹ and ∼3000 cm⁻¹ were observed. These changes in the vibrational modes clearly indicate the NaOH mediated interaction with NDI 1.

Fig. 3 IR spectra of NDI 1 (acetonitrile) with the addition of NaOH (in equiv).

Nuclear magnetic resonance (NMR) studies

NMR spectroscopic investigation further confirms the structural changes occurring during the course of NDI 1 aggregation. ¹H NMR spectra for the solvent-dependent aggregation of NDI 1 (aqueous CD₃CN solution) is shown in Fig. 4. The hydrophobic-effect-induced stacking of NDI 1 did not show significant changes in the naphthalene core proton resonances. The aromatic component (indole) of tryptophan shows interesting changes. The indole N–H proton resonating at δ = 8.91 ppm in CD₃CN undergo a downfield shift to δ = 9.72 ppm in 65% aqueous CD₃CN. The labeled protons Hb, Hc, Hd and He experience an upfield shift, while Ha undergoes a downfield shift. Therefore, the J-type aggregation observed has more effect on the tryptophan moieties. However, the structural changes in NDI 1 in the presence of NaOH are clearly evident from the ¹H NMR studies (Fig. 5). On portionwise addition of NaOH, the indole N–H proton shifts toward lower δ values. At 4 equiv of NaOH the indole N–H disappears, indicating the abstraction of protons by NaOH. The naphthalene core protons undergo an upfield shift. The αCH also experiences an upfield shift from δ = 5.99 ppm to δ = 5.69 ppm, while protons Hb, Hc and Hd show minimal variations in the chemical shift. However, Ha and He protons have slight upfield and downfield shifts respectively. Addition of more than 4 equiv NaOH brings about significant changes in the ¹H NMR of NDI 1. Chemical shifts observed in the aromatic region above 4 equiv of NaOH are a clear indication of the structural changes occurring due to the presence of NaOH. We attribute these structural changes to sodium-mediated variations which also involve cation–π interactions.²⁹¹H NMR studies shows the major variations in the proton resonances of tryptophan, and hence tryptophan and its indole moieties play a crucial role in determining the mode of aggregation.

Fig. 4 Chemical structure of NDI 1 with proton assignments (top) and ¹H NMR spectra of NDI 1 in CD₃CN with varying percentage of added water (0–65%).

Fig. 5 ¹H NMR spectra of NDI 1 in 10% aqueous CD₃CN with increasing amounts of added NaOH (0–100 equiv). All the ¹ H NMR spectra were recorded with different equiv of added NaOH while maintaining the solvent composition of 10% aqueous CD₃CN.

Morphological studies

NDI 1 forms spherical aggregates from acetonitrile solution as shown in the FESEM micrograph (Fig. 6a). TEM micrograph clearly shows nanospheres of NDI 1 (Fig. 6b). We obtained a maximum of 6 nm bathochromic shift (NDI electronic transitions) in absorption as well as CD studies, along with an 8 nm hypsochromic shift of the 221 nm band (CD) for NDI 1 in 90% aqueous acetonitrile. Consequently, the FESEM micrograph showed the formation of particles of NDI 1 from 90% aqueous acetonitrile (Fig. 6c and 6d). AFM studies revealed the particles as triangular aggregates of 400 nm dimension with a typical height of ∼40 nm (ESI†).

Fig. 6 (a) FESEM micrograph of NDI 1 nanospheres obtained from 100% acetonitrile solution. (b) The corresponding TEM micrograph of NDI 1 nanospheres. (c) FESEM micrograph of NDI 1 particles obtained from 90% aqueous acetonitrile solution. (e) FESEM micrograph of NDI 1 fractals formed by 10% aqueous acetonitrile solution containing 2 equiv of NaOH. (d) and (f) are corresponding high-magnification micrographs of (c) and (e) respectively.

The interplay between hydrogen bonding, solvophobic forces and aromatic stacking decides the molecular organization and hence their morphology. The intermolecular hydrogen bonding between carboxylic acid groups of NDI 1 in cooperation with the solvophobic and aromatic interaction leads to nanospheres in acetonitrile solution. The presence of water in aqueous acetonitrile solution of NDI 1 disrupts the intermolecular hydrogen bonding. The mutual interactions involving solubility due to carboxylic acid groups and the hydrophobic-effect-induced aromatic interaction leads to J-type aggregation. The particles are believed to be formed from disordered organization of J-type aggregates, as indicated by absorption and morphological studies.

NDI 2 also forms spherical aggregates from acetonitrile solution as shown in Fig. 7a and 7b. The presence of the methyl ester of tryptophan in NDI 2 results in no significant changes in the aggregation mode with respect to NDI 1 from acetonitrile solution. From the absorption spectroscopic study we had observed a bathochromic shift of not more than 2 nm for the NDI π–π* transitions, a hypsochromic shift of 1 nm for the 221 nm (CD) band. Surprisingly, the molecular self-assembly of NDI 2 from 60% aqueous acetonitrile differs distinctly. NDI 2 forms fibers from 60% aqueous acetonitrile. The FESEM micrograph with a high aspect ratio fibers (bundle of nanobelts) is shown in Fig. 7c and 7d. AFM data revealed the transformation of NDI 2 nanospheres (D = a few hundred nm) into nanobelts and then into microfibers (see ESI†). The nanobelts bundle together to form long fibers that have a thickness of a few 100 nm to a few micrometers (see ESI†). The formation of spherical aggregates from NDI 2 suggests that intermolecular hydrogen bonding (carboxylic groups) need not be necessary, as the aggregation is mainly facilitated by solvophobic and aromatic interactions. However, the presence of the methyl ester reduces the solubility in water, and moreover the dominance of hydrophobic forces in cooperation with aromatic interactions leads to 1D aggregation.

Fig. 7 FESEM micrographs of NDI 2 nanospheres obtained from 100% acetonitrile solution (a) and fibers (bundle of nanobelts) obtained from 60% aqueous acetonitrile (c). (b) and (d) are corresponding high-magnification micrographs of (a) and (c) respectively.

Analogous to the morphological changes induced by the bathochromic shift due to hydrophobic effect, NaOH too was found to affect the morphology. Deprotonation of the carboxylic acid protons in NDI 1 was found to result in hardly any change in the absorption spectra. However, deprotonation results in drastic changes in the morphology of NDI 1. With 2 equiv of NaOH, the formation of the sodium salt of NDI 1 results in the formation of fractals (Fig. 6e, 6f). For 10 equiv of NaOH, distinct microstructures of NDI 1 are formed (see ESI†). With further increase of NaOH (100 equiv), agglomeration of NDI 1 was observed (see ESI†). NDI 2 also results in similar fractals for 2 equiv of NaOH and agglomerated masses for 100 equiv of NaOH. We also studied the morphological changes as a function of concentration of NDIs. At higher concentration (1 mM) NDI 1 agglomerates (see ESI†), while 1 mM NDI 2 transforms into 1D nanobelts (Fig. S5, ESI†). The reduced solubility and the enhanced solvophobic forces are believed to enforce 1D assembly of NDI 2 (1 mM) aromatic cores into J-type aggregates, as indicated by CD studies (Fig. 2f).

Conclusions

In an attempt to mimic Nature's versatility in terms of molecular recognition and the specific assembly properties embodied in amino acids, we have synthesized tryptophan-appended naphthalenediimides NDI 1 and NDI 2. The tryptophan, as the imide substituent, possesses all the characteristics of a conventional substituent in a simple yet compact system. We were able to induce J-type aggregation by the hydrophobic effect, and H-type aggregation by sodium interaction. We were successful in tuning the morphology of NDIs to well-defined architectures including nanospheres, particles, nanobelts, fibers and fractals. Such drastic change in the morphologies of NDI 1 and NDI 2 is an illustration of the possibilities for extending morphology control mediated by molecular recognition. Our current work on tryptophan-appended NDI is an intriguing approach that, with further chemical modifications, has scope for electronic as well as biomedical applications. Tryptophan-appended NDIs are examples of compact model systems that should enable us to understand the relative contributions of non-covalent forces and to exploit them for useful applications.

Acknowledgements

Authors thank Prof. C. N. R. Rao, FRS, for constant support and encouragement, and JNCASR and the Department of Science and Technology, Government of India, New Delhi, for financial support. We thank Prof. G. U. Kulkarni for FESEM, the VIN lab for the AFM facility, Prof. Hemalatha Balaram for CD measurements, and we acknowledge Vasu, Selvi, Mahesh, Basavaraja and Usha for their help in IR, FESEM, AFM and TEM measurements.

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Footnote

† Electronic supplementary information (ESI) available: Experimental methods, synthesis, NMR, IR, FESEM, TEM and AFM. See DOI: 10.1039/c0nr00766h

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