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The supramolecular helicity in the self-assembled nanostructures of two perylene bisimide bichromophoric systems could be controlled by varying the ...
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Inversion of Supramolecular Chirality in Bichromophoric Perylene Bisimides: Influence of Temperature and Ultrasound Jatish Kumar, Takuya Nakashima,* and Tsuyoshi Kawai* Graduate School of Materials Science, Nara Institute of Science and Technology, NAIST, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan S Supporting Information *

ABSTRACT: The supramolecular helicity in the selfassembled nanostructures of two perylene bisimide bichromophoric systems could be controlled by varying the preparatory methods. The self-assembly of the compounds under different conditions was investigated in detail by using absorption, fluorescence, CD, FTIR, XRD, TEM, and SEM techniques. These studies reveal that the heating−cooling method results in aggregates with ordered molecular packing and enhanced optical chirality. Ultrasonication leads to molecular aggregates with less ordered packing wherein the supramolecular chirality was reversed relative to the sample prepared via a heating− cooling method. This heating−cooling method proved to be superior in terms of nanofiber synthesis, yielding fibers with extended length and a prominent helical twist. At higher concentration, both compounds exhibited a gelation property in benzonitrile. The tunable chiroptical properties in these supramolecular systems make them potential candidates for applications in the field of optical and electronic device fabrication based on organic nanostructures.

1. INTRODUCTION Controlled self-assembly of functional organic π-conjugated molecular systems is a challenging topic of interdisciplinary research due to the intriguing possibilities available for these nanomaterials to be used in the field of optical, electrical, and mechanical nanodevices.1−4 The functions and properties of self-assembled chromophoric aggregates are dependent on their shape and size, and hence a large amount of research has been focused on the development of shape-shifting nanostructures.5,6 The precise control of molecular arrangements is an essential criterion for obtaining well-defined nanoscopic architectures with a specific shape.7,8 Several methodologies have been adopted for changing the shape of the organic nanosolids by using temperature,9,10 concentration,11 guest molecules,12,13 ultrasound,14−16 and solvents.17−20 Even though the modulations in the morphological features of assembled nanostructures have been reported, there have been a few investigations on the inversion of supramolecular chirality when subjected to external stimuli.10,19−23 The helicity of the self-assembled structures can be inverted by varying the preferences toward the available intermolecular interactions such as hydrogen bonding, π−π stacking, and hydrophobic forces that arise from the typical nature of molecular systems.24−26 Control of the helicity in self-assembled molecular systems has far-reaching implications in the field of supramolecular chemistry, particularly due to its relation to chirality transmission, which is of prime importance in biology. The inversion of supramolecular chirality becomes particularly important in light of © 2014 American Chemical Society

the fact that this can mimic life systems. For example, a small change in molecular packing leading to the reversal of supramolecular chirality is represented by double-stranded DNAs. Among different classes of organic chromophoric systems investigated for self-assembly, perylene bisimide (PBI) has gained much attention27,28 due to the wide range of possibilities available for fine-tuning their properties by the introduction of suitable functional groups at the core as well as the periphery.29−32 The nature of chirality in the aggregates of PBIs is deeply sensitive to their external environmental conditions. Different PBI-based molecular systems have been designed, which include the monochromophoric, bichromophoric, and multichromophoric systems.33−38 Among them, the bichromophoric systems have gained special attention due to their ability to exhibit interesting excited-state properties, both in monomeric and aggregate states. The introduction of a chiral center into the core unit bridging two PBIs helps to enhance the chiroptical properties of these systems in their selfassembled state.35−37 Bichromophoric PBI molecules having a chiral cyclohexane diamide core (Chart 1) showed an enhancement of dissymmetry in circular dichroism (CD) and circularly polarized luminescence (CPL) in self-assembled structures.35 Self-assembly and the enhancement of chiroptical Received: February 6, 2014 Revised: May 1, 2014 Published: May 7, 2014 6030

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Chart 1. Molecular Structures of the S Isomers of Compounds 1 and 2

carrying out any spectroscopic investigations. In the ultrasonication method, the stock solution of the compound was prepared by subjecting the sample (2 × 10−4 M) to sonication for 30 min in a water-bath sonicator and was equilibrated at room temperature for 1 h. The high boiling point of the solvent facilitated the easy handling of the solution during various modes of sample preparation. The stock solutions with high concentration prepared by two different processes were diluted to a certain value of concentration prior to spectroscopic study. No precipitates were observed in the solution during the spectroscopic measurements, and the experiments were performed on clear solutions. The ground-state chirality of the compounds in monomeric as well as self-assembled states was probed with the help of concentration-dependent absorption and CD investigations. At low concentration (1 μM), the absorption spectrum of the solution of 1 prepared by the ultrasonication method exhibited a high A0−0/A0−1 peak intensity ratio of 1.36 (Figure 1c). The

properties were strongly dependent on the molecular structure, especially on the central unit including spacers bridging between PBIs and the chiral core. In this study, we investigate the process-dependent shape-shifting behavior of selfassembled structures formed by these bichromophoric PBIs. The morphological features and supramolecular chirality are sensitive to external stimuli such as the solvent, heating, and ultrasound. The inversion of supramolecular chirality is demonstrated by successive processing with ultrasound followed by thermal treatment.

2. RESULTS AND DISCUSSION The molecules under investigation are 1,2-diamide-cyclohexane38 derivatives bearing two perylene bisimides arranged in a chiral fashion (Chart 1).35 The two compounds differ in the nature of the spacer group between the PBI chromophores and the central cyclohexane moiety. Molecule 1 has a glycine unit as the linker, whereas for 2 the length of the spacer is increased by a methylene unit with β-alanine as the spacer. The variation in the length of the methylene spacer controls the degree of overlap between the PBI units as well as helps in reversing the direction of chiral arrangement of PBIs connected to the identical chiral center.35 Spectral features corresponding to the monomeric state of the compounds were exhibited in chloroform wherein absorption bands specific to different vibrational transitions of the PBI unit were observed (Figure S1, Supporting Information). The sign of circular dichroism (CD) was in agreement with the configuration of the PBIs as shown in Chart 1; the S isomer of 1 exhibited a negative first Cotton effect at 530 nm whereas the same isomer of 2 exhibited a positive Cotton effect at the corresponding wavelength. The chirality on the molecular level is governed by the relative configuration of the chromophoric units. Under suitable conditions, the molecules undergo self-assembly through the hydrogen-bonding network developed between amide groups and the concomitant intermolecular π−π stacking of PBIs.35 All experiments in the present investigation were carried out in benzonitrile wherein compounds 1 and 2 showed moderate solubility. The molecules could be dispersed in this solvent at low concentration (10 μM) easily. Above this concentration, the solution has to be subjected either to heating or ultrasonication for the dissolution of the compounds. The sample preparation followed two different methods, including heating−cooling and ultrasonication. In the heating−cooling method, a concentrated solution of the compound in benzonitrile (2 × 10−4 M) was heated to 150 °C for 30 min to dissolve molecules completely. The solution was allowed to cool and was maintained at room temperature for 1 h before

Figure 1. Concentration-dependent absorption (a, c) and the corresponding CD spectra (b, d) of R-1 after subjecting the solution to heating−cooling (a, b) and ultrasonication (c, d) in benzonitrile.

CD spectrum showed a positive first Cotton effect for R-1 at 530 nm (Figure 1d) derived from the intramolecular exciton coupling between the bridged PBI units. These spectral features are specific to the monomeric state of the molecule as evident from the studies on dilute solution of 1 in chloroform at varying CD intensity (Figure S1a, Supporting Information).35 Interestingly, at higher concentrations (above 5 μM), spectral features corresponding to the aggregated state emerged: (i) broadening of the absorption bands along with the decrease in the A0−0/ A0−1 peak intensity ratio (1.09), which is an indication of the cofacial arrangement of molecules,39,40 and (ii) additional strong positive and negative peaks at 545 and 490 nm, respectively, in CD spectra, corresponding to the intermolecular exciton coupling of PBIs in supramolecular assemblies (Figure 1c,d). Similar studies on the solution of R-1 prepared by the heating−cooling method showed aggregation at a concentration below 5 μM with clear negative and positive CD peaks at 550 and 490 nm, respectively (Figure 1b). Further evidence for the aggregation of the compounds under different experimental conditions was obtained from the concentrationdependent fluorescence measurements. Fluorescence spectra exhibited a reversal in the intensity ratio between the peaks at 550 and 590 nm and an enhancement of a peak at 630 nm with increasing concentration (Figure S2, Supporting Information). 6031

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The spectral features indicate the self-assembly of molecules in solution; however, it is evident that the changes are not as prominent as reported for similar PBI systems.28,41 This may be indicative of the presence of a certain number of monomers in solution along with the aggregated species. A deep analysis of the CD spectra showed that the chiroptical properties of bichromophoric molecules are more complicated in comparison to monochromiophoric systems. In the monomeric state, PBI units are the subject of exciton coupling, and the sign of the Cotton effect was in accord with the relative configuration (P or M) of PBIs attached to the chiral center (Figure S1, Supporting Information). The intramolecular coupling between the chromophoric units within a molecule is the source of the CD signal in this state.35 During self-assembly, the molecules are brought close together and the interaction between the molecules results in the addition of intermolecular exciton coupling. Hence the CD spectra in Figure 1 arises from a combination of different excitonic couplings: (i) intramolecular coupling of the monomers, (ii) intermolecular coupling between two molecules in an aggregate, and (iii) intramolecular coupling within the same molecule in an aggregate. Intermolecular exciton coupling between the molecules partly results in enhanced CD in the longest-wavelength region of the spectrum at around 550 nm. Interestingly, for experiments carried out at the same concentration, the CD spectra exhibited differences in the sign, position, and intensity of peaks between the solutions prepared by the heating−cooling and ultrasonication methods (Figure 1b,d). The signs of a pair of the strong CD bands at around 550 and 490 nm were reversed between the solution samples of the same stereoisomer of compound 1 prepared by the two methods. Those peaks could attribute to the intermolecular exciton coupling in selfassembled structures since they were not observed in a monomeric state in chloroform and exhibited marked progression with increasing concentration. The sign inversion of these peaks is indicative of the inversion in supramolecular chirality of the self-assembled structures formed in these solution samples. In the shorter-wavelength region at around 460 nm, which has contributions from intramolecular coupling, the spectral changes are minimal. Moreover, the sample prepared via the heating−cooling method showed larger amplitudes both at 550 and 490 nm in CD spectra relative to the sample prepared by the sonication method. These features suggest that higher-order exciton coupling is observed in solution when the sample is subjected to the heating−cooling procedure rather than ultrasonication. The nature of the self-assembled structures in the solution was investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). For imaging, 75 μL of the solutions used for spectroscopic investigations were drop-cast on a carbon grid and allowed to dry overnight in vacuo. TEM micrographs showed typical fibrous network structures for the aggregates of 1 prepared by both methods (Figure 2). The fibrous nature of the aggregates was confirmed with the help of SEM micrographs (Figure S4, Supporting Information). X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis showed that the molecules self-assemble through the combined effects of π−π stacking and hydrogen-bonding interactions, leading to the formation of extended fibers (vide infra). The solution prepared by the heating−cooling method exhibited fibrous structures with a width of 15−25 nm and a length of a few micrometers. Right-handed P helices were observed for the R isomer whereas

Figure 2. TEM micrographs of the R (a, c) and S (b, d) isomers of 1 prepared by heating−cooling (a, b) and by ultrasonication (c, d).

the S isomer exhibited left handed M helices for fibers (Figure 2a,b). It should be noted that the negative Cotton effect observed in CD for R-1 suggests the left-handed arrangement of PBIs, which was opposite to the helical handedness of fibers found in TEM micrographs. Such a reversal in the handedness during the formation of hierarchical superstructures has already been reported for some chiral assemblies of PBI dyes as well as the same molecule.35,42 In fact, the thickness of the fibers is far larger than the molecular length of 1, suggesting the supercoiled structure in the fibrous aggregates as in the previous cases. On the contrary, the solution prepared by the ultrasonication method exhibited thinner fibers (10−15 nm) with the opposite twist in helices for the corresponding isomers; left (M) and right (P) handed twists for the R and S isomers, respectively (Figure 2c,d). The fibers formed via the sonication method were shorter in length and had less evident helicate structures in comparison to the samples prepared by the heating−cooling method. The morphological investigations are indicative of the fact that higher-order chiral induction occurs in the fibers formed through the heating−cooling method, and these features are in good agreement with results obtained from CD measurements. Similar spectral and morphological investigations were carried out on 2. Unlike for the case of 1, the solution of 2 prepared through both methods showed spectral features corresponding to the self-assembled state even at low concentrations in benzonitrile. With increasing concentration, the absorption spectra exhibited red-shifted bands with a reversal in the intensity ratio between the A0−0/A0−1 peaks (Figure 3a,c). The CD spectrum showed a gradual increase in the intensity of peaks with an increase in concentration. The solution of R-2 prepared via the sonication method showed the first positive Cotton band at 540 nm whereas the solution sample of the same isomer via the heating−cooling method showed the first negative Cotton band at 546 nm (Figure 3b,d). These spectral features are indicative of the inversion in supramolecular chirality of the self-assembled nanostructures. 6032

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handedness of fibers was contradictory to the sign of the Cotton effect in CD, suggesting the formation of hierarchical superstructures.35,42 The solution prepared by the sonication method showed thin fibers (4−8 nm) which underwent no bundling (Figure 4c,d). The helical twists in these structures were not clear from the TEM micrographs due to the thin nature of the fibers. However, it is evident from the CD spectral investigations that the local twist of these fibers should be opposite to that observed in the samples prepared by the heating−cooling method. The observation of the similar nature of the twist for both compounds prepared under similar conditions confirms the role of the central cyclohexane moiety in modulating the nature of helicity in the self-assembled nanostructures. In order to gain more insight into the molecular arrangement within the aggregated structures under different experimental conditions, we have carried out FTIR and XRD studies of both derivatives in their self-assembled states. Both measurements gave only minor variations in profiles between the samples prepared by two different methods. FTIR analysis on the aggregates of 1 and 2 showed the appearance of the amide I and II bands at 1646−1653 and 1538−1552 cm−1, respectively, suggesting the existence of the amide hydrogen bonds (Figure S5, Supporting Information).43,44 The strong and broad vibrational peak observed at around 3272−3280 cm −1 corresponding to the amide N−H confirmed the presence of a hydrogen-bonding network in the aggregated structures of both compounds.45,46 The small variation in peak positions for aggregates formed by different preparatory methods shows a conformational change leading to minor differences in the packing of the molecules within the nanostructures. The selfassembled nanoaggregates from both compounds exhibited peaks corresponding to νs(CH2) and νa(CH2) at approximately 2849−2854 and 2920−2926 cm−1, respectively, indicating that the alkyl chains are stretched, resulting in multilayer fibers through interdigitation.47,48 The peak positions for νs(CH2) and νa(CH2) modes of the aggregates of both 1 and 2 formed by the heating−cooling method were found at a slightly lower wavenumber than for those formed by the ultrasonication method, suggesting stronger van der Waals interaction and a more ordered arrangement of side chains in the former case.49−51 The XRD spectra of 1 prepared via the heating− cooling method showed diffraction peaks at 2θ = 5.08 and 7.16° corresponding to d spacings of 17.4 and 12.3 Å, respectively (Figure S6, Supporting Information). These peaks can be ascribed to the average length along the long axis of the molecule and the thickness along the perpendicular direction. An additional peak observed at 2θ = 2.54° which corresponds to a d spacing of 34.7 Å suggests the existence of the molecule in the dimeric form along the N−N axis. Peaks observed at 2θ = 7.16° (12.3 Å), 9.96° (8.9 Å), and 13.1° (6.76 Å) corresponding to the higher-order diffraction originate from the ordered stacking periodicity of the structural units.52 In the wide-angle region, the diffraction peak observed at 3.8 Å (2θ = 22.74°) is in agreement with the loose π−π stacking distance between the PBI units in the self-assembled aggregates.53−56 The self-assembled structures of 2 displayed broad peaks, suggesting the formation of amorphous aggregates relative to 1.35 Two bands at 2θ = 4.72 (18.7 Å) and 2.44° (36.2 Å) corresponding to the length of the single and bilayer forms of the molecule were observed. It may be noted that the length is slightly larger than 1 owing to the larger spacer in the case of 2. The bilayer thickness of the molecule was in agreement with

Figure 3. Concentration-dependent absorption (a, c) and the corresponding CD spectra (b, d) of the R-2 after subjecting the solution to heating−cooling (a, b) and ultrasonication (c, d) in benzonitrile.

Meanwhile, both samples prepared by different methods exhibited similar profiles below 500 nm. The CD spectrum at lower wavelengths depicts the contribution from the intramolecular coupling as well, which arises from both the monomeric form of the molecule in solution and the molecules within the aggregates. For the same isomer of the molecule, the intramolecular coupling is dependent on the relative configuration between the PBI units and not on the helicity of the fibers.35 The morphological features of the aggregated structures of 2 were found to be different from that of 1. The TEM micrographs of the sample prepared through the heating− cooling method showed long bundled and entangled fibers (Figure 4a,b). The nature of the helicity in these fibrous bundles resembled that of 1 under similar conditions; right- and left-handed helical twists were observed for the R and S isomers, respectively, for the aggregates prepared by the heating−cooling method. In this case also the helical

Figure 4. TEM micrographs of the R (a, c) and S (b, d) isomers of 2 prepared by heating−cooling (a, b) and by ultrasonication (c, d). 6033

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the width of the fibers (∼3.8 and 4 nm) calculated from the high-resolution TEM micrographs (Figure S7, Supporting Information). The presence of weak band at 3.8 Å suggests the existence of loose π−π stacking interactions between the PBI units. A greater number of peaks observed in the low-angle region imply a relatively higher periodicity and compact arrangement of molecular side chains in the aggregates formed by the heating−cooling method.49,52 This method leads to the formation of structures with ordered packing and extended exciton coupling. The diffraction pattern of samples prepared by the heating−cooling method showed a broad peak in the 2θ range of 15°−25° while this was not observed for the sonicated samples. This indicates that the alkyl chains are packed to higher order in the former case relative to the samples prepared via the sonication method.57 The results of XRD and FTIR clearly suggested that only a slight change in the mode of molecular packing had a strong impact on the resulting selfassembled structures as well as their chiroptical properties.21 The small change in the hydrogen-bonding network and the packing of alkyl chains was reported to lead to polymorphism in the self-assembling structures of artificial lipids.58 More commonly, the reversal of supramolecular chirality caused by a small change in molecular packing is represented by the natural double-helical structure of DNAs. Compounds 1 and 2 were found to exhibit gelation properties in benzonitrile above a certain concentration. The fluorescent gels were prepared by dissolving the compounds in benzonitrile at elevated temperature and allowing them to cool to room temperature in a vial. The failure of the soft mass to flow on inverting the vial was indicative of the formation of gel. The critical gelation concentrations were found to be 8.8 and 8.4 mM for 1 and 2, respectively. Spectral investigations of the gels were carried out in a cuvette with a 1 mm optical path. The absorption spectra of the gels exhibited features corresponding to the aggregated state of the molecules (Figure 5a,c). The absorption spectra reveal that aggregates are predominant in the gel state when compared to partial aggregation observed in solution, and this may be due to the higher concentration of the molecule in gel. The gels of both compounds exhibited mirror

image CD spectra for their stereoisomers. Careful analysis of the CD spectra of the gel and the solution prepared via the heating−cooling method showed a resemblance to one another; R and S isomers exhibited the first negative and positive Cotton bands, respectively (Figure 5b,d). This is indicative of the fact that the nature of self-assembled structures formed via the heating−cooling method is similar to those formed in the gels. SEM or TEM analysis revealed the nature of the self-assembled structures in the gels. For the electron microscope investigations, the gels were diluted and 75 μL of the diluted solutions were drop-cast onto carbon grids. The electron microscope images showed typical fibrous network structures observed for organogels on major portions of the grid (Figures S8 and S9, Supporting Information). The nature and twist of helices for both compounds were similar to those obtained via the heating−cooling method. Moreover, the gels were synthesized by adopting the heating−cooling procedure, which resulted in nanostructures with similar morphology to the aggregates formed by the heating−cooling method. At higher concentrations, the interlinked fibers trap solvent molecules, and these higher-order network structures are responsible for the observed gelation properties of the compounds. The reversibility in chirality of the nanostructures was investigated by subjecting the samples to simultaneous heating and sonication cycles. At high temperature, the absorption and CD spectra of the compounds exhibited similar features for samples prepared by both methods. The absorption spectrum of 1 showed a blue shift and a narrowing of peaks with an increase in the A0−0/A0−1 ratio (Figure 6a,c). The CD

Figure 6. Temperature-dependent absorption (a, c) and the corresponding CD spectra (b, d) of R-1 at 25 °C (red lines), at 85 °C (blue lines), and after cooling the solution to room temperature (green lines) for the samples prepared by heating−cooling (a, b) and ultrasonication (c, d). Note that the green lines in (b) and (d) are identical, suggesting that the molecules revert back to the aggregates prepared via the heating−cooling method.

responses of the stereoisomers displayed a gradual decrease in the peak intensities corresponding to the aggregates with the progressive peaks in the monomeric region (blue traces in Figure 6b,d). Similar spectral features were exhibited by 2 at elevated temperatures: a gradual decrease in the intensity of bands with a blue shift in peak position (Figure S10, Supporting Information). The spectral features for both the compounds at

Figure 5. Absorption (a, c) and the corresponding CD spectra (b, d) of the gels of R (red lines) and S (blue lines) isomers of 1 (a, b) and 2 (c, d) in benzonitrile. The inset shows the photographs of the corresponding gels under white light and 365 nm UV light illumination. 6034

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85 °C resembled those in chloroform at room temperature, confirming that the aggregated structures dissociate to monomers at elevated temperatures. However, the dissociation is not complete, and the molecules partly remain in the selfassembled state, as is evident from the peak at 550 nm in the CD spectra. It may require further heating for complete dissociation, but due to the limitations of the instrument, we could not monitor the spectral changes at higher temperatures. Moreover, the dissociation of the self-assembled structures of 1 and 2 at higher temperature was confirmed with the help of TEM micrographs. Furthermore, upon cooling the solution to room temperature, the molecules gradually self-assemble to the chiral aggregated structures. The rate of reassembly to nanoaggregates was dependent on the concentration of the solution; dilute solutions form chiral aggregates at a slower rate. On cooling the solution, the samples prepared by both methods exhibited CD spectral features corresponding to the aggregates formed via the heating−cooling method (green traces in Figure 6). This indicates the heat-induced helicity inversion in the chiral arrangement in the sample prepared by ultrasonication during the heat treatment. The formation of aggregates during cooling is a time-dependent process, hence the difference in intensity between the initial and final spectra. On the contrary, sonication retained the chirality of the samples prepared by the heating−cooling method (Figure S11, Supporting Information). This is indicative of the fact that the inversion in the handedness of supramolecular chirality occurs only in one direction with the heat treatment of the sample prepared by the ultrasonication method but not the other way (by the sonication of the sample prepared by the heating−cooling method), suggesting the thermodynamic control of the supramolecular chirality in the heating−cooling procedure. Aparicio et al. and others have recently demonstrated that the three driving factors that decide the kinetic or thermodynamic pathway in the formation of helical structures are concentration, temperature, and time.59,60 In the present case, the heating−cooling method provides high temperature, and the nanostructures are formed on a longer time scale during the slow cooling of the solution. This procedure results in the formation of more stable nanostructures which are thermodynamically driven. On the contrary, ultrasonication is a fast process leading to a kinetically stable product on a shorter time scale. However, no change was observed in the CD spectra for the samples prepared via the ultrasonication method after being aged for 1 month at room temperature (Figure S12), suggesting that activation energy is required for the conversion to thermodynamically stable assemblies. The investigations of the reversibility of the self-assembled structures show that the kinetically controlled product (formed by ultrasonication) evolves to a thermodynamically controlled one by modifying the temperature instead of time. The activation energy required for the conversion of one form to another is provided by the heat treatment in the present system. A comparison of the nature of aggregates formed by the two methods indicates a higher-order chiral induction for the fibers formed by the heating−cooling method. The gCD values for the aggregates of 1 formed by the heating−cooling and ultrasonication procedures were found to be 1.05 × 10−3 and 8.5 × 10−4, respectively. The corresponding values obtained for 2 were 3.1 × 10−4 and 2.1 × 10−4 for the two different methods of preparation. These results were in agreement with the TEM micrographs and the XRD data which showed extended fibers and more ordered molecular packing for the aggregates formed

by the heating−cooling method. During the heating−cooling cycle, the molecules are thermodynamically driven to the most stable arrangement to form self-assembled structures with higher order. The helicity of the fibers is driven by the nature of the chiral center present on the molecule. On the contrary, sonication induces pressure and heat which kinetically enforce the aggregation process leading to disordered packing of the molecules and the rupture of the higher-order structures (Scheme 1). The nature of chirality in the aggregates of 1 and 2 Scheme 1. Schematic Illustration of the Difference in Molecular Packing Leading to the Reversal in Supramolecular Chirality of the Aggregates Formed by the Two Methods

were similar for the same preparatory methods, indicating that the central cyclohexane-diamide unit is responsible for the supramolecular helicity of the self-assembled nanostructures.

3. CONCLUSIONS We have demonstrated the successfully tuning of supramolecular chirality in chiral bichromophoric PBI systems by adopting suitable preparatory methods. The inversion of supramolecular chirality was achieved by applying external stimuli such as heat and ultrasound. With the heating−cooling method, the R isomer of the compounds showed a negative first Cotton effect with a right-handed twist for the fibers. The supramolecular chirality observed in CD and the twist in the fibers could be reversed by adopting ultrasonication as the method of preparation. The induction of higher-order supramolecular chirality and a larger extent of excitonic coupling as evident from the CD spectra make the heating−cooling method superior in terms of chiral nanofiber synthesis. The heating−cooling cycle stimulates the formation of the thermodynamically stable helical arrangement of the molecules. The ample amount of heat and pressure during sonication kinetically induces the spontaneous aggregation of molecules forming a less ordered packing structure. Moreover, the aggregates could be relaxed to the more favored thermodynamic product by suitable heat treatment. The controlled modulation of chiral properties of the self-assembled aggregates helps in the design of optical devices based on organic nanostructures with the desired chiroptical properties. 6035

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ASSOCIATED CONTENT

S Supporting Information *

Spectral and morphological investigations of the aggregated structures under different experimental conditions. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Hitachi High-Technolgies for high-resolution SEM measurements with SU9000. This work was partially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan under the Green Photonics Project at NAIST and a Grant-in-Aid for Scientific Research A (25248019).

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