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Oct 15, 2014 - Self-Assembly Mechanism of 1,3:2,4-Di(3,4-dichlorobenzylidene)-d-sorbitol and Control of the Supramolecular Chirality. Jingjing Li† ...
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Self-Assembly Mechanism of 1,3:2,4-Di(3,4dichlorobenzylidene)‑D‑sorbitol and Control of the Supramolecular Chirality Jingjing Li,† Kaiqi Fan,† Xidong Guan,† Yingzhe Yu,‡ and Jian Song*,†,§ †

School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, P. R. China R&D Center for Petrochemical Technology, Tianjin University, Tianjin, 300072, P. R. China § The Co-Innovation Center of Chemistry and Chemical Engineering of Tianjin, Tianjin, 300072, P. R. China ‡

S Supporting Information *

ABSTRACT: Dibenzylidene-D-sorbitol (DBS) and its derivatives are known to form gels in organic solvents; however, the mechanism of the gel formation has been a subject of much debate. The present work is undertaken to elucidate the organization mechanism of a DBS derivative, 1,3:2,4-di(3,4dichlorobenzylidene)-D-sorbitol (DCDBS), by taking into account the solvent effects and comparing the experiment data with theoretical calculation. These molecules form smooth nonhelical fibers with a rest circular dichroism (CD) signal in polar solvents, in contrast to rope-liked left-helical fibers with a strong negative CD signal observed in nonpolar solvents. The molecular complexes thus formed were characterized by means of Fourier transform infrared spectra, ultraviolet−visible spectra, X-ray diffraction patterns, static contact angles, and theoretical calculations. It was proposed that the interactions between the gelator and the solvents could subtly change the stacking of the molecules and hence their self-assembled nanostructures. In nonpolar solvents, the gelator molecules appear as a distorted Tshaped structure with the 6-OH forming intermolecular hydrogen bonds with the acetal oxygens of adjacent gelator molecule. In addition, because of differential stacking interactions on both sides of the 10-member ring skeleton of the gelator, the oligomers may assemble in a helix fashion to minimize the energy, leading to helical fibers. In polar solvents, however, the gelator molecules show a rigid planelike structure and thus stack on top of each other because of strong parallel-displaced π interactions. The balanced driving force on both sides of the 10-member ring skeleton made it difficult for the dimers to bend, thus resulting in nonhelical nanostructure. As expected from the mechanisms proposed here, twisted ribbon fibers with a medium strength CD signal were obtained when solvents of different polarities were mixed. Thus, solvent effects revealed in this work represent an effective means of realizing in situ tuning of nanostructures and control of the expression of chirality at supramolecular levels.

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INTRODUCTION

Despite enormous efforts directed to the studies of these low molecular weight gelators (LMWGs) over the past hundred years,7−13 the self-assembly mechanism has not been well understood. Yamasaki et al.14 proposed a conceptual model for DBS aggregation in which the 6-OH group forms an intermolecular hydrogen bond with an acetal oxygen on a neighboring DBS molecule. Watase et al.,15 however, proposed that the DBS molecules stack neatly on top of each other, separated by a distance of no more than 3.5 Å between adjacent phenyl rings. The two models were considered to be in fundamental conflict. In order to determine which models best described the intermolecular interactions between DBS molecules, Wilder et al.16 performed both molecular mechanics calculations and molecular dynamics simulations on DBS and confirmed that both intermolecular hydrogen bonding

Chiral molecule dibenzylidene-D-sorbitol (DBS) (Scheme 1, without chlorine substituents) and its derivatives are wellknown gelators with many applications, such as nucleating agents,1 gel electrolytes,2 light stabilizers,3 light-scattering displays,4 adsorption of dyes,5 and controlled drug delivery.6 Scheme 1. Chemical Structure of the Gelator, 1,3:2,4-Di(3,4dichloro benzylidene)-D-sorbitol (DCDBS)

Received: August 27, 2014 Revised: October 13, 2014 Published: October 15, 2014 © 2014 American Chemical Society

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(between the 6-OH group of one molecule and free acetal oxygens of another) and π interactions (between the phenyl rings of adjacent molecules) play an important role in DBS selforganization. However, their studies also revealed that the phenyl rings of DBS molecules prefer to orient in a coplanar fashion and perpendicular to the sorbitol backbone not only in the single molecule state but also in the dimer situation. Therefore, their theory failed to give a reasonable explanation or models to combine the intermolecular hydrogen bonding with the π interactions. Most importantly, all their studies were performed in vacuum with solvent effects totally ignored. VanderHart et al.17 utilized solid-state 13C and 1H NMR techniques to characterize the structures of both in situ and dried DBS gels. Because the sample, obtained by melt crystallization and subsequent slow cooling, was a mixed crystal with a fraction of small domains structurally similar to the fibrils, their NMR analysis is considered incompetent to reveal the structural features of the fibrils. Taken together, the previous studies seem to indicate that the self-organization of DBS and its derivatives is so complicated that a complete understanding has yet to be achieved. On the other hand, within the field of supramolecular gels, supramolecular chirality has generated increasing interest because of the tunability of the noncovalent interactions such as π−π stacking, hydrogen bonding, van der Waals and hydrophobic interactions, etc.18,19 On the basis of appropriate molecular design and self-assembly processes, many helical or twist nanostructures have been produced through organogels and hydrogels.20−23 In addition, when the temperature, pH, light, cooling rate, additives, and even the solvents are changed, the self-assembled chiral nanostructures could be regulated both in organogels and hydrogels.24−32 Sun and co-workers20 reported an interesting effect of solvent-triggered chiralinteraction reversion and successfully applied this chiral effect in the selective separation of quinine enantiomers, thus imparting “smart” merits to the gel materials. Despite these reports, in situ tuning of helical structures through external stimulators and controlling over the expression of chirality at supramolecular levels remain a challenge.33,34 Therefore, despite the fact that helical nanostructures of the DBS organogel have been reported since 1990,35 the morphological development of the helical nanostructures and control over the expression of chirality are still not clear. Therefore, it is important to clarify the underlying chiral self-assembly mechanism of DBS or its derivative gels, thus realizing control of the chirality and fabrication of chiral materials. The objectives of the present work are to elucidate selfassembly mechanisms and to explore means of controlling the chirality of supramolecular gels comprising a DBS derivative 1,3:2,4-di(3,4-dichlorobenzylidene)- D -sorbitol (DCDBS, Scheme 1). DCDBS shows good gelation ability in both polar solvents and nonpolar solvents due to the cooperative effect of hydrogen bonding and π−π stacking interaction. However, the morphologies, optical properties, and selfassembly mechanisms are totally different in polar solvents and nonpolar solvents, as shown by scanning electron microscopy (SEM), circular dichroism (CD), Fourier transform infrared (FTIR) spectra, ultraviolet−visible, static contact angles, X-ray diffraction (XRD) patterns, and theoretical calculation. It turns out that the solvents play a central role in not only the formation of the supramolecular gels but also the control of the chirality.

Article

EXPERIMENTAL SECTION

Chemicals and Materials. The reagents used in the studies were purchased from Tianjin Chemical Reagent Factory and used without further purification. Synthesis of the Gelator. The gelator DCDBS was synthesized by a condensation reaction between D-sorbitol and 3,4-dichlorobenzylidene (Scheme S1, Supporting Information). Scanning Electron Microscopy Measurements. SEM images were taken on a Hitachi S-4800 operating at 3.0 kV and 10 μA. A sample of the xerogel was prepared by drying the gel in air and then coating with platinum for 60 s. Rheology Measurements. Measurements were performed using a stress-controlled rheometer (Stress Texh) equipped with stainless steel parallel plates (25 mm diameter, 1 mm gap). The strain sweeps experiment to obtain the storage modulus (G′, associated with energy storage) and the loss modulus (G″, associated with the loss of energy) was performed at a constant oscillation frequency of 10 rad/s at 20 °C. FTIR Measurements. The FTIR spectra of the xerogels and solutions were collected from 3900−3000 cm−1 with a Bruker Equinox 55 infrared spectrophotometer using KBr pellets. UV−Vis and CD Spectra Measurements. UV−vis absorption spectra were obtained with a Varian Cary 1E UV−vis spectrometer. The path length of the quartz cell was 1 cm. CD spectra were recorded on JASCO J-810 CD spectrophotometers. A hot sol of DCDBS (0.5% w/v) was poured into a 5 mm path length quartz cell, which was sealed with a Teflon cap to avoid loss of liquid, and cooled to room temperature to form a stable gel. The time-dependent spectra were recorded from hot sol to stable gel at room temperature in increments of 5 min, allowing a 2 h equilibration time after heating. Wide-Angle XRD Measurements. The X-ray diffraction patterns were recorded using a Rigaku D/Max 2200-PC diffractometer with Cu Kα radiation (λ = 0.15418 nm) and a graphite monochromator at ambient temperature. The samples were tested from 3° to 20° at a scanning rate of 0.5 s/step. The gels were spread on a glass slide as a film and allowed to dry in air prior to data collection. Static Contact Angles Measurements. Static contact angle measurements were performed on POWEREACH JC2000D1 contact angle measuring device. A drop of a hot solution/sol was placed onto a microslide, cooled to room temperature (to form a gel), and vacuum dried. Water droplets were placed on the vacuum-dried surface via a microsyringe, and images were captured to measure the angle of the liquid−solid interface. Each sample was recorded at three different points on a surface. Calculation Methods. Energy calculations were performed on single DCDBS molecules using Gaussian 09 program suited with hybrid density functional theory (DFT) method at the 6-31g (d) level to elucidate the three-dimensional structure of DCDBS. To simulate the solvent environment, a conductor-like polarizable continuum model (C-PCM) was used in the calculation. To simplify the calculation, water and benzene were selected as the typical representatives of polar and nonpolar solvents. All the stationary points were checked with frequency analysis at the same level. Energy minimizations were also conducted on DBS dimers using Discover Module of Materials Studio 6.6 (Accelrys, Inc.) with the CVFF force field in an effort to reveal the intermolecular interactions between DCDBS molecules.

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RESULTS AND DISCUSSION Gelation Behaviors and Morphologies. The gelation behaviors of DCDBS in various solvents were examined, and the results are shown in Table 1. DCDBS was found to be an effective gelator that could gel in a wide range of solvents, from nonpolar solvents like o-dichlorobenzene and n-octanol to polar solvents like ethylene glycol and DMSO−H2O (7:3 v/v) mixtures, thus allowing the effects of solvents on DCDBS gel properties to be studied systematically. Moreover, all the critical gelator concentrations (CGCs) are less than 1%, indicating that DCDBS is a powerful gelator. A strict definition of a gel must 13423

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Table 1. Appearancea and Critical Gelator Concentration (CGC, % (g/mL)) of DCDBS in Different Solvents solvent chlorobenzene o-dichlorobenzene n-octanol n-hexanol ethylene glycol glycero DMSO:H2O (7:3 v/v) a

solvent G G G G G G G

(0.17%) (0.12%) (0.39%) (0.65%) (0.22%) (0.03%) (0.25%)

DMF DMSO acetone methanol acetonitrile water

S S I I I I

G, gel; S, solution; I, insoluble.

include specific mechanical behavior. For that reason, DCDBS gel in n-octanol is selected as typical representative to carry out the rheology experiment. As shown in Figure S1 in the Supporting Information, the initial G′ (5.85 × 104 Pa) is greater than G″ (3.91 × 103 Pa) by 1 order of magnitude, and the difference is maintained over a large range of shear strain. This behavior is as expected for a true gel phase.36 The morphologies of some typical gels were examined by SEM. As shown in Figure 1, smooth fibers with a 21 nm

Figure 2. CD spectra of 0.5% DCDBS in (a) different solvents and (b) n-octanol during gelation at different times (within 2 h from solution to gel).

because the molecules are not aggregated. When the temperature is lowered, the DCDBS molecules begin to aggregate and significant increases in the CD signal are observed. This observation indicates that the CD bands do not originate from the intrinsic molecular chirality but rather result from chiral aggregates of DCDBS.37 Moreover, the CD spectra continually red shift during the whole gel process. The negative CD signals show two transition states centered at 254, 289 nm and 260, 292 nm and finally shift to 301 nm. This observation indicates a gelation-induced helix transition during the hierarchical selfassembly, which is rarely reported in the literature.38 UV Absorption Spectra. The self-assembly of DCDBS was examined by solvent-dependent and concentration-dependent UV−visible spectroscopy. Figure 3a shows the absorption spectra of different concentrations of DCDBS in DMSO−H2O. The DCDBS does not form aggregates at low concentration (0.1% w/v); thus, the major absorption band shows the wellresolved vibronic structure ranging from 200 to 300 nm. With the increase of the concentration of DCDBS, aggregation is observed, as evidenced by the spectral changes (Figure 3a). The most prominent features are an increase in the peak intensity along with a broadening of the absorption spectra and a loss of the fine structure. Additionally, a new broad absorption appeared in the longer wavelength region. These features suggest the formation of a ground-state dimer, due to the resonance of a π-electron between the benzylidene moieties.15 For DCDBS in n-octanol, very similar π stacking was observed (Figure 3b), except for the appearance of a new peak at 231 nm and the resolution of the peaks from 260 to 285 nm. These results demonstrate that π−π stacking between phenyl groups of DCDBS plays an important role in the gel formation. FTIR Spectra. FTIR spectra of DCDBS in solution state and xerogel state were measured, and the results are shown in Figure 4. The FTIR spectrum of DCDBS in the solution phase

Figure 1. SEM images of DCDBS xerogels made from (a) DMSO− H2O (7:3 v/v), (b) ethylene glycol, (c) n-octanol and, (d) odichlorobenzene.

average diameter are observed in polar solvents like DMSO− H2O and ethylene glycol (Figure 1a,b). In contrast, ropelike left-handed helical structures with a pitch of about 90 nm and a width of about 60 nm are obtained in nonpolar solvents like odichlorobenzene and n-octanol (Figure 1c,d). These results suggest that the polarity of solvents may have important effects on the morphology of DCDBS gels. Circular Dichroism Spectra. To study the chiral structure of the aggregate in the gel, CD spectra of the DCDBS gels were measured. As shown in Figure 2a, in n-octanol, a strong CD signal with a negative Cotton Effect center at 301 nm was observed, indicating a left-handed helical assembly of DCDBS. However, compared with that of n-octanol, the CD signals of DCDBS in DMSO−H2O mixtures were almost invisible, indicating a nonhelical aggregation, which was in good agreement with the results of SEM. To exclude artifacts that may confuse the interpretation of the CD spectra, variable temperature CD spectroscopy is often conducted to demonstrate the formation of nanoscale chiral aggregates.37 As shown in Figure 2b, almost no CD signals are observed at high temperature for n-octanol gel in solution state 13424

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polarity, and become virtually invisible, as shown in Figure 4b− e and Table S1 in the Supporting Information. Moreover, the intensity ratio of the two bands (Il/Ih) also increases with decreasing solvent polarity (Table S1 in the Supporting Information), and a new band at around 3264 cm−1 appears (Figure 4e). These data suggest that with decreasing solvent polarity the intermolecular hydrogen bonding between DCDBS molecules becomes more predominant over the hydrogen bonding between DCDBS and solvents. In other word, the gel formation involving DCDBS in nonpolar solvents is driven by strong hydrogen bonding interactions between gelator molecules, and these interactions are weakened by completion from solvent molecules when polar solvents are used.25 Wide-Angle XRD and Static Contact Angle. To explore the possible packing modes of the gelator molecules in various solvents, wide-angle XRD (WXRD) and the contact angles of the xerogels were examined. As shown in Figure 5a,b, XRD

Figure 3. UV−vis absorption spectra of different concentrations of DCDBS in (a) DMSO−H2O (7:3 w/v) and (b) n-octanol.

Figure 5. WXRD spectra of 2% DCDBS xerogels made from (a) noctanol, (b) o-dichlorobenzene, (c) ethylene glycol, and (d) DMSO− H2O.

patterns of the xerogel from n-octanol and o-dichlorobenzene show three main diffraction peaks centered at 2θ = 4.62° (d = 1.91 nm), 2θ = 8.99° (d = 0.98 nm), and 2θ = 16.81° (d = 0.52 nm), while the xerogels from ethylene glycol and DMSO−H2O (Figure 5b) exhibit four diffraction peaks at 2θ of 5.27° (d = 1.62 nm), 9.39° (d = 0.94 nm), 13.69° (d = 0.64 nm), and 17.09° (d = 0.52 nm). More ordered molecular packing in the xerogel obtained from nonpolar solvents is expected from the number of diffraction peaks observed.41 The different packing modes were further confirmed by static contact angle experiments. As shown in Figure S3 in the Supporting Information, the contact angle of a water droplet on a film of the xerogel made from o-dichlorobenzene and n-octanol is 99.38° and 95.23°, respectively, indicating a hydrophobic nature. The contact angle of the xerogel prepared from ethylene glycol and DMSO−H2O is 76.66° and 73.53°, respectively, indicating a hydrophilic nature.42,43 The above results indicate that in nonpolar solvents, the DCDBS molecules form a molecular complex in which the polar groups are buried inside the structure and its phenyl

Figure 4. FTIR spectra of 2% DCDBS in (a) a solution state in DMF and in xerogel states made from (b) DMSO−H2O (7:3 v/v), (c) ethylene glycol, (d) n-octanol, and (e) o-dichlorobenzene.

(a fresh solution in DMF) shows two bands at 3503 and 3287 cm−1, which shift to lower wavenumbers in all the xerogels, indicating that the O−H groups participate in the formation of hydrogen bonds among the gelator molecules.39 Roughly speaking, two kinds of νOH bands were observed in the xerogel state; the higher-wavenumber band is assigned to the hydrogen bonding between DCDBS and solvents and the lowerwavenumber band to the DCDBS molecules with each other.14,40 The O−H bands were identical in all the xerogels at low wavenumbers (νl = 3193 cm−1) but different at higher wavenumbers (νh), gradually red shift with decreasing solvent 13425

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Scheme 2. Possible Self-Assembly Models of DCDBS in Different Solvents Obtained by Molecular Mechanics Calculation

The two acetal oxygens form hydrogen bonds in turn to keep a fixed d-spacing (1.918 nm), as verified by XRD (Figure 5a,b), in agreement with the conceptual model proposed by Yamasaki et al.14 More importantly, the R2 ring of the middle DCDBS molecules in the oligomers is more or less perpendicular to the other two R2 benzene rings and forms weak T-shaped benzene ring π interactions (ΔEbonding = 8.16 kJ mol−1 according to McGaughey45). Conversely, R1 benzene rings in the oligomers are so far away (7.2 or 7.7 nm) that they cannot form paralleldisplaced π interactions with each other. Because of the energy difference of the two sides, the oligomers may assemble in a helix fashion to minimize the energy, leading to fibrils that contain spiral grooves. In polar solvents, the rigid planelike structures stack on top of each other and form strong paralleldisplaced π interactions (ΔEbonding = 11.51 kJ mol−1 according to McGaughey45), in agreement with the conceptual model proposed by Watase et al.15 The balanced driving force made it difficult for the dimers to bend, thus resulting in a nonhelical nanostructure. From the above discussion we can conclude that the selfassembly models proposed by Yamasaki et al. and Watase et al. are actually not in fundamental conflict as previously perceived. They can be used to account for the different packing modes in different solvents. An important lesson we have learned is that the solvent effects could not be neglected when considering self-assembly-related problems. Supramolecular Chirality Control. As indicated earlier, a transition in stacking modes of DCDBS molecules can occur when the polarity of the solvent is changed. This phenomenon can be exploited for the fine-tuning of the supramolecular nanostructures of DCDBS gels. Because THF has polarity (ε = 7.39) similar to that of odichlorobenzene (ε = 9.93) and is miscible with water (ε = 80.4), the two solvents were selected as the representatives in our initial attempt to tune the nanostructures of DCDBS. Figure 6 shows the dependence of the morphologies of DCDBS on the composition of THF and H2O. DCDBS was soluble in a mixed solvent with the composition range of 0% to 30% H2O−THF (v:v) mixtures, as shown in Table S2 in the Supporting Information. In pure THF and 20% H2O−THF solution (Figure 6a,b), helical fibers with mean widths of 30 and 50 nm and mean helical pitches of 70 and 80 nm, respectively, were formed. When the water content increased to

groups are extended outside, resulting in a hydrophobic surface (contact angle exceeding 90°). The hydrogen bonding between hydroxyl groups of the DCDBS molecules was the principal driving force for the gelation, as verified from FTIR spectra. Conversely, in polar solvents, the polar groups packed on the outside of the structure and the phenyl groups remained inside of the structures, resulting in a hydrophilic surface (contact angle less than 90°). The π−π stacking plays much important roles during the process of the self-assembly. To further our understanding of DCDBS gel formation in different solvents, quantum chemistry calculation (Gaussian 2009, density function theory, at 6-31g d level) at the singlemolecule level44 and molecular mechanics calculation at the oligomer level were performed. To simplify the calculation, water and benzene were selected as representatives of polar and nonpolar solvents in the course of quantum chemistry calculation. As shown in Scheme S2 in the Supporting Information, different optimum conformations of DCDBS in different solvents were obtained starting with the same initial conformation, as reported previously.20 Regardless of what kind of initial structure is involved, the minimum energy of the DCDBS molecule in water is always lower than that in benzene. The energy differences between two conformations are −20.98, −21.99, −22.18, and −24.27 kJ mol−1. From this point of view, DCDBS may be much more stable in water than in benzene. This brings an obvious advantage in energy for the subsequent self-assembly, thus resulting in a more ordered stacking as indicated by XRD results. Additionally, in nonpolar solvents, the 5-OH group tends to form an intramolecular hydrogen bond with the nearest acetal oxygen. Because the R1 phenyl ring is perpendicular to the 10-member ring skeleton and the R2 phenyl ring is oblique at some angle, the whole molecule is thus featured with a distorted T-shaped structure, as previously reported.14,16 In polar solvents, no intramolecular hydrogen bonds are formed because of the strong solvation effects. The two phenyl rings look like two open wings of a butterfly with a dihedral angle of about 150°; thus, the whole molecule shows a rigid planelike structure. On the basis of the optimal conformation described above, two possible self-assembly models in polar or nonpolar solvents are proposed, as illustrated in Scheme 2. In nonpolar solvents, the 6-OH of DCDBS tends to form intermolecular hydrogen bonds with the acetal oxygens of adjacent DCDBS molecule. 13426

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Figure 7. CD spectra of 0.5% DCDBS in different H2O−THF mixtures.

through changing solvents and control over the expression of chirality at supramolecular levels are realized. Figure 8 shows the WXRD pattern of the xerogels fromed from 50% H2O−THF mixtures. Note that the d-spacing at 2θ =

Figure 6. SEM images of 0.5% DCDBS xerogels made from different H2O−THF mixtures.

30%, twisted ribbons with many fluted edges were observed, indicating a less chiral order.46 The mean width and pitches of the twisted ribbons are 70 and 140 nm, respectively. With further increase in the content of water to 40% and 50%, gels were formed and the morphologies were similar to that in 30% water solutions. The mean width and pitches of the twisted ribbons are 90 and 130 nm, respectively. Upon increasing water content up to 60% or more, DCDBS molecules were no longer soluble in the mixtures. Both short rods and twisted ribbons with mean helical pitch of 110 nm were observed. The widths of the rods and the twisted ribbons are 70 and 50 nm, respectively. In pure water, only short rods with mean widths of 80 nm were observed. It is evident that the morphology of the self-assembled DCDBS depends on THF−H2O solvent composition as well as the widths and helical pitches of the fibers. Because the gel structures of DCDBS are fragile, we were unalbe to obtain more information about the morphology changes with higher-resolution SEMs. Assembly of DCDBS in H2O/THF mixtures was further investigated by circular dichroism spectroscopy and WXRD. As indicated in Figure 7, the organization and the resulting chirality of the aggregates of DCDBS molecules are strongly controlled by the composition of H2O and THF. The aggregation promoted in 50% H2O, for example, results in the formation of gels featuring signals much stronger than those obtained at a lower water proportion (0−30% H2O). However, the molar ellipticity in 50% THF−H2O (Figure 7, −23 mdeg at λmax = 303 nm) was smaller than that in n-octanol (Figure 2, −84 mdeg at λmax = 301 nm), suggesting a disordered chiral packing structure of DCDBS molecules,46,47 which is consistent with the SEM results. Thus, in situ tuning of helical structures

Figure 8. WXRD spectrum of 0.5% DCDBS xerogel from 50% H2O− THF mixture.

5.27° (d = 1.62 nm) is the same as that from ethylene glycol, while the characteristic peak at 2θ = 13.69° (d = 0.64 nm) in the ethylene glycol pattern is not shown in this pattern, just as the pattern from o-dichlorobenzene and n-octanol. Therefore, the XRD pattern of xerogel from THF−H2O mixture has characteristic patterns of both polar and nonpolar solvents, indicating a mixed and complicated molecular packing. As indicated in Scheme 2b, the self-assembly of DCDBS in polar solvents was mainly driven by the π−π stacking. The driving force was so strong that the chirality of DCDBS could not be expressed at a higher level. Because the hydrophobic phenyl groups of the amphiphile are highly solubilized with the addition of THF, the hydrogen bonds take over the π−π stacking, thus enabling the expression of molecular chirality in the final morphology. To provide further evidence, FTIR spectra of xerogel from 50% THF−water mixture solvents were investigated. As shown in Figure S4 in the Supporting Information, the characteristic hydroxyl group at low wavenumber centered at 3193 cm−1 was identical to that in other solvents. However, compared with DMSO−water (3417 cm−1, Il/Ih = 1.03) and ethylene glycol (3407 cm−1, Il/Ih = 0.91) (Table S1 in the Supporting Information), the higher wavenumber of the hydroxyl group 13427

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centered at 3406 cm−1 and the Il/Ih was 0.87. A larger red shift and a lower Il/Ih value indicate increasing hydrogen bonding interactions. The increased hydrogen bonding interactions has a larger chiral influence, which might be the origin of chirality expression at higher levels induced by the addition of THF.

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CONCLUSIONS We have systematically investigated the effect of the solvent on the morphology, self-assembly mechanism, and supramolecular chirality of DCDBS gels. In polar solvents, the gelator selfassembled with strong π−π stacking interaction resulting in a hydrophilic surface and smooth nonhelical fibers with a rest CD signal, whereas in nonpolar solvents the gelator self-assembled with strong hydrogen bonding interactions resulting in a hydrophobic surface and helical fibers with a strong negative CD signal. The significant solvent effects observed have been utilized to affect the chirality and the morphology of the selfassembled DCDBS nanostructures. The present work exemplifies that, by simply changing solvents, in situ tuning of nanostructures and control over the expression of chirality at supramolecular levels can be achieved, providing an important design consideration in nanofabrication via molecular selfassembly.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis, rheology measurements, UV−vis spectra, static contact angle measurements, FTIR spectra, and fluorescence emission spectra of DCDBS in different solvents. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21276188) for financial support. We are grateful to Professor Xueguang Shao of Nankai University for the Gaussian calculation and Professor Minhua Zhang of Tianjin University for the Materials Studio calculation.

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REFERENCES

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