ARTICLE pubs.acs.org/JPCB
Multiresponsive Chiroptical Switch of an Azobenzene-Containing Lipid: Solvent, Temperature, and Photoregulated Supramolecular Chirality Pengfei Duan,† Yuangang Li,‡ Liangchun Li,§ Jingen Deng,§ and Minghua Liu*,† †
Beijing National Laboratory for Molecular Science (BNLMS), Key Laboratory of Colloid, Interface, Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ College of Chemistry and Chemical Engineering, Xi0 an University of Science and Technology, Xi0 an 710054, China § National Engineering Research Center of Chiral Drugs and Key Laboratory of Asymmetric Synthesis, Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China ABSTRACT: An azobenzene-containing lipid was designed as a functional organogelator, and its self-assembly as well as the chiroptical properties were investigated. The gelator shows good gelation ability in various organic solvents ranging from polar to nonpolar solvents. Although the molecule did not show a CD signal in the absorption band of azobenzene in solution, supramolecular chirality was observed upon gel formation. Moreover, the supramolecular chirality exhibited a multiresponse to temperature, photoirradiation, and the solvent polarity. Particularly, positive supramolecular chirality was observed in polar solvents, while it inverted to a negative one in nonpolar solvents. All the responses in relating to the supramolecular chirality were reversible and thus produced a multiresponsive chiroptical switch.
1. INTRODUCTION Chirality is a basic character of nature and expressed at hierarchical scales from molecular, macromolecular, and supramolecular levels to nano-/macroscopical level.1 The transmission of chirality from the molecular level to the macro- or supramolecular level in the form of chiral structure is a general issue, as it allows control of chiral organization by molecular design. Recently, much research effort has been devoted to the control of the chirality at various hierarchical levels.2 On the other hand, the chirality can be used as an additional dimension in data storage, and the change of the chirality extends to many possibilities in designing functional materials such as chiroptical switches, which combine optical activity and switchable properties of a system.3 Chiroptical switches are stimuli responsive smart objects which display reversible changes in their optical activity upon external stimuli, e.g., temperature, light, pressure, gases, and so on. They can be achieved at both the molecular and the supramolecular level. However, in each case an intrinsically chiral component, either a chiral trigger, a chiral inducer, or a chiral matrix, is required for the operation of the switch. Outstanding achievements in this research area have been summarized in several excellent reviews and papers.3,4 In comparison with the chiroptical switch at a molecular level, a supramolecular chiroptical switch is sometimes easier to control.5 At the supramolecular level, chirality is strongly related to the molecular r 2011 American Chemical Society
assembly that all kinds of molecules, either chiral or achiral, could be possibly assembled into chiral assemblies through the noncovalent bond, thus providing many possibilities in realizing the functional chiral materials. Low-molecular-weight gelators (LMWG), as an important class of soft matter, offer many opportunities not only for the formation of nanostructures but also for the regulation of the supramolecular chirality.6 In general, the supramolecular chirality of a chiral superstructure is reflected by the underlying chirality of the subclass building blocks. If one enantiomer gives a righthanded aggregate, the other gives a left-handed one.7 However, opposite cases are frequently found in supermolecules.8 The supramolecular chirality of the formed nanostructures does not necessarily relate to the molecular chirality of the component molecules. In addition, the external conditions, such as the heat, light, and even the solvents, either achiral or chiral, could alter the supramolecular chirality of the whole system. Therefore, it is an important issue to clarify the relationship between the supramolecular chirality of the certain system and the molecular chirality. Further, the clarification of the underlying mechanism would
Received: November 7, 2010 Revised: February 9, 2011 Published: March 15, 2011 3322
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The Journal of Physical Chemistry B significantly help to control the chirality and fabricate chiral materials. In this paper, we report the chiral functionalization based on an azobenzene-containing lipid. Azobenzene is well-known as a photoisomerizable building block and frequently used as the photofunctional materials.9 Upon incorporating the unit into the scaffold made from glutamic acid derivative, we have found that the lipid showed good gelation ability in organic solvents ranging from polar solvents to nonpolar solvents due to strong intermolecular interactions. Although the azobenzene was not chiral in the designed lipid, the gel formation generated the supramolecular chirality for the azobenzene part, which is suggested to be due to the chiral transfer by self-assembly. More interestingly, the supramolecular chirality of the organogel showed active response to the temperature, light irradiation, and even polarity of the solvents, the process of which is reversible and thus realizes multiresponsive chiroptical switches. So far, various chiroptical switches based on single stimulus such as light or temperature are largely reported; we tuned the supramolecular chirality in multichannels by using this azobenzene-containing lipid. The results will provide an important clue to the design of functional soft materials.
2. EXPERIMENTAL SECTION 2.1. Materials. All starting materials were obtained from
commercial suppliers and used as received. Solvents were purified and dried according to standard methods. Thin-layer chromatography (TLC) was performed on silica gel HF254 flake, and column chromatography was carried out with 230400 mesh silica gel. 1H NMR spectra were recorded with a Bruker ARX400 (400 MHz) spectrometer in chloroform (CDCl3) using Me4Si as internal standard. Mass spectra were determined with BEFLEX III for MALDI-TOF mass spectrometer. Elemental analyses were performed on a Carlo-Erba-1106 instrument. 2.2. Synthesis of the Target Compound azo-LG2C18. In previous reports, we have demonstrated the synthesis of an organogelator containing a Boc group: N,N0 -bisoctadecyl-L-Bocglutamic diamide (LBG2C18). In this paper, we changed the Boc group to a 4-(phenyldiazenyl)benzoic acid. The Boc group of LBG2C18 was made to free amino by mixing with trifluoroacetic acid in dichloromethane. A 1.50 g amount of LBG2C18 was dispersed in dichloromethane under an icy bath. Then 5 mL of trifluoroacetic acid was added to the above mixture and stirred at 0 °C for 3 h. After that, the solvent was removed by rotary evaporation and an oily product was obtained. The oily compound was dissolved in 10 mL of tetrahydrofuran (THF) and poured into a 300 mL aqueous saturated solution of NaHCO3. After filtration, the product was purified by reprecipitation in THF to give a white solid (1.28 g, 98%). N,N-Bisoctadecyl-L-amino-glutamic diamide (LGA) (0.65 g, 1 mmol) was dispersed in dichloromethane (60 mL) and stirred for 30 min. Then, 4-(phenyldiazenyl)benzoic acid (0.23 g, 1 mmol) was added into the above mixture and stirred at 0 °C for 30 min. After that, 1-ethyl3-(3-(dimethylamino)propyl)carbodiimide hydrochloride (EDC 3 HCl; 0.23 g, 1.2 mmol) and 1-hydroxybenzotrizole (HOBt; 0.16 g, 1.2 mmol) were added to the mixture. The obtained mixture was stirred for 5 days at room temperature. After that, the solvent was removed by rotary evaporation and orange solid was obtained. The crude product was dissolved in 10 mL of THF and poured into a 300 mL aqueous saturated solution of NaHCO3. After filtration, the product was purified by reprecipitation in ethanol to give an orange solid (0.80 g, 90%). 1H NMR (400 MHz, CDCl3,
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Table 1. Gelation Property of azo-LG2C18 in Various Organic Solvents solvent
phasea CGC (% (m/v)) 0.26
solvent
DMSO
G
DMF methanol
P P
ethyl acetate P ethyl ether G
ethanol
P
CCl4
TG
THF
S
chloroform
S
NMP
P
n-propanol PG n-butanol
P 0.3 0.33
cyclohexane
TG
0.22
benzene
TG
0.27
toluene
TG
0.36
n-pentanol G
1.3
chlorobenzene TG
0.29
n-hexanol G n-dodecanol G
1.2 0.9
o-xylene m-xylene
TG TG
0.41 0.32
p-xylene
TG
0.31
nitrobenzene TG
0.52
acetone
PG
acetonitrile
phasea CGC (% (m/v))
P
1,4-dioxane P a
G = turbid gel; TG = transparent gel; PG = partial gel (gelator concentration, 2% (w/v)); P = precipitate (gelator concentration, 2% (w/v)). For gels, the minimum gelation concentrations at room temperature are shown in parentheses.
ppm): δ 8.18 (d, J = 6.3 Hz, 1H), 8.038.05 (m, 2H), 7.947.98 (m, 4H), 7.527.53 (m, 3H), 7.05 (s, 1H), 6.07 (s, 1H), 4.58 (q, J = 6.2, 1H), 3.26 (t, J = 6.1, 4H), 2.542.61 (m, 1H), 2.342.40 (m, 1H), 2.192.22 (m, 2H), 1.491.51 (m, 4H), 1.24 (br, 64H), 0.860.89 (m, 6H). MALDI-TOF-MS. Calcd for C54H91N5O3: 857.7. Found: 880.7 (Mþ þ Na), 896.7 (Mþ þ K). Anal. Calcd for C54H91N5O3: C, 75.56; H, 10.69; N, 8.16. Found: C, 75.72; H, 10.66; N, 8.31. 2.3. Characterization. The gel formation and their properties were characterized by a series of methods such as field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), UVvis absorption, circular dichroism (CD), and Fourier transform infrared spectroscopy (FT-IR) measurements. X-ray diffraction (XRD) was achieved on a Rigaku D/Max-2500 X-ray diffractometer with Cu KR radiation (λ = 1.5406 Å), which was operated at 45 kV, 100 mA. SEM was performed using a Hitachi S-4300 system with an accelerating voltage of 15 kV. Samples for FESEM were prepared by spinning the gels on silicon slices and dried under vacuum for 2 days. After that, all samples were coated with gold. FT-IR spectra were obtained by a JASCO FT/IR-660 plus spectrophotometer. UVvis and CD spectra were obtained using JASCO UV-550 and JASCO J-810 spectrophotometers, respectively. The hot solution was poured into a 0.1 mm quartz cell and cooled to room temperature to form a stable gel.
3. RESULTS 3.1. Formation of the Organogels in Various Solvents. The gelator molecule, abbreviated as azo-LG2C18, was synthesized by amidation of a 4-(phenyldiazenyl)benzoic acid with a glutamic acid based lipid having a free amino group. The azobenzene moiety is linked directly to the chiral center by an amide bond so that pronounced chiral induction can be transmitted to the ground-state orientation of the azobenzene moiety. It was found that the gelator has good solubility in THF and chloroform, while forming precipitate in most of the polar solvents such as dimethylformamide (DMF), methanol, ethanol, acetone, and acetonitrile, presumably because of the disruption of the 3323
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Figure 1. Plots of Tgel versus the concentration of azo-LG2C18 (A) and 1/Tgel versus the natural logarithm of the concentration of azo-LG2C18 in DMSO (b) and toluene (2).
Figure 3. FT-IR spectra of the xerogels obtained from DMSO (a) and toluene (b).
Figure 2. SEM images of azo-LG2C18 gels in various solvents: (A) DMSO, (B) ethyl ester, (C) toluene, and (D) CCl4.
hydrogen bonds and weak solubility of the azobenzene moiety. However, it is expected to be capable of forming a one-dimensional molecular arrangement by means of intermolecular hydrogen bonding through three amide linkages, ππ stacking interaction of the azobenzene moieties, and even by the van der Waals interaction involved in long alkyl chains. The gelation ability of azo-LG2C18 in various organic solvents was investigated by the “stable to inversion in a test tube” method. The gelator and the solvent were put in a screw-capped sample tube, and the solution was heated until the solid was dissolved. The solution was then cooled to room temperature, and the sample was inversed to determine whether the solution flowed. The resulting state of each solution is summarized in Table 1. Transparent gels were formed in several nonpolar solvents, such as cyclohexane, and aromatic solvents, whereas diemthyl sulfoxide (DMSO), n-pentanol, n-hexanol, n-dodecanol, and ethyl ester produced translucent or turbid gels. The gelling table shows that the gelator has good gelling ability in nonpolar solvents, especially in aromatic solvents, and the critical gelation concentration (CGC) is lower respectively. It is known that gelation is a balance between solubility and precipitation, which suggests that the gelator azo-LG2C18 has a good solubility in aromatic solvents compared with polar solvents. 3.2. Properties of the Organogel. All these gels are very stable at room temperature. They remain stable at room temperature at least months without obvious phase separation. To elucidate the inherent strength and stability of the systems, we
have selected two solvents: DMSO (polar solvent) and toluene (nonpolar solvent) as the typical representatives, and have examined their gel melting temperatures (Tgel). Tgel were determined by convenient “falling ball method” experiments. Figure 1A gives the plot of the gelator concentration versus the Tgel both in DMSO and toluene. The results are presented in Figure 1A. Typically, as the molar concentration of the gelator was increased, Tgel also increased nonlinearly. Compared with the toluene gel, the Tgel values of DMSO gel is higher under the same gelator concentration. The Tgel values of DMSO gels were about 20 K higher than theses of toluene gels at the same gelator concentrations. The thermodynamic parameters ΔH° and ΔS° can be calculated using gel melting temperatures Tgel and molar concentration (c) according to eq 1,10 in which ΔH° and ΔS° are the standard enthalpy and entropy for the solgel transition of the gel, respectively, and R is the gas constant. ln c ¼
ΔH° ΔS° þ RTgel R
ð1Þ
The ΔH° and ΔS° values for the DMSO and toluene gel melting processes, calculated from the linear ln c vs 1/Tgel plots (Figure 1B), are shown in the following: ΔH°DMSO = 208.01 kJ mol1, ΔS°DMSO = 626.11 J mol1 K1, ΔH°toluene = 51.13 kJ mol1, and ΔS°toluene = 104.31 J mol1 K1. Comparison of the enthalpy and entropy differences for the gelator in DMSO and toluene shows that the differences for DMSO gels are larger than those calculated for the toluene gels. The somewhat larger energy differences between DMSO gels and toluene gels suggest their different supramolecular organization. The Gibbs free energy change ΔG° for the gel melting process of the gelator in DMSO and toluene give close values as ΔG°DMSO = 20.18 kJ mol1 and ΔG°toluene = 19.84 kJ mol1, indicating a similar stability of the gels. However, the significant differences of the enthalpy and entropy between DMSO gels and toluene gels strongly suggest totally different assembly modes. The microscopic structures of the gels were investigated by field emission scanning electron microscopy. Figure 2 shows representative images of xerogels obtained from polar solvents (DMSO, ethyl ether) and nonpolar solvents (toluene, CCl4). These morphologies strongly depend on the solvents. The images obtained from DMSO were well-defined ribbon structures of 0.12 μm in width (Figure 2A), whereas uniform fibrous 3324
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Table 2. Xerogels Characteristic Vibrations (cm1) of the Gelators Obtained from Various Solvents gelling solvents
ν(NH)
νas, νs(CH2)
ν(CdO)
δ(NH)
δas(CH2)
DMSO
3334, 3292
2916, 2850
1657, 1636
1552, 1532
1470
ethyl ether
3333, 3292
2916, 2850
1657, 1638
1552, 1532
1471
CCl4
3299
2919, 2851
1657, 1633
1552, 1529
1467
cyclohexane
3299
2919, 2851
1632
1552
1467
toluene
3299
2919, 2851
1633
1552
1467
Figure 4. XRD patterns of the xerogels obtained from DMSO gel (4.3 103 mol L1) and toluene gel (5.0 103 mol L1); (B) models for the gelator arrangements in the DMSO gel with a one-above-the-other bilayer structure; (C) models for the gelator arrangements in the toluene gel with a interdigitated bilayer structure.
structures were obtained from ethyl ether with a diameter of 200 nm. The ribbons from DMSO xerogels were more than 100 μm in length, suggesting outstanding organization in onedimensionality. The nanofibers obtained from ethyl ether xerogels were also very long and with a large aspect ratio. However, in toluene or CCl4 gels, no regular morphologies were observed. Some bulky superstructures, which seemed to be scrolling from the film structure, were observed both in toluene and CCl4 xerogels. The distinct differences in morphologies between polar solvents and nonpolar solvents suggest different microcosmic assembly modes. 3.3. FT-IR Spectra of the Organogels. The hydrogen-bond pattern of the amide enabled the gel formation to be followed by monitoring the FT-IR. Figure 3 shows characteristic IR absorption for hydrogen bond between NH and CdO. It is obvious to find that shoulder peaks are observed at all of the characteristic peaks for hydrogen bonding in DMSO xerogel: 3334, 1657, and 1532 cm1 for ν(NH), ν(CdO), and δ(NH), respectively. These shoulder peaks could be ascribed to the isolated amide groups.11 That means that the hydrogen bonding strength in DMSO gel is weaker than in toluene gel. This phenomenon can be observed at other gels. As shown in Table 2, shoulder peaks exist in polar solvents, whereas there are less shoulder peaks in nonpolar solvents. From the above observations, we can safely conclude that the gel formation in nonpolar solvents was driven by a strong hydrogen bond, while weak H-bonds were formed in organogels obtained from polar solvents. The polar solvents probably limit the participation of hydrogen bonding. However, longrange ordered structures observed in polar solvent suggest wellordered molecular interactions exist in the systems. Considering the molecular structure of the gelator, the driving forces for the gelation are expected to be the hydrogen bonding and ππ interactions. Thus, the interactions between the azobenzene chromophore are expected to be the major driving force in polar solvents.
Figure 5. CD spectra of azo-LG2C18 gel in various states: DMSO gel (2) with a concentration of 4.3 103 mol L1, DMSO xerogel (Δ), toluene gel (9) with a concentration of 5.0 103 mol L1, toluene xerogel (0), and toluene solution at 40 °C (b) with a concentration of 5.0 103 mol L1.
3.4. X-ray Diffraction of the Assemblies. The stacking of azoLG2C18 in gels was further addressed by powder XRD. Figure 4 shows the powder XRD pattern of the xerogels obtained from DMSO and toluene gels. There were three diffraction peaks corresponding to the d spacings of 6.21, 3.13, and 1.54 nm in the low-angle region. The d spacing ratio of 1:1/2:1/4 was consistent with a lamellar structure. The length of 6.21 nm was larger than the extended molecular length of azo-LG2C18 (3.7 nm estimated by CPK molecular modeling) but smaller than twice the length. To satisfy the intermolecular strong ππ stacking interaction and weak hydrogen bonding interaction, we have approximated a typical columnar arrangement, as presented in Figure 4B, in which the gelator molecules are stacked in a one-above-the-other fashion.12 The width of the column as calculated from the energy minimized orientations is longer (≈6.5 nm) than the distance we obtained from the low-angle diffraction peaks present in DMSO xerogel. To rationally explain this deviation, we considered some degree of tilt for the stacking planes. However, in toluene gel sample, two wide diffraction peaks corresponding to the d spacings of 4.01 and 1.84 nm in the lowangle region were observed. The d spacing ratio of 1:1/2 was consistent with a lamellar structure too. The length of 4.01 nm was commensurate neither with the width of the extended molecular length of azo-LG2C18 nor with twice the length (≈5.5 nm). It is believed that chiral molecules form bilayer membranes with some favored tilt, and therefore, the only way for molecules to twist with respect to their neighbors is for the entire membrane to twist. To satisfy the intermolecular strong hydrogen bonding interaction, we argue that an interdigitated bilayer structure was obtained (Figure 4C). The aliphatic tails interdigitated together, and the amide moieties organized into 3325
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Table 3. CD Spectral Parameters of azo-LG2C18 Gels gelling solvent
CD exciton type
λθ = 0 (nm)
λmax (nm)
λmin (nm)
DMSO
positive
305
344
276
ethyl ether
positive
300
319
282
1-pentanol
positive
301
325
287
1-dodecanol
positive
304
331
287
1-hexanol
positive
301
325
289
cyclohexane
negative
295
287
319
toluene
negative
300
287
332
benzene o-xylene
negative negative
304 289
293 297
329 326
m-xylene
negative
290
299
326
p-xylene
negative
289
297
326
chlorobenzene
negative
289
298
325
CCl4
negative
327
well-defined arrangement which favors the strong hydrogen bonding interaction and H-type aggregates. Similar results were obtained in other gel systems. 3.5. Solvent Regulated Inversion of the Supramolecular Chirality. To probe the chiral stacking at the molecular level, CD spectroscopy might be considered an appropriate method due to its extremely sensitivity to the intermolecular chiral order at the molecular level. Interestingly, totally opposite CD signals are observed in DMSO and toluene gels: DMSO gel exhibits positive Cotton effect while negative Cotton effect is observed in toluene gel. As shown in Figure 5, in the DMSO gel, an intensive positive CD splitting band intersected the θ = 0° line at 305 nm, which is corresponding to the λmax of the absorption maximum obtained in the gel phase, and a maximum positive band at 344 nm and a minimum negative band at 276 nm. In toluene gel, a negative CD splitting band is located at 300 nm with an intensity negative band at 287 nm and positive band at 332 nm. However, toluene solution at 40 °C is totally CD-silent which definitely points to the existence of supramolecular chirality that originates from the molecular chirality. The opposite Cotton effect obtained from different solvents implies the reversed supramolecular chirality resulted from different molecular orientation at the molecular level.13 This might be in relation to the different stacking modes speculated from XRD data. To affirm the authenticity of the CD spectra, we have measured xerogels spectra. The gel samples were cast on a quartz slide, and then the solvents were evaporated under vacuum. The obtained quartz slides were used for CD spectra measurement. In the process of CD spectra measurement, the slides were placed perpendicular to the light path and rotated within the film plane to avoid polarization-dependent reflections and eliminate the possible angle dependence of the CD signals. As shown in Figure 5, the obtained CD spectra of xerogels nearly keep the same peak shape with the gel states, which reveals that the CD spectra of the gel states are authentic. Thus, the gelator has taken the opposite stacking orientation in these two solvents, leading to totally opposite CD signals. The most interesting phenomenon is the unusual inversion of CD spectra in different gelling solvents. As recorded in Table 3, among the gelling solvents, the gelator azo-LG2C18 exhibits positive exciton coupling in polar solvents, such as DMSO and acyclic alcohol, whereas negative exciton coupling is seen in nonpolar solvents such as aromatic solvents and acyclic solvents. Shinkai et al. have reported a series of cholesterol based
Figure 6. Temperature-dependent UVvis absorption spectra of azoLG2C18: (A) DMSO gel (4.3 103 mol L1); (B) toluene gel (5.0 103 mol L1).
organogelators containing a variety of azobenzene moieties which exhibited CD inversion in different gelling solvents.8a However, there was not an obvious rule such as that with azoLG2C18 gelator which exhibits positive exciton coupling in polar solvents whereas negative exciton coupling in nonpolar solvents. Moreover, the CD inversion in different organic solvents was still unclear in their gel systems. Here, in our case, the CD signal orientation is definitely controlled by the different stacking modes of the gelator in different organic solvents. 3.6. Thermoregulated Supramolecular Chirality. It is wellknown that organogels are responsive to stimuli such as temperature, followed by a reversible process of self-assembly and disassembly. The disassembly process will lead to a gel-to-sol phase transition. In the chromophore-bearing organogelator system, we can speculate the chromophore stacking modes by monitor the optical properties of the resulting assemblies. Thus, we have investigated the absorption of the different solvent systems in the gel state and also during the gel-to-sol phase transition with varying temperature. We also selected two representative solvent (DMSO and toluene) systems to discuss. As shown in Figure 6, totally different temperature-dependent UVvis absorption spectra are observed. In the DMSO gel system, a maximum absorption peak at 305 nm, which can be ascribed to the ππ* transition originated from the long axis transition moment of azobenzene, gradually decreased with increasing temperature. At 50 °C, the absorption reached to the lowest value and the peak largely broadened, suggesting that the organogel has been partially dissociated. Further increasing the temperature, the absorption peak increased rapidly accompaniad by a dramatic red shift of 23 nm (Figure 6A). One can consider this trend as that, in the process of sol-to-gel, the aggregation of chromophores will lead to a blue shift and hypochromic effect of the absorption peak. Though the exact nature of the self-assembled aggregates formed in a gel structure is difficult to establish unambiguously, the blueshifted spectral features suggest that the H-type aggregate structure is likely to be formed in the assembled state. Furthermore, the dramatic hypochromic effect of the azobenzene absorption suggests a strong interaction of interchromophores. However, in the toluene gel system, a red shift of 18 nm was observed in the gel-to-sol process without obvious hypochromic or hyperchromic effect. Also, one can consider that, in the process of sol-to-gel, the absorption peak blue-shifted, which suggests the H-aggregates formed in the toluene gel structure. But the interaction strength of interchromophores might be weaker than DMSO gel because of the small blue-shift values. However, both of the UVvis spectra of DMSO gel and toluene 3326
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Figure 7. Temperature-dependent CD spectra of DMSO: (A) gel (4.3 103 mol L1); toluene (B) gel (5.0 103 mol L1).
gel indicate that exciton coupling and H-aggregates existed in the gel system. We have further investigated the temperature-dependent CD spectra of azo-LG2C18 in DMSO gel and toluene gel. Representative CD spectra of different gel systems with varying temperatures are presented in Figure 7. In all cases, the CD signals gradually diminished with heating, which can be explained as the gradual disassembly of the gelator aggregation. On account of the thermal responsiveness of gelator aggregation, variabletemperature CD spectroscopy provides an ideal approach for demonstrating the formation of nanoscale chiral aggregates. Thus, the heating process is a sequential disassembly of chiral aggregates. From the above discussions, it is clear that the molecular chirality of the gelator molecule is indeed transmitted to the nanoscale supramolecular chirality through the ordering of the chromophore in the assembled phase. Moreover, on the basis of the property of the supramolecular gels, we can realize a thermodriven chiroptical switch using these gels. When the organogel was heated into a clear solution, the CD signals disappeared. When they were cooled, the chirality reappeared. Such process can be repeated many times without changing the intensity of the CD signals. Thus, a thermodriven chiroptical switch can be realized. 3.7. Light Irradiation-Regulated Supramolecular Chirality. In the following we will demonstrate that the solgel transition for the organogels formed with azo-LG2C18 can be reversibly tuned by UV/visible light irradiations. As an example, the organogel of azo-LG2C18 in toluene in a quartz cell was gradually collapsed and transformed into a viscous solution after UV light (365 nm) irradiation for 40 min at room temperature, as shown in Figure 8A. Trans-to-cis isomerization of the azobenzene groups occurred as indicated by the typical spectral changes, the decrease in the ππ* absorption band of the trans-azobenzene moieties at 310 nm (Figure 8B). For the CD measurements, significant Cotton effect was observed in the gel state. It decreased as the gel-to-sol transition proceeded under UV irradiation (Figure 8C). Also, gel regeneration was achieved by the subsequent irradiation of visible light in just 5 min, and the trans-azobenzene was restored up to more than ca. 95%. The CD spectra could recovered to the primary intensity (Figure 8D). This photoinduced phase transition was totally reversible and could be repeated many times. The supramolecular chirality could be controlled by the photoirradiation. Thus, a photodriven chiroptical switch can be realized. As shown in Figure 8E, the reversible CD intensity at 332 nm could be achieved at several irradiation cycles. However, when we took the same method to investigate the DMSO gel, no photoinduced gel-to-sol transition happened. Also, the UVvis spectrum rarely diminished after 2 h UV irradiation. This might be explained by the different
Figure 8. (A) Photographs of the reversible tuning of the toluene gel by UV irradiation and subsequent visible light irradiation; (B) UVvis spectral changes of the toluene gel under UV 365 nm irradiation; (C) CD spectral changes of the toluene gel under UV 365 nm irradiation and (D) visible light irradiation; (E) CD intensity at 332 nm as a function of irradiation cycle.
chromophore stacking modes in different solvents. Keep in mind that ππ interaction in DMSO gel was stronger than in toluene gel because of the dramatic hypochromic effect and large blueshift values during the sol-to-gel process. Therefore, the azobenzene chromophores were restricted heavily and could not respond to the UV irradiation effectively. For toluene gel, the major driving force for gelator assembly was hydrogen bonding while the interaction of interchromophores was relatively weak. The azobenzene chromophores were unrestricted and can respond to the UV photoirradiation. On the other hand, the differences of photoresponse ability between DMSO and toluene gels revealed the differences of interaction strengthen of interchromophores.
4. DISCUSSION The above multiresponsive chiroptical switch can be summarized in Figure 9. It has been confirmed from the FT-IR and the UVvis spectra that both hydrogen bond and ππ stacking play important roles in the formation of organogels. However, the strength of these noncovalent interactions is different in polar and nonpolar solvents. In polar solvent, poor solubility of the azobenzene group leaded to strong interchromophores interaction that confines the isomerization of azobenzene groups. Thus, it is hard to response to UV photoirradiation for the organogels formed in the polar solvents. In nonpolar solvent, weak interchromophores interaction offers the feasibility for photoisomerization, as shown in Figure 9. However, in nonpolar solvent, too strong intermolecular hydrogen bond led to the formation of the bulky structures. In short, the gel formation is the result of the synergy between hydrogen bond and ππ stacking whether in polar solvents or nonpolar solvent. 3327
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Figure 10. Energy optimized azo-LG2C18 packing model: (A) benzenes groups packed face to face (ππ stacking); (B) amino acids packed by forming H-bonds; (C) B3LYP/3-21g* simulated ECD spectra of these two models (solid line (model A) and dashed line (model B) in gas). Figure 9. Illustration of the multichannel supramolecular chiroptical switches composed by azo-LG2C18. The gelator molecules formed organogels showing negative CD in nonpolar solvents and a positive one in polar solvents. Both of the gels showed reversible changes in chirality upon heating and cooling cycles. The organogels formed in the nonpolar solvents could show additional photoresponse upon alternative UV/vis irradiation.
Supramolecular chirality of the organogels is the chirality of assembled aggregates transferred from molecular chirality. It is related to the assembly model of gelator molecules. Thus, it is understandable that the supramolecular chirality of organogels is thermoresponsive. It disappears when the gel state changes to the sol state. In the solution or molecular state, the chirality of the chiral center, which is far from the chromophore, did not transfer to the chromophore. Therefore, whether in polar or nonpolar solvents, supramolecular chiroptical switches can be organized upon thermodriving. Photoisomerization of azobenzene groups will change the assembly method of gelator aggregates which will lead to the change of supramolecular chirality. In polar solvent, strong interchromophores interaction confined the activity of azobenzene groups. Thus supramolecular chirality could not respond to photoirradiation. However, it is active in nonpolar solvent because of the weak interchromophores interaction. Photodriven supramolecular chiroptical switches can be proposed. Keep in mind that supramolecular chirality is related to assembly models of chromophores. Thus, chiral inversion in different solvents is understandable. To further investigate the CD inversion in different conditions, we conducted a model calculation using density functional theory (DFT) method.14 To simplify the model, we truncated the molecule by deleting the alkyl side chain. First, the conformational search of the monomer was carried out with the Macromodel 7.5 program, and Amber94 force field was used with default setting. Conformational search was performed with use of the systematic torsional sampling (SPMC) method. 5000 starting structures were generated and minimized until the gradient was less than 0.05 (kJ/mol)/Å1, using the truncated NewtonRaphson method implemented in MacroModel. Duplicate conformations and those with energy greater than 50 kJ mol1 above the global minimum were discarded. The optimized molecules could then be used to simulate the energy minimized packing model structure by using the M052X/ 6-31G (D) method.15 The stacking molecules were built via two methods: (a) the benzene groups packed face to face (ππ stacking); (b) the amino acids packed by forming H-bonds. Two stable model structures were found, as shown in Figure 10.
The two DFT-optimized stacking molecules were then calculated using TDDFT methodology of Gaussian 03.16 The simulated ECD spectra at the B3LYP/3-21G* level present Cotton effect with a mirror image shape (Figure 10C). The calculated CD spectrum of model A is considered to be in satisfactory agreement with the observed spectra of azo-LG2C18 gels in polar solvents, while the spectrum of model B agreement with the spectra in nonpolar solvents. This result further confirms the different packing mode of azo-LG2C18 in various solvents concluded from XRD experiment. The inversion of CD signal in polar and nonpolar solvents is clear and considerable. Therefore, we can propose solvent polarity-driven supramolecular chiroptical switches based on this system.
5. CONCLUSION It has been found that gelator azo-LG2C18 shows good gelation ability in organic solvents ranging from polar solvents to nonpolar solvents due to the cooperative effect of hydrogen bonding and ππ stacking as well as the van der Waals interaction. Two kinds of packing models were proposed for gelation in polar solvents and nonpolar solvents. In polar solvents the gelator self-assembled into H-aggregates with strong interchromophores interaction by a one-above-the-other stacking mode, whereas in nonpolar solvents the gelator also self-assembled into H-aggregates but with weak interchromophore interactions and strong hydrogen bonding interactions by an interdigitated stacking mode. Totally conversely supramolecular chirality were observed due to these two different stacking models which are affirmed by semiempirical quantum mechanical method. The organogel exhibits reversible assembly and disassembly or reassembly when stimulated by the temperature and photoirradiation. As a result, we can conveniently obtain multiresponsive supramolecular chiroptical switches by temperature, light, and solvent polarity. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ86-10-82615803. Fax: þ86-10-62569564. E-mail: liumh@ iccas.ac.cn.
’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant Nos. 91027042 and 21021003), the Basic Research Development Program (Grant Nos. 2007CB808005 3328
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The Journal of Physical Chemistry B and 2009CB930802), and the Fund of the Chinese Academy of Sciences. The support of the Supercomputing Center, CNIC, CAS for computer time is acknowledged too.
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