Two Types of Two-Component Gels Formed from Pseudoenantiomeric

Jul 20, 2012 - Koji Yamamoto†, Naohiro Oyamada†, Marie Mizutani†, Zengjian An†, Nozomi Saito†, Masahiko Yamaguchi*†, Motohiro Kasuya‡, a...
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Two Types of Two-Component Gels Formed from Pseudoenantiomeric Ethynylhelicene Oligomers Koji Yamamoto,† Naohiro Oyamada,† Marie Mizutani,† Zengjian An,† Nozomi Saito,† Masahiko Yamaguchi,*,† Motohiro Kasuya,‡ and Kazue Kurihara‡ †

Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University, Aoba 6-3, Sendai 980-8578, Japan ‡ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-Ku, Sendai 980-8577, Japan S Supporting Information *

ABSTRACT: Two-component gels formed from pseudoenantiomeric ethynylhelicene oligomers in toluene exhibited two different properties depending on difference in numbers of helicenes in the two components. The combinations (M)-5/ (P)-4, (M)-6/(P)-4, and (M)-7/(P)-4, which contained oligomers with comparable numbers of helicenes, formed transparent gels (Type I gels). The combinations (M)-6/(P)3, (M)-7/(P)-3, and (M)-8/(P)-3, which contained oligomers with considerably different numbers of helicenes, formed turbid gels (Type II gels). Negative Cotton effects were observed for the Type I gels in the region between 350 and 450 nm, and were positive for the Type II gels, despite the use of (M)-oligomers for the longer components. UV/vis exhibited absorption maxima at 350 nm for the Type I gels and at 338 nm for the Type II gels. Different behaviors in gel formation processes were observed by fluorescence studies. Atomic force microscopy analysis showed fiber structures of 25−50 nm diameter for Type I gels and bundles of 100−150 nm diameter for Type II gels. The stoichiometry in gel formation also differed: The Type I gels showed 1:1 stoichiometry of the two components; the Type II gels showed no 1:1 stoichiometry, likely 1:2 stoichiometry. Using the Type I and II gels, two-layer gel systems were constructed.

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INTRODUCTION Gel formation by the assembly of small molecules has attracted interest for the development of a method of constructing nanostructures for stimuli-responsive materials.1 Such gels are formed from fibrous assemblies of molecules, which trap large amounts of solvents in a fiber network. The method has an advantage that, in principle, various small molecules with different structures can be used to modify and control gel properties. Unfortunately, the diversity in such assembled gels has not been very great in conventional studies, because changes in molecular structures often resulted in the loss of gelation ability. In order to develop functional assembled gels, a general method, which can fine-tune gel properties is required. Two-component gel systems, in which fiber formation occurs only in the presence of two compounds, have apparently greater diversity and tunability than single-component gels.2 Gel properties in principle can be tuned by changing the combination of the components.3−6 However, two-component systems with great diversity were not known, and tuning of gel properties in these systems has not been well realized. Several comparative studies were conducted in the systems of carboxylic acids and amines.7 The structures of one component were generally changed, and often gel formation ability was lost. It may be partly because the conventional examples employed two components quite different in molecular structure and properties, which limited the variation in the structure of each © 2012 American Chemical Society

component for gel formation. To obtain great diversity, the use of two compounds with similar structures and properties was considered attractive, because both components could be changed in a diverse and systematic way. We previously reported two-component gel formation using pseudoenantiometic ethynylhelicene oligomers containing different numbers n of helicene units, 1,12-dimethylbenzo[c]phenanthrene.8 Mixtures of (M)- and (P)-oligomers with n, namely, (M)-n/(P)-n systems, formed gels provided that n for both components was 3 or larger. Because both components were equally essential for gel formation, the structure and ratio of both components could be varied to a large extent, thus providing an opportunity to control gel properties. In this paper, we report that two different properties of gels were obtained in this two-component system: Gels formed from the combinations (M)-5/(P)-4, (M)-6/(P)-4, and (M)-7/ (P)-4 were named Type I, which contained oligomers with small differences between the number of helicenes; gels formed from (M)-6/(P)-3, (M)-7/(P)-3 and (M)-8/(P)-3 were named Type II, in which the differences between the numbers of helicenes were larger. Different properties were observed for the Types I and II in gel appearance, circular dichroism (CD), Received: July 9, 2012 Revised: July 18, 2012 Published: July 20, 2012 11939

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Figure 1. (a) CD and (b) UV−vis spectra of various gels (toluene, 1:1, 25 °C). The spectra were obtained after heating at 70 °C [(M)-6/(P)-3], 80 °C [(M)-7/(P)-3], 90 °C [(M)-8/(P)-3], 90 °C [(M)-5/(P)-4], 70 °C [(M)-6/(P)-4], and 110 °C [(M)-7/(P)-4], cooling to 25 °C, and holding at that temperature for 24 h.

UV/vis, fluorescence, viscosity, and atomic force microscopy (AFM) analyses. Also the stoichiometry in the complexation of the pseudoenantiomeric ethynylhelicene oligomers was observed to be different between the two types of the gel: Type I gels were formed in 1:1 stoichiometry, and Type II, non-1:1 stoichiometry. The results were consistent with our recent observation that the two-component gels and vesicles were formed by 1:1 complex hetero double-helices of (P)-4 and (M)-5 and that side chain structures could be used to fine-tune gel properties.9

upon inverting the test tube. Analyses of gel properties were conducted at 25 °C, unless otherwise noted. The appearances of gels differed depending on the combination. The (M)-5/(P)-4, (M)-6/(P)-4, and (M)-7/ (P)-4 (1.0 mM total, 1:1) gels were transparent, whereas the (M)-6/(P)-3, (M)-7/(P)-3, and (M)-8/(P)-3 (1.0 mM total, 1:1) gels were turbid (Figure S1). The (M)-6/(P)-3, (M)-7/ (P)-3, and (M)-8/(P)-3 gels were classified as Type II and differentiated from the Type I (M)-5/(P)-4, (M)-6/(P)-4, and (M)-7/(P)-4 gels. Note that all these gels employed the longer (M)-oligomers as one of the two components. UV−vis analyses also revealed considerable differences between Type I and II gels (Figure 1b). As noted in our previous work, the Type I (M)-5/(P)-4, (M)-6/(P)-4, and (M)-7/(P)-4 (1.0 mM total, 1:1) gels had UV−vis spectra with λmax at 350 nm and a shoulder at 360 nm. In contrast, the Type II (M)-6/(P)-3, (M)-7/(P)-3, and (M)-8/(P)-3 (1.0 mM total, 1:1) gels showed λmax at 339 nm. The CD spectra also exhibited differences. The (M)-7/(P)-3 (1.0 mM total, 1:1) gel exhibited a positive maximum at 402 and 370 nm and a negative maximum at 327 nm. The spectra were similar in shape to that of the (M)-6/(P)-3 gel previously reported.8 The (M)-8/(P)-3 (1.0 mM total, 1:1) gel showed negative Cotton effects in the region of 350−400 nm, which was different from the (M)-6/(P)-3 and (M)-7/(P)-3 gels. The presence of a homo double-helix of (M)-8, which exhibited extremely strong negative Cotton effects in this region, was suspected.10 The mixing ratio of (M)-8 and (P)-3 was then changed, keeping the total concentration at 1.0 mM, and CD analysis was carried out (Figure S2). (M)-8 and (P)-3 were mixed at ratios of 1:2, 1:3, and 1:9. These mixtures were considered not to contain (M)-8 homo double-helices, because hetero double-helix formation was much stronger than homo double-helix formation.9 As expected, (M)-8/(P)-3 (1.0 mM total, 1:2, and 1:3) gels exhibited positive Cotton effects in the

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RESULT AND DISCUSSION CD/UV−Vis Analysis. A notable difference in CD spectra was observed in our previous work8 depending on the combination of the pseudoenantiomeric oligomers. The (M)4/(P)-3, (M)-5/(P)-3, (M)-5/(P)-4, (M)-6/(P)-4, and (M)-6/ (P)-5 gels exhibited negative Cotton effects in the region of 350−450 nm, whereas the (M)-6/(P)-3 gel showed positive Cotton effects in the same region, despite the use of (M)oligomers for the longer component for all the combinations. It was suggested that different structures formed in the (M)-6/ (P)-3 gel compared with other gels, and it was interesting to consider whether such abnormalities were specific to this combination. Because the difference was presumed to be derived from the larger difference in the number of helicenes in the (M)-6/(P)-3 gel, namely, six versus three helicenes, (M)-7/ (P)-3, (M)-7/(P)-4, and (M)-8/(P)-3 gels, which also possess larger numbers of helicenes, were examined (Figure 1a). Toluene solutions of 1.0 mM (M)-7 and 1.0 mM (P)-3 were prepared and mixed in 1:1 ratio at room temperature. This resulting solution contained (M)-7 and (P)-3 at a total concentration of 1.0 mM, and such a solution is denoted (M)-7/(P)-3 (1.0 mM total, 1:1) in this paper. The mixture was heated at 110 °C for 3 min to disperse aggregates formed in the solid states and then cooled to 25 °C. A turbid gel was formed after 12 h as indicated by the lack of gravitational flow 11940

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Figure 2. AFM images (left, height mode; middle, phase mode; and right, cross-sectional analysis) of dried (a) (M)-5/(P)-4 (1.0 mM total, 1:1), (b) (M)-6/(P)-4 (1.0 mM total, 1:1), (c) (M)-7/(P)-4 (1.0 mM total, 1:1), (d) (M)-6/(P)-3 (1.0 mM total, 1:2), (e) (M)-7/(P)-3 (1.0 mM total, 1:2), and (f) (M)-8/(P)-3 (1.0 mM total, 1:2) gels (xerogels) formed in toluene.

for the gelation of the Type I (M)-5/(P)-4, (M)-6/(P)-4, and (M)-7/(P)-4 gels was 1:1, and that of the (M)-6/(P)-3, (M)-7/ (P)-3, and (M)-8/(P)-3 gels was not 1:1, but likely 1:2. Taking the stoichiometry into consideration, 1:1 mixtures of two components were used for the Type I gels and 1:2 mixtures for the Type II gels in the following experiments. AFM Analysis. Fiber structures obtained by the AFM analysis of the dried gels (xerogels) were also different between the two types (Figure 2). The Type I (1.0 mM total, 1:1) gels and Type II (1.0 mM total, 1:2) gels were heated at 110 °C for

region of 350−400 nm, which may be these gels themselves, which do not contain (M)-8 homo double-helices. Because the mixing ratio of (M)-8 and (P)-3 affected the CD spectra of the (M)-8/(P)-3 gel (Figure S2), the effect of the ratio for the Type II (M)-6/(P)-3 and (M)-7/(P)-3 gels was examined (Figures S3 and S4). The (M)-7/(P)-3 gel showed similar CD and UV−vis spectra at ratios of 1:1 and 1:2, and (M)-6/(P)-3 gel showed somewhat different CD spectra. However, the regions of the negative and positive Cotton effects were similar. As will be discussed later, the stoichiometry 11941

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Figure 3. (a) Plots of viscosities (2.0 mM total, toluene) of 1:1 (M)-5/(P)-4, 1:1 (M)-6/(P)-4, 1:1 (M)-7/(P)-4, 1:2 (M)-6/(P)-3, 1:2 (M)-7/(P)-3, and 1:2 (M)-8/(P)-3 gels at a shear rate of 10 Hz. (b) Enlarged view of (a). The gels were prepared after heating at 120 °C for 2 min, cooling to rt, and holding for 1 h. As an exception, 1:1 (M)-6/(P)-4 gel was prepared after heating at 120 °C for 2 min, cooling to rt, and holding for 4 h.

Figure 4. Emission spectra of (a) (M)-5/(P)-4 (1.0 mM total, 1:1, toluene, 25 °C) gel with excitation at 365 nm. The mixture was heated at 95 °C and cooled to 25 °C for analyses. (b) Time dependence of emission of (M)-5/(P)-4 (1.0 mM total, 1:1, toluene, 25 °C) gel at 447 nm. Emission spectra of (c) (M)-7/(P)-3 (1.0 mM total, 1:2, toluene, 25 °C) gel with excitation at 365 nm. The mixture was heated at 90 °C and cooled to 25 °C for analyses. (d) Time dependence of emission of (M)-7/(P)-3 (1.0 mM total, 1:2, toluene, 25 °C) gel at 448 nm. (e) Emission spectra of (M)-5/ (P)-4, (M)-6/(P)-4, (M)-7/(P)-4 (1.0 mM total, 1:1, toluene, 25 °C), (M)-6/(P)-3, (M)-7/(P)-3, and (M)-8/(P)-3 (1.0 mM total, 1:2, toluene, 25 °C) gels with excitation at 365 nm. The spectra were obtained after heating at 95 °C [(M)-5/(P)-4], 90 °C [(M)-6/(P)-4], 100 °C [(M)-7/(P)-4], 70 °C [(M)-6/(P)-3], 90 °C [(M)-7/(P)-3], 90 °C [(M)-8/(P)-3], cooling to 25 °C, and holding at that temperature for 3 min.

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emission was observed, which reached a minimum after 30 min (Figure 4a and b). Then the intensity of the emission increased, and reached a steady state after 24 h with maxima at 447 and 463 nm, which did not change after 48 h. Similar decreases and increases were observed for the Type I (M)-6/(P)-4 and (M)7/(P)-4 gels (Figures S8 and S9). In the case of the Type II (M)-7/(P)-3 gel, the intensity of the emission maxima at 447 and 463 nm increased after cooling to 25 °C and reached a maximum in 3 min. Then the intensity of the emission decreased, which continued for 6 h (Figure 4c and d). The intensity again increased and reached a steady state after 36 h. The same observations were made for (M)-6/(P)-3 and (M)-8/(P)-3 gels (Figures S10 and S11). To confirm the difference, the fluorescence spectra obtained after cooling to 25 °C for 3 min were compared. Three of the Type I gels showed a maximum at 468 nm with emission up to 700 nm, and the Type II gels showed a maximum at 447 nm (Figure 4e). The results indicated different mechanisms of gel formation between the Type I and II gels. The differences in the stoichiometry in two-component gel formation described in the following is likely the origin of these phenomena. Stoichiometric Type I Gel Formation. The stoichiometries of gel formation were different between the Type I and II gels. The study was conducted on the basis of the previous observations that the formation of (M)-6/(P)-3 gel (1 mM total) took place when (M)-6 and (P)-3 were mixed at a ratio between 1:9 and 2:1, and that different CD spectra were obtained depending on the ratio.8 The results suggested the formation of different structures of gels depending on the ratio for the Type II gel. These gels were compared with the Type I gels. The total minimal gelation concentration (MGC), at which thermoreversible gelation occurred, was obtained using 1:1 mixtures of (M)-n and (P)-n (n = 3−8), (Table 1). The MGCs

3 min, left at room temperature for 24 h, placed on mica, and dried in vacuum. AFM analysis of the dried gels was conducted under ambient conditions. The Type I (M)-5/(P)-4, (M)-6/ (P)-4, and (M)-7/(P)-4 gels showed fibers with diameters of about 25−50 nm. In contrast, the Type II (M)-6/(P)-3, (M)-7/ (P)-3, and (M)-8/(P)-3 gels formed bundles of fibers, in which the diameters of the fibers were 50−100 nm and those of the bundles, 150−400 nm. Viscosity. The Type I and Type II gels exhibited different viscosities when touched: Type I gels were much harder than the Type II gels. The viscotic nature was quantitatively analyzed using a cone and plate viscometer, which uses a cone of very shallow angle in bare contact with a flat plate (Figure 3). With this system, the shear rate of the cone is made constant for precise analyses. A graph of shear stress against shear rate yields the viscosity in a straightforward manner. Viscosity was defined by η = τ /υ

where η is the viscosity of the samples, τ is shear stress, and υ is the shear rate of the cone. The gels (2.0 mM total) were formed on the plate after heating at 120 °C for 2 min, cooling to rt, and holding for 1 h. The (M)-6/(P)-4 gel needed 4 h for gelation, which was confirmed by the appearance of a mixture of (M)-6/(P)-4 (2.0 mM total) prepared in a test tube. The viscosity of the gels was measured using a rotating cone plate with a shear rate of 10 Hz. The Type I gels showed high values of stress overshoot, and the viscosity of the Type I (M)-7/(P)-4 gel increased rapidly to 6930 mPa·s in 3 s, which decreased to 200 mPa·s after 5 s owing to the deformation of the hard gel. The (M)-5/(P)-4 and (M)-6/(P)-4 gels showed similar behaviors. AFM analyses of the Type I gels indicated the formation of highly dense fibrous aggregates, which may be the origin of the hardness of Type I gels. In contrast, the viscosity of the Type II (M)-6/(P)3, (M)-7/(P)-3, and (M)-8/(P)-3 gels was in the range of 180−520 mPa·s in 20 s, which indicated the softness of the gels. The Type II gels formed bundles of fibers, which might be brittle. Fluorescence in the Gel Formation Process. Different fluorescent behaviors of the Type I and II gels were observed. The oligomers (P)-3, (P)-4, (M)-5, (M)-6, and (M)-7 showed blue emission in toluene (1.0 mM) on irradiation at 365 nm and provided similar fluorescence spectra with a maximum at 447 nm and a shoulder at 463 nm (Figure S5). The slightly different spectrum obtained for (M)-7 with a maximum at 455 nm may be due to the partial formation of homo double helices. The Type I (M)-5/(P)-4, (M)-6/(P)-4, and (M)-7/ (P)-4 (1.0 mM total, 1:1) gels, and the Type II (M)-6/(P)-3, (M)-7/(P)-3, and (M)-8/(P)-3 (1.0 mM total, 1:2) gels were formed in toluene and held at 25 °C for 24 h. Both Type I and II gels showed weaker emission at steady states compared with sol states (Figure S6); with emission maxima at 447 and 463 nm and weak emissions up to 700 nm (Figure S7). The Type I gels showed a weaker greenish emission by appearance, whereas Type II showed a stronger blue emission. The latter emissions up to 700 nm were slightly stronger for the Type II gels than for the Type I gels, which may be the origin of the slight differences in appearance. The fluorescence spectra (λex = 365 nm) during the gel formation process were different between the Type I and II gels (Figure 4). The Type I (M)-5/(P)-4 gel was heated at 95 °C and cooled to 25 °C. A rapid decrease in the intensity of the

Table 1. Minimal Gelation Concentration (MGC, mM) and Appearance of (M)-n/(P)-n (n = 3−8) Gelsa

a

Gelation experiment performed in toluene. bcG = clear gel. ctG = turbid gel.

of the (M)-7/(P)-3, (M)-7/(P)-4, and (M)-8/(P)-3 gels were obtained in this study, and other data were cited from our previous work.7 Two compounds were mixed in toluene in a test tube and heated to 110 °C. The mixture was cooled to room temperature, and after 12 h gel formation was analyzed by checking the gravitational nonflow of the contents by inverting the test tube. Experiments were conducted with the Type I gels to examine the stoichiometry of each compound in gel formation. The mixing ratio of two components in the (M)-5/(P)-4 gel was changed between 59:1 and 1:59, keeping the total concentration at 0.5 mM (Table 2a). Gel formation was observed between the ratios of 9:1 and 1:9. The symmetrical nature of the ratio was consistent with the 1:1 stoichiometry of twocomponent gel formation. At the critical ratio of 9:1, which was the maximum ratio for gelation at 0.5 mM total, a mixture of 0.45 mM (M)-5 and 0.05 mM (P)-4 was used. Assuming that a 11943

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(M)-5 (0.4 mM) solution. Because no gel was formed at a ratio > 9:1, 0.1 mM total concentration should coincide with the MGC. Accordingly, the gel forming concentration of 0.1 mM in this critical 9:1 mixture was in good agreement with the MGC of 0.1 mM. The same result was obtained for the other critical gel forming ratio of 1:9. The experiments were consistent with the formation of a 1:1 stoichiometric gel in the (M)-5/(P)-4 gel system. The The 1:1 stoichiometry was confirmed by CD analysis of (M)-5/(P)-4 (1.0 mM total, 1:3) gel. Assuming 1:1 gel formation, the calculated spectrum was obtained by adding 1/2 of the CD spectrum for the (M)-5/(P)-4 (0.5 mM total, 1:1) gel and 1/2 of the spectrum of (P)-4 (0.5 mM). The mixture was considered to contain the (M)-5/(P)-4 (0.25 mM total, 1:1) gel and random coil of (P)-4 (0.25 mM). Because it was confirmed that the CD spectra of the (M)-5/(P)-4 (1:1) gel showed no concentration dependence above 0.25 mM, the CD spectrum of the (M)-5/(P)-4 (0.5 mM total, 1:1) gel was used for calculation as a fully gelated state (Figure S12). The experimental and calculated spectra are in good agreement (Figure S13a). Analogously, the experimental and calculated CD spectra of the (M)-5/(P)-4 (0.5 mM total, 1:2, and 3:1) gels at different ratios of components are in good agreement with the calculated spectra (Figure S13b and c). The Job plots using the Δε values (0.5 mM total) at 370 nm of (M)-5/(P)-4 gel at various ratios indicated the formation of a 1:1 complex (Figure 5). The results confirmed that (M)-5 and (P)-4 formed a complex at a 1:1 ratio. Essentially the same results were obtained for the (M)-6/(P)-4 gel (Figures S14−S16 and Table S1). The experiments indicated that the Type I (M)-5/(P)-4 and (M)-6/(P)-4 gels were formed by a 1:1 stoichiometric amount of two component oligomers. These results are consistent with our observation of hetero double-helices in the mixtures of pseudoenantiomeric oligomers of (M)-5 and (P)-4 (Figure 7a).9 The properties of the (M)-7/(P)-4 gel were slightly different from the Type I (M)-5/(P)-4 and (M)-6/(P)-4 gels. When the ratio of (M)-7 to (P)-4 was changed, keeping the total concentration at 1.0 mM, (M)-7/(P)-4 (1.0 mM total) gel formation was observed between 9:1 and 1:2, which was not quite symmetrical (Table S2). When the (M)-7/(P)-4 (1.0 mM total) gel at the critical 9:1 ratio was assumed to be a mixture of (M)-7/(P)-4 (0.2 mM total, 1:1) gel and 0.8 mM (M)-7, the gel forming concentration of 0.2 mM was obtained, which was lower than the MGC of 1.0 mM. When the (M)-7/(P)-4 (1.0 mM total, 1:2) gel at the critical ratio was assumed to be a mixture of (M)-7/(P)-4 (0.67 mM total, 1:1) and 0.33 mM

Table 2. Gelation Experiment with Mixtures of (a) (M)-5/ (P)-4 (0.5 mM Total) and (b) (M)-7/(P)-3 (1.0 mM Total) at Various Ratios

a

All states indicated in the experiments are as follows: 1) as-mixed, 2) heated at 110 °C, 3) cooled to rt, 4) heated at 110 °C, and 5) cooled to rt. G: gel. pG: partial gel. tG: turbid gel. tpG: turbid partial gel. S: solution. tS: turbid solution.

gel was formed in a 1:1 stoichiometry of (M)-5 and (P)-4, the mixture contained (M)-5/(P)-4 (0.1 mM total, 1:1) gel and

Figure 5. (a) CD spectra of (M)-5/(P)-4 (toluene, 0.5 mM total, 25 °C) gels at various ratios. (b) Plots of Δε (toluene, 0.5 mM total, 25 °C) at 370 nm against the ratio (M)-5/(P)-4. Lines were connected point to point. 11944

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Figure 6. (a) CD spectra of (M)-7/(P)-3 (toluene, 1.0 mM total, 25 °C) gels at various ratios. (b) Plots of Δε (toluene, 1.0 mM total, 25 °C) at 365 nm against the ratio (M)-7/(P)-3. Lines were connected point to point.

Figure 7. Proposed mechanisms of formation of Type I and Type II gels.

MGC of 1.0 mM. The results suggested that the gel was not formed in a 1:1 stoichiometric ratio. CD analyses were conducted for the (M)-7/(P)-3 gel. The experimental CD spectrum of the (M)-7/(P)-3 (1.0 mM total, 1:3) gel was different from the calculated spectrum, which was obtained by adding the spectrum of the (M)-7/(P)-3 gel (0.5 mM total, 1:1) and 1/2 of the CD spectra (1.0 mM) of (P)-3 (Figure S21a). The CD spectra of (M)-7/(P)-3 (1.0 mM total, 1:5) gel at another ratio of components also differed considerably from the calculated spectrum (Figure S21b). These results again indicated that the (M)-7/(P)-3 gel was not formed in a 1:1 stoichiometric ratio. The Job plots using the Δε values (1.0 mM total) at 365 nm of (M)-7/(P)-3 gel at various ratios suggested formation of a 1:2 complex (Figure 6). Similar results were obtained for (M)-6/(P)-3 and (M)-8/(P)-3 systems (Figures S20, S22, and S23, and Tables S3 and S4). The Type II (M)-7/(P)-3 gel was not formed by the 1:1 complexation of two oligomers, and a 1:2 complex was suggested (Figure 7b). It is likely that (M)-7 interacted with two molecules of (P)-3, as determined on the basis of the different numbers of helicenes in the two component, that is, seven and three. Two-Layer System of the Type I and II Gels. Two types of gels in hand, layer systems were constructed using different properties of the Type I and II gels. The Type I (M)-5/(P)-4

(M)-4, again the gelation concentration was lower than the MGC of 1.0 mM of (M)-7/(P)-4 gel. These experiments showed that Type I (M)-5/(P)-4 and (M)-6/(P)-4 gel formation took place by 1:1 complexation, and that the (M)-7/(P)-4 gel appeared to exhibit intermediate properties between the Type I and II gels. These differences may arise because of the difference in the numbers of helicenes, that is, seven and four. Nonstoichiometric Type II Gel Formation. In the formation of the Type II (M)-6/(P)-3, (M)-7/(P)-3, and (M)-8/(P)-3 gels, the experimental results on MGC and CD were quite different from the calculations obtained by assuming 1:1 stoichiometry. Using 1:1 mixtures of (M)-7/(P)-3, MGC of 1.0 mM total was obtained. The experiment on the change in the (M)-7 to (P)-3 mixing ratio (1.0 mM total) showed gel formation between 3:1 and 1:5, which was not symmetrical (Table 2b). When the critical (M)-7/(P)-3 (1.0 mM total, 1:5) gel was assumed to form a mixture of the (M)-7/(P)-3 gel (0.33 mM total, 1:1) and 0.67 mM (P)-3, the 1:1 gel should form at a total concentration of 0.33 mM, which was much lower than a total MGC of 1.0 mM. The other critical (M)-7/ (P)-3 gel (1.0 mM total, 3:1) was calculated as a mixture of the (M)-7/(P)-3 (0.5 mM total, 1:1) gel and (M)-7 (0.5 mM), and again the gel was formed at a concentration lower than the 11945

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Type I gel and upper Type II gel was formed as observed by the appearance. Type II (M)-7/(P)-3 gel (1:1, 1.0 mM total) in toluene (0.15 mL) was formed at room temperature. A toluene solution (0.15 mL) containing 1:1 (M)-5 (1.0 mM) and (P)-4 (1.0 mM) was heated at 110 °C, cooled to 50−60 °C, slowly added over the gel. The mixture was cooled to room temperature. A two-layer system of lower Type II gel and upper Type I gel was formed as observed by the appearance.

gel (1:1, 1.0 mM total) in toluene was formed in a test tube at room temperature. A toluene solution containing 1:1 (M)-7 (1.0 mM) and (P)-3 (1.0 mM) was heated at 110 °C, cooled to 50−60 °C, and slowly added over the gel. The mixture was then cooled to room temperature. A two-layer system of lower Type I gel and upper Type II gel was formed as observed by the appearance. Similarly, another two-layered system of the lower Type II (M)-7/(P)-3 gel and the upper Type I (M)-5/(P)-4 gel was obtained (Figure 8). These results showed an advantage of two-component gels, which provides gel layers with different properties but with similar molecular structures.

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Chemical structure of ethynylhelicene oligomer, experimental detail, appearance of Type I and Type II gels (Figure S1), stoichiometry in (M)-8/(P)-3 gel formation (Figure S2), CD/ UV−vis Spectra of (M)-6/(P)-3 (1:1 and 1:2) and (M)-7/(P)3 (1:1 and 1:2) gel (Figures S3 and S4), fluorescence of original oligomers (Figure S5), comparison of fluorescence in sol and gel state (Figure S6), fluorescence in gel state (Figure S7), (M)6/(P)-4, (M)-7/(P)-4, (M)-6/(P)-3, and (M)-8/(P)-3 gel in the gel formation process (Figures S8, S9, S10, and S11), results on gelation experiment of (M)-6/(P)-4, (M)-7/(P)-4, (M)-6/(P)-3, and (M)-8/(P)-3 gel at various ratios (Tables S1, S2, S3, and S4), and CD measurements of (M)-5/(P)-4, (M)6/(P)-4, (M)-7/(P)-4, (M)-6/(P)-3, (M)-7/(P)-3, and (M)-8/ (P)-3 gel for stoichiometry analyses (Figures S12, S13, S14, S15, S16, S17, S18, S19, S20, S21, S22, and S23). This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Pictures of two-layer gels composed of the Type I (M)-5/ (P)-4 (1:1, 1.0 mM total) gel as lower layer and the Type II (M)-7/ (P)-3 (1:1, 1.0 mM total) gel as upper layer (left), and the Type II (M)-7/(P)-3 (1:1, 1.0 mM total) gel as lower layer and the Type I (M)-5/(P)-4 (1:1, 1.0 mM total) gel as upper layer (right).

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CONCLUSION In conclusion, two-component gels formed by the pseudoenantiomeric ethynylhelicene oligomers were classified into Type I (M)-5/(P)-4, (M)-6/(P)-4, and (M)-7/(P)-4 gels, and Type II (M)-6/(P)-3, (M)-7/(P)-3, and (M)-8/(P)-3 gels. The gel appearance, UV/vis, CD, fluorescence, and AFM analyses showed different properties. The Type I gels showed 1:1 stoichiometry of two components, and the Type II gels showed no 1:1 stoichiometry, likely 1:2 stoichiometry. This is a notable example of a two-component gel showing different properties depending on the combination; the advantage of the twocomponent system is also explicitly shown. It should also be noted that the method provides a diversity of gels, the properties of which can be tuned by changing the components.

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ASSOCIATED CONTENT

S Supporting Information *

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by a Grant-in-Aid for Scientific Research (No. 21229001), the GCOE program, and the WPI Initiative from Japan Society for the Promotion of Science. K.Y. thanks the GCOE program for financial supports and JSPS for a fellowship for young Japanese scientists. Financial support from WPI-AIMR to Z.A. is also acknowledged.

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EXPERIMENTAL SECTION

REFERENCES

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