Stereochemical effects on dynamics in two-component systems of gelators with perfluoroalkyl and alkyl chains as revealed by vibrational circular dichroism

Hisako Sato *a, Tomoko Yajima b and Akihiko Yamagishi c
aGraduated School of Science and Engineering, Ehime University, Matsuyama, Ehime 790-8577, Japan. E-mail:; Fax: +81-89-927-9590; Tel: +81-89-927-9599
bFaculty of Science, Department of Chemistry, Ochanomizu University, Tokyo 112-8610, Japan
cDepartment of Medicine, Toho University, Ota-ku, Tokyo 143-8540, Japan

Received 13th September 2017 , Accepted 9th October 2017

First published on 9th October 2017

A mixture of chiral low-molecular weight gelators, (1R,2R)- or (1S,2S)-N,N′-diperfluoroheptanoyl-1,2-diaminocyclohexane (denoted as RR- or SS-CF7, respectively) and (1R,2R)- or (1S,2S)-N,N′-diheptanoyl-1,2-diaminocyclohexane (denoted as RR- or SS-CH7, respectively), was used as a two-component gelator in acetonitrile solvent. The process of gelation was investigated by time-step vibrational circular dichroism (VCD) spectroscopy. The method enabled us to study the dynamical behavior of CF7 and CH7 molecules independent of their characteristic vibrational bands. We focused on the effects of the chirality relation between the two gelators. In the case of the enantiomeric mixtures (RR-CF7/RR-CH7 or SS-CF7/SS-CH7), the two components exhibited different time-courses in their VCD spectra. As for CF7, the couplet peak intensities assigned to C[double bond, length as m-dash]O stretching increased with time, while those for CH7 remained low. In the case of the racemic mixture (RR-CF7/SS-CH7 or SS-CF7/RR-CH7), the intensities of the peaks assigned to the C[double bond, length as m-dash]O stretching vibration for both CF7 and CH7 molecules maintained a constant level with time. The VCD results were compared with the SEM images of the freeze-dried gel samples taken at various time intervals. Furthermore, the mechanisms for the gelation of two-component systems are discussed.


A chiral gelator is of particular interest because its gelation often leads to fibrils helically wound at a micrometer scale.1 A helical fibril can adopt several conformations such as a tape and a ribbon. However, the mechanism by which molecular chirality is amplified to give such chiral architectures on a large scale is yet to be clarified.1–11

Recently, attention has been paid to behaviors of multicomponent low-molecular weight gelators. Gelators of different kinds may combine or self-sort to form a fibril.3,4 In these studies, two components often form a molecular associate in an isotropic solution so that it acts as a more or less efficient gelator than those of the individual components. The rate of gelation is, for example, controlled by selecting an appropriate pair of components. There have been, however, few reports on the effect of chirality in two- or multi-component systems. It would be interesting to monitor in situ the behavior of each chiral component during gelation; that is, to determine whether they combine or segregate.

Vibrational circular dichroism (VCD) spectroscopy is the extension of circular dichroism into the infrared and near-infrared regions.12–15 Recently, VCD has been applied to reveal the kinetic and thermodynamic aspects of gelation.16 In our group, VCD was used to elucidate the molecular organization in the fibrils of chiral gelators.17–21 In the case of (1R,2R)- or (1S,2S)-N,N′-diperfluoroheptanoyl-1,2-diaminocyclohexane (denoted as RR- or SS-CF7), for example, the molecular conformation of a gelator in a fibril was derived by analyzing the VCD peaks of its CD3CN gel. The results led us to a model of the helical arrangement of gelators. The same approach was taken for analogous gels such as (1R,2R)- or (1S,2S)-N,N′-diheptanoyl-1,2-diaminocyclohexane (denoted as RR- or SS-CH7) (Chart 1). It should be emphasized that in these studies, the VCD peaks enhanced remarkably (Δε > 10−3) when the gelator molecules formed a gel, while they were much lower for an isotropic solution (Δε < 10−4).18,19 A similar enhancement in VCD signals was reported elsewhere for the organization of chiral molecules into macromolecular architectures.22–27 The structural origin for such an enhancement was recently postulated by considering the cooperative vibrational motions among neighboring molecular arrays.24

image file: c7cp06264h-c1.tif
Chart 1 Molecular structures of RR-CF7 and RR-CH7.

For the types of gelators shown in Chart 1, the molecules are thought to be connected through two anti-parallel sequences of hydrogen bonding between C[double bond, length as m-dash]O and NH at the cyclohexyl rings.5 Such molecular chains are strengthened by the aggregation of perfluoroalkyl or alkyl chains through fluorophilic or hydrophobic interactions, respectively.28 When the gelation abilities of CF7 and CH7 were compared, the critical gel concentration of CF7 was obtained as 0.0064 mol L−1, which was two times lower than that of CH7 (0.012 mol L−1). This implied that the perfluoroalkyl chains were more effective in strengthening the fibrils compared to alkyl chains with the same carbon number.18

The above results prompted us to study the effects of the coexistence of these two gelators in the same solvent. If the perfluoroalkyl and alkyl chains mix uniformly to form a single fibril, CF7 might assist the gelation of CH7. If these chains repel each other, CF7 and CH7 would form different fibrils to yield heterogeneous gel phases. Practically, the dilution of perfluorinated substances by alkylated ones would result in the reduction of environmental harm.

The present work examined the possibility of collaboration effects between two kinds of gelators with different aggregation properties of perfluoroalkyl and alkyl chains. In particular, attention was focused on the stereochemical contribution by comparing the gelation behaviors of the pairs of CF7 and CH7 with the same and opposite chirality (denoted as enantiomeric and racemic mixture, respectively).

Results and discussion

Gelation by the enantiomeric mixture of CF7 and CH7

Firstly, an isotropic solution was prepared by dissolving a mixture of RR-CF7/RR-CH7 (0.007 mol L−1/0.018 mol L−1) or SS-CF7/SS-CH7 into CH3CN at 80 °C. The total concentrations were a little over the critical gel concentration. The solution was cooled down to room temperature until it formed an opaque gel. A small portion of the gel was taken out at various intervals after the appearance of the gel phase. Each sample was freeze-dried and its SEM image was measured. The results are shown in Fig. 1.
image file: c7cp06264h-f1.tif
Fig. 1 The SEM images of the CH3CN xerogels containing RR-CF7/RR-CH7 (left) or SS-CF7/SS-CH7 (right), respectively. The samples were taken (top) 10 minutes, (middle) 1 h and (bottom) 24 h after the appearance of gels.

For the samples taken 10 min after gelation, network structures of fibrils were observed, which seemingly consisted of two portions with different shapes: thread-like fibrils with less than 1 μm width and flat ribbon-like fibrils with 1 μm thickness. For the samples taken 1 h after gelation, the fraction of thread-like fibrils increased while that of the ribbon-like fibrils decreased. For the samples taken at 24 h after gelation, all the fibrils adopted a thread-like morphology. The SEM images indicated that the fibrils changed their shape even after the formation of a gel phase. In other words, the initially formed flat ribbon-like fibrils separated into thinner thread-like fibrils with time until the entire gel eventually consisted of uniform thread-like fibrils.

The same gelation process was monitored by measuring the VCD spectra of the same enantiomeric mixture using CD3CN as a solvent instead of CH3CN (Fig. 2). One advantage of the VCD method was that the behavior of CF7 or CH7 could be monitored separately when the time course of each characteristic peak was followed. Fig. 2 shows the VCD spectra recorded at 0 and 15 min after the appearance of gels. The main peaks in the VCD spectra had opposite signs between RR-CF7/RR-CH7 (black) and SS-CF7/SS-CH7 (grey) although their intensity was not strictly equal. No such relation of opposite signs depending on the absolute configuration of an enantiomer would occur through the scattering or linear dichroic effects. The relationship was considered as evidence for the circular dichroism of the samples. More detailed time-courses are provided in the ESI. It was confirmed that the VCD spectra changed with time, while the IR spectra remained unchanged.

image file: c7cp06264h-f2.tif
Fig. 2 Observed IR (lower) and VCD (upper) spectra of CD3CN gels of RR-CF7/RR-CH7 (black solid line) or SS-CF7/SS-CH7 (grey line), respectively. The spectra were recorded (a) at the initial state, (b) after 3 min, and (c) at the final state of gelation. The VCD spectra were measured when the isotropic solutions of CD3CN containing the gelators at 80 °C were mounted onto CaF2 plates and left at room temperature. The initial and final spectra were recorded 1 and 15 min after the sample attained room temperature. More detailed time courses are given in the ESI.

The VCD peaks in the wavenumber region of 1600–1800 cm−1 were assigned to the C[double bond, length as m-dash]O stretching vibrations of CF7 and CH7. At the initial stage, small couplet peaks appeared around 1700 cm−1 and 1650 cm−1 due to C[double bond, length as m-dash]O stretching in CF7 and CH7, respectively. The intensity of the former band increased monotonously with the simultaneous widening of peak splitting, while that of the latter band remained low. The multiplet peaks around 1200 cm−1 were assigned to the stretching vibrations of C–F in the perfluoroalkyl chains. The intensity of the peaks increased with little change in spectral shape.

On the basis of the previous calculation, the couplet peaks assigned to the stretching vibration of C[double bond, length as m-dash]O implied that the molecules were connected through two anti-parallel hydrogen bonds at the head groups.18 Based on this, in the case of CF7, the fraction of such a molecular sequence increased with time even after gelation. In contrast, in the case of CH7, no structural change occurred once it formed a gel. From the view-point of the gelation rate, the VCD results reflected the difference between the kinetic behaviors of these two gelators. CH7 formed a stable gel structure directly, while CF7 took time to reach the stable state.

By comparing the VCD results with the SEM images, the following mechanism was proposed. At the early stage of gelation, CH7 molecules formed a thread-like fibril (Fig. 1(top)), whereas the CF7 molecules formed a flat ribbon-like fibril. As time elapsed, CF7 changed from flat ribbon-like fibrils to thread-like fibrils. This process might be related to the increase in the intensity of the VCD band assigned to the stretching vibration of C[double bond, length as m-dash]O in CF7. The transient formation of ribbon-like fibrils by CF7 might be caused by the scarcity of solvent molecules. That is, most solvent molecules were entrapped in the network of the initially formed thread-like fibrils of CH7. As time elapsed, the solvent molecules moved to the ribbon-like fibrils formed by CF7 and caused a structural change into thread-like fibrils.

In the present application of VCD to the dynamic aspects of gelation, the enhancement of signals played a decisive role. The high intensity of signals made it possible to obtain a reliable spectrum within a short time (or several minutes). In the case of conventional measurements, a signal is obtained after 104–105 times scanning. Recently, the enhancement of VCD signals has been reported in several studies.22–27 As for the origin of the enhancement, one possible reason is the role of metal ions. In fact, theoretical analyses have established that the presence of low-lying electronic states of d-orbitals in the near-infrared region contributed to the increase in the intensity of VCD signals.22 Another possible reason is the collaborative interaction in vibrational motion between the molecules in the neighboring molecular fibers. The mechanism was proposed to explain the enhanced signals during the aggregation of amyloids.23 The simulation of the VCD spectra of amyloid fibrils leads to the conclusion that the intersheet exciton coupling resulted in the increase in VCD signals.24 In the case of gels, it was reported that the VCD intensity depended on the incubation time.25 The role of solvent molecules was also suggested to be important.26 At present, the mechanism of signal enhancement in our gel systems remains to be clarified. For this, a theoretical approach including solvent molecules is required.

Gelation by the racemic mixture of CF7 and CH7

In the next step, gelation experiments were performed using CF7 and CH7 with opposite chirality (or the racemic mixture). An isotropic solution was prepared by dissolving RR-CF7/SS-CH7 (0.016 mol L−1/0.038 mol L−1) or SS-CF7/RR-CH7 in CH3CN at 80 °C. In both the cases, a turbid gel formed almost instantly when the solution was cooled down to room temperature.

A small portion of the gel was taken out after gelation. The samples were freeze-dried, and their SEM images were recorded, as shown in Fig. 3. Thread-like fibrils were observed over the entire imaged region with no indication of heterogeneous fibril formation. These results suggested that both the components were included in thread-like fibrils.

image file: c7cp06264h-f3.tif
Fig. 3 SEM images of CH3CN xerogels of (a) RR-CF7/SS-CH7 and (b) SS-CF7/RR-CH7.

The process of gelation was monitored by VCD. An isotropic solution was prepared by dissolving RR-CF7/SS-CH7 (0.005 mol L−1/0.01 mol L−1) or SS-CF7/RR-CH7 in CD3CN at 80 °C. Clear VCD peaks appeared on the formation of turbid gels, as shown in Fig. 4. The same IR and VCD spectra were recorded at 1 min and 15 min after the onset of gelation. Coupling peaks were observed both at 1700 cm−1 due to C[double bond, length as m-dash]O stretching in CF and at 1650 cm−1 due to C[double bond, length as m-dash]O stretching in CH. The results implied that both CF7 and CH7 molecules were connected through two anti-parallel hydrogen bonds, as determined on the basis of the previous calculation.18–21

image file: c7cp06264h-f4.tif
Fig. 4 Observed IR (lower) and VCD (upper) spectra of CD3CN gels of RR-CF7/SS-CH7 (black line) or SS-CF7/RR-CH7 (grey line), respectively.

Since SS-CF7 and RR-CH7 molecules were unable to connect in the same direction due to steric interference, they would not be included in the same molecular sequences. Instead, it was suggested that CF7 and CH7 molecules formed different molecular sequences and aggregated to form a thread-like fibril. The results of VCD measurements are summarized in Table 1.

Table 1 Signs of C–O couplet for the mixed gel, SS-CF7/SS-CH7, and SS-CF7/RR-CH7a

C[double bond, length as m-dash]O couplet


(Enantiomeric mixture)


(Racemic mixture)

a Low/high wavenumber.
1700 cm−1


Initial: +/−


Stable: +/−


+/− +/−
1650 cm−1


Initial: +/−

Stable: +/−

−/+ +/−

Comparison of sol–gel transition temperature

The temperatures of the sol–gel transition of the following two samples were compared using acetonitrile as a solvent: (a) the enantiomeric mixture of CF7 and CH7 (or RR-CF7/RR-CH7) and (b) the racemic mixture of CF7 and CH7 (RR-CF7/SS-CH7). Additionally, the following two samples were investigated as references: (c) enantiopure CF7 and (d) enantiopure CH7. For all the samples, the concentration of the gelator was chosen at a little (ca. 5%) over the critical gel concentration (CGC). The results are summarized in Table 2.
Table 2 Sol–gel temperatures of the gelators
Sample Concentration (mol L−1) Average temp. (°C)
RR-CF7/RR-CH7 0.007/0.02 45.3
RR-CF7/SS-CH7 0.0075/0.018 43.4
RR-CF7 0.010 44.2
RR-CH7 0.017 41.6

As can be seen, enantiomeric CF7 exhibited a higher temperature for sol–gel transition; that is, its gel was more stable than that of enantiomeric CH7. The results were rationalized in terms of the higher self-aggregation tendency of perfluoroalkyl chains than that of the alkyl chains. The tendency was reduced by diluting with alkyl chains, as seen for a mixture of CF7 and CH7, although the lowering of stability was less than expected from the average of these two components (ESI).

The time needed for gelation was compared among the following four cases: enantiopure CF7, enantiopure CH7, their enantiomeric mixtures, and their racemic mixtures. An isotropic solution was prepared by dissolving one or both gelators in CH3CN at 100 °C. It was then left at room temperature, and the order of gelation rates was determined from visual observation (the photos are provided in the ESI). The gel phase appeared in the following order: enantiopure CH7 > racemic mixture > enantiomeric mixture > enantiopure CF7. A remarkable feature was that the enantiomeric mixtures required a longer time for gelation than the racemic ones did. This was an example of enhancing the gelation rate by mixing two kinds of chiral gelators.3,4 A possible reason was that RR-CF7 and SS-CH7 (or SS-CF7 and RR-CH7) formed a molecular sequence separately; that is, they formed a stereo-complex as a fibril with the formation of segregated perfluoroalkyl and alkyl domains. Two homo-chiral molecular chains thus formed might co-assemble to lead to the thicker fibrils.



The syntheses of (1R,2R)- or (1S,2S)-N,N′-diperfluoroheptanoyl-1,2-diaminocyclohexane and (1R,2R)- or (1S,2S)-N,N′-diheptanoyl-1,2-diaminocyclohexane have been described in a previous paper.18,19 All the solvents were used as purchased.

Sample preparation

The VCD samples were prepared as follows: a mixture of solid CF7 and CH7 was dissolved in 100–150 μl of CD3CN on a hot plate (80 °C) at the critical gel concentration (CGC). About 30 μl of the viscous solution was sandwiched between two CaF2 plates with a 50 μm spacer and left at room temperature.


VCD spectra were measured using a spectrometer, PRESTO-S-2007 and PRESTO-S-2016 (JASCO Co. Ltd, Japan). The signals were accumulated at a rate of 100 scans per min with 4 cm−1 resolution using a liquid-nitrogen-cooled MCT infrared detector equipped with ZnSe windows. No dichroic effect was observed when the samples were rotated with respect to incident light using cell rotation. SEM observation was performed with either an S-3100H (Hitachi Ltd, Japan) or a JSM-7001F (JEOL Ltd, Japan) electron microscope. The gel sample was deposited onto a silicon wafer and freeze-dried following air-drying. The samples were then coated with platinum for the measurements.


The collaboration of two-component systems of gelators was examined. The used gelators were diaminocyclohexane derivatives with perfluoroalkyl and alkyl chains (denoted as CF7 and CH7, respectively). Since both of them were chiral, the stereochemical effects were taken into account. We observed that the gelation depended on whether the mixture of the two gelators was enantiomeric or racemic. The results provided an approach for tuning the gelation ability by using molecular chirality. For this investigation, time-step VCD was effectively applied since the chiral vibrational peak assigned to each component made it possible to follow their behaviors independently. We are now planning to perform large-scale calculation of aggregates of perfluorinated molecules to further elucidate the organization of two-component systems together with the enhancement phenomena of VCD signals.

Conflicts of interest

There are no conflicts of interest to declare.


Thanks are due to Ms Miwa Ochi (Ehime University) for sample preparation and Prof. Sho Shirakata (Ehime University), Prof. Takahiro Nakae (Kyoto Univ) and Ms Eri Kondo (Ochanomizu University) for obtaining SEM images. This work has been financially supported by the JSPS MEXT KAKENHI Grant-in-Aid for Exploratory Research (Number JP26620068) and Innovative Areas (Number JP16H00840).

Notes and references

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Electronic supplementary information (ESI) available: Photos for gelation; time change of VCD spectra for a CD3CN gel containing an enantiomeric mixture of CF7 and CH7; Sol–gel temperature of one- or two-component gels. See DOI: 10.1039/c7cp06264h

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