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2008, 112, 15521–15524 Published on Web 11/13/2008
Two-fold Helical Inversion in the Chiral SmC Phase of Optically Active Materials Derived from (R)-(+)-1-(1-Phenyl)ethylamine Masakane Muto, Hideyuki Suzuki, Seng Kue Lee, Sungmin Kang, Masatoshi Tokita, and Junji Watanabe* Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan ReceiVed: September 12, 2008; ReVised Manuscript ReceiVed: October 31, 2008
The transfer of chirality from molecules to an anisotropic liquid crystal phase has been a subject of significant interest. A typical example is the spontaneous formation of helical structures in chiral nematic and chiral SmC* phases. One of the most interesting chiral effects in these helical liquid crystal systems is the helical inversion arising from the sensitivity of the chirality to temperature variation. In this study we found a distinct 2-fold helical inversion in the chiral SmC* phase of materials with single chiral centers, in which the chiral phenylethylamine is introduced to the end of the mesogens through the imine linkage and the alkyl tail is attached to the other end. As far as we know, this is the first example of 2-fold helical inversion in a singlecomponent system. The transfer of chirality from molecules to an anisotropic liquid crystalline (LC) phase has been a subject of significant interest.1 The chiral transmission takes place in a whole system over distances many times the molecular length. A typical example is the spontaneous formation of helical structures in chiral nematic and chiral SmC* phases. One of the most interesting chiral effects in these helical LC systems is the helical inversion arising from the sensitivity of the chirality to temperature variation. Various helical inversion behaviors have been observed in polymers2-4 and low molar mass molecules,5-11 some of which have been analyzed by calculations to elucidate their chirality effects at the microscopic level.12-18 In this paper we report a distinct 2-fold helical inversion in the chiral SmC* phase of materials with single chiral centers. These materials shared the following structure.
The chiral phenylethylamine (PEA) is introduced to the end of the mesogens through the imine linkage, and the alkyl tail is attached to the other end.19 As far as we know, this is the first example of 2-fold helical inversion in a single-component system. The materials were denoted M-n, where n is the carbon number in the aliphatic tail and ranged from 4 to 16. The phase sequence was determined from the observations of optical microscopic textures and X-ray patterns on both planar and homeotropic samples, together with the DSC observations. The transition temperatures and phase structures are listed in Table 1. * To whom correspondence should be addressed.
10.1021/jp808109d CCC: $40.75
TABLE 1: Phase Transition Temperatures (°C) for a Homologous Series of M-n transition temperature/°C (enthalpy/kJ/mol) n
Cry
4
b
6
b
8
b
10
b
12
b
14
b
16
b
SmC* 150.2 (27.0) 104.6 (20.6) 94.5 (32.1) 69.2 (17.9) 71.2 (23.3) 42.9 (22.3) 50.1 (27.7)
SmA b
b b b b b
148.2 (0.5) 159 (0.3) 162.4 (0.3) 163.1 (0.2) 159.4 (0.3)
b
b b b b b
Iso 190.2 (1.8) 174.3 (1.4) 170.5 (2.8) 167.4 (3.3) 169.3 (4.1) 163.7 (3.9) 157.4 (5.9)
b b b b b b b
The notable chiral SmC* phase is formed for M-6∼M-16 just after the SmA phase. It was identified from X-ray observations of molecules tilted to the layer and electro-optical observations of the ferroelectric switching. The tilt angle (θ) and spontaneous polarization (Ps) determined for M-12 are plotted against the temperature in Figure 1. A typical temperature dependence can be seen; as the temperature falls, θ and Ps increase steadily from zero at the SmA-SmC* phase transition point (Tc), exhibiting a power law behavior with a critical exponent of about 0.3, and ultimately reach about 20° and 80 nC/cm2, respectively. The chiral PEA group attached to the molecular end is known to induce a high helical twisting power.19,20 This is also the case in the present materials; the chiral SmC* phase forms the helical structure with the small pitches corresponding to the wavelengths of visible light. Because of this, the pitch and helical sense can be reliably determined from a circular dichroism (CD) 2008 American Chemical Society
15522 J. Phys. Chem. B, Vol. 112, No. 49, 2008
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Figure 1. Spontaneous polarization Ps (open circles) and tilt angle (closed circles) versus temperature for the chiral SmC* phase of M-12.
measurement of the reflection band and the induced CD (ICD) band for the homeotropically aligned sample. Typical CD spectra are shown for M-10 in Figure 2. As the temperature descends in the SmC* temperature region from 160 to 147 °C during cooling from the SmA, the wavelength of the reflection CD (RCD) band (i.e., the optical pitch, nP) increases from 400 to 500 nm (see Figure 2a). The negative RCD and positive ICD at around 340 nm indicate that the helical sense is right-handed. Interestingly, a drastic change occurs in the short temperature span from 147 to 146 °C. The negative RCD band disappears abruptly, but appears again with the opposite sign, that is, a positive sign (compare Figure 2 panel a with panel b). The ICD spectra, meanwhile, show a sign inversion from positive to negative. Thus, we find that a helical twist inversion from a right-handed helix to a left-handed helix takes place at 147 °C. As the temperature decreases from 146 to 85 °C, the optical pitch increases from 370 nm to infinite (Figure 2b). No RCD band appears during further cooling to temperatures below 85 °C (Figure 2c), but the sign of the ICD band changes from negative to positive, undergoing a second helical inversion at 85 °C. M-12 also undergoes a 2-fold helical inversion (see Supporting Information, Figure S1). In this case, RCD bands with the same sign are observed in the highest temperature zone of 160∼132 °C and the lowest temperature zone of 90∼75 °C. Though no RCD band is observed in the intermediate temperature zone of 130∼90 °C, the sign change in ICD indicates a 2-fold helical inversion similar to that observed in M-10. Figure 3 shows the temperature dependence of the helical twisting power (reciprocal optical pitch) and intensity of ICD for M-6∼M-16. M-10 and M-12 undergo a 2-fold inversion, while M-8 undergoes a single inversion. The short tail homologue of M-6 and long tail homologues of M-14 and M-16 undergo no inversion at any point during the temperature region, though the helical senses of these two groups are opposite. This effect of the alkyl tail length can be simply surveyed from Figure 4, where the helical inversion temperatures in the chiral SmC* zone are shown together with the phase transition temperatures. A clear peninsula-like region of the left-handed helix penetrates the right-handed helix region, and we find that the 2-fold helical inversion disappears at a critical tail length of between 12 and 14. In summary, we observed an interesting helical inversion in the chiral SmC* phase of M-n homologues. This helical inversion was distinct in the following ways. First, it took place
Figure 2. CD spectra observed for the chiral SmC* phase of M-10 in three characteristic temperature regions. (a) 160-147 °C, (b) 147-85 °C, and (c) 85-80 °C. The measurement was performed for a homeotropically aligned sample with a light irradiation parallel to the helical axis of the chiral SmC* phase.
in a 2-fold fashion. Never before has a 2-fold inversion of this type been observed in the chiral SmC* phase of a single pure material, although one has been seen in the chiral nematic phase of a two component mixture.21 The second distinct feature was an abrupt higher-temperature helical inversion between helices with large twisting powers within a very narrow temperature span of 1 °C (see Figure 2 and Figure 3b-d). A helical inversion generally occurs through a definite infinite-pitch system between helices with low helical twisting powers since the instigating force is the competition between two helices of opposite handedness. We thus can expect a drastic structural change following this higher-temperature inversion, though the basic structural characteristics of the chiral SmC* phase, that is, tilting of molecules and spontaneous polarization, continuously change during the helical inversion, as mentioned above. As a third feature, the longer homologues of M-14 and M-16 show no helical inversion. This suggests that the molecular length has a significant effect on the 2-fold helical inversion. Two general explanations for the helical inversion have been suggested. The first is based on the conformation by Goodby et al.6,8 Goodby and colleagues speculate that molecules conform only to the uniaxial field in LC systems, and that several chiral conformations are possible. The twist senses and dipole moment
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J. Phys. Chem. B, Vol. 112, No. 49, 2008 15523
Figure 4. Dependence of transition temperatures on the alkyl tail length of M-n. The helical inversion temperatures in the chiral SmC* zone are also presented by the dashed curve together with the helical sense.
Thus, the helical twist inversion may result from the different weighting of certain molecular conformations at different temperatures, that is, a temperature-dependent equilibrium between conformers with opposite contributions to the macroscopic twist.14 This explanation takes us only so far, as it offers no apparent explanation for a 2-fold inversion. The second explanation is based on the competition between the dispersion attraction and steric repulsion.15-17 The combination of these two forces is proposed to elicit a special effective potential that leads to various helical sense inversions in different classes of chiral nematics. This approach has proven to be especially successful in explaining the helical inversion in nematics comprising chiral rigid-rod polymers.2-4 Recent evidence also suggests that 2-fold inversion may be possible when the biaxiality of the molecules is coupled.18 This may help us understand our present result, as the phenyl ring of the chiral end group sticking out of the mesogenic axis may attach the biaxiality to the molecules. Current studies seek to clarify the relationship between the 2-fold inversion and molecular structure using several molecular systems known to undergo the 2-fold inversion. Acknowledgment. This research was supported by the Grantin-Aid for Creative Scientific Research from the Ministry of Education, Science, Sports and Culture in Japan. Supporting Information Available: Scheme S1 showing the synthetic route to the target materials, Figure S1 showing the polarization reversal current in the chiral SmC* phase at 120 oC under the application of a 180 Vpp triangular wave voltage of 40 Hz, and Figure S2 showing the CD spectra observed for the chiral SmC* phase of M-12 in three characteristic temperature regions. This material is available free of charge via the Internet at http://pubs.acs.org. Figure 3. Temperature dependences of the (a-f) optical pitch and (g-l) intensity of induced CD observed for the chiral SmC* phase of M-6∼M16. The arrow gives the temperature region of the chiral SmC* phase and the dashed lines indicate the position of helical inversion.
directions of these conformations can oppose each other, and their statistical distributions shift with changing temperatures.
References and Notes (1) Eelkema, R.; Feringa, B. L. Org. Biomol. Chem. 2006, 4, 3729. (2) Uematsu, I.; Uematsu, Y. AdV. Polym. Sci. 1984, 59, 37. (3) Watanabe, J.; Nagase, T. Macromolecules 1988, 21, 171. (4) Yamagishi, T.; Fukuda, T.; Ichizuka, T.; Miyamoto, T.; Watanabe, J. Liq. Crys. 1990, 7, 155. (5) Stegemeyer, H.; Siemensmeyer, K.; Scurow, W.; Appel, L. Z. Naturforsch. 1989, A 44, 1127.
15524 J. Phys. Chem. B, Vol. 112, No. 49, 2008 (6) Slaney, A. J.; Nishiyama, I.; Styring, P.; Goodby, J. W. J. Mater. Chem. 1992, 805. (7) Dierking, I.; Giesselmann, F.; Zugenmaier, P.; Kuczynski, W.; Lagerwall, S. L.; Stebler, B. Liq. Cryst. 1993, 13, 45. (8) Styring, P.; Vuijk, J. D.; Nishiyama, I.; Slaney, A. J.; Goodby, J. W. J. Mater. Chem. 1993, 3, 399. (9) Dierking, I.; Giesselmann, F.; Zugenmaier, P.; Mohr, K.; Zaschke, H.; Kuczynski, W. Liq. Cryst. 1995, 18, 443. (10) Styring, P.; Vuijk, J. D.; Wright, S. A.; Takatoh, K.; Goodby, J. W. J. Mater. Chem. 1994, 4, 1365. (11) Kasper, M.; Gorecka, E.; Sverenyak, H.; Hamplova, V.; Glogarova, M.; Pakhomov, S. A. Liq. Cryst. 1995, 19, 589. (12) Ferrarini, A.; Moro, G. J.; Nordio, P. L. Liq. Cryst. 1995, 19, 397. (13) Memmer, R.; Janssen, F. Liq. Cryst. 1998, 24, 805. (14) Earl, D. J.; Wilson, R. J. Chem. Phys. 2003, 119, 10280.
Letters (15) Kimura, H.; Hoshino, M.; Nakano, H. J. Phys. Soc. Jpn. 1982, 51, 1584. (16) Osipov, M. A.; Semenov, A. M.; Khokhlov, A. R. SoV. J. Chem. Phys. 1987, 6, 1312. (17) Osipov, M. A. Liquid crystalline and Mesogenic Polymers; Springer-Verlag: Berlin, 1993. (18) Emelyanenko, A. V. Phys. ReV. 2003, E 67, 031704. (19) Fukuda, K.; Suzuki, H.; Ni, J.; Tokita, M.; Watanabe, J. Jpn. J. Appl. Phys. 2007, 46, 5280. (20) Rindai, P. L.; Naido, M. R. S.; Conaway, W. E. J. Org. Chem. 1982, 47, 3987. (21) Finkelmann, H.; Stegemeyer, H. Z. Naturforsch. 1973, A 28, 1046.
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