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Nuclear magnetic resonance study of the smectic-cholesteric phase transition in a dimesogenic liquid crystal
Current Applied Physics 14 (2014) 1356e1359
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Nuclear magnetic resonance study of the smectic-cholesteric phase transition in a dimesogenic liquid crystal Jun Hee Han a, J.S. Kim a, Jun Kue Park a, Kyu Won Lee a, J.-I. Jin b, E.H. Choi c, Cheol Eui Lee a, * a b c
Department of Physics, Korea University, Seoul 136-713, Republic of Korea Department of Chemistry, Korea University, Seoul 136-713, Republic of Korea Department of Electrophysics, Kwangwoon University, Seoul 130-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 April 2014 Received in revised form 10 July 2014 Accepted 28 July 2014 Available online 7 August 2014
Nuclear magnetic resonance (NMR) spectroscopy has provided a unique opportunity to study the characteristic smectic-A to chiral nematic phase transition in a dimesogenic liquid crystal (“KI-5S”). The order parameters in the liquid crystalline phases were obtained from the 2H NMR quadrupole splitting and 13C NMR chemical shift measurements, manifesting a first-order smectic-nematic phase transition. © 2014 Elsevier B.V. All rights reserved.
Keywords: Dimesogenic liquid crystal Smectic to chiral nematic phase transition Order parameters Deuterium NMR and carbon NMR
1. Introduction Mesomorphic states of matter have been among the most attractive topics in physical sciences [1]. Since de Gennes first addressed the nematic to smectic transition, its exact nature has been under intense investigation [2]. Liquid crystals have been applied for liquid crystal display, nano-material devices, nanooptics devices, optical memories, helical filaments, and multistable liquid crystals [3]. Chiral smectic liquid crystals, with their visible light pitch, can exhibit ferro- and antiferro-electric properties, possessing high operational speed and resolution far superior to those of the nematic liquid crystals. It is also well known that the functional groups attached on the rigid core of the mesogens may dictate the liquid crystal properties [4]. Nano-functional metals in the mesogen may give rise to novel properties [5]. The amphiphilic liquid crystals comprise enantiotropic liquid crystals forming liquid crystal phase both on cooling and heating, and monotropic liquid crystals forming liquid crystal only on cooling. The smectic phase possesses both orientational order and positional order, the positional order having to do with the center of mass of the rod-like molecules. The thermal energy may be
* Corresponding author. Tel.: þ82 2 3290 3098; fax: þ82 2 927 3292. E-mail address: [email protected] (C.E. Lee). http://dx.doi.org/10.1016/j.cap.2014.07.018 1567-1739/© 2014 Elsevier B.V. All rights reserved.
sufficient to destroy the positional order but still not sufficient to disrupt the orientational order [4,6]. For most thermotropic nematics known so far, the nematic phase is uniaxial, with rotational symmetry around the director. The cholesteryl 6-(40 -butylstilbenyl4-oxy)hexanoate (“KI-5S”) dimesogenic compound, studied in this work, has a chiral (cholesteric) nematic and a smectic-A phase depending on the temperature. The pitch of the helix interacts with visible light in the nematic phase. Increased temperature diminishes the degree of the twist, changing the color of the sample [7]. Besides the orientational order of the long molecular axes, another common feature of the various smectic phases is the onedimensional density wave, the molecular center of mass being, on average, arranged in equidistant planes [8]. Liquid crystals have recently attracted renewed interest for applications in soft matter nano-, bio- and microtechnology such as organic electronics, nanotemplating and nanoparticle organization, photonics, and metamaterials [9e13]. NMR has been employed as a powerful tool to study the lattice dynamics and orientational order in liquid crystals. Recent reviews on NMR of liquid crystals address application of NMR in investigating the biaxial nematics, chiral smectics, V-shaped mesogens, liquid crystalline elastomers, and micelle-templated mesoporous solids. In particular, positional order parameter was obtained from dipolar coupling constants, and topological defects and various phases and phase transitions have been elucidated by means of NMR spectroscopy [14e18]. In this
J.H. Han et al. / Current Applied Physics 14 (2014) 1356e1359
1357
paper, we have employed deuterium and carbon NMR to study the smectic to chiral nematic phase transition in the KI-5S system by means of the order parameter measurements [19]. Results of this work may be used to better understand the nature of the nematic and smectic phases and phase transitions in other related liquid crystal systems [20]. 2. Experiment The liquid crystalline samples of the KI-5S, whose molecular structure is shown in Fig. 1, were synthesized as previously reported [20,21]. For the 2H NMR measurements, the compound was selectively deuterated in the ortho positions of the aromatic C atoms (see Fig. 1). The 2H and 13C NMR spectra were first taken upon cooling from the isotropic phase and then upon heating by using a Varian 200 Innova Unity Solid NMR spectrometer with a 4.7 T magnet at Korea Basic Science Institute in Seoul as a function of temperature. The NMR line shapes, nearly identical on cooling and on heating, were obtained by Fourier-transforming the freeinduction decay (FID) signals. The 13C NMR spectra were taken by using the cross-polarization (CP) method employing hexamethylbenzene as the reference. 3. Results and discussion Fig. 2 shows the differential scanning calorimetry (DSC) measurements of the KI-5S sample, in which the crystalline, smectic-A, nematic, and isotropic phases are identified. The smectic A to nematic phase transition in the vicinity of Tc ¼ 460 K is shown both on heating and cooling cycles, an endothermic peak indicating a first-order phase transition [20]. Fig. 3(a) shows the distinct 2H NMR spectra in different phases, coexistence of peaks from both the smectic and nematic phases over some temperature range being characteristic of our sample showing a first-order phase transition. The deuterium NMR quadrupole splitting, which reflects the orientational order in the liquid crystal phases, is manifested in the smectic phase below the smectic-cholesteric phase transition temperature Tc. The smectic phase spectra are characteristic of an oriented sample, the director being aligned along the magnetic field according to our simulation, and the orientation appears to arise from cooling from the isotropic phase in a strong magnetic field used for the NMR measurements. It is to be noted that the molecular chain is rotating around the director D (see Fig. 1) and that the liquid crystal is aligned in the magnetic field with the director being parallel to the magnetic field due to the magnetic susceptibility arising from the electric current generated by the rotation of the benzene rings about their para axes [22]. The NMR spectra in the nematic phase above Tc, on the other hand, were not well resolved apparently due to the relatively smaller order parameter.
Fig. 2. The DSC thermogram of the KI5S liquid crystal on heating and cooling, in which the crystalline, smectic-A, chiral nematic, and isotropic phases are identified.
The order parameter SD, which can be obtained from the quadrupole splitting in the spectra in the smectic phase below Tc, may be defined as [23]
SD ¼
nSQ $3 cos2 b1 2
;
(1)
where the quadrupole coupling constant nSQ is taken to be 185 kHz for the aromatic deuterons, and nLC Q is the quadrupole splitting in the liquid crystal phases [24,25]. b is the time-averaged angle between the CD bond axis and the director axis, which becomes the angle between the CD bond axis and the para axis of the benzene ring, p/3 (see inset of Fig. 1). The order parameter SD, which is directly proportional to the quadrupole splitting according to Eq. (1), decreased with increasing temperature in the smectic phase, exhibiting an abrupt drop at Tc [Fig. 3(b)] [22]. The order parameter SD here is for the CD bond, assuming that the electric-field gradient is axially symmetric about the bond. In Fig. 3(a), the quadrupole splitting corresponding to SD ¼ 1 is indicated in order to show how the order parameters scale the 2H quadrupole interactions. Besides, a Landau theory or the Maier-Saupe theory, a mean-field approximation theory explaining the discontinuity at the phase transition temperature, are shown to successfully fit the temperature dependence of the order parameter SD [Fig. 3(b)] [26]. Fig. 4(a) shows the temperature evolution of the 13C NMR spectra, reflecting a first-order phase transition behavior with signals from both the smectic and nematic phases near Tc. The order parameter SC may be obtained from the 13C NMR chemical shift Ds according to.
Ds ¼
Fig. 1. The molecular structure of the KI-5S [cholesteryl 6-(40 -butylstilbenyl-4-oxy) hexanoate] liquid crystal, in which the aromatic dimesogen and the terminal chain structure are identified. The distinct carbon sites, giving rise to the distinct 13C NMR chemical shifts in Fig. 4(a), are identified. The deuteration sites (“2H”) and the director (“D”) are also indicated, as well as the angles f and b.
nLC Q
2 S sk s⊥ ; 3
(2)
where sk and s⊥ are the principal elements of the chemical shift tensor s, parallel and perpendicular to the molecular director, respectively. It may not be easy to relate Ds to the molecular director as the latter changes with conformational changes and the molecule is not really axially symmetric. However, only one line observed from each pair of orthocarbon positions (see Fig. 1) indicates rapid reorientation of the benzene rings about their para axes, thus enabling Ds to represent the molecular order. The molecules reorient rapidly about the director D, and each 13C tensor element is motionally averaged with components sk along D and s⊥
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J.H. Han et al. / Current Applied Physics 14 (2014) 1356e1359
dependencies of the chemical shifts are noticed for the distinct aromatic carbon sites in Fig. 4(a). The order parameter SC obtained from 13C NMR, quite comparable to the values of SD obtained by 2H NMR in the smectic phase [Fig. 3(b)], also exhibits a discontinuous drop at Tc indicating a firstorder nature [Fig. 4(b)]. Besides, the temperature dependence of the order parameter SC is also well fitted by the Landau theory or the Maier-Saupe theory. While the order parameter SC shows relatively high values of ~0.8 in the smectic phase, quite low values
Fig. 3. (a) The deuterium NMR spectra at various temperatures, the quadrupole splitting being apparent in the smectic phase. The quadrupole splitting corresponding to SD ¼ 1 is indicated in order to show how the order parameters scale the 2H quadrupole interactions. (b) The order parameter SD on cooling and then on heating obtained from the quadrupole splitting in the deuterium NMR spectra according to Eq. (1). The dashed line indicates a fit to the Landau theory or the Maier-Saupe theory.
perpendicular to D, thus enabling sk and s⊥ to define the chemical shift anisotropy. Thus, Eq. (2) is valid, and the values of sk ¼ 36 ppm and s⊥ ¼ 16 ppm can be used for our system, calculated by using the average values of the chemical tensor elements in model aromatic solids, with f ¼ 11 (see Fig. 1) [23]. The Ds can be calculated in reference to the resonance frequency of the NMR peak in the isotropic phase of KI-5S [see Fig. 4(a)]. Thus, the order parameter SC was obtained from Eq. (2) for a representative aromatic site (“C2” in Fig. 1) by using the corresponding values of Ds in reference to the resonance frequency of the NMR peak (130.1 ppm for the C2 site indicated in Fig. 1) in the isotropic phase of KI-5S [Fig. 4(a)] [23]. Similar but slightly different temperature
Fig. 4. (a) The 13C NMR spectra at various temperatures. Assignments of the NMR peaks to the distinct carbon sites in Fig. 1 are indicated. The vertical dashed line, corresponding to the resonance peak of the “C2” site in the isotropic phase (solution), indicates the reference to the 13C NMR chemical shift Ds of that aromatic carbon site (see Fig. 1). The order parameter SC, corresponding to the “C2” aromatic carbon site, obtained on cooling and then on heating from the carbon NMR spectra according to Eq. (2). The dashed line indicates a fit to the Landau or the Maier-Saupe theory.
J.H. Han et al. / Current Applied Physics 14 (2014) 1356e1359
of ~0.1 are noticed in comparison the usual values of S ¼ 0.4 ~ 0.8 in the nematic phase [27]. It is also to be noted that the order parameter Szz in this work can be well defined as the error possibly arising from the molecular or phase biaxiality can be safely neglected due to the fast rotations of about the molecular axis (director) and the benzene para axes. In summary, order parameters in a dimesogenic compound possessing smectic-A and chiral nematic liquid crystalline phases were obtained by means of 2H and 13C NMR measurements. Distinct 2H NMR spectra with quadrupole splitting characteristic of the liquid crystalline order were obtained in different temperature ranges. Besides, chemical shifts in the 13C NMR spectra sensitively reflected the distinct liquid crystalline phases. The order parameters obtained from the 2H and 13C NMR measurements showed compatible behaviors, both manifesting a first-order nature of the smectic-cholesteric phase transition. Acknowledgments This work was supported by the National Research Foundation of Korea (Project No. 2013057555, Proton Users Program 2014M2B2A4030835, and NRF-2010-0027963). The measurements at the Korean Basic Science Institute (KBSI) are acknowledged. References [1] G. Friedel, Ann. de Phys. 18 (1922) 273. [2] P.G. deGennes, Solid State Commun. 10 (1972) 753.
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[3] G.G. Wells, C.V. Brown, Appl. Phys. Lett. 91 (2007) 223506. [4] H.K. Bisoyi, S. Kumar, Chem. Soc. Rev. 40 (2011) 306. [5] S. Chandrasekhar, Liquid Crystals, second ed., Cambridge University Press, Inc., 1992. [6] J.K. Cha, K.W. Lee, C.E. Lee, J.I. Jin, Appl. Phys. Lett. 96 (2010) 092903. [7] G. Chidichimo, Z. Yaniv, Phys. Rev. A 25 (1982) 1077. [8] W.L. McMillan, Phys. Rev. A 7 (1973) 1419. [9] J.P.F. Lagerwall, G. Scalia, Curr. Appl. Phys. 12 (2012) 1387. [10] D.K. Yoon, G.P. Smith, E. Tsai, M. Moran, D.M. Walba, T. Bellini, I.I. Smalyukh, N.A. Clark, Liq. Cryst. 39 (2012) 571. [11] O.V. Manyuhina, G. Tordini, W. Bras, J.C. Maan, P.C.M. Christianen, Phys. Rev. E 87 (2013) 050501(R). [12] H. Coles, S. Morris, Nat. Photonics 4 (2010) 676. [13] F. Castles, S.M. Morris, J.M.C. Hung, M.M. Qasim, A.D. Wright, S. Nosheen, S.S. Choi, B.I. Outram, S.J. Elston, C. Burgess, L. Hill, T.D. Wilkinson, H.J. Coles, Nat. Mater.. doi:10.1038/nmat3993. [14] B. Alonso, C. Marichal, Chem. Soc. Rev. 42 (2013) 3808. [15] M. Cifelli, V. Domenici, C.A. Veracini, Curr. Opin. Coll. Inter. Sci. 18 (2013) 190. [16] M.E. Di Pietro, G. Celebre, G. De Luca, H. Zimmermann, G. Cinacchi, Eur. Phys. J. E 35 (2012) 112. [17] A.C.J. Weber, X. Yang, R.Y. Dong, W.L. Meerts, E.E. Burnell, Chem. Phys. Lett. 476 (2009) 116. [18] V. Domenici, Prog. Nucl. Magn. Reson. Spectr. 63 (2012) 1. [19] J.W. Doane, R.S. Parker, B. Cvikl, Phys. Rev. Lett. 28 (1972) 1694. [20] F. Hardouin, M.F. Achard, J.-I. Jin, Y.-K. Yun, S.-J. Chung, Eur. Phys. J. B 1 (1998) 47. [21] S.W. Cha, J.I. Jin, Liq. Cryst. 26 (1999) 1325. [22] A. Pines, J.J. Chang, Phys. Rev. A 10 (1974) 946. [23] R.Y. Dong, Nuclear Magnetic Resonance of Liquid Crystals, Spinger-Verlag Press, Inc, 2005. [24] E.H. Hardy, R. Witt, A. Dolle, M.D. Zeidler, J. Magn. Reson. 134 (1998) 300. [25] A. Pines, M.G. Gibby, J.S. Waugh, Chem. Phys. Lett. 15 (1972) 373. [26] M.J. Stephen, J.P. Straley, Rev. Mod. Phys. 46 (1974) 617. [27] K.W. Lee, C.H. Lee, S.H. Yang, J.K. Cha, C.E. Lee, J. Kim, Curr. Appl. Phys. 1 (2001) 529.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Nuclear magnetic resonance study of the smectic-cholesteric phase transition in a dimesogenic liquid crystal Jun Hee Han a, J.S. Kim a, Jun Kue Park a, Kyu Won Lee a, J.-I. Jin b, E.H. Choi c, Cheol Eui Lee a, * a b c
Department of Physics, Korea University, Seoul 136-713, Republic of Korea Department of Chemistry, Korea University, Seoul 136-713, Republic of Korea Department of Electrophysics, Kwangwoon University, Seoul 130-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 April 2014 Received in revised form 10 July 2014 Accepted 28 July 2014 Available online 7 August 2014
Nuclear magnetic resonance (NMR) spectroscopy has provided a unique opportunity to study the characteristic smectic-A to chiral nematic phase transition in a dimesogenic liquid crystal (“KI-5S”). The order parameters in the liquid crystalline phases were obtained from the 2H NMR quadrupole splitting and 13C NMR chemical shift measurements, manifesting a first-order smectic-nematic phase transition. © 2014 Elsevier B.V. All rights reserved.
Keywords: Dimesogenic liquid crystal Smectic to chiral nematic phase transition Order parameters Deuterium NMR and carbon NMR
1. Introduction Mesomorphic states of matter have been among the most attractive topics in physical sciences [1]. Since de Gennes first addressed the nematic to smectic transition, its exact nature has been under intense investigation [2]. Liquid crystals have been applied for liquid crystal display, nano-material devices, nanooptics devices, optical memories, helical filaments, and multistable liquid crystals [3]. Chiral smectic liquid crystals, with their visible light pitch, can exhibit ferro- and antiferro-electric properties, possessing high operational speed and resolution far superior to those of the nematic liquid crystals. It is also well known that the functional groups attached on the rigid core of the mesogens may dictate the liquid crystal properties [4]. Nano-functional metals in the mesogen may give rise to novel properties [5]. The amphiphilic liquid crystals comprise enantiotropic liquid crystals forming liquid crystal phase both on cooling and heating, and monotropic liquid crystals forming liquid crystal only on cooling. The smectic phase possesses both orientational order and positional order, the positional order having to do with the center of mass of the rod-like molecules. The thermal energy may be
* Corresponding author. Tel.: þ82 2 3290 3098; fax: þ82 2 927 3292. E-mail address: [email protected] (C.E. Lee). http://dx.doi.org/10.1016/j.cap.2014.07.018 1567-1739/© 2014 Elsevier B.V. All rights reserved.
sufficient to destroy the positional order but still not sufficient to disrupt the orientational order [4,6]. For most thermotropic nematics known so far, the nematic phase is uniaxial, with rotational symmetry around the director. The cholesteryl 6-(40 -butylstilbenyl4-oxy)hexanoate (“KI-5S”) dimesogenic compound, studied in this work, has a chiral (cholesteric) nematic and a smectic-A phase depending on the temperature. The pitch of the helix interacts with visible light in the nematic phase. Increased temperature diminishes the degree of the twist, changing the color of the sample [7]. Besides the orientational order of the long molecular axes, another common feature of the various smectic phases is the onedimensional density wave, the molecular center of mass being, on average, arranged in equidistant planes [8]. Liquid crystals have recently attracted renewed interest for applications in soft matter nano-, bio- and microtechnology such as organic electronics, nanotemplating and nanoparticle organization, photonics, and metamaterials [9e13]. NMR has been employed as a powerful tool to study the lattice dynamics and orientational order in liquid crystals. Recent reviews on NMR of liquid crystals address application of NMR in investigating the biaxial nematics, chiral smectics, V-shaped mesogens, liquid crystalline elastomers, and micelle-templated mesoporous solids. In particular, positional order parameter was obtained from dipolar coupling constants, and topological defects and various phases and phase transitions have been elucidated by means of NMR spectroscopy [14e18]. In this
J.H. Han et al. / Current Applied Physics 14 (2014) 1356e1359
1357
paper, we have employed deuterium and carbon NMR to study the smectic to chiral nematic phase transition in the KI-5S system by means of the order parameter measurements [19]. Results of this work may be used to better understand the nature of the nematic and smectic phases and phase transitions in other related liquid crystal systems [20]. 2. Experiment The liquid crystalline samples of the KI-5S, whose molecular structure is shown in Fig. 1, were synthesized as previously reported [20,21]. For the 2H NMR measurements, the compound was selectively deuterated in the ortho positions of the aromatic C atoms (see Fig. 1). The 2H and 13C NMR spectra were first taken upon cooling from the isotropic phase and then upon heating by using a Varian 200 Innova Unity Solid NMR spectrometer with a 4.7 T magnet at Korea Basic Science Institute in Seoul as a function of temperature. The NMR line shapes, nearly identical on cooling and on heating, were obtained by Fourier-transforming the freeinduction decay (FID) signals. The 13C NMR spectra were taken by using the cross-polarization (CP) method employing hexamethylbenzene as the reference. 3. Results and discussion Fig. 2 shows the differential scanning calorimetry (DSC) measurements of the KI-5S sample, in which the crystalline, smectic-A, nematic, and isotropic phases are identified. The smectic A to nematic phase transition in the vicinity of Tc ¼ 460 K is shown both on heating and cooling cycles, an endothermic peak indicating a first-order phase transition [20]. Fig. 3(a) shows the distinct 2H NMR spectra in different phases, coexistence of peaks from both the smectic and nematic phases over some temperature range being characteristic of our sample showing a first-order phase transition. The deuterium NMR quadrupole splitting, which reflects the orientational order in the liquid crystal phases, is manifested in the smectic phase below the smectic-cholesteric phase transition temperature Tc. The smectic phase spectra are characteristic of an oriented sample, the director being aligned along the magnetic field according to our simulation, and the orientation appears to arise from cooling from the isotropic phase in a strong magnetic field used for the NMR measurements. It is to be noted that the molecular chain is rotating around the director D (see Fig. 1) and that the liquid crystal is aligned in the magnetic field with the director being parallel to the magnetic field due to the magnetic susceptibility arising from the electric current generated by the rotation of the benzene rings about their para axes [22]. The NMR spectra in the nematic phase above Tc, on the other hand, were not well resolved apparently due to the relatively smaller order parameter.
Fig. 2. The DSC thermogram of the KI5S liquid crystal on heating and cooling, in which the crystalline, smectic-A, chiral nematic, and isotropic phases are identified.
The order parameter SD, which can be obtained from the quadrupole splitting in the spectra in the smectic phase below Tc, may be defined as [23]
SD ¼
nSQ $3 cos2 b1 2
;
(1)
where the quadrupole coupling constant nSQ is taken to be 185 kHz for the aromatic deuterons, and nLC Q is the quadrupole splitting in the liquid crystal phases [24,25]. b is the time-averaged angle between the CD bond axis and the director axis, which becomes the angle between the CD bond axis and the para axis of the benzene ring, p/3 (see inset of Fig. 1). The order parameter SD, which is directly proportional to the quadrupole splitting according to Eq. (1), decreased with increasing temperature in the smectic phase, exhibiting an abrupt drop at Tc [Fig. 3(b)] [22]. The order parameter SD here is for the CD bond, assuming that the electric-field gradient is axially symmetric about the bond. In Fig. 3(a), the quadrupole splitting corresponding to SD ¼ 1 is indicated in order to show how the order parameters scale the 2H quadrupole interactions. Besides, a Landau theory or the Maier-Saupe theory, a mean-field approximation theory explaining the discontinuity at the phase transition temperature, are shown to successfully fit the temperature dependence of the order parameter SD [Fig. 3(b)] [26]. Fig. 4(a) shows the temperature evolution of the 13C NMR spectra, reflecting a first-order phase transition behavior with signals from both the smectic and nematic phases near Tc. The order parameter SC may be obtained from the 13C NMR chemical shift Ds according to.
Ds ¼
Fig. 1. The molecular structure of the KI-5S [cholesteryl 6-(40 -butylstilbenyl-4-oxy) hexanoate] liquid crystal, in which the aromatic dimesogen and the terminal chain structure are identified. The distinct carbon sites, giving rise to the distinct 13C NMR chemical shifts in Fig. 4(a), are identified. The deuteration sites (“2H”) and the director (“D”) are also indicated, as well as the angles f and b.
nLC Q
2 S sk s⊥ ; 3
(2)
where sk and s⊥ are the principal elements of the chemical shift tensor s, parallel and perpendicular to the molecular director, respectively. It may not be easy to relate Ds to the molecular director as the latter changes with conformational changes and the molecule is not really axially symmetric. However, only one line observed from each pair of orthocarbon positions (see Fig. 1) indicates rapid reorientation of the benzene rings about their para axes, thus enabling Ds to represent the molecular order. The molecules reorient rapidly about the director D, and each 13C tensor element is motionally averaged with components sk along D and s⊥
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J.H. Han et al. / Current Applied Physics 14 (2014) 1356e1359
dependencies of the chemical shifts are noticed for the distinct aromatic carbon sites in Fig. 4(a). The order parameter SC obtained from 13C NMR, quite comparable to the values of SD obtained by 2H NMR in the smectic phase [Fig. 3(b)], also exhibits a discontinuous drop at Tc indicating a firstorder nature [Fig. 4(b)]. Besides, the temperature dependence of the order parameter SC is also well fitted by the Landau theory or the Maier-Saupe theory. While the order parameter SC shows relatively high values of ~0.8 in the smectic phase, quite low values
Fig. 3. (a) The deuterium NMR spectra at various temperatures, the quadrupole splitting being apparent in the smectic phase. The quadrupole splitting corresponding to SD ¼ 1 is indicated in order to show how the order parameters scale the 2H quadrupole interactions. (b) The order parameter SD on cooling and then on heating obtained from the quadrupole splitting in the deuterium NMR spectra according to Eq. (1). The dashed line indicates a fit to the Landau theory or the Maier-Saupe theory.
perpendicular to D, thus enabling sk and s⊥ to define the chemical shift anisotropy. Thus, Eq. (2) is valid, and the values of sk ¼ 36 ppm and s⊥ ¼ 16 ppm can be used for our system, calculated by using the average values of the chemical tensor elements in model aromatic solids, with f ¼ 11 (see Fig. 1) [23]. The Ds can be calculated in reference to the resonance frequency of the NMR peak in the isotropic phase of KI-5S [see Fig. 4(a)]. Thus, the order parameter SC was obtained from Eq. (2) for a representative aromatic site (“C2” in Fig. 1) by using the corresponding values of Ds in reference to the resonance frequency of the NMR peak (130.1 ppm for the C2 site indicated in Fig. 1) in the isotropic phase of KI-5S [Fig. 4(a)] [23]. Similar but slightly different temperature
Fig. 4. (a) The 13C NMR spectra at various temperatures. Assignments of the NMR peaks to the distinct carbon sites in Fig. 1 are indicated. The vertical dashed line, corresponding to the resonance peak of the “C2” site in the isotropic phase (solution), indicates the reference to the 13C NMR chemical shift Ds of that aromatic carbon site (see Fig. 1). The order parameter SC, corresponding to the “C2” aromatic carbon site, obtained on cooling and then on heating from the carbon NMR spectra according to Eq. (2). The dashed line indicates a fit to the Landau or the Maier-Saupe theory.
J.H. Han et al. / Current Applied Physics 14 (2014) 1356e1359
of ~0.1 are noticed in comparison the usual values of S ¼ 0.4 ~ 0.8 in the nematic phase [27]. It is also to be noted that the order parameter Szz in this work can be well defined as the error possibly arising from the molecular or phase biaxiality can be safely neglected due to the fast rotations of about the molecular axis (director) and the benzene para axes. In summary, order parameters in a dimesogenic compound possessing smectic-A and chiral nematic liquid crystalline phases were obtained by means of 2H and 13C NMR measurements. Distinct 2H NMR spectra with quadrupole splitting characteristic of the liquid crystalline order were obtained in different temperature ranges. Besides, chemical shifts in the 13C NMR spectra sensitively reflected the distinct liquid crystalline phases. The order parameters obtained from the 2H and 13C NMR measurements showed compatible behaviors, both manifesting a first-order nature of the smectic-cholesteric phase transition. Acknowledgments This work was supported by the National Research Foundation of Korea (Project No. 2013057555, Proton Users Program 2014M2B2A4030835, and NRF-2010-0027963). The measurements at the Korean Basic Science Institute (KBSI) are acknowledged. References [1] G. Friedel, Ann. de Phys. 18 (1922) 273. [2] P.G. deGennes, Solid State Commun. 10 (1972) 753.
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