Comparative study of local structure of two cyanobiphenyl liquid crystals by molecular dynamics method Egor D. Gerts, Andrei V. Komolkin, Vladimir A. Burmistrov, Victor V. Alexandriysky, and Sergey V. Dvinskikh Citation: The Journal of Chemical Physics 141, 074503 (2014); doi: 10.1063/1.4892877 View online: http://dx.doi.org/10.1063/1.4892877 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/141/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in An ab initio molecular dynamics study of the liquid-vapor interface of an aqueous NaCl solution: Inhomogeneous density, polarity, hydrogen bonds, and frequency fluctuations of interfacial molecules J. Chem. Phys. 141, 194705 (2014); 10.1063/1.4901118 Hydrogen bonded structure, polarity, molecular motion and frequency fluctuations at liquid-vapor interface of a water-methanol mixture: An ab initio molecular dynamics study J. Chem. Phys. 141, 134703 (2014); 10.1063/1.4896233 Ab initio and classical molecular dynamics studies of the structural and dynamical behavior of water near a hydrophobic graphene sheet J. Chem. Phys. 138, 204702 (2013); 10.1063/1.4804300 A molecular dynamics study of nitric oxide in water: Diffusion and structure J. Chem. Phys. 123, 054505 (2005); 10.1063/1.1992482 Structure and speciation of liquid 2HF/KF: A molecular dynamics study J. Chem. Phys. 117, 3772 (2002); 10.1063/1.1494795
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THE JOURNAL OF CHEMICAL PHYSICS 141, 074503 (2014)
Comparative study of local structure of two cyanobiphenyl liquid crystals by molecular dynamics method Egor D. Gerts,1,a) Andrei V. Komolkin,1,b) Vladimir A. Burmistrov,2 Victor V. Alexandriysky,2,3 and Sergey V. Dvinskikh4,5 1
Physical Faculty, Saint Petersburg State University, Saint Petersburg 198504, Russia Ivanovo State University of Chemical Technology, Ivanovo 153000, Russia 3 Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo 153045, Russia 4 Laboratory of Biomolecular NMR, Saint Petersburg State University, Saint Petersburg 199034, Russia 5 Department of Chemistry, KTH Royal Institute of Technology, Stockholm SE-100 44, Sweden 2
(Received 21 April 2014; accepted 31 July 2014; published online 18 August 2014) Fully-atomistic molecular dynamics simulations were carried out on two similar cyanobiphenyl nematogens, HO-6OCB and 7OCB, in order to study effects of hydrogen bonds on local structure of liquid crystals. Comparable length of these two molecules provides more evident results on the effects of hydrogen bonding. The analysis of radial and cylindrical distribution functions clearly shows the differences in local structure of two mesogens. The simulations showed that anti-parallel alignment is preferable for the HO-6OCB. Hydrogen bonds between OH-groups are observed for 51% of HO-6OCB molecules, while hydrogen bonding between CN- and OH-groups occurs only for 16% of molecules. The lifetimes of H-bonds differ due to different mobility of molecular fragments (50 ps for N···H–O and 41 ps for O···H–O). Although the standard Optimized Potentials for Liquid Simulations - All-Atom force field cannot reproduce some experimental parameters quantitatively (order parameters are overestimated, diffusion coefficients are not reproduced well), the comparison of relative simulated results for the pair of mesogens is nevertheless consistent with the same relative experimental parameters. Thus, the comparative study of simulated and experimental results for the pair of similar liquid crystals still can be assumed plausible. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4892877] I. INTRODUCTION
Liquid crystalline materials, as a part of one of the mainstream directions of modern supramolecular chemistry, are of particular interest for researchers as they possess some degree of orientational order while retaining their mobility. This unique feature is often exploited to design new functional materials with predefined properties.1 The examination of molecular structure can provide information on different parameters such as mobility and stability of the mesophase and molecular orientation that influence greatly the physical properties. Liquid crystals (LCs) are often used in modern electrooptical devices, and a significant amount of investigations has been carried out during the last decade in order to decrease their switching time.2 Cyanobiphenyls are the most common materials for that purpose due to high value of dielectric anisotropy and good electrooptical characteristics. These are highly polar complexes having large dipole moment along the long molecular axis. Dipole contribution to the dielectric properties is prevailing for such LCs.3 The interest in supramolecular LC materials is greatly increased during last years.4–8 In particular, hydrogen-bonded systems are of great practical importance as they modify the molecular mobility and ordering of the material.1, 9–15 To increase the value of dielectric anisotropy, thus increasing the efficiency of electrooptical compositions, some attempts are a) Electronic mail: [email protected] b) Electronic mail: [email protected]
0021-9606/2014/141(7)/074503/7/$30.00
made to influence the dipole-dipole association of highly polar mesogens. Hydrogen bonding seems to be the most suitable for that purpose due to its strength and selectivity.16 Hydrogen bonding as well as the high molecular conformational mobility makes it difficult to analyze the experimental data in terms of conformational and local structure of the LCs. To get realistic structure of LCs different computer simulation techniques are used. Molecular dynamics (MD) simulations have become indispensable tool for the structure study of different liquid crystalline materials along with experimental techniques. Atomistic MD simulations are gaining popularity nowadays due to the increase of computer performance, as they allow researchers to get plausible results on the structure of the substances, their macro- and microscopic characteristics. Phase transitions studies and magnetic resonance spectra interpretations are also made via MD simulations. Several interesting works in this field were done recently. In the paper,17 the types of mesophases and transition temperatures of sexithiophene was determined using united atoms (UA) model and specifically optimized force field (FF). Liquid crystal 4-n-octyl-4 cyanobiphenyl (8CB) was thoroughly investigated.18, 19 In the paper,18 authors used fully atomistic (FA) MD with AMBER force field parameters and were able to reproduce the experimental ESR spectrum and the local order and motional parameters, however they cannot get realistic transition temperatures. The UA model with the specifically developed force field for cyanobiphenyls was used19 in order to obtain
141, 074503-1
© 2014 AIP Publishing LLC
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.91.169.193 On: Sun, 23 Nov 2014 16:36:31
074503-2
Gerts et al.
J. Chem. Phys. 141, 074503 (2014)
NC
O
affect the atoms are calculated at each time step. Equations of motion are integrated with the step t in order to calculate coordinates and velocities. Step value depends on the implemented model of the molecules. Fully atomistic model of a molecule consists of both backbone atoms and lightweight hydrogen atoms. Interactions of peripheral hydrogen atoms influence the local structure and conformations of molecules. Also interpretation of 1 H, 2 H, and 13 C-1 H NMR spectra, which can provide information on conformational structure, is possible when one knows positions of all atoms in the system under study. However, since hydrogen atoms make up almost half of all atoms in LC molecules, FA models are computationally rather expensive. Also the simulation step t should be less than the correlation times of motion of the fastest atoms (hydrogens), that means t ≤ 1 fs for FA models. In order to speed up the simulations UA model is used. In this model bonded carbon and hydrogen atoms are joined to one interaction cite called “united atom.” Hydrogen atoms which involved in H-bonds still used as “independent” interaction sites. This model as compared with FA model decreases the number of interacting sites in a molecule and increases the simulation step t up to 2–3 fs. The simulation speed is increased, but UA model may lead to some misrepresentation of local and conformational structures. In this work, the simulated cells were created and equilibrated using UA model retaining the hydrogen atom of the OH-group. When the equilibrium was reached the MultiScale Transitions method (MuScaT)36 was applied in order to replace all UA molecules with FA molecules. This method allows one to add hydrogen atoms while retaining the current conformations of the molecules. Thus the equilibration of the systems takes less computational time. The analysis of simulated parameters has been done in FA model. In this paper, local structure and dynamics is presented. Conformational structure and NMR spectra analysis will be published separately. The intra- and intermolecular interactions were taken into account: the energies of bond angles, torsional angles, non-bonded intramolecular interactions, Van der Waals’ and electrostatic intermolecular atom-atom interactions. For both models the potential energy is given by
(CH2)6 R
FIG. 1. Chemical structure.
macroscopic parameters: orientational, positional, and mixed order parameters, layer spacing, translational diffusion tensor components, and their temperature dependence. The results show good agreement with experiments. In the work,20 isomerization and molecular orientation of two pure liquid crystals and their mixture were studied using non-modified fully atomistic Optimized Potentials for Liquid Simulations - AllAtom (OPLS-AA)21 force field. Yan and Earl22 were able to reproduce experimental results for the liquid crystal mixtures of alkoxy substituted phenylpyrimidines via fully atomistic MD simulations using the Liquid Crystal Force Field (LCFF). Although specifically derived or optimized force field for one certain substance can reproduce the experimental parameters rather accurately, there is always the problem of transferability of this FF to other substances. On the other hand, the same force field can be applied to the homologue series.23, 24 In this paper, local structures of two nematic thermotropic LCs are studied: NC–C6 H4 –C6 H4 –O–(CH2 )6 –R, where R = OH (ω-hydroxy-4-hexyloxy-4 -cyanobiphenyl, HO-6OCB, also referred as HOC6B, or H6CBP) and R = CH3 (4-heptyloxy-4 -cyanobiphenyl, 7OCB) as shown in Fig. 1. Both liquid crystals were synthesized at the Ivanovo State University of Chemical Technology, Russia. Crystal to Nematic and Nematic to Isotropic transition temperatures are TKN = 367 K and TNI = 384 K for HO-6OCB and TKN = 327 K and TNI = 348 K for 7OCB.16 HO-6OCB can form hydrogen bonds by virtue of its hydroxyl group. Possible hydrogen bonds are shown in Fig. 2. Liquid crystal 7OCB is well investigated,25–31 while only few experimental and simulation studies of HO-6OCB were reported. The structure of HO-6OCB and its homologues in crystal phase was published by Zugenmaier and Heiske.32, 33 Burmistrov et al. examined reological and dielectric properties, density and order parameter.3, 16 Ojha and Praveen simulated intermolecular interactions between two HO-6OCB molecules in benzene solution at the temperature of nematicisotropic phase transition.34 Some investigations of pair interaction energies between two HO-6OCB molecules can also be found in the paper of Thakur and Roychoudhury.35
Etotal =
Kr (r − req )2 +
bonds
II. SIMULATION DETAILS
Kθ (θ − θeq )2
angles
V n + [1 − cos n( − γ )]2 2 dihedrals
A. Simulation models
12 6 qi qj σij σij 4εij + , (1) + − rij rij 4π 0 rij i
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.91.169.193 On: Sun, 23 Nov 2014 16:36:31
THE JOURNAL OF CHEMICAL PHYSICS 141, 074503 (2014)
Comparative study of local structure of two cyanobiphenyl liquid crystals by molecular dynamics method Egor D. Gerts,1,a) Andrei V. Komolkin,1,b) Vladimir A. Burmistrov,2 Victor V. Alexandriysky,2,3 and Sergey V. Dvinskikh4,5 1
Physical Faculty, Saint Petersburg State University, Saint Petersburg 198504, Russia Ivanovo State University of Chemical Technology, Ivanovo 153000, Russia 3 Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo 153045, Russia 4 Laboratory of Biomolecular NMR, Saint Petersburg State University, Saint Petersburg 199034, Russia 5 Department of Chemistry, KTH Royal Institute of Technology, Stockholm SE-100 44, Sweden 2
(Received 21 April 2014; accepted 31 July 2014; published online 18 August 2014) Fully-atomistic molecular dynamics simulations were carried out on two similar cyanobiphenyl nematogens, HO-6OCB and 7OCB, in order to study effects of hydrogen bonds on local structure of liquid crystals. Comparable length of these two molecules provides more evident results on the effects of hydrogen bonding. The analysis of radial and cylindrical distribution functions clearly shows the differences in local structure of two mesogens. The simulations showed that anti-parallel alignment is preferable for the HO-6OCB. Hydrogen bonds between OH-groups are observed for 51% of HO-6OCB molecules, while hydrogen bonding between CN- and OH-groups occurs only for 16% of molecules. The lifetimes of H-bonds differ due to different mobility of molecular fragments (50 ps for N···H–O and 41 ps for O···H–O). Although the standard Optimized Potentials for Liquid Simulations - All-Atom force field cannot reproduce some experimental parameters quantitatively (order parameters are overestimated, diffusion coefficients are not reproduced well), the comparison of relative simulated results for the pair of mesogens is nevertheless consistent with the same relative experimental parameters. Thus, the comparative study of simulated and experimental results for the pair of similar liquid crystals still can be assumed plausible. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4892877] I. INTRODUCTION
Liquid crystalline materials, as a part of one of the mainstream directions of modern supramolecular chemistry, are of particular interest for researchers as they possess some degree of orientational order while retaining their mobility. This unique feature is often exploited to design new functional materials with predefined properties.1 The examination of molecular structure can provide information on different parameters such as mobility and stability of the mesophase and molecular orientation that influence greatly the physical properties. Liquid crystals (LCs) are often used in modern electrooptical devices, and a significant amount of investigations has been carried out during the last decade in order to decrease their switching time.2 Cyanobiphenyls are the most common materials for that purpose due to high value of dielectric anisotropy and good electrooptical characteristics. These are highly polar complexes having large dipole moment along the long molecular axis. Dipole contribution to the dielectric properties is prevailing for such LCs.3 The interest in supramolecular LC materials is greatly increased during last years.4–8 In particular, hydrogen-bonded systems are of great practical importance as they modify the molecular mobility and ordering of the material.1, 9–15 To increase the value of dielectric anisotropy, thus increasing the efficiency of electrooptical compositions, some attempts are a) Electronic mail: [email protected] b) Electronic mail: [email protected]
0021-9606/2014/141(7)/074503/7/$30.00
made to influence the dipole-dipole association of highly polar mesogens. Hydrogen bonding seems to be the most suitable for that purpose due to its strength and selectivity.16 Hydrogen bonding as well as the high molecular conformational mobility makes it difficult to analyze the experimental data in terms of conformational and local structure of the LCs. To get realistic structure of LCs different computer simulation techniques are used. Molecular dynamics (MD) simulations have become indispensable tool for the structure study of different liquid crystalline materials along with experimental techniques. Atomistic MD simulations are gaining popularity nowadays due to the increase of computer performance, as they allow researchers to get plausible results on the structure of the substances, their macro- and microscopic characteristics. Phase transitions studies and magnetic resonance spectra interpretations are also made via MD simulations. Several interesting works in this field were done recently. In the paper,17 the types of mesophases and transition temperatures of sexithiophene was determined using united atoms (UA) model and specifically optimized force field (FF). Liquid crystal 4-n-octyl-4 cyanobiphenyl (8CB) was thoroughly investigated.18, 19 In the paper,18 authors used fully atomistic (FA) MD with AMBER force field parameters and were able to reproduce the experimental ESR spectrum and the local order and motional parameters, however they cannot get realistic transition temperatures. The UA model with the specifically developed force field for cyanobiphenyls was used19 in order to obtain
141, 074503-1
© 2014 AIP Publishing LLC
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.91.169.193 On: Sun, 23 Nov 2014 16:36:31
074503-2
Gerts et al.
J. Chem. Phys. 141, 074503 (2014)
NC
O
affect the atoms are calculated at each time step. Equations of motion are integrated with the step t in order to calculate coordinates and velocities. Step value depends on the implemented model of the molecules. Fully atomistic model of a molecule consists of both backbone atoms and lightweight hydrogen atoms. Interactions of peripheral hydrogen atoms influence the local structure and conformations of molecules. Also interpretation of 1 H, 2 H, and 13 C-1 H NMR spectra, which can provide information on conformational structure, is possible when one knows positions of all atoms in the system under study. However, since hydrogen atoms make up almost half of all atoms in LC molecules, FA models are computationally rather expensive. Also the simulation step t should be less than the correlation times of motion of the fastest atoms (hydrogens), that means t ≤ 1 fs for FA models. In order to speed up the simulations UA model is used. In this model bonded carbon and hydrogen atoms are joined to one interaction cite called “united atom.” Hydrogen atoms which involved in H-bonds still used as “independent” interaction sites. This model as compared with FA model decreases the number of interacting sites in a molecule and increases the simulation step t up to 2–3 fs. The simulation speed is increased, but UA model may lead to some misrepresentation of local and conformational structures. In this work, the simulated cells were created and equilibrated using UA model retaining the hydrogen atom of the OH-group. When the equilibrium was reached the MultiScale Transitions method (MuScaT)36 was applied in order to replace all UA molecules with FA molecules. This method allows one to add hydrogen atoms while retaining the current conformations of the molecules. Thus the equilibration of the systems takes less computational time. The analysis of simulated parameters has been done in FA model. In this paper, local structure and dynamics is presented. Conformational structure and NMR spectra analysis will be published separately. The intra- and intermolecular interactions were taken into account: the energies of bond angles, torsional angles, non-bonded intramolecular interactions, Van der Waals’ and electrostatic intermolecular atom-atom interactions. For both models the potential energy is given by
(CH2)6 R
FIG. 1. Chemical structure.
macroscopic parameters: orientational, positional, and mixed order parameters, layer spacing, translational diffusion tensor components, and their temperature dependence. The results show good agreement with experiments. In the work,20 isomerization and molecular orientation of two pure liquid crystals and their mixture were studied using non-modified fully atomistic Optimized Potentials for Liquid Simulations - AllAtom (OPLS-AA)21 force field. Yan and Earl22 were able to reproduce experimental results for the liquid crystal mixtures of alkoxy substituted phenylpyrimidines via fully atomistic MD simulations using the Liquid Crystal Force Field (LCFF). Although specifically derived or optimized force field for one certain substance can reproduce the experimental parameters rather accurately, there is always the problem of transferability of this FF to other substances. On the other hand, the same force field can be applied to the homologue series.23, 24 In this paper, local structures of two nematic thermotropic LCs are studied: NC–C6 H4 –C6 H4 –O–(CH2 )6 –R, where R = OH (ω-hydroxy-4-hexyloxy-4 -cyanobiphenyl, HO-6OCB, also referred as HOC6B, or H6CBP) and R = CH3 (4-heptyloxy-4 -cyanobiphenyl, 7OCB) as shown in Fig. 1. Both liquid crystals were synthesized at the Ivanovo State University of Chemical Technology, Russia. Crystal to Nematic and Nematic to Isotropic transition temperatures are TKN = 367 K and TNI = 384 K for HO-6OCB and TKN = 327 K and TNI = 348 K for 7OCB.16 HO-6OCB can form hydrogen bonds by virtue of its hydroxyl group. Possible hydrogen bonds are shown in Fig. 2. Liquid crystal 7OCB is well investigated,25–31 while only few experimental and simulation studies of HO-6OCB were reported. The structure of HO-6OCB and its homologues in crystal phase was published by Zugenmaier and Heiske.32, 33 Burmistrov et al. examined reological and dielectric properties, density and order parameter.3, 16 Ojha and Praveen simulated intermolecular interactions between two HO-6OCB molecules in benzene solution at the temperature of nematicisotropic phase transition.34 Some investigations of pair interaction energies between two HO-6OCB molecules can also be found in the paper of Thakur and Roychoudhury.35
Etotal =
Kr (r − req )2 +
bonds
II. SIMULATION DETAILS
Kθ (θ − θeq )2
angles
V n + [1 − cos n( − γ )]2 2 dihedrals
A. Simulation models
12 6 qi qj σij σij 4εij + , (1) + − rij rij 4π 0 rij i
