Molecular Dynamics Investigation of Bent-Core Molecules on a Water

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Molecular Dynamics Investigation of Bent-Core Molecules on a Water Surface Nathan Duff,† Ji Wang,‡ Elizabeth K. Mann,‡ and Daniel J. Lacks*,† Department of Chemical Engineering, Case Western ReserVe UniVersity, CleVeland, Ohio 44106, and Department of Physics, Kent State UniVersity, Kent, Ohio 44242 ReceiVed July 12, 2006. In Final Form: September 12, 2006 Molecular dynamics simulations are carried out for bent-core molecules at water surfaces. The water surface is shown to alter the equilibrium molecular structure significantly by causing a different class of torsional states to become more favorable. The equilibrium structure is also altered by the substitution of chlorine atoms for hydrogen atoms on the central phenyl ring in that this substitution forces the bent core to remain in a single torsional state rather than be delocalized among several torsional states. The consequences of these structural changes on the chirality and packing of these molecules on water surfaces are discussed.

Introduction Bent-core molecules can exhibit chiral anti-ferroelectric liquidcrystalline phases.1,2 This behavior is promising for a number of applications, including light shutters,3 artificial muscles,4 and nonlinear optics.2,5 However, the large-scale alignment of the molecules necessary for most applications has proven difficult.6 Bulk-phase alignment of liquid crystals is determined by alignment at the surface, which is often facilitated by an alignment layer. The self-assembly and packing of bent-core molecules on a water surface, in a Langmuir layer, provides an example of the influence of a surface on these systems. Transferred to a solid surface, such a layer may itself serve as an alignment layer.7 Experiments have shown striking differences in Langmuir layers of similar bent-core molecules. In particular, Langmuir layers composed of the bent-core class shown in Figure 1a were examined with X ) H (denoted Bc-2H) and X ) Cl (denoted Bc-2Cl). The Bc-2H molecule formed layers with no in-plane optical anisotropy,8 whereas the Bc-2Cl molecule formed layers with macroscopic, centimeter-scale anisotropy.9 This markedly different behavior occurs even though the two molecules differ only in the substitution of two hydrogen atoms by chlorine atoms. Note that these substitutions are also known to alter the bulkphase behavior.2 The present investigation addresses the structures of these bent-core molecules at water surfaces in order to elucidate the experimental observations.

Computational Methods The present investigation uses atomistic molecular dynamics (MD) simulations with classical force fields. This methodology was chosen because more coarse-grained or more rigorous † ‡

Case Western Reserve University. Kent State University.

(1) Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.; Korblova, E.; Walba, D. M. Science 1997, 278, 1924. (2) Pelzl, G.; Diele, S.; Weissflog, W. AdV. Mater. 1999, 11, 707. (3) Ja´kli, A.; Chien, L. C.; Kru¨erke, D.; Sawade, H.; Heppke, G. Liq. Cryst. 2002, 29, 377. (4) Ja´kli, A.; Kru¨erke, D., Nair, G. G. Phys. ReV. E 2003, 67, 051702. (5) Reddy, R. A.; Tschierske, C. J. Mater. Chem. 2006 16, 907. (6) Takezoe, H.; Takanishi, Y. Jpn. J. Appl. Phys. 2006, 45 597. (7) Chen, J.; Vithana, H.; Johnson, D.; Albarici, A.; Lando, J.; Mann, J. A.; Kakimoto, M. A. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1996, 275, 49. (8) Zou, L.; Wang, J.; Beleva, V. J.; Kooijman, E. E.; Primak, S. V.; Risse, J.; Weissflog, W.; Ja´kli, A.; Mann, E. K. Langmuir 2004, 20, 2772. (9) Wang, J.; Zou, L.; Weissflog, W.; Ja´kli, A.; Mann, E. K. Langmuir 2006, 22, 3198.

Figure 1. (a) Molecule Bc-2X, where X is hydrogen in BC-2H and chlorine in Bc-2Cl. Torsion angles Φ1L and Φ1R are shown. (b) Annealed structure of Bc-2Cl in vacuo. Note that the carbonyl oxygens are on opposite sides of the plane of the central phenyl ring. (c) Snapshot from the MD simulation of the Bc-2Cl molecule on the water surface. Note that both carbonyl oxygens are on the same (water) side of the plane of the central phenyl ring. Images b and c were created using the VMD 1.84 software package.29

methods are unable to address the questions of interest here. On one hand, the highly simplified molecular representations (e.g., cylinders, chains of beads, etc.)10-17 that have been used to address the phase behavior in bent-core systems are not able to predict the effects of subtle differences in atomic composition. On the other hand, the quantum chemical calculations that have addressed the structure of isolated bent-core molecules18-21 are not able to (10) Schiller, P.; Schlacken, H. Liq. Cryst. 1998, 24, 619. (11) Xu, J. L.; Selinger, R. L. B.; Selinger, J. V.; Shashidhar, R. J. Chem. Phys. 2001, 115, 4333. (12) Johnston, S. J.; Low, R. J.; Neal, M. P. Phys. ReV. E 2002, 65, 051706. (13) Johnston, S. J.; Low, R. J.; Neal, M. P. Phys. ReV. E 2002, 66, 061702. (14) Lansac, Y.; Maiti, P. K.; Clark, N. A.; Glaser, M. A. Phys. ReV. E 2003, 67, 011703. (15) Dewar, A.; Camp, P. J. Phys. ReV. E 2004, 70, 011704. (16) Orlandi, S.; Berardi, R.; Steltzer, J.; Zannoni, C. J. Chem. Phys. 2006, 124, 124907. (17) Dewar, A.; Camp, P. J. J. Chem. Phys. 2005, 123, 174907. (18) Imase, T.; Kawauchi, S.; Watanabe, J. J. Mol. Struct. 2001, 560, 275. (19) Dong, R. Y.; Fodor-Csorba, K.; Xu, J.; Domenici, V.; Prampolini, G.; Veracini, C. A. J. Phys. Chem. B 2004, 108, 7694.

10.1021/la0620253 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/28/2006

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Figure 2. Φ1L torsion angles during MD simulations for bent-core molecules on a water surface. (a) Bc-2H and (b) Bc-2Cl. The dashed lines represent approximate positions of minimum-energy positions of discrete torsional states.

address the interaction with a water surface (because of the excessive computational demands). The MD simulations are carried out on bent-core molecules both in vacuo and on a water surface. The bent-core molecules are modeled using the general AMBER force field (GAFF)22 with atomic partial charges assigned using the BCC chargefitting method.23 The water molecules are modeled with the TIP3P water potential.24 All simulations are carried out with the AMBER 8 package.25 First, an annealing procedure is carried out in vacuo for each type of bent-core molecule to get starting configurations for subsequent simulations. This procedure is necessary because some of the torsional barriers are very large and the system would not equilibrate at 300 K on the time scale of the simulations. For the annealing, a series of MD simulations are run starting at T ) 800 K and decreasing to 0 K in steps of 50 K. The high initial temperature allows all degrees of freedom to equilibrate (which makes the initial configuration inconsequential). The total duration of these annealing simulations is 110 ns for each molecule, where the step changes in temperature are evenly spaced during the simulation. The final configurations from these annealing simulations are then used as starting configurations for 15 ns in vacuo simulations at 300 K. The simulations of a bent-core molecule on the water surface are carried out with periodic boundary conditions in all three dimensions. The size of the simulation cell is 67.1 Å × 66.9 Å × 150.3 Å; the large z dimension limits the effects of periodicity along the z direction. A total of 2883 water molecules are included in the simulation, which creates a water slab ∼20 Å thick. An MD simulation is first run on a water slab to obtain an equilibrated water slab system (i.e., independent of the initial configuration). A single bent-core molecule with the structure obtained from the in vacuo annealing simulations is then placed ∼10 Å above the water surface; the bent-core molecule moves to the water surface within the first 50 ps of the simulation. (20) Cacelli, I.; Prampolini, G. Chem. Phys. 2005, 314, 283. (21) Krichnan, S. A. R.; Weissflog, W.; Pelzl, G.; Diele, S.; Kresse, H.; Vakhovskaya, Z.; Friedemann, R. Phys. Chem. Chem. Phys. 2006, 8, 1170. (22) Wang, J. M.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A. J. Comput. Chem. 2004, 25, 1157. (23) Jakalian, A.; Jack, D. B.; Bayly, C. I. J. Comput. Chem. 2002, 23, 1623. (24) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J.; Impey, R. W.; Klein, M. L. J. Chem. Phys. 1983, 79, 926. (25) Case, D. A.; Darden, T. A.; Cheatham, T. E., III; Simmerling, C. L.; Wang, J.; Duke, R. E.; Luo, R.; Merz, K. M.; Wang, B.; Pearlman, D. A.; Crowley, M.; Brozell, S.; Tsui, V.; Gohlke, H.; Mongan, J.; Hornak, V.; Cui, G.; Beroza, P.; Schafmeister, C.; Caldwell, J. W.; Ross, W. S.; Kollman, P. A. AMBER 8; University of California: San Francisco, 2004.

The simulations are run at constant temperature using a Langevin thermostat26 and are carried out at 300 K. (The annealing procedure also includes runs at other temperatures.) The Coulombic interactions are calcuated with the particle mesh Ewald method,27 and non-Coulombic interactions are cut off at a distance of 10 Å to reduce the computational demands of the simulations. The SHAKE algorithm28 is used to constrain the C-H and O-H bonds, which allows a larger time step to be used. The equations of motion are integrated with the leapfrog method; a time step of 2 fs is used in the simulations with the water (this time step is recommended by the AMBER User’s Manual for simulations of organic and aqueous systems at 300 K with the SHAKE algorithm25), and a time step of 0.5 fs is used in the in vacuo simulations (a smaller time step is necessary because of the higher temperatures used during the annealing procedure). The simulations in vacuo are carried out for 15 ns, and the simulations with water are carried out for 10 ns.

Results At 300 K, the bent-core portion of the molecule remains relatively rigid while the terminal alkyl chains undergo floppy motion. The structure of the bent core is determined by the torsion angles along the bonds that connect the phenyl rings. Torsion angles Φ1L and Φ1R (shown in Figure 1a) have the largest impact on the molecular structure because they exhibit multiple torsional states with similar energies. Figure 2 shows that multiple torsional states are visited for Φ1L and Φ1R during the simulation, which implies that the states are similar in energy and that the barriers between states are low enough for the system to move between states and equilibrate on the time scales of the simulation. (These results are for simulations on the water surface; similar results are obtained in vacuo.) In contrast, the other torsions in the bent-core portion have a single energy state that is substantially lower in energy than the others, with large barriers between states. The convention used here in defining these torsion angles is such that Φ1L and Φ1R are equal to zero for the configuration shown in Figure 1a and Φ1L and Φ1R have the same sign when the carbonyl oxygens are on opposite sides of the plane of the central phenyl ring. The Bc-2H molecule in vacuo is considered first. The results for the probability distributions of Φ1L and Φ1R are shown in (26) Pastor, R. W.; Brooks, B. R.; Szabo, A. Mol. Phys. 1988, 65, 1409. (27) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen, L. G. J. Chem. Phys. 1995, 103, 8577. (28) Ryckaert, J. P.; Ciccotti, G.; Berendsen, H. J. C. J. Comput. Phys. 1977, 23, 327. (29) Humphrey, W.; Dalke, A.; Schulten, K. J. Mol. Graph. 1996, 14, 33.

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π Figure 3. Contour plots showing the probability P(Φ1L,Φ1R) defined such that ∫-π P(Φ1L,Φ1R) dΦ1L,Φ1R ) 1. Each contour line represents an increase of 0.0004 in probability. Results for the full Φ1L - Φ1R phase space have been folded onto the two unique quadrants. (a) Bc-2H in vacuo, (b) Bc-2H on the water surface, (c) Bc-2Cl in vacuo, and (d) Bc-2Cl on the water surface.

Figure 4. Position and orientation of a molecule with respect to the water surface. Dashed lines, Bc-2H; solid lines, Bc-2Cl. (a) Probability P(θ1), where θ1 is the angle between the plane of the phenyl ring and the plane of the water surface. P(θ1) is defined such that (180/π) ∫π/2 0 P(θ1) sin(θ1) dθ1 ) 1. The top, middle, and bottom frames represent the results for the central, inner, and outer phenyl rings, respectively. (b) Probability P(z - z0), where z - z0 is the height above the water surface for different parts of the molecule relative to the height of central phenyl ring. The top, middle, and bottom frames represent the results for the inner phenyl ring of the wing, the outer phenyl ring, and the terminal carbon atom of the alkyl chain.

Figure 3a. These results indicate the presence of distinct states characterized by local maxima in the probability distribution. Torsions Φ1L and Φ1R each have 4 stable states (see also Figure 2a), as also found in previous investigations,15,17 thus there are 16 stable states in Φ1L - Φ1R space. Because the states related by inversion in Φ1L - Φ1R space are equivalent, only eight of these states are unique. (Furthermore, two pairs of these eight states are geometrically equivalent.) Note that the probability of one of these states is very low, so its peak is not evident in Figure 3a. Four of these states have the carbonyl oxygens on the opposite side of the plane of the central phenyl ring, and the other four states have the carbonyl oxygens on the same side of the plane of the central phenyl ring. The states with carbonyl oxygens on the opposite side are more probable and account for 64% of the configurations occurring during the simulation (e.g., Figure 1b). This result agrees with density functional theory (DFT) studies that show that the lowest-energy configurations in vacuo occur when the carbonyl oxygens are on opposite sides of the plane of the central phenyl ring.15,17 This agreement with results from

a very different (and more rigorous) methodology supports the reliability of our results. The interaction of the molecule with the water surface has a significant effect on the structure of the molecule. Figure 3b shows the results for the probability distributions of Φ1L and Φ1R when Bc-2H is on the water surface. These results are fundamentally different than the results in vacuo in that the probability is much higher for states in which Φ1L and Φ1R have the opposite sign; these states now account for 88% of the configurations occurring during the simulation. Therefore, the carbonyl oxygens tend to be on the same side of the plane of the central phenyl ring when the molecule is on a water surface (e.g., Figure 1c), in direct contrast to the structure in vacuo. The effects of chlorine substitution (at points X in Figure 1) on the molecular structure can be seen most strikingly in the torsion angles given for Bc-2Cl in Figure 3c (in vacuo) and d (on the water surface). As for Bc-2H, for Bc-2Cl the carbonyl oxygens tend to be on the same side of the plane of the central phenyl ring when the molecule is on a water surface (Figure 1c)

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but tend to be on opposite sides in vacuo (Figure 1b). However, the Bc-2Cl structure is fundamentally different from the Bc-2H structure in that torsions Φ1L and Φ1R each have two stable states rather than four (see also Figure 2b). Thus, there are only two unique stable states in Φ1L - Φ1R space: one of these states is strongly favored in vacuo (85% probability), and the other is strongly favored on the water surface (96% probability). The position and orientation of the molecules with respect to the water surface are similar for the Bc-2H and Bc-2Cl molecules. Figure 4a shows that the central and outer phenyl rings tend to be flat with respect to the water surface whereas the inner wing phenyl ring takes on a broad range of orientations. Figure 4b shows that the alkyl chain sits farthest above the water and that the wing phenyl rings sit below the central phenyl ring. Although it is expected that the alkyl chain sits farthest above the water, it is at first surprising that the wing phenyl rings sit lower than the central phenyl ring. However, the reason for this effect is clear from the results presented above: (i) the central phenyl ring lies flat on the water surface and (ii) the Φ1L and Φ1R torsions are such that both carbonyl oxygens are directed toward the water surface. A consequence of these two results is that the wing phenyl rings sit below the central phenyl ring (Figure 1c)

Discussion and Conclusions The present simulations demonstrate two significant effects regarding the structure of bent-core molecules on water surfaces: (1) The structures of bent-core molecules are fundamentally different on a water surface than in vacuo. Previous theoretical work addressed only the bent-core molecules in vacuo and showed that the lowest-energy configurations occur when the carbonyl oxygens are on opposite sides of the plane of the central phenyl ring.15,17 However, the in vacuo results are not relevant when the molecules are on a water surface, and the interaction with the water surface causes both carbonyl oxygens to be located on the same side of the plane of the central phenyl ring. The physical origin of this water-surface-induced conformational change can be rationalized in the following way. The carbonyl oxygen atoms are the most hydrophilic groups on the molecule. Flipping one of the Φ1 torsion angles so that the two carbonyl oxygen atoms are on the same side of the plane of the benzene ring allows these hydrophilic groups to interact more directly with the water surface.

The interaction with the water surface can affect the chirality of the structuressBc-2Cl is in a chiral configuration in vacuo but in an achiral configuration on the water surface. For Bc-2H, the molecule is delocalized between chiral and achiral configurations both in vacuo and on the water surface. Similar structural changes may be induced by other surfaces, including solid surfaces. Such structural changes may partially explain why these materials are difficult to align in that frustration can arise if one type of configuration is favored in the bulk but a different type of configuration is favored on a surface. (2) The substitution of two chlorine atoms for hydrogen atoms on the central phenyl ring has an important effect on the nature of the structure at room temperature in that it forces the bent core to remain in a single state rather than be delocalized over several states. The Bc-2H molecule has a delocalized structure in which the system moves through several torsional states. In contrast, the Bc-2Cl molecule is localized to a single torsional state. The reason for this difference is the bulkiness of the chlorine atom, which destabilizes torsional states that have the carbonyl oxygen near the chlorine atom. A previous DFT study in vacuo for a bent-core molecule with a single chlorine substitution also showed that chlorine substitution destabilizes torsional states with the carbonyl oxygen near the chlorine atom;17 however, the singly substituted molecule will still have a delocalized bent-core structure due to the existence of multiple torsional states on the nonsubstituted side of the molecule. This torsional delocalization may elucidate experimental observations in regard to the formation of aligned monolayers on water surfaces. It has been found that well-aligned monolayers can be formed for Bc-2Cl on water7 but not for Bc-2H.6 Our results suggest that a possible explanation for this observation is that the torsional delocalization in the Bc-2H molecule inhibits the molecular packing necessary for alignment. Acknowledgment. This material is based upon work supported by the National Science Foundation under grant no. DMR0402867 and in part by a grant of computing time from the Ohio Supercomputer Center. We thank Professor J. Adin Mann for helpful discussions. LA0620253