Design of Potentially Photorefractive Liquid Crystalline Materials

May 27, 2005 - Maura Belloni, Benson M. Kariuki,* M. Manickam, John Wilkie, and. Jon A. Preece*. School of Chemistry, University of Birmingham, Edgbas...
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Design of Potentially Photorefractive Liquid Crystalline Materials: Derivatives of 3,6-Disubstituted Carbazole Maura Belloni, Benson M. Kariuki,* M. Manickam, John Wilkie, and Jon A. Preece*

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 4 1443-1450

School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom Received December 13, 2004;

Revised Manuscript Received March 24, 2005

ABSTRACT: The incorporation of photorefractive molecular units, such as carbazole, into anisotropic materials (liquid crystals) may offer many advantages over conventional electrical poling of photorefractive polymers. Thus, a series of symmetric 3,6-disubstituted carbazole derivatives with a bent molecular shape have been synthesized and characterized. The influence of the nature of the symmetric substituents on the carbazole is investigated through comparison of crystal packing and electrostatic potentials. Introduction In synthetic chemistry, carbazole has been extensively exploited as a convenient building block for the development of biologically active compounds,1-3 photorefractive materials,4-15 and more recently, liquid crystals.16-23 The interest in photorefractive materials24 lies in their numerous potential technological applications,25 such as high-density optical data storage, multiple image processing, phase conjugated mirrors,26 simulation of neural networks, dynamic holography, programmable optical interconnectors, optical computing, parallel optical logic, and pattern recognition.27 Much research has recently been devoted to organic photorefractive materials24 to circumvent the limitations27 associated with crystalline inorganic compounds.28-30 One of the limitations of organic photorefractive materials is the requirement for noncentrosymmetric alignment by application of a high electric field (as large as 900 kV cm-1)16 for the device to function. Because of the potential for degradation of the material, any development toward the elimination of the need for high electric fields would be desirable. For instance, the combination of photorefractivity with liquid crystallinity would represent a significant step forward in materials chemistry of photorefractive materials. In such materials, the molecules would already be anisotropically aligned and, therefore, require only the application of small electric fields to achieve noncentrosymmetric alignments. The carbazole molecular moiety4-15 has been exploited in organic photorefractive materials in both polymeric and low-molecular weight systems.9-14 Introduction of substituents on the 3- and 6-positions of carbazole represents a possible approach for designing carbazolebased photorefractive materials.24 Additionally, by extending the central aromatic core, and introducing flexible alkyl chains into the periphery of the molecular architecture as required for liquid crystals design,31 it should be possible to obtain a structure that is both * Corresponding authors. (B.M.K.) Telephone: +44 (0) 12 1 414 7481; fax: +44 (0) 121 414 4403; e-mail: [email protected]. (J.A.P.) Telephone: + 44 (0) 12 1 414 3528; fax: + 44 (0) 121 414 4403; e-mail: [email protected].

photorefractive and liquid crystalline and, therefore, superior for photorefractive applications.32 Previously, we have described33 our molecular design criteria for chemically modifying carbazole at the 3- and 6-positions with 4-phenyl-alkoxy moieties (1a-f in Scheme 1) to create mesogenic banana-shaped carbazole derivatives, in the hope that these materials will exhibit liquid crystalline phases. Unfortunately, no mesophases have been observed in this series of compounds. Structure analysis highlighted to us that there were potentially at least two reasons why these materials were not able to support a mesophase. First, the exocyclic angle of the molecular bond is ca. 90°, which is much smaller than known banana mesogens that support mesophases.34,35 Second, the occurrence of the N-H‚‚‚π interaction between adjacent molecules leads to a herringbone packing arrangement, which clearly is not consistent with mesophase formation. Therefore, we have redesigned our molecular structure36 to address these shortcomings. To allow the exocyclic bend angle to open out, an imine unit has been incorporated between the substituent phenylene rings at the 3- and 6-positions of the carbazole. In addition, the N-H‚‚‚π interaction has been eliminated by methylating the carbozole nitrogen atom. The effect of methylation is illustrated using 3-5 (see Scheme 1), and the combined effect of both the presence of the imine group and the methylation is shown using 2a, 2d, and 2e. A variety of molecules incorporating the two modifications have been synthesized (2a-g), but none of the materials are liquid crystalline.36 However, they serve to illustrate the crystal design aspect. Experimental Procedures The sample of 3,6-dibromo-9H-carbazole36 (3) was synthesized as an intermediate in the preparation of methoxy (1a) and propyloxy (1b) derivatives. These are obtained by the symmetric introduction of 4-alkyloxyphenyl33 on the 3- and 6-positions of the central carbazole core via a standard Suzuki coupling.37,38 On the other hand, compounds 2a-g are based upon the introduction, at the 3- and 6-positions, of two aldehyde groups on 9-methyl-9H-carbazole36 (6), followed by treatment with the appropriate alkyl-aniline or alkyloxyaniline derivatives.36 3,6-Dibromo-9-methyl-carbazole (4) and 3-bromo-9H-carbazole (5) were obtained from commercial sources.

10.1021/cg049580s CCC: $30.25 © 2005 American Chemical Society Published on Web 05/27/2005

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Scheme 1. Derivatives of Carbazole

Table 1. Crystallographic Data for the Carbazole Derivatives

FW T (K) λ (Å) system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dc (M gm-3) µ (mm-1) size (mm) collected unique Rint R1 [2σ(I)] wR2 Flack parameter

2a

2d

2e

3

C35H37N3 499.68 200(2) 1.54178 monoclinic C2/c 17.661(4) 8.0855(19) 39.543(8) 90.065(8) 5647(2) 8 1.176 0.523 0.5 × 0.4 × 0.16 14991 3986 0.046 0.090 0.246

C59H85N3 836.30 296(2) 1.54178 monoclinic P21/c 9.0912(4) 7.8877(3) 72.467(3) 91.103(3) 5195.6(4) 4 1.069 0.453 0.4 × 0.25 × 0.25 23644 7265 0.032 0.062 0.139

C35H37N3O2 531.68 296(2) 0.71069 monoclinic P21/n 6.5057(10) 8.2178(11) 55.034(9) 91.781(2) 2940.8(8) 4 1.201 0.075 0.4 × 0.2 × 0.05 5696 2112 0.054 0.072 0.163

C12H7Br2N 325.01 296(2) 0.71069 A monoclinic P21 11.902(4) 11.061(3) 3.9951(13) 90.849(7) 525.9(3) 2 2.052 7.668 0.5 × 0.2 × 0.1 2403 1457 0.066 0.070 0.176 -0.11(5)

4 C13H9Br2N 339.03 296(2) 1.54178 orthorhombic Pca21 22.6571(3) 4.0725(1) 12.5717(2) 1160.0(4) 4 1.941 8.603 0.24 × 22 × 16 5767 1995 0.035 0.028 0.072 0.10(3)

5 C12H8BrN 246.10 293(2) 0.71069 monoclinic P21/a 7.9369(15) 5.8669(9) 20.412(3) 91.607(6) 950.1(3) 4 1.720 4.278 0.05 × 0.5 × 0.5 3581 1703 0.068 0.060 0.154

Figure 1. (a) Ortep representation of a molecule of 3. Views of the crystal structure (b) down the c axis with the NH‚‚‚Br contacts shown as dotted lines and (c) down the a axis.

Liquid Crystals: 3,6-Carbazole Derivatives

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Figure 2. (a) Ortep representation of a molecule of 5; (b) the crystal structure viewed down the b axis with the NH‚‚‚π contacts represented by dotted lines; and (c) a segment of the structure showing molecular packing in one layer.

Figure 3. (a) Ortep representation of a molecule of 4; (b) the crystal structure of 4 viewed down the b axis with CH‚‚‚Br contacts shown as dotted lines; and (c) a segment of the structure showing the tapes. Crystals for structure determination were grown by either the vapor diffusion method or slow evaporation of solvent (hexane/EtOAc) in a narrow tube. Single-crystal diffraction data for 2e, 3, and 5 were recorded on a Rigaku R-Axis IIc diffractometer equipped with an image plate detector system and a molybdenum rotating anode source. Data for 2a, 2d, and 4 were recorded on a Bruker Smart 6000 diffractometer equipped with a CCD detector and a copper tube source. The structures were solved and refined using SHELX97.39 A riding model was used for the hydrogen atoms with default bond distances. Crystallographic data are presented in Table 1.

Energy minimizations were carried out using the Gaussian98 program40 and 6-31G** basis set.41 Electrostatic potentials, based on the Gaussian charges, were displayed at the solvent accessible surface using GRASP.42 In the case of 2, R ) Me to reduce the orbital count and degrees of freedom, simplifying the calculation.

Results and Discussion N-H‚‚‚π Interaction and Crystal Packing. The crystal structures of 3,6-bis-(4-methoxyphenyl)-9H-car-

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Figure 4. Electrostatic potentials of 4-6 at the solvent accessible surface. Red regions are negative, and blue is positive.

Figure 5. (a) Ortep molecular representation of 2a; (b) a segment of the structure showing the double tapes; and (c) the crystal structure viewed down the b axis. Hydrogen atoms in panels b and c and some carbon atoms in panel b have been omitted for clarity.

bazole (1a) and 3,6-bis-(4-propyloxyphenyl)-9H-carbazole (1b) have been discussed previously.33 This previous study indicated that the presence of the N-H group in the pyrrole ring of 1a, 1b, and carbazole43 tends to favor the formation of N-H‚‚‚π intermolecular interactions and hence a herringbone-type arrangement of molecules in the crystal. To investigate this structural control further, a number of small carbazole derivatives has been characterized. These are 3 and 5 (which contain the pyrollo N-H group) and 4 (in which a methyl group has replaced the hydrogen atom). In the crystal structure of 3 (Figure 1), the molecules form stacks by translation through ca. 4 Å, along the c axis (Figure 1c). A view of the structure down the a axis (Figure 1c) shows a herringbone packing arrangement of molecules. However, in this case, the N-H bond is directed toward the C-Br bond of a molecule in a neighboring stack, with a distance from the N atom to the center of the C-Br bond of 3.80(1) Å (the H‚‚‚Br distance is 2.91(1) Å, and the N-H‚‚‚Br angle is 155.3(1)°). The N-H‚‚‚C-Br contact links the stacks to

form layers parallel to the bc plane. The closest intermolecular Br‚‚‚Br contact in the structure is 3.69(1) Å. In the crystal structure of 5 (Figure 2), the molecules form stacks parallel to the b axis (Figure 2b). The N-H bond is directed toward the unsubstituted aromatic ring of the carbazole moiety of a molecule in a neighboring stack, leading to the formation of layers parallel to the ab plane. The distance between the N atom and the center of the aromatic ring is ca. 3.67 Å (the distance for the H atom is 3.08(1) Å). Within a layer, a herringbone arrangement of the molecules is observed (Figure 2c). In the structure, the closest Br‚‚‚Br contact is 3.79(1) Å, and the molecules are oriented such that only Br‚‚‚Br contacts occur on one side of the layer. In the crystal structure of 4 (Figure 3), the molecules form stacks parallel to the b axis by translation through ca. 4 Å (perpendicular to the view in Figure 3b). These stacks are, in turn, arranged side by side to form layers parallel to the bc plane. Within each layer, the CH3 groups are oriented toward the Br atoms in adjacent stacks, resulting in an almost parallel arrangement

Liquid Crystals: 3,6-Carbazole Derivatives

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Figure 6. (a) Ortep molecular representation of 2d; (b) a segment of the structure showing molecular tapes; and (c) the crystal structure viewed down the b axis. Hydrogen atoms in panels b and c and some carbon atoms in panel b have been omitted for clarity.

Figure 7. Electrostatic potential of 2 (core) at the solventaccessible surface shown from above (left panel) and below (right panel). Red is negative, blue positive, contours as for Figure 4.

(ribbons) of molecules (Figure 3c). The angle between ribbons in neighboring layers is ca. 60°. The closest contact between Br atoms in this structure is 3.97(1) Å, but a close intermolecular H‚‚‚Br contact is observed (H‚‚‚Br distance is 2.98(1) Å, and C-H‚‚‚Br angle is 152.5(1)°). As expected, the crystal structures of 3 and 5 show the herringbone type arrangement consistent with the formation of the N-H‚‚‚π type interaction. Replacement of the N-H proton in 3 by a methyl group (leading to 4) results in the transformation of the crystal structure to the desired planar arrangement. However, the same effect is not observed for carbazole. It also has herringbone molecular arrangement,43 but substitution of the N-H proton by a methyl group gives 6, which has

a rather complex structure,44 in contrast to 4 in which the molecular packing is planar. These observations suggest that substitution of one or both protons in the 3- and 6-positions by larger groups may favor planar structures. Comparison of the electrostatic potentials of 4 and 5 with that of 6 shows that inclusion of Br at positions 3 or 6 extends the region of negative electrostatic potential to the periphery of the molecule rather than concentrating it in the core of the aromatic system (Figure 4). In addition, there is an increase in the positive potential between positions 4 and 5 on the aromatic system. These two effects combine to produce a linear quadrupole on the rear edge of the carbazole core in 4 that is not present in simple unsubstituted carbazoles and Nmethylated carbazoles. A similar linear quadrupole can be observed in 3. Molecular Bend and 3,6-Substitution. The molecules of 2a-g have the structural flexibility to provide a wider exocyclic bond angle when compared to the more rigid structures of 1a and 1b (which have exocyclic bond angles of ca. 90°). Samples of 2a, 2d, and 2e have been structurally characterized to probe this aspect. In the crystal structure of 2a, the phenyl-alkyl groups are disordered with the major and minor components having occupancies of ca. 52 and 48% (the figures shown are for the 52% component). The molecule (Figure 5a) is not planar, the angles between the plane of the carbazole moeity and those of the two phenyl rings being ca. 41 and -58°.

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Figure 8. (a) Ortep molecular representation of 2e; (b) a segment of the structure showing the tapes; and (c) the crystal structure viewed down the a axis. Hydrogen atoms in panels b and c and some carbon atoms in panel c have been omitted for clarity.

The carbazole moieties form ribbons in which the N-CH3 groups occupy the U slots of the adjacent banana-shaped molecules (Figure 5b). Parallel ribbons are stacked along the a axis resulting in layers parallel to the ab plane, composed of the carbazole groups on the inside and alkyl groups on the outside (Figure 5c). Neighboring layers are antiparallel. Within each layer, pairs of ribbons are offset with respect to the two neighboring pairs (Figure 5b). In addition, the phenyl groups of the substituents in neighboring molecules within the layer are almost perpendicular to each other. However, the exocyclic bond angle is ca. 90°, a value similar to those observed for the more rigid 1a and 1b, despite the less rigid molecular components in 2a. The conformation is clearly constrained by the crystal environment in this case. In the crystal structure of 2d, the molecule (Figure 6a) is also nonplanar. The angles between the carbazole moieties and the two phenyl rings are ca. 10 and -34°. Each of the alkyl chains adopts an all-trans conformation. One chain is located above the plane of the carbazole group and the other one below the plane, resulting in a torsion angle of ca. 47° between the two chains. The N-CH3 groups in the structure of 2d are oriented toward the U slots of adjacent molecules to form ribbons of carbazole moieties (Figure 6b). This is consistent with

the electrostatic potential (Figure 7), which shows a far greater accumulation of positive potential at N-Me as compared with molecules 4-6 (Figure 4). The positive potential at N-Me is then able to form favorable electrostatic interactions with a similarly large negative potential in the region of the imine group. Parallel ribbons are stacked to form layers parallel to the ab plane (Figure 6c), with the parallel alkyl chains located on either side of every layer. Within the layer, a given ribbon is offset (in the direction of the ribbon) by half a molecule relative to its two neighbors, and the ribbons in neighboring layers run in opposite directions. Although the phenyl rings are not perpendicular (ca. 135°), an edge-to-face relationship is observed between neighboring ribbons within the layer. Similar to 2a, crystal contraints do not allow the molecule of 2d to open up, the exocyclic bond angle being ca. 70°. In the crystal structure of 2e, the molecule (Figure 8a) is more planar when compared to the other structures discussed. The angles between the carbazole moeity and the two phenyl rings are ca. 13 and -12°. This is possibly because resonance is enhanced by the alkoxy oxygen, combined with the fact that the introduction of the imine unit removes any potential steric hindrance to ring coplanarity. In the crystal of 2e, the molecules form ribbons that, in this case, are not planar (Figure 8b)sthe plane of

Liquid Crystals: 3,6-Carbazole Derivatives

the carbazole moeity is twisted ca. 25° from the plane of the ribbon. The ribbons run parallel to the a axis and form layers by stacking parallel to the b axis (Figure 8c). Each ribbon is offset laterally (i.e., perpenducular to the direction of the ribbon) relative to its two neighbors, leading to interdigitation of the layers. In the structure, neighboring ribbons in the stack are antiparallel, and a given molecule is almost perpendicular to the nearest two molecules in neighboring ribbons. Thus, the phenyl ring of the substituent is almost perpendicular to the nonheterocyclic rings of the carbazole moeities of the two neighboring molecules. The exocyclic bond angle is ca. 100°, a value greater than those observed for 1a and 1b. This is proof that substitution of flexible groups into the 3- and 6-positions can result in greater exocyclic angles in favorable circumstances. Conclusion Modification of the molecular structure to exclude NH‚‚‚π interactions (by methylating the carbozole nitrogen atom) reduces the likelyhood of herringbone crystal packing. The methylcarbazole moiety tends to favor the formation of ribbons (as observed in 4), which are further stabilized by the imine group, as shown by energy minimization (2a and 2d). The ribbons are stacked to form layers that interact mainly through van der Waals interactions. Modification to enable the exocyclic bond angle to open out (by incorporation of an imine unit in the substituent) did not produce the desired results for 2a and 2d. The angles for the reported structures are the same or less than those for the more rigid 1a and 1b, but as the molecules are flexible, other phases could possibly be obtained under different crystallization conditions. The exocyclic bond angle for 2e is larger, showing that the desired structural properties have been designed into the molecules. Although the materials are not liquid crystalline,36 they illustrate that, at least in the crystalline state, our design criteria have partly succeeded. Acknowledgment. This work was supported by grants from the EPSRC and Leverhulme Trust. Supporting Information Available: X-ray crystallographic information files (CIF) as well as tables of coordinates, bond lengths, and angles for compounds 2a, 2d, 2e, 3, 4, and 5. This material is available free of charge via the Internet at http://pubs.acs.org.

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