Enforced stacking in crowded arenes.

Disk-shaped π-surfaces that stack to form columnar structures1 are prototypes of molecular-scale wires that have an insulating hydrocarbon sheath surrounding a conductive aromatic core.2 Typically, the strengths of the associations between the molecules, formed through contacts between aromatic surfaces, are weak. Previous schemes to modulate these strengths are based on metalligand interactions,3 π-donor/acceptor pairs,4 and hydrogen bonds.5 The study described below considers whether substituents in the 2,4,6-positions can force 1,3,5-triamides into conformations favorable for intermolecular hydrogen bonding.6 Suprisingly, there are no examples of benzene rings with secondary amides at the 1,3,5-positions with any substituents other than hydrogen at the remaining positions.7 Described below are syntheses of the first members of this new class of molecules (1a-d) and studies showing that they self-assemble into columns. Their physical properties show that 1d forms a liquid crystalline phase with its columns perpendicular to the surface and that 1b forms a highly ordered phase whose columns are parallel to the surface. Shown in Figure 1 is the lowest energy8 dimer of a benzene ring that is alternatingly substituted with methoxyls and methylamides. To relieve steric congestion, the amides twist out of the aromatic plane by ca. 45° allowing three intermolecular hydrogen bonds, while π-surfaces are “in-registration”, stacked 3.8 Å apart. Key to the synthesis of this class of molecules9 (Scheme 1) was the discovery that 1,3,5-tribromo-2,4,6-tridodecyloxylbenzene (2) undergoes a triple lithium/halogen exchange at -78 °C.10 After quenching with methyl chloroformate, 4 is produced in an unoptimized 30% yield on a 4-g scale. The target structures 1a-d are then synthesized in three steps: saponification, conversion to 5, and reaction with primary amines (75-81% yield). The synthesis is both expeditious and flexible. As shown in Table 1, 1a-d undergo an initial thermal transition between 47 and 98 °C. At higher temperatures, both 1a and 1c form isotropic liquids. In contrast, 1b (at 176-232 °C) and 1d (at 85-200 °C) form another phase before becoming isotropic. Upon cooling the isotropic liquids, 1b and 1d undergo phase transitions (1b, 3 kJ/mol, and 1d, 8.6 kJ/mol) with enthalpies similar to those observed for discotic liquid crystals (ca. 1-20 kJ/mol).1,3-5 Indicative of hydrogen bonds forming in the mesophases,5f the N-H stretching frequency of 1b shifts from 3295 (at 200 °C) to 3361 cm-1 (at 250 °C), and for 1d it shifts from 3282 (at 135 °C) to 3374 cm-1 (at 220 °C). Displayed in Figure 2 is the diffraction pattern of synchrotron radiation (λ ) 1.151 Å) by 1b at 200 °C. The diffractogram is dominated by a single sharp peak at low angle, diagnostic of columnar assemblies.11 Remarkably, diffraction peaks up to fifthorder are seen that can be indexed to a hexagonal lattice. The diffuse reflection at ca. 4.5 Å arises from the fluidlike packing of side chains.11 The lateral core-to-core separation12 is 21 Åsin ‡ Columbia University. § State University of New York at Stony Brook. (1) (a) Guillon, D. Struct. Bonding 1999, 95, 41-82. (b) Chandrasekhar, S.; Ranganath, G. S. Rep. Prog. Phys. 1990, 53, 57-84. (2) (a) Van de Craats, A. M.; Warman, J. M.; Fechtenkotter, A.; Brand, J. D.; Harbison, M. A.; Mullen, K. AdV. Mater. 1999, 11, 1469-1472. (b) Chandrasekhar, S.; Prasad, S. K. Contemp. Phys. 1999, 40, 237-245. (c) Boden, N.; Bushby, R. J.; Clements, J.; Movaghar, B. J. Mater. Chem. 1999, 9, 2081-2086. (3) (a) Metallomesogens; Serrano, J. L., Ed.; VCH: New York, 1996. (b) Simon, J.; Bassoul, P. In Phthalocyanines: Properties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: New York, 1989; Vol. 2, Chapter 6. (c) Serrette, A. G.; Lai, C. K.; Swager, T. M. Chem. Mater. 1994, 6, 225268. (4) (a) Bengs, H.; Ebert, M.; Karthaus, O.; Kohne, B.; Praefcke, K.; Ringsdorf, H.; Wendorff, J. H.; Wuestefeld, R. AdV. Mater. 1990, 2, 141-4. (b) Weck, M.; Dunn, A. R.; Matsumoto, K.; Coates, G. W.; Lobkovsky, E. B.; Grubbs, R. H. Angew. Chem., Int. Ed. Engl. 1999, 38, 2741-2745. (5) (a) Paleos, C. M.; Tsiourvas, D. Angew. Chem., Int. Ed. Engl. 1995, 34, 1696-711. (b) Brienne, M.-J.; Gabard, J.; Lehn, J.-M.; Stibor, I. J. Chem. Soc., Chem. Commun. 1989, 1868. (c) Goldmann, D.; Dietel, R.; Janietz, D.; Schmidt, C.; Wendorff, J. H. Liq. Cryst. 1998, 24, 407-411. (d) Ungar, G.; Abramic, D.; Percec, V.; Heck, J. A. Liq. Cryst. 1996, 21, 73-86. (e) Percec, V.; Ahn, C.-H.; Bera, T. K.; Ungar, G.; Yeardley, D. J. P. Chem.-Eur. J. 1999, 5, 1070-1083. (f) Matsunaga, Y.; Miyajima, N.; Nakayasu, Y.; Sakai, S.; Yonenaga, M. Bull. Chem. Soc. Jpn. 1988, 61, 207-10. (g) Brunsveld, L.; Zhang, H.; Glasbeek, M.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 6175-6182 and references therein. (h) Malthete, J.; Levelut, A. M.; Liebert, L. AdV. Mater. 1992, 4, 37-41. (i) Pucci, D.; Veber, M.; Malthete, J. Liq. Cryst. 1996, 21, 153-155. (6) Benzenes with meta-disposed secondary amides form columnar liquid crystals (refs 5f-i). (7) No associative properties were reported for examples with primary amides: (a) Wallenfels, K.; Witzler, F.; Friedrich, K. Tetrahedron 1967, 23, 1845-55. (b) Kolotuchin, S. V.; Thiessen, P. A.; Fenlon, E. E.; Wilson, S. R.; Loweth, C. J.; Zimmerman, S. C. Chem.-Eur. J. 1999, 5, 2537-2547. (8) MacroModel v.7.0 (Amber*): Mohamadi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.; Lipton, M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C. J. Comput. Chem. 1990, 11, 440-67. (9) Experimental details are in the Supporting Information. (10) With methyls ethers this reaction was inoperably low yielding: Engman, L.; Hellberg, J. S. E. J. Organomet. Chem. 1985, 296, 357-66. (11) For diffraction from discotics: (a) Levelut, A. M. J. Chim. Phys. Phys.Chim. Biol. 1983, 80, 149-61. (b) The citations in refs 1 and 3-5. (12) Given by d100/cos30°. Figure 1. Energy minimized dimeric model. Methyls on the ether oxygens were included in the minimization and removed to clarify the view.