Enforced stacking in crowded arenes [21]

Full text of DOI:10.1021/ja0104148

J. Am. Chem. Soc. 2001, 123, 8157-8158
Enforced Stacking in Crowded Arenes
8157
Mark L. Bushey,‡ Austin Hwang,‡ Peter W. Stephens,§ and
Colin Nuckolls*,‡
Department of Chemistry, Columbia UniVersity
New York, New York 10027
Department of Physics and Astronomy
State UniVersity of New York at Stony Brook
Stony Brook, New York 11974
Figure 1. Energy minimized dimeric model. Methyls on the ether
oxygens were included in the minimization and removed to clarify the
view.
ReceiVed February 16, 2001
ReVised Manuscript ReceiVed May 8, 2001
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 metal-
ligand 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.
Scheme 1
Table 1. Transition Temperatures (°C) and Enthalpies (kJ/mol) for
1a-d
heating cycle
cooling cycle
1a 98 (57.1)
1b 82 (11.1) 176 (7.6) 232 (4.0)
1c 91 (2.9)
294 (35.1) 247 (-5.6) 83 (-39.1)
227 (-3.0)
123 (27.4) 107 (-21.3)
104 (-7.1)
66 (-4.4)
1d 47 (25.0)
85 (6.3) 200 (18.6) 189 (-8.6)
Shown in Figure 1 is the lowest energy8 dimer of a benzene
ring that is alternatingly substituted with methoxyls and methyl-
amides. 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).
‡ 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, 2252-
68.
(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.
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 fifth-
order 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
(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°.
10.1021/ja0104148 CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/31/2001

8158 J. Am. Chem. Soc., Vol. 123, No. 33, 2001
Communications to the Editor
Figure 2. X-ray diffraction from 1b at 200 °C.
Figure 4. (a) X-ray diffraction of 1d at 120 °C; (b) polarized light
micrograph of 1d at 180 °C; (c) hexagonal features (2× magnification);
and (d) homeotropic alignment.
frustrates hexagonal packing, or are bulky, which tilts or offsets
the subunits.
The birefringent texture of 1d is shown in Figure 4b. In thicker
regions of the sample, linear defects occur, resembling those seen
in other homeotropically grown discotic liquid crystals.19 In
thinner regions, the material is weakly birefringent with large areas
of blackness. The hexagonal pattern in Figure 4c is diagnostic19,20
of structures having columnar axes perpendicular to surfaces.21
Therefore, the orientations of the columns are opposite for 1d
and 1b. Possibly, the ester-containing side chains act as anchors
to the surface.15b
Figure 3. (a) Optical micrograph of 1b cooled from an isotropic liquid;
(b) thinner sample with a unit retardation plate at a 45° angle; and (c)
orientation of the columns.
agreement with the ca. 27 Å expected for columns with nonin-
terdigitating, extended side chains. There is one reflection (4.91
Å) that could not be indexed.
The polarized light micrograph of 1b shown in Figure 3a is
seen when samples are cooled from their clearing temperatures.
It is uniform over seVeral hundred micrometers. The areas of
extinction in the pattern are aligned along the polarizer/analyzer
axes and are invariant with the sample’s rotation (spherulitic
domains).13 A unit retardation plate allows the direction of column
growth to be determined.14 As shown in Figure 3b, the pattern is
shifted to yellow along the slow vibration ray of the plate and
blue along the fast vibration ray.14a The lower index of refraction,
and therefore the long axis of the columns,15 radiates outward
from a central defect point to the edge of the pattern (Figure 3c).13
The pattern is not typical of a discotic liquid crystal and may
indicate that 1b forms plastic or soft crystals.
For 1d the two phases below 85 °C are shown by X-ray
diffraction to be crystalline polymorphs.9 Above 85 °C, a fluid
phase develops that is easily aligned by shearing.16 The diffraction
pattern seen at 120 °C (Figure 4a) has reflections that index to a
rectangular lattice with parameters a ) 38 Å and b ) 22 Å.
Rectangular packing has been seen in other columnar liquid
crystals17 and results from a distortion of the lattice. For 1d the
lattice deviates by only 1% from hexagonal.18 This distortion could
arise because the side chains are mismatched in size, which
The birefringence that develops for 1a, after supercooling for
47 °C, is similar to that of 1b. Diffraction from 1a after it was
cooled from an isotropic liquid showed a two-dimensional
hexagonal lattice of columns.9 Diffraction from 1c at 100 °C, on
the heating or the cooling cycle, is consistent with a columnar
organization, but a multitude of other peaks indicate a crystalline
phase.9
In summary, a new class of molecules is put forth that assemble
into columnar superstructures whose exteriors are coated with a
variety of functional groups. They may have useful properties22
because each subunit’s dipole moment should be amplified when
it assembles into stacks.
Acknowledgment. A portion of this work was supported by seed
funding from the MRSEC Program of the National Science Foundation
(DMR-9809687). The Brookhaven National Laboratory NSLS is sup-
ported by the U.S. D.O.E., Divisions of Chemical and Materials Sciences.
The SUNY X3 beamline at the NSLS is supported by the Division of
Basic Energy Sciences of the U.S. D.O.E. (DE-FG02-86ER45231).
Supporting Information Available: Experimental details and X-ray
diffraction patterns (1a,c,d) (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
JA0104148
(13) Billard, J. Springer Ser. Chem. Phys. 1980, 11, 383-95.
(14) (a) Hartshorne, N. H.; Stuart, A. Crystals and the Polarising Light
Microscope, 3rd ed.; Edward Arnold Ltd.: London, 1960; p 290ff. (b)
Lovinger, A. J.; Nuckolls, C.; Katz, T. J. J. Am. Chem. Soc. 1998, 120, 264-
268.
(19) (a) Destrade, C.; Foucher, P.; Gasparoux, H.; Nguyen, H. T.; Levelut,
A. M.; Malthete, J. Mol. Cryst. Liq. Cryst. 1984, 106, 121-46. (b) Billard,
J.; Dubois, J. C.; Nguyen H. T.; Zann, A. NouV. J. Chim. 1978, 2, 535-40.
(20) Bouligand, Y. J. Phys. (Orsay, Fr.) 1980, 41, 1307-15.
(21) Similar to: (a) Hudson, S. D.; Jung, H. T.; Percec, V.; Cho, W.-D.;
Johansson, G.; Ungar, G.; Balagurusamy, V. S. K. Science 1997, 278, 449-
452. (b) Percec, V.; Johansson, G.; Ungar, G.; Zhou, J. J. Am. Chem. Soc.
1996, 118, 9855-9866.
(22) For example: (a) Kilian, D.; Knawby, D.; Athanassopoulou, M. A.;
Trzaska, S. T.; Swager, T. M.; Wrobel, S.; Haase, W. Liq. Cryst. 2000, 27,
509-521. (b) Jaworek, T.; Neher, D.; Wegner, G.; Wieringa, R. H.; Schouten,
A. J. Science 1998, 279, 57-60.
(15) (a) Crystals: Sundararajan, K. S. Z. Kristallogr. 1936, 93, 238-40.
(b) Discotics: Vauchier, C.; Zann, A.; Le Barny, P.; Dubois, J. C.; Billard, J.
Mol. Cryst. Liq. Cryst. 1981, 66, 423-33.
(16) Similar to discotics: (a) Takabatake, M.; Iwayanagi, S. Jpn. J. Appl.
Phys., Pt. 2 1982, 21, 685-6. (b) Reference 21.
(17) (a) Lai, C. K.; Serrette, A. G.; Swager, T. M. J. Am. Chem. Soc. 1992,
114, 7948-7949. (b) Billard, J.; Dubois, J. C.; Vaucher, C.; Levelut, A. M.
Mol. Cryst. Liq. Cryst. 1981, 66, 115-122.
(18) The distortion is given by 2(d20 - d11)/(d20 + d11) (ref 17b).