Structural factors controlling the self-assembly of columnar liquid crystals

Article abstract of DOI:10.1021/ja0613198

A series of disc-shaped molecules were prepared by the condensation of 1,2-diamines with 2,3,6,7-tetrakis(hexyloxy)phenanthrene-9,10-dione to investigate the relationship between changes in molecular structure and the self-assembly of columnar liquid crystalline phases. A comparison of compounds with different core sizes indicated that molecules with larger aromatic cores had a greater propensity to form columnar phases, as did compounds substituted with electron-withdrawing groups. In contrast, molecules with electron-donating substituents were nonmesogenic. The clearing temperature of columnar phases increased linearly with the electron-withdrawing ability of the substituents, as quantified by Hammett σ-values. The observed trends can be rationalized in terms of the strength of π-π interactions between aromatic cores in the liquid crystalline phases and suggest that both electrostatic interactions and dispersion forces play important roles in the self-assembly of these materials.

Full text of DOI:10.1021/ja0613198

Published on Web 06/09/2006
Structural Factors Controlling the Self-Assembly of Columnar
Liquid Crystals
E. Johan Foster, R. Bradley Jones, Christine Lavigueur, and Vance E. Williams*
Contribution from the Department of Chemistry, Simon Fraser UniVersity, 8888 UniVersity
DriVe, Burnaby, British Columbia, Canada V5A 1S6
Received February 23, 2006; E-mail: vancew@sfu.ca
Abstract: A series of disc-shaped molecules were prepared by the condensation of 1,2-diamines with
2,3,6,7-tetrakis(hexyloxy)phenanthrene-9,10-dione to investigate the relationship between changes in
molecular structure and the self-assembly of columnar liquid crystalline phases. A comparison of compounds
with different core sizes indicated that molecules with larger aromatic cores had a greater propensity to
form columnar phases, as did compounds substituted with electron-withdrawing groups. In contrast,
molecules with electron-donating substituents were nonmesogenic. The clearing temperature of columnar
phases increased linearly with the electron-withdrawing ability of the substituents, as quantified by Hammett
σ-values. The observed trends can be rationalized in terms of the strength of π-π interactions between
aromatic cores in the liquid crystalline phases and suggest that both electrostatic interactions and dispersion
forces play important roles in the self-assembly of these materials.
Introduction
nanostructures represents a striking example of self-assembly
driven largely by π-π interactions. Any factor that alters the
Columnar liquid crystals have emerged as a promising class
of materials for light-emitting diodes,1 photovoltaic devices,2
and field effect transistors.3 These liquid crystals exhibit a host
of attractive properties, including high charge carrier mobilities,4
a lack of grain boundaries, and the potential to be uniformly
aligned.3,5-10 In addition to their practical importance, the ability
of disc-shaped molecules to spontaneously form columnar
strength of π-stacking between neighboring molecules should
therefore have a dramatic impact on the propensity of these
molecules to form columnar mesophases. Studying the relation-
ship between a molecule’s structure and its tendency to self-
assemble into columns can therefore provide valuable insight
into the nature and strength of noncovalent interactions between
discotic mesogens, in addition to facilitating the design of new
liquid crystalline materials.
Despite the large number of discotic mesogens that have been
prepared in the thirty years since their discovery, only a few
studies have attempted to relate phase behavior to factors such
as core size11-14 or substituents effects.15-20 Most series of
substituted discotic mesogens examined to date have been
(1) Freudenmann, R.; Behnisch, B.; Hanack, M. J. Mater. Chem. 2001, 11,
1618.
(2) Schmidt-Mende, L.; Fechtenkotter, A.; Mu¨llen, K.; Moons, E.; Friend, R.
H.; MacKenzie, J. D. Science 2001, 293, 1119.
(3) Van de Craats, A. M.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Watson,
M.; Mu¨llen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend, R. H. AdV. Mater.
2003, 15, 495.
(4) (a) Warman, J. M.; Van de Craats, A. M. Mol. Cryst. Liq. Cryst. 2003,
396, 41. (b) Van de Craats, A. M.; Warman, J. M. AdV. Mater. 2001, 13,
130. (c) Van de Craats, A. M.; Warman, J. M.; Mu¨llen, K.; Geerts, Y.;
Brand, J. D. AdV. Mater. 1998, 10, 36. (d) Van de Craats, A. M.; Warman,
J. M.; Fechtenkoetter, A.; Brand, J. D.; Harbison, M. A.; Mu¨llen, K. AdV.
Mater. 1999, 11, 1469. (e) Struijk, C. W.; Sieval, A. B.; Dakhorst, J. E.;
Van Dijk, M.; Kimkes, P.; Koehorst, R. B.; Donker, H.; Schaafsma, T. J.;
Picken, S. J.; Van de Craats, A. M.; Warman, J. M.; Zuilhof, H.; Sudholter,
E. J. R. J. Am. Chem. Soc. 2000, 122, 11057.
(5) (a) Vauchier, C.; Zann, A.; Lebarny, P.; Dubois, J. C.; Billard, J. Mol.
Cryst. Liq. Cryst. 1981, 66, 423. (b) Eichhorn, S. H.; Adavelli, A.; Li, H.
S.; Fox, N. Mol. Cryst. Liq. Cryst. 2003, 397, 347. (c) Hatsusaka, K.; Ohta,
K.; Yamamoto, I.; Shirai, H. J. Mater. Chem. 2001, 11, 423. (d)
Shklyarevskiy, I. O.; Jonkheijm, P.; Stutzmann, N.; Wasserberg, D.;
Wondergem, H. J.; Christianen, P. C. M.; Schenning, A.; de Leeuw, D.
M.; Tomovic, Z.; Wu, J. S.; Mu¨llen, K.; Maan, J. C. J. Am. Chem. Soc.
2005, 127, 16233. (e) Terasawa, N.; Monobe, H.; Kiyohara, K.; Shimizu,
Y. Chem. Commun. 2003, 1678.
(6) (a) Monobe, H.; Terasawa, N.; Kiyohara, K.; Shimizu, Y.; Azehara, H.;
Nakasa, A.; Fujihira, M. Mol. Cryst. Liq. Cryst. 2004, 412, 1839. (b)
Zimmermann, S.; Wendorff, J. H.; Weder, C. Chem. Mater. 2002, 14, 2218.
(7) Tracz, A.; Jeszka, J. K.; Watson, M. D.; Pisula, W.; Mu¨llen, K.; Pakula,
T. J. Am. Chem. Soc. 2003, 125, 1682.
(8) (a) Pisula, W.; Menon, A.; Stepputat, M.; Lieberwirth, I.; Kolb, U.; Tracz,
A.; Sirringhaus, H.; Pakula, T.; Mu¨llen, K. AdV. Mater. 2005, 17, 684. (b)
Pisula, W.; Tomovic, Z.; El Hamaoui, B.; Watson, M. D.; Pakula, T.;
Mu¨llen, K. AdV. Funct. Mater. 2005, 15, 893. (c) Pisula, W.; Tomovic, Z.;
Stepputat, M.; Kolb, U.; Pakula, T.; Mu¨llen, K. Chem. Mater. 2005, 17,
2641.
(9) Piris, J.; Debije, M. G.; Stutzmann, N.; Laursen, B. W.; Pisula, W.; Watson,
M. D.; Bjornholm, T.; Mu¨llen, K.; Warman, J. M. AdV. Func. Mater. 2004,
14, 1053.
(10) (a) Bunk, O.; Nielsen, M. M.; Solling, T. I.; Van de Craats, A. M.;
Stutzmann, N. J. Am. Chem. Soc. 2003, 125, 2252. (b) Laursen, B. W.;
Norgaard, K.; Reitzel, N.; Simonsen, J. B.; Nielsen, C. B.; Als-Nielsen, J.;
Bjornholm, T.; Solling, T. I.; Nielsen, M. M.; Bunk, O.; Kjaer, K.;
Tchebotareva, N.; Watson, M. D.; Mu¨llen, K.; Piris, J. Langmuir 2004,
20, 4139.
(11) Lau, K.; Foster, J.; Williams, V. Chem. Commun. 2003, 2172.
(12) Yatabe, T.; Harbison, M. A.; Brand, J. D.; Wagner, M.; Mu¨llen, K.; Samori,
P.; Rabe, J. J. Mater. Chem. 2000, 10, 1519.
(13) (a) Mu¨ller, G. R. J.; Meiners, C.; Enkelmann, V.; Geerts, Y.; Mu¨llen, K.
J. Mater. Chem. 1998, 8, 61. (b) Van de Craats, A. M.; Warman, J. M.
AdV. Mater. 2001, 13, 130. (c) Pisula, W.; Tomovic, Z.; Simpson, C.;
Kastler, M.; Pakula, T.; Mu¨llen, K. Chem. Mater. 2005, 17, 4296.
(14) (a) Msayib, K.; Mahkseed, S.; McKeown, N. B. J. Mater. Chem. 2001,
11, 2784. (b) Boden, N.; Bushby, R. J.; Headdock, G.; Lozman, O. R.;
Wood, A. Liq. Cryst. 2001, 28, 139.
(15) For recent reviews of structure property relationships of discotic mesogens
see: (a) Cammidge, A. N.; Bushby, R. J. In Handbook of Liquid Crystals;
Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-
VCH: New York, 1998; Vol. 2B, p 693. (b) Kumar, S. Liq. Cryst. 2004,
31, 1037.
(16) (a) Rose, B.; Meier, H. Z. Naturforsch. B: Chem. Sci. 1998, 53, 1031. (b)
Cammidge, A. N.; Gopee, H. Chem. Commun. 2002, 966.
9
10.1021/ja0613198 CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 8569-8574
8569

A R T I C L E S
Foster et al.
Scheme 1. Synthesis of Compounds 2-4a
Table 1. Phase Behavior of 2, 3a-k, and 4
a Reagents and conditions: (a) 1,2-ethylenediamine, AcOH; (b) AcOH;
(c) 3,4-diaminonaphthalene, AcOH.
obtained by post-functionalization of an existing triphenylene
core, an approach that tends to place a practical constraint on
the number of compounds that can be easily prepared and that
limits the scope of structure-property studies. In an attempt to
address this problem, we have undertaken the study of disc-
shaped molecules derived from the coupling of a series of 1,2-
diamines with a versatile precursor molecule, 2,3,6,7-tetra-
(hexyloxy)phenanthrene-9,10-dione21,22 1 (Scheme 1). The
intrinsic modularity of our synthetic approach, taken together
with the large number of ortho-diamines available as starting
materials, greatly facilitates the preparation of a broad family
of potential mesogens that should allow us to systematically
a Transition temperatures and enthalpies were determined by DSC (scan
rate ) 10 °C/min.). b Cr ) crystal, Colh ) columnar hexagonal, I )
isotropic. c Ref 22b.
probe the effects of functional groups and core size on columnar
self-assembly.
Results
(17) (a) Boden, N.; Bushby, R. J.; Lu, Z. B.; Cammidge, A. N. Liq. Cryst. 1999,
26, 495. (b) Henderson, P.; Kumar, S.; Rego, J. A.; Ringsdorf, H.;
Schumhmacher, P. J. Chem. Soc. Chem. Commun. 1995, 1059. (c) Rego,
J. A.; Kumar, S.; Ringsdorf, H. Chem. Mater. 1996, 8, 1402.
(18) (a) Praefcke, K.; Eckert, A.; Blunk, D. Liq. Cryst. 1997, 22, 113. (b) Boden,
N.; Bushby, R. J.; Cammidge, A. N.; Duckworth, S.; Headdock, G. J. Mater.
Chem. 1997, 7, 601. (c) Bushby, R. J.; Boden, N.; Kilner, C. A.; Lozman,
O. R.; Lu, Z. B.; Liu Q.; Thornton-Pett, M. A. J. Mater. Chem. 2003, 13,
470. (d) Boden, N.; Bushby, R. J.; Cammidge, A. N. Liq. Cryst. 1995, 18,
673. (e) Boden, N.; Bushby, R. J.; Cammidge, A. N.; Headdock, G. J.
Mater. Chem. 1995, 5, 2275. (f) Boden, N.; Bushby, R. J.; Cooke, G.;
Lozman, O. R.; Lu, Z. B. J. Am. Chem. Soc. 2001, 123, 7915.
(19) Debije, M. G.; Chen, Z. J.; Piris, J.; Neder, R. B.; Watson, M. M.; Mu¨llen,
K.; Wurthner, F. J. Mater. Chem. 2005, 15, 1270.
(20) (a) Babuin, J.; Foster, J.; Williams, V. E. Tetrahedron Lett. 2003, 44, 7003.
(b) Mohr, B.; Wegner, G.; Ohta, K. J. Chem. Soc. Chem. Commun. 1995,
995.
(21) Foster, E. J.; Babuin, J.; Nguyen, N.; Williams, V. E. Chem. Commun.
2004, 2052.
(22) (a) Mohr, B.; Enkelmann, V.; Wegner, G. J. Org. Chem. 1994, 59, 635.
(b) Foster, E. J.; Lavigueur, C.; Ke, Y. C.; Williams, V. E. J. Mater. Chem.
2005, 15, 4062.
2, 3a-k, and 4 were prepared by the condensation of the
appropriate 1,2-diamines with 1 in refluxing acetic acid. The
diamines employed in this study were all either obtained
commercially or prepared in one or two steps according to
literature procedures. The phase behaviors of these condensation
products were examined by polarized optical microscopy
(POM), differential scanning calorimetry (DSC), and variable
temperature X-ray diffraction (XRD) experiments, the results
of which are summarized in Tables 1 and 2. 2, 3a, 3g, 3h, 3j,
and 3k failed to exhibit any liquid crystalline phases but instead
melted directly from crystalline solids to isotropic liquids. The
fluorinated derivative 3b is not liquid crystalline on heating but
forms a monotropic liquid crystal phase upon cooling from the
isotropic state. On the basis of the dendritic texture observed
9
8570 J. AM. CHEM. SOC. VOL. 128, NO. 26, 2006

Columnar Self-Assembly
A R T I C L E S
Table 2. X-ray Diffraction Data of Liquid Crystalline Compounds
temperature
C)
d-spacing
(Å)
Miller index
(hkl)
phase
(
°
(lattice constant)
3c
170
150
150
16.3
4.9
3.5
17.3
4.3
3.5
(100)
alkyl halo
π-π
(100)
alkyl halo
π-π
(100)
Colh
(a ) 18.8 Å)
3da
3e
Colh
(a ) 20.0 Å)
16.4
9.6
(110)
Colh
4.4
3.5
16.7
9.3
alkyl halo
π-π
(a ) 19.0 Å)
Figure 2. Polarized optical photomicrographs of (a) 3i at 235 °C and (b)
4 at 167 °C (100× magnification).
3f
150
170
130
(100)
(110)
Colh
4.7
3.5
16.9
10.2
4.5
3.5
16.7
4.5
alkyl halo
π-π
(a ) 19.2 Å)
lattices. Broad peaks were also observed for all molecules at
larger angles that correspond to distances of approximately 4.5
and 3.5 Å, which were attributed to the alkyl chain halo and
π-π stacking distances, respectively. The presence of a π-π
peak in the XRD indicates that there is a degree of ordering
within the columns.
3i
(100)
(110)
Colh
(a ) 19.6 Å)
alkyl halo
π-π
(100)
alkyl halo
π-π
4
Colh
(a ) 19.3 Å)
3.5
Discussion
a Ref 22b.
The trends in phase behavior, summarized in Table 1,
demonstrate that even relatively small changes in the core
structure can have a dramatic impact on the ability of the
molecules to form liquid crystalline phases. Because the addition
of one or more substituents is necessarily accompanied by an
increase in the size of the molecule, we first wanted to address
the effect of core size on mesophase behavior. 2, 3a, and 4,
which were prepared by the condensation of 1 with 1,2-
ethylenediamine, 1,2-phenylenenediamine, and 3,4-diamino-
naphthalene, respectively, comprise a series in which the core
size increases progressively from four to six rings. Although
the two smaller members of this series, 2 and 3a, are non-
mesogenic, the naphthalene derivative 4 is liquid crystalline over
a relatively large temperature range (122.5-171.5 °C). This
implies that increased core size leads to a greater propensity to
form columnar phases, which is consistent with earlier observa-
tions that discotic mesogens with large cores tend to be liquid
crystalline over broad temperature ranges.11-14,16b,23
Figure 1. Polarized optical photomicrograph of monotropic phase of 3b
at 158 °C (100× magnification).
The effect of core size on the mesogenic behavior can be
understood in terms of the interactions between neighboring
molecules within a columnar phase. Current models of π-π
interactions indicate that electrostatic and dispersion forces are
the most important contributors to the overall energy of most
π-stacked structures.24-29 Because dispersion forces are favored
by POM (Figure 1), this monotropic phase was identified as a
hexagonal columnar phase (Colh). Unfortunately, due to the
thermal instability of this phase, we were unable to confirm its
identity by XRD experiments.
Examination of the remaining compounds by DSC revealed
that each undergoes two phase transitions upon heating, which
were identified by polarized optical microscopy as solid-to-liquid
crystal and liquid crystal-to-isotropic liquid transitions, respec-
tively. Samples cooled slowly into their liquid crystalline phase
from the isotropic state exhibited dendritic textures when viewed
by polarized optical microscopy. Representative optical micro-
graphs are shown in Figure 2. These textures are typical of
columnar phases, and the observation of domains with ap-
proximately 6-fold symmetry suggests that these are Colh phases.
XRD experiments corroborate this assignment. Small-angle
X-ray diffractograms of the liquid crystalline phases of 3e, 3f,
and 3i each exhibit one intense peak and a second, smaller peak
that index to the (100) and (110) peaks of a hexagonal lattice.
Each of the XRD patterns of the liquid crystalline phases of
3c, 3d, and 4 have only a single intense peak in the low-angle
region, which was assigned to the (100) spacings of hexagonal
(23) (a) Herwig, P.; Kayser, C. W.; Mu¨llen, K.; Spiess, H. W. AdV. Mater. 1996,
8, 510. (b) Makhseed, S.; Bumajdad, A.; Ghanem, B.; Msayib, K.;
McKeown, N. B. Tetrahedron Lett. 2004, 45, 4865. (c) Liu, C.-Y.;
Fechtenkoetter, A.; Watson, M. D.; Mu¨llen, K.; Bard, A. J. Chem. Mater.
2003, 15, 124.
(24) Cozzi, F.; Ponzini, F.; Annunziata, R.; Cinquini, M.; Siegel, J. S. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1019.
(25) Williams, V. E.; Lemieux, R. P.; Thatcher, G. R. J. J. Org. Chem. 1996,
61, 1927.
(26) (a) Cockroft, S. L.; Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J.
J. Am. Chem. Soc. 2005, 127, 8594. (b) Meyer, E. A.; Castellano, R. K.;
Diederich, F. Angew. Chem., Int. Ed. 2003, 42, 1210. (c) Hoeben, F. J.
M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. Chem. ReV. 2005, 105,
1491. (d) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112,
5525. (e) Smith, C. E.; Smith, P. S.; Thomas, R. L.; Robins, E. G.; Collings,
J. C.; Dai, C. Y.; Scott, A. J.; Borwick, S.; Batsanov, A. S.; Watt, S. W.;
Clark, S. J.; Viney, C.; Howard, J. A. K.; Clegg, W.; Marder, T. B. J.
Mater. Chem. 2004, 14, 413. (f) Cozzi, F.; Annunziata, R.; Benaglia, M.;
Cinquini, M.; Raimondi, L.; Baldridge, K. K.; Siegel, J. S. Org. Biomol.
Chem. 2003, 1, 157. (g) Guckian, K. M.; Schweitzer, B. A.; Ren, R. X.-F.;
Sheils, C. J.; Tahmassebi, D. C.; Kool, E. T. J. Am. Chem. Soc. 2000, 122,
2213.
9
J. AM. CHEM. SOC. VOL. 128, NO. 26, 2006 8571

A R T I C L E S
Foster et al.
by both increased surface areas and higher polarizabilities,30 it
is reasonable that larger molecules, such as 4, show a greater
tendency to π-stack and, hence, to assemble into columnar liquid
crystalline phases.
Although 2 is nonmesogenic, its dicyano derivative, 5,
possesses a Colh phase over an extremely broad temperature
range (72-256 °C).21 The greater tendency of nitrile-substituted
derivatives to form columnar phases has also been observed
for a longer-chain analogue of 520 and demonstrates the
importance of functional groups to the self-assembly process.
To carry out a more comprehensive investigation of substituent
effects, we turned our attention to 3a-k, which were prepared
by the condensation of 1 with appropriately substituted 1,2-
phenylenediamines.
Figure 3. Plot of Hammett σp versus clearing temperature (Tc). Best-fit
line derived from linear regression on monosubstituted derivatives 3c-f
(9) only; disubstituted compound 3i (2) was excluded from this analysis
(see text).
Approximately half of the compounds in series 3 were found
to assemble into ordered columnar hexagonal phases, whereas
the remainder melted directly from crystalline solids into
isotropic liquids. There is a striking correlation between the
tendency of these molecules to form columnar phases and the
electron-withdrawing or -donating ability of the functional
groups attached to the core. Compounds with electron-
withdrawing groups (-F, -Cl, -CO2CH3, -CN, and -NO2)
all formed Colh phases, whereas those with relatively electron-
donating groups (-H, -CH3, and -OCH3) were all found to
be nonmesogenic. It is worth noting that compound 3b, which
bears only a weakly electron-withdrawing fluoro substituent,
is unique in this series in that it forms only a metastable
monotropic phase. Derivatives with more strongly electron-
withdrawing groups than fluorine all exhibit thermodynamically
stable enantiotropic columnar phases, whereas more electron-
rich analogues failed to form mesophases.
It is perhaps also notable that the nitro-derivative 3f, which
is the most electron-deficient mesogen in this series, also has
the highest clearing temperature. A quantitative treatment of
the relationship between the substituent effects and phase
stability can be obtained by comparing Hammett parameters,31
which provide a convenient measure of the withdrawing or
donating ability of a functional group, with the clearing
temperatures, Tc. A linear correlation was obtained when the
Tc of the monosubstituted mesogens 3c-f were plotted versus
Figure 4. Plot of Hammett σm versus clearing temperature (Tc). Best-fit
line derived from linear regression on monosubstituted derivatives 3c-f
(9) only; disubstituted compound 3i (2) was excluded from this analysis
(see text).
either Hammett σp (Figure 3) or σm values (Figure 4), with R2
values of 0.90 and 0.88, respectively. This observation provides
quantitative evidence that the liquid crystalline ordering is related
to the electron-withdrawing character of the substituents, because
a higher clearing temperature corresponds to a more thermo-
dynamically stable phase. A similar relationship has previously
been reported for the columnar phases formed by tetrahedral
metallomesogens,32 but to the best of our knowledge, no
correlation of this type has previously been investigated for
discotic mesogens.
As with the core-size effects described above, the correlation
between clearing temperatures and Hammett parameters can be
rationalized in terms of π-π interactions within the columnar
phases. Cozzi and Seigel have shown that π-stacking is favored
by the addition of electron-withdrawing groups, which help to
minimize the repulsive interactions between adjacent aromatic
π-systems.24,28 Several groups have shown that σp parameters
correlate well with the strength of arene-arene interactions in
the gas phase,25 solution,28 and calamitic liquid crystalline
phases.29 The observation of a similar linear relationship
suggests that the same mechanism is responsible for stabilizing
columnar phases. The relationship between σm values and
π-stacking is less well-established,33 although these parameters
(27) (a) Sinnokrot, M. O.; Sherrill, C. D. J. Am. Chem. Soc. 2004, 126, 7690.
(b) Sinnokrot, M. O.; Sherrill, C. D. J. Phys. Chem. A 2003, 107, 8377;
(c) Williams, V. E.; Lemieux, R. P. J. Am. Chem. Soc. 1998, 120, 11311.
(28) (a) Cozzi, F.; Cinquini, M.; Annunziata, R.; Dwyer, T.; Siegel, J. S. J. Am.
Chem. Soc. 1992, 114, 5729. (b) Cozzi, F.; Cinquini, M.; Annuziata, R.;
Siegel, J. S J. Am. Chem. Soc. 1993, 115, 5330.
(29) Williams, V. E.; Lemieux, R. P. Chem. Commun. 1996, 2259.
(30) Stone, J. A. The Theory of Intermolecular Forces; Clarendon Press: Oxford,
1997; Vol. 32, p 264.
(31) Hammett σp and σm parameters are obtained by taking the difference
between the pKa of a substituted benzoic acid and that of the parent
compound, benzoic acid. For example, the σp and σm values for NO2 are
derived from the pKas of p-nitrobenzoic acid and m-nitrobenzoic acid,
respectively. See: Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91,
165.
(32) Chien, C. W.; Liu, K. T.; Lai, C. K. J. Mater. Chem. 2003, 13, 1588.
9
8572 J. AM. CHEM. SOC. VOL. 128, NO. 26, 2006

Columnar Self-Assembly
A R T I C L E S
investigation. Because group polarizabilities are additive,30 the
relative contribution of the functional groups to the overall
molecular polarizabilty will be much smaller in the present case
than for simple benzene derivatives, for which there is a
discernible correlation between arene-arene interactions and
group polarizabilities.25,27 Any effects arising from the variation
in dispersion forces across the series 3a-k therefore may be
masked by the competing electrostatic perturbations induced
by the functional groups.
Figure 5. Schematic representation of antiferroelectric ordering in columnar
phases. Adjacent molecules within a column adopt an antiparallel orientation.
Arrows represent direction of molecular dipole.
do perform better than σp values in at least one theoretical study
of π-π interactions25 and have been found to correlate well
with the strength of cation-π interactions.34
The dichlorinated derivative, 3i, which was the only disub-
stituted mesogenic compound studied, was excluded from our
initial analysis of clearing temperatures. Hammett parameters
for this compound can be obtained by assuming that substituents
effects are strictly additive and employing σp and σm values
that are twice those used for the monochlorinated derivative
3c. On the basis of these assigned values of σp ) 0.46 and σm
) 0.74 and extrapolating from the trends obtained for the
monosubstituted series, the clearing temperature of 3i is
predicted to be either 202 (σp) or 230 °C (σm), as compared to
the observed value of 237.5 °C. Likewise, when the data for 3i
is included with the monosubstituted derivatives 3c-f, the
correlation of clearing temperature with σp becomes dramatically
worse (R2 ) 0.34) whereas the correlation with σm is slightly
improved (R2 ) 0.91). This suggests that σm is a better descriptor
of substituent effects in these systems than σp.
Although the least-squares fit is better for σm than σp, it is
not clear a priori whether a better correlation should be obtained
when a more symmetrical molecule such as 3i is included
alongside its lower symmetry analogues. Reducing the symmetry
of a discotic mesogen has been shown to depress the clearing
temperature,21 presumably because unsymmetrical molecules do
not pack as well as symmetrical ones. We would therefore
expect that the more symmetrical mesogen 3i would have a
higher clearing temperature than predicted based on the behavior
of its lower-symmetry analogues. Such an effect could explain
the discrepancy between the observed clearing temperature of
3i and its σp value. This argument needs to be treated with some
caution, because studies of symmetry effects on discotic
mesogens have invariably focused on the effects of desymme-
trizing the flexible side chains. Whether a similarly large
symmetry effect would occur in the case of rigid groups attached
to the core remains an unanswered question.
Dipole-dipole interactions may also contribute to the pref-
erential formation of columnar phases by molecules bearing
electron-withdrawing groups. The intercolumnar distances ob-
tained by XRD studies are consistent with molecules within
the columns having antiferroelectric ordering; in other words,
the adjacent mesogens will be approximately antiparallel with
respect to one another (Figure 5). Such an arrangement would
explain how these low-symmetry molecules are able to form
fairly symmetrical columns, which can then pack into hexagonal
arrays. The addition of electron-withdrawing groups should lead
to an increase in the molecular dipole moment, which will favor
the antiparallel orientation of adjacent molecules within the
columns. Dipole-dipole interactions are also believed to play
a significant role in the stabilization of mesophases formed by
bent-rod35 and calamitic mesogens36 and have been suggested
as an important stabilizing feature in other dipolar discotic
mesogens.19,37
No significant correlation was found between the group
polarizabilities39 and the clearing temperatures of the mesogenic
compounds 3c-f, suggesting that dispersion forces play only a
secondary role in the observed trends. This conclusion is also
supported by the failure of 3g-h and 3j-k to form columnar
phases, despite the methyl and methoxy substituents having
comparable polarizabilities to chloro or cyano groups, respec-
tively. The absence of an observed relationship between group
polarizabilities and the tendency to organize into columnar
phases may be due to the large size of the aromatic cores under
(33) Reek, J. N. H.; Priem, A. H.; Engelkamp, H.; Rowan, A. E.; Elemans, J.
A. A. W.; Nolte, R. J. M. J. Am. Chem. Soc. 1997, 119, 9956.
(34) Mecozzi, S.; West, A. P.; Dougherty, D. A. J. Am. Chem. Soc. 1996, 118,
2307.
(35) (a) Kishikawa, K.; Harris, M. C.; Swager, T. M. Chem. Mater. 1999, 11,
867. (b) Levitsky, I. A.; Kishikawa, K.; Eichhorn, S. H.; Swager, T. M. J.
Am. Chem. Soc. 2000, 122, 2474. (c) Eichhorn, S. H.; Paraskos, A. J.;
Kishikawa, K.; Swager, T. M. J. Am. Chem. Soc. 2002, 124, 12742.
(36) Demus, D. In Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray,
G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: New York, 1998; Vol.
1, p 133.
If the above discussion of symmetry effects correctly explains
the apparent failure of σp values when 3i is included in the series,
then the accurate prediction of this compound’s transition
temperature by σm values may be merely coincidental. There
are some reasons to distrust the reliability of σm values in the
present context. The value of σm is 0.37 for fluorine, which is
virtually identical to that of chlorine. On the basis of these
values, compounds 3b and 3c are expected to have similar phase
behavior, which they clearly do not. In contrast, a fluorine
substituent is only mildly electron withdrawing according to
the σp scale, which is consistent with 3b forming only a
monotropic phase with a relatively low clearing temperature.
A similar argument can be made with respect to the methoxy-
substituted derivatives 3h and 3k, because alkoxy groups are
electron withdrawing according to σm scale but electron donating
according to σp. The failure of these compounds to form
columnar phases is, therefore, more consistent with σp than σm.
(37) Kishikawa, K.; Furusawa, S.; Yamaki, T.; Kohmoto, S.; Yamamoto, M.;
Yamaguchi, K. J. Am. Chem. Soc. 2002, 124, 1597.
(38) Although it is in principle possible to calculate dipole moments using
molecular modeling, it is difficult in practice to obtain meaningful values
for these systems. Gas phase AM1 calculations indicate that these
compounds possess several nearly isoenergetic conformational isomers
associated with different orientations of the alkoxy chains. The magnitude
and, in some cases, the sign of the molecular dipole moment depend strongly
on the conformation of these chains and, in particular, on the orientation
of the oxygen lone pair electrons. Because we lack a detailed knowledge
of which of these conformers is favored in the liquid crystal phase, these
calculations cannot be reliably applied to the quantitative determination of
the dipole moment of the mesogens within the columns. We do note,
however, that the observed trends in Tc do qualitatively parallel the
experimentally determined dipole moments of the corresponding substituted
benzene derivatives.
(39) Group polarizabilities were estimated from the molecular polarizabilities
of the corresponding monosubtituted benzene derivatives. See: David R.
Lide, Ed. CRC Handbook of Chemistry and Physics; CRC: Boca Raton,
FL, 2005; Chapter 10.
9
J. AM. CHEM. SOC. VOL. 128, NO. 26, 2006 8573

A R T I C L E S
Foster et al.
derivatives tend to form columnar phases. In this case, we found
a weaker correlation to Hammett σp and σm parameters (R2 )
0.82 and 0.73, respectively), although, perhaps significantly, the
linear fit improves to R2 ) 0.90 (σp) and 0.89 (σ m) when the
highly polarizable TMS-acetylene derivative is excluded from
this analysis. Several triphenylene derivatives substituted at the
1-position (8a-g)18 have also been reported. Although the
clearing points in this series show a marked dependence on the
nature of the functional group, a less than straightforward
relationship exists between the electronic properties of these
substituents and the liquid crystal-to-isotropic transition tem-
peratures; nitro groups, however, were found to increase the
clearing temperature. The lack of a clear correlation in this series
is likely due to the additional complication introduced by steric
effects, because the presence of functional groups at this
hindered position causes large distortions of the core away from
planarity.
In conclusion, we have examined a large number of disc-
shaped molecules to elucidate how changes in the core structure
alter the propensity of these compounds to self-assemble into
columnar liquid crystalline phases. There is both a qualitative
relationship between the functional groups and whether a
columnar phase is observed as well as a quantitative correlation
of clearing temperatures of the mesogenic compounds with the
electron-withdrawing ability of the substituents. This trend can
most likely be explained by the changes in electrostatic
interactions between molecules within the columns, which favor
stacking between electron-deficient aromatic rings.
The observed relationship between the nature of the functional
group and the mesophase stability in series 3 prompted us to
reexamine the trends obtained by other researchers working with
substituted discotic mesogens. Indeed, it has often been noted
that electron-withdrawing groups often tend to promote the
formation of columnar phases. For example, although the
unsubstituted tetrakis(hexyloxy)triphenylene 6a is nonmesogen-
ic, its bromo and cyano derivatives 6b-c both form columnar
phases.16 Moreover, replacing a bromine atom with the more
electron-withdrawing cyano group was found to cause an
increase in the clearing temperature. Only limited conclusions
can be made from such a small number of mesogenic com-
pounds spanning a narrow range of Hammett parameters. A
more comprehensive treatment is possible with the series 7a-
g.17 Again, the parent compound (7a) melts directly from the
solid state to an isotropic liquid, whereas its substituted
Acknowledgment. We gratefully acknowledge the Natural
Sciences and Engineering Research Council of Canada (NSERC)
and Simon Fraser University for funding and Mr. Mikki Yang
for carrying out microanalysis.
Supporting Information Available: Full synthetic and ana-
lytical details for all new compounds; XRD I(Q) curves for
liquid crystalline phases. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA0613198
9
8574 J. AM. CHEM. SOC. VOL. 128, NO. 26, 2006