Tunable columnar mesophases utilizing C2 symmetric aromatic donor-acceptor complexes

Article abstract of DOI:10.1021/ja061649s

Derivatives of relatively electron rich 1,5-dialkoxynaphthalene (Dan) donors and relatively electron deficient 1,4,5,8-naphthalenetetracarboxylic diimide (Ndi) acceptors have been exploited in the folding and self-assembly of a variety of complex molecular systems in solution. Here, we report the use of Dan and Ndi derivatives to direct assembly of extended columns with alternating face-centered stacked structure in the solid state. A variety of 1:1 Dan:Ndi mixtures produced mesophases that were found to be stable over temperature ranges extending up to 110 °C. Analysis of these mesophases indicates mixtures with soft/plastic crystal phases and a few mixtures with the thermodynamic properties of true liquid crystals, all composed of alternating donor-acceptor columns within. Importantly, a correspondence was found between the clearing and crystallization points of the mesophase mixtures and the melting/clearing points of the component Ndi and Dan units, respectively. This correspondence enables the predictable tuning of mesophase phase transition temperatures. The study of sterically hindered derivatives led to a set of mixtures in which a dramatic and sudden color change (deep red to yellow) was observed upon crystallization of the mesophase due to a phase separation of the component donor and acceptor units.

Full text of DOI:10.1021/ja061649s

Published on Web 05/26/2006
2
Tunable Columnar Mesophases Utilizing C Symmetric
Aromatic Donor-Acceptor Complexes
†
‡
†
‡
Joseph J. Reczek, Karen R. Villazor, Vincent Lynch, Timothy M. Swager, and
,
†
Brent L. Iverson*
Contribution from the Department of Chemistry and Biochemistry, The UniVersity of Texas at
Austin, Texas 78712, and the Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Received March 9, 2006; E-mail: biverson@mail.utexas.edu
Abstract: Derivatives of relatively electron rich 1,5-dialkoxynaphthalene (Dan) donors and relatively electron
deficient 1,4,5,8-naphthalenetetracarboxylic diimide (Ndi) acceptors have been exploited in the folding and
self-assembly of a variety of complex molecular systems in solution. Here, we report the use of Dan and
Ndi derivatives to direct assembly of extended columns with alternating face-centered stacked structure in
the solid state. A variety of 1:1 Dan:Ndi mixtures produced mesophases that were found to be stable over
temperature ranges extending up to 110 °C. Analysis of these mesophases indicates mixtures with soft/
plastic crystal phases and a few mixtures with the thermodynamic properties of true liquid crystals, all
composed of alternating donor-acceptor columns within. Importantly, a correspondence was found between
the clearing and crystallization points of the mesophase mixtures and the melting/clearing points of the
component Ndi and Dan units, respectively. This correspondence enables the predictable tuning of
mesophase phase transition temperatures. The study of sterically hindered derivatives led to a set of mixtures
in which a dramatic and sudden color change (deep red to yellow) was observed upon crystallization of
the mesophase due to a phase separation of the component donor and acceptor units.
Introduction
shown to promote intermolecular hetero-duplex formation in
water. As a continuation of our studies of the folding and
7
Alternating stacks of electron-rich and electron-deficient
aromatic moieties have proven to be a versatile tool for
molecular self-assembly in solution.1-4 Examples include the
catenane and rotaxane assemblies that are designed around
electron-rich and electron-deficient aromatics to produce com-
plex preorganized structures.2 We have previously reported that
oligomers containing tethered electron-rich 1,5-dialkoxynaph-
thalene (Dan) and electron-deficient 1,4,5,8-naphthalenetetra-
carboxylic diimide (Ndi) exhibit intramolecular folding in water
based upon an alternating column of aromatics stacked in a face-
assembly of molecular systems based upon complementary Dan
and Ndi stacking, we report here various mesophases created
from the 1:1 mixtures of several Dan and Ndi derivatives.
The solid state assembly of aromatic donor-acceptor com-
plexes has previously attracted much attention. For example,
in 1960, a crystalline phase was reported in a 1:1 mixture of
benzene and hexafluorobenzene in which the two molecules
,3
8
were found to be in long alternating stacks. Work with liquid
crystalline materials has shown that electron-rich mesogens can
be modified by the introduction of electron-deficient moieties.9
Equimolar mixtures have been key in developing novel films
with linear charge-transfer (CT) channels, and improving the
4
centered geometry. This folding is largely driven by desolvation
of the aromatic faces (the hydrophobic effect), directed by the
complementary geometry and electrostatics of the Dan:Ndi
complex.5 Independent Dan and Ndi oligomers have also been
1
0,11
efficiency of organic photovoltaic devices.
Recently, 1:1
,6
(
6) For reviews of aromatic interactions, see: (a) Waters, M. L. Curr. Opin.
Chem. Biol. 2002, 6, 736. (b) Hunter, C. A.; Lawson, K. R.; Perkins, J.;
Urch, C. J. J. Chem. Soc., Perkin Trans. 2, 2001, 651-669. (c) Hunter, C.
A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525-5534.
†
Univeristy of Texas at Austin.
Massachusetts Institute of Technology.
‡
(
1) For examples, see: (a) Gabriel, G. J.; Sorey, S.; Iverson, B. L. J. Am.
Chem. Soc. 2005, 127, 2637-2640. (b) Wang, X.-Z.; Jiang, X.-K.; Li, Z.-
T. Youji Huaxue 2004, 24(7), 753-760. (c) Zhao, X.; Jia, M.-X.; Jiang,
X.-K.; Wu, L.-Z.; Li, Z.-T.; Chen, G.-J. J. Org. Chem. 2004, 69, 270-
(7) Gabriel, G. J.; Iverson, B. L. J. Am. Chem. Soc. 2002, 124, 15174-15175.
(8) Patrick, C. R.; Prosser, G. S. Nature (London) 1960, 1021.
(9) For examples, see: (a) Lee, S. J.; Chang, J. Y. Tetrahedron Lett. 2003, 44,
7493-7497. (b) Arikainen, E.; O.; Boden, N.; Bushby, R. J.; Lozman, O.
R.; Vinter, J. G.; Wood, A. Angew. Chem., Int. Ed. 2000, 39, 2333-2336.
(c) Weck, M.; Dunn, A. R.; Matsumoto, K.; Coates, G. W.; Lobkovsky, E.
B.; Grubbs, R. H. Angew. Chem., Int. Ed. 1999, 38, 2741-2745. (d)
Praefcke, K.; Singer, D. In Handbook of Liquid Crystals; Demus, D.;
Goodby, J.; Gary, G. W.; Spiess, H.-W.; Vill, V., Eds.; Wiley-VCH:
Weinheim, 1998; Vol. 2B, pp 945-967. (e) Bengs, H.; Ebert, M.; Karthaus,
O.; Kohne, B.; Praefcke, K.; Ringsdorf, H.; Wendorff, J.; W u¨ stefeld, R.
AdV. Mater.1990, 2, 141-144. (f) Ringsdorf, H.; W u¨ stefeld, R.; Zerta, M.;
Ebert, J.; Wendorff, J. Angew. Chem. 1989, 101, 934-938.
2
79. (d) Zhou, Q.-Z.; Jiang, X.-K.; Shao, X.-B.; Chen, G.-J.; Jia, M.-X.;
Li, Z.-T. Org. Lett. 2003, 5, 1955
(
2) Vignon, S. A.; Jarrosson, T.; Iijima, T.; Tseng, H.-R.; Sanders, J. K.;
Stoddart, J. F. J. Am. Chem. Soc. 2004, 126, 9884-9885. (b) Ortholand, J.
Y.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Williams, D. J. Angew.
Chem., Int. Ed. Engl. 1989, 28, 1394-1395.
(
3) Raehm, L.; Hamilton, D. G.; Sanders, J. K. M. Synlett 2002, 11, 1743-
1
761. (b) Hamilton, D. G,; Davies, J. E.; Prodi, L.; Sanders, J. K. M.
Chem.sEur. J. 1998, 4, 608.
(
(
4) Lokey, R. S.; Iverson, B. L. Nature (London) 1995, 375, 303.
5) Cubberley, M. S.; Iverson, B. L. J. Am. Chem. Soc. 2001, 123, 7560-
(10) Okabe, A.; Fukushima, T.; Ariga, K.; Aida, T. Angew. Chem., Int. Ed.
2002, 41, 3414-3417.
7
563.
10.1021/ja061649s CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 7995-8002
9
7995

A R T I C L E S
Reczek et al.
Chart 1. Dan and Ndi Derivatives Used in This Study.
Figure 1. Color change from light yellow to deep red upon melting of the
1:1 Dan:Ndi mixture 1b:2b, which is characteristic of the Dan:Ndi mixtures
reported here.
(Nda) and the corresponding alkylamines. A range of alkyl
substituents were chosen in order to investigate the role played
by representative side chains on mesophase formation (Chart
mixtures of structurally complementary electron-rich and electron-
deficient aromatics have been shown to greatly alter and/or
1). In particular, the location and extent of branching was varied
to evaluate steric constraints on face-centered stacking.
Mixtures Dan:Ndi mixtures with the desired molar equiva-
lents were stirred during melting. In general, the Dan derivatives
melted first into a liquid that dissolved the Ndi. During the
dissolution of the Ndi derivative, there was usually a rapid
change in color from off-white or light yellow to a deep red
characteristic of the Dan-Ndi charge transfer (CT) band (Figure
1).
Differential Scanning Calorimetry. Differential scanning
calorimetry (DSC) was used to characterize the phase transitions
of the different Dan and Ndi derivatives, as well as their
mixtures. Upon heating, all crystalline Dan derivatives in pure
form (1a-1f) undergo a single transition to the isotropic phase.
The phase transition temperatures measured for the molecules
and their mixtures are given in Table 1. Two of the pure Ndi
derivatives (2c and 2d) also undergo a single transition to an
isotropic phase, although the Ndi derivatives with the longest
unbranched alkyl chains, 2a and 2b, exhibit a mesophase over
modest temperature ranges (20-50 °C range).
The 1:1 mixtures of all Dan:Ndi combinations for which
evaporation of the Dan component was not a problem produced
at least two transitions, indicating mesophase formation over a
sometimes relatively large temperature range (i.e., 1e:2c). Figure
2 shows representative DSC data measured for compounds 1b,
2b, and a 1:1 molar mixture of the two cooling at 5 °C/min.
The mixture exhibits two phase transitions, one of lower energy
at 135 °C, and one at 70 °C. Closer inspection of the phase
transition at 135 °C reveals several overlapping small peaks.
1:1 mixtures of 1b with 2c and 2d exhibited transition enthalpies
indicative of a liquid crystal phase along with a third phase
transition, the nature of which is not clear this time.
12
create phase behavior unique from individual components. The
complementary electrostatics and comparable geometries of Dan
and Ndi lend themselves to moving from folding and self-
assembly in solution to mesophase assembly in bulk phase.
Advantages of using aromatic donor-acceptor interactions
to create mesophases from two different molecules include: (1)
having a driving force for the creation of columnar aromatic
stacks with face-centered geometry, (2) opportunities for tuning
phase transition temperatures and mesophase geometries by
altering side chains on one component at a time, and (3) the
ability to adjust the ratio of donors and acceptors to further affect
and tune properties. For mesophase formation to occur, it is
assumed that interactions between Dan and Ndi molecules are
preferred over Ndi:Ndi and Dan:Dan self-association in the solid
state, and that the Dan:Ndi stacks are face-centered columns.
For comparison, crystal structures have revealed that indeed in
4
the solid state, Dan:Ndi stacks are face-centered, while Dan:
Dan association is dominated by relatively weak edge-to-face
interactions (vide infra) (a so-called herringbone structure), and
Ndi alone packs in an offset face-to-face geometry (vide infra).
Herein, we report the synthesis and initial characterization
of several novel and highly colored mesophases, including
examples of soft/plastic crystals and liquid crystals, produced
from mixtures of various Dan and Ndi derivatives. The
temperatures associated with the mesophase phase transitions
were found to depend in a predictable way on the melting points
or in some cases the clearing points of the component Dan and
Ndi units. In addition, one set of mixtures displayed a sudden
and dramatic change in color when undergoing a sharp
mesophase to crystalline phase transition.
The phase transitions of many Dan:Ndi mixtures follow an
interesting trend in that the clearing point transition is generally
similar to the melting point of the Ndi component (or the
clearing points in the cases of Ndi 2a and 2b), whereas the
crystallization temperature of the mixture is generally very
similar to the Dan component melting points. Of note are the
transition temperatures of Ndi enantiomers 2c and 2d with chiral
derivative Dan 1e. Although 2c and 2d have identical phase
behavior individually and when mixed with other nonchiral Dan
derivatives, the 1e:2c mixture has phase transitions ∼20 °C
higher that of the 1e:2d mixture. Combining a racemic mixture
Results.
Synthesis. The Dan derivatives were synthesized through
deprotonation of dihydroxynaphthalene (Dhn) followed by
reaction with various alkyl bromides. Ndi derivatives were
synthesized from 1,4,5,8-naphthalenetetracarboxylic dianhydride
(
11) (a) Schmidt-Mende, L.; Fechtenk o¨ tter, A.; M u¨ llen, K.; Moons, E.; Friend,
R.; MacKenzie, J. Science, 2001, 293, 1119-1122. (b) Arikainen, E. O.;
Boden, N.; Bushby, R. J.; Lozman, O. R.; Vinter, J. G.; Wood, A. Angew.
Chem., Int. Ed. 2000, 39, 2333-2336.
(12) (a) Park, L. Y.; Hamilton, D. G.; McGehee, E. A.; McMenimen, K. A. J.
Am. Chem. Soc. 2003, 125, 10586-10590.
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A R T I C L E S
Table 1. Results of the Thermal Phase Behavior Characterization of Several Dan and Ndi Derivatives as Well as Their 1:1 Mixtures
a
Temperatures (°C) and enthalpies (kJ/mol) for phase transitions upon cooling from the isotropic phase to the mesophase (left), and from the mesophase
to the crystalline phase (right) determined by DSC (5 °C/min).
mixture of 1d:2c. Similar results were seen with a 1:1:2 mixture
of 1b:1d:2c.
Titrations. Both the clearing and crystallization points were
monitored as titrations were carried out with mixtures of 1b:2b
and 1c:2c. For both titrations, a relatively slow linear increase
was seen in the clearing point with increasing concentration of
the Ndi component until 33% of the mixture was the Ndi
derivative, at which point a steeper increase in clearing point
was seen until 66% was reached, after which there was a return
Figure 2. DSC on cooling (5 °C/min) of (a)1:1 mixture of 1b:2b (b) 1b
to the slower linear increase. For the crystallization point, only
(c) 2b.
a relatively slow decrease in temperature with increasing
amounts of the Ndi derivative was seen. Data are shown for
the molar titration of 1b with 2b (Figure 3). Note the two
clearing points at high molar ratios of 2b, no doubt due to the
fact that this Ndi derivative has its own mesophase in the pure
state.
Optical Microscopy. The bulk optical characteristics of the
Dan:Ndi mixtures were observed by melting small amounts of
sample between glass coverslips, then cooling slowly (5 °C/
min) under a polarizing microscope. Cooling from the isotropic
liquid, dendritic domains were seen to grow over about a 10
°
C temperature range. Onset of domain growth occurred at a
temperature corresponding to the clearing point measured by
DSC. Shearing pressure was applied to the slides upon comple-
tion of the phase transition and the coverslips slid past each
other with little resistance. Upon reaching the crystallization
point, the coverslips could no longer move relative to each other.
Shown in Figure 4 are representative polarizing microscope
images of textures of several different mixtures in their
respective mesophase temperature regions. Complexes involving
chiral Ndi 2c (or 2d) exhibit long, thin, sheetlike patterns of
various size and order with all of the Dan derivatives (panels
a-c in Figure 4) characteristic of a columnar liquid crystals.
Complexes involving Ndi 2b gave a mosaic of small or
elongated rectangles also representative of a columnar me-
sophase (panels d-f in Figure 4).
Figure 3. (a) DSC showing the three phase transitions of the 1:1:2 mixture
of 1c:1d:2c. (b) Phase Behavior on Cooling of the Titration of Dan 1b with
Ndi 2b.
of Ndi 2c and 2d with Dan 1e results in no mesophase, only an
isotropic to crystalline phase transition at 25 °C upon cooling.
Mixing two Dan components,1c and 1d, with Ndi 2c in a
1
3
:1:2 ratio resulted in three phase transitions, shown in Figure
. The clearing point is consistent with other 1:1 mixtures
involving Ndi 2c. The middle temperature transition is 15 °C
lower than the crystallization transition of 1:1 1c:2c, and the
lower temperature transition is correspondingly 15 °C higher
in temperature than the crystallization temperature of a 1:1
Thermochromic Behavior. For most mixtures exhibiting
mesophase behavior in the DSC studies, the characteristic deep
red color of the Dan:Ndi CT absorbance was seen throughout
the isotropic liquid phase, the mesophase, and the crystalline
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A R T I C L E S
Reczek et al.
the loss of a CT band in mixture 1c:2c upon crystallization
(Figure 6d).
The absorbance of the CT band at the λmax was compared to
the Ndi absorbance in the various phases. In general, the 1:1
Dan:Ndi isotropic phases exhibited Ndi:CT absorbance ratios
of 3:1. This ratio is observed regardless of the side chains on
either component. Upon cooling to the mesophase, these ratios
shifted to an average of 1.7:1, again regardless of the particular
mesogens involved. The Ndi:CT absorption ratios did not change
significantly when crystallizing from the mesophase, remaining
at about 1.7:1, with the noted exception of mixtures including
Dan 1c in which evidence of the CT band disappears completely
upon crystallization.
X-ray Crystallography. Single crystal data were obtained
for the homo-crystals of 1c, 1d, and 2c (Figure 7). In these
homo-crystals, 1c and 1d adopt a herringbone structure, while
2
c shows an offset face-to-face packing. Both of these solid
state arrangements are as expected based either on previously
reported structures or electrostatic considerations.6c
A cocrystal of suitable quality for single crystal determination
could not be grown from solution using any combination of
Dan:Ndi derivatives with hydrocarbon side chains. Likewise,
when slowly cooled from melts, no suitable crystal could be
isolated. We have previously reported that hydrophilic side
chains yielded diffracting 1:1 cocrystals of 1f:2e from 20% DMF
Figure 4. Optical texture of 1:1 mixtures at 110 °C of (a) 1b:2c, (b) 1c:
2
c, (c)1d:2c, (d) 1b:2b, (e) 1c:2b, (f) 1e:2b.
3
in water (Figure 7d-f). The resulting structure shows a
monoclinic cell, with columns of alternating Dan and Ndi
molecules stacked in a face-centered orientation.
For comparison purposes, UV-vis spectra were taken of a
:1 mixture of 1f:2e in both the isotropic and crystalline phases
1
(Figure 6g,h). The Ndi:CT absorbance ratio is once again 3:1
in the isotropic phase, and 1.7:1 in the crystal. These measure-
ments are significant because single crystal X-ray analysis
confirmed essentially complete alternating Ndi-Dan, face-
centered stacking in this crystalline phase.
X-ray Powder Diffraction. X-ray powder diffraction studies
were carried out on the 1f:2e cocrystal for which the structure
was known from single-crystal data. In addition, X-ray powder
diffraction was used to investigate the mesophase and crystalline
phases for two different mixtures (1c:2c and 1d:2c) for which
the single-crystal data are not available. For the 1f:2e sample
Figure 5. Color of bulk samples of 1:1 mixtures. (a) deep red 1b:2b
crystalline phase (60 °C). (b) deep red 1b:2b mesophase (110 °C). (c) deep
red 1b:2b liquid phase (160 °C). (d) light yellow 1c:2c crystalline phase
(60 °C). (e) deep red 1c:2c mesophase (110 °C). (f) deep red 1c:2c liquid
phase (160 °C).
solid phase during cooling. The top three panels of Figure 5
show representative colors of all three phases, in this case a
1:1 1b:2b mixture.
(Figure 8h), the two most intense peaks in the X-ray powder
On the other hand, a dramatic color change was observed
pattern, (100) and (10-2), correspond to the planes between
columns. Mapping these reflections to the single crystal data,
they are seen to represent the short plane (smaller distance
between side chains within a column) and the long plane (greater
distance between side chains within a column) as shown in
Figure 7f. Importantly, the short plane corresponds closely with
the (001) of Ndi 2e, which is essentially the length of the Ndi
molecule, while the long plane similarly corresponds to the
length of Dan 1f. In other words, the spacing between inter-
column planes (100 and 10-2) of the cocrystal appear to be
dictated by the lengths of the Ndi and Dan units, respectively.
The X-ray powder pattern of the mesophases of 1:1 mixtures
of 1c:2c and 1d:2c (Figure 8a,b) both show at least two-dimen-
sional order. Additionally, the mesophase of the 1d:2c mixture
produced sharp reflections at wide angles possibly indicating
for mixtures involving the Dan derivative 1c. In these cases,
the isotropic phase and mesophase exhibited the characteristic
deep red color of the Dan:Ndi CT absorbance. However, upon
cooling to the crystallization point, an instant change to a light
yellow color was observed (Figure 5d). This dramatic color
change is not accompanied by any significant distortions of the
texture at low magnification (i.e., 10×), but inspection under
greater magnification (100×) revealed that the transition to the
yellow color was accompanied by extensive small fractures and
blurring of once sharp boundaries.
UV-Vis Spectroscopy. The Dan:Ndi CT band was further
investigated using UV-vis spectroscopy. Data were acquired
by sandwiching premelted samples of the 1:1 mixtures between
glass coverslips. The samples were then subjected to several
melt and cool cycles while taking spectra continuously, moni-
toring temperature with a thermocouple. Representative spectra
from the isotropic liquid phase, mesophase, and crystalline phase
of mixtures 1b:2b and 1c:2c are shown in Figure 6a-f. Note
1
3
three-dimensional ordering. Strong 100 reflections are seen
(
13) Prasad, S. K.; Rao, D. S. S.; Chandrasekhar, S.; Kumar, S. Mol. Cryst.
Liq. Cryst. 2003, 396, 121-139.
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A R T I C L E S
Figure 6. UV-vis Spectra comparing Ndi adsorption (380 nm) to CT band adsorption (475 nm) of 1:1 mixtures. (a) 1b:2b crystalline phase (60 °C). (b)
b:2b mesophase (110 °C). (c) 1b:2b liquid phase (160 °C). (d) 1c:2c crystalline phase (60 °C). (e) 1c:2c mesophase (110 °C). (f) 1c:2c liquid phase (160
C). (g) 1f:2e crystalline phase (60 °C). (h) 1f:2e liquid phase (180 °C).
1
°
Figure 7. X-ray single-crystal structure of (a) 2b, (b) 1c, (c) 1d, (d) 1f:2e cocrystal, alternating face-centered columns (e) 1f:2e cocrystal, oblique monoclinic
cell (f) 1f:2e cocrystal, planes between columns, and their Miller indices.
for both mesophases that are similar to the 200 reflection seen
in the X-ray powder pattern of the Ndi component 2c (Figure
In the 1d:2c crystalline phase (Figure 8d), the low angle 100
and 010 reflections seen in the mesophase remain. Again, these
correspond to essentially the lengths of the Ndi and Dan
components, respectively. However, the remaining reflections
measured for the 1d:2c crystalline phase are distinct from either
component.
For the 1c:2c crystalline phase (Figure 8c), the positions of
reflections closely resemble the sum of the two components,
the only independent peak being the broad one at ∼2θ ) 6. In
8g), corresponding to essentially the length of the Ndi molecule.
In addition, less intense 010 reflections are seen in both
mesophases corresponding to the 100 reflections of the com-
ponent Dan molecules 1c and 1d, respectively (Figure 8e,f),
which in turn reflect the length of the Dan component. Beyond
these two prominent reflections, neither mesophase contains the
remaining diffraction patterns of the individual components.
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A R T I C L E S
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Figure 8. X-ray Powder Diffraction Data including expansions over the same degree range (top spectra) where informative for (a) 1c:2c mesophase (110
C) (b) 1d:2c mesophase (100 °C) (c) 1c:2c crystalline phase (35 °C) (d) 1d:2c crystalline phase (27 °C) (e) 1c crystalline phase (35 °C) (f) 1d crystalline
°
phase (27 °C) (g) 2c crystalline phase (35 °C) (h) 1f:2e crystalline phase (27 °C).
particular, the reflections corresponding to the Ndi 2c mesogen
are all present, albeit less intense and broader in the mixture.
The exception is the reflection corresponding to the 200
reflection of 2c, which is of similar intensity and sharpness in
the mixture. Reflections in the mixture corresponding to the
Dan 1c pattern are present and sharp, although the 100 reflection
is significantly higher in relative intensity in the mixture. Recall
that crystalline 1c:2c is yellow even though the mesophase of
the same mixture is deep red.
Ndi face-centered stacks (Figure 7d), showed an Ndi:CT
absorbance ratio of 1.7:1 in the crystalline phase. This value is
identical to most of the Dan:Ndi complexes investigated. We
therefore associate the Ndi:CT absorbance ratio of 1.7:1 with
complete face-centered columnar order in the crystalline phase,
consistent with other aromatic donor-acceptor systems.14
Significantly, the same Ndi:CTabsorbance ratio of 1.7:1 is
seen in the mesophases of the 1:1 mixtures reported here. By
analogy to the crystalline phases, it is reasonable to propose
that these mesophases also have columnar, alternating face-
centered stacking of the Dan and Ndi molecules. Thus, it appears
that the mesophase to crystalline transition of most of our
mixtures does not involve a significant reorganization of our
columnar stacked aromatic donor and acceptor cores, the
exception being mixtures with Dan 1c.
Disscusion.
Mesophase Face-Centered Assembly of Extended Col-
umns. The small amount of order indicated by the powder
diffraction data for most Dan/Ndi mixtures in the temperature
region between the clearing and crystallization points, as well
as the ability to shear and deform slides of the mixtures in that
region, but not below the crystallization point, are indicative of
a true mesophase formation and not crystal-crystal transitions.
The deep red color seen upon melting of the Dan:Ndi mixtures,
interpreted as an aromatic donor-acceptor CT band, is clear
evidence that the Dan and Ndi molecules are stacking in an
alternating, face-centered geometry. The UV-vis spectrum of
The X-ray powder diffraction data of the two 1:1 mixtures
investigated, 1c:2c and 1d:2c, are also consistent with columnar
stacks in their mesophases, separated by distances corresponding
to the lengths of the component Dan and Ndi molecules, just
as was seen in the 1f:2e cocrystal structure. According to this
interpretation, both complexes analyzed by X-ray powder
(
14) (a) Pyzhtina, E. V.; Cherednichenko, L. V.; Kardash, I. E.; Pravednikov,
A. N. Zh. Fiz. Khim. 1975, 49(3), 652-656. (b) Lawrey, D. M. G.;
McConnell, H. J. Am. Chem. Soc. 1952, 74, 6175-6177.
a 1:1 mixture of 1f:2e (Figure 6g), for which single-crystal X-ray
data reveals a cocrystal structure consisting of alternating Dan:
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Tunable Mesophases
A R T I C L E S
diffraction show the short plane reflection corresponding to the
length of the Ndi 2c, which has a d spacing calculated to be
the aromatic ring, as in derivative 1d, eliminates the thermo-
chromic behavior.
2
1
0.5 Å. The calculated d spacing for the long plane of complex
c:2c (with isopropyl side chains on the Dan) is 9.1 Å,
Tuning Phase Transition Temperatures. As seen in Table
1, the relative clearing points of the various donor-acceptor
mixtures track with the relative melting/clearing points of the
Ndi component. For example, in 1:1 mixtures containing Dan
1b as the donor, the clearing points range from 143 °C with 2a
to 127 °C with enantiomers 2c and 2d, with 2b coming in
between. This relative order coincides with the relative melting/
clearing points of the Ndi components.
Along the same lines, the relative crystallization points of
the 1:1 mixtures track with the melting points of the Dan
derivatives. For example, with Ndi 2b, the crystallization points
range from 79 °C with 1c to 26 °C with 1d, with the other
derivatives coming between; again in the same relative order
as their individual melting points. In the case of the crystal-
lization points, the values for the mixtures are also similar
corresponding to the length of Dan 1c, whereas the longer side
chains on the Dan in the 1d:2c complex give a somewhat longer
distance of 10.5 Å, corresponding to the length of Dan 1d
(Figure 9).
The preceding structural analysis indicates that the mesophase
of Dan:Ndi mixtures should be classified as columnar oblique
rectangular (ColOb) mesophases. This conclusion is supported
by the optical textures (See Figure 4d-f) in which rectangular
domains are seen.15 The transition enthalpies, optical textures,
and powder diffraction data suggest soft/plastic liquid crystals
for most of the mesophases, while a few (1b:2c and 1b:2d)
exhibit properties of true columnar liquid crystals. Note that
with chiral Ndi 2c, evidence for a ColOb B1 phase comes from
the more sheetlike domains in the optical texture (See Figure
(
within ∼5 °C) in absolute magnitude to the Dan component
1
4a,16
melting points. The exception is the derivative 1c, in which the
crystallization points of the 1:1 mixtures are around 20 °C lower
than the melting point of 1c, presumably a phenomenon that is
related to the phase separating behavior seen in these mixtures
upon crystallization.
4
a-c).
Thermochromic Behavior of Mixtures Containing Dan 1c.
The fact that all the complexes investigated in this study exhibit
the same deep red color in the mesophase and isotropic phase
suggest that the side chains investigated do not affect the
aromatic stacking arrangement in these less ordered phases.
However, the mixtures involving Dan 1c with various Ndi
derivates all show striking thermochromic behavior in which
the deep red mesophase rapidly turns to a light yellow color
upon crystallization. Thus, it appears that the packing of the
side chains can make a difference in the crystalline phase. Steric
interactions involving the Dan 1c isopropyl chains apparently
force the aromatic faces apart and thereby disrupt the CT
absorbance upon freezing into a crystalline phase (yellow color),
although these interactions are not significant enough to prevent
face-centered interactions in the isotropic or mesophases (deep
red color).
A straightforward interpretation of these trends is that the
1:1 Dan:Ndi mixture clearing points track in a relative way with
restriction of motion of the Ndi side chains, and the crystal-
lization points track with restriction of motion of the Dan side
chain component. In other words, there is more order associated
with Ndi long axes because the Ndi side chains have more
restricted motion in the mesophase, and less order along the
Dan long axes because the Dan residues maintain greater
freedom of motion in the mesophase. Consistent with this, the
mesophase X-ray powder diffraction data for both the 1c:2c
and 1d:2c mixtures show substantially more order (much more
intense reflection) corresponding to the short plane, due to the
long axis of the Ndi component. The ability to tune phase
transition temperatures based on knowledge of component
melting/clearing points is a relatively unique and highly desirable
feature of Dan:Ndi mixtures.
The X-ray powder diffraction data for the 1c:2c mixture are
consistent with crystallization-induced phase separation in
mixtures containing Dan 1c. In the crystalline phase of the 1c:
2
1
c mixture, reflections corresponding to those seen in the Dan
c homo-crystal appear as sharp peaks, and reflections corre-
Further temperature tuning of the clearing point was affected
by altering the molar ratios of the component mixtures as was
exhibited in the titration of 1b with 2b. While the crystallization
temperature remained fairly constant, the clearing point ranged
from 95 °C at 66% 1b to 140 °C at 33% 1b. Presumably, the
sponding to those seen in the Ndi 2c homo-crystal appear as
well, although they are of low intensity and diffuse. These data
are consistent with the Dan molecules forcing themselves out
of contact with the Ndi molecules in what is likely a steric
induced phase separation. The Dan molecules reorganize and
pack with themselves in what appears to be the homo-crystal
herringbone arrangement, and the Ndi molecules are forced to
reorganize and pack with themselves as well. This leads to many
structural defects seen at high magnification. The only new peak
in the 1c:2c crystal phase is a very broad one at 2θ ) 6.2° that
could be due to the boundaries between phase separated Dan
and Ndi molecules.
6
6% and 33% values indicate 2:1 and 1:2 stoichiometries, in
which each aromatic unit is still interacting with at least one
complementary unit, and there is still long-range face-centered
stacking. At mixtures with more extreme molar compositions
(>66% or < 33%), this arrangement is not possible, and the
clearing temperatures of the mixtures change more gradually,
taking on more the characteristics of the aromatic unit in highest
concentration.
Chiral Side Chains. Phase transition temperatures were
further modulated by manipulating the chirality of the side
chains. While Ndi enantiomers 2c and 2d behave identically
by themselves and with any achiral Dan, the diastereomeric
mixtures they create with chiral Dan 1e produced similar
mesophase textures, but their phase transition temperatures
differed significantly. In particular, the 1:1 mixture of 1e:2d
Note that the steric effect proposed to be responsible for the
thermochromic behavior seen with Dan 1c is highly sensitive
to structure. Moving the branch one methylene unit farther from
(
15) (a) Dierking, I. Textures of Liquid Crystals; Wiley-VCH: Weinheim, 2003
pp147-148, 207-208. (b) Kouwer, P. H. J.; Jager, W. F.; Mijs, W. M.;
Picken, S. J. Macromolecules 2002, 35, 4322-4329.
(16) (a) Prasad, V.; J a´ kli, A, Liq. Cryst. 2004, 31(4), 473-479. (b) Lagerwall,
J. P. F.; Giesselmann, F.; Wand, M. D.; Walba, D. M. Chem. Mater. 2004,
(the S,S:S,S diastereomeric complex) displayed clearing and
1
6, 3606-3615.
crystallization points of 125 °C and 38 °C, respectively. The
J. AM. CHEM. SOC.
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VOL. 128, NO. 24, 2006 8001

A R T I C L E S
Reczek et al.
Figure 9. Computer models of the proposed oblique rectangular packing for the mesophase of (a) 1c:2c, (b) 1d:2c (Hyperchem, Hypercube, Inc.).
1
:1 mixture of 1e:2c (the S,S:R,R diastereomeric complex)
exhibited clearing and crystallization points of 115 °C and 17
C, respectively. These latter two values represent the lowest
Conclusion
As a logical extension of our efforts to use the preferred
alternating, face-centered stacking of Dan and Ndi donor and
acceptor units to control folding and recognition in aqueous
solution, we have explored their use in the formation of novel
mesophases. Detailed characterization of 1:1 mixtures indicated
that long, alternating stacks of Dan and Ndi derivatives produced
columnar mesophases with a deep red color and inter-stack
distances predictable in a straightforward way from the side
chains of the Dan and Ndi components. Importantly, we found
that the temperatures of the crystallization and clearing point
phase transitions correlated with the melting/clearing points of
the Dan and Ndi components, respectively, allowing for what
we believe may be an unprecedented degree of predictable
control over the tuning of these important properties.
Several interesting materials were produced, including ones
based on mixtures containing Dan 1c that undergo a rapid color
change from deep red to yellow upon crystallization from the
mesophase, due to separation of the components into domains
presumably due to steric interactions caused by the isopropyl
side chains of 1c. This change not only affects the color of the
material, but should have a dramatic influence over other
properties such as conductivity. Current efforts are directed
toward elucidating these and other practical properties, as well
as characterizing other mesophases produced by novel mixtures
of Dan and Ndi derivatives.
°
transition temperatures measured of any Dan:Ndi complex in
the present study. Interestingly, using a racemic mixture of 2c
and 2d with 1e (in a 1:1:2 ratio) resulted in no mesophase being
formed. The single isotropic to crystal phase transition occurs
at 25 °C, between the crystallization temperatures of the single
diastereomeric mixtures implying that the crystallization event
is an average of the two. However, it appears that any long
range order at higher temperature is disrupted by the opposing
chirality of the Ndi units, disallowing the existence of a
mesophase.
Terniary Mixtures. To investigate an alternate method of
controlling phase transition temperatures, more than one Dan
was mixed with a particular Ndi; for example a 1:1:2 mixture
of 1b:1c:2c. The result was not a single mesophase with
intermediate temperature crystallization points, but rather, two
separate sets of crystallization points (Figure 3a) corresponding
roughly to the crystallization points of independent 1b:2c and
1
d:2c mixtures, respectively. The clearing point was that
expected for a mixture containing Ndi 2c. Similar results were
obtained with a 1:1:2 mixture of 1b:1d:2c.
Taken together, our results indicate that a surprising amount
of predictable control can be exerted over the phase transition
temperatures of 1:1 Dan:Ndi mixtures. The success of this
approach is no doubt related to not only their complementary
geometries (both aromatic units are of a similar size) but also
their C2 symmetries that facilitate alternating, face-centered
stacks with side chains oriented in orthogonal directions. This
orthogonal directionality apparently allows the side chain
motions of individual Dan and Ndi units to behave indepen-
dently, as well as retain their behaviors in homo-crystalline
materials. Such independent behavior is probably the basis for
the general correspondence of mixture phase transition temper-
atures with the component melting/clearing points. Note that
the orthogonal directionality of the side chains also makes it
possible to predict inter-stack distances in the columnar mes-
ophases and crystalline phases of 1:1 mixtures.
Acknowledgment. We thank Dr. Grant Willson for provid-
ing access to a polarized optical microscope and DSC. This
work was supported by funds from the Robert A. Welch
Foundation (F1188) and the National Institutes of Health (GM-
0
69647).
Supporting Information Available: Experimental section,
spectral data, and Crystallographic information (CIF).This
material is available free of charge via the Internet at
http://pubs.acs.org.
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