Engineering solid-state morphologies in carbazole-ethynylene macrocycles

Article abstract of DOI:10.1021/ja204795q

We present a crystallographic study that systematically investigates the effects of the n-alkyl side-chain length on the crystal packing in shape-persistent macrocycles. The solid-state packing of carbazole-ethynylene- containing macrocycles is sensitive to the alkyl-chain length. In macrocycles containing n-alkyl side chains up to nine carbons in length, face-on aromatic π interactions predominate, while the addition of one carbon leads to a completely different packing arrangement. Macrocycles with C₁₀ or C₁₁ chains exhibit a novel packing motif wherein the alkyl chains intercalate between macrocycles, leading in one case to continuous solvent-filled channels. When crystals of the C₁₀ macrocycle are bathed in solvent, the included molecules exchange with the external solvent, and the alkyl chain disorder changes in response to changes in the guest volume in order to retain crystallinity. Powder X-ray diffraction data indicate that alkyl-macrocycle interactions in the longer chains "emulate" the distances typical of face-to-face π interactions, leading to deceptive indicators of π stacking.

Full text of DOI:10.1021/ja204795q

ARTICLE
pubs.acs.org/JACS
Engineering Solid-State Morphologies in CarbazoleꢀEthynylene
Macrocycles
Aaron D. Finke,† Dustin E. Gross, Amy Han, and Jeffrey S. Moore*
Department of Chemistry, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801,
United States
S Supporting Information
b
ABSTRACT: We present a crystallographic study that system-
atically investigates the effects of the n-alkyl side-chain length on
the crystal packing in shape-persistent macrocycles. The solid-
state packing of carbazoleꢀethynylene-containing macrocycles
is sensitive to the alkyl-chain length. In macrocycles containing
n-alkyl side chains up to nine carbons in length, face-on aromatic
π interactions predominate, while the addition of one carbon
leads to a completely different packing arrangement. Macro-
cycles with C10 or C11 chains exhibit a novel packing motif
wherein the alkyl chains intercalate between macrocycles,
leading in one case to continuous solvent-filled channels. When crystals of the C10 macrocycle are bathed in solvent, the included
molecules exchange with the external solvent, and the alkyl chain disorder changes in response to changes in the guest volume in
order to retain crystallinity. Powder X-ray diffraction data indicate that alkylꢀmacrocycle interactions in the longer chains “emulate”
the distances typical of face-to-face π interactions, leading to deceptive indicators of π stacking.
’ INTRODUCTION
attachment of alkyl chains to large aromatic moieties is typically
utilized to enhance solubility and ease of processing. However,
such derivatization can lead to a dramatic change in the solid-
state properties. One such example reported by Anthony is the
functionalization of pentacene with silylalkynes, which changes
the solid-state packing from the orthogonal “herringbone” motif
to one in which cofacial interactions are dominant, leading to an
increase in the solid-state charge-carrier mobility.25,26
One major advantage of using functional small molecules
instead of heterogeneous polymeric materials is the ability to
explore intermolecular interactions systematically and with
high resolution using X-ray crystallography. The first aryleneꢀ
ethynylene macrocycle characterized by X-ray crystallography, a
m-phenyleneꢀethynylene [6]cycle bearing phenol groups, as-
sembles to form porous cavities generated by intermolecular
hydrogen bonding in two-dimensional (2D) layers.27 Subsequent
crystallographic investigations of SPMs have demonstrated a
significant diversity in the solid-state packing of this class of
materials.18,28ꢀ34
In this article, we present an extensive crystallographic in-
vestigation of SPMs containing a carbazoleꢀethynylene back-
bone (1) (Figure 1). We recently discovered that SPMs of this
type readily form nanofibril aggregates whose fluorescence is
rapidly quenched in the presence of explosive vapors such as
TNT and DNMB.35 It has been proposed that the quenching
event is due to electron transfer from the electron-rich π-stacked
macrocycles to the electron-deficient explosive compound.
In the world of molecular electronics, where polymeric
architectures have dominated the chemical space, discrete small
molecules are starting to find their own place as uniquely effective
materials.1ꢀ5 Among architectures found in the small-molecule
regime, shape-persistent macrocycles (SPMs) constitute a dis-
tinct class of architectures whose materials properties can
emulate those of high polymers, but with better control of
solid-state organization.6ꢀ11 The use of SPMs as functional
materials has benefited from recent advancements in their
preparative methods.12ꢀ15 Traditionally, widespread utilization
of SPMs was hindered by tedious preparation, often requiring
dilute conditions, small scales, and difficult separations. The
recent implementation of methodologies wherein the bond-
formation event is under thermodynamic control, otherwise
known as dynamic covalent chemistry, has enabled a diverse
array of macrocycles to be generated from simple precursors in a
less synthetically demanding manner.16,17 Advancing the original
work of Bunz,18,19 our laboratory has developed an alkyne-
metathesis-based approach to the dynamic covalent formation
of aryleneꢀethynylene macrocycles. This method has also
proven useful and efficient for large-scale preparations.20ꢀ22
The functional applications of many conjugated systems are
directly tied to their solid-state packing; thus, rigorous character-
ization of functional small molecules in the solid state is critical
for optimization of their bulk properties.23 Despite major efforts,
the development of molecular design rules to relate chemical
identity to a particular solid-state organization remains difficult.
Each component of a molecule contributes to geometric con-
straints and intermolecular interactions.24 For example, the
Received: May 25, 2011
Published: July 20, 2011
r
2011 American Chemical Society
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the alkyl chain; longer alkyl chains led to better solubility of the
material.
Crystals suitable for X-ray diffraction were obtained by slow
diffusion of ether into solutions of 2ꢀ5 in a halogenated solvent
[typically dichloromethane (DCM)] at room temperature over
several days. The crystals were uniform in quality, without
apparent polymorphism (except for 2). The lack of ordered heavy
atoms (Z > 8) in 1 precluded the use of Mo radiation for all but
the largest of crystals; the use of Cu sources was required for the
others.38 A summary of the crystallographic data is presented in
Table 1.
Crystals of the C10[4]cycle. Crystals of 2 formed in two
distinct morphologies: long, thin rods (2a DCM), which con-
3
tain one long crystal axis, and hexagonal plates (2b), which
Figure 1. Carbazoleꢀethynylene macrocycle 1 with various alkyl
chains 2ꢀ6.
contain two long crystal axes. Crystals of 2a DCM were indefi-
3
nitely stable after removal from solution; in contrast, 2b rapidly
decomposed upon removal from solution, becoming striated
within a few minutes and opaque within 1 h. However, crystals of
2b were stable when immersed in inert oil and rapidly cooled to
ꢀ100 °C.
However, the atomistic details of these interactions have not
been rigorously determined with high-resolution structure de-
termination methods. Moreover, we sought to probe the effects
of alkyl-chain length in a systematic way. Herein we demonstrate
using X-ray crystallographic analysis that the packing of 1 in the
solid state is quite sensitive to the choice of side chain, leading
to the formation of a diverse array of morphologies, some of
which are novel to SPMs.
The predominant polymorph of the C10[4]cycle, rods
2a DCM, crystallized in the monoclinic space group P21. The
3
macrocycle is mostly planar with a macrocycle plane deviation of
0.10 Å.39 The four alkyl chains are grouped into two sets of two
on the basis of their relative orientation with respect to the
macrocycle. The molecule as a whole, including the two sets of
alkyl chains and the macrocycle, adopts a chairlike orientation,
with each set of alkyl chains propagating nearly perpendicular to
the macrocycle plane. The packing consists of two stacks of
parallel macrocycles whose macrocycle planes intersect at an
angle of 75.5° (Figure 2a). The macrocycle planes are parallel to
the crystallographic a axis, forming macrocycle rows. Along the
b axis, there are alternating stacks of macrocycles and alkyl chains;
the alkyl chains are also arranged in parallel rows propagating
down the a axis. The distance between two macrocycles in 2a is
7.23 Å as a result of the alternating alkyl chain/macrocycle
packing. The ends of two of the alkyl chains on one side are
slightly disordered. Rows of macrocycles are not parallel down
the c axis, leading to a “staggered” arrangement that appears as
pleated sheets. Macrocycle staggering results in an AꢀB stacking
’ RESULTS
Macrocycles 2ꢀ6 were prepared via precipitation-driven
alkyne metathesis of precursors bearing functionalities whose
byproducts precipitate from solution upon cross-metathesis, a
process developed in our laboratory (Scheme 1).21 The synthetic
protocol is modular and allows for a variety of monomer units
with differing alkyl chains to be prepared in a straightforward
manner. Following F€urstner’s reports,36,37 we have found that
Mo alkylidynes bearing triphenylsilyloxy ligands are superior
alkyne metathesis catalysts for the preparation of macrocycles 1,
exhibiting greater efficiency and tolerance than previously used
catalysts bearing electron-deficient phenoxy ligands. The macro-
cyclization yields were generally good. As expected, the solubi-
lities of the macrocycles were highly dependent on the identity of
Scheme 1. General Synthetic Scheme for the Preparation of Macrocycles 1
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Table 1. Crystallographic Data for All Compounds
2a DCM
2a MeOH
2b
3
4 DCM
4 DCE
5
3
3
3
3
moiety formula
C
96H108N4
C
96H108N4
C
96H108N4
C
100H116N4
C80H76N4
C
80H76N4
C92H100N4
3
3
3
3
3
(CH2Cl2)
(CH3OH)
2(CH2Cl2)
2(CH2Cl2)
C2H4Cl2
space group
a (Å)
P21
P21
C2/c
Cc
P1
P1
P1
19.3461(6)
8.9264(3)
24.7549(8)
90
19.4024(7)
8.9759(3)
24.5083(7)
90
42.960(17)
9.082(4)
24.092(10)
90
19.277(3)
48.012(7)
10.382(2)
90
9.0398(9)
14.0715(13)
14.9754(14)
68.419(3)
77.031(3)
73.791(4)
1685.1(3)
1
9.2308(4)
13.2487(6)
15.2331(7)
68.242(2)
74.840(2)
75.250(2)
1643.98(13)
1
9.6728(2)
12.2684(2)
15.8148(3)
95.9930(10)
98.7200(10)
102.3650(10)
1793.71
1
b (Å)
c (Å)
R (deg)
β (deg)
γ (deg)
V (Å3)
Z
105.939(2)
90
106.389(3)
90
105.579(11)
90
122.58
90
4110.6(2)
2
4094.8(2)
2
9055(7)
4
8096(2)
4
Dcalcd (g cmꢀ1
)
1.065
1.070
1.091
1.127
173
1.245
1.204
1.168
T (K)
100
100
193
120
100
100
total reflns
30444
7630
23349
7383
68724
8238
37445
9823
33246
18509
10274
ind. reflns
7441
5777
4346
R(F) [I > 2σ(I)]a
Rw(F2) (all data)b
GOF S
MC plane deviation (Å)c
disordered solvent volume (Å3)e
0.0877
0.2442
1.063
0.0892
0.2529
1.038
0.0567
0.1564
0.903
0.0450
0.1183
1.029
0.1789
n/a
0.0531
0.0484
0.0952
0.2814
1.050
0.1552
0.1383
1.045
1.043
0.1134; 0.2154d
0.1001
0.1126
0.0770
0.0548
0.0783
332
266
2
1347
2
n/a
n/a
2
n/a
a R(F) = ∑ Fo| ꢀ |Fc /∑|Fo| for Fo2 > 2σ(Fo)2. b Rw(Fo ) = [∑w(Fo ꢀ Fc2)2/∑wFo4]1/2, where wꢀ1 = σ2(Fo)2 + [M(Fo )]2 + [N(Fo2 + 2Fc2)/3] for
Fo2 g 0 and wꢀ1 = σ2(Fo ) for Fo2 < 0. M and N are weighting variables. c The macrocycle(MC) plane isthe least-squaresplane ofthe macrocycleatoms, not
2
including hydrogens or pendant alkyl chains. The MC plane deviation is the standard deviation from the MC plane for all of the atoms that define it.
d Each value corresponds to a disorder in one carbazole of the macrocycle. e The total disordered solvent-containing volume per unit cell as calculated
using SQUEEZE/PLATON.
Figure 3. Packing of 2a MeOH viewed down the crystallographic
3
b axis. Highly disordered alkyl chains are shown in red, other alkyl chains
in blue, and macrocycles in black. H atoms have been omitted for clarity.
Despite the fact that 2a contains free, disordered solvent, the
crystals do not decompose even after several weeks of sitting in
oil under ambient conditions. This could be due to several
factors: the solvent may be “trapped” in the macrocycle cavity
and thus be unable to escape; alternatively, if the solvent were to
escape, the overall crystallinity would not be affected and the
resulting void space would be filled by something else.
The following experiment illustrates that both of the above
factors may be in play. Crystals of 2a were immersed in methanol
Figure 2. Packing of 2a DCM looking down (a) the crystallographic
a axis and (b) the crystallographic b axis. H atoms have been omitted for
clarity. In (b), alkyl chains are shown in blue and macrocycles in black.
3
repeat of the pleated sheets. Importantly, a consequence of this
arrangement is that an alkyl chain lies above and below each
macrocycle (Figure 2). This arrangement turns out to be critical
for understanding solvent effects in the crystal.
The interior cavity of each macrocycle contains one disor-
dered DCM molecule. The solvent positions in 2a could not be
accurately determined, and thus, the electron density present in
the disordered solvent regions was removed using the solvent
bypass (“SQUEEZE”) procedure in the PLATON program.40
for several weeks; the resulting crystals 2a MeOH were then
3
characterized by X-ray diffraction. The DCM was replaced by
disordered MeOH, as evidenced by the electron count given by
SQUEEZE; as in the case of 2a, the solvent positions in
2a MeOH could not be accurately determined, and thus, the
3
electron density present in the disordered solvent regions was
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Figure 4. Visualization of the solvent volumes in (a) 2a DCM and (b) 2a MeOH. Solvent volumes are shown in red. The solvent void area in
3
3
2a MeOH is significantly smaller than that in 2a DCM as a result of the alkyl-chain disorder.
3
3
removed. Crystals of 2a MeOH crystallized in the space group
crystals of 2a are indefinitely stable because alkyl-chain disorder
prevents the destruction of the crystallinity. A visualization of the
solvent volumes in 2a DCM and 2a MeOH (Figure 4) is
3
P21, the same as in 2a DCM, with similar unit cell parameters;
3
the unit cell volume of 2a MeOH (V = 4094.8 Å3) is smaller than
3
3
3
that of 2a DCM (V = 4110.6 Å3) by only 15.8 Å3.
illustrative: in 2a MeOH, the disordered alkyl chain appears to
3
3
Most notable in the structure of 2a MeOH is the extreme
be responsible for the diminished void volume.
3
disorder of the alkyl chain C(67)ꢀC(76), which is much more
The morphology of 2b, a polymorph of 2a, is fascinating. The
macrocycles in crystalline2b are arranged in the space group C2/c;
half the macrocycle is related by inversion symmetry. As with
2a, the macrocycle is mostly planar with a macrocycle plane
deviation of 0.077 Å. The molecule as a whole again adopts a
chairlike shape, and the overall packing motif is similar along the
a and b axes. However, the packing along the c axis is not
staggered as in 2a but instead is aligned; this means that alkyl
chains lie along the edges of the macrocycles rather than near the
macrocycle cavities. This leads to contiguous solvent “channels”
in the crystal, which propagate indefinitely along the b axis
(Figure 5). The solvent channels are created by alternating rows
of macrocycles and alkyl chains; there are significant interactions
between the alkyl chains and macrocycles (∼3.5 Å). The DCM
solvent prevents the collapse of the alkyl chains into the cavities;
disordered than that in 2a DCM (see the Supporting In-
3
formation). A model wherein the full alkyl chain is disordered
about two positions with isotropic displacement restraints gave
the best refinement. The disorder can be rationalized as follows.
In the crystal, the disordered alkyl chains lie above and below the
macrocycle cavity (Figure 3). The change in unit cell volume
(15.8 Å3) is less than the difference ΔV between the volume of
two DCM molecules (VDCM = 76.8 Å3) and the volume of two
MeOH molecules (VMeOH = 51.2 Å3):
ΔV ¼ 2ð76:8 Å3 ꢀ 51:2 Å3Þ ¼ 51:2 Å3
Thus, it is likely that the alkyl chains are disordered to account for
the insufficient space-filling by MeOH. This may indicate that
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Figure 5. Visualization of the solvent channels in 2b DCM viewed down (a) the a axis and (b) the c axis. Solvent channels are shown in red.
3
Crystals of the C11[4]cycle. In light of the fascinating
morphologies we found above, it was of interest to see whether
the packing arrangement of the C10[4]cycle was unique to that
particular alkyl-chain length. It should be noted that the length of
an ordered C10 chain is approximately the same as the macro-
cycle’s lateral NꢀN distance (11.435 and 12.298 Å, respectively).
Large prisms of C11[4]cycle 3 were grown following the general
protocol. These plates crystallized in the monoclinic space group
Cc. The packing morphology is very similar to that of 2a, with a
staggered arrangement along the b axis and AꢀB pleated sheets
of macrocycles propagating along the c axis. This indicates that
similar packing forces are in play for the C10 and C11 cases.
However, unlike the case of 2a, there is no solvent cavity present
in 3 because of the longer alkyl chain. The ends of the alkyl chains
are slightly distorted from the typical anti conformation, leading
to a banana-shaped conformation (Figure 6). The slightly curved
alkyl chains fill the macrocycle cavities, where in the case of 2a
and 2b the cavities were filled with solvent. This further indicates
that the staggered, pleated sheet arrangement is a favorable mode
of packing for these macrocycles.
Crystals of the C6[4]cycle. The effects of shorter alkyl chains
on the solid-state packing were also investigated. The hexyl-
substituted macrocycle 4 had the shortest alkyl chain that gave a
macrocycle with any appreciable solubility. Slow diffusion of ether
Figure 6. Space-filling representation of the banana-shaped distortion
of the alkyl chains in 3.
thus, when crystals of 2b are removed from the solvent, the
crystallinity is quickly lost. It is likely that collapse of 2b leads to
the packing motif seen in 2a; however, we have not been able to
verify that such a crystal-to-crystal transformation occurs.
It should be stressed that the crystals of 2a and 2b grew
simultaneously under the same conditions. The remarkably large
solvent channels present in 2b are absent in 2a; this should serve
as a caveat that a single-crystal study may not tell the entire story
of the solid-state chemistry of a novel material.41
into a solution of 4 in DCM gave large prisms 4 DCM. Crystals of
3
4 DCM lost their crystallinity within hours of being removed from
3
the solvent, indicative of a sensitive, solvated complex.
4 DCM crystallizes in the triclinic space group P1. The
3
macrocycles form ordered slip-stacks that run parallel to the axis
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Figure 7. Packing of 4 DCM viewed down [111]. H atoms and DCM
molecules have been omitted for clarity.
Figure 8. Packing of 5 viewed down [111]. H atoms have been omitted
for clarity.
3
of the alkyl chains, resulting in two different orientations with
50% occupancy. While the macrocycle in the crystal is related by
inversion symmetry, this alkyl chain is not; the alkyl chain and its
inversion-related partner must have different orientations. Inter-
calation of one of the alkyl chains through the cavity of another
macrocycle leads to the absence of guest solvent molecules in
crystals of 5.
The packing of 5 is similar to that of 4 (Figure 8), with aromatic
cofacial interactions predominant. The packing of 5 indicates that
there does appear to be an upper bound for the alkyl-chain length
beyond which van der Waals interactions between alkyl chains and
macrocycles dominate the packing motif.
perpendicular to the [111] plane. In stark contrast to 2, cofacial
interactions between adjacent macrocycles dominate the packing
motif, as indicated by the short distance between macrocycle
planes (3.296 Å) (Figure 7). In contrast, the distance between
two macrocycles in 2a and 2b is much longer (7.23 Å) as a result
of the alternating alkyl chain/macrocycle packing. The presence
of ordered DCM solvent in 4 DCM is unique; as in the other
3
crystals, DCM molecules were located within the cavities of the
macrocycle. However, 4 DCM was the only case in this study
3
where positions for a guest solvent molecule could be acceptably
refined. Hirschfeld surface analysis42 indicated that close contacts
from hydrogens in the DCM guest to the macrocycle host may be
responsible for the ordered solvent.
’ DISCUSSION
Crystals of 4 DCE were grown from diffusion of ether into a
3
solution of 4 in 1,2-dichloroethane (DCE). The guest solvent
Crystallographic investigations of SPMs are in high demand,
not only because of the unique insights such investigations give
but also because of the relative paucity of crystallographic data for
such materials. Solid-state structure determination of SPMs are
quite challenging for several reasons. Many systems do not
crystallize at all; for example, attempts to grow single crystals
of macrocycles containing 1 with pendant tetradecyl or
ꢀCO2C(CH3)2C11H23 chains, both of which are of great inter-
est because of their application as fluorescent sensors, have been
unsuccessful. Crystalline SPMs having disordered alkyl chains
often diffract poorly, leading to difficulties in their struc-
ture refinement. Furthermore, the shape persistence and large
size of SPMs generally lead to the formation of cavities in which
volatile, disordered solvent resides. Such “solvated” crystals, if
not treated properly, may decompose in seconds upon removal
from solution.31
was much more disordered in this case relative to 4 DCM.
3
Crystals of 4 DCE are unique in that one of the carbazoles in the
3
macrocycle is disordered away from the macrocycle plane. This
causes the pendant alkyl chain to be disordered as well. This is
likely an effect of the solvent molecule, as the disordered
carbazole is adjacent to the solvent-containing cavity of another
macrocycle and there appears to be close contact between that
carbazole and DCE.
That cofacial aromaticꢀaromatic interactions become impor-
tant as the alkyl-chain length decreases is a sensible explanation
for the changes in crystal morphology in going from 2 and 3 to 4.
It is reasonable to assume that as these alkyl chains get shorter,
aromatic interactions increasingly dominate the packing motif;
upon complete removal of the alkyl chain, one may expect
columnar architectures like those found in phenol-containing
m-phenyleneꢀethynylene [6]cycles.27 However, because of their
low solubility, our attempts to prepare carbazoleꢀethynylene
macrocycles with alkyl chains shorter than C6 have to date been
unsuccessful.
Crystals of the C9[4]cycle. In an attempt to probe the “upper
bound” between the intercalated packing in 2 and 3 and the
cofacial arrangements seen in 4, C9[4]cycle 5 was prepared. In
contrast to the examples shown above, crystallization of 5 was
difficult; only very small plates of 5 were isolable. Macrocycle 5
crystallizes in the space group P1; half the macrocycle is related
by inversion symmetry. It contains an unusual disordering of two
The lamellar 2D arylꢀalkyl arrangement seen in 2 and 3 is
similar to a 2D motif that has been observed in scanning
tunneling microscopy studies of square-shaped aryleneꢀ
ethynylene macrocycles.43 The relative order of the n-alkyl chains
in these crystals is indicative of strong van der Waals interactions
between alkyl groups in a sheet. In 2a DCM, for example, the
3
distance between two parallel alkyl chains in one macrocycle is
12.5 Å, which is large enough to accommodate two more alkyl
chains. These four alkyl chains are evenly spaced; the distance
between hydrogens in adjacent alkyl chains is 2.6ꢀ2.9 Å. Clearly,
the spacing of the alkyl chains as dictated by the size of the SPM
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(12.2 Å to an edge) is optimal for this type of lamellar arrange-
ment. The van der Waals interactions are maximized when the
alkyl chains reach the approximate length of the macrocycle; the
As evidenced by 2a and 2b, polymorphism of large aromatic
systems may belie the full story of the solid-state chemistry of a
novel material. Additionally, correlating the powder and single-
crystal X-ray diffraction data for 6 has enabled us to suggest that
its solid-state structure is similar to that of 2 and 3. Additional
crystallographic studies on various aryleneꢀethynylene macro-
cycles should give a more complete picture of the nature of
these interesting materials and shed light on their functional
properties.
length of the C10 chain in 2a DCM is 12.2 Å, allowing for a
3
regular arrangement.
In the course of the analysis of the above compounds, it
became clear that the alkylꢀaryl interactions present in crystals
of 2 and 3 contribute significantly to the solid-state packing.
Analysis of simulated powder diffraction data of the crystals
indicated an unusual effect: the van der Waals interactions
between the macrocycles and the alkyl chains present in 2 and
3 result in d spacings that closely resemble those for π stacking.
The values of these d spacings deceptively appear to be associated
with πꢀπ stacking, while in reality πꢀπ stacking does not exist
in these solids!
’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic procedures and
b
characterization for all compounds, crystallographic data for
2ꢀ5 (CIF), and powder diffraction data for 3 and 6. This
material is available free of charge via the Internet at http://
pubs.acs.org.
The simulated powder diffraction data for the crystals illus-
trate this phenomenon. In all cases, the Miller plane that
corresponds to the d spacing of ∼3.5 Å is an intense reflection
in the powder data. However, in the case of the staggered
alkylꢀaryl motif present in 2 and 3, there also exists a reflection
at double this d spacing that corresponds to the actual d spacing
between aromatic macrocycles [except in 2b, where the (h/2,
k/2, l/2) peak is systematically absent]. In the cases where
aromatic π stacking is present, the (h/2, k/2, l/2) reflection
does not exist. Thus, the presence of a reflection at 2d in the
powder diffraction data is indicative of staggered layers, that is,
layers consisting of alkyl chains followed by layers consisting of
aromatic rings (however, as indicated by the systematic absences
seen in 2b, the reverse is not true). We were able to confirm this
experimentally with powder diffraction data for crystalline 3 (see
the Supporting Information). Crystals of macrocycle 3 are easily
grown in large quantities and are sufficiently stable that they
could be ground for the powder diffraction experiment. Using the
single-crystal data for 3, we were able to index the reflections in
the powder diffraction data for 3. We found that the reflection at
d = 3.62 Å corresponds to the Miller plane parallel to alternating
layers of macrocycles and alkyl chains. As expected, there was also
a peak at 2d = 7.24 Å corresponding to the distance between
layers of macrocycles. Therefore, powder diffraction data can be
used to determine the presence of staggered versus cofacial
packing motifs in cases where single crystals cannot be grown.
The C14[4] cycle 6 was prepared previously by us; however, as
stated above, we were unable to grow crystals suitable for single-
crystal analysis. Nonetheless, polycrystalline 6 was readily ob-
tained by growth from slow diffusion of ethyl acetate into a DCM
solution, and the resulting powder was analyzed by X-ray
diffraction (see the Supporting Information). The presence of
a staggered packing arrangement was apparent from the powder
diffraction data, according to the observations above. An intense
peak at d = 3.62 Å was present, as well as a peak at 2d = 7.24 Å;
thus, we predict a staggered alkyl/aromatic packing arrangement
in the case of polycrystalline 6.
’ AUTHOR INFORMATION
Corresponding Author
jsmoore@illinois.edu
Present Addresses
†ETH-Z€urich, H€onggerberg, CH-8093 Z€urich, Switzerland.
’ ACKNOWLEDGMENT
This work was supported by the National Science Foundation
(CHE-1010680). The Materials Chemistry Laboratory at the
University of Illinois was supported in part by Grants NSF CHE
95-03145 and NSF CHE 03-43032 from the National Science
Foundation. The authors thank Dr. Danielle Gray (Materials
Chemistry Laboratory, University of Illinois) and Charlotte
Stern and Dr. Amy Sarjeant (Northwestern University) for
helpful discussions and assistance with single-crystal and powder
X-ray diffraction experiments.
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