Columnar self-assembly of rhomboid macrocyclic molecules via step-like intermolecular interaction. Crystal formation and gelation

Article abstract of DOI:10.1039/c1cc15311k

Two macrocyclic compounds with a rhomboid molecular shape, composed of a π-conjugated framework and an imine or amine functionality, were synthesized. The amine-containing macrocycle crystallizes with step-like interaction of each molecule, forming a colu

Full text of DOI:10.1039/c1cc15311k

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COMMUNICATION
Columnar self-assembly of rhomboid macrocyclic molecules via step-like
intermolecular interaction. Crystal formation and gelationw
Tomohito Ide, Daisuke Takeuchi and Kohtaro Osakada*
Received 26th August 2011, Accepted 3rd November 2011
DOI: 10.1039/c1cc15311k
Two macrocyclic compounds with a rhomboid molecular shape,
composed of a p-conjugated framework and an imine or amine
functionality, were synthesized. The amine-containing macrocycle
crystallizes with step-like interaction of each molecule, forming a
columnar arrangement, although dispersion of the imine gelates
upon ultrasonification.
Macrocyclic molecules composed of p-conjugated units are
expected to have rigid structures and to form a structurally
regulated assembly constructed by intermolecular attractive
interaction of the arylene, thienylene and alkynylene groups.1
Stacking of the macrocycle forms its multi-decker type low-
dimensional aggregates. Most of such compounds, reported so
far, have molecular shapes close to a regular polygon or circle.2
Macrocyclic p-conjugated molecules with a rhomboid shape
also have planar structures but with anisotropic aspects.
Although such compounds are much less common, they would
form one-dimensional aggregates based on p–p stacking, as
shown in Fig. 1.3 In this communication, we report crystal
formation and gelation of the rhomboid macrocycles depending
on the partial structure and relevance of the molecular structures
to the molecular interaction in the assembled structure.
As an extension of our study on the rhomboid macrocycles,4
we prepared a compound composed of 2,20-substituted biphenylene
and bis(phenylethynyl)benzene units according to the procedure
Scheme 1 Synthetic route of macrocycles. (a) p-TsOH, acetonitrile,
shown in Scheme 1. Macrocycle 1 was prepared by 2 : 2
cyclocondensation of biphenyl-2,20-dicarbaldehyde with
0 1C, 92%; (b) Zn(Cu), DMF, 0 1C, then p-TsOH, rt.
1,3-bis(4-aminophenylethynyl)benzene. Intramolecular cyclizative
pinacol coupling using Zn(Cu) converts 1 into the cyclic
compound 2, containing two trans-9,10-diamino-9,10-dihydro-
phenanthrene groups. The 1H-NMR, 13C{1H}-NMR, and
FAB–MS spectra were consistent with the cyclic structures
of 1 and 2, although low solubility of 1 prevented it from
13C{1H}-NMR measurement.
Although attempted recrystallization of 1 was not successful, its
molecular structure was optimized by density functional theory
computation at the B97-D/TZVP6,7 level of theory by using
Gaussian 09,8 as shown in Fig. 2(a). The torsion angles between
central m-phenylene and outer p-phenylene groups are 27.51.
X-Ray crystallography of single crystals of 2 revealed a
macrocyclic structure with orthogonal 9,10-diamino-9,10-
dihydrophenanthrene units (Fig. 2(b)).5 The formed six-membered
ring has two NH groups with trans chemistry. The central
m-phenylene and two other p-phenylene groups are almost
Fig. 1 Illustration of (a) common stacking of a cyclic macrocycle and
(b) edge-to-edge stacking of a rhomboid macrocycle.
Chemical Resources Laboratory, Tokyo Institute of Technology,
4359 Nagatsuda, Midori-ku, Yokohama 226-8503, Japan.
E-mail: kosakada@res.titech.ac.jp; Fax: +81 45-924-5224;
Tel: +81 45-924-5224
w Electronic supplementary information (ESI) available: Analytical
and spectroscopic characterization for the new compounds and
computation details. CCDC 836760. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1cc15311k
c
278 Chem. Commun., 2012, 48, 278–280
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Fig. 2 (a) DFT optimized structure of 1 at the B97-D/TZVP level of
theory. (b) Structure of 2 determined by X-ray crystallography.
Hydrogen atoms are omitted for clarity.
Fig. 4 Gelation of macrocycle 1 in CH2Cl2.
planar (torsion angle = 5.51). The dihydrophenanthrene
plane is almost perpendicular to the planar macrocycle core.
A cyclohexane molecule is included in the cavity of the
macrocyclic molecule.
Fig. 3(a) shows packing of the molecules of 2 in the crystal.
Two pairs of p-phenylene and diphenylethynylene groups in
the neighbouring molecules have a short contact with the
distance between the centroid of the aromatic ring and the
alkynylene of 3.7 A. This can be ascribed to p–p interaction
between the ethynylene and phenylene groups. The p–p inter-
action between the aromatic and alkynylene groups is estimated to
be more stable than p–p interaction between aromatic groups.9
This interaction forms a step-like linkage of the molecules, and
further aggregation of the molecules results in a columnar struc-
ture formed by the linearly arranged macrocyclic compounds. A
9,10-dihydrophenanthrene group and a p-phenylene group of the
molecule contained in the neighbouring columnar assembly have
a contact due to CH–p interaction with d(CHꢀꢀꢀp plane) = 2.6 A,
while an aromatic group of the phenanthrene accepts CH–p
interaction of the phenylene group of the molecule within the
column (2.7 A). All the columnar assemblies of 2 in the crystals
are aligned with the same orientation and have herringbone
packing as shown in Fig. 3(b).
Fig. 5 Optical microscope image (ꢂ2000) of CH2Cl2 gel.
30 min yields opaque gel (Fig. 4). Gelation in DMAc and THF
is completed within 1 min and that in DMF requires 20 min.
The amount of 1 suited for gel formation in CH2Cl2 is within
0.5–8 wt%. Mechanical stirring of 1 in the solvents for 2 days
by using a magnetic stirrer or heating of a solid of 1 in DMF
(4120 1C) and cooling it to room temperature did not give
any gels, but left the original suspension. Hence, this gelation
is induced only by ultrasound irradiation.
Fig. 5 shows the optical microscope image of the gel, formed
in CH2Cl2, after spreading on a glass plate. It contains fibrous
objects with length of several micrometres order and width of
submicron order. Therefore, the gels are formed by a network
composed of micrometre order fibrous aggregates of 1.
Drying the gel of 1 in vacuo yields its xerogel. Comparison
of the SEM (scanning electron microscope) image of 1 before
ultrasonification and of its xerogel indicated clear difference of
the microstructures, as shown in Fig. 6. The submicron fibrous
structures are observed in the xerogel, and have a typical width
of 0.1–0.5 mm. Therefore, the gels are formed by a network
composed of micrometre order fibrous aggregates of 1.
The CH–p interaction of the 9,10-dihydrophenanthrene
group of 2 serves to stabilize attractive interaction between
the molecules within the columnar structure and that between
a molecule and that in the neighbouring columnar aggregate.
Attempts to crystallize 1 by ultrasonification with organic
solvents resulted in formation of its organogel as mentioned below.
Ultrasound irradiation of the suspension of 1 (15 mg mLꢁ1) in
CH2Cl2 for 1 min and allowing the mixture to stand for further
Fig. 3 (a) Intermolecular interaction in the crystal of 2. The dotted
lines denote p–p (blue) and CH–p interactions (red). (b) Crystal
packing of 2, view along the a-axis. Hydrogen atoms and cyclohexanes
are omitted for clarity.
Fig. 6 SEM images and XRD patterns of 1 (a) before ultrasound
irradiation, (b) xerogel prepared from CH2Cl2 gel.
c
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Chem. Commun., 2012, 48, 278–280 279

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In addition, IR spectra of 1 before ultrasound irradiation and the
xerogel of 1 are almost identical, suggesting that chemical
reaction did not occur during the ultrasound irradiation. Powder
X-ray diffraction patterns of 1 before the ultrasonification and
the xerogels of 1 are included in Fig. 6. Xerogel of 1 exhibits clear
diffraction patterns due to the crystalline structure. Maximum of
the diffraction intensity was observed at 2y = 23.61 (d = 3.8 A)
for the CH2Cl2 gel, which may correspond to intermolecular p–p
stacking in the same manner as macrocycle 2 in the crystal. This
result indicated that structurally regulated aggregation via p–p
stacking between the macrocycles is an important factor in the
gelation of 1. Crystals of 2 show a complex diffraction pattern
compared with the xerogels, and their major peaks are similar to
that of xerogel from CH2Cl2. The obtained one-dimensional
aggregates from CH2Cl2 may have a three-dimensional structure
similar to 2.
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5 Crystal data for 2: C90H84N42: M = 1221.68, monoclinic,
space group P21/n (no. 14), a = 9.424(14) A, b = 17.14(3) A,
c = 23.45(4) A, b = 91.541(18)1, V = 3786(10) A3, Z = 2,
Dx = 1.072, 31 070 reflections measured, 8020 unique reflections,
1709 observations (I 4 2.0s(I)), R = 0.1181 (I 4 2.0s(I)),
Rw = 0.2957 (I 4 2.0s(I)). CCDC 836760. Low quality of the
crystals hindered discussion of detailed bond parameters of the
compound.
Sol–gel transition induced by change of temperature and pH
as well as photoirradiation was reported. These physical
stimuli reorganize interactions such as hydrogen bonds, p–p
and CH–p interactions, and van der Walls interactions
between the molecules, forming the network structure suited
for gelation.10 Solution of various compounds and their
mixture was reported to form gels upon ultrasonification.11–16
Most of them are caused by change of conformation of the
molecules or of the intermolecular interaction of local functional
groups. Macrocycle 1 undergoes ultrasound-induced gelation,
and it is based on formation of fibrous aggregates of the
molecules during the irradiation.
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9 Interaction energy of the benzene–diphenylethyne p–p complex is
ꢁ18.3 kJ molꢁ1 and the benzene–benzene parallel-displaced stacking
p–p complex is ꢁ11.2 kJ molꢁ1 at the B97-D/TZVP level of theory.
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In summary, macrocycles 1 with twisted biphenyl-2,20-diimine
units and 2 composed of a planar core and orthogonal 9,10-
dihydrophenanthrene groups at the two acute corners aggregate
linearly in the crystals or gel. A rhomboid core composed of
p-conjugated groups and 9,10-dihydrophenanthrene side
group of 2 forms the columnar aggregates and their bundles,
as observed by X-ray crystallography. Detailed aggregated
structures of 1 are not clear but formation of the fibrous
structure, observed in the gels formed by ultrasonification, is
attributed to linear aggregation of the molecules via p–p and
CH–p interactions.
We thank Dr Daling Lu of the Chemical Resources
Laboratory, Tokyo Institute of Technology for SEM measure-
ments. We also thank Prof. Shigeru Machida of Tokyo
National College of Technology for optical microscope and
photophysical measurements. This work was supported by the
Global COE Program ‘‘Education and Research Center for
Emergence of New Molecular Chemistry’’. T. I. acknowledges
Research Fellowship for Young Scientists from Japan Society
for the Promotion of Science.
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280 Chem. Commun., 2012, 48, 278–280
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