Gelation or crystallization? Subtle balance of structural factors for assembly of DBA derivatives with methyl esters

Article abstract of DOI:10.1039/c5ce00778j

Morphology of molecular aggregates is crucially affected by molecular shape and intermolecular interactions. In 2009, we reported the unusual gelation behaviour of a curved-butadiynyl-bridged cyclic pi-conjugated molecule, octadehydrodibenzo[12]annulene ([12]DBA), possessing two methyl ester groups in noncentrosymmetric positions. Aggregation of the [12]DBA derivative occurred anisotropically to give crystalline nanofibers, which finally formed an organogel. To investigate the aggregation behaviour of the related analogues, we synthesized octadehydrotribenzo[14]annulene ([14]DBA) derivatives with two and four methyl ester groups and revealed that the [14]DBAs gave not gels but crystals, in which the molecules are stacked not in a unidirectional manner but in an alternating way. The results indicate that the gelation and crystallization of the present molecular system is derived from the subtle balance of molecular shape and intermolecular interactions.

Full text of DOI:10.1039/c5ce00778j

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Gelation or Crystallization? Subtle Valance of Structural Factors
*
*
for Assembly of DBA Derivatives with Methyl Esters
Received 00th January 20xx,
Accepted 00th January 20xx
a*
a
a
a*†
Ichiro Hisaki, Eriko Kometani, Norimitsu Tohnai, Mikiji Miyata
DOI: 10.1039/x0xx00000x
Morphology of molecular aggregates is crucially affected by molecular shape and intermolecular interactions. In 2009, we
reported unusual gelation behaviour of curved-butadiynl-bridged cyclic pi-conjugated molecule,
a
www.rsc.org/
octadehydrodibenzo[12]annulene ([12]DBA), possessing two methyl ester groups in noncentrosymmetric positions.
Aggregation of the [12]DBA derivative occurred anisotropically to give crystalline nanofibers, which finally form a
organogel.
octadehydrotribenzo[14]annulene ([14]DBA) derivatives with two and four methyl ester groups and revealed that the
14]DBAs gave no gels but crystals, in which the molecules are stacked without unidirectional manner but with alternate
To
investigate
aggregation
behaviour
of
the
related
analoges,
we
synthesized
[
way. The results indicate that gelation and crystallization of the present molecular system is laid on subtle balance of
molecular shape and intermolecular interactions.
Introduction
One dimensionally pi-stacked fibrous assemblies have been
one of attractive supramolecular architectures because they
are expected to show exotic optoelectronic properties
1
-8
originating in a morphology with high aspect ratio. Organo-
or hydrogels based on such pi-stacked fibers are thus
promising candidates of optoelectronically active soft
9
-13
materials.
Generally, a design principle of low molecular
weight organic gelators with optoelectronic properties is to
introduce long alkyl or alcoxy chains with hydrogen bonding
moieties into periphery of a rigid pi-conjugated functional core.
The resulting molecules can aggregate via phase segregation of
rigid and flexible parts, combined with directional
intermolecular interactions such as pi-pi stacking and
hydrogen bonds, to give anisotropically-grown supramolecular
fibers, which then form a three-dimensional network to hold
solvent molecules within it. The principle is quite universal,
and thus, is widely applied by huge numbers of pi-conjugated
systems.
Scheme 1. Octadehydrodibenzo[12]annulene ([12]DBA) derivatives 1a and
b and octadehydrotribenzo[14]annulene ([14]DBA) derivatives 2a-1, 2a-2,
and 2b.
1
On the other hand, in 2009 we reported that butadiyne-
bridged acene like pi-conjugated molecule so-called
1
4
octadehydrodibenzo[12]annulene
([12]DBA)
derivative
possessing two methyl ester groups 1a (Scheme 1) exhibited
excellent gelation ability for chlorinated solvents such as 1,2-
5,16
1
dichloroethane.
The phenomenon is quite unusual because
Figure 1. Aggregation status of DBAs 1a, 1b and 2a-1.
a.
Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita Osaka
65-0871, Japan
the compound has neither long substituent groups, steroidal
groups, or hydrogen bonding groups but just two methyl ester
groups at noncentrosymmetric positions. Such simple
molecule, however, yielded organogel, successfully.
5
†
Present address: The Institute of Scientific and Industrial Research, Osaka
University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047 (Japan).
DBA = Dehydrobenzoannulene
*
*
1
13
Electronic Supplementary Information (ESI) available: [ H and C NMR spectra of
the newly synthesized compounds]. See DOI: 10.1039/x0xx00000x
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Figure 2. Structural factors providing varied molecular packing of [12]- and [14]DBA derivatives. (a) Conformational flexibility of methyl ester groups. (b)
Weak hydrogen bonds (CH/O interactions) with various manners formed among methoxycarbonylphenyl moieties.
Inspired by gelation of 1a, we have started investigating further purification. The oil was added to a mixture of 3a (1.46
assembly behaviour of its analogue. We then synthesized g, 4.07 mmol), Pd(PPh
3 4
) (107 mg, 92.5 μmol), CuI (35.0 mg,
[12]DBA derivative with four methyl ester groups 1b and 185 μmol), and triisopropylamine (1.3 mL) in deoxygenated
octadehydrotribenzo[14]annulene ([14]DBA) derivatives with anhydrous tetrahydrofuran (THF) (17 mL) under nitrogen
two methyl ester groups 2a-1, which has more expanded pi- atmosphere. After stirred over night at room temperature, the
conjugated core. As a result, [12]DBA 1b was revealed to give reaction mixture was concentrated in vacuo. The mixture was
1
7
no gels but crystals with three polymorphic forms. [14]DBA extracted with dichloromethane and washed with water and
a-1 was also revealed to yield as many as five polymorphic brine. The organic layer was dried over anhydrous MgSO and
crystalline forms, one of which formed a fragile gelly concentrated in vacuo. Purification by column chromatography
2
4
1
8
1
9
materials at low temperature (
polymorphs of 1b and 2a-1 were provided from flexibility of 79%) as a yellow solid. H NMR (270 MHz, CDCl
the methyl ester group regarding with its conformations and 1.6 Hz, 2H, ArH), 7.93 (dd, J = 1.6 Hz, J = 8.1 Hz, 2H, ArH), 7.59
intermolecular interactions as shown in Figure 2. Namely, (m, 4H, ArH), 7.37 (m, 2H, ArH), 3.79 (s, 6H, OCH ), 0.24 (s,
165.8, 133.2,
−19 °C). The observed (silica gel, chloroform / hexane = 10 / 1) gave 4a (85.9 mg,
1
3
): δ 8.20 (d, J =
1
2
3
1
3
3 3
rotational flexibility of the methyl ester can provide 18H, SiCH ) ppm. C NMR (67.5 MHz, CDCl ): δ
conformers (Figure 2a), bringing about changes of dipole 132.2, 131.9, 129.7, 129.6, 128.8, 128.3, 126.3, 125.8, 102.6,
moments of the molecule and electrostatic potential surface 102.2, 92.8, 91.7, 52.2, -0.2 ppm.
on the molecule. Moreover, the carbonyl oxygen atom in the Synthesis
of
Acyclic
Tetrayne
4b.
1,2-
group can provide weak and less directional CH/O bis(trimethylsilylethynyl)benzene (313 mg, 1.16 mmol) was
2
0-22
interactions
Figure 2b), allowing the molecules to pack into various product was added to a mixture of 3b (1.40 g, 3.37 mmol),
arrangements. Pd(PPh (134 mg, 116 μmol), CuI (25 mg, 130 μmol), and
In this manuscript, we describe synthesis and molecular triethylamine (50 mL) under nitrogen atmosphere. After
assemblies of other two analogues, 2a-2 and 2b. [14]DBA 2a-2 stirred over night at 40 C, the reaction mixture was
has two methyl ester groups in different positions from those concentrated in vacuo. The mixture was extracted with
of 2a-1. [14]DBA 2b has four methyl ester groups similar to 1b dichloromethane and washed with water and brine. The
These systematic series enable us to consider correlation organic layer was dried over anhydrous MgSO and
between assembly behaviour, molecular shape, and concentrated in vacuo. Purification by column chromatography
with electrostatically-positive hydrogen atoms desilylated by the same procedure described above. The
(
3 4
)
°
.
4
intermolecular interaction such as weak hydrogen bonds.
(silica gel, hexane / ethyl acetate = 10 / 1 to 6 / 4) followed by
preparative HPLC gave 4b (536 mg, 66%) as a dark yellow solid.
1
H NMR (400 MHz, CDCl
3
):
δ 7.87 (s, 2H ArH), 7.83 (s, 2H, ArH),
.62-7.59 (m, 2H, ArH), 7.40-7.38 (m, 2H, ArH), 3.91 (s, 3H,
Experimental
7
1
3
OCH
3
), 3.82 (s, 3H, OCH
3 3
), 0.22 (s, 18H, SiCH ) ppm. C NMR
Synthesis
bis(trimethylsilylethynyl)benzene (500 mg, 1.85 mmol) and
anhydrous K CO (2.56 g, 18.5 mmol) in methanol/ether (32/8
of
Acyclic
Tetrayne
4a.
1,2-
(
150 MHz, CDCl
3
): δ 167.2, 166.7, 132.8, 132.2, 131.6, 131.2,
1
5
29.0, 128.7, 128.5, 125.7, 103.0, 101.8, 95.0, 91.4, 53.04,
3.00 ppm.
2
3
mL) was stirred at room temperature for 1 h. The mixture was
extracted with dichloromethane and washed with water and
Synthesis of [14]DBA 2a-2. A 1.0 M THF solution of
tetrabutylammomium fluoride (0.32 mL) was added dropwise
to a solution of 4a (183 mg, 0.312 mmol) dissolved in THF (8.5
mL) at room temperature. After stirred for 1 h, the mixture
4
brine. The organic layer was dried over anhydrous MgSO and
concentrated in vacuo to yield 1,2-diethynylbenznene as a
yellow oil. The product was used in the next reaction without
2
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was extracted with chloroform and washed with water and Table 1. Crystal data for 2a-2 and 2b.
4
brine. The organic layer was dried over anhydrous MgSO and
2
a-2
2b
concentrated in vacuo to yield a light brown solid. The
resulting material was used for the following step without
further purification. A drop of the product dissolved in pyridine
Formula
C
30
H
16
O
4
34 20 8
C H O
M
r
440.43
orthorhombic
Pna2 (#33)
556.53
monoclinic
P2 /c (#14)
Crystal system
(20 mL) was added into CuCl (309 mg, 3.12 mmol) dissolved in
Space group
1
1
pyridine/methanol (160/160 mL) for 1h. After stirred for 2 h at
room temperature, the reaction mixture was concentrated in
vacuo. The mixture was extracted with chloroform and washed
with water and brine. The organic layer was dried over
a/Å
b/Å
c/Å
α/°
21.2337(4)
7.94640(14)
25.6281(5)
90
9.8740(1)
13.6348(1)
19.5627(2)
90
anhydrous MgSO
4
and concentrated in vacuo. Purification by
β/°
90
91.5660(5)
90
γ/°
90
column chromatography (silica gel, dichloromethane) gave 2a-
2
3
1
(102 mg, 74%) as a pale yellow solid. H NMR (270 MHz,
V/Å
4324.27(14)
8 / 2
2632.74(4)
4 / 1
Z / Z’
T / K
CDCl3): δ 8.51 (d, J = 1.6 Hz, 2H, ArH), 8.05 (dd, J = 1.6 Hz, J =
.4 Hz, 2H, ArH), 7.95 (m, 2H, ArH), 7.64 (d, J= 8.1 Hz, 2H, ArH),
1
2
153
153
8
1
.48 (m, 2H, ArH), 3.98 (s, 6H, OCH ) ppm. C NMR (67.5 MHz,
3
d
calcd.
1.353
1.404
7
3
GOF
1.08
1.05
CDCl3): δ 165.8, 136.2, 134.0, 130.2, 129.8, 129.3, 128.7, 128.4,
26.4, 123.1, 95.6, 92.2, 86.2, 82.5, 52.5 ppm. HR-MS (FAB)
Reflections collected
No. of unique data [R(int)
61425
14389
1
]
7884 [0.069]
4243
4289 [0.035]
1731
+
m/z calcd for [M] C H O 440.1049, found 440.1048.
3
0
16
4
No. of data with I > 2σ(I)
R1 (I > 2σ(I))
Synthesis of [14]DBA 2b. 2b was synthesized according to the
synthetic procedure of 2a-2 with starting from 4b (930 mg,
0.077
0.099
Rw(all data)
0.249
0.319
2
6
1.32 mmol). Purification by column chromatography (silica gel,
CCDC
1060477
1060476
chloform), followed by preparative HPLC gave 2b (296 mg,
1
0%) as a gray solid. H NMR (400 MHz, CDCl ):
4
δ
ArH), 7.97-7.95 (m, 2H+2H, ArH), 7.55-7.53 (m, 2H, ArH), 3.98
8.21(s, 2H,
3
1
s, 6H, OCH ), 3.96 (s, 6H, OCH ) ppm. C NMR (150 MHz,
3
(
3
3
CDCl3): δ 167.1, 166.8, 136.7, 133.6, 132.3, 132.1, 131.0, 130.2,
1
29.2, 125.0, 123.4, 96.9, 92.0, 85.7, 82.7, 53.3, 53.2 ppm. HR-
+
MS (FAB) m/z calcd for [M+H] C H O 557.1237, found
3
4 20 8
557.1236.
Crystallography. For crystal of 2a-2, diffraction data were
collected on a Rigaku R-AXIS RAPID diffractometer with a 2-D
area detector using graphite-monochromatized Cu-Kα
radiation (λ = 1.54187 Å). The cell refinements were
performed with software equipped in a Rigaku R-AXIS RAPID
system. For crystal of 2b, diffraction data were collected on a
Scheme 2. Synthesis of [14]DBAs 2a-2 and 2b.
CCD (Rayonix/MX225HE) with the synchrotron radiation (λ = tetraynes 4a and 4b followed by copper-mediated
.8000 Å) monochromated by the fixed exit Si(111) double intramolecular oxdative cyclization gave
crystal. The cell refinements were performed with HKL2000 dehydrobenzo[14]annlene derivative 2a-2 and 2b in 74% and
0
2
3
24
software. Direct methods (SHELXD2013) were used for the 40% yields for 2 steps, respectively.
structure solution of the crystals. All calculations were
To investigate aggregation behaviour, the synthesized
performed with the observed reflections [I > 2σ(I)] with the [14]DBAs 2a-2 and 2b were then dissolved in various organic
2
5
program CrystalStructure crystallographic software packages,
except for refinement which was performed using The resultant aggregates were not gels but crystalline powders
solvents with heating, followed by rapidly cooled to precipitate.
2
4
Shelxl2013. All non-hydrogen atoms were refined with or needles. Single crystals of 2a-2 and 2b suitable for X-ray
anisotropic displacement parameters, and hydrogen atoms crystallographic analysis were successfully obtained by slow
were placed in idealized positions and refined as rigid atoms evaporation of ethyl acetate and dichloromethane solutions,
with the relative isotropic displacement parameters.
respectively.
1
14]]DBA 2a-2 crystallized into the space group Pna2 .
[
Crystallographically independent two molecules with the up-
down conformation of the carbonyl groups (coloured with
yellow and blue) were stacked in an alternate arrangement
with an averaged pi-stacking distance of 3.43 Å (planarity
deviation of each of the DBA rings are upto 0.07Å and 0.10 Å)
as shown in Figure 3a. The stacked dimer was stabilized with
electrostatically favourable head-to-tale arrangements of the
Results and discussion
[14]DBAs 2a-2 and 2b were synthesized as shown in Scheme 2.
Iodobenzene derivatives 3a and 3b were cross-coupled with
the freshly-prepared o-diethynylbenzene in the presence of
Pd(0) to give the corresponding acyclic tetrayne derivatives 4a
and 4b in 79% and 66% yields, respectively. Desilylation of the
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Figure 3. Crystal structure of 2a-2. (a) Stacked dimer composed of crystallographically independent two molecules with 50% probability ellipsoid plot.
b) the other stacked pair with smaller overlap of the molecular planes. (c) Packing diagrams viewed from the b axis (top) and the c axis (bottom). (d)
(
CH/O Contacts observed in the crystal. S-1: single CH/O interaction between the carbonyl oxygen and aromatic hydrogen atoms, S-2: single CH/O
interaction between the carbonyl oxygen and methoxy hydrogen atoms. (e) Photograph of crystals of 2a-2. Scale bar: 500 μm. Symmetry code: (A, B) x,
y, z; (A’) x, -1+y, z; (A’’) 1-x, 3-y, 1/2+z; (B’) 1/2-x, -1/2+y, 11/2+z; (B’’) 1-x, 2-y, -1/2+z.
Figure 4. Crystal structure of 2b. (a) Stacked dimer composed of inverted two molecules with 50% probability ellipsoid plot. (inset) conformation of the
methyl phthalate moieties I and II. (b) Packing diagrams viewed from the b axis, in which the dimers colored with bule and yellow are related by
twofold screw axis opperation. (c) Twofold helical motif running along the b axis. (d) CH/O Contacts observed in the crystal. S-1: single CH/O interaction
between the carbonyl oxygen and aromatic hydrogen atoms, S-2: single CH/O interaction between the carbonyl oxygen and methoxy hydrogen atoms,
B-1: branched CH/O interaction among the carbonyl oxygen and two aromatic hydrogen atoms, B-3: branched CH/O interaction among the carbonyl
oxygen and aromatic and methoxy hydrogen atoms. (e) Photograph of crystals of 2b. Scale bar: 250 μm. Symmetry code: (A) x, y, z; (A’) 1-x, -y, -z; (B) x,
-1+y, z; (C) -x, -1/2+y, 1/2-z; (D) -1+x, 1/2-y, 1/2+z; (E) -1+x, 1+y, z.
adjacent carbonyl groups, marked with arrows. This kind of overlap of the [14]DBA plane with the averaged interplanar
carbonyl arrangements are often observed in crystalline distance of ca. 3.41 Å (Figure 3b,c). The slipping angle is 64.0°.
1
states. The dimer was stacked along the b axis with smaller The crystal exhibits the beta-type packing structure. The
9
27
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Table 2. Structural characteristics of the assembies of [12]- and [14]DBAs 1a, 2a-1, 2a-2, 1b, and 2b.
compound
assembly status
morphology
conformation
stacking direction)
CH/O interaction
ref.
(
a
c
1
2
a
organo gel
nanofiber
I: plate
ver-ver (Uni.)
S-2
ref. 15, 16
refs. 18, 19
d
a-1
polymorphic crystals
I: ver-ver (alt )
I: B-1, S-1
II: S-2
e
(5 forms)
II: block
II: ver-ver (others )
b
e
fragile gel at low temp.
II’: block
III: thin needle
IV: thin needle
column
II’: ver-ver(others )
II’: S-2
c
III: ver-ver (uni )
III: B-1, C-1
IV: B-1, C-1
S-1, S-2
c
IV: hol-ver (uni )
d
2
1
a-2
b
crystal
up-down (alt )
this work
ref.17
f
polymorphic crystals
I: plate
I: anti (-)
I: B-2
f
(3 forms)
II: column
III: spear
block
II: syn (-)
II: S-1, C-2
III: C-1, B-2
S-1, S-2, B-1, B-3
f
III: anti (-)
c
e
2
b
crystal
syn (alt , others )
this work
a) Molecular conformation and packing manner of 1a in the gel was estimated based on comparison between the observed powder X-ray diffraction pattern of the dried gel and
that simulated for a model generated by the crystal structure prediction method. b) Fragile gel was formed only the 1,2-dichloroethane solution was rapidly cooled to -19 °C. c)
“
uni” indicates unidirectional stacking. d) “alt” indicates alternate stacking. e) “others” indicates stacking ways different from “alt” or “uni”. (f) Stacking direction was not defined.
methyl ester groups form S-1 and S-2 CH/O contacts as shown conclude the assembly behaviours of
in Figure 3d. dehydrobenzoannulene derivatives as followed:
a
series of
The alternate stacking manner of 2a-2 is the completely 1. When the molecules are stacked unidirectionally, the
opposite to that of polymorphs III and IV of 2a-1, in which the morphology of the resultant aggregates tends to be highly
molecules slip and stack unidirectionally to give needle-like anisotropic, like fibre and thin needle. On the other hand,
crystals with high aspect ratio (Figure 1). Crystalline habit of alternate stacking often yields high symmetric aggregation
2a-2, on the other hand, is columnar shape with a relatively motifs, resulting in crystals with lower aspect ratio.
lower aspect ratio as shown in Figure 3e. Thus the position of 2. Methyl ester groups can provide wide range of CH/O
the two methyl ester groups is crucial for stacking manner of interactions in terms of directionality and topology. This is one
[
14]DBA core, and consequently, for crystalline habit. of remarkable factors for generation of polymorphic crystals.
DBA 2b crystallized into space group P2 /c. Contrary to 2a-2 The present systems 2a-2 and 2b, which yield one unique
the [14]DBA core is distorted with upto 0.25 Å of displacement crystalline form at the present, also have potential to yield
1
,
2
8-29
from the means plane of the core, due to a steric effect of the polymorphs in the future.
ester moieties. The both dimethyl phthalate moieties ( and II 3. Gelation behaviour of 2a is quite specific among those of
have the syn-conformation; dihedral angles of the carbonyl related compounds, which probably derived by subtle balance
I
)
3
0,31
group against fused phenylene ring are 48.4° and 36.1° for
I
of molecular shape and intermolecular interactions.
and 57.8° and 16.4° for II as shown in Figure 4a. This
1
7
conformation is also observed in crystals of 1b
.
Two
Conclusions
molecules form alternately-stacked dimer with an inversion
center between them. Due to the steric hindrance of the
methyl ester groups, overlapping of the [14]DBA cores is
smaller than that of 2a-2. The interplanar distance of the
dimer is ca. 3.39 Å. As shown in Figure 4b, the neighbouring
dimers in the packing diagram are related by twofold screw
Two [14]DBA derivatives, analogs of a unusual gelator 1a, were
synthesized and their assembly behaviour was investigated.
[
14]DBA with two methyl ester groups 2a-2 gave a Pna2
crystal, in which the molecules are alternately stacked.
14]DBA with four methyl ester groups 2b gave P2 /c crystal, in
1
[
1
axis (2
Figure 4c, where no significant pi-interaction between the
14]DBA cores were observed, while the motif was stabilized
1 1
) operation. The 2 helical motif of 2b were shown in
which the molecules are also stacked in alternate fashion.
Because of the alternate stacking manner, crystals grow less
anisotropic to yield no fibers or thin needles but columnar or
block-like crystals. These results, in addition to assembly
behaviours of other reported derivatives, indicate that
appropriate positioning of the methyl ester groups is
significantly important for changing molecular arrangements
and consequently morphology of the crystalline products.
Particularly, gelation behaviour of 1a previously reported was
achieved through quite specific balance of molecular shape
and intermolecular interactions. Deep understanding and
tuning of such valance, which is the next challenging target, is
conducting in our laboratory.
[
by branched CH/O interaction involving one carbonyl group
and two aromatic hydrogen atoms (type B-1) and by single
CH/O interaction between a carbonyl group and an aromatic
hydrogen atom (type S-1). In the crystal, the methyl ester
groups participate in totally four kinds of CH/O interactions: B-
1,
S-1, B-3, and S-2 as shown in Figure 4d. Crystalline habit of
2b is rhombic block, indicating that crystal growth occurred
less anisotropically (Figure 4e).
In Table 2, structural characteristics of the [12]- and [14]DBA
derivatives are summarized. Based on the results, we can
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obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/.
Acknowledgements
This work was supported by Grant-in-Aid for Young Scientists 27 G. R. Desiraju and A. Gavezzotti, Acta. Crystallogra. Sect. B,
989, 45, 473-482.
1
(A) (24685026) and for Scientific Research on Innovative Areas:
2
2
3
8 McCrone, W. C. Polymorphism. In Physics and chemistry of
the organic solid state; Fox, D., Labes, M. M., Weissberger,
A., Eds.; Wiley In-terscience: New York, 1965, pp. 725–767.
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-System Figuration (15H00998) from MEXT. This work is partly
supported by JSPS and NSF under the JSPS-NSF International
Collaborations in Chemistry (ICC). Crystallographic data was
partly collected using a synchrotron radiation at the BL38B1 in
the SPring-8 with approval of JASRI (proposal Nos. 2009B1969
and 2014A1252).
3
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DOI: 10.1039/C5CE00778J
Aggregation of a series of strained dehydrobenzoannulene derivatives with methyl ester groups is
revealed.