Exploring the minimal structure of a wholly aromatic organogelator: Simply adding two β-cyano groups to distyrylbenzene

Article abstract of DOI:10.1039/c1jm14558d

The minimal structure of a wholly π-conjugated aromatic organogelator was explored in this work to show that distyrylbenzene with simple β-cyano substitution (β-DCS) is highly efficient for gelation which is attributed to the cooperative interplay of π-π

Full text of DOI:10.1039/c1jm14558d

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 18971
www.rsc.org/materials
COMMUNICATION
Exploring the minimal structure of a wholly aromatic organogelator: simply
adding two b-cyano groups to distyrylbenzene†
Seong-Jun Yoon, Jong H. Kim, Jong Won Chung and Soo Young Park*
Received 14th September 2011, Accepted 10th October 2011
DOI: 10.1039/c1jm14558d
The minimal structure of a wholly p-conjugated aromatic organo-
gelator was explored in this work to show that distyrylbenzene with
simple b-cyano substitution (b-DCS) is highly efficient for gelation
which is attributed to the cooperative interplay of p–p stacking and
secondary bonding interactions of dipolar cyano groups.
3
chains or CF groups. Self-assembly in this system is attributed to the
cooperative interplay of p–p stacking and secondary bonding
interaction of strongly dipolar CN group.
Scheme 1 shows the molecular structure of distyrylbenzene deri-
0
0
vative with b-CN groups, (2Z,2 Z)-3,3 -(1,4-phenylene)bis(2-phenyl-
acrylonitrile) (b-DCS), investigated in this study. b-DCS was
synthesized by the Knoevenagel reaction of 2-phenylacetonitrile with
In recent years, one-dimensional (1D) self-assembled structures of
p-conjugated organic molecules have been actively investigated
because they offer a useful strategy for realizing functional devices
1
terephthalaldehyde, in a good yield. Full synthetic details, H NMR,
13
C NMR, mass spectroscopy, and elemental analysis characteriza-
tion are described in the ESI†.
1
based on a regular array of the building block component. In
Interestingly, it was found that b-DCS could form a stable gel at
a concentration as low as 0.8 wt% in 1,2-dichloroethane (DCE) or
dichloromethane (DCM) (Fig. 1a). This gelation process was
particular, ‘p-conjugated organogel’, a functional soft matter, which
is formed by the entanglement between self-assembled 1D architec-
tures is drawing special attention in view of innovative optoelectronic
applications with advantages of flexibility and low-cost mass
completely thermoreversible with the gel melting temperature of
ꢀ
82 C as determined by a dropping ball method. This high gel melting
2
production.
temperature is most probably attributed to the specific molecular
structure of b-DCS which eliminates solubilizing and hydrophobic
alkyl chains. Most peculiarly, b-DCS organogel exhibited bright
green fluorescence emission (Fig. 1a and b, vide infra for the detailed
discussion). The aggregation morphology of b-DCS in the organogel
was determined by the field emission scanning electron microscopy
The general design principle of molecular gelators is known to
incorporate the long alkyl chains or steroidal groups into the rigid
aromatic segments to secure the balanced hydrophobic interaction
3
and solubility. Recently, however, it was found that specific aromatic
molecules even without long alkyl chains and steroidal groups could
be similarly effective as molecular gelators, if they incorporate func-
(FE-SEM) observation. The SEM image in Fig. 1a reveals that 1D
3 2
tional groups like CF or H-bonding units (e.g. CONH , COOH) or
fibrous aggregates entangle each other to organize the three-dimen-
sional (3D) organogel networks. These fibrous aggregates showed
clear birefringence under cross-polarized condition, indicating that
the self-assembled 1D supramolecules are highly crystalline presum-
ably due to the extremely simple molecular structure which is
favourable for compact stacking and crystallization (Fig. 1c).
At this point, it is worth noting that b-DCS is the smallest wholly
p-conjugated aromatic molecular gelator without alkyl chains and
steroidal groups ever reported. The molecular weight of b-DCS is
4
boomerang-shaped structures. These rare examples are of great
importance because they offer clues to understanding peculiar inter-
molecular interactions in the self-assembly process. Moreover, the
strategy of eliminating long alkyl chains and steroidal groups in the
design of p-conjugated organogelator is considered essential for
the high performance optoelectronic application, since they are
inactive towards optical absorption and charge transport. Therefore,
research on simpler structures of the p-conjugated organogelator is
worth exploring. In this communication, we demonstrate the smallest
and wholly p-conjugated aromatic molecular gelator system based
on b-cyano-substituted distyrylbenzene which lacks either long alkyl
ꢁ1
only 332.4 g mol . To understand the driving force for the
outstanding gelation ability of this simple-structured organogelator,
we have decided to systematically explore its properties. First, we
have investigated the self-assembled architectures of a series of
Creative Research Initiative Center for Supramolecular Optoelectronic
Materials and WCU Hybrid Materials Program, Department of
Materials Science and Engineering, Seoul National University, Seoul,
1
+
51-744, Korea. E-mail: parksy@snu.ac.kr; Fax: +82 2 886 8331; Tel:
82 2 880 8327
Electronic supplementary information (ESI) available: Material
†
synthesis and characterizations, experimental methods, SEM images,
fluorescence decay profile, optical microscopy observation, I–V curves,
photoluminescence spectra. See DOI: 10.1039/c1jm14558d
Scheme 1 Molecular structure of b-DCS.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 18971–18973 | 18971

View Online
Fig. 1 Dried gel of b-DCS obtained from DCE: (a) SEM image, (b)
fluorescence microscopy image, and (c) optical microscopy image under
crossed polarizers. Insets in (a) show the images of b-DCS organogel (1
wt% in DCE) under room light (left) and UV light (right). (d) FT-IR
spectrum of b-DCS dried gel.
Fig. 2 SEM images of b-DCS: (a) vacuum-deposited film on a glass
substrate (thickness ¼ 500 nm), (b) drop-cast film (0.25 wt% in DCE), (c)
recrystallized film (0.05 wt% in DCM/MeOH (1 : 9)), and (d) dried gel (1
wt% in DCE). (e) XRD patterns of b-DCS: vacuum-deposited film (line
A), drop-cast film (line B), recrystallized film (line C), and dried gel (line
D). (f) Schematic illustration of a molecular packing model with lamellar
structure in b-DCS supramolecules.
b-DCS analogues (b-DCS, DSB, and a-DCS) in the simple drop-
casting process from DCE solution to gain an insight into the
structural origin of self-assembly. As shown above, clear 1D wire
structures were obtained for b-DCS (see ESI†). In the case of DSB,
which lacks C^N units but is otherwise the same as b-DCS, on the
other hand, unstructured and featureless agglomerates were obtained
(
see ESI†). In the FT-IR spectrum of b-DCS dried gel (Fig. 1d), the
strong intermolecular secondary bonding interaction was evidenced
ꢀ
ꢁ
1
using Gaussian 09 software) and the d-spacing (d100 ¼ 18.1 A,
by the appearance of a sharp peak at ca. 2212 cm corresponding to
5
the strong intermolecular C^N interaction band. These findings
observed value from XRD), we could infer the lamellar structure of
ꢀ
b-DCS with a tilt angle of 74 , indicating a substantial intermolecular
p–p overlap (Fig. 2f).
indicate that the strongly dipolar CN group plays a significant role in
inducing well-defined supramolecular self-assembled architectures.
On the other hand, we have previously reported that a-DCS, which
incorporates C^N units to the a position but otherwise is the same
as b-DCS, exhibited two-dimensional (2D) crystalline morphologies
attributed to the multiple C–H/N and C–H/p interactions (see
Third, to gain further insight into the self-assembled crystalline b-
DCS, we have carefully analyzed the optical properties using UV-vis
absorbance and fluorescence spectroscopy (Fig. 3). To avoid artificial
effects induced by high optical density, a vacuum-deposited thin film
(thickness of 100 nm) was used for the optical measurements. The
large hypsochromic shift (60 nm) and asymmetric band shape in the
absorption spectrum of a crystalline b-DCS film compared to those
6
ESI†). Therefore, it is importantly noted that even a subtle molecular
structural change of p-conjugated molecule can significantly alter the
supramolecular architecture of an organic solid, due to the geometric
8
7
of the solution are attributed to H-type aggregation, which is
and electronic factors of functional group interaction.
consistent with the face-to-face molecular packing structure analyzed
from XRD patterns (Fig. 3a and 2f). Thus, the strong 1D self-
assembling tendency of b-DCS is most likely originating from the
Second, we have conducted XRD measurements to monitor the
molecular packing structure of crystalline b-DCS. For the XRD
measurement, we have fabricated different b-DCS samples prepared
by vacuum-deposition, drop-casting, recrystallization, as well as
gelation to explore the effect of the fabrication conditions on the
molecular stacking structure and crystallinity. It was interesting that
we could observe the 1D self-assembled morphology of b-DCS, even
in the vacuum-deposited sample (see ESI† for the 1D growth prog-
ress depending on the thickness). As shown in Fig. 2a–d, b-DCS
molecules exhibited an outstanding 1D self-assembling tendency
irrespective of the fabrication methods. However, the general char-
acteristics of 1D architectures, such as the shape, dimension, and
aspect ratio, were influenced by the fabrication methods and condi-
tions. The XRD patterns of b-DCS in various conditions comprised
Fig. 3 UV-visible absorption (a) and photoluminescence (b) spectra of
ꢁ1
ꢁ
5
the same specific peaks featuring an identical lamellar structure
ꢀ
Fig. 2e). According to the calculated molecular length (18.5 A, by
b-DCS in THF solution (c ¼ 2 ꢂ 10 mol L , open symbols) and
a vacuum-deposited thin film (thickness ¼ 100 nm, filled symbols).
(
1
8972 | J. Mater. Chem., 2011, 21, 18971–18973
This journal is ª The Royal Society of Chemistry 2011

View Online
cooperative interplay of p–p stacking and secondary bonding
interaction of dipolar CN group. Like other cyano-substituted dis-
This work was supported by Basic Science Research Program
(CRI; RIAMI-AM0209(0417-20090011)) and WCU (World Class
University) project (R31-2008-000-10075-0) through the National
Research Foundation of Korea funded by the Ministry of Education,
Science and Technology.
6,9
tyrylbenzene derivatives reported earlier by us, b-DCS exhibits
a great increase (32.5 times) in fluorescence efficiency from practically
ꢁ2
nonfluorescent THF solution (F
F
¼ 1.6 ꢂ 10 ) to the strongly
¼ 0.52) due to the character-
fluorescent supramolecular phase (F
F
istic aggregation-induced enhanced emission (AIEE) process. The
AIEE process is generally known to be originating not only from the
spatial confinement effect but also from the formation of specific
supramolecular stacking architecture associated with the unique
electronic and geometric characteristics of the designed molecules.
This highly fluorescent property in the supramolecular phase will
certainly play a beneficial role in the fluorescence-based application of
b-DCS. In the emission spectra, the peak maximum of the crystalline
phase is bathochromically shifted to 514 nm with respect to that of
solution at 445 nm (Fig. 3b). The large bathochromic shift and the
Notes and references
1 (a) D. O’Carroll, D. Iacopino, A. O’Riordan, P. Lovera,
ꢁ
E. O’Connor, G. A. O’Brien and G. Redmond, Adv. Mater., 2008,
2
0, 42; (b) A. L. Briseno, S. C. B. Mannsfeld, X. Lu, Y. Xiong,
S. A. Jenekhe, Z. Bao and Y. Xia, Nano Lett., 2007, 7, 668; (c)
Y. Che, A. Datar, X. Yang, T. Naddo, J. Zhao and L. Zang, J.
Am. Chem. Soc., 2007, 129, 6354; (d) Y. Yamamoto, T. Fukushima,
Y. Suna, N. Ishii, A. Saeki, S. Seki, S. Tagawa, M. Taniguchi,
T. Kawai and T. Aida, Science, 2006, 314, 1761; (e) L. Schmidt-
Mende, A. Fechtenk o€ tter, K. M u€ llen, E. Moons, R. H. Friend and
J. D. MacKenzie, Science, 2001, 293, 1119.
10
2
(a) S. Diring, F. Camerel, B. Donnio, T. Dintzer, S. Toffanin,
R. Capelli, M. Muccini and R. Ziessel, J. Am. Chem. Soc., 2009,
relatively long lifetime of the emission (lshift ¼ 69 nm, sav ¼ 19.2 ns)
in crystalline b-DCS indicate that efficient excimer formation is
1
31, 18177; (b) S. Varghese, N. S. S. Kumar, A. Krishna,
11
favored by pronounced overlap of the p-systems. These
observations represent that intermolecular p–p stacking
interactions play an important role in determining optical properties
of b-DCS as well as the gelation ability and the formation of 1D
architectures.
D. S. S. Rao, S. K. Prasad and S. Das, Adv. Funct. Mater., 2009,
19, 2064; (c) J. P. Hong, M. C. Um, S. R. Nam, J. I. Hong and
S. Lee, Chem. Commun., 2009, 310; (d) S. Prasanthkumar, A. Saeki,
S. Seki and A. Ajayaghosh, J. Am. Chem. Soc., 2010, 132, 8866; (e)
S. Prasanthkumar, A. Gopal and A. Ajayaghosh, J. Am. Chem.
Soc., 2010, 132, 13206; (f) S. Zhang, M. A. Greenfield, A. Mata,
L. C. Palmer, R. Bitton, J. R. Mantei, C. Aparicio, M. O. de la
Cruz and S. I. Stupp, Nat. Mater., 2010, 9, 594; (g) S. S. Babu,
K. K. Kartha and A. Ajayaghosh, J. Phys. Chem. Lett., 2010, 1, 3413.
(a) P. Terech and R. G. Weiss, Chem. Rev., 1997, 97, 3133; (b)
A. Dawn, T. Shiraki, S. Haraguchi, S. Tamaru and S. Shinkai,
Chem.–Asian J., 2011, 6, 266.
Finally, we have briefly investigated the electrical property of
crystalline b-DCS samples for the practical use of their 1D
morphology and face-to-face p–p stacking motif between p-conju-
gated rigid aromatic molecules. The I–V measurements were carried
out for crystalline b-DCS samples grown directly onto a vertically
3
4
(a) B. K. An, D. S. Lee, J. S. Lee, Y. S. Park, H. S. Song and
S. Y. Park, J. Am. Chem. Soc., 2004, 126, 10232; (b) X. Yang,
R. Lu, T. Xu, P. Xue, X. Liu and Y. Zhao, Chem. Commun., 2008,
453; (c) J. W. Chung, B. K. An and S. Y. Park, Chem. Mater.,
2
placed SiO /Si substrate by the simple solvent evaporation method
(see ESI† for the detailed fabrication method). 1D supramolecules of
b-DCS showed a clear birefringence under cross-polarized condition,
indicating that they had a highly oriented molecular ordering (see
ESI†). For comparison, we have also fabricated a vacuum-deposited
thin film (b-DCS, thickness of 50 nm) device. The electrical
conductivity of 1D supramolecules of b-DCS was measured to be as
2
008, 20, 6750; (d) I. Hisaki, H. Shigemitsu, Y. Sakamoto,
Y. Hasegawa, Y. Okajima, K. Nakano, N. Tohnai and M. Miyata,
Angew. Chem., Int. Ed., 2009, 48, 5465; (e) B. K. An, S. H. Gihm,
J. W. Chung, C. R. Park, S. K. Kwon and S. Y. Park, J. Am.
Chem. Soc., 2009, 131, 3950; (f) J. W. Chung, S. J. Yoon, S. J. Lim,
B. K. An and S. Y. Park, Angew. Chem., Int. Ed., 2009, 48, 7030;
ꢁ
6
ꢁ1
ꢁ6
ꢁ1
high as 9.7 ꢂ 10 S cm (for 7 devices, s ¼ 4.2 ꢂ 10 S cm ). On
av
(
g) X. Yang, R. Lu, F. Gai, P. Xue and Y. Zhan, Chem. Commun.,
010, 46, 1088; (h) X. Zhang, R. Lu, J. Jia, X. Liu, P. Xue, D. Xu
and H. Zhou, Chem. Commun., 2010, 46, 8419.
5 K. Jang, A. D. Ranasinghe, C. Heske and D. C. Lee, Langmuir, 2010,
6, 13630.
S. J. Yoon and S. Y. Park, J. Mater. Chem., 2011, 21, 8338.
the other hand, it was obtained that those of the vacuum-deposited b-
S
2
ꢁ7
DCS thin film were an order of magnitude lower to be 6.9 ꢂ 10
ꢁ1
ꢁ7
ꢁ1
cm (maximum value, for 5 devices, sav ¼ 3.6 ꢂ 10 S cm ) (see
ESI†). In this respect, it should be mentioned that 1D alignment of b-
DCS molecules induced by the supramolecular self-assembly process
gives the more crystalline sample with extended pathways for charge
carrier transport. The electrical conductivity value of 1D supra-
molecules of b-DCS is comparable with one of the well-known
organic semiconductors, poly(3-hexylthiophene) (P3HT) (s ¼ 7.5 ꢂ
2
6
7 (a) R. Li, W. Hu, Y. Liu and D. Zhu, Acc. Chem. Res., 2010, 43, 529;
(b) K. Balakrishnan, A. Datar, T. Naddo, J. Huang, R. Oitker,
M. Yen, J. Zhao and L. Zang, J. Am. Chem. Soc., 2006, 128, 7390;
(
c) Y. Zhao, X. Mu, C. Bao, Y. Fan, J. Zhang and Y. Wang,
Langmuir, 2009, 25, 3264; (d) Y. Fan, Y. Zhao, L. Ye, B. Li,
G. Yang and Y. Wang, Cryst. Growth Des., 2009, 9, 1421; (e)
L. Huang, Q. Liao, Q. Shi, H. Fu, J. Ma and J. Yao, J. Mater.
Chem., 2010, 20, 159; (f) S. Varghese and S. Das, J. Phys. Chem.
Lett., 2011, 2, 863.
ꢁ6
ꢁ1
ꢁ6
ꢁ1
10
S cm ) and oligothiophene (s ¼ 9.9 ꢂ 10 S cm ) prepared by
12
a solution method.
In summary, we have synthesized and successfully demonstrated
the smallest wholly p-conjugated aromatic molecular gelator based
on distyrylbenzene with b-cyano groups (b-DCS) showing highly
8
9
M. Kasha, H. R. Rawls and M. L. El-Bayoumi, Pure Appl. Chem.,
1965, 11, 371.
S. J. Yoon, J. W. Chung, J. Gierschner, K. S. Kim, M. G. Choi,
D. Kim and S. Y. Park, J. Am. Chem. Soc., 2010, 132, 13675.
0 The fluorescence decay of b-DCS in solution is too short to measure
enhanced fluorescence emission (F
F
¼ 0.52) in the self-assembled
1
crystalline phase. The cooperative interplay of p–p stacking and
secondary bonding interactions of dipolar CN groups was found to
be responsible for both the outstanding 1D self-assembling tendency
and the unique optoelectronic properties (enhanced emission and
semiconductivity) of b-DCS.
by using our lifetime measurement instrument sets (s < 70 ps). A very
F
short fluorescence lifetime (s
a solution of related compound (see ref. 9).
F
¼ 4.2 ps) has been observed for
1
1
1 T. F o€ rster and K. Kasper, Z. Elektrochem., 1955, 59, 976.
2 D. A. Stone, A. S. Tayi, J. E. Goldberger, L. C. Palmer and
S. I. Stupp, Chem. Commun., 2011, 47, 5702.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 18971–18973 | 18973