H-Bonding-driven gel formation of a phenylacetylene macrocycle

Article abstract of DOI:10.1039/c1ob05441d

An amide-containing phenylacetylene macrocycle (PAM) has been synthesized and its gelation properties were studied in different solvents. Surprisingly, this macrocycle forms organogels at low concentration in many polar and apolar solvents. XRD and FTIR analysis suggest that this macrocycle forms stable supramolecular assemblies owing to H-bonding. Scanning electron microscopy analyses show the formation of bundles of nanofibrils, demonstrating the long-range organization of this material.

Full text of DOI:10.1039/c1ob05441d

View Article Online / Journal Homepage / Table of Contents for this issue
Organic &
Biomolecular
Chemistry
Dynamic Article Links
Cite this: Org. Biomol. Chem., 2011, 9, 4440
www.rsc.org/obc
COMMUNICATION
H-Bonding-driven gel formation of a phenylacetylene macrocycle†
Katy Cantin, Simon Rondeau-Gagne´, Jules Rome´o Ne´abo, Maxime Daigle and Jean-Francois Morin*
Received 21st March 2011, Accepted 19th April 2011
DOI: 10.1039/c1ob05441d
An amide-containing phenylacetylene macrocycle (PAM) has
been synthesized and its gelation properties were studied
in different solvents. Surprisingly, this macrocycle forms
organogels at low concentration in many polar and apolar
solvents. XRD and FTIR analysis suggest that this macro-
cycle forms stable supramolecular assemblies owing to H-
bonding. Scanning electron microscopy analyses show the
formation of bundles of nanofibrils, demonstrating the long-
range organization of this material.
functions that are expected to improve intermolecular interactions,
our ultimate goal being to prepare more robust organic nano-
objects for further utilization in devices. We report herein the
synthesis and gelation properties of a new PAM (1, Fig. 1) showing
excellent gelation properties in many organic solvents owing to
the presence of two amide groups that increased intermolecular
interactions through H-bonding. H-bonding has proven to be an
efficient strategy to increase intermolecular interactions between
macrocycles.8
The face-to-face stacking of p-conjugated systems is a convenient
and elegant way of building well-ordered molecular architectures
that could be used as efficient active components in electronic
devices. In fact, high levels of organization in organic semi-
conducting materials can help in improving charge mobility in
bulk and thin-film configurations. Rigid, 1D conjugated building
blocks that have been used to create such ordered arrays in the solid
state are hexa-peri-hexabenzocoronene,1 perylenes,2 porphyrins,3
phthalocyanines,4 p-conjugated oligomers5 and macrocycles.6
Among others, macrocycles, especially phenylacetylene macro-
cycles (PAMs), have attracted a lot of attention in the past ten
years because supramolecular assemblies of PAMs in a face-
to-face configuration can lead to porous materials analogous
to carbon nanotubes. However, a good and reliable method to
assemble PAMs in a face-to-face configuration still needs to be
developed, the biggest challenge being to find a good balance
between PAMs’ solubility and intermolecular interactions. Self-
association of PAMs in solution has been demonstrated several
times,5 but relatively low binding constants have been reported
and the architectures thus formed through this process are usually
made of dimers or trimers at most.
Fig. 1 Phenylacetylene macrocycles 1 and 2.
The structures of the two PAMs prepared for this study are
presented in Fig. 1. The only structural difference between PAMs 1
and 2 is the functional group introduced onto the head and the tail
of the macrocycle to generate intermolecular H-bonding. In PAM
1, a very common amide group was introduced while urethane
was used for PAM 2. Because our long-term goal is to prepare
covalently linked nanotubes from these architectures, diyne unit
was introduced within the structure of the PAMs to allow further
photopolymerization to form polydiacetylene (PDA).9 Four octyl
chains per PAM were also added to increase the solubility and van
der Waals intermolecular interactions of the resulting PAMs.
Macrocycles 1 and 2 were prepared using similar synthetic
pathways, which involved a ring closing reaction by the Eglinton–
Glaser coupling between identical symmetrical meta-substituted
trimers in highly diluted conditions.10 The synthetic strategies
for the synthesis of 1 and 2 are depicted in Scheme 1 and
in Scheme S1 in the ESI,† respectively. For 1, the amide
Recently, Moore et al. have developed an efficient way of
assembling carbazole-based PAMs using the sol–gel process in
cyclohexane.7 The gel thus obtained was made of microns-long
nanofibrils owing to p–p and hydrophobic interactions. In order to
extend the scope of this strategy to other types and sizes of PAMs,
we have undertaken the preparation of PAMs containing chemical
De´partement de chimie et Centre de Recherche sur les Mate´riaux Avance´s
(CERMA), Pavillon A.-Vachon, 1045 Ave de la Me´decine, Universite´ Laval,
Que´bec, Canada, G1V 0A6. E-mail: jean-francois.morin@chm.ulaval.ca;
Fax: +1-418-656-7916; Tel: +1-418-656-2812
† Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for all the new compounds,
synthetic scheme for PAM 2 and X-ray diffractograms. See DOI:
10.1039/c1ob05441d
4440 | Org. Biomol. Chem., 2011, 9, 4440–4443
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Table 1 Gelation properties of PAM 1 at 1.0 w/v%
Solvent
Observationa
toluene
1,2-dichlorobenzene
benzene
G
S
G
p-xylene
chlorobenzene
pyridine
G
G
S
tetrahydrofuran
methanol
acetone
S
P
P
ethyl acetate
dimethylformamide
acetonitrile
cyclohexane
1,4-dioxane
hexanes
G
S
S
G
G with precipitate
G
G
S
1,2-dichloroethane
chloroform
carbon disulfide
2-propanol
ethanol
S
P
P
dichloromethane
decalin
methyl tert-butyl ether
P
G
P
a G = gel; S = soluble; P = precipitate.
et al.13 As reported by Breslow et al.,14 the high-dilution conditions
usually needed to perform macrocyclization reactions can be
avoided when Cu(I) and Cu(II) are used in anhydrous pyridine,
making the reaction mixture easier to workup. The macrocycle 1
was thus obtained in 26% yield, which is low but comparable to
other non-templated ring closing reactions using this method. The
macrocycle 1 is readily soluble in THF and chloroform and partly
soluble in all the other common organic solvents.
For the gelification process, powder of 1 and 2 was dissolved in
solvent (1.0 w/v%) by heating the solvent near its boiling point
and the clear solution thus obtained was allowed to cool down
at room temperature, resulting in a gel which was either opaque
or translucent depending on the solvent used for the gelification
process. The gelification properties of PAM 1 are summarized in
Table 1. Surprisingly, PAM 1 formed a gel in 10 out of the 23
solvents tested, which included aromatic as well as polar (ethyl
acetate) and apolar (hexanes) solvents. The astonishing ability
of PAM 1 to form a gel in different solvents can be ascribed
to its ability to participate in different kinds of intermolecular
interactions, presumably H-bonding, van der Waals and p–p
interactions.
Scheme 1 Synthesis of PAM 1.
groups were place at the “top” and “bottom” junctions of
the macrocycle. Starting from 3,5-diaminobenzoic acid, the
amino groups were replaced by diazonium intermediates using
sodium nitrite in acidic conditions. The bis(diazonium) salt was
then reacted with potassium iodide to provide compound 5
in 43% yield.11 Amide was formed by reacting compound 5
with dodecylamine using N,N¢-diisopropylcarbodiimide (DIC)
and 1-hydroxybenzotriazole (HOBt) to provide compound 6
in good yield (82%). It is noteworthy that in our hands, the
coupling reaction using dicyclocarbodiimide (DCC) and N,N¢-
dimethylaminopyridine (DMAP) gave only a poor yield of
compound 6 since the DCC-activated carboxylic acid derivatives
cannot be converted to the final product efficiently. Standard
Castro–Stephen–Sonogashira12 coupling, using trimethylacety-
lene (TMSA), PdCl2(PPh3)2 and CuI, was then used to install
protected alkynes in the meta positions to give compound 7 in
excellent yield (98%).
The two alkyne groups were then deprotected using potassium
hydroxide in tetrahydrofuran (THF) and directly coupled to 1-
octyl-3,5-diiodobenzene by Castro–Stephen–Sonogashira reac-
tion to give compound 9 in 32% yield. Because the use of an
excess of 1-octyl-3,5-diiodobenzene was not sufficient to avoid
the formation of longer oligomers, the yield obtained for this
reaction was quite low. The alkyne groups used for the Eglinton
ring closing reaction were introduced using Castro–Stephen–
Sonogashira reaction using triisopropylsilylacetylene (TIPSA) to
yield compound 10 in 88% yield. In the final step, the two alkyne
groups were deprotected using tetrabutylammonium fluoride
(TBAF) in THF, followed by the Eglinton–Glaser ring closing
reaction in high dilution conditions (10-3 M) as adapted by Ho¨ger
PAM 2 shows much better solubility in most of the solvents
listed in Table 1 compared to PAM 1, resulting in very poor
gelation ability at the concentrations tested (1–10 w/v%). This
can be explained by the lower ability of the urethane function to
participate in H-bonding compared to amides and by the higher
rigidity of chemical functions surrounding the amide groups in
PAM 1. This result demonstrates how important the structural
parameters are for the gelification process and how difficult it is to
predict self-assembly behavior based on structural analysis.
In order to study the nanoscale morphology of the organogel of
PAM 1, a small portion of it was slowly dried in ambient conditions
onto a metallic substrate and subjected to scanning electron
microscopy (SEM) analysis. As shown in Fig. 2a, organogel of
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 4440–4443 | 4441
©

View Article Online
To study the molecular organization that leads to the fibrillar
structure observed in the SEM images, X-ray diffraction (XRD)
analyses were performed on films of PAM 1 obtained from a
cyclohexane gel (1.0% w/v) and compared to a film made from
PAM 1 in a THF solution for which no gelation was observed
at this concentration. The diffractogram (see Figure S2 in the
ESI†) of the dried gel showed a sharp intense peak at 2q =
2.5, which corresponds to 3.52 nm. The data extracted from
the diffractogram are not clear enough to confirm any particular
arrangements within the gel. Nonetheless, based on other reports
on PAM assemblies,6,15 it can be hypothesized that the macrocycles
stacked on top of each other to form columnar assemblies with an
intercolumnar distance of ca. 4 nm, which is in good agreement
with the calculated width of PAM 1. The columnar arrangement
is also in good agreement with the fibrillar structure observed by
SEM. Interestingly, no feature except a broad band centered at
2q ª 24, which is representative of an amorphous material, was
observed when a film of PAM 1 was formed from a THF solution.
This is indicative of the importance of the gelification process for
obtaining a well-ordered array of macrocycles.
In summary, we have prepared a phenylacetylene macrocycle
that shows excellent gelation properties in many organic solvents.
Characterization reveals the formation of nanofibrils that consist
of stacks of macrocycles owing to intermolecular hydrogen bond-
ing as showed by the FTIR analysis as a function of temperature.
The interactions strengthen the supramolecular organization and
open the way to the utilization of PAMs as robust building blocks
for the preparation of nanoscale functional materials. PAM 1
is now being studied as a potential precursor in topochemical
polymerization to build covalently linked organic nanotubes.
Fig. 2 SEM images of the organogel from PAM 1 (a) in toluene, scale
bar = 1 mm, and (b) in cyclohexane, scale bar = 50 mm.
PAM 1 in toluene (1.0 w/v%) is made of microns-long 1D wirelike
fibers with diameters of few tens to few hundreds of nanometres.
These nanofibers assembled into much larger bundles, which is
indicative of strong interfiber interactions. Interestingly, the size
of the fiber obtained after the gelification process is dependant
on the nature of the solvent. As shown in Fig. 2b, much larger
fibers than those obtained from toluene gel (Fig. 2a) are observed
when cyclohexane is used at the same gel concentration (1.0%
w/v). In this particular case, microns-wide fibers are obtained, and
although the internal structure of the fibers is difficult to observe,
one can argue that the fibers are made of aligned smaller fibers.
At this point, the parameters that lead to different assembly from
one solvent to another are not well understood and a systematic
study of fiber width–solvent dependence is being conducted.
To assess whether or not the amide groups allow the formation
of intermolecular H-bonding, Fourier transform infrared (FTIR)
spectra were recorded on a gel of PAM 1 at different temperatures.
The results are shown in Fig. 3. For this study, the gel was prepared
in decalin because a high boiling point solvent was needed to avoid
solvent loss during the experiment.
Notes and references
1 For reviews and representative examples, see: (a) A. Fechtenko¨tter, K.
Saalwa¨chter, M. A. Harbison, K. Mu¨llen and H. W. Spiess, Angew.
Chem. Int. Ed., 1999, 38, 3039; (b) M. D. Watson, A. Fechtenko¨tter
and K. Mu¨llen, Chem. Rev., 2001, 101, 1267; (c) C. D. Simpson, J. Wu,
M. D. Watson and K. Mu¨llen, J. Mater. Chem., 2004, 14, 494; (d) J.
Wu, W. Pisula and K. Mu¨llen, Chem. Rev., 2007, 107, 718.
2 (a) K. Sugiyasu, N. Fujita and S. Shinkai, Angew. Chem. Int. Ed., 2004,
43, 1229; (b) X.-Q. Li, X. Zhang, S. Ghosh and F. Wu¨rthner, Chem.
Eur. J., 2008, 14, 8074; (c) Y. Avlasevich, C. Li and K. Mu¨llen, J. Mater.
Chem., 2010, 20, 3814.
3 (a) S.-i. Tamaru, M. Nakamura, M. Takeuchi and S. Shinkai, Org. Lett.,
2001, 3, 3631; (b) M. Shirakawa, S.-i. Kawano, N. Fujita, K. Sada and
S. Shinkai, J. Org. Chem., 2003, 68, 5037; (c) M. Shirakawa, N. Fujita
and S. Shinkai, J. Am. Chem. Soc., 2005, 127, 4164.
4 H. Engelkamp, S. Middelbeek and R. J. M. Nolte, Science, 1999, 284,
785.
5 (a) A. Ajayaghosh, R. Varghese, V. K. Praveen and S. Mahesh, Angew.
Chem. Int. Ed., 2006, 45, 3261; (b) S. Wang, W. Shen, Y. Feng and H.
Tian, Chem. Comm., 2006, 1497; (c) R. Abbel, R. Van Der Weegen, W.
Pisula, M. Surin, P. Lecle`re, R. Lazzaroni, E. W. Meijer and A. P. H. J.
Schenning, Chem. Eur. J., 2009, 15, 9737; (d) B.-K. An, D.-S. Lee, Y.-S.
Park, H.-S. Song and S. Y. Park, J. Am. Chem. Soc., 2004, 126, 10232.
6 (a) J. Zhang and J. S. Moore, J. Am. Chem. Soc., 1992, 114, 9701l;
(b) S. S. Shetty, J. Zhang and J. S. Moore, J. Am. Chem. Soc., 1996, 118,
1019; (c) A. J. Gallant and M. MacLachlan, Angew. Chem. Int. Ed.,
2003, 42, 5307; (d) D. Zhao and J. S. Moore, Chem. Comm., 2003, 807;
(e) L. Shu and M. Mayor, Chem. Comm., 2006, 4134; (f) B. N. Boden, J.
K.-H. Hui and M. J. MacLachlan, J. Org. Chem., 2008, 73, 8069; (g) P.
D. Frischmann, S. Guieu, R. Tabeshi and M. J. MacLachlan, J. Am.
Chem. Soc., 2010, 132, 7668.
Fig. 3 FTIR of a 1.0 w/v% organogel of PAM 1 in decalin as a function
of the temperature.
At 20 ◦C, the associate Amide I and Amide II bands were
observed at 1640 and 1542 cm-1, respectively. Upon heating, the
bands gradually disappeared, as the H-bonds broke, and a new
band at 1685 cm-1 attributed to the free Amide I band appeared.
Unfortunately, the band associated with the free Amide II band
was hidden by the decalin peak in the 1400–1520 cm-1 region.
These spectral changes strongly support the formation of inter-
molecular H-bonding that contributed to the gelation ability of
PAM 1 in addition to possible p–p and van der Waals interactions.
7 (a) K. Balakrishnan, A. Datar, W. Zhang, X. Yang, T. Naddo, J. Huang,
J. Zuo, M. Yen, J. S. Moore and L. Zang, J. Am. Chem. Soc., 2006, 128,
4442 | Org. Biomol. Chem., 2011, 9, 4440–4443
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
6576; (b) N. Naddo, Y. Che, W. Zhang, K. Balakrishnan, X. Yang, J.
Zhao, J. S. Moore and L. Zang, J. Am. Chem. Soc., 2007, 129, 6978;
(c) L. Zhang, Y. Che and J. S. Moore, Acc. Chem. Res., 2008, 41, 1596.
8 H. Shimura, M. Yoshio and T. Kato, Org. Biomol. Chem., 2009, 7, 3205.
9 (a) M. Suzuki, A. Comito, S. I. Khan and Y. Rubin, Org. Lett., 2010,
12, 2346; (b) Y. Xu, M. D. Smith, M. F. Geer, P. J. Pellechia, J. C. Brown,
A. C. Wibowo and L. S. Shimizu, J. Am. Chem. Soc., 2010, 132, 5334.
10 (a) G. Eglinton and W. McCrae, Adv. Org. Chem., 1963, 4, 225; (b) P.
Siemsen, R. C. Livingston and F. Diederich, Angew. Chem. Int. Ed.,
2000, 39, 2632.
11 X. Zhao and K. S. Schanze, Langmuir, 2006, 22, 4856.
12 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron Lett., 1975,
50, 4467.
13 S. Ho¨ger, A.-D. Meckenstock and S. Mu¨ller, Chem. Eur. J., 1998, 4,
2423.
14 D. O’Krongly, S. R. Denmeade, M. Y. Chiang and R. Breslow, J. Am.
Chem. Soc., 1985, 107, 5544.
15 (a) Y. Tobe, N. Utsumi, K. Kawabata, A. Nagano, K. Adachi, S. Araki,
M. Sonoda, K. Hirose and K. Naemura, J. Am. Chem. Soc., 2002, 124,
5350; (b) S. Ho¨ger, Chem. Eur. J., 2004, 10, 1320.
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 4440–4443 | 4443
©