Planar macrocyclic fluoropentamers as supramolecular organogelators

Article abstract of DOI:10.1021/ol201361f

Despite their great diversities, 2D-shaped macrocycles that can serve as the organogelators have been surprisingly rare; two planar macrocyclic fluoropentamers designed by us were highly able to gelate organic solvents, largely derived from their strong tendency to form 1D stacked fibrillar structures stabilized by both interplanar H-bonds and π-π stacking forces.

Full text of DOI:10.1021/ol201361f

ORGANIC
LETTERS
2011
Vol. 13, No. 15
3840–3843
Planar Macrocyclic Fluoropentamers as
Supramolecular Organogelators
Changliang Ren,† Shengyu Xu,† Jun Xu,‡ Hongyu Chen,‡ and Huaqiang Zeng*,†
Department of Chemistry and NUS MedChem Program of the Office of Life Sciences,
3 Science Drive 3, National University of Singapore, Singapore 117543, and Division of
Chemistry & Biological Chemistry, School of Physical & Mathematical Sciences,
Nanyang Technological University, 21 Nanyang Link, Singapore 637371
chmzh@nus.edu.sg
Received May 19, 2011
ABSTRACT
Despite their great diversities, 2D-shaped macrocycles that can serve as the organogelators have been surprisingly rare; two planar macrocyclic
fluoropentamers designed by us were highly able to gelate organic solvents, largely derived from their strong tendency to form 1D stacked fibrillar
structures stabilized by both interplanar H-bonds and πÀπ stacking forces.
Low-molecular-weight organogelators (LMOGs) have
promising applications in diverse areas ranging from
structure-directing agents, drug delivery systems, sensors,
catalysis, to electronic nanodevices.1 Compared to the
currently available large structural diversity of acyclic
gelators,1aÀi the emerging macrocyclic gelators have been
much less studied.1j The hitherto discovered macrocyclic
gelators or their simply modified derivatives mostly have
been based on well-known three dimensionally (3D)
shaped molecules including calixarene,2a,b cyclodextrin,2c
cyclophane,2d resorcinarene,2e cucurbit[7]uril,2f crown
ether,2g and diimide2h- or dehydrobenzoannulene2i-based
macrocycles, while 2D planar macrocyclic organogelators
remainverylimitedwith onlyone recentexample known to
us that derives from arylene ethynylene macrocycles.2j,k
This inability of 2D-shaped macrocycles to form gels is in
sharp contrast with their ability to form vesicles of varying
sizes3aÀd and with the significant gellating ability of other
planar polycyclic molecules.3eÀg Given the availability of
diverse 2D-shaped macrocycles,3h the rare occurrence of
their use as organogelators can be attributed to the general
difficulty1e,2h,2j in the rational design of organogelators
(2) (a) Aoki, M.; Murata, K.; Shinkai, S. Chem. Lett. 1991, 1715. (b)
Becker, T.; Yong Goh, C.; Jones, F.; McIldowie, M. J.; Mocerino, M.;
Ogden, M. I. Chem. Commun. 2008, 3900. (c) Kida, T.; Marui, Y.;
Miyawaki, K.; Kato, E.; Akashi, M. Chem. Commun. 2009, 3889. (d)
Becerril, J.; Burguete, M. I.; Escuder, B.; Luis, S. V.; Miravet, J. F.;
Querol, M. Chem. Commun. 2002, 738. (e) Haines, S. R.; Harrison, R. G.
Chem. Commun. 2002, 2846. (f) Hwang, I.; Jeon, W. S.; Kim, H.-J.; Kim,
D.; Kim, H.; Selvapalam, N.; Fujita, N.; Shinkai, S.; Kim, K. Angew.
Chem., Int. Ed. 2007, 46, 210. (g) Jung, J. H.; Kobayashi, H.; Masuda,
M.; Shimizu, T.; Shinkai, S. J. Am. Chem. Soc. 2001, 123, 8785. (h)
Nakagaki, T.; Harano, A.; Fuchigami, Y.; Tanaka, E.; Kidoaki, S.;
Okuda, T.; Iwanaga, T.; Goto, K.; Shinmyozu, T. Angew. Chem., Int.
Ed. 2010, 49, 9676. (i) Hisaki, I.; Shigemitsu, H.; Sakamoto, Y.;
Hasegawa, Y.; Okajima, Y.; Nakano, K.; Tohnai, N.; Miyata, M.
Angew. Chem., Int. Ed. 2009, 48, 5465. (j) Balakrishnan, K.; Datar, A.;
Zhang, W.; Yang, X.; Naddo, T.; Huang, J.; Zuo, J.; Yen, M.; Moore,
J. S.; Zang, L. J. Am. Chem. Soc. 2006, 128, 6576. For porphyrin-based
metal-containing 2D planar macrocyclic gelators, see: (k) Norstrum,
C. F. v.; Picken, S. J.; Schouten, A.-J.; Nolte, R. J. M. J. Am. Chem.
Soc. 1995, 117, 9957. (l) Iavicoli, P.; et al. J. Am. Chem. Soc. 2010
132, 9350.
† National University of Singapore.
‡ Nanyang Technological University.
(1) (a) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237. (b)
Weiss, R. G.; Terech, P. Molecular Gels: Materials with Self-Assembled
Fibrillar Networks; Springer: New York, 2006. (c) Hirst, A. R.; Escuder, B.;
Miravet, J. F.; Smith, D. K. Angew. Chem., Int. Ed. 2008, 47, 8002. (d)
Llusar, M.; Sanchez, C. Chem. Mater. 2008, 20, 782. (e) Dastidar, P.
Chem. Soc. Rev. 2008, 37, 2699. (f) Piepenbrock, M.-O. M.; Lloyd, G. O.;
Clarke, N.; Steed, J. W. Chem. Rev. 2009, 110, 1960. (g) Smith, D. K.
Chem. Soc. Rev. 2009, 38, 684. (h) Suzaki, Y.; Taira, T.; Osakada, K.
J. Mater. Chem. 2010, 21, 930. (i) Steed, J. W. Chem. Commun. 2011, 47,
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P.; Yuan, L.; Gong, B. Org. Lett. 2010, 12, 2958.
r
10.1021/ol201361f
Published on Web 07/11/2011
2011 American Chemical Society

and to the lack of suitable designer protocols allowing the
strong intermolecular interactions among macrocycles to
be directionally controllable and precisely balanced to
favor the formation of not only one-dimensional (1D)
fibrillar nanostructures but also 3D entangled networks
for extensive solvent entrapment.1e,2hÀ2j,3i Accordingly,
new gelling agents often have been discovered serendipi-
tously or produced by modifying the serendipitously ob-
tained gelators.1e
A few new dimensions recently were added into the
design aspects of supramolecular gelling agents. Dastidar1e,4a
and McNeil4b,c adopted a de novo crystal engineering
approach to create new gelators by design. This approach
emphasizes the importance of 1D intermolecular interac-
tions in inducing and maintaining gel formation,3i and
such interactions believably can be extracted from the
known crystal structures and applied to identify new types
of gelators. Another de novo design strategy explores the
hostÀguest chemistry to design responsive supramolecular
gels. The prominent examples along this line include the
use of cyclodextrin by Harade,4d crown ether by Huang
and Liu,4e cucurbit[8]uril by Scherman,4f and proteinÀ
peptide interactions by Regan.4g
with its nearest pentamer, leading to a dimeric ensemble
˚
where the average interplanar distance is as short as 3.1 A.
These H-bonds form as a result of comparatively much
weaker CÀF HÀN H-bonds, causing the two amide
3 3 3
bonds in every pentamer to twist out of the pentameric
plane to form stronger CdO HÀN H-bonds with
3 3 3
the adjacent nearest pentamer. The weakness of
CÀF HÀN H-bonds can be further illustrated by a
3 3 3
dimer molecule 1d (Figure 1c) whose amide bond is twisted
out of the aromatic plane by 45° in order to form stronger
˚
CdO HÀN H-bonds (2.17 A) even in the presence of
3 3 3
two stabilizing CÀF HÀN H-bonds (F Hdistances=
3 3 3
3 3 3
˚
2.50 and 2.54 A).
We recently designed and crystallized C5-symmetric
fluoropentamer 15 with its circular pentameric backbone3h,6
enforced by intramolecular CÀF HÀN H-bonds
3 3 3
(Figure 1aÀb).6e,f The aromatic backbone of 1 was re-
vealed to be quite planar which should enhance interplanar
πÀπ interactions. In addition, every pentamer forms two
Figure 1. (a) Structures of fluoropentamers 1À3. (b) Top and
side views of crystal structure of 1,5 illustrating the formation of
˚
intermolecularCdO HÀNH-bonds(2.50A, Figure1b)
3 3 3
˚
interplanar H-bonds of 2.50 A in length. (c) Structure of dimer
1d, and top and side views of its crystal structure, illustrating the
˚
formation of intermolecular H-bonds of 2.17 A in length.
(3) (a) Shklyarevskiy, I. O.; Jonkheijm, P.; Christianen, P. C. M.;
Schenning, A. P. H. J.; Meijer, E. W.; Henze, O.; Kilbinger, A. F. M.;
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Pouderoijen, M. J.; Engelkamp, H.; Schenning, A. P. H. J.; Christianen,
P. C. M.; Maan, J. C.; Meijer, E. W. Angew. Chem., Int. Ed. 2006, 45,
1232. (d) Ajayaghosh, A.; Varghese, R.; Praveen, V. K.; Mahesh, S.
Angew. Chem., Int. Ed. 2006, 45, 3261. (e) Zang, L.; Che, Y.; Moore, J. S.
Acc. Chem. Res. 2008, 41, 1596. (f) Wu, J.; Pisula, W.; Mullen, K. Chem.
Rev. 2007, 107, 718. (g) Krieg, E.; Shirman, E.; Weissman, H.; Shimoni,
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Li, Z.; Xue, F.; Su, H. B.; Chass, G. A.; Zeng, H. Q. J. Am. Chem. Soc.
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Iwasawa, T.; Rebek, J. J. Am. Chem. Soc. 2009, 131, 7402.
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J. D.; Dufresne, E. R.; Regan, L. J. Am. Chem. Soc. 2010, 132, 14024.
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H. Q. Angew. Chem., Int. Ed. 2011, in revision.
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H. B.; Zeng, H. Q. J. Am. Chem. Soc. 2010, 132, 5869. (c) Qin, B.; Chen,
X. Y.; Fang, X.; Shu, Y. Y.; Yip, Y. K.; Yan, Y.; Pan, S. Y.; Ong, W. Q.;
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Dotted cycles in (b) indicate the amide bonds that are twisted
out of the plane to form stronger intermolecular H-bonds that
enhance the interplanar aggregations.
Inferred from the existence of these interplanar
H-bonds, shortened interplanar distance, and the pla-
nar aromatic backbone as found in pentamer 1 that
exhibits very poor solubilities in all of the organic
solvents, we envisioned that macrocyclic analogs de-
rived from 1 that bear suitably modified hydrocarbon
chains and enhanced solubilities could be gelators.
Their gelating ability should derive first from their
tendency to form 1D stacked fibrillar structures that
are stabilized by both interplanar H-bonds and πÀπ
stacking forces (Figure 1c), followed by the interco-
lumnar association via hydrophobic hydrocarbon
chains to form a 3D gelling network, resulting in the
gel by trapping organic solvents through surface ten-
sion and capillary forces.
To test our hypothesis, pentamers 2 and 3 each can be
synthesized after 15 steps starting from the commercially
available 2-fluoro-3-nitrobenzoic acid (Scheme S1). The
ability of 2 and 3 toserve as2Dplanarmacrocyclic gelators
was examinedin a variety of organic solventsbythe “stable
to inversion” method. In brief, the gelators and solvents
were mixed in a sealed sample vial and heated in an oil bath
Org. Lett., Vol. 13, No. 15, 2011
3841

until all of the gelators were dissolved. The solution was
then cooled to 25 °C under ambient conditions. The
samples were regarded as a gel if no flow was observed
within 30 s after inverting the vial. By this method, solvents
that can be gelated by 2 and 3 at room temperature are
summarized in Table 1. In other solvents, 2 and 3 are either
insoluble or too soluble (>25 mM), resulting in no gel
formation. In both n-hexane and ethyl acetate, 3 carrying
longer aliphatic side chains functions as a better gelator
than 2 carrying shorter ones. Pentamer 2, however, pos-
sesses a better gelating ability than 3 when diethyl ether,
cyclohexane, and dioxane are used as the solvent, probably
due to the much enhanced solubility of 3 in these solvents.
The minimum gelation concentrations (MGCs) for 2 and 3
in n-hexane are as low as 2.67 and 1.61 mM, which
correspond to 2.8 Â 103 and 4.8 Â 103 solvent molecules
being efficiently trapped on average by just one macro-
cyclic gelator molecule of 2 and 3, respectively.
Table 1. Minimum Gelation Concentrations for Pentamers 2
and 3 in Different Solvents at Room Temperaturea
solvents
pentamer 2 (mM/wt %) pentamer 3 (mM/wt %)
Figure 2. Temperature-dependent UV absorption spectra of (a)
2 and (b) 3 at 2 Â 10À5 M in n-hexane. The determined
gelÀsolution transition temperatures (Tg) for 2 and 3 in n-
hexane at various concentrations. The data were the averaged
value of two runs. The relative error for all of the Tg’s is within
0.6 °C.
n-Hexane
2.67/0.51 (tr)
6.40/0.88 (op)
4.72/0.82 (op)
5.57/0.89 (tr)
5.70/0.69 (op)
1.61/0.37 (tr)
5.03/0.86 (op)
14.88/3.18 (op)
S
Ethyl acetate
Diethyl ether
Cyclohexane
Dioxane
20.40/3.02 (op)
a Abbreviations: S = soluble, tr = transparent, op = opaque.
temperature (Table 1), the UVÀvis data allow us to
surmise that 2 possibly can gelate n-hexane better than 3
at 25 °C. To verify this postulate, gelÀsolution transition
temperatures (Tg) for 2 and 3 in n-hexane were determined
by the “falling drop” method.7 Indeed, at the lower tem-
perature end, 2 is a better gelator (Figure 2c). The intersec-
tion point of the two curves has a (1.42 mM, 23.3 °C)
coordinate, suggesting that 2 and 3 are equal gelators and
capable of gelating n-hexane at an MGC of 1.42 mM at
23.3 °C. At temperatures below 23.3 °C, 2 becomes more
capable of gelating n-hexane. For instance, at 19.5 °C, the
MGC of 2 was determined to be 0.75 mM, while the MGC
of 3 extrapolated from the curve is 1.09 mM. Consistent
with the very strong intermolecular aggregation that does
not change significantly between 0 and 18 °C (Figure 2a),
MGC values for 2 essentially remain constant around
0.70 mM over the same temperature range. In further
accord with the dramatically increased λmax values and
apparently much weakened intermolecular aggregation
from 18 to 25 °C, the MGC value of 2 increases radically
from ∼0.70 to 2.67 mM. This trend in intermolecular aggre-
gation and MGC values in n-hexane was completely reversed
in ethyl acetate. Specifically, the intermolecular aggregation
inferred from UVÀvis spectra is considerably enhanced for 3,
rather than 2, between 0 and 25 °C (Figures S10 and S11).
Given that the absorption spectrum of the H-aggregate
typically consists of a blue-shifted band with respect to the
absorption by monomer, self-assembly of 2 and 3 in the
organic solvents shown in Table 1 was investigated by
UVÀvis spectroscopy. Compared to the maximum ad-
sorption (λmax) value of 274 nm for both 2 and 3 in
chloroform where no gelation occurs, a blue shift of 4À9nm
is observed in all the solvents in which they can form the
gels (Table 1 and Figures S6 and S7). This blue shift is in
accordwiththe H-type packingseeninthe crystal structure
(Figure 1b) and suggests both 2 and 3 assemble into the
H-aggregates in forming the gels. Furthermore, except for
3 in dioxane, the λmax values for 2 and 3 in n-hexane, ethyl
acetate, cyclohexane, and dioxane are lower than those
of 2 and 3 in chloroform (Figures S6 and S7), likely due to
the better aggregation in these solvents with respect to
chloroform whereby extensive aggregations of 2 also occur
at ∼1 mM and above (Figures S30 and S31).
Further examinations of variable temperature UVÀvis
data interestingly show that the absorbance of λmax drops
very quickly for 2 in n-hexane when temperature decreases
from 25 to 0 °C (Figure 2a), suggesting stronger intermo-
lecular aggregations occurring at lower temperature, while
much smaller changes in λmax value for 3 in n-hexane were
observed (Figure 2b), indicating similar aggregation ex-
tents in 3 between 25 and 0 °C. Despite the fact that 3
actually is a better gelator than 2 in n-hexane at room
(7) Debnath, S.; Shome, A.; Dutta, S.; Das, P. K. Chem.;Eur. J.
2008, 14, 6870.
3842
Org. Lett., Vol. 13, No. 15, 2011

Accordingly, 3 remains as a consistently better gelator of
ethyl acetate than 2 at <25 °C (Figure S5).
formed in n-hexane possibly may enhance the retention of
solvent molecules, accounting for the lower MGC values in
n-hexane compared to those in ethyl acetate (Table 1).
To establish the 3D arrangement of 1D H-aggregates
responsible for the fiber formation, powder X-ray diffrac-
tion (XRD) analysis was carried out. However, the ob-
tained XRD data are of low resolution and do not allow
for the unambiguous assignment of the packing pattern by
1D columns.8 Still, the presence of predominant major
peaks below 5° (Figures S27 and S28) may allow us to
estimate the interpenetrating depth by exterior side chains.
From 2θ = 3.70° in Figure S28 for pentamer 3, the value of
The solid state morphology of the gelling networks for the
as-formed gels was visualized using the transmission elec-
tron spectroscopy (TEM) technique. Typically, the TEM
grids were immersed in the gels, and images of the gels
spotted onto the TEM grids were then captured by TEM.
The TEM images demonstrate the extensive formation
of nanofibers, presumably resulting from intercolumnar
associations of the 1D H-aggregates, for 2 in n-hexane
(Figure 3a), ethyl acetate (Figure 3b), and cyclohexane
(Figure 3c). These seemingly endless nanofibers typi-
cally measure between 100 and 250 nm in width and are
structured in a 3D knotted network able to “freeze”
solvent molecules to form the gel. For 3, fiber formation
is also observed in ethyl acetate (Figure 3d) and n-hexane
(Figure S26) even though the image quality seems not to
be very excellent.
d100 spacing for gels formed by pentamer 3 can be calcu-
˚
lated to be 23.86 A, corresponding to an intercolumnar
distanceofdhex =2.76nmor dtetra = 3.37nm, respectively,
for the hexagonal and tetragonal arrangements that are the
two most common packing patterns. In light of a radius of
∼2.13 nm for the 1D columns formed from 3 (0.68 nm
from its pentamer core plus 1.45 nm from its dodecyl side
chain), the overlapping among the 1D columns can be
calculated to be about 0.75 and 0.45 nm, respectively, for
the hexagonal and tetragonal arrangements. This suggests
that the exterior dodecyl side chains may possibly penetrate
into each other by a four- to seven-carbon linker length.
˚
Similarly, on the basis of the value for d100 = 18.70 A
(2θ = 4.72°, Figure S27) for gels formed by 2 and an
overall radius of 1.65 nm for the 1D columns by 2, an
intercolumnar distance of dhex =2.16nmanddtetra =2.64nm
and so an overlapping of 0.57 and 0.33 nm among the
exterior octyl side chains, respectively, for the hexagonal
and tetragonal arrangements can be estimated, suggesting
that it is highly likely that the octyl side chains penetrate
into each other by a three- to five-carbon linker length.
In summary, a crystallographic observation of the
H-bond enhanced intermolecular aggregation occurring
in a 2D-shaped macrocyclic fluoropentamer enables us to
design two macrocyclic organogelators with high gelling
abilities in organic solvents such as hexane, cyclohexane,
ethyl acetate, and dioxane. Experimental analyses based
on TEM and XRD data suggest that the gelling networks
contain 3D entangled nanofibers formed from the inter-
columnar association of 1D H-aggregates possibly via
hydrophobic interactions among interpenetrating alkyl
side chains. These fluoropentamers represent rare exam-
ples of 2D-shaped macrocyclic organogelators.
Figure 3. TEM images of the as-formed gels of 2 in (a) n-hexane,
(b) ethyl acetate, and (c) cyclohexane and (d) of 3 in ethyl acetate
as well as SEM micrographs of the as-formed gels of (e) 2 in n-
hexane, (f) 3 in n-hexane, (g) 2 in ethyl acetate, and (h) 3 in ethyl
acetate.
A micromorphological investigation by scanning elec-
tron microscopy (SEM) reveals similar surface topogra-
phies in the microstructure of the gels formed from the
same solvent (Figure 3eÀ3h). While a highly porous sur-
face with a three-dimensional lattice of many voids and
possibly channels is found for gels formed by both 2 and 3
in n-hexane (Figure 3e and 3f), a smoother surface with
significantly less voids embedded within the 3D lattice
appears for those formed in ethyl acetate by 2 or 3
(Figure 3g and 3h). Such an increase in porosity for gels
Acknowledgment. Financial support of this work to
H.Z. by the NUS AcRF Tier 1 grants (R-143-000-375-112
and R-143-000-398-112) and A*STAR BMRC research
consortia (R-143-000-388-305) is gratefully acknowledged.
Supporting Information Available. Synthetic proce-
dures and a full set of characterization data including
1H NMR, 13C NMR, HRMS, and crystal data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(8) The XRD data of the gels formed from 2 and 3 differ significantly
from the XRD of single crystals of 1 (Figure S29) that packs hexagonally
in a 2D space but not in a 3D space. Comparison of XRD data among
1À3 does not permit us to confidently deduce the possible arrangements
of 1D columns formed by 2 and 3.
Org. Lett., Vol. 13, No. 15, 2011
3843