Gels of shape-persistent macrocycles: The role of the interior

Article abstract of DOI:10.1039/c2cc32804f

The aggregation of shape-persistent macrocycles with an empty cavity, an undecyldiether strand and a tetraethylene glycol strand leads in all cases to a macroscopic gelation of the solvent. However, the gelation temperatures are fine-tuned by the intraannular substituents.

Full text of DOI:10.1039/c2cc32804f

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 6547–6549
www.rsc.org/chemcomm
COMMUNICATION
Gels of shape-persistent macrocycles: the role of the interiorw
Joscha Vollmeyer,a Stefan-S. Jester,*a Friederike Eberhagen,a Thomas Prangenberg,b
b
a
¨
Werner Mader and Sigurd Hoger*
Received 19th April 2012, Accepted 9th May 2012
DOI: 10.1039/c2cc32804f
The aggregation of shape-persistent macrocycles with an empty
cavity, an undecyldiether strand and a tetraethylene glycol
strand leads in all cases to a macroscopic gelation of the solvent.
However, the gelation temperatures are fine-tuned by the intra-
annular substituents.
Self-assembly of molecular building blocks towards complex
predictable architectures has become manifested as an estab-
lished concept in chemistry.1 Among the manifoldness of
supramolecular structures, rod-shaped and tubular aggregates
have received increased attention.2 More particularly, they
were recently discussed as sensor materials,3 electrically
conductive molecular wires,4 and ion channels.5 An approved
design principle of molecular building blocks forming the
designated highly anisotropic structures in solution is based
on oblate shape-persistent backbones, surrounded by a flexible
periphery, which—due to different solubility—aggregate by
non-specific interactions (that is without the help of adjunct
functional groups).6–8 Association of tubular aggregates
may also lead to the formation of ribbon-like or vesicular
structures.7c,9 Beyond polycyclic aromatic hydrocarbons,10
comparably large arylene–alkynylene-macrocycles11 allow—
beside the tailorable design of the rigid core dimensions and
shapes—the implementation of central cavities and the intra-
annular attachment of functional units. This should nominally
facilitate an independent host–guest chemistry within the
tubular channels, ideally without influencing the aggregate
structure.6a Whereas the influence of the electronic structure
of the shape-persistent macrocyclic backbones on the aggrega-
tion behavior has been intensively investigated,8d–f the cavity
filling has largely been unexplored.
Fig. 1 Structures of the gel forming macrocycles.
ribbons and even networks. The latter leads to a macroscopically
observable gelation.13 During our studies on the template
directed macrocycle synthesis we observed that macrocycle 1,
with an intraannular ring-crossing aliphatic bridge and extra-
annular octadecyloxy side chains, is capable of forming gels in
selected non-polar solvents (Fig. 1). To evaluate the effect of the
interior filling on gelation, we additionally synthesized two
derivatives of 1 comprising an empty cavity (2) and a tetra-
ethylene glycol strand (3).14 Both of them are also gel-forming.
The gels are obtained after dissolving the macrocycles in hot
cyclohexane (CHX), methylcyclohexane (MCH), or decaline,
and subsequent cooling to room temperature. Within 1 min
(2 and 3) or 10 min (1) slightly yellow, translucent gels are
formed. The critical gelation concentration (cgc) in CHX is
0.5 wt% for all three compounds, as determined by stable-to-
inversion tests. Below cgc—stored at room temperature or
below—a gel phase separates from a solution of 1. Under
identical conditions, 2 or 3 form a cloudy (not gel-like)
precipitate. Gels of 1 and 3 (1 wt% in CHX) maintain unaltered
consistence for days and weeks, respectively, while the corre-
sponding gel of 2 significantly loses stiffness overnight.
Recently, we demonstrated that shape-persistent macro-
cycles carrying extraannular bulky oligostyrene or oligoalkyl
side chains form nanoscale tubular aggregates under specific
solvent conditions.6,12 In some cases, due to interactions
between individual tubes, there are formed bundles and/or
The atomic force microscopy (AFM) investigations of
solution and gel samples cast on mica allow a detailed
characterization of the aggregate morphology. All compounds
(1, 2, and 3) form self-assembled networks, consisting of
randomly oriented ribbons (Fig. 2a and b and ESIw) with a
minimum height of 4.0 Æ 0.5 nm (AFM height measurements,
Fig. 2d). At crossing points a height addition (to 7.5 Æ 0.5 nm;
11.0 Æ 0.5 nm) is observed.
a
´
Rheinische Friedrich-Wilhelms-Universitat Bonn, Kekule-Institut fur
¨
¨
Organische Chemie und Biochemie, Gerhard-Domagk-Str. 1,
53121 Bonn, Germany. E-mail: stefan.jester@uni-bonn.de,
hoeger@uni-bonn.de; Fax: +49 228 73 5662; Tel: +49 228 73 3495
b Rheinische Friedrich-Wilhelms-Universitat Bonn, Institut fur
Anorganische Chemie, Romerstr. 164, 53117 Bonn, Germany
¨
¨
¨
Fusing and splitting of individual ribbons along their direction
occurs frequently (e.g. arrows in Fig. 2b). Transmission electron
microscopy (TEM) investigations of a dried gel of 1 (from a
diluted 1 wt% CHX-gel) on a holey carbon film confirm the
w Electronic supplementary information (ESI) available: Synthesis of
the compounds, DSC measurements, UV/Vis spectra, TEM and AFM
images. See DOI: 10.1039/c2cc32804f
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6547–6549 6547

View Article Online
Fig. 3 DSC cooling (1st scan) and heating curves (2nd scan) of gels of
1 (9.2 wt%), 2 (9.6 wt%) and 3 (8.7 wt%) in CHX (heating/cooling
rates 5 K minÀ1).20
the ribbon height (Fig. 2d). Therefore, the monolayer pattern
resembles a cross section through a multilayer of several
ribbons of the gel. UV/Vis spectroscopy of MCH-solutions
of 1–3 at different temperatures supports this hypothesis. At
low temperatures, the absorption spectra are red-shifted
compared to the spectra at higher temperatures, indicating
p–p-interaction of the aromatic macrocyclic backbones upon
aggregate formation (see ESIw).17
Differential scanning calorimetry (DSC) investigations of
gels of 1–3 (9 to 10 wt% in cyclohexane) allow the determina-
tion of the gel–sol- and sol–gel-transition temperatures and
enthalpies.18 Fig. 3 shows the DSC heat flux curves of the gel
samples. The gel–sol transition temperatures (Tgs), identified
by endothermic peaks during heating the sample, are 37 1C,
34 1C, and 52 1C for 1, 2, and 3, respectively (Table 1). Hence,
the intraannular substitution pattern has a significant influ-
ence on the thermal stability of the gels. The gel formed by the
macrocycle with an empty internal cavity (2) has a lower
melting point compared to the respective gel of 1, which
contains the inner linear alkyl chain. This is attributed to
additional van der Waals contacts between the intraannular
substituents. The polar inner chain in 3 further increases the
gel melting point, most probably due to even enhanced
dipole–dipole interactions as well as solvophobic interactions
between the polar interior and the nonpolar solvent.19 Upon
cooling, the sol–gel-transition (Tsg) shows in the case of 1
in CHX a hysteresis of 17 K with respect to the gel–sol-
transition. Similarly, the gelation point of 3 in CHX is about
10 K below Tgs, whereas 2 immediately gelates CHX when
approaching Tgs.
Fig. 2 (a) and (b) Atomic force microscopy (AFM) images of
aggregates of 1 on mica. (a) A dense fibrous network is observed near
the edge of a gel film of 1 (2.5 Â 10À3 M, 1.0 wt%, CHX), slightly
spread by spinning. (b) Several ribbons with varying widths are cast
from a dilute solution (1.5 Â 10À4 M, 0.06 wt%, MCH), which
randomly fuse/split (arrows) and cross. Inset: proposed aggregate
model. (c) Scanning tunneling microscopy (STM) image of 1 at the
TCB–HOPG interface (c = 10À5 M, VS = À1.2 V, It = 30 pA). The
distance w between two rows is 4.2 Æ 0.2 nm, which is closely related to
the height of the ribbons measured by AFM along the white dotted
line in (b) (see topography cross section in (d)). (e) and (f) Trans-
mission electron microscopy (TEM) images of aggregates obtained
from a gel of 1 in CHX (2.5 Â 10À3 M, 1.0 wt%, diluted with CHX) on
holey carbon. Folding of the aggregates is observed (arrows), which
confirms their ribbon-like structure.
ribbon-like aggregate structure (Fig. 2e and f) with a variable
width of about 50 to 200 nm (see ESIw).
The results obtained by DSC are consistent with the visual
observations of the macroscopic gel samples. Conclusively, the
combination of microscopic and thermal investigations of the
gels has shown that the ring interiors of the macrocycles
Based on previous results2b,9,15 we also assume here the
phase separation of rigid and flexible parts of the molecules.
The macrocycles aggregate and stack on top of each other to
form columns that assemble tightly. As a consequence, a sheet-
like structure is formed, in which adjacent stacks of rigid
backbones arrange as a layer embedded between the flexible
side chain layers.16 Scanning tunneling microscopic (STM)
investigations of 1 at the interface of 1,2,4-trichlorobenzene
(TCB) and highly oriented pyrolytic graphite (HOPG) show
self-assembled monolayers. The rows of adjacent, densely
packed backbones are separated by interdigitating alkoxy side
chains (Fig. 2c). They are 4.2 Æ 0.2 nm apart, a value similar to
Table 1 Transition temperatures and enthalpies determined by DSC
measurements of CHX gels of 1, 2, and 3
Heating
Tgs/1C (DHtrs/kJ molÀ1
Cooling21
Tsg/1C
Hysteresis
Tgs À Tsg/1C
CHX-gel
)
1 (9.6 wt%)
2 (9.2 wt%)
3 (8.7 wt%)
37.3 (À133)
33.5 (À42)
51.8 (À75)
19.9
31.6
41.6
À17.4
À1.9
À10.2
c
6548 Chem. Commun., 2012, 48, 6547–6549
This journal is The Royal Society of Chemistry 2012

View Article Online
interact intermolecularly in the stacked one-dimensional
aggregates and therefore influence the gel properties.
5350–5364; (f) L. Shu, M. Muri, R. Krupke and M. Mayor, Org.
Biomol. Chem., 2009, 7, 1081–1092.
9 S. Rosselli, A.-D. Ramminger, T. Wagner, G. Lieser and S. Hoger,
Chem.–Eur. J., 2003, 9, 3481–3491.
10 (a) J. P. Hill, W. Jin, A. Kosaka, T. Fukushima, H. Ichihara,
T. Shimomura, K. Ito, T. Hashizume, N. Ishii and T. Aida,
Science, 2004, 304, 1481–1483; (b) W. Pisula, X. Feng and
K. Mullen, Adv. Mater., 2010, 22, 3634–3649.
In summary, we synthesized shape-persistent macrocycles
with extraannular octadecyloxy chains which are gel forming
in apolar solvents due to non-specific interactions. A function-
alized cavity is not crucial for gel formation, and the aggregate
structure is similar for all compounds, but the intraannular
filling influences the gel melting points and aggregation
dynamics. The ring interior is therefore clearly a powerful
handle to fine-tune the gel properties. Future investigations
will include the role of size and flexibility of the intramolecular
strands on the supramolecular assembly in one dimension as
well as possible guest molecule organization.
11 M. Iyoda, J. Yamakawa and M. J. Rahman, Angew. Chem., 2011,
123, 10708–10740 (Angew. Chem., Int. Ed., 2011, 50, 10522–10553).
12 (a) X. Cheng, A. Ver Heyen, W. Mamdouh, H. Uji-i, F. De
Schryver, S. Hoger and S. De Feyter, Langmuir, 2006, 23,
1281–1286; (b) J. C. Gielen, A. Ver Heyen, S. Klyatskaya,
W. Vanderlinden, S. Hoger, J. C. Maan, S. De Feyter and
P. C. M. Christianen, J. Am. Chem. Soc., 2009, 131, 14134–14135.
13 For recent reviews and reports about organogelators, see also:
(a) Molecular Gels: Materials with Self-Assembled Fibrillar
Networks, ed. R. G. Weiss and P. Terech, Springer, Dordrecht,
2006; (b) J. J. van Gorp, J. A. J. M. Vekemans and E. W. Meijer,
J. Am. Chem. Soc., 2002, 124, 14759–14769; (c) I. Hwang,
W. S. Jeon, H.-J. Kim, D. Kim, H. Kim, N. Selvapalam,
N. Fujita, S. Shinkai and K. Kim, Angew. Chem., 2007, 119,
214–217 (Angew. Chem., Int. Ed., 2007, 46, 210–213);
(d) V. Percec, M. Peterca, M. E. Yurchenko, J. G. Rudick and
P. A. Heiney, Chem.–Eur. J., 2008, 14, 909–918; (e) I. Hisaki,
H. Shigemitsu, Y. Sakamoto, Y. Hasegawa, Y. Okajima,
K. Nakano, N. Tohnai and M. Miyata, Angew. Chem., 2009,
121, 5573–5577 (Angew. Chem., Int. Ed., 2009, 48, 5465–5469);
(f) J. A. Foster and J. W. Steed, Angew. Chem., 2010, 122,
6868–6874 (Angew. Chem., Int. Ed., 2010, 49, 6718–6724);
(g) T. Ide, D. Takeuchi and K. Osakada, Chem. Commun., 2012,
48, 278–280.
Financial support for this project by the DFG (particularly
the SFB 624) and the MIWFT in the frame of the consortium
ENQUETE is gratefully acknowledged.
Notes and references
1 (a) G. Whitesides, J. Mathias and C. Seto, Science, 1991, 254,
1312–1319; (b) J.-M. Lehn, Chem. Soc. Rev., 2007, 36, 151–160.
2 (a) M. A. Balbo Block, C. Kaiser, A. Khan and S. Hecht, Top.
Curr. Chem., 2005, 245, 89–150; (b) L. C. Palmer and S. I. Stupp,
Acc. Chem. Res., 2008, 41, 1674–1684; (c) Y. Yan, Y. Lin, Y. Qiao
and J. Huang, Soft Matter, 2011, 7, 6385–6398.
3 (a) L. Zang, Y. Che and J. S. Moore, Acc. Chem. Res., 2008, 41,
1596–1608; (b) T. Nakagaki, A. Harano, Y. Fuchigami, E. Tanaka,
S. Kidoaki, T. Okuda, T. Iwanaga, K. Goto and T. Shinmyozu,
Angew. Chem., 2010, 122, 9870–9873 (Angew. Chem., Int. Ed.,
2010, 49, 9676–9679).
14 For details on the synthetic procedures, see ESIw.
15 (a) E. R. Zubarev, M. U. Pralle, E. D. Sone and S. I. Stupp, J. Am.
Chem. Soc., 2001, 123, 4105–4106; (b) H. J. Kim, T. Kim and
M. Lee, Acc. Chem. Res., 2010, 44, 72–82.
4 B. R. Kaafarani, Chem. Mater., 2011, 23, 378–396.
5 A. J. Helsel, A. L. Brown, K. Yamato, W. Feng, L. Yuan,
A. J. Clements, S. V. Harding, G. Szabo, Z. Shao and B. Gong,
J. Am. Chem. Soc., 2008, 130, 15784–15785.
16 The observation of ribbon-like structures gives a hint on the
hierarchical nature of the aggregation process of these macrocycles.
17 Due to precipitation or the formation of two phases, temperature
and concentration dependent UV/Vis spectroscopy did not yield
reliable data, inevitable to analyze the aggregation process in
detail. The UV/Vis spectrum in chloroform does not exhibit any
thermochromism, and is similar to the high temperature spectra in
MCH (see ESIw).
6 (a) S. Rosselli, A.-D. Ramminger, T. Wagner, B. Silier,
S. Wiegand, W. Haußler, G. Lieser, V. Scheumann and
S. Hoger, Angew. Chem., 2001, 113, 3233–3237 (Angew. Chem.,
Int. Ed., 2001, 40, 3137–3141); (b) N. Dingenouts, S. Klyatskaya,
S. Rosenfeldt, M. Ballauff and S. Hoger, Macromolecules, 2009,
42, 5900–5902; (c) M. Fritzsche, S.-S. Jester, S. Hoger, C. Klaus,
N. Dingenouts, P. Linder, M. Drechsler and S. Rosenfeldt, Macro-
molecules, 2010, 43, 8379–8388.
18 S. R. Raghavan and B. H. Cipriano, in Molecular Gels: Materials
with Self-Assembled Fibrillar Networks, ed. R. G. Weiss and
P. Terech, Springer, Dordrecht, 2006, pp. 241–254.
7 (a) J.-H. Ryu, N.-K. Oh and M. Lee, Chem. Commun., 2005,
1770–1772; (b) J. Couet and M. Biesalski, Soft Matter, 2006, 2,
1005–1014; (c) J.-K. Kim, E. Lee, M.-C. Kim, E. Sim and M. Lee,
J. Am. Chem. Soc., 2009, 131, 17768–17770.
19 The measured transition enthalpies increase in the order 2 o 3 o 1
and thus do not correlate with the trend of the thermal stability.
This demonstrates that entropic and solvation effects play a
significant role in the aggregation process. However, the Hildeb-
rand solubility parameters indicate stronger interactions between
oligo(ethyleneoxide) chains as compared to oligoethylene chains,
see: Polymer Handbook, ed. J. Brandrup, E. H. Immergut,
E. A. Grulke, A. Abe and D. R. Bloch, John Wiley & Sons, 4th
edn, 2005.
20 The slight oscillation of the cooling curve of 3 is caused by the
cooling system of the calorimeter and sometimes cannot be
eliminated. The transition temperatures do not change when the
scans are repeated.
8 Other driving forces for aggregation are, for example, ionic inter-
actions: (a) J. K. H. Hui, P. D. Frischmann, C. H. Tso,
C. A. Michal and M. J. MacLachlan, Chem.–Eur. J., 2010, 16,
2453–2460, donor–acceptor interactions: see 3b; hydrogen bonding:
(b) C.-H. Lin and J. Tour, J. Org. Chem., 2002, 67, 7761–7768;
(c) K. Cantin, S. Rondeau-Gagne, J. R. Neabo, M. Daigle and
J.-F. Morin, Org. Biomol. Chem., 2011, 9, 4440–4443, or p–p
interactions: (d) A. S. Shetty, J. Zhang and J. S. Moore, J. Am.
Chem. Soc., 1996, 118, 1019–1027; (e) 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,
21 The transition enthalpies could not be determined correctly for the
cooling scan.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6547–6549 6549