From vesicles to helical nanotubes: A sergeant-and-soldiers effect in the self-assembly of oligo(p-phenyleneethynylene)s

Article abstract of DOI:10.1002/anie.200603238

Coassembly of short molecular wires OPE1 with chiral analogues OPE2, both of which are CD silent, exhibited a sergeant-and-soldiers effect that resulted in transformation of vesicular aggregates of OPE1 into CD-active helical nanotubes (see schematic pict

Full text of DOI:10.1002/anie.200603238

Angewandte
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Supramolecular Chemistry
and CD spectral features. This behavior of OPE2 is different
from that of an analogous chiral oligo(p-phenylenevinylene),
OPV, for which formation of helical structures in decane leads
to gelation.[19] These observations reveal the differences in the
self-organization propensities of OPEs and OPVs, which
reiterate that a subtle difference in p interaction is sufficient
to induce a large difference in the morphological features.
DOI: 10.1002/anie.200603238
From Vesicles to Helical Nanotubes: A Sergeant-
and-Soldiers Effect in the Self-Assembly of
Oligo(p-phenyleneethynylene)s**
Ayyappanpillai Ajayaghosh,* Reji Varghese,
Sankarapillai Mahesh, and Vakayil K. Praveen
The past few years have witnessed significant progress toward
the design of supramolecular architectures of nano- to
micrometer dimensions with intriguing properties.[1,2] Of
particular interest to chemists is the self-assembly of rigid p-
conjugated molecules with controlled size and shape, as active
components of organic electronic devices.[3,4] In this context,
self-assembly of oligo(p-phenyleneethynylene)s (OPEs) has
received considerable attention.[5] Control of the morphology
of self-assembled structures requires rational design of
molecular components.[6] For example, spherical to tubular
or cylindrical assemblies are obtained by structural variation
of the self-assembling units,[7] whereas nonhelical to helical
transformation can be achieved by attaching chiral han-
dles[8–11] or by the “sergeant-and-soldiers” coassembly
approach.[12–14] However, transitions of linear p-conjugated
molecules from vesicles to helical tubules or vice versa are
rare.[15] Herein we reveal a spectacular chirality-amplification
effect and an unprecedented transition from vesicles to helical
tubules during the sergeant-and-soldiers coassembly of chiral
and achiral OPEs.
We chose OPE1 and its chiral analogue OPE2 for our
studies. These compounds were synthesized by palladium-
catalyzed Sonogashira–Hagihara cross-coupling reactions[16]
and characterized by 1H NMR, 13C NMR, and FAB-MS
techniques.[17] Compound OPE1 self-assembles in nonpolar
hydrocarbon solvents to form nanoparticles, microspheres,
giant superstructures, and blue-light-emitting organogels.[18]
Surprisingly, OPE2 failed to form aggregates in nonpolar
hydrocarbon solvents, as evident from absorption, emission,
The UV/Vis absorption spectrum of OPE1 in decane (1 ”
10À5 m) at 258C exhibited a maximum at 384 nm with a
shoulder at 420 nm. At this concentration, a decrease in the
intensity of the p–p* transition band with first-order kinetics
(k = 47.41 minÀ1) is observed over 6 h (Figure 1a). Interest-
ingly, addition of OPE2 (5–30 mol%) to this solution at a
total concentration of 1 ” 10À5 m resulted in reversal of the
absorption, which indicates probable destruction of the initial
OPE1 aggregates to form coassembled or molecularly
dissolved species. On the other hand, addition of OPE2 (5–
30 mol%) to a fresh solution of OPE1 in decane (without
aging) at a total concentration of 0.7 ” 10À5 m led to a decrease
in the intensity of the aggregate band at 420 nm.[17] After
heating to 808C and cooling to room temperature, this
solution did not show any considerable change in the
absorption spectrum, that is, the time-dependent process is
arrested in this case (Figure 1b).
Circular dichroism (CD) measurements on OPE1 and
OPE2 in decane (1 ” 10À5 m) did not show any detectable
[*] Dr. A. Ajayaghosh, R. Varghese, S. Mahesh, V. K. Praveen
Photosciences and Photonics Group
Chemical Sciences and Technology Division
Regional Research Laboratory, CSIR
Trivandrum 695019 (India)
Fax: (+91)471-249-1712
E-mail: aajayaghosh@rediffmail.com
[**] We thank the Department of Science and Technology, New Delhi for
financial support. R.V. and V.K.P. are grateful to CSIR and S.M.
acknowledges UGC, Government of India for research fellowships.
We acknowledge the help by the Biomedical Technology Wing of the
Sree Chitra Tirunal Institute for Medical Sciences and Technology
with DLS and the Rajiv Gandhi Centre of Biotechnology with TEM
analyses. We thank the anonymous referees for critical comments
which helped us in improving the quality of the work. This is
contribution No. RRLT-PPG-232.
Figure 1. a) Time-dependent absorption changes of a) OPE1
(1”10À5 m) and b) OPE1 with 30 mol% of OPE2 (0.7”10À5 m) in
decane at 298 K.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Cotton effect. Though this is expected for OPE1, the CD
phology (Figure 3a).[20] The histogram shows Lorentzian
distribution (R2 = 0.9103) with an average circular width of
94 nm (full width at half-maximum 34 nm) and height of 4 Æ
1 nm after accounting for the tip-broadening factor.[21] The
silence of OPE2 was rather surprising and indicates that it is
not able to form aggregates in decane. However, addition of
OPE2 (5–30 mol%) to a solution of OPE1 (1 ” 10À5 m) in
decane, followed by heating and cooling, resulted in a positive
Cotton signal and two negative signals (Figure 2a). Since both
individual components are CD-silent, the observed CD of the
coassembly is due to the induced chirality from OPE2 in a
Figure 3. a) Tapping-mode AFM height (z scale=20 nm) and b) TEM
images (unstained) of OPE1 from decane (1”10À6 m) under ambient
conditions. The inset shows the zoomed portion of the marked area.
AFM analysis of OPE1 from chloroform on a mica surface
revealed significantly different morphology when compared
to that from decane, that is, the solvent has some role in the
formation of spherical aggregates.[17] The transmission elec-
tron microscopy (TEM) images of an evaporated decane
solution of OPE1 on carbon-coated grid revealed that the
particles are vesicular with contrast difference between the
inner part and the periphery (Figure 3b).
Figure 2. a) CD spectra of OPE1 with different fractions of OPE2
recorded after heating and cooling. The inset shows the corresponding
spectra of sonicated solutions without heating and cooling. b) Plots of
CD intensity at 420 nm against concentration of OPE2. All experiments
were carried out in decane (1”10À5 m, l=10 mm) at 298 K.
Interestingly, OPE2 does not show any specific morphol-
ogy from decane.[17] This observation is in agreement with the
absorption and CD studies, which indicated that OPE2 does
not aggregate in decane. However, AFM pictures of the
coassemblies of OPE2 with OPE1 revealed the formation of
both nanoparticles and helical structures at 8 mol% of the
former (Figure 4a). The width of the particles obtained under
these condition varies in the range of 30–60 nm with average
heights of 3–16 nm. However, at 25 mol% of OPE2 almost all
the spherical assemblies were transformed into helical
structures (Figure 4b). Section analysis of the helical fibers
revealed a width of 90 nm for the smallest fiber after
accounting for the tip-broadening effect,[21] with almost
uniform pitch of 140 nm. The height of the fibers varies
between 6 and 25 nm, and this indicates considerable flat-
tening of the fibers. However, TEM analysis of the coassem-
bly revealed that the helical fibers are tubular in nature with
diameters of 55–90 nm (Figure 4c). The width of the inner
hollow tubular space is almost uniform in all cases. Above
50 mol% of OPE2, an ill-defined morphology could be seen,
in agreement with the complete disappearance of the CD
helical sense. Interestingly, simple mixing of the two compo-
nents in decane followed by sonication resulted in weak CD
signals (Figure 2a, inset), that is, heating and cooling are
crucial to the formation of helical coassemblies. The intensity
of the induced CD (ICD) signals increases with increasing
fraction of OPE2 up to 30 mol%, then decreases, and finally
disappears at 50 mol%, which is clear from the plots of the
ICD intensity at 420 nm against the concentration of OPE2
(Figure 2b). The increase in the ICD intensity reveals that up
to 30 mol% OPE2 is able to induce chirality in OPE1. Above
this concentration, coassembly is destabilized, as indicated by
the decreased ICD intensity, which finally disappears at
higher concentrations of OPE2 (50 mol%).
Dynamic light scattering (DLS) experiments on the aged
solution of OPE1 in decane (1 ” 10À6 m) revealed the forma-
tion of particles with sizes in the range of 50–180 nm with
peak intensity at 93 nm and a polydispersity of 0.24.[17]
Tapping-mode atomic force microscopy (AFM) images of
OPE1 from decane (1 ” 10À6 m) on a freshly cleaved mica
surface showed characteristics of flattened spherical mor-
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A schematic representation of the coassembly processes
of OPE1 and OPE2 is shown in Figure 5. In decane, OPE1
forms vesicular structures, as indicated by DLS, AFM, and
TEM studies. However, the coassembly of OPE2 with OPE1
Figure 5. Self-assembly of vesicles and subsequent transformation into
helical tubes in decane.
facilitates transcription of the molecular chirality of the
former in a helical sense and results in transition from
vesicular assemblies to helical tubular structures, as con-
firmed by CD, AFM, and TEM data. In summary, coassembly
of OPE1 with the chiral analogue OPE2 retards the
formation of vesicular assemblies and facilitates the evolution
of helical structures. Though OPE2 alone is unable to self-
assemble, when mixed with OPE1, the former participates in
coassembly and thereby transfers chiral information to the
latter. To the best of our knowledge, this is the first report of
the sergeant-and-soldiers approach to chirality-induced trans-
formation from vesicles to helical tubules.
Received: August 9, 2006
Revised: September 11, 2006
Published online: October 25, 2006
Keywords: circular dichroism · helical structures · nanotubes ·
.
self-assembly · supramolecular chemistry
[1] a) J.-M. Lehn, Supramolecular Chemistry, Concepts and Per-
spectives, VCH, Weinheim, 1995; b) J. W. Steed, J. L. Atwood,
Supramolecular Chemistry, Wiley, Chichester, 2000.
[2] For recent reviews, see a) S. I. Stupp, M. U. Pralle, G. N. Tew, L.
Li, M. Sayar, E. R. Zubarev, MRS Bull. 2000, 42 – 48; b) M. Lee,
B.-K. Cho, W.-C. Zin, Chem. Rev. 2001, 101, 3869 – 3892;
c) J. A. A. W. Elemans, A. E. Rowan, R. J. M. Nolte, J. Mater.
Chem. 2003, 13, 2661 – 2670; d) S. I. Stupp, Chem. Rev. 2005, 105,
1023 – 1024.
[3] For a review, see F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer,
A. P. H. J. Schenning, Chem. Rev. 2005, 105, 1491 – 1546.
[4] a) J. M. Tour, Molecular Electronics: Commercial Insights,
Chemistry, Devices, Architectures and Programming, World
Scientific, River Edge, NJ, 2003; b) M. van der Auweraer, F. C.
De Schryver, Nat. Mater. 2004, 3, 507 – 508; c) A. P. H. J. Schen-
ning, E. W. Meijer, Chem. Commun. 2005, 3245 – 3258; d) A. C.
Grimsdale, K. Müllen, Angew. Chem. 2005, 117, 5732 – 5772;
Angew. Chem. Int. Ed. 2005, 44, 5592 – 5629.
Figure 4. Tapping-mode AFM height images of the coassembly of
OPE1 with different concentrations of OPE2 from decane (1”10À5 m).
a) 8 mol% and b) 25 mol% under ambient conditions on freshly
cleaved mica surface (z scale=50 nm). c) TEM images (unstained) of
25 mol% OPE2 with OPE1 under ambient conditions. The inset is a
zoomed image of the marked area showing the helical-tubular nature
of the coassembly.
signal in Figure 2b.[17] Heating and cooling of the two
components resulted in significant morphological differences
in the AFM images when compared to those of samples
prepared by simple mixing and sonication.[17]
[5] a) P. Samorí, V. Francke, K. Müllen, J. P. Rabe, Chem. Eur. J.
1999, 5, 2312 – 2317; b) P. Samorí, I. Sikharulidze, V. Francke, K.
Müllen, J. P. Rabe, Nanotechnology 1999, 10, 77 – 80; c) U. H. F.
Bunz, Chem. Rev. 2000, 100, 1605 – 1644; d) U. H. F. Bunz, Acc.
Angew. Chem. Int. Ed. 2006, 45, 7729 –7732
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7731

Communications
Chem. Res. 2001, 34, 998 – 1010; e) S. Zahn, T. M. Swager,
Nature 1999, 398, 498 – 502; f) L. J. Prins, P. Timmerman, D. N.
Reinhoudt, J. Am. Chem. Soc. 2001, 123, 10153 – 10163.
[14] A. Ajayaghosh, R. Varghese, S. J. George, C. Vijayakumar,
Angew. Chem. 2006, 118, 1159 – 1162; Angew. Chem. Int. Ed.
2006, 45, 1141 – 1144.
[15] J.-H. Ryu, H.-J. Kim, Z. Huang, E. Lee, M. Lee, Angew. Chem.
2006, 118, 5430 – 5433; Angew. Chem. Int. Ed. 2006, 45, 5304 –
5304.
Angew. Chem. 2002, 114, 4399 – 4404; Angew. Chem. Int. Ed.
2002, 41, 4226 – 4230; f) J. Xu, C.-Z. Zhou, L. H. Yang, N. T. S.
Chung, Z.-K. Chen, Langmuir 2004, 20, 950 – 956.
[6] For a review, see T. Shimizu, M. Masuda, H. Minamikawa,
Chem. Rev. 2005, 105, 1401 – 1444, and references therein.
[7] B.-S. Kim, D.-J. Hong, J. Bae, M. Lee, J. Am. Chem. Soc. 2005,
127, 16333 – 16337.
[16] a) K. Sonogashira, Y. Tohda, N. Hagihara, Tetrahedron Lett.
1975, 16, 4467 – 4470; b) J.-F. Nierengarten, T. Gu, G. Had-
ziioannou, D. Tsamouras, V. Krasnikov, Helv. Chim. Acta 2004,
87, 2948 – 2966.
[8] For reviews on helical structures, see a) “Supramolecular
chirality”: Top. Curr. Chem. 2006, 265; b) A. E. Rowan,
R. J. M. Nolte, Angew. Chem. 1998, 110, 65 – 71; Angew. Chem.
Int. Ed. 1998, 37, 63 – 68; c) J. J. L. M. Cornelissen, A. E. Rowan,
R. J. M. Nolte, N. A. J. M. Sommerdijk, Chem. Rev. 2001, 101,
4039 – 4070; d) L. Brunsveld, B. J. B. Folmer, E. W. Meijer, R. P.
Sijbesma, Chem. Rev. 2001, 101, 4071 – 4098; e) C. Schmuck,
Angew. Chem. 2003, 115, 2552 – 2556; Angew. Chem. Int. Ed.
2003, 42, 2448 – 2452; f) E. Yashima, K. Maeda, T. Nishimura,
Chem. Eur. J. 2004, 10, 43 – 51.
[9] a) E. D. Sone, E. R. Zubarev, S. I. Stupp, Angew. Chem. 2002,
114, 1781 – 1785; Angew. Chem. Int. Ed. 2002, 41, 1705 – 1709;
b) P. Jonkheijm, F. J. M. Hoeben, R. Kleppinger, J. van Herri-
khuyzen, A. P. H. J. Schenning, E. W. Meijer, J. Am. Chem. Soc.
2003, 125, 15941 – 15949; c) P. Jonkheijm, A. Miura, M. Zda-
nowska, F. J. M. Hoeben, S. De Feyter, A. P. H. J. Schenning,
F. C. De Schryver, E. W. Meijer, Angew. Chem. 2004, 116, 76 –
80; Angew. Chem. Int. Ed. 2004, 43, 74 – 78; d) J. J. D. de Jong,
L. N. Lucas, R. M. Kellogg, J. H. van Esch, B. L. Feringa, Science
2004, 304, 278 – 281; e) S.-i. Kawano, N. Fujita, S. Shinkai, Chem.
Eur. J. 2005, 11, 4735 – 4742; f) A. Ajayaghosh, C. Vijayakumar,
R. Varghese, S. J. George, Angew. Chem. 2006, 118, 470 – 474;
Angew. Chem. Int. Ed. 2006, 45, 456 – 460.
[17] See Supporting Information.
[18] A. Ajayaghosh, R. Varghese, V. K. Praveen, S. Mahesh, Angew.
Chem. 2006, 118, 3339 – 3342; Angew. Chem. Int. Ed. 2006, 45,
3261 – 3264.
[19] S. J. George, A. Ajayaghosh, P. Jonkheijm, A. P. H. J. Schenning,
E. W. Meijer, Angew. Chem. 2004, 116, 3504 – 3507; Angew.
Chem. Int. Ed. 2004, 43, 3422 – 3425.
[20] M. Yang, W. Wang, F. Yuan, X. Zhang, J. Li, F. Liang, B. He, B.
Minch, G. Wegner, J. Am. Chem. Soc. 2005, 127, 15107 – 15111.
[21] P. Samorí, V. Francke, T. Mangel, K. Müllen, J. P. Rabe, Opt.
Mater. 1998, 9, 390 – 393.
[10] a) H. Engelkamp, S. Middelbeek, R. J. M. Nolte, Science 1999,
284, 785 – 788; b) F. Würthner, S. Yao, U. Beginn, Angew. Chem.
2003, 115, 3368 – 3371; Angew. Chem. Int. Ed. 2003, 42, 3247 –
3250; c) M. Kimura, T. Kuroda, K. Ohta, K. Hanabusa, H. Shirai,
N. Kobayashi, Langmuir 2003, 19, 4825 – 4830; d) T. Yamaguchi,
T. Kimura, H. Matsuda, T. Aida, Angew. Chem. 2004, 116, 6510 –
6515; Angew. Chem. Int. Ed. 2004, 43, 6350 – 6355; e) J. Wu,
M. D. Watson, L. Zhang, Z. Wang, K. Müllen, J. Am. Chem. Soc.
2004, 126, 177 – 186; f) J. P. Hill, W. Jin, A. Kosaka, T. Fukushi-
ma, H. Ichihara, T. Shimomura, K. Ito, T. Hashizume, N. Ishii, T.
Aida, Science 2004, 304, 1481 – 1483.
[11] a) T. Gulik-Krzywicki, C. Fouquey, J.-M. Lehn, Proc. Natl. Acad.
Sci. USA 1993, 90, 163 – 167; b) A. P. H. J. Schenning, P. Jonk-
heijm, E. Peeters, E. W. Meijer, J. Am. Chem. Soc. 2001, 123,
409 – 416; c) J. Bae, J.-H. Choi, Y.-S. Yoo, N.-K. Oh, B.-S. Kim,
M. Lee, J. Am. Chem. Soc. 2005, 127, 9668 – 9669; d) B. W.
Messmore, P. A. Sukerkar, S. I. Stupp, J. Am. Chem. Soc. 2005,
127, 7992 – 7993; e) A. Lohr, M. Lysetska, F. Würthner, Angew.
Chem. 2005, 117, 5199 – 5202; Angew. Chem. Int. Ed. 2005, 44,
5071 – 5074.
[12] a) M. M. Green, M. P. Reidy, R. D. Johnson, G. Darling, D. J.
OꢀLeary, G. Willson, J. Am. Chem. Soc. 1989, 111, 6452 – 6454;
b) M. M. Green, J.-W. Park, T. Sato, A. Teramoto, S. Lifson,
R. L. B. Selinger, J. V. Selinger, Angew. Chem. 1999, 111, 3328 –
3345; Angew. Chem. Int. Ed. 1999, 38, 3138 – 3154.
[13] a) A. R. A. Palmans, J. A. J. M. Vekemans, E. E. Havinga, E. W.
Meijer, Angew. Chem. 1997, 109, 2763 – 2765; Angew. Chem. Int.
Ed. Engl. 1997, 36, 2648 – 2651; b) R. B. Prince, J. S. Moore, L.
Brunsveld, E. W. Meijer, Chem. Eur. J. 2001, 7, 4150 – 4154;
c) A. P. H. J. Schenning, A. F. M. Kilbinger, F. Biscarini, M.
Cavallini, H. J. Cooper, P. J. Derrick, W. J. Feast, R. Lazzaroni,
Ph. Leclꢁre, L. A. McDonel, E. W. Meijer, S. C. J. Meskers, J.
Am. Chem. Soc. 2002, 124, 1269 – 1275; d) A. J. Wilson, M.
Masuda, R. P. Sijbesma, E. W. Meijer, Angew. Chem. 2005, 117,
2315 – 2319; Angew. Chem. Int. Ed. 2005, 44, 2275 – 2279; e) L. J.
Prins, J. Huskens, F. de Jong, P. Timmerman, D. N. Reinhoudt,
7732
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