Hierarchical self-assembly of donor-acceptor-substituted butadiene amphiphiles into photoresponsive vesicles and gels

Article abstract of DOI:10.1002/anie.200602088

(Figure Presented) It all gels well: Amphiphilic donor-acceptor-substituted butadienes undergo spontaneous concentration-dependent self-assembly into highly fluorescent vesicles and gels. The vesicles and gels exhibit light-induced destruction and self-repair as a result of the photochromic nature of the butadiene chromophore.

Full text of DOI:10.1002/anie.200602088

Angewandte
Chemie
Photoisomerization
DOI: 10.1002/anie.200602088
Hierarchical Self-Assembly of Donor–Acceptor-
Substituted Butadiene Amphiphiles into
Photoresponsive Vesicles and Gels**
N. S. Saleesh Kumar, Shinto Varghese,
Gopinathan Narayan, and Suresh Das*
The construction of supramolecular architectures by the
spontaneous self-assembly of molecules is currently a subject
of great interest in areas that range from chemistry and
biology to materials science.[1] Of particular interest is the
design of externally addressable molecules that contain self-
assembly elements, as these can give rise to so-called “smart
materials”.[2] Such addressable molecules could include
among others, photochromics, which can be used to bring
about light-induced changes in the self-assembly[3,4] or
environment-sensitive luminophores, which can provide val-
uable insight into the nature of the molecular aggregation
within such assemblies.[5] Herein, we present a novel class of
donor–acceptor-substituted
amphiphilic
butadienes
(Scheme 1) capable of undergoing spontaneous concentra-
tion-dependent hierarchical self-assembly from vesicles to
gels. The self-assembly process was associated with unique
changes in the fluorescence of the system. Moreover, the
presence of a photoisomerizable chromophore makes these
Scheme 1. Structures of the molecules used in this study.
[*] N. S. S. Kumar, S. Varghese, G. Narayan, Dr. S. Das
Photosciences and Photonics Section
Chemical Sciences and Technology Division
Regional Research Laboratory, CSIR
Trivandrum, 695019 Kerala (India)
Fax: (+91)471-249-1712
E-mail: sdaas@rediffmail.com
[**] This study was supported by the Department of Science and
Technology (DST), Government of India (New Delhi), and Council of
Scientific and Industrial Research (CSIR, Task Force Program, CMM
22). The authors thank Robert Philip for assistance with AFM
studies, M. R. Chandran for SEM measurements, Dr. J. D. Sudha for
the rheological studies, and Dr. M. R. Pillai (Director of the Rajiv
Gandhi Centre for Biotechnology, Trivandrum) for TEM and
confocal laser scanning microscopy experiments. N.S.S.K., S.V.,
and G.N. thank the CSIR (Government of India) for research
fellowships (This is manuscript no. RRLT-PPS-218).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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materials photoresponsive. Such stimuli-responsive vesicles
millimolar concentration range, in which BINC18 formed
transparent gels, AFM analysis indicated that the large
spherical aggregates interconnected to form a continuous
globular network (Figure 1c).
and gels are becoming increasingly important because of their
obvious implications in controlled-release systems.[4d,6,7]
The butadiene derivatives were synthesized in a multistep
process, and their structures were established by FTIR and 1H
and 13C NMR spectroscopy, high-resolution mass spectro
metry, and elemental analysis.[8] 1H NMR spectroscopic
analysis revealed that the compounds existed as their
E isomers.[8] All the derivatives formed transparent gels in
polar solvents, such as alcohols, acetonitrile, and dimethyl
sulfoxide (DMSO).[8] As BINC18 gels were the most stable,
detailed studies were carried out on this molecule. BINC18
was observed to be capable of gelating polar solvents at very
low concentrations. Its critical gelator concentration (CGC),
which ranged from 1.3 to 1.7 ” 10À3 m,[8] was comparable to
that of some of the best low-molecular-weight gelators.[5f,6i,9]
Microscopic investigations of solutions of BINC18 pro-
vided clear evidence for a hierarchical build-up of supra-
molecular assemblies, which started from small-sized vesicles
at lower concentrations to larger vesicles at intermediate
concentrations, until gel formation occurred in the millimolar
concentration range. The formation of aggregates was spon-
taneous and did not require any special effects, such as
ultrasonication or the use of mixed solvents. Atomic force
microscopy (AFM) imaging of films deposited on mica sheets
from freshly prepared 5 ” 10À6 m solutions of BINC18 in
methanol indicated the formation of small nanospheres of
nearly uniform size, with an average diameter of 200 nm
(Figure 1a). Films cast from the same solutions, but which
were aged for about 24 hours, indicated the formation of
much larger polydisperse spherical aggregates, with sizes that
ranged from 1 to 2 mm, many of which were found to be
adhered together (Figure 1b). Apart from aging, the size of
the spherical aggregates was also observed to be dependent
on the initial concentration of the solution. Thus AFM images
of films deposited from a freshly prepared 1 ” 10À4 m solution
of BINC18 in methanol indicated the formation of polydis-
perse spherical structures, with an average diameter of about
500 nm.[8] Aging of these solutions also resulted in an increase
in the size of the spherical aggregates from 1 to 2 mm.[8] In the
Scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) images indicated a vesicular
structure for the spherical aggregates (Figure 1d,e). SEM
analysis of samples prepared from 1 ” 10À4 m aged solutions of
BINC18 in methanol (Figure 1d) showed spheres with
diameters that ranged from 1 to 2 mm, which was similar to
that observed by AFM images under identical conditions.[8] In
several cases, half-opened vesicular aggregates that resulted
from wall collapse were observed, which can be attributed to
the high vacuum applied during the preparation of the
sample. In the millimolar concentration range, in which
gelation occurred, the SEM images indicated a morphology
that matched closely that of the AFM images.[8]
TEM images of samples prepared by drop-casting aged
1.2 ” 10À4 m solutions onto a Formvar-coated copper grid also
indicated circular structures with diameters in the range 1–
1.5 mm (Figure 1e), similar to that observed by SEM and
AFM. The hollow nature of the spherical aggregates was
clearly indicated by the sharp contrast between the periphery
and center.[6e,g] The images also revealed that the vesicles were
sufficiently robust enough to retain their spherical shape
under the conditions of the TEM experiments. Further
support for the concentration-dependent hierarchical forma-
tion of vesicles, joined vesicles, and gels was obtained by
confocal laser scanning microscopy (CLSM) (Figure 1 f–h).
Self-assembly of BINC18 was also indicated by changes in
the absorption and emission spectra as a function of temper-
ature (Figure 2a). With decreasing temperature, a reduction
in the intensity of the main peak at 432 nm and concomitant
increase in absorption at 330 nm, with clear isosbestic points
at 372 and 274 nm, were observed. The process was com-
pletely reversible on heating. The temperature-dependent
changes in the absorption spectrum of BINC18 in methanol
were also accompanied by changes in its fluorescence
behavior. Whereas the solution of BINC18 (1.2 ” 10À4 m)
exhibited a weak fluorescence band centered at 540 nm at
room temperature (quantum yield,
Ff = 0.004),
enhancement
a
nearly 40-fold
in fluorescence
accompanied by a shift in the max-
imum to 602 nm was observed at
78C (Figure 2b).
A
significant
enhancement in the fluorescence
lifetimes was also observed with a
decrease in temperature (Figure 2b,
inset). Mainly a long-lived compo-
nent with a lifetime of 1.0 ns was
observed at 138C, whereas the life-
time of the species was within the
time resolution of the instrument
(< 100 ps) at 408C.[8] These effects
can be attributed to the formation of
H aggregates, which involves a par-
allel interaction mode of the chro-
mophores.[10] Excitonic splitting in
Figure 1. Microscopic images of aggregates of BINC18 deposited from methanolic solutions. The concen-
tration of the solutions and conditions are indicated. (a–c) AFM height images on mica: a) 5”10À6 m;
b) 5”10À6 m, aged 24 h; and c) 1.5”10À3 m. d) SEM image at 1”10À4 m, aged 24 h; e) TEM image at
1.2”10À4 m, aged 24 h. (f–h) CLSM images (lex =488 nm; lem =500–800 nm): f) 1”10À4 m; g) 1”10À4 m, aged
24 h; and h) dried gel.
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supramolecular assembly of BINC18 that leads to gel
formation is provided in Figure 3.
Photoirradiation of a solution of BINC18 in methanol
with 445-nm light resulted in a decrease in intensity of the
main absorption band and was accompanied by a slight
increase in absorption intensity at the lower wavelengths
(Figure 4a). These changes can be attributed to a photo-
isomerization process (Figure 4b) similar to that reported
earlier for donor–acceptor-substituted butadienes.[13] The rate
of thermal reversal to the E isomer in such butadienes is
known to depend on the electron-donating and -accepting
strengths of the substituents.[13] For BINC18, a lifetime of 8.2 ”
103 s was determined for the recovery of the E isomer by
Figure 3. Schematic representation of the hierarchical organization
from small to large vesicles and finally to gels. The molecular self-
assembly of only one bilayer is indicated.
monitoring the growth of the 432-nm peak (Figure 4c).
Photochemical isomerization and complete thermal reversal
of BINC18 could be achieved over several cycles.
Figure 2. Temperature-dependent changes in a) absorption (l=1 mm)
and b) emission spectra (lex =370 nm; 1 cm”1 cm) of a 1.2”10À4
solution of BINC18 in methanol. The inset shows the fluorescence
decay profile monitored at 138C (lex =375 nm; lem =600 nm).
m
Photoirradiation of BINC18 also resulted in disruption of
its supramolecular aggregates. Figure 5a,b depicts the effect
of photoisomerization and thermal recovery of vesicles of
BINC18. The AFM image of a film obtained from a non-
irradiated aged 5 ” 10À6 m solution of BINC18 showed the
existence of large joined vesicular aggregates described
earlier (Figure 1b). On irradiation, destruction of the larger
vesicles and formation of small spherical aggregates was
observed (Figure 5a), whereas AFM images obtained from
the irradiated solutions kept in the dark for 24 hours clearly
indicated recovery of the large vesicular aggregates (Fig-
ure 5b).
The effects of photoisomerization and thermal reversal on
gel formation are shown in Figure 5c. Increased transparency
and loss of fluorescence in a region of the gel photoirradiated
at 445 nm through a mask (Figure 5c, film 2) suggested the
destruction of the gel nature. Recovery of the fluorescence
and scattering nature in the irradiated portion was observed
on keeping the film in the dark for 24 hours, although
persistence of a faint residual image indicated that the
recovery was not complete under these conditions (Figure 5c,
film 3).
H aggregates is known to give rise to a high-energy allowed
transition and a low-energy forbidden band. In such systems,
absorption is dominated by the high-energy band, which
results in a blue shift relative to the monomer absorption
band, whereas long-lived and strongly red-shifted fluores-
cence occurs from the low-energy band.[5e,10]
The formation of H aggregates indicates a close p-stacked
arrangement of the molecules. These results are also in
support of the presence of an outer-membrane-like structure
in the supramolecular aggregates.[11] The thickness of the wall
of the vesicles ( ꢀ 38 nm) measured from TEM images
(Figure 1e) was almost ten times larger than the contour
length of BINC18 (3.72 nm), thus suggesting a multilamellar
structure for the walls of the vesicles. A model for the vesicle
that is consistent with the spectroscopic studies and the TEM
images would consist of about three to four closed bilayers,
with the solvent layers existing in the center and between the
bilayer membranes.[12] A schematic representation of the
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Figure 5. Effect of photoirradiation on vesicles and gels of BINC18.
a) AFM tapping mode height image of film deposited from aged
5”10À6 m solution in methanol irradiated with 445-nm light for
2 minutes. b) AFM height image of film obtained from the same
photoirradiated solution kept in the dark for 24 h. c) Photographic
image of BINC18 gel in n-butanol kept under low-intensity 365-nm
light which indicates yellow fluorescence of the gelated region
(l=1 mm, c=9.5”10À3 m): 1) gel film before irradiation; 2) gel film
irradiated with 445-nm light[8] for 30 minutes using a mask; and
3) irradiated gel film kept in the dark for 24 h.
in a gel-to-sol conversion, as observed by the free-flowing
nature of the irradiated sample. The viscosity changes of the
gel phase before and after irradiation were measured using a
stress-controlled rheometer at low shear (0.02–1 sÀ1) at
258C.[14] At a shear rate of 0.027 sÀ1, the viscosity of the gel
was 2.7 ” 105 cP. Following irradiation,
a considerable
decrease in viscosity (5.1 ” 104 cP) was observed at the same
shear rate. Keeping the irradiated solution in the dark for
24 hours resulted in the reformation of the gel, as indicated by
its nonflowing nature and increased viscosity (9.4 ” 104 cP at a
shear rate of 0.027 sÀ1).
In conclusion, we have synthesized a novel class of
amphiphilic donor–acceptor butadienes capable of sponta-
neous hierarchical self-assembly, which first involves the
formation of smaller vesicles that merge to form larger ones
until they finally link together to form gels that consist of
globular aggregates with solvent entrapped within them. This
type of gelation is rather uncommon, as with a very few recent
exceptions[15] most reported gels are formed through self-
aggregation of gelator molecules into long intertwined fibrous
networks that hold the solvents in between predominantly by
surface tension. The photoresponsive nature of these materi-
als makes them potentially useful in controlled release
systems, and we are currently exploring some of these
possibilities.
Figure 4. a) The effect of irradiation on the absorption spectrum of
BINC18 in methanol (c) before and (a) after irradiation with
445-nm light for 2 minutes (c=5”10À6 m). b) Scheme depicting photo-
and thermal isomerization of BINC18. c) Time-dependent recovery of
the absorbance at 432 nm in the irradiated solution.
Received: May 25, 2006
Published online: August 21, 2006
Keywords: gels · isomerization · photoresponsive materials ·
.
self-assembly · vesicles
The reversible photoinduced destruction of the gel was
also confirmed by viscosity measurements. Rheological
studies on BINC18 (4 ” 10À3 m) in n-butanol were indicative
of the formation of fairly stable gels.[8] Frequency-sweep
measurements showed that the elastic modulus (G’) and the
loss modulus (G’’) were almost independent of frequency
over a wide range. The studies also indicated that these gels
were not thixotropic, which is a common feature with most
known low-molecular-weight gelators.[9a] Irradiation of
BINC18 gels (3 ” 10À3 m) in n-butanol at wavelengths greater
than 385 nm from a 200-W mercury lamp for 3 hours resulted
[1] a) J.-M. Lehn, Supramolecular Chemistry: Concepts and Per-
spectives, VCH, Weinheim, 1995.
[2] a) S. I. Stupp, V. Le Bonheur, K. Walker, L. S. Li, K. E. Huggins,
M. Kesser, A. Amstutz, Science 1997, 276, 384 – 389; b) M.
Muthukumar, C. K. Ober, E. L. Thomas, Science 1997, 277,
1225 – 1232; c) J. H. K. K. Hirschberg, L. Brunsveld, A. Ramzl,
J. A. J. M. Vekemans, R. P. Sijbesma, E. W. Meijer, Nature 2000,
407, 167 – 170; d) T. Kato, Science 2002, 295, 2414 – 2418;
e) G. M. Whitesides, B. Grzybowski, Science 2002, 295, 2418 –
6320 www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chemie
2421; f) A. C. Grimsdale, K. Müllen, Angew. Chem. 2005, 117,
5732 – 5772; Angew. Chem. Int. Ed. 2005, 44, 5592 – 5629; g) T.
Shimizu, M. Masuda, H. Minamikawa, Chem. Rev. 2005, 105,
1401 – 1443; h) J. M. J. Paulusse, R. P. Sijbesma, Angew. Chem.
2006, 118, 2392 – 2396; Angew. Chem. Int. Ed. 2006, 45, 2334 –
2337.
[9] a) P. Terech, R. G. Weiss, Chem. Rev. 1997, 97, 3133 – 3159;
b) J. H. van Esch, B. L. Feringa, Angew. Chem. 2000, 112, 2351 –
2354; Angew. Chem. Int. Ed. 2000, 39, 2263 – 2266; c) L. A.
Estroff, A. D. Hamilton, Chem. Rev. 2004, 104, 1201 – 1217;
d) N. M. Sangeetha, U. Maitra, Chem. Soc. Rev. 2005, 34, 821 –
836.
[10] a) M. Kasha, H. R. Rawls, M. A. El-Bayoumi, Pure Appl. Chem.
1965, 11, 371 – 392; b) D. G. Whitten, Acc. Chem. Res. 1993, 26,
502 – 509; c) K. C. Hannah, B. A. Armitage, Acc. Chem. Res.
2004, 37, 845 – 853; d) S. Y. Ryu, S. Kim, J. Seo, Y.-W. Kim, O.-H.
Kwon, D.-J. Jang, S. Y. Park, Chem. Commun. 2004, 70 – 71; e) H.
Yao, K. Domoto, T. Isohashi, K. Kimura, Langmuir 2005, 21,
1067 – 1073.
[11] a) T. Kunitake, Angew. Chem. 1992, 104, 692 – 710; Angew.
Chem. Int. Ed. Engl. 1992, 31, 709 – 726; b) D. G. Whitten, L.
Chen, H. C. Geiger, J. Perlstein, X. Song, J. Phys. Chem. B 1998,
102, 10098 – 10111; c) J.-M. Pedrosa, M. T. M. Romero, L.
Camacho, D. Möbius, J. Phys. Chem. B 2002, 106, 2583 – 2591.
[12] Y. Shang, Yi. Xu, H. Liu, Y. Hu, J. Dispersion Sci. Technol. 2006,
27, 105 – 108.
[3] a) K. Murata, M. Aoki, T. Suzuki, T. Harada, H. Kawabata, T.
Komori, F. Ohseto, K. Ueda, S. Shinkai, J. Am. Chem. Soc. 1994,
116, 6664 – 6676; b) M. S. Vollmer, T. D. Clark, C. Steinem, M. R.
Ghadiri, Angew. Chem. 1999, 111, 1703 – 1706; Angew. Chem.
Int. Ed. 1999, 38, 1598 – 1601; c) S. A. Ahmed, X. Sallenave, F.
Fages, G. Mieden-Gundert, W. M. Müller, U. Müller, F. Vögtle,
J.-L. Pozzo, Langmuir 2002, 18, 7096 – 7101; d) L. Frkanec, M.
ˇ
´
´
´
Jokic, J. Makarevic, K. Wolsperger, M. Zinic, J. Am. Chem. Soc.
2002, 124, 9716 – 9717; e) M. Moriyama, N. Mizoshita, T. Yokota,
K. Kishimoto, T. Kato, Adv. Mater. 2003, 15, 1335 – 1338; f) F.
Rakotondradany, M. A. Whitehead, A.-M. Lebuis, H. F. Slei-
man, Chem. Eur. J. 2003, 9, 4771 – 4780.
[4] a) J. J. D. de Jong, L. N. Lucas, R. M. Kellogg, J. H. van Esch,
B. L. Feringa, Science 2004, 304, 278 – 281; b) N. Koumura, M.
Kudo, N. Tamaoki, Langmuir 2004, 20, 9897 – 9900; c) S. Yagai,
T. Nakajima, K. Kishikawa, S. Kohmoto, T. Karatsu, A.
Kitamura, J. Am. Chem. Soc. 2005, 127, 11134 – 11139; d) J.
Eastoe, A. Vesperinas, Soft Matter 2005, 1, 338 – 347.
[5] a) C. Thalacker, F. Würthner, Adv. Funct. Mater. 2002, 12, 209 –
218; b) S.-J. Lim, B.-K. An, S. D. Jung, M.-A. Chung, S. Y. Park,
Angew. Chem. 2004, 116, 6506 – 6510; Angew. Chem. Int. Ed.
2004, 43, 6346 – 6350; c) J. Wu, M. D. Watson, Li. Zhang, Z.
Wang, K. Müllen, J. Am. Chem. Soc. 2004, 126, 177 – 186; d) T.-
Q. Nguyen, R. Martel, P. Avouris, M. L. Bushey, L. Brus, C.
Nuckolls, J. Am. Chem. Soc. 2004, 126, 5234 – 5242; e) B. W.
Messmore, J. F. Hulvat, E. D. Sone, S. I. Stupp, J. Am. Chem.
Soc. 2004, 126, 14452 – 14458; f) S. J. George, A. Ajayaghosh,
Chem. Eur. J. 2005, 11, 3217 – 3227.
[13] a) H. Braatz, S. Hecht, H. Seifert, S. Helm, J. Bendig, W. Rettig,
J. Photochem. Photobiol. A 1999, 123, 99 – 108; b) R. Davis, S.
Das, M. George, S. Druzhinin, K. A. Zachariasse, J. Phys. Chem.
A 2001, 105, 4790 – 4798; c) R. Davis, V. A. Mallia, S. Das, Chem.
Mater. 2003, 15, 1057 – 1063.
[14] C. T. Lee, K. A. Smith, T. A. Hatton, Macromolecules 2004, 37,
5397 – 5405.
[15] a) J. H. Jung, Y. Ono, K. Sakurai, M. Sano, S. Shinkai, J. Am.
Chem. Soc. 2000, 122, 8648 – 8653; b) F. M. Menger, A. V.
Peresypkin, J. Am. Chem. Soc. 2003, 125, 5340 – 5345; c) G.
Battaglia, A. J. Ryan, Nat. Mater. 2005, 4, 869 – 876; d) A.
Ajayaghosh, R. Varghese, V. K. Praveen, S. Mahesh, Angew.
Chem. 2006, 118, 3339 – 3342; Angew. Chem. Int. Ed. 2006, 45,
3261 – 3264.
[6] a) K. Tsuda, G. C. Dol, J. Hofkens, L. Latterini, J. W. Weener,
E. W. Meijer, F. C. De Schryver, J. Am. Chem. Soc. 2000, 122,
3445 – 3452; b) F. M. Menger, J. S. Keiper, Angew. Chem. 2000,
112, 1980 – 1996; Angew. Chem. Int. Ed. 2000, 39, 1906 – 1920;
c) D. E. Discher, A. Eisenberg, Science 2002, 297, 967 – 973;
d) Y. J. Jeon, P. K. Bharadwaj, S. W. Choi, J. W. Lee, K. Kim,
Angew. Chem. 2002, 114, 4654 – 4656; Angew. Chem. Int. Ed.
2002, 41, 4474 – 4476; e) D. M. Vriezema, J. Hoogboom, K.
Velonia, K. Takazawa, P. C. M. Christianen, J. C. Maan, A. E.
Rowan, R. J. M. Nolte, Angew. Chem. 2003, 115, 796 – 800;
Angew. Chem. Int. Ed. 2003, 42, 772 – 776; f) J.-H. Fuhrhop, T.
Wang, Chem. Rev. 2004, 104, 2901 – 2937; g) M. Lee, S.-J. Lee, L.-
H. Jiang, J. Am. Chem. Soc. 2004, 126, 12724 – 12725; h) R.
Charvet, D.-L. Jiang, T. Aida, Chem. Commun. 2004, 2664 –
2665; i) T. Ishi-i, S. Shinkai, Top. Curr. Chem. 2005, 258, 119 –
160.
[7] a) O. Uzun, A. Sanyal, H. Nakade, R. J. Thibault, V. M. Rotello,
J. Am. Chem. Soc. 2004, 126, 14773 – 14777; b) Z. Hu, A. M.
Jonas, S. K. Varshney, J.-F. Gohy, J. Am. Chem. Soc. 2005, 127,
6526 – 6527; c) T. Akutagawa, K. Kakiuchi, T. Hasegawa, S.-i.
Noro, T. Nakamura, H. Hasegawa, S. Mashiko, J. Becher, Angew.
Chem. 2005, 117, 7449 – 7453; Angew. Chem. Int. Ed. 2005, 44,
7283 – 7287; d) 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; e) B.-S. Kim, W.-Y. Yang, J.-H. Ryu, Y.-S. Yoo, M.
Lee, Chem. Commun. 2005, 2035 – 2037; f) F. J. M. Hoeben, I. O.
Shklyarevskiy, M. J. Pouderoijen, H. Engelkamp, A. P. H. J.
Schenning, P. C. M. Christianen, J. C. Maan, E. W. Meijer,
Angew. Chem. 2006, 118, 1254 – 1258; Angew. Chem. Int. Ed.
2006, 45, 1232 – 1236; g) G. Ma, A. M. Müller, C. J. Bardeen, Q.
Cheng, Adv. Mater. 2006, 18, 55 – 60; h) S. Wang, W. Shen, Y.
Feng, H. Tian, Chem. Commun. 2006, 1497 – 1499.
[8] See the Supporting Information.
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