Gelation-assisted control over excitonic interaction in merocyanine supramolecular assemblies

Article abstract of DOI:10.1002/anie.200702263

(Figure Presented) Gelling together: Merocyanine dyes possessing a cholesterol group as a gelforming auxiliary self-assemble into nanoscopic fibers in apolar cyclohexane and polar acetone to form organogels, showing dramatically distinct optical propertie

Full text of DOI:10.1002/anie.200702263

Angewandte
Chemie
DOI: 10.1002/anie.200702263
Dye Assemblies
Gelation-Assisted Control over Excitonic Interaction in Merocyanine
Supramolecular Assemblies**
Shiki Yagai,* Manabu Ishii, Takashi Karatsu, and Akihide Kitamura
One of the most fascinating aspects of functional dye
assemblies is their variable optical properties, which depend
on the chromophore packing arrangements through excitonic
interactions.[1] Archetypal examples might be chlorophyll
arrays in light-harvesting complexes of photosynthetic bac-
teria, in which the chromophore packing can be rationally
controlled by protein scaffolds, thereby diversifying their
optical properties.[2] Thus, the exploitation of artificial dye
assemblies in which the chromophore packing arrangements
and the resulting optical properties can be controlled by some
means is a fascinating avenue of research.[3] One possible
approach might be the application of the “organogelation”
phenomenology by which fibrous supramolecular assemblies
can be obtained from low-molecular-weight compounds as
solvent-incorporated soft materials, because a very subtle
balance between the structures of gelators and the solvent
characters can have a dramatic impact on local packing
arrangements of the building blocks as well as self-assembled
nanostructures.[4]
Merocyanines are an important class of dyes and repre-
sent promising nonlinear optical and photorefractive materi-
als owing to their donor–p–acceptor (D–p–A) structures.[5]
This structural character also makes these dyes attractive as
supramolecular components because different chromophore
packing or spatial arrangements result in prominent changes
in their optical properties through excitonic interactions.[6–8]
Covalently tethered merocyanine dimers are interesting
examples in that their optical properties can be tuned through
the control of the relative orientation of two merocyanine
chromophores by selecting the media.[7] Other examples
include merocyanine assemblies supported by specific non-
covalent interactions such as multiple hydrogen bonds, which
show drastic changes in their optical and electronic proper-
ties.[8] We report herein that the aggregation of cholesterol-
appended merocyanines with minor structural differences in
various organic solvents results in the formation of organogels
with dramatically different chromophore packing, thus giving
rise to distinct optical properties through excitonic interac-
tions.
To endow merocyanines with definitive gelation ability in
organic solvents,[9] we selected the cholesterol group as a
gelation auxiliary. The cholesterol group has been effectively
utilized to endow functional dyes with excellent gelation
ability for various organic solvents.[10] Shinkai and co-workers
conjugated cholesterol to azobenzene,[11] a porphyrin,[12]
perylene bisimide,[13] and oligothiophene[14] to fabricate
various functional organogels. Ajayaghosh et al. reported on
cholesterol-appended oligo(p-phenylenevinylene) gelators
for which the chromophore packing can be controlled by
the substitution patterns of the cholesterol group.[15] These
studies corroborate the versatility of the cholesterol group for
organizing functional chromophores to gel-like materials. In
this work we combine a barbituric acid type merocyanine dye
with the cholesterol group as in compounds 1 and 2[16] to
address gelation-assisted control over the packing arrange-
ments of merocyanine self-assemblies.
[*] Dr. S. Yagai, M. Ishii, Prof. Dr. T. Karatsu, Prof. Dr. A. Kitamura
Department of Applied Chemistry and Biotechnology
Faculty of Engineering
Chiba University
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522 (Japan)
Fax: (+81)43-290-3039
E-mail: yagai@faculty.chiba-u.jp
While investigating proper substituents R on the barbitu-
ric acid moiety of merocyanines for gelating organic solvents,
we found that the N,N’-dimethyl derivative 1 gelates cyclo-
hexane whereas N,N’-dibutyl derivative 2 gelates acetone.[16]
This result demonstrates that the substituents on the barbi-
turic acid moiety have a great impact on their gelation
properties. Polarized optical microscopy (POM) showed that
the acetone gel of 2 is more birefringent than the cyclohexane
gel of 1, indicating a higher degree of molecular anisotropy in
the former (insets in Figure 1a,b). Scanning electron micro-
scopy (SEM) and atomic force microscopy (AFM) of the spin-
coated gels reveal that the acetone gel of 2 consists of
Dr. S. Yagai
PRESTO
Japan Science and Technology Agency (JST)
4-1-8 Honcho Kawaguchi, Saitama (Japan)
[**] We are grateful to Dr. Takashi Nakanishi of Organic Nanomaterials
Center, NIMS, Tsukuba, Japan and Ms. Kanako Unoike of Chiba
University for the AFM measurements, and to the referees for
valuable suggestions on revising the paper.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2007, 46, 8005 –8009
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8005

Communications
Figure 1. Microscopy images of the cyclohexane gel of 1 (a,c) and the
acetone gel of 2 (b,d). a,b) FE-SEM images of the spin-coated gels;
insets show the corresponding POM images of the pristine gels
(c=2”10À3 m), 600”600 mm2; c,d) AFM height images of the spin-
coated gels.
Figure 2. UV/Vis absorption (solid lines, left axes (e in 104 mÀ1 cmÀ1))
and fluorescence spectra (dotted lines, right axes, lex =400 nm) of a) 1
in cyclohexane, b) 1 in acetone, c) 2 in cyclohexane, and d) 2 in
acetone. Concentration=1.0”10À5 m.
nanofibers with an average width of 100 nm (Figure 1b,d).[16]
In contrast, SEM images of the cyclohexane gel of 1 only
shows fused, irregular structures (Figure 1a). The fine struc-
tures could be visualized only with AFM, whereby thinner
fibers of about 20 nm in width are entangled (Figure 1c).[16]
These observations at the microscopic level suggest that
elemental aggregates of 2 in acetone have ordered structures
capable of hierarchically organizing into larger nanoscopic
fibers. In contrast, the elemental aggregates of 1 in cyclohex-
ane are dispersed at a level closer to the molecular scale (the
molecular width of 1 is 4 nm), and the gels are thereby
supported by much finer fibrous networks. This interpretation
is supported by the significantly higher gel-to-sol transition
temperature (Tgel) and lower minimum gelation concentra-
tion (cmin) for the cyclohexane gel of 1 (Tgel = 658C at c = 2 ”
10À3 m, cmin = 1 ” 10À3 m) compared to the acetone gel of 2
(Tgel = 408C at c = 2 ” 10À3 m, cmin = 2 ” 10À3 m).[17]
UV/Vis absorption spectra of molecularly dissolved 1 and
2 in cyclohexane and acetone under diluted conditions (1 ”
10À5 m) show intramolecular charge transfer (ICT) absorption
bands of the merocyanine chromophore in the 350—500-nm
wavelength region which are almost independent of the
substituents on the barbituric acid moiety in the respective
solvents (solid lines in Figure 2). The absorption maxima in
cyclohexane are located at l = 440 nm, whereas in acetone
they move to l = 460 nm because the excited state is more
charge-separated than the ground state (positive solvato-
chromism).
to the emission from the monomer at lmax = 520 nm (dotted
lines in Figure 2b,d). This new emission from 2 is attributable
to an excimer fluorescence from its large Stokes shift
(4847 cmÀ1) and long lifetime (t = 4.9 ns) in comparison
with that of the monomer (t = 0.3 ns).[7b] The formation of
excimer species is rationalized in terms of the increased
intermolecular charge-transfer interaction in the polar sol-
vent. The absence of excimer species of 1 in acetone can be
explained by the decreased solvophobicity on the barbituric
acid moiety lacking butyl groups.
A significant consequence arising from the gelation by 1
and 2 is the quite distinct optical properties of the gels. The
absorption spectrum of the cyclohexane gel of 1 is charac-
terized by a new maximum at 460 nm, red-shifted by 20 nm
from the molecularly dissolved state in the same solvent (red
solid line in Figure 3a). In sharp contrast, the maximum of the
acetone gel of 2 is located at 425 nm (red solid line in
Figure 3b), blue-shifted by 35 nm from the molecularly
dissolved state in the same solvent. In addition, significant
absorption tailing is observed up to 600 nm, making this gel
more of a red color (left insets in Figure 3a,b). These spectral
features are not observed for concentrated solutions (c = 5 ”
10À3 m) of 1 in acetone and 2 in cyclohexane, indicating that
they are characteristic for the aggregation of the respective
dyes in specific solvents. Although the observed spectral
changes are reminiscent of J- and H-type aggregation, they
are considerably different from those reported for J- and H-
aggregates of (mero)cyanines in view of their small absorp-
tion shifts. We rather ascribe these spectral changes to the
formation of aggregates consisting of exciton-coupled anti-
parallel dimeric units in which the degrees of rotational
displacements around the stacking axis are significantly
different.[1,18] Judged from the shifted values, rotational
displacement is more pronounced for 1, leading to weaker
exciton coupling. Such a difference in packing arrangements
Although the emission intensities of molecularly dissolved
1 and 2 are very weak (Ff < 0.01) owing to the nonradiative
=
decay associated with the twisting of the C C bond between
D and A groups in the excited state, an explicit difference was
observed between the two. In cyclohexane the fluorescence
maxima of the two compounds emerge at l = 486 nm (dotted
lines in Figure 2a,c), and no difference is detected between
the two. In acetone, however, an emission at lmax = 592nm
contributes significantly only to the spectrum of 2 in addition
8006 www.angewandte.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 8005 –8009

Angewandte
Chemie
images are consistent with the morphological insights given
by SEM and AFM.
Thus, aggregation of 1 and 2 occurs in different solvents,
affording fibrous nanostructures consisting of antiparallel
dimeric units with different degrees of rotational displace-
ment. Although such a difference in the degree of rotational
displacement between 1 and 2 can partly be attributed to the
steric effect of the substituents on the barbituric acid moiety,
we believe that the degree of charge localization in the
merocyanine D–p–A system may have some contribution:
The charge-localized character of the present merocyanine
skeleton is more pronounced in polar acetone (permittivity
e = 2.0) than in cyclohexane (e = 20.7), thus reducing the
rotational displacement by stronger intermolecular D–A
interaction.
Finally, we present the unique morphological transforma-
tion observed for the nanofibers of 2 in acetone. While the
cyclohexane gel of 1 is thermodynamically stable, the acetone
gel of 2 proved to be a kinetic product that changes into
thermodynamically more stable materials upon aging for a
few days after gelation. This result was recognized by the
increase of turbidity, indicating the formation of macroscopic
aggregates. Surprisingly, SEM revealed that the turbid gels
consist of globular objects with diameters at around 10 mm
(Figure 4a).[22] The globular objects are abirefringent, indi-
Figure 3. a,b) UV/Vis (red solid lines) and fluorescence spectra (red
dotted lines) of a) the cyclohexane gel of 1 and b) the acetone gel of 2
(c=2”10À3 m, l=0.1 mm, lex =430 nm). The spectra of corresponding
diluted solutions (c=1.0”10À5 m) are also shown by blue lines for
comparison. Insets: photographs of the gels under daylight (left) and
under UV light (right). c,d) Fluorescence microscopy images
(lex =365 nm, lem >450 nm) of c) the cyclohexane gel of 1 and d) the
acetone gel of 2.
alters the emission properties of the merocyanine skeleton as
described below.
When the above gels were illuminated with UV light, the
cyclohexane gel of 1 exhibited a yellow emission whereas the
acetone gel of 2 exhibited a red emission despite the fact that
the emissions come from the same merocyanine skeleton
(right insets in Figure 3a,b). The fluorescence spectra of the
gels measured with a front-face illumination setup display
maxima at 550 nm for 1 and at 592nm for 2 (red dotted lines
in Figure 3a,b). The 550-nm emission of 1 was shifted by
64 nm from the molecularly dissolved state in diluted
cyclohexane solutions (Figure 2a), indicating that the emis-
sion is derived from exciton-coupled chromophores, which
was further supported by the fluorescence excitation spectra.
On the contrary, the 592-nm emission of 2 was already
observed for the corresponding diluted acetone solutions as
excimer fluorescence (Figure 2d), suggesting that the excimer
formation is promoted by aggregation. Aggregation-induced
excimer formation[19] has been reported for several dyes such
as perylene bisimides[20] and a triphenylene derivative[21] as
well as covalently tethered merocyanines.[7b]
Figure 4. a) SEM image and b) fluorescence spectrum (lex =430 nm)
of the thermodynamically stable acetone gel of 2. The SEM image was
taken after vacuum drying. Inset in (a) is a magnified image of a
globule. Inset in (b) is the corresponding fluorescence microscopy
image (lex =365 nm, lem >450 nm). c,d) SEM images of transient gels.
Inset in (d) is the fluorescence microscopy image in the corresponding
state.
Fluorescence microscopy of the pristine (nondried) gels
provides further direct evidence for the occurrence of differ-
ent exciton coupling in 1 and 2. As shown in Figure 3c,d,
fibrous entities emanating green and red fluorescence could
be visually observed for the cyclohexane gel of 1 and the
acetone gel of 2, respectively, the colors of which are
consistent with the emission maxima of the respective gels.
Above the gel-to-sol transition temperatures, these fluores-
cent fibers become invisible. Notably, 1 shows uniformly
dispersed fluorescent fibers, which are in clear contrast to
agglomerated fibrous networks of 2. These highly contrasting
cating a lack of long-range molecular anisotropy. Magnified
imaging allowed visual observation of their grainy surface
(inset in Figure 4a), suggesting that they are constructed
through the hierarchical association of smaller objects. Of
particular interest is their emission behavior: They no longer
emanate the original red fluorescence but emit in green (inset
in Figure 4b). The fluorescence spectra of the thermodynamic
gels are dominated by the emission with a maximum at
518 nm (Figure 4b), which corresponds to the fluorescence of
Angew. Chem. Int. Ed. 2007, 46, 8005 –8009
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 8007

Communications
the monomer in acetone. Although we could not obtain
Keywords: exciton coupling · gels · merocyanine dyes ·
self-assembly · solvatochromism
.
reliable UV/Vis data of these globular micro-objects, the
observed fluorescence property verifies that the fiber-to-
globule transition is not a simple morphological transforma-
tion of nanoscopic objects, but is a result of the transition from
kinetically formed exciton-coupled aggregates to thermody-
namically stable non-exciton-coupled aggregates.[23]
[1] M. Kasha, H. R. Rawls, M. A. El-Bayoumi, Pure Appl. Chem.
1965, 11, 371 – 392.
[2] a) T. Pullerits, V. Sundström, Acc. Chem. Res. 1996, 29, 381 – 389;
b) X. Hu, A. Damjanovic, T. Ritz, K. Schulten, Proc. Natl. Acad.
Sci. USA 1998, 95, 5935 – 5941; c) T. S. Balaban, H. Tamiaki,
A. R. Holzwarth, Top. Curr. Chem. 2005, 258, 1 – 38.
[3] a) Top. Curr. Chem. 2005, 258 (Ed.: F. Würthner); b) K. C.
Hannah, B. A. Armitage, Acc. Chem. Res. 2004, 37, 845 – 853;
c) Y. Sagara, T. Mutai, I. Yoshikawa, K. Araki, J. Am. Chem. Soc.
2007, 129, 1520 – 1521; d) M. Numata, S. Tamesue, T. Fujisawa, S.
Haraguchi, T. Hasegawa, A.-H. Bae, C. Li, K. Sakurai, S.
Shinkai, Org. Lett. 2006, 8, 5533 – 5536; e) S. Yagai, J. Photo-
chem. Photobiol. C 2006, 7, 164 – 182.
[4] a) P. Terech, R. G. Weiss, Chem. Rev. 1997, 97, 3133 – 3159;
b) D. J. Abdallah, R. G. Weiss, Adv. Mater. 2000, 12, 1237 – 1247;
c) J. H. van Esch, B. L. Feringa, Angew. Chem. 2000, 112, 2351 –
2354; Angew. Chem. Int. Ed. 2000, 39, 2263 – 2266; d) O.
Gronwald, S. Shinkai, Chem. Eur. J. 2001, 7, 4328 – 4334;
e) L. A. Estroff, A. D. Hamilton, Chem. Rev. 2004, 104, 1201 –
1217; f) K. Hanabusa in Springer Series, Materials Science,
Vol. 78 (Eds.: N. Ueyama, A. Harada), Springer, Heidelberg,
2004, pp. 118 – 137; g) N. M. Sangeetha, U. Maitra, Chem. Soc.
Rev. 2005, 34, 821 – 836; h) Top. Curr. Chem. 2005, 256 (Ed.: F.
Fages); i) M. de Loos, B. L. Feringa, J. H. van Esch, Eur. J. Org.
Chem. 2005, 3615 – 3631; j) T. Ishi-i, S. Shinkai, Top. Curr. Chem.
2005, 258, 119 – 160; k) A. R. Hirst, D. K. Smith, Chem. Eur. J.
2005, 11, 5496 – 5508; l) M. George, R. G. Weiss in Molecular
Gels. Materials with Self-Assembled Fibrillar Networks (Eds.:
R. G. Weiss, P. Terech), Springer, Heidelberg, 2006, 449 – 551;
m) A. Ajayaghosh, V. K. Praveen, Acc. Chem. Res. 2007, 40,
644 – 656.
To get some mechanistic insight into the above fiber-to-
globule transition, transient gels obtained by different aging
periods were spin-coated and observed by SEM.[16] The
collected images clearly showed that the fiber-to-globule
transition progresses in a manner reminiscent of fluff balls
appearing on a sweater. Initially, the elemental fibers with
100-nm width start to entangle into globular clumps with
diameters less than 1 mm (Figure 4c). These globular clumps
subsequently fuse quasi-one-dimensionally, affording fibrous
clumps composed of entangled elemental fibers (Figure 4d,
and Figure S3c in the Supporting Information). Finally, these
fibrous clumps further cohere and entangle to give the final
products (Figure S3d in the Supporting Information). Fluo-
rescence microscopic observation of a transient stage impres-
sively showed the coexistence of fibrous networks exhibiting
either red or green emissions, respectively (inset in Figure 4d,
and Figure S4 in the Supporting Information). Thus, the
kinetic assembly of merocyanine 2 into gel-forming nano-
fibers in acetone is presumably driven by the equal contribu-
tion of the dipole–dipole interaction between the merocya-
nine parts and the solvophobic interactions between the
cholesterol groups. Molecular packing of the cholesterol
moieties in such nanofibers is, however, considered not
optimal, so that once a thermodynamic process toward
optimal packing of cholesterol moieties starts, as demon-
strated by the fibers–globule transition, exciton-coupled
aggregates might be broken, thereby showing monomeric
fluorescence.
[5] a) F. Nüesch, J. E. Moser, V. Shklover, M. Grätzel, J. Am. Chem.
Soc. 1996, 118, 5420 – 5431; b) F. Würthner, S. Yao, T. Debaer-
demaeker, R. Wortmann, J. Am. Chem. Soc. 2002, 124, 9431 –
9447; c) F. Würthner, R. Wortmann, K. Meerholz, ChemPhys-
Chem 2002, 3, 17 – 31.
In summary, the results described here demonstrate that a
minor structural alteration of the cholesterol-appended
merocyanine dyes results in the formation of organogels in
organic solvents with different polarities, affording nano-
scopic fibers with pronouncedly distinct absorption and
fluorescence properties. This gelation-assisted control over
the optical properties of the merocyanine chromophores is a
consequence of different chromophore packing arrangements
within the nanofibers through excitonic interactions. We have
also presented an interesting example of the morphological
transition from kinetically formed nanofibers to thermody-
namically stable globules with the disappearance of excitonic
interactions between the chromophoric building blocks. Thus,
this study can expand the potential applicability of organo-
gelation for fabricating functional dye assemblies featuring
unique chromophore packing arrangements and optical
properties which cannot be achieved by solution and solid-
state chemistries.
[6] a) F. Würthner, S. Yao, Angew. Chem. 2000, 112, 2054 – 2057;
Angew. Chem. Int. Ed. 2000, 39, 1978 – 1981; b) F. Würthner, S.
Yao, U. Beginn, Angew. Chem. 2003, 115, 3368 – 3371; Angew.
Chem. Int. Ed. 2003, 42, 3247 – 3250.
[7] a) T. Katoh, Y. Inagaki, R. Okazaki, J. Am. Chem. Soc. 1998, 120,
3623 – 3628; b) L. Lu, R. J. Lachicotte, T. L. Penner, J. Perlstein,
D. G. Whitten, J. Am. Chem. Soc. 1999, 121, 8146 – 8156; c) S.
Zeena, K. G. Thomas, J. Am. Chem. Soc. 2001, 123, 7859 – 7865.
[8] a) L. J. Prins, C. Thalacker, F. Würthner, P. Timmerman, D. N.
Reinhoudt, Proc. Natl. Acad. Sci. USA 2001, 98, 10042– 10045;
b) S. Yagai, M. Higashi, T. Karatsu, A. Kitamura, Chem.
Commun. 2006, 1500 – 1502; c) F. Würthner, J. Schmidt, M.
Stolte, R. Wortmann, Angew. Chem. 2006, 118, 3926 – 3930;
Angew. Chem. Int. Ed. 2006, 45, 3842– 3846.
[9] Merocyanine-based organogels: a) S. Yao, U. Beginn, T. Gress,
M. Lysetska, F. Würthner, J. Am. Chem. Soc. 2004, 126, 8336 –
8348; b) S. Yagai, M. Higashi, T. Karatsu, A. Kitamura, Chem.
Mater. 2004, 16, 3582– 3585; c) S. Yagai, M. Higashi, T. Karatsu,
A. Kitamura, Chem. Mater. 2005, 17, 4392– 4398; d) S. Yagai, T.
Karatsu, A. Kitamura, Langmuir 2005, 21, 11048 – 11052.
[10] a) Y. C. Lin, R. G. Weiss, Macromolecules 1987, 20, 414 – 417;
b) Y. Iwashita, K. Sugiyasu, M. Ikeda, N. Fujita, S. Shinkai,
Chem. Lett. 2004, 33, 1124 – 1125; c) C. Geiger, M. Stanescu, L.
Chen, D. G. Whitten, Langmuir 1999, 15, 2241 – 2245; d) K. J. C.
van Bommel, A. Friggeri, S. Shinkai, Angew. Chem. 2003, 115,
1010 – 1030; Angew. Chem. Int. Ed. 2003, 42, 980 – 999; e) M.
Received: May 22, 2007
Revised: July 2, 2007
Published online: September 17, 2007
ˇ
´
Zinic, F. Vögtle, F. Fages, Top. Curr. Chem. 2005, 256, 39.
8008 www.angewandte.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 8005 –8009

Angewandte
Chemie
[11] a) K. Murata, M. Aoki, T. Nishi, A. Ikeda, S. Shinkai, J. Chem.
Soc. Chem. Commun. 1991, 1715 – 1718; b) J. H. Jung, Y. Ono, K.
Sakurai, M. Sano, S. Shinkai, J. Am. Chem. Soc. 2000, 122, 8648 –
8653.
[12] H. J. Tian, K. Inoue, K. Yoza, T. Ishi-I, S. Shinkai, Chem. Lett.
1998, 871 – 872.
[13] K. Sugiyasu, N. Fujita, S. Shinkai, Angew. Chem. 2004, 116,
1249 – 1253; Angew. Chem. Int. Ed. 2004, 43, 1229 – 1233.
[14] S.-i. Kawano, N. Fujita, S. Shinkai, Chem. Eur. J. 2005, 11, 4735 –
4742.
[15] A. Ajayaghosh, C. Vijayakumar, R. Varghese, S. J. George,
Angew. Chem. 2006, 118, 1159 – 1162; Angew. Chem. Int. Ed.
2006, 45, 456 – 460.
[22] For examples of stimuli-responsive transitions from fibrous to
globular nano-objects, see: a) A. Ajayaghosh, R. Varghese, S.
Mahesh, V. K. Praveen, Angew. Chem. 2006, 118, 7893 – 7896;
Angew. Chem. Int. Ed. 2006, 45, 7729 – 7732; b) A. Ajayaghosh,
P. Chithra, R. Varghese, Angew. Chem. 2007, 119, 234 – 237;
Angew. Chem. Int. Ed. 2007, 46, 230 – 233; c) 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 – 5307.
[23] There has been growing interest in kinetically controlled
assemblies: a) B. Hasenknopf, J. M. Lehn, N. Boumediene, E.
Leize, A. Van Dorsselaer, Angew. Chem. 1998, 110, 3458 – 3460;
Angew. Chem. Int. Ed. 1998, 37, 3265 – 3268; b) V. Paraschiv, M.
Crego-Calama, T. Ishi-i, C. J. Padberg, P. Timmerman, D. N.
Reinhoudt, J. Am. Chem. Soc. 2002, 124, 7638 – 7639; c) P.
Jonkheijm, A. Miura, M. Zdanowska, 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) A. Hori, K. Yamashita, M. Fujita,
Angew. Chem. 2004, 116, 5126 – 5129; Angew. Chem. Int. Ed.
[16] See the Supporting Information.
[17] a) R. Wang, C. Geiger, L. Chen, B. Swanson, D. G. Whitten, J.
Am. Chem. Soc. 2000, 122, 2399 – 2400; b) A. R. Hirst, D. K.
Smith, J. P. Harrington, Chem. Eur. J. 2005, 11, 6552– 6559.
[18] U. Rösch, S. Yao, R. Wortmann, F. Würthner, Angew. Chem.
2006, 118, 7184 – 7188; Angew. Chem. Int. Ed. 2006, 45, 7026 –
7030.
´
2004, 43, 5016 – 5019; e) J. D. Badjic, S. J. Cantrill, J. F. Stoddart,
[19] Gel-to-sol excimer formation was also reported; see: Y. Kami-
kawa, T. Kato, Langmuir 2007, 23, 274 – 278.
[20] Z. Chen, V. Stepanenko, V. Dehm, P. Prins, L. D. A. Siebbeles, J.
Seibt, P. Marquetand, V. Engel, F. Würthner, Chem. Eur. J. 2007,
13, 436 – 449.
J. Am. Chem. Soc. 2004, 126, 2288 – 2289; f) H. Tamiaki, H.
Kitamoto, A. Nishikawa, T. Hibino, R. Shibata, Bioorg. Med.
Chem. 2004, 12, 1657 – 1666; g) A. Lohr, M. Lysetska, F.
Würthner, Angew. Chem. 2005, 117, 5199 – 5202; Angew.
Chem. Int. Ed. 2005, 44, 5071 – 5074.
[21] M. Ikeda, M. Takeuchi, S. Shinkai, Chem. Commun. 2003, 1354 –
1355.
Angew. Chem. Int. Ed. 2007, 46, 8005 –8009
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 8009