Stable supramolecular helical structure of C6-symmetric hydrogen-bonded hexakis(phenylethynyl)benzene derivatives with amino acid pendant groups and their unique fluorescence properties

Article abstract of DOI:10.1039/b803532f

A highly stable supramolecular helical structure was formed by the self-assembly of novel C₆-symmetric hydrogen-bonded discotic molecules, hexakis(phenylethynyl)benzene derivatives with chiral alanine parts, and exhibited orange excimer emission with a large Stokes shift. The Royal Society of Chemistry.

Full text of DOI:10.1039/b803532f

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Stable supramolecular helical structure of C6-symmetric
hydrogen-bonded hexakis(phenylethynyl)benzene derivatives with
amino acid pendant groups and their unique fluorescence propertiesw
Koichi Sakajiri,* Takeshi Sugisaki and Keiichi Moriya
Received (in Cambridge, UK) 29th February 2008, Accepted 24th April 2008
First published as an Advance Article on the web 4th June 2008
DOI: 10.1039/b803532f
A highly stable supramolecular helical structure was formed by
the self-assembly of novel C6-symmetric hydrogen-bonded
discotic molecules, hexakis(phenylethynyl)benzene derivatives
with chiral alanine parts, and exhibited orange excimer emission
with a large Stokes shift.
the stability of the supramolecular helix (Fig. 1). The L-1
compound consists of a large hexakis(phenylethynyl)benzene
central core, bearing chiral L-alanine parts contributing to the
formation of hydrogen bonds and a one-handed helix, and
peripheral hydrophobic dodecyl chains. L-1 gives rise to
viscous solutions when dissolved in n-alkane solvents. L-1
gives a relatively low critical gelation concentration, 4.2 Â
10À4 M (1.0 mg mlÀ1), at a temperature of about 10 1C in
n-dodecane and exhibits a lyotropic liquid crystalline gel over
approximately 6 wt% in n-hexane.
Biological polymers, such as DNA and polypeptides, adopt
regular one-handed helical structures composed of their homo-
chiral components (D-sugars and L-amino acids), which further
self-assemble into lyotropic liquid crystals because of their stiff
helical backbones.1 Since the discovery of biological helices and
their liquid crystal properties, significant attention has been paid
to developing helical architectures2 with controlled helicity and
liquid crystallinity that mimic the structures and functions of
biological helices. In particular, supramolecular helical assemblies
constructed using various types of noncovalent bonding interac-
tions,3 as utilized in biological systems, are of great interest from
fundamental and biological viewpoints, and offer potential appli-
cations in chiral materials science.2
To investigate the self-assembly of L-1, UV-visible absorp-
tion (UV-vis) and circular dichroism (CD) spectra were mea-
sured in several solvents (4.20 Â 10À5 M). As shown in Fig. S1
and 2,w L-1 shows two absorption bands at 285 (e = 7.23 Â
104 MÀ1 cmÀ1) and 359 nm (e = 1.71 Â 105 MÀ1 cmÀ1), at
25 1C in the relatively polar CHCl3, and at 286 (e = 7.00 Â 104
M
À1 cmÀ1) and 362 nm (e = 1.65 Â 105 MÀ1 cmÀ1) in 1,1,2,2-
tetrachloroethane (TCE). On the other hand, the absorption
bands at a shorter wavelength shifted bathochromically to
300 nm in both n-hexane (e = 6.30 Â 104 MÀ1 cmÀ1) and
n-dodecane (e = 6.04 Â 104 MÀ1 cmÀ1), while the absorption
bands at a longer wavelength were essentially unchanged at
362 (e = 1.28 Â 105 MÀ1 cmÀ1) and 361 nm (e = 1.24 Â 105
Among such supramolecular assemblies, most of the ordered
structures have been obtained from disk-shaped molecules, which
offers the possibility of forming highly ordered columns. To
stabilize the p–p stacking among aromatic groups and enforce
the intracolumnar stacking order, it is important to enlarge the
disk size4 and to introduce hydrogen bonds,4d,5 respectively. The
majority of studies on hydrogen-bonded helical columns have
been performed on 1,3,5-benzene tricarboxamide derivatives,
which are C3-symmetric molecules consisting of a central benzene
ring and three peripheral chiral segments connected via amide
bonds.3f,4e,f,6 The p–p interactions of the central benzene cores are
enforced by 3-fold intermolecular hydrogen bonding. To enable
efficient packing, the intermolecular hydrogen bonds rotate out of
the plane, inducing a preferred one-handed helicity in the columns.
Amino acid units are also widely used as effective structuring
elements which set up hydrogen bonds in self-assembled architec-
tures such as liquid crystalline dendritic peptides,7a,b functional
cyclic peptides,7c and C3-symmetric benzene and cyclohexane
derived organogelators.3g
M
À1 cmÀ1), respectively. The red shift observed for the shorter
wavelength band in nonpolar solvents suggests that L-1 stacks
through p–p interactions between the large central cores.8
From this background, we have designed a new C6-sym-
metric hydrogen-bonded discotic molecule, L-1, to enhance
Department of Chemistry, Faculty of Engineering, Gifu University,
Yanagido, Gifu 501-1193, Japan. E-mail: sakajiri@apchem.
gifu-u.ac.jp; Fax: +81 58 293 2794; Tel: +81 58 293 2565
w Electronic supplementary information (ESI) available: Experimental
details and characterization of new compounds. See DOI: 10.1039/
b803532f
Fig. 1 Molecular structures of L-1 and D-1.
Chem. Commun., 2008, 3447–3449 | 3447
ꢀc
This journal is The Royal Society of Chemistry 2008

View Article Online
Fig. 2 Temperature dependent UV-vis spectra of L-1 in n-dodecane
(4.2 Â 10À5 M) at 25, 40, 70 and 100 1C. Arrows indicate the change
that occurs with increasing temperature. The UV-vis spectrum in TCE
at 25 1C is also shown (thin solid line).
Fig. 3 Temperature dependent CD spectra of L-1 in n-dodecane
(4.2 Â 10À5 M) at 25, 40, 70 and 100 1C. Arrows indicate the change
that occurs with increasing temperature. The CD spectrum of L-1 in
TCE at 25 1C is also shown (thin solid line). The inset shows the
concentration dependence of the chiral anisotropy g factor of D-1 in
n-dodecane at 25 1C.
In haloalkanes, L-1 shows no CD in the UV-vis range (Fig.
S2 and S3w). This indicates that the molecules do not form the
predominantly preferred chiral structures. L-1, however,
shows three major strong Cotton effects with a negative first
Cotton effect (De = À100 MÀ1 cmÀ1 at 364 nm), a negative
second Cotton effect (De = À270 MÀ1 cmÀ1 at 305 nm) and a
positive third Cotton effect (De = 89 MÀ1 cmÀ1 at 292 nm) at
25 1C in n-hexane, and similarly in n-dodecane (first: De =
À108 MÀ1 cmÀ1 at 365 nm; second: De = À266 MÀ1 cmÀ1 at
307 nm; and third: De = 85 MÀ1 cmÀ1 at 293 nm). The slight
difference in the CD spectra in n-alkanes could be due to a
minor change in solvophobic interactions. In addition, the CD
spectrum of the other enantiomer, D-1, in n-dodecane was the
mirror image of that of L-1 as shown in Fig. S2.w These results
indicate that L-1 and D-1 form opposite preferred-handed
helical structures in nonpolar n-alkanes.
absorption maximum from 300 to 298 nm (Fig. 2). Corres-
pondingly, there was a gradual decrease in CD intensity with
increasing temperature (Fig. 3), which may be attributed to a
decrease in positional order of the molecules within the
In a n-dodecane solution of L-1 (4.20 Â 10À4 M), an NH
stretching vibration of amide A was observed at 3262 cmÀ1; a
similar wavenumber was observed for a solid film of L-1 on
KBr (3257 cmÀ1), whereas a higher wavenumber was observed
(3433 cmÀ1) for a TCE solution of L-1. A similar trend was
also observed for the CQO stretching vibration of amide I;
1634 cmÀ1 in n-dodecane, 1633 cmÀ1 in a solid film, and 1655
cmÀ1 in TCE. The combined results of the UV-vis, CD and IR
indicate that L-1 forms a rigid helical columnar assembly in
nonpolar solvents, which is stabilized by strong hydrogen
bonds among the large disk cores, while the molecules in
haloalkanes are mostly in a molecularly dispersed state.
To investigate the stability of the helical structure, tempera-
ture and concentration dependent UV-vis and CD spectro-
scopy were performed from 25–100 1C (4.20 Â 10À5 M) and
from 4.20 Â 10À4–4.20 Â 10À7 M (at 25 1C) in n-dodecane
solutions of 1. A gradual decrease in intensity was observed
with increasing temperature along with a small blue shift of the
Fig. 4 FL spectra of L-1 in CHCl3, TCE and n-hexane (4.2 Â 10À7 M)
at 25 1C. The inset shows an expanded view of the temperature
dependent FL spectra in n-dodecane at 25, 40 and 70 1C. The arrow
indicates the change that occurs with increasing temperature. FL spectra
(lex = 360 nm) were normalized using the absorption values at 360 nm.
ꢀc
This journal is The Royal Society of Chemistry 2008
3448 | Chem. Commun., 2008, 3447–3449

View Article Online
columns as a result of thermal motion. Importantly, the
apparent CD spectrum at 100 1C reveals that the helical
structure of 1 is exceptionally thermally stable. Subsequent
cooling to 25 1C restored the original CD spectrum. In
addition, the chiral anisotropy g factor (De307/e300 = 0.0044)
scarcely changes from low concentrations (4.20 Â 10À7 M) to
higher concentrations (4.20 Â 10À4 M; Fig. 3, inset), suggest-
ing strong association among the molecules.
This work was partly supported by the Sasakawa Scientific
Research Grant from The Japan Science Society.
Notes and references
1. (a) C. Robinson and J. C. Ward, Nature, 1957, 180, 1183–1184;
(b) F. Livolant and Y. Bouligand, J. Phys. (Paris), 1986, 47,
1813–1827; (c) T. E. Strzelecka, M. W. Davidson and R. L. Rill,
Nature, 1988, 331, 457–460; (d) F. Livolant, A. M. Levelut,
J. Doucet and J. P. Benoit, Nature, 1989, 339, 724–726.
To further investigate the differences in the intermolecular
interactions, fluorescence (FL) spectra (4.20 Â 10À7 M) were
measured at an excitation of 360 nm (Fig. 4). Emission maxima
in CHCl3 and TCE solutions at 25 1C were observed at 457 nm
(fluorescence quantum yield, f = 0.26) and 459 nm (f = 0.32),
respectively. In contrast to the blue fluorescence observed for
these nonhelicogenic solutions, the emission in helicogenic n-
hexane and n-dodecane solutions was orange, and their max-
ima were observed at 604 (f = 0.020) and 607 nm (f = 0.036),
respectively. For the helicogenic solutions, the broad peaks at
around 605 nm are approximately 1/9–1/16 the intensity and
are considerably red-shifted with respect to the emission peaks
at around 458 nm for the nonhelicogenic solutions. This
behavior also indicates that the molecules form columnar
structures by strong p–p stacking in the helicogenic solvents.9
In addition, close stacking of 1 in n-alkanes is also supported
from the fact that the non-aggregated hexakis[4-(hexyloxycar-
bonyl)phenylethynyl]benzene, 2, which does not include amide
groups, shows no solvatochromism (Fig. S3w) and its UV-vis
and FL spectra in n-hexane were essentially the same as those
of L-1 in nonhelicogenic solutions.10 Consequently, the FL
property is clearly dependent on the aggregation state of 1, and
the emission changes from blue in the molecularly dispersed
state to orange in the helical columnar state. Here, the broad
emission in the helical columnar state is shifted by approxi-
mately 5300 cmÀ1 to lower energy compared to that of mono-
mer emission, considered to be excimer emission, because this
energy difference is comparable to those of other excimers
generated among the phenylethynyl units.11 With heating, the
excimer emission peak hypsochromically shifts by about 4 nm
and the intensity gradually decreases; the monomer emission
was not observed at all (Fig. 4, inset). This result also supports
the strong association of 1 in n-alkane. The decrease in
fluorescence intensity is probably due to several factors in
combination: (1) a decrease in positional order of the molecules
within the columns by thermal motion, (2) breaking of larger
columnar stacks into smaller stacks but not monomers,12a and
(3) thermally activated nonradiative processes occurring via the
long chain alkyl group.12b
2. (a) The entirety of Top. Stereochem., 2003, 24; (b) The entirety of
Top. Curr. Chem, 2006, 265.
3. Reviews: (a) D. J. Hill, M. J. Mio, R. B. Prince, T. S. Hughes and
J. S. Moore, Chem. Rev., 2001, 101, 3893–4011; (b) J. L. Serrano and
T. Sierra, Coord. Chem. Rev., 2003, 242, 73–85; (c) M. A. Mateos-
Timoneda, M. Crego-Calama and D. N. Reinhoudt, Chem. Soc. Rev.,
2004, 33, 363–372; (d) F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer
and A. P. H. J. Schenning, Chem. Rev., 2005, 105, 1491–1546;
(e) M. Albrecht and R. Frohlich, Bull. Chem. Soc. Jpn., 2007, 80,
¨
797–808; (f) T.-Q. Nguyen, R. Martel, M. Bushey, P. Avouris,
A. Carlsen, C. Nuckolls and L. Brus, Phys. Chem. Chem. Phys.,
2007, 9, 1515–1532; (g) M. de Loos, J. H. van Esch, R. M. Kellogg
and B. L. Feringa, Tetrahedron, 2007, 63, 7285–7301.
4. Large discotic molecules: (a) M. Kastler, W. Pisula, D. Wasserfallen,
T. Pakula and K. Mullen, J. Am. Chem. Soc., 2005, 127, 4286–4296;
¨
(b) O. F. Aebischer, A. Aebischer, P. Tondo, B. Alameddine,
M. Dadras, H.-U. Gudelb and T. A. Jenny, Chem. Commun., 2006,
¨
´
4221–4223; (c) Z. Tomovic, J. van Dongen, S. J. George, H. Xu,
W. Pisula, P. Leclere, M. M. J. Smulders, S. De Feyter, E. W. Meijer
and A. P. H. J. Schenning, J. Am. Chem. Soc., 2007, 129,
16190–16196; (d) X. Dou, W. Pisula, J. Wu, G. J. Bodwell and
K. Mullen, Chem.–Eur. J., 2008, 14, 240–249; Core-extended discotics;
¨
(e) A. R. A. Palmans, J. A. J. M. Vekemans, E. E. Havinga and
E. W. Meijer, Angew. Chem., Int. Ed. Engl., 1997, 36, 2648–2651;
(f) J. J. van Gorp, J. A. J. M. Vekemans and E. W. Meijer, J. Am.
Chem. Soc., 2002, 124, 14759–14769.
5. (a) M. Ikeda, M. Takeuchi and S. Shinkai, Chem. Commun., 2003,
1354–1355; (b) T. Ishi-i, R. Kuwahara, A. Takata, Y. Jeong,
K. Sakurai and S. Mataka, Chem.–Eur. J., 2006, 12, 763–776;
(c) C. Bao, R. Lu, M. Jin, P. Xue, C. Tan, T. Xu, G. Liu and
Y. Zhao, Chem.–Eur. J., 2006, 12, 3287–3294.
6. (a) L. Brunsveld, A. P. H. J. Schenning, M. A. C. Broeren,
H. M. Janssen, J. A. J. M. Vekemans and E. W. Meijer, Chem. Lett.,
2000, 29, 292–293; (b) A. J. Wilson, J. van Gestel, R. P. Sijbesma and
E. W. Meijer, Chem. Commun., 2006, 4404–4406; (c) K. P. van den
Hout, R. Martın-Rapu´ n, J. A. J. M. Vekemans and E. W. Meijer,
´
Chem.–Eur. J., 2007, 13, 8111–8123; (d) R. Hameren, P. Schon,
¨
A. M. Buul, J. Hoogboom, S. V. Lazarenko, J. W. Gerritsen,
H. Engelkamp, P. C. M. Christianen, H. A. Heus, J. C. Maan,
T. Rasing, S. Speller, A. E. Rowan, J. A. A. W. Elemans and R. J.
M. Nolte, Science, 2006, 314, 1433–1436.
7. (a) V. Percec, A. E. Dulcey, V. S. K. Balagurusamy, Y. Miura,
J. Smidrkal, M. Peterca, S. Nummelin, U. Edlund, S. D. Hudson,
P. A. Heiney, H. Duan, S. N. Magonov and S. A. Vinogradov,
Nature, 2004, 430, 764–768; (b) Y. Kamikawa, M. Nishii and
T. Kato, Chem.–Eur. J., 2004, 10, 5942–5951; (c) S. E. Gibson and
C. Lecci, Angew. Chem., Int. Ed., 2006, 45, 1364–1377.
8. Shoulders at around 283 nm in n-alkanes also indicate that free
molecules are contained. It is, however, considered that the
shoulders might not be due to free molecules but factors of (1)
and (2) described in the discussion on FL results.
9. (a) B. S. Gaylord, S. Wang, A. J. Heeger and G. C. Bazan, J. Am.
Chem. Soc., 2001, 123, 6417–6418; (b) L. Brunsveld, J. A. J.
M. Vekemans, J. H. K. K. Hirschberg, R. P. Sijbesma and
E. W. Meijer, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4977–4982.
10. These strong FL intensities imply that the molecules are mostly in
a molecularly dispersed state9.
In summary, we have succeeded in the synthesis of a
C6-symmetric hydrogen-bonded discotic molecule. Stacking
among large central cores of 1 is enforced by six intermolecular
hydrogen bonds, resulting in a highly stable supramolecular helical
structure, which exhibits excimer emission due to strong inter-
molecular interactions. In addition, the type of intermolecular
association is dependent on the solvent, and the observed fluor-
escence changes from orange in a helical assembly state to blue in
a molecularly dispersed state. Detailed investigations into aspects
such as the fluorescence life-time, the self-assembled structure and
the chiral amplification,4e,f,5b,6a,b,9b will be performed to further
understand features of the supramolecular helicity.
11. (a) C.-H. Lee and T. Yamamoto, Tetrahedron Lett., 2001, 42,
3993–3996; (b) J. Wu, J. Li, U. Kolb and K. Mullen, Chem.
¨
Commun., 2006, 48–50; (c) A. Hayer, V. de Halleux, A. Kohler,
¨
A. El-Garoughy, E. W. Meijer, J. Barbera, J. Tant, J. Levin,
´
M. Lehmann, J. Gierschner, J. Cornil and Y. H. Geerts, J. Phys.
Chem. B, 2006, 110, 7653–7659.
12. (a) C. V. Yelamaggad, A. S. Achalkumar, D. S. S. Rao and
S. K. Prasad, J. Mater. Chem., 2007, 17, 4521–4529;
(b) I. A. Levitsky, K. Kishikawa, S. H. Eichhorn and
T. M. Swager, J. Am. Chem. Soc., 2000, 122, 2474–2479.
ꢀc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 3447–3449 | 3449