Supramolecular helical columnar structures formed by hydrogen-bonded disk-like (Phenylethynyl)benzene derivatives with L-Alanine pendant groups: Helix stability and supramolecular helical sense inversion

Article abstract of DOI:10.1246/bcsj.20130105

To investigate the influence of the number of substituents on supramolecular assemblies of hydrogen-bonded disklike (phenylethynyl)benzene derivatives with chiral L-alanine dodecyl groups, a tris(phenylethynyl)benzene derivative 3 and a tetrakis(phenylethynyl)benzene derivative 4 were synthesized. The behavior of these supramolecular assemblies was compared with that of the previously reported hexakis(phenylethynyl)benzene derivative 6. It is clearly shown that the stability of the supramolecular helical columnar structure is enhanced by increasing the number of substituents, namely the number of amide groups contributing to the hydrogen bonding. In addition, along with 6, 4 also exhibits a solvent-induced supramolecular helical sense inversion. Furthermore, 4 exhibits a thermally reversible supramolecular helical sense inversion at a critical solvent composition.

Full text of DOI:10.1246/bcsj.20130105

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Bull. Chem. Soc. Jpn. Vol. 86, No. 8, 940-946 (2013)
© 2013 The Chemical Society of Japan
Selected Papers
Supramolecular Helical Columnar Structures Formed
by Hydrogen-Bonded Disk-Like (Phenylethynyl)benzene
Derivatives with L-Alanine Pendant Groups: Helix Stability
and Supramolecular Helical Sense Inversion
Koichi Sakajiri,*1,2 Takahiro Hirama,1 Keiichi Yasuda,1 Shoichi Kutsumizu,1 and Junji Watanabe2
1Department of Chemistry, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193
2Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
Ookayama, Meguro-ku, Tokyo 152-8552
Received April 17, 2013; E-mail: ksakajiri@polymer.titech.ac.jp
To investigate the influence of the number of substituents on supramolecular assemblies of hydrogen-bonded disk-
like (phenylethynyl)benzene derivatives with chiral L-alanine dodecyl groups, a tris(phenylethynyl)benzene derivative 3
and a tetrakis(phenylethynyl)benzene derivative 4 were synthesized. The behavior of these supramolecular assemblies
was compared with that of the previously reported hexakis(phenylethynyl)benzene derivative 6. It is clearly shown that
the stability of the supramolecular helical columnar structure is enhanced by increasing the number of substituents,
namely the number of amide groups contributing to the hydrogen bonding. In addition, along with 6, 4 also exhibits
a solvent-induced supramolecular helical sense inversion. Furthermore, 4 exhibits a thermally reversible supramolecular
helical sense inversion at a critical solvent composition.
It is well-known that biological helices such as DNA and
parts and peripheral hydrophobic dodecyl chains. The stacking
among large central cores of 6 is enforced by six intermolecular
hydrogen bonds, leading to an exceptionally stable supra-
molecular helical columnar structure that is maintained even
at 100 °C in dilute nonpolar n-alkane solutions. The rigidity of
the helical column also produces lyotropic liquid crystalline
properties at a relatively low concentration (ca. 6 wt %) in n-
alkane. However, 6 is molecularly dispersed in relatively polar
haloalkanes. Interestingly, we found a solvent-induced supra-
molecular helical sense inversion in a binary solvent system
consisting of a helicogenic n-alkane and a nonhelicogenic
haloalkane.5b
Such a chirality inversion is a topic of particular inter-
est.1c,1d,6 The majority of studies on this subject have involved
polymers7 and host-guest complexes.8 There are, however, a
few reports on supramolecular helical columns constructed
using the noncovalent bonding interactions.9 With our system,
it is easy to prepare various analogous compounds, which
enables us to systematically examine the supramolecular
chirality inversion of helical columnar structures and the
stability of each helix.
polypeptides adopt regular one-handed helical structures com-
posed of homochiral components. Because the structures and
properties of these biological helices are highly attractive,
the development of various helical architectures has been
accelerated.1-4 Among them, supramolecular helical assemblies
constructed using various types of noncovalent bonding inter-
actions (such as ³-³ stacking, hydrogen-bonding, and hydro-
phobic interactions),4 as utilized in biological systems, are of
great interest from fundamental and biological viewpoints and
have potential applications in chiral materials science.1a,1b,1e
Recently, we reported the supramolecular assembly of the
six-armed hydrogen-bonded disk-like molecule 6, as shown
in Figure 1 (left).5a Compound 6 consists of a large hexakis-
(phenylethynyl)benzene central core bearing chiral L-alanine
In this study, we synthesized two new compounds to
investigate the influence of the number of substituents on the
supramolecular assembly of hydrogen-bonded disk-like (phen-
ylethynyl)benzene derivatives with chiral L-alanine dodecyl
groups, namely, the three-armed tris(phenylethynyl)benzene
derivative 3 and the four-armed tetrakis(phenylethynyl)benzene
derivative 4, as shown in Figure 1. Herein, we report their helix
Figure 1. Molecular structures of 6, 4, and 3.
Published on the web August 15, 2013; doi:10.1246/bcsj.20130105

K. Sakajiri et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 8 (2013)
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Figure 2. Temperature-dependent (a) UV-vis and (b) CD
spectra of 4 in n-dodecane (4.20 © 10¹5 M) at 25, 50, 70,
80, and 90 °C.
Figure 3. Temperature-dependent (a) UV-vis and (b) CD
spectra of 4 in n-dodecane (4.20 © 10¹6 M) at 25, 50, 70,
80, and 90 °C.
stabilities and helical sense inversion behaviors together with
the results of UV-visible absorption (UV-vis), circular dichro-
ism (CD) and fluorescence (FL) measurements.
nm) and positive at short wavelengths (¦¾2 = 809 M¹1 cm¹1 at
306 nm) near this UV-vis absorption. The same spectra were
also obtained in Hx solution (4.20 © 10¹5 M), as shown in
Figure S1 (see the Supporting Information). These profiles (so-
called negative coupling) suggest the presence of a left-handed
helical arrangement of 4 molecules.8b,8c,9b An increase in
temperature slightly decreased the UV-vis absorption. Corre-
spondingly, there was a gradual decrease in the CD intensity
with increasing temperature. Importantly, the apparent CD
spectrum at 90 °C reveals that the helical structure of 4 is
definitely maintained at this concentration. Subsequent cooling
to 25 °C restored the original CD spectrum.
Results and Discussion
Stability of the Supramolecular Helices. To investigate
the self-assembly of 3 and 4, UV-vis and CD spectra were
measured (4.20 © 10¹5 M). Unfortunately, 3 did not dissolve in
a pure helicogenic n-alkane. Although 3 was dissolved in n-
hexane (Hx) containing 5 vol % of chloroform (CHCl3), a CD
signal was not observed. As described in the next section, both
4 and 6 form helical columnar structures under these condi-
tions. Consequently, it is concluded that the construction of the
supramolecular chiral self-assembly is more difficult in 3 than
in 4 and 6. On the other hand, an n-alkane solution of 4 showed
an obvious CD signal. Hereinafter, the self-assembly behavior
of 4 will be described in detail. Figures 2a and 2b show the
temperature-dependent UV-vis and CD spectra of 4 in n-
When the solution was more dilute (4.20 © 10¹6 M), the
thermal behavior of 4 changed. Figures 3a and 3b show the
temperature-dependent UV-vis and CD spectra of 4 in n-
dodecane (4.20 © 10¹6 M), respectively. In addition, to easily
see its thermal behavior, the variation in Kuhn’s anisotropy
factor (g = ¦¾1/¾) as a function of temperature are shown
in Figure S2 (see the Supporting Information). Compound 4
shows a major absorption band at 316 nm (¾ = 1.18 © 105
dodecane (4.20 © 10¹5 M), respectively. Compound 4 shows a
¹1
major absorption band at 316 nm (¾ = 1.34 © 105 M¹1 cm
)
at 25 °C and a bisignate Cotton effect with the sign being
M
¹1 cm¹1) at 25 °C, which is at the same position as that in the
spectrum of the 4.20 © 10¹5 M solution. In addition, this CD
¹1
negative at long wavelengths (¦¾1 = ¹763 M¹1 cm at 321

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Bull. Chem. Soc. Jpn. Vol. 86, No. 8 (2013)
Controlled Supramolecular Helicity
approximately 5600 cm¹1 to a lower energy compared to that of
the monomer emission, considered to be an excimer emission,
because this energy difference is comparable to those of other
excimers generated among the phenylethynyl units.11 With
heating, the intensity of the excimer emission peak decreased
gradually. Concomitantly, the monomer emission appeared at
395 nm above 50 °C, and its peak intensity gradually increased
with temperature. This result indicates that the helical columnar
structure of 4 collapses with an increase in temperature.
Furthermore, the stability of the helical column of 4 was
evaluated on the basis of IR measurements. In a n-dodecane
solution of 4 (4.20 © 10¹4 M), the C=O stretching vibration of
amide I was observed at 1637 cm¹1, whereas a higher wave-
number was observed (1662 cm¹1) for this vibration in a CHCl3
solution of 4 (4.20 © 10¹4 M). The combined results of the
UV-vis, CD, FL, and IR analyses indicate that 4 forms a rigid
helical columnar structure at lower temperature in nonpolar
n-alkane solvents that is stabilized by strong hydrogen bonds
among the large disk cores. With heating, the self-assembled
helical columnar structure is deconstructed because of the
breaking of the hydrogen bond.
Compound 6, on the other hand, forms a thermally stable
supramolecular helical columnar structure without any free
monomers even in a more dilute n-alkane solution (4.20 ©
10¹7 M).5a Therefore, these results have given direct evidence
that the stability of the supramolecular helical columnar
structure is enhanced by increasing the number of substituents,
namely the number of amide groups contributing to the hydro-
gen bonding.
Solvent-Induced Supramolecular Helical Sense Inver-
sion. We previously reported that the six-armed disk-like
molecule 6 exhibited a solvent-induced supramolecular helical
sense inversion in a binary solvent system using helicogenic
Hx and nonhelicogenic CHCl3.5b In the present study, we also
investigated the supramolecular assembly state of 4 in mixed
solvents of various CHCl3/Hx ratios (v/v). The UV-vis and
CD spectra of 4 (4.20 © 10¹6 M) measured at 25 °C are shown
in Figure S3 (see the Supporting Information) and Figure 5,
respectively. In addition, to easily see the self-assembled
behaviors, the variation in the maximal absorption wavelength
Figure 4. Temperature-dependent FL spectra of 4 in n-
dodecane (4.20 © 10¹6 M) at 25, 50, 60, and 70 °C. The FL
spectrum in CHCl3 at 25 °C is also shown.10 FL spectra
were measured at excitations of 316 and 329 nm (corre-
sponding to the absorption maxima) for n-dodecane and
CHCl3 solutions, respectively, and normalized using the
corresponding absorption values.
spectrum also has the same features as those in the spectrum of
the 4.20 © 10¹5 M solution at 25 °C, namely a bisignate Cotton
effect with the sign being negative at long wavelengths (¦¾1 =
¹1
¹665 M¹1 cm at 321 nm) and positive at short wavelengths
(¦¾2 = 713 M¹1 cm¹1 at 306 nm) near this UV-vis absorption.
This result indicates that 4 forms a left-handed supramolecular
helical structure in a 4.20 © 10¹6 M solution at 25 °C.
An increase in temperature, however, gradually decreased
the UV-vis absorption, CD intensities, and «g« value. At 80 °C,
the main absorption band shifted bathochromically. At 90 °C,
no CD signal was observed and the g value dropped to zero.
Finally, the position of the UV-vis absorption band shifted to
326 nm (¾ = 1.11 © 105 M¹1 cm¹1). This wavelength is quite
¹1
close to that (¾ = 1.45 © 105 M¹1 cm at 329 nm) in a CHCl3
solution, as shown in Figure S1a (see the Supporting Informa-
tion). In addition, 4 does not show any CD in CHCl3, as can
easily be seen in Figure S1b (see the Supporting Information).
Therefore, 4 does not form a self-assembled structure but
is molecularly dispersed at high temperature in this dilute
4.20 © 10¹6 M solution. This transition was further confirmed
by investigating the temperature dependence of the intermo-
lecular interactions of 4 in the same 4.20 © 10¹6 M solution.
Figure 4 shows the FL spectra of 4 measured at excitations of
316 and 329 nm (corresponding to the absorption maxima) for
n-dodecane and CHCl3 solutions, respectively. The emission
maximum in the CHCl3 solution at 25 °C was observed at
around 400 nm.10 In contrast to the blue fluorescence observed
for this nonhelicogenic solution, the emission in the helico-
genic n-dodecane solution was green, and this maximum was
observed at around 517 nm at 25 °C. In addition, this broad
peak for the helicogenic solution is weak with respect to
the sharp emission peak for the nonhelicogenic solution. This
behavior indicates that the molecules form columnar structures
by strong ³-³ stacking in the helicogenic solvent. In fact, the
broad emission in the helical columnar state is shifted by
(-max) and the ¦¾ value, and the g value as a function of the
1
CHCl3 content are shown in Figure 6 and Figure S4 (see the
Supporting Information), respectively.
In pure Hx, i.e., CHCl3/Hx (0/100), 4 shows a major
absorption band at 315 nm (¾ = 1.26 © 105 M¹1 cm¹1) and a
bisignate Cotton effect with the sign being negative at long
¹1
wavelengths (¦¾1 = ¹733 M¹1 cm at 319 nm) and positive
¹1
at short wavelengths (¦¾2 = 752 M¹1 cm at 306 nm) near
this UV-vis absorption. These spectra are essentially the same
as those of the helical columnar assembly in n-dodecane.
At CHCl3/Hx (5/95), 4 also shows a major absorption band
at 316 nm (¾ = 1.21 © 105 M¹1 cm¹1) and a bisignate Cotton
effect with the sign being negative at long wavelengths (¦¾1 =
¹1
¹694 M¹1 cm at 320 nm) and positive at short wavelengths
(¦¾2 = 709 M¹1 cm¹1 at 306 nm) near this UV-vis absorption.
These spectra are almost the same as those of the helical
columnar assembly in CHCl3/Hx (0/100). When the CHCl3
content was higher than CHCl3/Hx (5/95), however, the CD
intensity and «g« value decreased drastically. Subsequently, at

K. Sakajiri et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 8 (2013)
943
4
(4.20 © 10¹6 M at 25 °C)
Figure 5. CD spectra of
4
(4.20 © 10¹6 M at 25 °C)
Figure 7. FL spectra of
measured in different solvent compositions (CHCl3/Hx):
(0/100), (5/95), (8.5/91.5), (9/91), (12.5/87.5), (20/80),
and (25/75).
measured in different solvent compositions ((CHCl3/Hx):
(5/95), (9/91), (12.5/87.5), (15/85), (20/80), and (25/
75)) at an excitation of 316 nm and normalized using the
absorption values.
of free monomer was observed at around 400 nm in the FL
spectra at 0-12.5 vol % of CHCl3, as shown in Figure 7.
On the contrary, free monomers of 6 were not observed
under these conditions,5b indicating that the inverted helical
columnar structure of 6 is more stable than that of 4, as is
the case for the original left-handed helices. Although a very
minor monomer emission is observed at around 400 nm in the
FL spectra of 4, these FL spectra are mainly constituted of a
major excimer-like emission at around 510 nm. Therefore, it
is concluded that 4 exhibits a solvent-induced supramolecular
helical sense inversion from a left-handed to a right-handed
helical columnar structure. When the CHCl3 content was fur-
ther increased in the solutions of 4, the CD intensity became
significantly weaker again (correspondingly, the g value also
decreased), and the value of -max was red-shifted toward 329
nm as observed in the molecularly dispersed state in CHCl3/Hx
(100/0). Concomitantly, the free monomer content drastically
increased, as shown by the sharp increase in the intensity of the
monomer emission, and the self-assembled helical columnar
structure was deconstructed, as seen from the decrease in the
excimer-like emission (Figure 7).
Interestingly, 6 also exhibited a solvent-induced supra-
molecular helical sense inversion at a similar solvent compo-
sition (CHCl3/Hx (8/92)) as that for 4. As previously reported,
the solvent polarity, namely the dielectric constant of the
solvents, seems to trigger the helical sense inversion.5b Some
polyacetylene derivatives bearing amino acid pendants also
tend to exhibit a solvent-induced helical sense inversion
depending on the solvent polarity.7c,7d It is likely that the
amino acid portion (alanyl groups in our materials) is sensitive
to the environment and play a key role in changing the helicity.
Thermally Reversible Supramolecular Helical Sense
Figure 6. Variation in the maximal absorption wavelength
(-max) (closed circles) and the ¦¾ value (open circles)
1
obtained from the UV-vis and CD spectra of 4 (4.20 ©
10¹6 M at 25 °C), respectively, as a function of the CHCl3
content in CHCl3/Hx mixtures.
CHCl3/Hx (9/91), the CD sign inversion occurred without
a change in the position of the UV-vis absorption band. At
CHCl3/Hx (12.5/87.5), the CD intensity with the opposite sign
reached a maximum. It is noteworthy that these two types of
CD spectra with opposite signs are exact mirror images. In
the CHCl3/Hx (12.5/87.5) solution of 4 (4.20 © 10¹4 M), the
C=O stretching vibration of amide I was observed at 1638
cm¹1. This value indicates that the inverted helical columns are
also stabilized by hydrogen bonds between the large disk cores,
as well as between the original left-handed helical columns.
In addition, at 0-12.5 vol % of CHCl3, the -max values are
almost independent of the nonhelicogenic CHCl3 content and
nearly constant, as can be clearly seen in Figure 6. This result
indicates that the supramolecular helical columns are robustly
maintained in the range from 0 to approximately 12.5 vol % of
nonhelicogenic CHCl3. It should be noted that a small amount
Inversion.
It is well known that the dielectric constant
depends on the temperature.12 Generally, the dielectric con-
stants of solvents decrease with an increase in temperature.
This behavior suggests that solvents become relatively polar

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Bull. Chem. Soc. Jpn. Vol. 86, No. 8 (2013)
Controlled Supramolecular Helicity
Figure 9. Temperature-dependent FL spectra of
4 in
(CHCl3/Hx): (8.5/91.5) (4.20 © 10¹6 M) at 20, 25, 40,
and 55 °C. The FL spectra were measured at an excitation
of 316 nm and normalized using the absorption values.
seen by the increase in the monomer emission (Figure 9).
Under this condition, the supramolecular helical columns par-
tially dissociate into monomers and these are at equilibrium.
Nevertheless, the CD intensity was enhanced. This suggests
that the helix has greater excess one-handedness than those
of helices with no monomers dissociated. In other words, it is
possible that a decrease in the solvent polarity with the increase
in temperature stabilized the original left-handed helical colum-
nar structure. When the temperature was increased to 55 °C,
however, the CD intensity decreased. In addition, the main
absorption was slightly red-shifted, and the monomer emis-
sion was dominant (Figures 8a and 9, respectively). These
results indicate that the helical columnar structures were being
disrupted into smaller assemblies and free monomers. This
temperature dependence of the self-assembled behavior is in
good correspondence with the change of the g value as shown
in Figure S5 (see the Supporting Information).
A CHCl3/Hx (12.5/87.5) solution of 4 was also investigated
as another example. Compound 4 in this solvent composition
forms an inverted right-handed helical columnar structure at
25 °C. An increase in the temperature to 55 °C (corresponding
to a decrease in the solvent polarity) gave rise to the inversion
of the CD sign, as shown in Figure S6b (see the Support-
ing Information). Although disruption into smaller assemblies
and free monomers is accompanied at this CHCl3 content
(Figures 7 and S6a in the Supporting Information), it is evident
that 4 also retransforms from an inverted right-handed to the
original left-handed helical regime. Moreover, it was confirmed
that all of the transitions were thermally reversible. This
result indicates that both of these opposite helical regimes are
thermodynamically stable9c and the transition in this system
is essentially different from the kinetically controllable transi-
tion (from metastable to thermodynamically stable helical
regimes).9a,9b Although the reversible supramolecular helical
sense inversion was recently reported in a helical tubule formed
from hexameric macrocycles,9c our material could be a valua-
ble example exhibiting such a phenomenon in typical disk-like
molecular systems.
Figure 8. Temperature-dependent (a) UV-vis and (b) CD
spectra of 4 in (CHCl3/Hx): (8.5/91.5) (4.20 © 10¹6 M) at
20, 25, 40, and 55 °C.
and nonpolar at low and high temperatures, respectively. At
a critical solvent composition, where 4 and 6 exhibit a helical
sense inversion, one therefore could expect that such a transi-
tion is also thermally induced. Although the reason has not yet
been clearly identified, only the thermally induced helical sense
inversion of 4 has been observed, as described below.
A CHCl3/Hx (8.5/91.5) solution of 4 was selected as the
critical sample solution because the CD intensity was signifi-
cantly decreased at this solvent composition, and then, the
helical sense inversion occurred at CHCl3/Hx (9/91) by further
addition of a small quantity of CHCl3. As expected, a decrease
in the temperature to 20 °C (corresponding to an increase in
the solvent polarity) gave rise to the inversion of the CD sign,
as shown in Figure 8b. Here, the UV-vis and FL spectra at
20 °C are almost the same as those at 25 °C (Figures 8a and 9,
respectively). Consequently, these results indicate that 4 exhib-
ited a thermally induced supramolecular helical sense inver-
sion. On the contrary, an increase in the temperature to 40 °C
led to an increase in the CD intensity. This result is surprising
because an increase in temperature generally disrupts a helical
columnar assembly and decreases the CD intensity. It is true
that the free monomer content increased at 40 °C, as can be

K. Sakajiri et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 8 (2013)
945
benzene (277 mg, 0.879 mmol), [Pd(PPh3)2Cl2] (111 mg, 0.158
mmol), CuI (111 mg, 0.582 mmol) and PPh3 (221 mg, 0.843
mmol) in TEA (40 mL) was refluxed under nitrogen for 1 week.
After the reaction mixture was evaporated under reduced pres-
sure, the residue was dissolved in CHCl3 (30 mL) and then
the dark brown solution was washed twice with 0.2 M HCl
(50 mL), three times with water (150 mL), dried over MgSO4,
filtered and evaporated to dryness. The residue was purified
by silica gel chromatography with CHCl3 as the eluent and
recrystallization several times from methanol to give a white
powder (48 mg, 4.4%). Mp 107.8 °C. 1H NMR (400 MHz,
45 °C): ¤H 0.88 (t, J = 6.8 Hz, 3 © 3H, 3 © CH3), 1.26-1.38
(m, 3 © 18H, 3 © 9 © CH2), 1.53 (d, J = 7.6 Hz, 3 © 3H, 3 ©
C¡CH3), 1.64-1.71 (m, 3 © 2H, 3 © OCH2CH2), 4.14-4.24 (m,
3 © 2H, 3 © OCH2), 4.75-4.82 (m, 3 © 1H, 3 © C¡H), 6.78
(d, J = 6.8 Hz, 3 © 1H, 3 © NH), 7.58 (d, J = 8.4 Hz, 3 © 2H,
peripheral aromatic protons), 7.68 (s, 3H, central aromatic
protons), 7.80 (d, J = 8.4 Hz, 3 © 2H, peripheral aromatic
protons). 13C NMR (100 MHz, 45 °C): ¤C 14.02, 18.70, 22.65,
25.83, 28.59, 29.18, 29.30, 29.47, 29.53, 29.60, 31.89, 48.75,
65.86, 89.91 (C¸C), 89.97 (C¸C), 123.89, 126.20, 127.16,
131.84, 133.93, 134.50, 165.93, 173.21. Anal. Calcd (%) for
C78H105N3O9: C, 76.25; H, 8.61; N, 3.42%. Found: C, 76.51;
H, 8.90; N, 3.20%.
Conclusion
To investigate the influence of the number of substituents
on the supramolecular assembly of hydrogen-bonded disk-
like (phenylethynyl)benzene derivatives with chiral L-alanine
dodecyl groups, 3 and 4 were synthesized, and these supra-
molecular assemblies were compared with that of the previ-
ously reported 6. The combined results showed that the number
of substituents, namely the number of amide groups contribu-
ting to the hydrogen bonding, is important for the stabilization
of supramolecular helical columns. In addition, 4 exhibited
a solvent-induced supramolecular helical sense inversion at a
similar solvent composition as that observed for 6, indicating
that the amino acid group may be sensitive to the solvent
polarity and play a key role in changing the helicity. Further-
more, 4 exhibited a thermally reversible supramolecular helical
sense inversion at a critical solvent composition. These behav-
iors have not been found in typical disk-like molecules.9 To
further understand the mechanism and the driving force of
such a transition, a systematic study including other analogous
compounds (for example, with different amino acid groups)
will be continued, together with detailed structural analyses of
these compounds.
Experimental
1,2,4,5-Tetrakis{4-[(S)-1-(dodecyloxycarbonyl)ethylcar-
Instruments. 1H NMR and 13C NMR spectra were recorded
on a JEOL JNM-A400 (400, 100 MHz) in CDCl3. Chemical
shifts of H and 13C are reported in ¤ values referred respec-
bamoyl]phenylethynyl}benzene (4):5a
A mixture of 4-
ethynylbenzoyl-L-alanine dodecyl ester (1.59 g, 4.12 mmol),
1,2,4,5-tetrabromobenzene (330 mg, 0.838 mmol), [Pd(PPh3)2-
Cl2] (116 mg, 0.166 mmol), CuI (107 mg, 0.560 mmol) and
PPh3 (216 mg, 0.824 mmol) in TEA (40 mL) was refluxed under
nitrogen for 8 days. After the reaction mixture was evaporated
under reduced pressure, the residue was dissolved in CHCl3 (50
mL) and then the dark brown solution was washed twice with
0.2 M HCl (100 mL), four times with water (150 mL), dried over
MgSO4, filtered and evaporated to dryness. The residue was
purified by silica gel chromatography with CHCl3 as the eluent,
and then the crude product was recrystallized several times
from 1,2-dichloroethane, ethyl acetate, and acetone to give a
1
tively to tetramethylsilane and CDCl3 as internal standards.
Elemental analyses were carried out by Elemental Analysis
Center of Kyoto University. IR spectra were recorded with
a Jasco FT-IR-460 Plus/IRT-30 spectrophotometer. UV-visible
absorption (UV-vis) and circular dichroism (CD) spectra were
measured using a Hitachi U-4000S spectrophotometer and a
Jasco J-820P spectropolarimeter, respectively. The temperature
was controlled with a Komatsu Electronics SPR-7 for UV-vis
spectral measurements and a Jasco PTC-423L apparatus for CD
spectral measurements. Fluorescence (FL) spectra were mea-
sured using a Hitachi F-4500 fluorescence spectrophotometer.
The temperature was controlled with a Komatsu Electronics
CTE32 apparatus.
Materials. Unless otherwise noted, reagents were commer-
cially available and used without purification. Triethylamine
(TEA) was obtained from Kishida Chemical Co., Ltd. 1,3,5-
Tribromobenzene, 1,2,4,5-tetrabromobenzene, bis(triphenyl-
phosphine)palladium dichloride ([Pd(PPh3)2Cl2]), copper(I)
iodide (CuI), and triphenylphosphine (PPh3) were obtained
from Tokyo Chemical Industry Co., Ltd. Silica gel used for
column chromatography was Silica gel C-200 from Wako.
4-Ethynylbenzoyl-L-alanine dodecyl ester and the six-armed
C6-symmetric hexakis(phenylethynyl)benzene derivatives were
prepared according to a previously reported method.5a The
new three-armed C3-symmetric tris(phenylethynyl)benzene and
four-armed C2-symmetric tetrakis(phenylethynyl)benzene anal-
ogous compounds, 3 and 4, respectively, were also synthesized
by the similar method described below.
1
yellowish white powder (0.30 g, 22%). Mp 178.9 °C. H NMR
(400 MHz, rt): ¤H 0.88 (t, J = 6.8 Hz, 4 © 3H, 4 © CH3), 1.22-
1.40 (m, 4 © 18H, 4 © 9 © CH2), 1.55 (d, J = 7.1 Hz, 4 © 3H,
4 © C¡CH3), 1.64-1.71 (m, 4 © 2H, 4 © OCH2CH2), 4.14-4.24
(m, 4 © 2H, 4 © OCH2), 4.76-4.83 (m, 4 © 1H, 4 © C¡H), 6.93
(broad, 4 © 1H, 4 © NH), 7.56 (d, J = 7.8 Hz, 4 © 2H, periph-
eral aromatic protons), 7.76 (s, 2H, central aromatic protons),
7.78 (d, J = 8.1 Hz, 4 © 2H, peripheral aromatic protons).
13C NMR (100 MHz, rt): ¤C 14.09, 18.52, 22.66, 25.80,
28.52, 29.20, 29.32, 29.49, 29.55, 29.60, 29.62, 31.88, 48.71,
65.79, 89.48 (C¸C), 94.93 (C¸C), 125.28, 126.10, 127.27,
131.70, 133.91, 135.10, 166.13, 173.20. Anal. Calcd (%) for
C102H138N4O12: C, 75.99; H, 8.63; N, 3.48%. Found: C, 76.09;
H, 8.67; N, 3.44%.
UV-vis, CD, FL, and IR Spectra Measurements. 4.20 ©
10¹5 M solutions were prepared in 100-mL flasks equipped
with stopcocks, and UV-vis and CD spectra were measured
in a 1-mm cell. Aliquots (2.5 mL) of the solutions were trans-
ferred to 25-mL flasks equipped with stopcocks and diluted,
and then UV-vis, CD, and FL spectra of the solutions (4.20 ©
10¹6 M) were measured in a 1-cm cell. Here, only the FL
1,3,5-Tris{4-[(S)-1-(dodecyloxycarbonyl)ethylcarbamoyl]-
phenylethynyl}benzene (3):5a A mixture of 4-ethynylbenzo-
yl-L-alanine dodecyl ester (1.16 g, 3.01 mmol), 1,3,5-tribromo-

946
Bull. Chem. Soc. Jpn. Vol. 86, No. 8 (2013)
Controlled Supramolecular Helicity
spectrum in CHCl3 was measured at more dilute 4.20 © 10¹7 M
because the FL intensity saturated at 4.20 © 10¹6 M solution.
IR spectral measurements of 4.20 © 10¹4 M solutions were
performed at about 25 °C in a 1-mm cell.
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The research is partly supported by a Grant-in-Aid for
Young Scientists (B) No. 23750229 from Japan Society for the
Promotion of Science.
Supporting Information
Characterization of new compounds. Figures S1-S6. This
material is available free of charge on the web at http://www.
csj.jp/journals/bcsj/.
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