COMMUNICATION
tors 2 and 3 also formed organogels in toluene, but no heli-
cal fibers were observed when only a chiral gelator was
present (Figure 1a, b). Various ratios (1:1, 2:1, 5:1, 10:1,
97:3, 98:2, and 99:1) of the achiral and chiral gelators were
tested. When the chiral portion increased too much, the
amount of the helical structure was not clear (Figure 3). Be-
cause the chiral methyl groups of 2 and 3 presumably inhibit
the stacking interactions, an excess of either 2 or 3 probably
obstructs the formation of helical structures. When the
amount of 2 or 3 is above 33%, helical structures are clearly
not formed (see Supporting Information).
The SEM images of xerogels exhibited the macroscopic
aggregation modes of the gelators. Figure 4 shows that the
xerogels obtained from complexes consist mainly of helical
structures. Figure 4a and b shows that all helices formed by
1 and 2, in the ratio of 99:1, reveal the characteristic left-
handed helical ribbon structures (M helices). In the case of
1 and 3, right-handed helical ribbon structures (P helices)
are exclusively formed (Figure 4c and d). Because the width,
length and average helical pitch length of helices are similar
to those of the helices formed by 1 alone, chiral triggers 2
and 3 did nothing but play a role in inducing the unidirec-
tional helicity.
CD experiments were performed to investigate the self-
assembly of 1 and chiral triggers from a microscopic view-
point. Gelator 1 was almost CD-inactive because organogels
were constructed by the random placement of nearly equal
numbers of P and M helices. In contrast, 2 and 3 show com-
plementary CD spectra because of the presence of the in-
polymeric structures of organogelator 1. This implies that
the p–p interaction in organogelator 1 exerted a strong in-
fluence on self-association and gelation. The H NMR sig-
1
nals of 1 are concentration-dependent in [D8]toluene (Fig-
ure 2b). The amide proton signal shifted downfield (d=
+0.15 ppm) upon increasing the concentration, whereas the
aromatic proton signals showed insignificant shift. This indi-
cated that hydrogen bonding is a crucial factor of self-associ-
ation rather than aromatic stacking in toluene.
The gelation behavior of the gelators was tested in various
organic solvents. Gelation occurred in aromatic solvents
such as toluene and p-xylene. The xerogel obtained from
achiral organogelator 1 in toluene (1.14% w/w) shows very
interesting features in which remarkably thick ribbon struc-
tures are twisted in both left- and right-handed helical struc-
tures (Figure 1c, d and e). The
width of the ribbons varies
from 4 to 5 mm and they are a
few hundred micrometers in
length, with an average helical
pitch length of 7–8.5 mm. The
aggregated structures were sta-
bilized presumably by coopera-
tive aromatic stacking, hydro-
gen bonding, and van der Waals
interactions. While aromatic
stacking between the phenyl
groups induces one-dimensional
aggregates, hydrogen bonds
among the amide groups propa-
gate along the aggregate axis
and enforce a helical mode of
the aggregate.
Figure 2. a) Stacked 1H NMR spectra of 17 mm achiral gelator 1 at 508C. The solvent ratio of CDCl3/CD3OD
(v/v) from bottom to top: 10:1; 5:1; 2:1; 1:1. b) Stacked 1H NMR spectra of 1 are concentration-dependent in
&
*
[D8]toluene at 808C. The concentration of 1 from bottom to top: 2, 4, 10, 20 mm. (Ha = , Hb = ).
For the purpose of control-
ling P and M helicity, chiral
triggers 2 and 3 were used. It
turned out that the chiral
methyl group of the d- or l-ala-
nine residue in 2 and 3 induced
the formation of homochiral
helical structures in the aggre-
gates consisting of the achiral
gelator and chiral gelator. Gela- Figure 3. SEM images of xerogels a) 1/3 1:1, b) 1/3 10:1 and c) 1/3 98:2 (scale bar a)–c)=5 mm).
Chem. Eur. J. 2008, 14, 6040 – 6043
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6041