Angewandte
Chemie
thus suggesting a low electronic density at the nodes of the
two-dimensional hexagonal lattice, and a strong (11) max-
imum, which can be accounted for by the disposition of the
acid counterparts in the outer part of the column. The
formation of a continuous ribbonlike structure in this com-
plex upon the incorporation of LiTf is supported by the fact
that it forms gels in dichloromethane (see the Supporting
Information).
In both salt-free and salt-containing supramolecular
organizations, efficient columnar assembly arises from p in-
teractions between the stacked melamine units self-associated
either in rosettes (T-A12, T-A(S)10*, and T-A(S)10*-LiTf) or
in helical ribbons (T-A12-LiTf). In this situation, the sur-
rounding V-shaped molecules can be tilted with respect to the
plane perpendicular to the column axis, to avoid steric
hindrance between their long rodlike arms and to optimize
space filling. This disposition favors an inherent helical
arrangement along the column. For the chiral complexes
T-A(S)10* and T-A(S)10*-LiTf, this kind of stacking gener-
ates respective systems that show superstructural chirality, as
revealed by CD (Figure 4a and b, solid lines)[18] and vibra-
tional CD (VCD; see the Supporting Information) spectra,
which suggest the formation of a helical stacking that strongly
involves the V-shaped acids. For the complexes with nonchiral
acids, the two helical senses should coexist in the same
proportion, thus yielding CD-silent materials (Figure 4c and
d, solid lines).
the architecture adopted by T-A(S)10*-LiTf. The sign of the
induced CD, which corresponds to absorption bands in the
UV/Vis spectra (see the Supporting Information), is depen-
dent on the CPL sign, and this indicates the possibility of
external modulation of the supramolecular chirality. Further-
more, the original CD spectra of both systems can be
recovered by heating at 908C for 5 seconds. This means that
it is not necessary to destroy the columnar organization (see
Table 1) to erase the chiral information recorded by irradi-
ation.
Finally, it was possible to transfer the chirality of CPL to
the achiral systems. Both systems, the rosette-type associa-
tion, T-A12, and the proposed helical ribbonlike H-bonded
organization, T-A12-LiTf, show intense CD bands (Figure 4c
and d) upon irradiation with CPL, thus indicating the
induction of chirality into the supramolecular systems. On
irradiation with light of the opposite handedness, the CD
shows the opposite sign, which indicates that the supramolec-
ular chirality of the mesophase can be inverted by the external
chiral radiation. The chiral photoresponse achieved upon
illumination is stable for long periods of time.
In summary, we have demonstrated the hierarchical self-
assembly of simple nonmesogenic building blocks into
hexagonal columnar mesophases. During the hierarchical
process, the orthogonal action of different noncovalent
interactions takes place. Indeed, two types of H-bonding
interaction, melamine–melamine and melamine–acid, oper-
ate in the plane of the macrocycle to form a rosettelike
stacking unit, whereas p–p interactions are mainly active in
the direction perpendicular to the rosette plane. These
interactions account for the formation of the columns that
organize within the Colh mesophase. Furthermore, it is shown
that these columns can accommodate the ions of a salt such as
lithium triflate after small architectural modifications, which
involve the formation of columns with long-range stacking
order.
Our interpretation of the observed structural changes
relies on the influence of the smallest Li+ ions. Ion–dipole
interactions between the N atoms of the triazine ring and Li+
are proposed to occur, and these allow the incorporation of
Li+ ions most likely sandwiched between rosettes. For the
chiral complex T-A(S)10*, which does not show a regular
stacking distance, the inclusion of Li+ cations in the proposed
way compels the rosettes to get closer along the column.
Complex T-A12, which shows a regular stacking distance, also
accommodates the Li+ cations without disrupting the colum-
nar mesomorphic order.
With regard to our proposal for the formation of inherent
helical structures along the column, it is shown that this self-
assembly process leads to functional materials, from simple
building blocks, which are capable of showing dynamic
supramolecular chirality and working as chirooptical switches.
In fact, during the light-induced reorientation process of
azobenzene groups, it is possible to tune the supramolecular
chirality at will by using CPL of different handedness.
Figure 4. CD spectra of cast films of T-A12, T-A12-LiTf, T-A(S)10*, and
T-A(S)10*-LiTf recorded at room temperature as fresh samples (c),
and after irradiation with a 488 nm Ar+ laser with left-handed CPL
(g) and right-handed CPL (a). The ellipticity measured strongly
depended on the cell thickness, which was smaller for the complex
T-A(S)10*-LiTf.
Subsequent experiments on the photomodulation and
photoinduction of supramolecular chirality were based on this
proposed helical model. Accordingly, illumination of the
chiral system T-A(S)10* with CPL led to either an increased
CD signal or the opposite sign depending on the handedness
of the CPL used (Figure 4a,b). Similar behavior was found for
Received: January 31, 2010
Revised: April 14, 2010
Published online: June 9, 2010
Angew. Chem. Int. Ed. 2010, 49, 4910 –4914
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4913