Chiral helical induction through h-bonding in supramoelcular polymers
J. Barberá, L. Puig, J.L. Serrano, T. Sierra
The significant role of helical organizations in nature ( i.e.
a -helical proteins, nucleic acids, Tobacco Mosaic Virus) has stimulated
the curiosity of researchers in materials chemistry. Many interesting
examples aimed at building and controlling supramolecular chiral architectures
based on helical superstructures have been reported. , Indeed, interesting
linear optical and nonlinear optical properties, electrooptical behavior
, , energy transfer, etc. have been found in synthetic helical superstructures,
which are often prepared according to the principles of supramolecular
We have developed an efficient strategy based on two types
of non-covalent intermolecular interaction, i.e . p-p interactions
and H-bonding, which have enabled the construction of a number of supramolecular
liquid crystalline assemblies9. Our approach consists on merging
mesomorphic arrangements based on molecular stacking, promoted by p-p interactions
of simple molecules, and the possibility of anchoring the structure of the mesophase
using H-bonding interactions with a second type of molecule. In this way we
should be able to build up stable helical polymeric assemblies in which chirality
can be induced via a stereogenic center and transferred to the mesophase
through hydrogen-bonding. The final supramolecular organization must be endowed
of a one-dimensional helical arrangement along the column.
For this purpose, we have chosen molecular units that are capable
of organizing into columnar mesophases, by virtue of a disk-like promesogenic
structure, and can also participate in intermolecular hydrogen-bonding interactions.
We therefore decided to employ 2,4,6-triarylamino-1,3,5-triazines (figure 1a).
It is also necessary to introduce chirality into the system and for this purpose
we have selected R-3-methyladipic acid as a chiral clip (figure 1b). It was
envisaged that this diacid would not only fix the supramolecular organization
by building a polymeric-like structure but would also be able to imprint its
chiral character onto the resulting self-assembled structure.
Figure 1. a) Molecular structure of the triazine derivative
(peripheral tails are omitted for the sake of clarity) b) Molecular structure
of (R)-3-methyladipic acid. c and d) Idealized cartoon representation of the
structure for two supramolecular materials prepared with different proportions
of (R)-3-methyladipic acid.
The use of techniques such as x-ray diffraction and circular
dichroism in the mesophase allow to confirm the success of the design. A reflection
maximum corresponding to a periodic distance within the column appears in the
x-ray diffraction patters of these supramolecular materials, especially those
containing A regular stacking within the columns is, indeed, induced by the
presence of R-3-methyladipic acid molecules fixing distances between disks by
means of hydrogen bonding. Moreover, additional experiments with adipic acid
have led to the conclusion that the methyl group in the stereogenic center of
the R-(3)-methyladipic acid performs a twofold role in the supramolecular organization.
Firstly, it must provide the acid a bent conformation that favors the possibility
of interdisk association along the column, which helps to keep the columnar
arrangement (figures 1b and 1c). Secondly, it is responsible of the transfer
of chirality to a helical superstructure within the column, which is detected
by circular dichroism experiments (figure 2). CD spectra recorded in the mesophase
demonstrate that there is a formal optical activity due to a helical superstructure
biased towards a chiral sense that is determined by the configuration of the
stereogenic center. This optical activity disappears when the material reaches
its transition temperature to the isotropic state.
Figure 2. CD spectral comparison of Tri-2OC 10 /MeAdip[2:1]
in the isotropic liquid (80°C), in solution (10 mm cell, 2.5 x 10 -5 M in
hexanes) and in the mesphase, both in the freshly formed mesophase and after
24 h . The Uv spectrum in the region under study in the mesophase is shown below
the corresponding CD spectrum.
Financial support was provided by MCyT ( MAT2003-07806-CO2-01 ) and DGA.
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