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Chiral Helical Induction

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 chemistry.


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.

Acknowledgements

Financial support was provided by MCyT ( MAT2003-07806-CO2-01 ) and DGA.

References

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Nuckolls, C.; Shao, R.; Jang, W.-G.; Walba, D.M.; Katz, T.J. Chem. Mater. 2002 , 14 , 773-776.

a) R. Iglesias, J.L. Serrano, T. Sierra, M.R. de la Fuente, B. Palacios, M.A. Pérez-Jubi ndo, J.T. Vázquez. J. Am. Chem. Soc. 1998 , 120 , 2908-2918 b) J. L. Serrano, T. Sierra , Chem. Eur. J., 2000 , 6 , 759-766,

Hoeben, F.J.M.; Herz, L.M.; Daniel, C.; Jonkheijm, P.; Schenning, A.P.H.J.; Silva, C.; Meskers, S.C.J.; Beljonne, D.; Phillips, R.T.; Friend, R.H.; Meijer, E.W. Angew. Chem., Int. Ed. Eng. 1983 , 22 , 565-566.

J.M.-Lehn, Principles of Supramolecular Chemistry

a) U. Beginn, Prog. Polym. Sci., 2003 , 28 , 1049-1105. b)T. Kato, N. Mizoshita, K. Kanie, Macromol. Rapid. Commun. 2001 , 22 , 797-868. c) C.M. Paleos, D. Tsiourvas, Angew. Chem. Int. Ed. 1995 , 34 , 1696-1711.

Hartgerink, J.D.; Zubarev, E.R.; Stupp, S.I. Curr. Opin. Solid State Mater. Sci. 2001 , 5 , 355-361.

 

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