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Photo-Induced Chirality On Achiral Side Chain Liquid Crystalline Polymethacrylates

L. Oriol, M. Piñol, R.M. Tejedor, J. L. Serrano, R. Alcalá, F.J. Rodríguez, B. Villacampa

Azobenzene side-chain liquid crystalline polymers (azo-SCLCPs) have received considerable attention as promising materials in the field of photonics, i.e. holographic storage, optical switching, or as light-driven aligning layers of liquid crystals. 1 All these applications derive from the possibility of changing the orientation of the azobenzene chromophores through the photoinduced trans-cis-trans isomerization of azobenzene units. The irradiation is usually carried out with linearly polarized light giving rise to a reorientation of the azo chromophores perpendicular to the laser polarization direction. Furthermore, in the case of azo-SCLCPs, annealing in the dark at mesophase temperatures leads to an increasing of the birefringence (thermotropic effect). Nevertheless, the heating above the isotropization temperature as well as irradiation with circular polarized light erases the written information.

On the other hand, helical arrangements on macromolecules are recently object of intense research because of the scientific curiosity on nature-imitating macromolecules and their predicted technological applications. 2,3 In this sense, the control of heliticity arises as an aspect of crucial importance on the development of helical polymeric devices. Chiroptical properties on azopolymers have been usually induced by introduction of stereogenic centers on the macromolecular structure. 4,5 For these polymers, the prevailing chiral geometry of the chromophores along the macromolecular backbone determines the extent and handedness of the resulting supramolecular helicity. However, Nikolova et al 6 have recently described the photoinduction of large circular anisotropy in films of an achiral mesogenic azopolyester on illumination with circularly polarized light (CPL) or amorphous polymers but illuminated with elliptically polarized light (EPL). 7-10 Furthermore, a reversible chiroptical switching has been described for these azopolymers, 11 which suppose a control of the photoinduced enantiomeric supramolecular structure in the material. Unusual effects are recently described on these materials as the switch of the sense of circular anisotropy during irradiation, 12 or the photofabrication of superhelix-like patterns using a conventional holographic optical set-up. 13

We have synthesized different series of polymers having a cianoazobenzene derivative as photoaddressable group for optical applications, such as holographic storage 14 or non linear optics. 15 Some of these materials are gathered in Figure1.


Figure 1

In order to study in deep the relationship between molecular structure, Tg and azo-content on the photoinduction of chirality, thin films of polymers shown in Figure 1 have been irradiated with circular polarized light and the chirooptical properties were examined by circular dichroism and polarimetry. The films were previously annealed at the mesophase temperature.

The CD spectra of these films are gathered in Figure 2

Several conclusions can be drawn from these results. Firstly, there is an important effect of the length of the flexible spacer on the chiroptical properties. The hexamethylenic spacer provides an adequate flexibility to the polymer and a relatively high Tg, giving rise to a huge ellipticity. Secondly, the incorporation of a low percentage of azo groups on mesogenic copolymers can induce the organization of the non-photoaddressable, mesogenic side groups in a supramolecular chiral structure. Furthermore, the photoinduced supramolecular helix could be easily switched by irradiating with circular polarized light with opposite handness.

The proposed model for explaining this optical behavior considers the polymeric films consisting of many layers. The presence of an optical axis in the first layer (intrinsically present on the liquid crystalline domains or induced by linear polarized light in amorphous materials) determines that CPL passing through the first layer is transmitted to the second layer as EPL. Consequently, molecules of the second layer reorient perpendicular to the long axis of the polarization ellipse and changes the azimuth of the polarization transmitted to the next layer. The overall effect is the formation of a supramolecular helical structure in which a cooperative effect is produced between light and mesogens in which a rotation of light is induced by mesogens as well as the light induced a rotation of mesogens.

Acknowledgements.

The support from the CICYT under Project MAT2002-04118-C02 is gratefully acknowledged.

References

  1. Natansohn, A.; Rochon, P. Chem. Rev. 2002 , 102 , 4139-4175.
  2. Masuda, M.; Jonkheijm, P.; Sijbesma, R. P.; Meijer, E. W. J. Am. Chem. Soc. 2003 , 125 , 15965.
  3. Hirschberg, J. H. K. K.; Brunsveld, L.; Ramzi, A.; Vekemans, J. A. J. M.; Sijbesma, R. P.; Meijer, E. W. Nature 2000 , 407 , 167.
  4. Yahima, E.; Noguchi, J.; Okamoto, Y. Macromolecules 1995 , 28 , 8368.
  5. Angiolini, L.; Caretti, D.; Giorgini, L.; Salatelli, E. Macromol. Chem. Phys. 2000 , 201 , 533-542.
  6. Nikolova, L.; Todorov, T.; Ivanov, M.; Andruzzi, F.; Hvilsted, S.; Ramanujam, P. S. Opt. Mater. 1997 , 8 , 255-258.
  7. Nikolova, L.; Nedelchev, L.; Todorov, T.; Petrova, T.; Tomova, N.; Dragostinova, Y. App. Phys. Lett. 2000 , 77 , 657-659.
  8. Ivanov, M.; Naydenova, I.; Todorov, T.; Nikolova, L.; Petrova, T.; Tomova, N.; Dragostinova, V. J. Mod. Opt. 2000 , 47 , 861-867.
  9. Nedelchev, L.; Nikolova, L.; Todorov, T.; Petrova, T.; Tomova, N.; Dragostinova, V.; Ramanujam, P. S.; Hvilsted, S. J. Opt. A: Pure Appl. Opt. 2001 , 3 , 304-310.
  10. Kim, M. J.; Shin, B. G.; Kim, J. J.; Kim, D. Y. J. Am. Chem. Soc. 2002 , 124 , 3504-3505.
  11. Iftime, G.; Lagugné-Labarthet, F.; Natansohn, A.; Rochon, P. J. Am. Chem. Soc. 2000 , 122 , 12646-12650.
  12. Hore, D. K.; Natansohn, A.; Rochon, P. J. Phys. Chem. B 2003 , 107 , 2506-2518.
  13. Kim, M. J.; Kumar, J.; Kim, D. Y. Adv. Mater. 2003 , 15 , 2005-2008.
  14. Sánchez, C.; Cases, R.; Alcalá, R.; López, A.; Quintanilla, M.; Oriol, L.; Millaruelo, M. J. Appl. Phys. 2001 , 89 , 5299-5306.
  15. Rodríguez, F.J.; Sánchez, Villacampa, B.; Alcalá, R.; Cases, R.; Millaruelo, M.; Oriol, L. Polymer 2004 , 45 , 2341-2348.

 

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