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