is less intense and blue shifted with respect to the homologous band of the trans
isomer.
Besides the chromophore itself, the chemical architecture of the polymer backbone as well as the way the photoresponsive units are incorporated in the material,
either dispersed or chemically linked (covalently or non-covalently) to the polymeric
scaffold, have also strong influence in the isomerization efficiency and in the
molecular reorientation dynamics; therefore, having also large impact in the overall
material photoresponse. Moreover, isomerization is strongly hindered in the solid
state as compared to the solution. This is mainly due to steric limitations coming
from the lack of free volume for the photochromic units to move. Besides,
intermolecular interactions between closely packed azobenzene chromophores can
lead to aggregation effects. On one hand, the electrostatic interactions between
adjacent chromophores lead to states that differ in energy and oscillator strength;
therefore, modifying the absorption spectra of the material. On the other hand, these
interactions between chromophores can strongly preclude conformational changes,
therefore hindering the isomerization process and slowing down the kinetics of the
photoresponse (Yager and Barrett 2009).
Macromolecular Architecture of Azobenzene Containing LCPs
As mentioned, the mesogenic character of azobenzene chromophores make them
suitable monomers for the preparation of polymeric systems showing mesomorphic
behavior. As a result, homopolymerization of azobenzene monomers have been widely
explored for the preparation of photoresponsive LCPs. Azobenzene chromophores can
be incorporated as part of the polymeric backbone, as schematically shown in Fig. 4a,
in linear main-chain polymer architecture. However, azo mesogens have most commonly been incorporated as pendant units in linear side chain polymers. Typically, the
chromophore is linked to the polymeric chain though a flexible methylenic spacer that
decouples to some extent the chromophore and polymer chain dynamics (Fig. 4b) as
done by Wendorff and coworkers (Eich et al. 1987). Actually, liquid crystalline phases
are favored by the molecular mobility provided by this spacer. Too short or no spacers
usually hinder liquid crystal phase development while spacers of six methylenic units
or more favor mesomorphism (Shibaev et al. 2003).
Traditionally, side-chain azobenzene polymers have been prepared by polymerization of azo functionalized monomers. Azobenzene liquid crystalline homopolymers
based on polyacrylates and polymethacrylates have been prepared by radical chain
polymerization of azobenzene (meth)acrylate monomers (Andruzzi et al. 1999). Conventional polymerization in solution using thermal initiators such as azobisisobutyronitrile (AIBN) have been extensively used to prepare this type of polymers (Fig. 5a, b).
Azobenzene photoresponsive promesogenic monomers have also been combined with
other non-photoresponsive mesogenic ones in the preparation of copolymers showing
liquid crystalline behavior and tailored and optimized properties and response (Fig. 5c).
The consolidation of Controlled Radical Polymerization (CRP) techniques has been of
16 Light-Sensitive Azobenzene-Containing Liquid Crystalline Polymers
453
isomer.
Besides the chromophore itself, the chemical architecture of the polymer backbone as well as the way the photoresponsive units are incorporated in the material,
either dispersed or chemically linked (covalently or non-covalently) to the polymeric
scaffold, have also strong influence in the isomerization efficiency and in the
molecular reorientation dynamics; therefore, having also large impact in the overall
material photoresponse. Moreover, isomerization is strongly hindered in the solid
state as compared to the solution. This is mainly due to steric limitations coming
from the lack of free volume for the photochromic units to move. Besides,
intermolecular interactions between closely packed azobenzene chromophores can
lead to aggregation effects. On one hand, the electrostatic interactions between
adjacent chromophores lead to states that differ in energy and oscillator strength;
therefore, modifying the absorption spectra of the material. On the other hand, these
interactions between chromophores can strongly preclude conformational changes,
therefore hindering the isomerization process and slowing down the kinetics of the
photoresponse (Yager and Barrett 2009).
Macromolecular Architecture of Azobenzene Containing LCPs
As mentioned, the mesogenic character of azobenzene chromophores make them
suitable monomers for the preparation of polymeric systems showing mesomorphic
behavior. As a result, homopolymerization of azobenzene monomers have been widely
explored for the preparation of photoresponsive LCPs. Azobenzene chromophores can
be incorporated as part of the polymeric backbone, as schematically shown in Fig. 4a,
in linear main-chain polymer architecture. However, azo mesogens have most commonly been incorporated as pendant units in linear side chain polymers. Typically, the
chromophore is linked to the polymeric chain though a flexible methylenic spacer that
decouples to some extent the chromophore and polymer chain dynamics (Fig. 4b) as
done by Wendorff and coworkers (Eich et al. 1987). Actually, liquid crystalline phases
are favored by the molecular mobility provided by this spacer. Too short or no spacers
usually hinder liquid crystal phase development while spacers of six methylenic units
or more favor mesomorphism (Shibaev et al. 2003).
Traditionally, side-chain azobenzene polymers have been prepared by polymerization of azo functionalized monomers. Azobenzene liquid crystalline homopolymers
based on polyacrylates and polymethacrylates have been prepared by radical chain
polymerization of azobenzene (meth)acrylate monomers (Andruzzi et al. 1999). Conventional polymerization in solution using thermal initiators such as azobisisobutyronitrile (AIBN) have been extensively used to prepare this type of polymers (Fig. 5a, b).
Azobenzene photoresponsive promesogenic monomers have also been combined with
other non-photoresponsive mesogenic ones in the preparation of copolymers showing
liquid crystalline behavior and tailored and optimized properties and response (Fig. 5c).
The consolidation of Controlled Radical Polymerization (CRP) techniques has been of
16 Light-Sensitive Azobenzene-Containing Liquid Crystalline Polymers
453