processing and mechanical properties of polymers. Photoresponsive LCPs incorporate light sensitive molecules that undergo reversible transformations between two
isomeric forms when exposed to light in their absorption bands. These transformations at the molecular level lead to changes in the electronic molecular structure and
therefore in the material absorption spectrum what is known as photochromism.
Beyond this color change, useful in the preparation of optical memories and
switches, light-induced isomerization of photochromic units results in molecular
motions that are amplified though cooperative effects in liquid crystalline materials
leading to different photoinduced effects and changes of different physical properties. Compared to other stimuli, light is a very powerful and attractive trigger for
these effects since it can be remotely applied to these materials with a high degree of
spatial and temporal control. For example, mask photolithographic or holographic
exposure as well as direct laser writing using focused beams (with size in the order of
its wavelength) can be used to apply light locally leading to patterned properties.
Light responsive molecular compounds that have been incorporated in polymeric
systems include fulgides and diarylethenes, spiropyrans and spirooxazines, as well
as azobenzene molecules. Figure 1 presents general structures of these photochromic
families. In all the cases, the isomerization processes are unimolecular and reversible. Besides, the switching between the two states leads to significant changes in
molecular structure affecting geometry, electronic properties, and dipole moments
among others, that are reflected as macroscopic changes in the properties of the
materials containing these molecules. Within each of these chromophore families,
the chemical nature of the molecular substituents, strongly affects the electronic
structure and therefore its optical properties. These include absorption spectra of the
isomers as well as the efficiency of the light-induced conversion between molecular
forms (Bamfield and Hutchings 2010; Schnabel 2007; Ercole et al. 2010).
Fulgides and diarylethenes type compounds (Fig. 1a, b) experience light-induced
isomerization between a colorless open form to a closed photocyclized colored form.
The interconversion is photochemicallly reversible; however, the right side isomers
are thermally stable in the dark at room temperature. Spiropyrans and spirooxazines
(Fig. 1c, d) transform upon irradiation from a colorless or slightly colored spirocyclic
closed form to strongly colored merocyanine open form by an electrocyclic ring
opening. In this case, the transformation from the photogenerated right side isomers
in Fig. 1c, d to the left side isomer can take place by thermal means. Finally,
azobenzene molecules (Fig. 1e), consisting on two phenyl rings linked by an azo
(-N=N-) bridge, can exist as two different isomers, trans and cis. As in the case of
spiropyrans, the photogenerated azobenzene cis isomer is unstable in the dark at
room temperature.
Among these previously mentioned photochromic molecules, azobenzenes have
been the most widely explored materials in combination with liquid crystalline
polymers. Azobenzene chromophores can be easily modified allowing their versatile
incorporation in different polymeric architectures. In addition, the thermodynamically stable azobenzene trans isomer is promesogenic allowing this to maintain the
liquid crystalline character of the materials, in which they are incorporated even at
large concentrations. On the other hand, the photogenerated cis isomer presents a
16 Light-Sensitive Azobenzene-Containing Liquid Crystalline Polymers
449
isomeric forms when exposed to light in their absorption bands. These transformations at the molecular level lead to changes in the electronic molecular structure and
therefore in the material absorption spectrum what is known as photochromism.
Beyond this color change, useful in the preparation of optical memories and
switches, light-induced isomerization of photochromic units results in molecular
motions that are amplified though cooperative effects in liquid crystalline materials
leading to different photoinduced effects and changes of different physical properties. Compared to other stimuli, light is a very powerful and attractive trigger for
these effects since it can be remotely applied to these materials with a high degree of
spatial and temporal control. For example, mask photolithographic or holographic
exposure as well as direct laser writing using focused beams (with size in the order of
its wavelength) can be used to apply light locally leading to patterned properties.
Light responsive molecular compounds that have been incorporated in polymeric
systems include fulgides and diarylethenes, spiropyrans and spirooxazines, as well
as azobenzene molecules. Figure 1 presents general structures of these photochromic
families. In all the cases, the isomerization processes are unimolecular and reversible. Besides, the switching between the two states leads to significant changes in
molecular structure affecting geometry, electronic properties, and dipole moments
among others, that are reflected as macroscopic changes in the properties of the
materials containing these molecules. Within each of these chromophore families,
the chemical nature of the molecular substituents, strongly affects the electronic
structure and therefore its optical properties. These include absorption spectra of the
isomers as well as the efficiency of the light-induced conversion between molecular
forms (Bamfield and Hutchings 2010; Schnabel 2007; Ercole et al. 2010).
Fulgides and diarylethenes type compounds (Fig. 1a, b) experience light-induced
isomerization between a colorless open form to a closed photocyclized colored form.
The interconversion is photochemicallly reversible; however, the right side isomers
are thermally stable in the dark at room temperature. Spiropyrans and spirooxazines
(Fig. 1c, d) transform upon irradiation from a colorless or slightly colored spirocyclic
closed form to strongly colored merocyanine open form by an electrocyclic ring
opening. In this case, the transformation from the photogenerated right side isomers
in Fig. 1c, d to the left side isomer can take place by thermal means. Finally,
azobenzene molecules (Fig. 1e), consisting on two phenyl rings linked by an azo
(-N=N-) bridge, can exist as two different isomers, trans and cis. As in the case of
spiropyrans, the photogenerated azobenzene cis isomer is unstable in the dark at
room temperature.
Among these previously mentioned photochromic molecules, azobenzenes have
been the most widely explored materials in combination with liquid crystalline
polymers. Azobenzene chromophores can be easily modified allowing their versatile
incorporation in different polymeric architectures. In addition, the thermodynamically stable azobenzene trans isomer is promesogenic allowing this to maintain the
liquid crystalline character of the materials, in which they are incorporated even at
large concentrations. On the other hand, the photogenerated cis isomer presents a
16 Light-Sensitive Azobenzene-Containing Liquid Crystalline Polymers
449