is changed in response to an external stimulus. This phenomenon illustrates that LC
molecules only require a small amount of energy to change the alignment: the energy
needed to induce an alignment change of only 1 mol% of the LC molecules is
enough to bring about the alignment change of the whole system. In other words, a
huge amplification is possible in LC systems. When a small amount of a photochromic molecule is added into LCs and the resulting guest/host mixture is irradiated to
cause photochemical reactions of the photochromic guest molecules, LC-isotropic
phase transition of the mixtures can be induced isothermally. Ikeda et al. reported the
first explicit example of nematic-isotropic phase transition induced by trans-cis
photoisomerization of a nematic LC with an azobenzene guest molecule dispersed
in it (Tazuke et al. 1987).
Azobenzene is a well-known chromophore which has two configurations. It
undergoes trans to cis photoisomerization upon exposure to UV irradiation, and
irradiation with visible light leads to cis to trans back-isomerization process. Therefore, azobenzene is the most frequently used photochromic moiety in photoresponsive polymers. The rodlike trans form of the azobenzenes stabilizes the
structure of LC phase, whereas its bent cis isomer tends to destabilize the phase
structure of mixture (Fig. 1a). As a consequence of two different conformations, the
LC-isotropic phase transition temperature (T c ) of the mixture with the cis form (T cc )
is much lower than that with the trans form (T ct ). If the temperature of the sample
(T) is between T ct and T cc and the sample is irradiated to cause trans-cis photoisomerization of the azobenzene guest molecules, T c decreases because of the
increasement of the cis form. When T c becomes lower than the irradiation
temperature T, LC-isotropic phase transition of the sample is induced. The sample
reverts to the initial LC phase through cis-trans back-isomerization due to reversible
photochromic reactions. Thus, phase transitions of LC systems can be induced
isothermally and reversibly by photochemical reactions of photoresponsive guest
molecules (Fig. 1b) (Ikeda 2003).
Finkelmann et al. reported pioneering work on photodeformation of a monodomain nematic CLCP, which had a polysiloxane main chain and azobenzene
chromophores at crosslinks. The CLCP film generated a contraction by 20% upon
irradiation with UV light to give rise to the trans-cis isomerization of the azobenzene
moieties (Fig. 2) (Finkelmann et al. 2001). It is necessary to take photomechanical
effects into consideration: the subtle variation in nematic order upon trans-cis
isomerization causes a significant uniaxial deformation of the LCs along the director
axis when the LC molecules are strongly associated by covalent crosslinking to form
a three-dimensional polymer network. The contracted elastomer thermally returned
to the original state due to the cis-trans back-isomerization after stopping irradiation.
Keller and coworkers synthesized oriented monodomain nematic side-on CLCPs
containing azobenzenes by photopolymerization with a near-infrared photoinitiator
(Li et al. 2003). The photopolymerization was performed with aligned azobenzene
monomers in conventional LC cells. The obtained thin films were found to show fast
(less than 1 min) photochemical contraction of up to 18% upon exposure to UV light
and a slow thermal recovering in the dark.
In order to observe a strong effect of the isomerization, high concentrations of the
photoresponsive molecules are required. This will lead to a high optical density of
13 Photodeformable Liquid Crystalline Polymers (LCPs)
363
molecules only require a small amount of energy to change the alignment: the energy
needed to induce an alignment change of only 1 mol% of the LC molecules is
enough to bring about the alignment change of the whole system. In other words, a
huge amplification is possible in LC systems. When a small amount of a photochromic molecule is added into LCs and the resulting guest/host mixture is irradiated to
cause photochemical reactions of the photochromic guest molecules, LC-isotropic
phase transition of the mixtures can be induced isothermally. Ikeda et al. reported the
first explicit example of nematic-isotropic phase transition induced by trans-cis
photoisomerization of a nematic LC with an azobenzene guest molecule dispersed
in it (Tazuke et al. 1987).
Azobenzene is a well-known chromophore which has two configurations. It
undergoes trans to cis photoisomerization upon exposure to UV irradiation, and
irradiation with visible light leads to cis to trans back-isomerization process. Therefore, azobenzene is the most frequently used photochromic moiety in photoresponsive polymers. The rodlike trans form of the azobenzenes stabilizes the
structure of LC phase, whereas its bent cis isomer tends to destabilize the phase
structure of mixture (Fig. 1a). As a consequence of two different conformations, the
LC-isotropic phase transition temperature (T c ) of the mixture with the cis form (T cc )
is much lower than that with the trans form (T ct ). If the temperature of the sample
(T) is between T ct and T cc and the sample is irradiated to cause trans-cis photoisomerization of the azobenzene guest molecules, T c decreases because of the
increasement of the cis form. When T c becomes lower than the irradiation
temperature T, LC-isotropic phase transition of the sample is induced. The sample
reverts to the initial LC phase through cis-trans back-isomerization due to reversible
photochromic reactions. Thus, phase transitions of LC systems can be induced
isothermally and reversibly by photochemical reactions of photoresponsive guest
molecules (Fig. 1b) (Ikeda 2003).
Finkelmann et al. reported pioneering work on photodeformation of a monodomain nematic CLCP, which had a polysiloxane main chain and azobenzene
chromophores at crosslinks. The CLCP film generated a contraction by 20% upon
irradiation with UV light to give rise to the trans-cis isomerization of the azobenzene
moieties (Fig. 2) (Finkelmann et al. 2001). It is necessary to take photomechanical
effects into consideration: the subtle variation in nematic order upon trans-cis
isomerization causes a significant uniaxial deformation of the LCs along the director
axis when the LC molecules are strongly associated by covalent crosslinking to form
a three-dimensional polymer network. The contracted elastomer thermally returned
to the original state due to the cis-trans back-isomerization after stopping irradiation.
Keller and coworkers synthesized oriented monodomain nematic side-on CLCPs
containing azobenzenes by photopolymerization with a near-infrared photoinitiator
(Li et al. 2003). The photopolymerization was performed with aligned azobenzene
monomers in conventional LC cells. The obtained thin films were found to show fast
(less than 1 min) photochemical contraction of up to 18% upon exposure to UV light
and a slow thermal recovering in the dark.
In order to observe a strong effect of the isomerization, high concentrations of the
photoresponsive molecules are required. This will lead to a high optical density of
13 Photodeformable Liquid Crystalline Polymers (LCPs)
363