rod-shaped trans-form to the non-planar bent-shaped cis form that increases the
solubility in polar solvents.
Macrocycles Containing Azobenzene Main-Chain Oligomers
In addition to the chain length, the polymeric topologies usually influence the properties and function of polymers to considerable extent. The cyclic polymers exhibit
exceptional properties as compared to their linear counterpart due to the presence of
chain-end free topology. By considering the influence of chain length and cyclic
topology, the molecularly defined cyclic polymers provide a good model for precisely
exploring the structure-property relationship (Binauld et al. 2009). By incorporating
one or more azobenzene moieties in a cyclic structure, the topological constraint
imposed by the ring strain can trigger a much larger impact on the photoswitching
properties of azobenzene as compared to non-cyclic structures (Reuter and Wegner
2013). Molecularly defined cyclic oligomers, containing alternate azobenzene and
tetraglycol moieties in the main chain, exhibit faster photo-isomerization from the
trans-form to cis-form and a slightly faster reversible relaxation from the cis-isomer to
trans-isomer than the linear analogue (Jiang et al. 2016). This is possible due to the
more stable conformation for cyclic cis-azobenzene relative to the linear analogues,
particularly for the small ring. Also, the better flexible nature of linear polymers than
the cyclic one, due to the absence of strain and the presence of swinging chain ends,
isomerization transition faces more resistance than the cyclic ones. The first factor
plays a dominating role in the faster trans-to-cis photo-isomerization for the cyclic
products. With increased chain length, progressive change of the cis isomer percentage, and the different self-quenching effect of fluorescence emission are common for
both linear and cyclic oligomers.
Stimulus-Induced Deformations in Various Geometries
Liquid crystal networks can be designed to undergo deformations in various geometries in response to light. Only bending motion and slow photoisomerization are
normally observed for photoresponsive moieties such as azobenzene incorporated
LC polymer films. By modifying the core or adding substituents to the azobenzene
moiety, substantial changes can be realized in photophysical and photochemical
properties (Bandara and Burdette 2012). Also the complex setup involving multiple
light sources, lenses, or mirrors can be avoided. Azobenzene derivatives with fast
cis-to-trans thermal relaxation can be incorporated into liquid crystal networks to
make photoactive polymer films, which exhibit continuous, directional, macroscopic
mechanical waves under constant light illumination (Gelebart et al. 2017). Azo
derivatives can be copolymerized with a mixture of LC methacrylate and acrylate,
which are first aligned in a splayed configuration over the cross-section of a thin film,
with homeotropic alignment at one surface and planar alignment at the other, and
14 Photoactive Liquid Crystalline Polymers
411
solubility in polar solvents.
Macrocycles Containing Azobenzene Main-Chain Oligomers
In addition to the chain length, the polymeric topologies usually influence the properties and function of polymers to considerable extent. The cyclic polymers exhibit
exceptional properties as compared to their linear counterpart due to the presence of
chain-end free topology. By considering the influence of chain length and cyclic
topology, the molecularly defined cyclic polymers provide a good model for precisely
exploring the structure-property relationship (Binauld et al. 2009). By incorporating
one or more azobenzene moieties in a cyclic structure, the topological constraint
imposed by the ring strain can trigger a much larger impact on the photoswitching
properties of azobenzene as compared to non-cyclic structures (Reuter and Wegner
2013). Molecularly defined cyclic oligomers, containing alternate azobenzene and
tetraglycol moieties in the main chain, exhibit faster photo-isomerization from the
trans-form to cis-form and a slightly faster reversible relaxation from the cis-isomer to
trans-isomer than the linear analogue (Jiang et al. 2016). This is possible due to the
more stable conformation for cyclic cis-azobenzene relative to the linear analogues,
particularly for the small ring. Also, the better flexible nature of linear polymers than
the cyclic one, due to the absence of strain and the presence of swinging chain ends,
isomerization transition faces more resistance than the cyclic ones. The first factor
plays a dominating role in the faster trans-to-cis photo-isomerization for the cyclic
products. With increased chain length, progressive change of the cis isomer percentage, and the different self-quenching effect of fluorescence emission are common for
both linear and cyclic oligomers.
Stimulus-Induced Deformations in Various Geometries
Liquid crystal networks can be designed to undergo deformations in various geometries in response to light. Only bending motion and slow photoisomerization are
normally observed for photoresponsive moieties such as azobenzene incorporated
LC polymer films. By modifying the core or adding substituents to the azobenzene
moiety, substantial changes can be realized in photophysical and photochemical
properties (Bandara and Burdette 2012). Also the complex setup involving multiple
light sources, lenses, or mirrors can be avoided. Azobenzene derivatives with fast
cis-to-trans thermal relaxation can be incorporated into liquid crystal networks to
make photoactive polymer films, which exhibit continuous, directional, macroscopic
mechanical waves under constant light illumination (Gelebart et al. 2017). Azo
derivatives can be copolymerized with a mixture of LC methacrylate and acrylate,
which are first aligned in a splayed configuration over the cross-section of a thin film,
with homeotropic alignment at one surface and planar alignment at the other, and
14 Photoactive Liquid Crystalline Polymers
411