chemistry (Lehn 1990). Recently, researches on supramolecular liquid crystalline
polymers (SLCPs) have become a hot spot in supramolecular chemistry. Combining
the photoresponsive properties of azobenzene and the self-assembled capability of
pyridine, azopyridine has been widely used in photoresponsive supramolecules (Cui
and Zhao 2004). Herein, we would mainly introduce the SLCPs formed by
hydrogen-bonding and halogen-bonding.
So far, hydrogen-bonding is one of the most common noncovalent interactions in
recognition processes (Priimagi et al. 2012a, b). Hydrogen bonds are everywhere in
our life, from the microscale DNA and RNA to the water we drink daily. The first
hydrogen-bonded supramolecular LC system was introduced by using the carboxylic acid and pyridyl group as the hydrogen-bond acceptor and donor (Kato and
Frechet 1989). Following these examples, Zhao and his coworkers synthesized
azopyridine side-chain homopolymers as well as BCs as the hydrogen bond acceptors. With several carboxylic acids as the hydrogen-bond donor, the amorphous
polymers can be easily converted into SLCPs. In the example of PS-b-PAzPy BCs,
three carboxylic acids were added to transform the amorphous AzPy block into the
LC block, which could further enhance the photoinduced orientation of the
azopyridine moieties (Cui et al. 2005).
The self-assembled morphologies of the hydrogen-bond supramolecules have
also become popular, which includes fibers, spherical particles, gels, and so
on. Zhou et al. has fabricated self-assembled nanofibers with different morphologies
based on amphiphilic low-molecular-weight azopyridine derivatives (Zhou et al.
2011; Zhou and Yu 2012, 2013). The nanofibers are successfully obtained with
different kinds of acids and salts, such as aniline hydrochloride, alkylbenzenesulfatebased anionic surfactants. These fibers are expected to have applications in conductive nanomaterials and biomaterials.
Yu et al. successfully get the self-assembled microparticles using SLCPs fabricated with one amorphous azopyridyl homopolymer and dicarboxylic acids (Liu
et al. 2011; Yu et al. 2011d and Zhang et al. 2014). The SLCP microparticles show
different morphologies with different alkyl chain lengths of the acids, as shown in
Fig. 20. The pure homopolymer assembled into microparticles with smooth surface,
and the supramolecules with long alkyl chain dicarboxylic acids show wrinkled
surfaces. Besides, the SLCP microparticles show photoresponsive characteristics as
the results of the photoactive azobenzene groups.
Chen et al. prepared multi-responsive reversible organogels with a carboxylic azo
polymer (PM6AzCOOH) and DMSO by the interaction of H-aggregation and
hydrogen-bonding (Chen et al. 2010 and Ren et al. 2015). The formed gel could
be destroyed by heating, the addition of less polar solvent and the irradiation of UV
light. Figure 21 demonstrates the reversible sol-gel process.
If the reverse process was performed, the gel could reform completely. In addition
to the self-assembled morphology, the application of hydrogen bonds has extended
to the field of holographic grating. As we all know, the azobenzene group is very
important in the process of mass transport upon irradiation of interference pattern
from two coherent later beams. However, as azobenzene group strongly absorb light,
it is not preferred when using the grating. Thus, Seki proposed a method to remove
15 Photoresponsive Liquid Crystalline Polymers
439
polymers (SLCPs) have become a hot spot in supramolecular chemistry. Combining
the photoresponsive properties of azobenzene and the self-assembled capability of
pyridine, azopyridine has been widely used in photoresponsive supramolecules (Cui
and Zhao 2004). Herein, we would mainly introduce the SLCPs formed by
hydrogen-bonding and halogen-bonding.
So far, hydrogen-bonding is one of the most common noncovalent interactions in
recognition processes (Priimagi et al. 2012a, b). Hydrogen bonds are everywhere in
our life, from the microscale DNA and RNA to the water we drink daily. The first
hydrogen-bonded supramolecular LC system was introduced by using the carboxylic acid and pyridyl group as the hydrogen-bond acceptor and donor (Kato and
Frechet 1989). Following these examples, Zhao and his coworkers synthesized
azopyridine side-chain homopolymers as well as BCs as the hydrogen bond acceptors. With several carboxylic acids as the hydrogen-bond donor, the amorphous
polymers can be easily converted into SLCPs. In the example of PS-b-PAzPy BCs,
three carboxylic acids were added to transform the amorphous AzPy block into the
LC block, which could further enhance the photoinduced orientation of the
azopyridine moieties (Cui et al. 2005).
The self-assembled morphologies of the hydrogen-bond supramolecules have
also become popular, which includes fibers, spherical particles, gels, and so
on. Zhou et al. has fabricated self-assembled nanofibers with different morphologies
based on amphiphilic low-molecular-weight azopyridine derivatives (Zhou et al.
2011; Zhou and Yu 2012, 2013). The nanofibers are successfully obtained with
different kinds of acids and salts, such as aniline hydrochloride, alkylbenzenesulfatebased anionic surfactants. These fibers are expected to have applications in conductive nanomaterials and biomaterials.
Yu et al. successfully get the self-assembled microparticles using SLCPs fabricated with one amorphous azopyridyl homopolymer and dicarboxylic acids (Liu
et al. 2011; Yu et al. 2011d and Zhang et al. 2014). The SLCP microparticles show
different morphologies with different alkyl chain lengths of the acids, as shown in
Fig. 20. The pure homopolymer assembled into microparticles with smooth surface,
and the supramolecules with long alkyl chain dicarboxylic acids show wrinkled
surfaces. Besides, the SLCP microparticles show photoresponsive characteristics as
the results of the photoactive azobenzene groups.
Chen et al. prepared multi-responsive reversible organogels with a carboxylic azo
polymer (PM6AzCOOH) and DMSO by the interaction of H-aggregation and
hydrogen-bonding (Chen et al. 2010 and Ren et al. 2015). The formed gel could
be destroyed by heating, the addition of less polar solvent and the irradiation of UV
light. Figure 21 demonstrates the reversible sol-gel process.
If the reverse process was performed, the gel could reform completely. In addition
to the self-assembled morphology, the application of hydrogen bonds has extended
to the field of holographic grating. As we all know, the azobenzene group is very
important in the process of mass transport upon irradiation of interference pattern
from two coherent later beams. However, as azobenzene group strongly absorb light,
it is not preferred when using the grating. Thus, Seki proposed a method to remove
15 Photoresponsive Liquid Crystalline Polymers
439