Bruce first introduced halogen-bonded LC system. This system was composed of
alkoxystilbazole and the diverse substituted phenols, and the interaction occurs
between N atoms on pyridine and I atoms on the substituted phenols. The molecular
structure shows the N-I distance of 2.811 Å, which is shorter than the common
distance of halogen-bonding (2.762–3.127 Å). Thermal analysis and measurements
of polarizing optical microscope were conducted to make sure the LC property of the
mixed compound (Nguyen et al. 2004).
Later, a systematic study on almost 100 kinds of halogen-bonded LC supramolecules was performed based on Bruce’s work. By halogen-bonding, new chiral
mesogens were also obtained from non-mesomorphic chiral compounds. The possible structure-function relationship was revealed in specific halogen-bonding system (Bruce et al. 2010). Then, researches on halogen-bonded LC supramolecules
were preceded to the system of alkoxystilbazoles and molecular iodine (I 2 )
(McAllister et al. 2013). Furthermore, Chen et al. first introduced the halogenbonded LCs using azopyridine compound and molecular bromide as well as the
reversible photoinduced phase transition of iodine-bonded LCs upon UV irradiation
(Chen et al. 2014), as shown in Fig. 22.
However, there are still distinctions between the two interactions. Firstly, halogen
bonds are more directional than hydrogen bonds. Secondly, the strength of halogenbonding can be tuned by changing halogen atoms. Then, halogen-bonding is usually
hydrophobic while hydrogen-bonding is hydrophilic. Lastly, the halogen atoms are
much bigger than hydrogen atoms (Priimagi et al. 2013). All these characteristics
render halogen-bonding a unique tool for the design of functional materials, such as
halogen-bonding-triggered supramolecular gels (Meazza et al. 2012), anion transport (Jentzsch et al. 2012), and so on.
One example of photoresponsive supramolecules is surface-relief gratings based
on hydrogen- and halogen-bonding. Due to the directionality and tunable properties
of halogen-bonding, halogen-bonded supramolecules have shown higher diffraction
efficiency that hydrogen-bonded ones in the formation of surface-relief gratings.
Priimagi et al. compared the diffraction efficiency of a series of molecules that differ
only in bond donor units and find that the iodoethynyl-containing azobenzene
becomes the best one to produce high efficiency of producing surface patterning
(Priimagi et al. 2012a; Saccone et al. 2015). Besides, photoalignment has been
combined with the surface-relief grating formation in halogen-bonded supramolecular systems, which shows the remarkable potential of halogen-bonding in photoresponsive functional material (Priimagi et al. 2012b).
Conclusion and Outlooks
In summary, photoresponsive LCP systems possess both the photoactive moieties
and the LC ordering, which could be easily controlled through adjusting the actinic
light. Researchers mainly focus on the relationship between light and ordered
structure of LCPs, especially the precise control of the structures. Through molecular
cooperative motions, the photo-inert molecules are aligned following the motion of
15 Photoresponsive Liquid Crystalline Polymers
441
alkoxystilbazole and the diverse substituted phenols, and the interaction occurs
between N atoms on pyridine and I atoms on the substituted phenols. The molecular
structure shows the N-I distance of 2.811 Å, which is shorter than the common
distance of halogen-bonding (2.762–3.127 Å). Thermal analysis and measurements
of polarizing optical microscope were conducted to make sure the LC property of the
mixed compound (Nguyen et al. 2004).
Later, a systematic study on almost 100 kinds of halogen-bonded LC supramolecules was performed based on Bruce’s work. By halogen-bonding, new chiral
mesogens were also obtained from non-mesomorphic chiral compounds. The possible structure-function relationship was revealed in specific halogen-bonding system (Bruce et al. 2010). Then, researches on halogen-bonded LC supramolecules
were preceded to the system of alkoxystilbazoles and molecular iodine (I 2 )
(McAllister et al. 2013). Furthermore, Chen et al. first introduced the halogenbonded LCs using azopyridine compound and molecular bromide as well as the
reversible photoinduced phase transition of iodine-bonded LCs upon UV irradiation
(Chen et al. 2014), as shown in Fig. 22.
However, there are still distinctions between the two interactions. Firstly, halogen
bonds are more directional than hydrogen bonds. Secondly, the strength of halogenbonding can be tuned by changing halogen atoms. Then, halogen-bonding is usually
hydrophobic while hydrogen-bonding is hydrophilic. Lastly, the halogen atoms are
much bigger than hydrogen atoms (Priimagi et al. 2013). All these characteristics
render halogen-bonding a unique tool for the design of functional materials, such as
halogen-bonding-triggered supramolecular gels (Meazza et al. 2012), anion transport (Jentzsch et al. 2012), and so on.
One example of photoresponsive supramolecules is surface-relief gratings based
on hydrogen- and halogen-bonding. Due to the directionality and tunable properties
of halogen-bonding, halogen-bonded supramolecules have shown higher diffraction
efficiency that hydrogen-bonded ones in the formation of surface-relief gratings.
Priimagi et al. compared the diffraction efficiency of a series of molecules that differ
only in bond donor units and find that the iodoethynyl-containing azobenzene
becomes the best one to produce high efficiency of producing surface patterning
(Priimagi et al. 2012a; Saccone et al. 2015). Besides, photoalignment has been
combined with the surface-relief grating formation in halogen-bonded supramolecular systems, which shows the remarkable potential of halogen-bonding in photoresponsive functional material (Priimagi et al. 2012b).
Conclusion and Outlooks
In summary, photoresponsive LCP systems possess both the photoactive moieties
and the LC ordering, which could be easily controlled through adjusting the actinic
light. Researchers mainly focus on the relationship between light and ordered
structure of LCPs, especially the precise control of the structures. Through molecular
cooperative motions, the photo-inert molecules are aligned following the motion of
15 Photoresponsive Liquid Crystalline Polymers
441