Dual Nature of Chromophore
Azobenzene
Azobenzene is a special molecule which undergoes rapid isomerization under light.
Additionally, one of the isomer (trans-) possesses rigid rod structure, akin to
mesogen, and fits well in conventional LC texture influencing photoresponse characteristics. On the other hand, the molecule alone can function as mesogen while
maintaining photoactive nature, thus exhibiting its dual nature.
Azobenzene can be designed by placing it in main chain, side chain, and together
both in main and side chain, respectively. The main-chain azobenzene polymers
developed in the initial stages do not have appreciable orientational and deformational effects, as the mobility of azobenzenes is mostly restricted by their inclusion
into the polymer backbone. This result prompted developing the other two types.
Similar to other side-chain LC polymers, azobenzene-containing side-chain LC
polymers mainly form low-ordered LC phases, namely, nematic (N), smectic A
(SmA), and C (SmC) phases. Their phase structure and transition depend on chemical
structure of azobenzene moieties, the spacer length, the tail of azo, etc. These factors
also influence their photoresponsive behavior. It has also been established that the
branched and even dendritic polymer structures exhibit photoresponsive effects
which are considerably smaller than those in linear azobenzene polymers.
Triblock copolymer having middle block with both LC and azo in combined
pendant group shows typical self-assembly characteristics in to lamellar phase (Deng
et al. 2008). The azobenzene-containing triblock copolymer exhibits surface alignment
of the lamellas perpendicular to the substrate, whereas the non-photoresponsive block
copolymer organizes the lamellas parallel to the substrate.
Bis(benzylidene)
A different class of polymer having dual functionality, such as photoresponsive and
mesogenic nature, is derived from bis(benzylidene) as building block. The architecture usually ensures higher thermal stability. The photoresponsive nature is exhibited
by occurrence of cross-linking via 2π + 2π cycloaddition. Faster photo-induced
behavior in hyperbranched architecture than the linear structure is usually observed.
Acyclic units, associated with bis(benzylidene), facilitate photo-cycloaddition, fivemembered rings showing better photoactivity as compared to six-membered rings
(Murali and Samui 2010). Reasonably high photoactivity qualifies it for potential
application as photoresponsive functional materials.
There can be alternate combinations of LC and chromophore, such as one small
molecule and other in polymer architecture. The types are as follows.
LCP/Chromophore Blend
During early design of smart materials based on LC and azo molecules, various
possibilities were envisaged as LC can be controlled by electric, magnetic, and
other fields. Dye can be controlled by radiation. When the polymer system was
made to get a stable film, many combinations were visualized. Azobenzene was the
398
A. B. Samui
Azobenzene
Azobenzene is a special molecule which undergoes rapid isomerization under light.
Additionally, one of the isomer (trans-) possesses rigid rod structure, akin to
mesogen, and fits well in conventional LC texture influencing photoresponse characteristics. On the other hand, the molecule alone can function as mesogen while
maintaining photoactive nature, thus exhibiting its dual nature.
Azobenzene can be designed by placing it in main chain, side chain, and together
both in main and side chain, respectively. The main-chain azobenzene polymers
developed in the initial stages do not have appreciable orientational and deformational effects, as the mobility of azobenzenes is mostly restricted by their inclusion
into the polymer backbone. This result prompted developing the other two types.
Similar to other side-chain LC polymers, azobenzene-containing side-chain LC
polymers mainly form low-ordered LC phases, namely, nematic (N), smectic A
(SmA), and C (SmC) phases. Their phase structure and transition depend on chemical
structure of azobenzene moieties, the spacer length, the tail of azo, etc. These factors
also influence their photoresponsive behavior. It has also been established that the
branched and even dendritic polymer structures exhibit photoresponsive effects
which are considerably smaller than those in linear azobenzene polymers.
Triblock copolymer having middle block with both LC and azo in combined
pendant group shows typical self-assembly characteristics in to lamellar phase (Deng
et al. 2008). The azobenzene-containing triblock copolymer exhibits surface alignment
of the lamellas perpendicular to the substrate, whereas the non-photoresponsive block
copolymer organizes the lamellas parallel to the substrate.
Bis(benzylidene)
A different class of polymer having dual functionality, such as photoresponsive and
mesogenic nature, is derived from bis(benzylidene) as building block. The architecture usually ensures higher thermal stability. The photoresponsive nature is exhibited
by occurrence of cross-linking via 2π + 2π cycloaddition. Faster photo-induced
behavior in hyperbranched architecture than the linear structure is usually observed.
Acyclic units, associated with bis(benzylidene), facilitate photo-cycloaddition, fivemembered rings showing better photoactivity as compared to six-membered rings
(Murali and Samui 2010). Reasonably high photoactivity qualifies it for potential
application as photoresponsive functional materials.
There can be alternate combinations of LC and chromophore, such as one small
molecule and other in polymer architecture. The types are as follows.
LCP/Chromophore Blend
During early design of smart materials based on LC and azo molecules, various
possibilities were envisaged as LC can be controlled by electric, magnetic, and
other fields. Dye can be controlled by radiation. When the polymer system was
made to get a stable film, many combinations were visualized. Azobenzene was the
398
A. B. Samui