Photochemical Phase Transition Behavior of Polymer Azobenzene Liquid Crystals with a Rigid Core Introduced at a Different Position in a Flexible Side Chain

Abstract

Photochemical phase transition behavior of polymer azobenzene liquid crystals (LCs) was explored by transmission-mode analysis and reflection-mode analysis. Four samples with different structures were used, which show LC behavior in the trans form but no LC phase in the cis isomer. Attention was focused on the effect of the position of the mesogenic azobenzene moiety in the flexible side chain on the photochemical phase transition behavior. Photoirradiation of a thin film of the azobenzene LCs (-200 nm) in the trans form resulted in the disappearance of the LC phase due to truns-cis photoisomerization of each mesogen. and the LC phase recovered quickly when the irradiated sample was kept in the dark because of thermal cis-trans back-isomerization. In all polymers, the photochemical LC to isotropic ( I ) phase transition was induced in 100-200 ps on laser pulse irradiation. In the sample with an azobenzene located apart from the main chain of the polymer by a spacer, the LC-I phase transition took place almost completely. In the sample with the azobe,nzene moiety in the vicinity of the main chain, however, the photochemical phase transition was induced only locally. These phenomena are interpreted in terms of the mobility of the azobenzene moiety in the side chain and the stability of the LC phase.

Photomodulation of liquid crystal orientations for photonic applications

“…To overcome this difficulty, a new system has been developed, in which every mesogen in LCs or PLCs is provided with a photosensitive moiety. [87][88][89][90][91][92][93][94][95][96][97] It has been found that some azobenzene derivatives exhibit LC behavior. In these azobenzene LCs, the azobenzene moiety could play both roles as a mesogen and as a photosensitive moiety (Fig.…”

“…90 The effect of the spacer between the polymer main chain and the azobenzene chromophore on the photochemical phase transition behavior was explored in detail. [91][92][93] Fig. 15 Examples of azobenzene LCs.…”

Bistable Phase Behavior and Kinetics of Nonisothermal Mesophase Formation in a Chiral Side Chain Polymethacrylate

“…LC poly-(meth)acrylates with azobenzene photochromic side chains. 11,[25][26][27][28] On the other hand, we have reported similar mesophases for numerous chiral homopolymers (polymethacrylates, polysiloxanes) [29][30][31][32] and their copolymers with nonchiral azodye comonomers. 24,33,34 The detailed study of the "transparent smectic mesophase" is presented in our recent paper; 32 here we will summarize them briefly.…”

Photo‐controlled alignment and helical organization in main‐chain liquid crystalline alternating polymers

“…32 To overcome this issue, next to the addition of plasticizers, 31 flexible polydimethylsiloxane chains have been used to improve the segmental mobility of the main chain and facilitate the molecular reorientation of the chromophores by light. [33][34][35][36][37] However, for these materials, the nanoscale organization is limited due to the polydispersity of the polydimethylsiloxane chains, which limits the cooperative motion of the photo-switches. Despite these promising results, design strategies to form flexible photo-responsive materials with highly ordered nanostructures are lacking.…”

“…Yet, the photo‐responsiveness of polymers containing azobenzene as part of the main chain is typically inefficient, particularly in thin film due to the restricted motion of the rigid polymer chains 32 . To overcome this issue, next to the addition of plasticizers, 31 flexible polydimethylsiloxane chains have been used to improve the segmental mobility of the main chain and facilitate the molecular reorientation of the chromophores by light 33–37 . However, for these materials, the nanoscale organization is limited due to the polydispersity of the polydimethylsiloxane chains, which limits the cooperative motion of the photo‐switches.…”