Large Thermally Irreversible Photoinduced Shift of Selective Light Reflection in Hydrazone-Containing Cholesteric Polymer Systems

Stimuli responsive liquid crystalline polymers are a unique class of so-called smart materials demonstrating various types of mesomorphic structures easily controlled by external fields, including light. In the present work we synthesized and studied a comb-shaped hydrazone-containing copolyacrylate exhibited cholesteric liquid crystalline properties with the pitch length of the helix being tuned under irradiation with light. In the cholesteric phase selective light reflection in the near IR spectral range (1650 nm) was measured and a large blue shift of the reflection peak from 1650 nm to 500 nm was found under blue light (428 or 457 nm) irradiation. This shift is related to the Z-E isomerization of photochromic hydrazone-containing groups and it is photochemically reversible. The improved and faster photo-optical response was found after copolymer doping with 10 wt % of low-molar-mass liquid crystal. It is noteworthy that both, the E and Z isomers of hydrazone photochromic group are thermally stable that enable to achieve a pure photoinduced switch without any dark relaxation at any temperatures. The large photoinduced shift of the selective light reflection, together with thermal bistability, makes such systems promising for applications in photonics.

Automated summary

Stimuli responsive liquid crystalline polymers are synthesized and studied in this work. The hydrazone-containing copolyacrylate exhibited cholesteric liquid crystalline properties and the helix pitch length can be tuned under light irradiation. The selective light reflection peak shifted from 1650 nm to 500 nm under blue light irradiation, which is related to the Z-E isomerization of photochromic hydrazone-containing groups and is photochemically reversible. The photo-optical response was improved and faster after copolymer doping with low-molar-mass liquid crystal, and both E and Z isomers of hydrazone photochromic group are thermally stable, enabling a pure photoinduced switch without any dark relaxation.