Synthesis and optical properties of cholesteric liquid‐crystalline oligomers displaying reversible thermochromism

Abstract

A series of side-chain liquid crystalline oligomers (P 1 -P 7 ) have been synthesized with cyclo(methylhydrogeno)siloxane and two cholesteric liquid crystalline monomers cholesteryl 4-(10-undecylen-1-yloxy)benzoate (M 1 ) and cholesterol 4-{6-[(4-(allyloxyl)benzoyl]-hexanoxocarbonyl}-benzoate (M 2 ). The chemical structures and liquid crystalline properties of the synthesized oligomers were investigated using various experimental techniques such as FTIR, 1 H-NMR, DSC, POM, and XRD. All monomers and chiral oligomers show a cholesteric mesophase with very wide mesophase temperature ranges. They appear highly thermally stable with decomposition temperatures (T d ) at 5% weight loss greater than 300 C. The optical properties of the oligmers have been characterized by reflection spectra and optical rotation analysis. All synthesized oligomers display colors at room temperature, and show reversible thermochromism within a wide temperature range (>120 C). The k max values of the oligomers also nearly coincide during the first, second, and third heating cycles. The specific rotation of each oligomer is very sensitive to temperature, and the specific rotation value of P 3 smoothly changes from 2 21.7 to 2 0.7 when it is heated. The optical properties of the oligomers offer tremendous potential for various optical applications.

Controlling the Phase Behavior and Reflection of Main-Chain Cholesteric Oligomers Using a Smectic Monomer

“…This effect is understood as the gradual formation of short-range smectic order in the cholesteric helix [ 20 , 32 , 33 , 34 ], which leads to unwinding of the cholesteric helix and enlarges the pitch [ 35 , 36 , 37 , 38 ]. To obtain temperature-responsive photonic devices showing good stability and a large difference in reflection band position between low and high temperatures (the band-shift range) as a result of the pre-transition effect, the use of polymers is desired to stabilize the performance of the photonic materials [ 4 , 10 , 11 , 39 , 40 , 41 , 42 ]. A polymer-stabilized cholesteric liquid crystal in a cell containing a 5 wt% cross-linked network only showed a 150 nm reflection band shift as the temperature decreased from 45 °C to 15 °C, indicating a poor pre-transitional effect [ 43 ].…”

Polymer Stabilized Cholesteric Liquid Crystal Siloxane for Temperature-Responsive Photonic Coatings

“…Alternative ways to make temperature-responsive photonic polymer coatings use either the humidity [20] or heat expansion of the materials [21,22], which are not able to induce a large reflection band shift without the presence of water. Soft polymers such as polysiloxane Ch-LC elastomers or semi-interpenetrating polymer networks based on oligosiloxanes or a Ch-LC polymer network have been reported that showed temperature-responsive optical changes [23,24], but the shift of the reflection band was limited. Additionally, the cholesterol group was selected as a chiral generator in these polymers, leading to their responding temperatures being much higher than room temperature (>100 • C).…”