Preparation And Investigation Of Photo-And Thermo-Responsive Behavior Of Chiral Liquid Crystals Composites

In this study, a new kind of hydrogen-bonded chiral molecular switches (H-bonded CMS) composed of azobenzene moieties as photo-responsive parts and H-bonded complexes as thermo-responsive parts was fabricated. The existence and thermal stability of the H-bonds were characterized by fourier transform infrared (FT-IR) spectroscopy and variable-temperature FT-IR spectroscopy. The results of circular dichroism (CD) spectroscopy and UV-vis spectroscopy demonstrate that H-bonded CMS undergo a trans-cis photoisomerization under UV light.Firstly, we doped ISIN-HABA with nematic liquid crystals (N-LCs) host to obtain cholesteric liquid crystal (Ch-LCs). According to the Cano-wedge method, the helical pitch of Ch-LCs will increase with increasing temperature as well as under UV irradiation.Secondly, we doped axially chiral molecular switches (BNAzo-MBA, BNAzo-DBA, BNAzo-(S)-OyBp, BNAzo-(R)-OyBp) with N-LCs host to obtain Ch-LCs with photo/thermo dual responsive behavior. Similar to ISIN-HABA system, the helical pitch of Ch-LCs will increase and the HTP of H-bonded CMS will decrease under UV irradiation. Meanwhile, the HTP of chiral dopants will also decrease with increasing temperature due to the modulation of the intermolecular forces between proton donors and acceptors. What’s more, the reflection bands of Ch-LCs exhibit red shift under UV light as well as with increasing temperature. According to a molecular simulation based on Gaussian03calculations at the B3LYP/6-31G(d) level, the molecular aspect ratio was found to be one of the key factors influencing the HTP of CMS.Finally, we developed H-bonded CMS (BNAzo-MHA and BNAzo-OCA) with high HTP value to stabilize liquid crystalline blue phases (BPs) and induce an optically tunable BPs. The results proved that the introduction of H-bonded CMS effectively expanded the temperature range of BPs and formed an optically tunable BP material system with a wide tuning range, in which the reflection wavelength of BP I was reversibly tuned through the visible region across red, green, and blue wavelengths. The light-induced behavior only happens in stable BP I and no phase transitions appeared during the light-controlled process.