Preparation Of Hydrogen-bonded Chiral Switches And Their Applications For Reversibly Tunable Helical Induction And Inversion Of Cholesteric Liquid Crystal

In this thesis, we fabricate a novel series of hydrogen-bonded (H-bonded) chiral switches bearing both chiral binaphthyl and azobenzene moieties, and investigate the multiple switching behaviors of chiral nematic liquid crystals (N*-LCs) induced by the H-bonded chiral switches. First, the chemical structures of H-bonded chiral switches are characterized by means of Nuclear magnetic resonance (NMR), Infrared spectra (IR), variable temperature IR, Mass spectra (MS), elemental analysis and molecular modeling. And then, the responsive behaviors of H-bonded chiral switches and the corresponding N*-LCs induced by them are investigated by UV absorption spectroscopy, circular dichroism (CD) and polarizing light microscope (POM).First of all, we prepare a new H-bonded chiral switch bearing ureido pyrimidinedione (UPy) group. The pH and photoresponse characteristics of the N*-LCs composed of N-LCs, E7 and the above H-bonded chiral switch are investigated by the use of Grandjean-Cano method. We find that the reflection color could be reversibly switched by the exposure of trifluoroacetic acid and triethylamine gas. This gas responsiveness should be attributed to the fracture and reformation of the intramolecular quadruple H-bond under the action of acid and alkali gas. Additionally, the N*-LCs induced by the H-bonded chiral switch also exhibits a reversible photoresponsive properties due to the trans-cis isomerization of the azobenzene groups under UV/vis light.Another type of chiral molecular switches are prepared, in which the binaphthyl azobenzene molecule is used as the proton acceptor, and binaphthyl acids with opposite chiral configuration are proton donors, We find that the helical twisted power (HTP) of H-bonded chiral switches and the helical handedness of induced N*-LCs are mainly determined by the terminal flexible chain length in proton donors of binaphthyl acids. Controlling the lengths of the terminal flexible chain leads to different photoswitching behaviors by light irradiation, the photoresponse behaviors of N*-LCs induced by n=3:(R,S,R)-a, n=6:(R,S,R)-b and n=8: (R,S,R)-d show a HTP decrease, a phase transition from N*-LCs to N-LCs and a HTP increase by UV/vis irradiation, respectively. To our surprise, the N*-LCs induced by n=7:(R,S,R)-c enable reversible helical inversion. This is mainly because of chiral counteraction and intensity attenuation of opposite chiral configurations between the proton acceptor and proton donor during UV-vis irradiation.Additionally, these four H-bonded chiral switches exhibit different thermal responsive behaviors based on effects of dihedral angle and H-bonded interaction. The pitch lengths of the N*-LCs doped with (R,S,R)-a and (R,S,R)-d increase with increasing temperature, which means that the HTP of (R,S,R)-a and (R,S,R)-d decrease with increasing temperature. While the pitch lengths of the N*-LCs doped with (R,S,R)-b and (R,S,R)-c decrease with increasing temperature, suggesting that the HTPs of (R,S,R)-b and (R,S,R)-c increase as the temperature is increased. To gain further insight into the structure-property relationships, we evaluate the optimized geometries of the H-bonded chiral switch (R,S,R)-a, b, c, d by Gaussian 03 calculations at B3LYP/6-31G(d) level. We find that H-bonded chiral switch (R,S,R)-a, b, c, d have similar molecular conformations before and after UV irradiation, respectively. Although the proton donors of binaphthyl acids in (R,S,R)-a, b, c, d have different terminal chain length, the molecular conformation of the central proton acceptor is not affected by them. What’s more, the molecular conformations of the binaphthyl acids have no change before and after UV irradiation, which means that no changes of the proton donors happen during the trans-cis isomerization of the central proton acceptor upon UV irradiation.