| With the rapid development of liquid crystal photonics and the emerging virtual reality/augmented reality(VR/AR)technologies,large birefringence liquid crystals(LCs)have become a hot research topic.Tolane LC is the most representative high birefringence type.Previous reports have shown that such LCs with large conjugated systems exhibit large birefringence and low viscosity.While they suffer from disadvantages such as narrow nematic phase interval,poor ultraviolet(UV)light stability,high melting point and the unwanted smectic phase,which limit the applications in the LC mixture.To tackle with the above-mentioned problems,through molecular design and synthesis,in this paper we systematically studied the effects of lateral substituents,bridges and terminal groups on tolane LCs’ properties,and explored the application of low-melting point diluters in high birefringence LC mixtures.The research contents are listed as follows:(1)Ten four-ring LCs with ethylene and internal alkyne bridge were synthesized via Grignard reaction,Huang Minglong reaction,iodination,Heck cross-coupling reaction and Sonogashira cross-coupling reaction with trans-4-alkyl-cyclohexane-carboxylic-acid as the raw material.The structures of these compounds were confirmed by infrared(IR)spectrometry,nuclear magnetic resonance(NMR)and mass(MS)spectra analyses.Differential scanning calorimetry(DSC),polarizing optical microscopy(POM)and Abbe refractometer were used to measure their properties.The effects of acetylene bond and ethylene-bridge on the LC mesophases were also studied.The results show that the compounds possess high clearing point(220-240 0C),broad nematic phase(59.6-99.6℃)and high birefringence(△n>0.3).(2)A series of ortho fluoro-tolane LCs was prepared via chemical structure modification by introducing a fluorine substituent ortho to the acetylene bond.The effects of ortho-fluorine substituents on the UV stability of tolane LCs were systematically investigated by DSC,UV-vis absorption,fluorescence emission spectra and POM.Meanwhile,the degradation mechanism of diphenylacetylene during UV exposure is deduced by means of IR,TGA,NMR and GC/MS.DFT calculations of molecular polarizabilities,triple bond lengths,electrostatic potential maps and molecular orbitals are used to correlate the experimental findings.The results show that the carbon-carbon triple bond in the diphenylacetylene is excited by UV photons and generates free radicals.Besides,the strong interaction between the lone electron pair of the ortho-fluorine atom and the π-electron of carbon-carbon triple bond is beneficial to improve the UV stability of tolane-LCs.This work paves the way for the applications of tolane LCs in LC photonics.(3)Thirty heterocyclic LCs were designed and synthesized using 1,3-dioxolane,4-pyridine ring or tetrahydropyran ring as terminal groups.To eliminate the smectic phase and to lower the melting point,a lateral fluorine substituent,a cyclohexyl group,an ethyl-bridge,and a tolane unit are all combined into the heterocyclic LC molecular structure.Meanwhile,the effects of alkyl chain,internal alkyne-bridge,lateral fluorine substituent and terminal heterocyclic ring on the LC phase,LC phase interval,birefringence(△n)and dielectric anisotropy(△ε)of these LCs were systematically studied.Finally,the application of heterocyclic LCs containing 1,3-dioxolane ring in the LC mixture was evaluated.The results show that the introduction of the lateral fluorine substituent,the internal alkyne-bridge and the 1,3-dioxolane terminal could enhance the stability of the nematic phase and improve the solubility of LC monomer in the LC mixture,and the 1,3-dioxolane terminal group can also simultaneously increase △n and △ε.In addition,the introduction of a phenyl ring in the 4-pyridyl LCs could broaden the nematic phase interval and increase the △n and △ε.The tetrahydropyran terminal group is not conducive to increasing △n and LC phase interval,while it can increase the negative △ε value.The application of 1,3-dioxolane-terminated LCs in the mixture was investigated and the results show that these LCs have a good application prospect in wearable LC device.(4)Based on quantum chemistry calculation,the molecular configuration was optimized by functional B3LYP and basis set 6-31g(d,p),and the dipole moment(μ)and polarizability parameter(Aa)of LC compounds were obtained.The relationship between the molecular structure of heterocyclic LCs and their properties was studied by combining Vuks equation with Maier-Meier theory.The experimental results are in good agreement with the DFT theoretical calculations.The results indicate that the △n and △ε of rod-like LCs mainly depend on the △α and μ of the molecules.Larger Aa andμ values contribute to larger △n and △ε values.(5)Four terminal olefin diluters were designed and synthesized,and their applications were investigated in a mixture P02-F containing fluoro-tolane or a mixture P12-ISO containing isothiocyano-tolane.The results show that the fluoro-tolane diluents 3PTPF2V and 3PTPFOV have a moderate rotational viscosity,while maintaining a larger △n and a low Tm,which are suitable for the high An LC mixture containing fluoro-tolane.Biphenyl diluters 2BBF2V and 3BFB2V could decrease the Tm of mixture P12-ISO to below-20℃,while maintain large △n and large △ε,which are suitable for the high-△n LC mixtures containing isothiocyano-tolane.(6)The diluter 3PTPFOV was used to form a fluoro-tolane LC mixture MCRI,which possessed a low Tm,broad LC phase interval,high △n,high resistivity,large △εand relatively low viscosity.This mixture enhances 2π phase modulation at 5 V operating voltage with 2.87 ms average phase-to-phase response time,therefore the phase-only LCoS device could employ it to realize field sequential color displays at 240 Hz frame rate,avoiding the problem of image flicker. |
