Synthesis And Circularly Polarized Luminescence Properties Of Chiral Luminescent Liquid Crystals Based On Aggregated Induced Luminescence And Cholesterol Groups

In recent years,circularly polarized light has been widely used in chemical sensors,biological probes,3D displays,molecular optical switches,optical data storage,advanced medical imaging technology and other fields,and has attracted more and more attention.Chiral light-emitting liquid crystals have the orientation structure,AIE luminescence characteristics and chirality of liquid crystals.They can be directly used as circularly polarized light emitters.Self-assembly of chiral light-emitting liquid crystals to obtain circularly polarized light is the simplest and most direct and effective method.Traditional liquid crystal units do not emit light.In order to obtain luminescent liquid crystals,Aggregation-Induced Emission(AIE)groups are used to modify the liquid crystal,which solves the problem of fluorescence quenching caused by aggregation.In this paper,two kinds of solid-state high-efficiency circularly polarized light-emitting liquid crystals containing cholesterol groups were synthesized through reasonable molecular design,namely,cholesterol derivatives based on tetraphenylethene(TPE)and cholesterol derivatives based on dicyanodistyrene.The chemical structure of the product was confirmed by 1H NMR,13 C NMR and high resolution mass spectrometry(MS).The fluorescence behavior of the products was investigated by UV-vis,PL,fluorescence quantum efficiency and transient fluorescence spectra.The phase structure and phase transition of liquid crystals were studied by thermogravimetric analysis(TGA),differential scanning calorimetry(DSC),polarizing microscope(POM),one-dimensional X-ray diffraction(1D-WAXD),two-dimensional X-ray diffraction(2D-WAXD)and small angle X-ray scattering(SAXS).Cyclodic dichroism(CD)and circular polarization spectroscopy(CPL)were used to study the circularly polarized luminescence behavior.Meanwhile,ordered anopatterns with high resolution were prepared by nanoimprinting technology,and observed by fluorescence microscopy and field emission electron scanning microscopy(SEM).The main contents are as follows:1.With TPE as the light-emitting unit and cholesterol containing multiple chiral centers as the chiral liquid crystal unit,four kinds of four-substituted solid-state high-efficiency circularly polarized liquid crystal materials with AIE characteristics,liquid crystal properties and circularly polarized light-emitting(CPL)were obtained by adjusting the length of spacer,which are TPE-Chol,TPE-2C-Chol,TPE-5C-Chol,TPE-8C-Chol,respectively.The results show that all four molecules have liquid crystalline properties,among which TPE-8C-Chol molecule has oil filament texture under polarizing microscope,while other compounds mainly have sector texture of cholesteric phase.All these molecules exhibit obvious AIE characteristics,and have high fluorescence quantum yield in solid state.The fluorescence quantum efficiency of TPE-5C-Chol solid is 55.8%.The circularly polarized luminescence(CPL)is superior to the non-liquid crystalline chiral molecules and exhibits good AICD and AICPL properties.Nano imprinting lithography(NIL)can be used to prepare ordered nano-patterns with different sizes and styles,which shows that the processability of these compounds is good.2.Dicyanostilbenzene(DCS)was used as the luminescent unit to connect cholesterol to DCS via carbonate bond.Because of the high conjugation degree of DCS,the emission wavelength of DCS is around 530 nm,which is yellow or yellow-green fluorescence.The properties of two kinds of DCS derivatives(?-DCS and ?-DCS)have been studied in this paper.The results show that the molecule with ?-DCS as the core exhibits obvious AIE effect,and its solid-state luminescence efficiency reaches 30.3%.The ?-DCS-8C-Chol with longer spacer group exhibits better chiral liquid crystal phase and stronger CPL signal in film state than the ?-DCS-5C-Chol with shorter spacer spacer group.The molecules with the core of ?-DCS exhibit strong luminescent properties in both solution and bulk states.The quantum efficiency of ?-DCS-5C-Chol solution and bulk reaches 45.8% and 73.7% respectively.Cholesteroid texture can be observed under polarized light microscopy,and the chiral liquid crystals with long spacer have better properties,and the CPL signals in the film state are also stronger.The series of molecules have good film-forming properties and are easy to process.High-resolution ordered nano-patterns with different sizes and styles can be prepared by nanoimprinting technology to improve their fluorescence quantum efficiency.