| Circularly polarized luminescence(CPL)refers to the discrepant emission of left-handed and right-handed circularly polarized light from helically assembled or chirally arranged luminophores after being excited.The research on CPL materials is conducive to comprehend the generation,transfer and amplification mechanism of chirality.In recent years,CPL materials have attracted great attention of researchers because of their great potential in asymmetric catalytic synthesis,LCD backlight,information storage and encryption,chemical probes and optoelectronic devices,etc.Luminescent quantum efficiency and luminescent asymmetry factor(glum)are two key parameters of evaluating the properties of CPL,which largely determine whether the CPL-active material has practical application value.However,the glum value of most organic CPL materials is small,and luminescence quantum efficiency is not high enough.To obtain CPL-active materials simultaneously with large glum value and high luminescent quantum efficiency,a series of novel CPL-active chiral liquid crystal small molecules and polymers with different chemical structures were designed and synthesized by using aggregation-induced emission enhanced(AIEE)cyanostilbene as fluorescent liquid crystal unit and connecting different kinds of chiral groups.The effects of the length of flexible spacer,the type and position of chiral groups on the CPL properties of side-chain chiral liquid crystal polymers were investigated in detail,and the application of these materials in stimulus responsive CPL field was explored.The detailed research work of this paper is as follows:(1)A novel CPL-active side-chain chiral liquid crystal polymers PGAC-m(m=4,6,10)were constructed by connecting chiral poly(L-glutamic acid)and cyanostilbene luminescent mesogen via different flexible spacer length.The effects of flexible spacer length on the liquid crystal phase structure,optical properties and chiroptical properties of PGAC-m were investigated in detail.The results show that PGAC-m can self-assemble to form chiral smectic C(Sm C*)phase liquid crystal featuring helical packing structure,and its clearing point temperature decreases with the extension of spacer length.PGAC-m features typical AIEE emission and high solid-state fluorescence quantum efficiency,and their values increase from 30.2%to34.1%with the extension of the flexible spacer length,while the glum values in the glass Sm C*film show the opposite change,from+4.5×10-2 to+3.9×10-3.(2)A new type of CPL-active side-chain chiral liquid crystal polymers PMCmCSChol(m=6,8,10)based on chiral cholesteryl group were prepared by linking polymethacrylic acid with cyanostilbene luminescent mesogen via different flexible spacer length.The effects of flexible spacer length on the liquid crystal phase structure,optical properties and chiroptical properties of PMCmCSChol were investigated in detail.The results show that PMCmCSChol can self-assemble into Sm C*liquid crystal with helical packing structure under the induction of chiral cholesteryl groups,and its clearing point temperature decreases with the extension of spacer length.PMCmCSChol features typical AIEE emission and high solid-state fluorescence quantum efficiency,and their values increase from 10.6%to 15.3%with the extension of the flexible spacer length.Meanwhile,the polymers present strong CPL signal in the glassy Sm C*film,and the glum values increase from-5.1×10-3 to-3.7×10-2 with the extension of the flexible spacer length.(3)A new type of CPL-active side-chain chiral liquid crystal polymers PMCmCSC*(m=4,8)based on chiral carbon tail-chain were prepared by linking polymethacrylic acid with cyanostilbene luminescent mesogen via different flexible spacer length.The effects of flexible spacer length on the liquid crystal phase structure,optical properties and chiroptical properties of PMCmCSC*were investigated in detail.The results show that the clearing point temperature of PMCmCSC*decreases with the extension of spacer length.PMC8CSC*has a helical Sm C*structure,and a strong CPL signal is detected in the glassy Sm C*film,with a glum value up to-1.1×10-1.However,PMC6CSC*with a shorter spacer length forms non-helical smectic A(Sm A)phase structure,showing silent CPL signal in the glassy Sm A film.Both PMC8CSC*and PMC8CSC*show typical AIEE properties and excellent solid-state fluorescence quantum efficiency,with values of 11.8%and 6.7%,respectively.(4)A novel CPL-active chiral liquid crystal NO2-CS-C6-Chol with abundant phase structure was prepared by covalently linking nitro-functionalized cyanostilbene luminescent mesogen with chiral cholesteryl group by six flexible methylene chains.The internal relationship between the phase structure,optical properties and chiroptical properties of NO2-CS-C6-Chol as well as the manipulation behavior of temperature on the CPL performance of NO2-CS-C6-Chol were investigated in detail.The results show that cholesteric phase,Sm C*phase and crystalline phase are formed successively during the annealing assembly of NO2-CS-C6-Chol.NO2-CS-C6-Chol exhibits typical AIEE emission feature and excellent solid-state fluorescence quantum efficiency,which are 15.2%,18.9%and 38.4%in the film state of cholesteric phase,Sm C*phase and crystalline,respectively.In addition,intense CPL signals are detected in cholesteric and Sm C*phase film,and the glum value is-1.5×10-1 and-2.5×10-2,respectively.However,no CPL signal is detected in the crystalline film.More importantly,the reversible switching of glum values under three different aggregation structures is successfully realized by manipulating the temperature.(5)The NO2-CS-C6-Chol prepared in the previous chapter was selected as luminescent chiral guest and doped into the room temperature nematic liquid crystal5CB host to fabricate a series of chiral nematic liquid crystals(N*-LCs)with helical stacking structure.The relationship among dopant concentration,pitch P and CPL performance of N*-LCs was investigated in detail,and revealing the dual switching behaviors of electric field and temperature on the CPL properties of N*-LCs.The results show that the pitch P of N*-LCs is inversely proportional to the doping concentration.All of the emissive N*-LCs possess very large glum value,which first increases and then decreases with the increase of doping concentration,and reaches the maximum of-0.38 when the doping concentration is 6 wt%.After injecting emissive N*-LC doped with 6 wt%NO2-CS-C6-Chol(N*-LC-6)into the conductive liquid crystal cell,the CPL signals can be switched repeatedly between“on”and“off”states by adjusting the applied voltage.Meanwhile,the CPL signals can be also switched repeatedly between“on”and“off”states in the process of adjusting the phase structure of N*-LC-6 by varying temperature. |
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