Photo- And Thermal-responsive Colors Of Doped Chiral Liquid Crystal Physical Gels

Doped chiral liquid crystals(CLCs)with unique helical superstructures and stimuli-responsive colors have been harnessed in sensors,cosmetic,high-tech anticounterfeit,and so on.On the one hand,it is still a formidable challenge to precisely tune the helical twisting power(HTP),to untilize clean stimulus sources and to realize long-lived colors in CLCs.On the other hand,the reported systems are mainly conducted in the fluid LC,so that the mechanical strength is urgent to be improved.Towards this end,this thesis focused on the following contents.First,a series of functional axially chiral azobenzene dopants were designed and synthesized.The relationship between the molecular structures and isomerization reactions of dopants upon UV and visible light irradiations was systematically investigated,which enabled precisely tuning the pitches and the reflection wavelengths/colors of CLCs.Then,the parent azobenzene was modified to be fluorinated azobenzene whose isomerization could be triggered by wholly visible lights.Based on a facile co-doping strategy,the reflection wavelengths/colors of CLCs could be rapidly and reversibly switched upon visible light irradiations and stabilized for a long time in dark.Further,the fluid LCs could be transformed into self-supporting and thermal-responsive gels via non-covalent bonds among gelators.Finally,wholly visible lights-and thermal-tunable colors of CLC physical gels were endowed by accommodating a functional gelator in LCs.Firstly,four parent azobenzene axially chiral dopants(parent azo,dopants 1~4)with different lengths of alkyl or alkoxyl substituent groups were successfully synthesized through diazo coupling,nucleophilic substitution and thiol-ene click reactions.On the basis of the photoisomerization behavior of dopants,in combination with theoretical calculation,the relationship between the lengths of alkyl or alkoxyl substituent groups and photoisomerization rate was established.With increasing the length of substituent group,the photoisomerization rate constant of dopants increased for trans-cis transformation upon UV irradiation and decreased for the reverse process upon visible light irradiation either in isotropic ethyl acetate or in anisotropic LCs.The miscibility between dopants and P0616A was conducted through the Teas solubility parameter and DSC results.Then,variable reflection colors in CLCs were achieved upon UV and visible light irradiations.Thus,the pitches and reflection wavelengths/colors of CLCs could be precisely tuned on the basis of their exponential decay(increased)behaviors.Secondly,four fluorinated azobenzene-derivatives(F-azo,namely,S-1F,D-1F,S-4F and D-4F)were designed and synthesized by controlling the reaction time of aniline oxidation through Mills reaction.Time dependent density functional theory(TD-DFT)computation and natural transition orbital(NTO)analysis were implemented to simulate the n-?*transition behaviors of D-4F and D-1F.Furthermore,UV-vis spectra certified the isomerization of F-azo could be triggered upon 550 nm green light and 450 nm blue light irradiations.Thus,the hazardous UV light could be avoided.Especially,the maximum content of cis-isomer and its half-life in dark at room temperature were as high as 49%and46 h for D-4F,respectively.In addition,we employed a solubility parameter oriented co-doping strategy to overcome the poor miscibility of D-4F with the non-fluorinated LC hosts.More interesting,the RGB reflection colors(red-green-blue)could be achieved,where each color range was stable for an average period of about 24 h in dark under ambient conditions.This color stability was enhanced remarkably.Thirdly,for maintaining the fast response of the LC hosts,isotropic organic-solvent-based physical gels and anisotropic LC-based physical gels were fabricated by a gelator(AG)with urea-based pyrimidine group(UPy).In particular,when the content of AG exceeded 3 wt%,the storage modulus G?of P0616A/AG gels could be more than10~4 Pa,showing good self-supporting ability.The enthalpic and entropic variations confirmed that the hydrogen-bond played a key role in the formation of gels.Besides,during the fabrication of gels,the effect of cooling rate on its morphologies was extensively investigated.Finally,in combination with F-azo and UPy,a novel gelator(S-4F-AG)with chirality,wholly visible lights-and thermal-responsive abilility was designed.Upon 550 and 450 nm light irradiations,S-4F-AG exhibited excellent photoswitchable behaviors.Notably,the maximum content of cis-isomer and its half-life as high as 35%and 62 h in acetonitrile,respectively.In addition,S-4F-AG was able to gel P0616A and dopant 2/P0616A with the minimum gelation contents(MGCs)were as low as 1.5 and 2.0 wt%,respectively.POM and FESEM images showed that the morphologies of both gels diaplayed excellent thermal-and wholly visible light-responsive behaviors.Notably,the reflection wavelengths/colors of these self-supporting CLC physical gels could be tuned across almost the visible region(470~800 nm)via heating/cooling or upon wholly visible light irradiations.