Construction And Chiral Modulation Of Nonreciprocal Chiroptical Film Materials Based On Helical Substituted Polyacetylenes

Chirality and helical structure are ubiquitous in nature,and both play indispensable roles in the generation and evolution of lives.Inspired by the helical structure of biological macromolecules,scientists have designed and synthesized many types of chiral helical polymers,among which chiral helical substituted polyacetylene is the most widely studied one.Particularly synthetic helical polymers have attracted great attention in the construction of circularly polarized luminescence（CPL）materials due to their unique chiral amplification effect and helical template effect.Nowadays,intelligent and controllable circularly polarized absorption and emission switching with high dissymmetric factors has attracted great attention,due to the potential applications in chiral chemistry,chiral photonics and electronics.However,the lack of examples to achieve freely switchable and adjustable chiroptical properties within a single material,especially in the stimulus-responsive solid film materials,significantly hampers the further continuous progress.Besides,controlling the chirality and investigating the underlying mechanism in chirality switching remains a highly challenging task,but is of great fundamental significance for understanding the origin of chirality and even for creating new functional materials.Meanwhlie,considering the needs of practical applications,it is urgent and signifcantly important in the field of chiral luminescence to develop a simple,low-cost and universal approach to fabricate high-performance and controllable CPL-active film materials with stimulus response.To solve this issue,in this thesis,the nonreciprocal controllable chiral film materials with tensile or angle response were prepared by spin-coating process and electrospinning technology,respectively,and their chiral inversion and modulation mechanism were investigated in detail.In addition,to solve the problem of limited types of chiral polymers and high price,we successfully prepared temperature-responsive chiral transparent film materials from completely achiral polymer casting system by using symmetry breaking strategy.Lastly,a series of controllable CPL film materials with satisfying luminescence dissymmetry factor(g_lum)values have been successfully prepared by introducing achiral fluorescent dyes into the above chiral film systems.In this thesis,three kinds of chiral functional film materials were constructed based on helical substituted polyacetylenes,namely,spin-coated film,nanofiber film and casting film,which realized the dynamic regulation of chiroptical signals.The detail research contents of the thesis are presented below:（1）A flexible,stretchable nonreciprocal spin-coated chiral film（SCF）was fabricated from chiral helical substituted polyacetylenes（PSA）and thermoplastic polyurethane（TPU）.After spin-coating and stretching processes,polymer molecular chains can self-organize into hierarchical chiroptical superstructures,and combine with the linear birefringence effect induced by stretching to realize the nonreciprocal transmission of chiral optical signals.In the absence of external force,the front-and back-sides of the original SCF both show the intrinsic chirality of PSA.Interestingly,the film can detect circular dichroism（CD）signals with opposite symbols and symmetry from its two faces via simple film stretching,thus realizing nonreciprocal circular polarization absorption of racemizing light source.Handedness inversion can be switched on by external pulling;and through adjusting the elongation rates of the stretched film,it is easy to allow for control of the intensity of dissymmetry in circularly polarized absorption in one single object.Moreover,after the stretched films recover,their CD signal intensity further enhances significantly.A possible formation mechanism of non-reciprocity of stretch-responsive chiral films was proposed,which laied a solid foundation for the further development of chiral polymer functional film materials.（2）A series of nonreciprocal,adjustable CPL flexible film materials with satisfying g_lum values(～10^-1)can be developed by introducing achiral fluorescent dyes into the system using the strategy established in part（1）above.We have developed a functional material of nonreciprocal circularly polarized emission with adjustable dissymmetry intensity,through stretching triggered CPL switch.On this basis,it is possible to further continuously regulate the g_lumvalues by adjusting the elongations of SCF after being stretched.Through two different construction methods of CPL film devices:physical isolation and physical blending,diversified artificial CPL regulation is realized,including signal reversal,g_lum regulation,chiral switching,etc.At the same time,CD and CPL signal strength can be further improved by annealing technology and repeated stretching orientation processing,which provides a new idea for the preparation of high g_lum chiral luminescent film materials.In addition,by regulating the blending ratio of RGB trichromatic fluorophores,the nonreciprocal flexible film with stable white CPL emission was successfully prepared.（3）A series of nonreciprocal nanofibrous film materials with angle response were prepared by electrospinning technique.We demonstrate that transparent nanofibrous films（NFs）infiltrated with poor solvents can show unique linear dichroism and linear birefringence（LDLB）properties,which allow achiral NFs to exhibit regularly opposite Cotton signals by simply flipping the two faces of the same film or even rotating testing angles of the sample around its optical axis in a single face.More importantly,we can quantitatively distinguish the contribution of LD/LB from true CD in the chiral composite nanofibrous system and even effectively integrate them into a single unity.Based on this,we provided a practical toolbox to recognize and quantify the relative contribution of true CD and LDLB in electrospun nanofibrous films,by working on transparent treatment of NFs and different CD test methods.Furthermore,by encapsulating the NFs with a transparent resin solution containing fluorescent dyes,the transparency of the NFs can be maintained and the integrity of nanofibrous physical stacking-optical structures can be preserved.Then,their large intrinsic LDLB feature,angle-dependent and nonreciprocal transmission for polarized lights are exploited to fabricate programmable polarized light-emitting devices through resin encapsulation process.This study may provide an easy and powerful way for construction and implementation of novel polarization optics materials towards future intelligent optical encryption and advanced anti-counterfeiting devices.（4）In order to solve the problem of limited and expensive types of chiral polymers used in the above-mentioned chapters,a kind of adjustable chiral transparent film material with temperature response was prepared from a completely achiral casting system.In the absence of any additional physical or chemical chiral source,spontaneous symmetry breaking has been acquired in the achiral polymer casting systems,optically active composite films are fabricated from casting racemic helical substituted polyacetylenes and achiral polar polymeric matrices.The induced dissymmetry intensity can be flexibly adjusted by controlling the temperature for fabricating the films and/or the recycling casting times.Based on the analysis of theoretical simulation and experimental data,it can be preliminarily speculated that the initial chiral bias in the achiral system occurs in the specific pendant structure（symmetry;suitable steric resistance and potential asymmetric carbon atom）,which induces the spontaneous symmetry breaking of racemic helices.Furthermore,the chiral casting film has good recyclable characteristics and corresponding chiral amplification effect.Excitingly,to demonstrate the utility of the as-obtained chiroptical films,CPL has been achieved by judiciously combining an achiral fluorescent source with the casting films.This also proves the feasibility of preparing polymer-based CPL-active materials from achiral polymers.