Chiral Aggregation-induced Emission Polymers With Multiple Signal Reversible Responses

Chiral polymers have attracted much attention in recent years because of their potential applications in chiral recognition,chiral sensor,chiral separation,and asymmetric catalysis,and special attention has been paid for chiral polymers with unique luminescence properties.Aggregation-induced emission(AIE)polymers,as a new type of luminescent functional polymer materials,have been gradually developed into promising stimuli-responsive intelligent materials,owing to their sensitive response to environmental changes such as temperature,p H,light,force,and solvent.In this dissertation,a series of stimuli-responsive AIE-active chiral polymers were designed which can realize multiple signal responses from a single signal input,including the responses on chiral signal and fluorescence signal of the AIE polymers.Based on this,salicylaldehydoethylenediamine(Salen)derivatives as functional units possess two C=N and two OH groups to serve as multidentate ligand and enjoy both fluorescence and chiral structures.A series of chiral Salen-containing AIE polymers are designed with multiple signals responded to metal ion,and their structure-property relationship was systematically investigated.The detailed work includes:In Chapter 2,salicylaldehyde derivatives and enantiomeric(1S,2S)/(1R,2R)-1,2-diphenylethylenediamine was as monomer,a pair of luminescent enantiomeric Salen-containing polymers(polysalens)were aynthesized.These polysalens possess same absorption spectra,opposite Cotton effect,and similar AIE characteristics.They can only selectively response to Zn²⁺among various metal ions,showing red-shifted absorption,weakened CD signal,dramatically enhanced fluorescence,and weakened circular polarization luminescence(CPL).Furthermore,the UV-vis spectra,CD spectra,fluorescence spectra,and CPL spectra of the polysalens in the presence of Zn²⁺can be restored to their original states after the addition of EDTA ligand into the mixed solution.The small molecular model compound study has revealed that these reversible changes from the spectra are attributed to the coordination and dissociation of Zn²⁺with Salen units in the polymer.A polymer spring model with reversible stimulus-responsive helical conformation was proposed.The single crystal analysis of small molecule model compounds and model complexes and the morphology of polysalens before and after Zn²⁺coordination in atomic force microscope(AFM)show that the spring pitch length of pure polysalen is relatively large,and the pitch length after coordination with Zn²⁺is shortened,symbolizing the“stretched”and“compressed”states of the polymer spring,respectively.The“stretching-compressing-stretching”movement of the spring can be realized by consecutive addition of Zn²⁺and EDTA.In Chapter 3,chiral polysalens with different connecting sites were prepared,and systematically explored their photophysical properties and conformational effects.4,4'-(9,9-dihexyl-9H-fluorene-2,7-diyl)bis(2-hydroxybenzaldehyde),6,6'-(9,9-dihexyl-9H-fluorene-2,7-diyl)bis(2-hydroxybenzaldehyde),5,5'-(9,9-dihexyl-9H-fluorene-2,7-diyl)bis(2-hydroxybenzaldehyde)and3,3'-(9,9-dihexyl-9H-fluorene-2,7-diyl)bis(2-hydroxybenzaldehyde)and(1S,2S)/(1R,2R)-1,2-diphenylethylenediamine were synthesized.Four pairs of polysalens with different connecting sites were synthesized.All of them show typical AIE characteristics and selective fluorescence"turn-on"behavior induced by Zn²⁺.When Zn²⁺and EDTA are added successively,the four pairs of polysalens can realize reversible responses on their UV-vis,CD,and PL spectra.Among them,the CD spectra of polysalens linked through the 4-position of salicylaldehyde showed a"first negative,then positive"Cotton effect,while the CD spectra of polysalens linked through the 5-position of salicylaldehyde showed a"first positive,then negative"Cotton effect.After the addition of Zn²⁺,the CD peak intensity of the polysalens linked through the 4-position of salicylaldehyde decreased by half,while that of the polysalens linked through the 5-position of salicylaldehyde dramatically decreased to the baseline,which may be attributed to their respective folding helical conformation and linear conformation.In Chapter 4,the influence of different steric hindrances of side chains on the photophysical properties and conformation of polysalens was investigated.Four pairs of polysalens with different steric hindrances were synthesized by adjusting the alkyl chain length and branched chain structures on the 9-position of the fluorene unit through the linkage from the 4-position of salicylaldehyde;Three pairs of polysalens with different side chain lengths on the 9-position of the fluorene unit were also synthesized through the linkage from the 5-position of salicylaldehyde.Each pair of polysalens have the opposite Cotton effect,and also show AIE characteristics.After the successive addition of Zn²⁺and EDTA,their UV-vis spectra,CD spectra,and fluorescence spectra show reversible optical response.For polysalens with the same connecting sites,the larger the steric hindrance of the side chain group,the larger the pitch length of the corresponding polymer helical conformation,and the stronger CD signal could be obtained.In Chapter 5,two pairs of chiral conjugated microporous polysalens were synthesized bythe polymerization of(1S,2S)/(1R,2R)-1,2-diphenylethylenediamine with tetraphenylethene(TPE)-and triphenylamine-containing multifunctional salicylaldehyde.The pore structure of these polysalens were studied by N₂ adsorption-desorption curves.When Zn²⁺and EDTA were added successively,the luminescence signals of these chiral conjugated microporous polysalens can responded reversibly.In comparison with the triphenylamine-containing polysalens,the TPE-containing polysalens shows blue shifted absorption maximum,Cotton peak,and emission maximum,indicating the existence of electron donor-receptor interaction in triphenylamine-containing polysalens.