Preparation And Properties Of Side-chain Fluorescent Alternative Copolymers With Large And Adjustable Pseudo-stokes Shift

Novel fluorescent materials have always been the frontier hotspots of theoretical research and an urgent need for industrial applications.For decades,fluorescent materials have been widely used in display lighting,chemical sensors,molecular probes,biological imaging,targeted therapy and other fields.Ideal fluorescent materials usually have the advantages of adjustable color,strong solid-state emission and large Stokes shift.However,it is difficult to take these advantages at the same time in the related researches reported so far.It is feasible to achieve the above advantages at the same time by constructing alternative copolymer with side chains contained the fluorophores with aggregation-induced emission(AIE)effect as energy donor and the fluorophores with intramolecular charge transfer(ICT)effect as energy acceptor.Firstly,the alternative structure makes the acceptor fluorophore which undergo aggregation caused quenching(ACQ)effect is separated by adjacent side chains to avoid emission quenching.Second,the alternative arrange of energy transfer donors and acceptors as side chains can greatly improve the efficiency of energy transfer.Third,the employment of donor fluorophore with ICT effect could realize tunable Stokes shift and stimulus responsiveness of the copolymer luminescence through rational design and introduction of luminescent groups.Therefore,a series of novel luminescent alternative copolymers were constructed where the fluorophores with AIE and ICT respectively were introduced as side chains.And the relationship between chemical structures of these copolymers and luminescent properties were discussed in detail.By creatively combining ICT effect,AIE effect and fluorescence/dark resonance energy transfer,a large and adjustable pseudo Stokes shift were realized both in solution and aggregated state.Moreover,their potential applications as agricultural light conversion films,white light materials and stimulated response fluorescent materials were discussed.In addition,a novel fluorescent small molecule was designed and synthesized,and its multiple stimulus response and the luminescent properties as a guest material in polymer were studied.This thesis is divided into the following parts:1.A side-chain alternative copolymer P(TPE-alt-NI)was designed and synthesized.Tetrastyrene(TPE)and 4-dimethylaminonaphthalimide(NI)groups with equal molar ratio were linked to styrene and maleimide respectively,and were introduced into the alternative copolymer as energy donor and energy acceptor respectively.The luminescent behavior of the copolymer was studied in a series of tetrahydrofuran solvents mixed with different contents of water or n-heptane.The results show that the copolymers of P(TPE-alt-NI)can emit the fluorescence of NI group both in solution and aggregated state,and the fluorescence intensity generated by energy transfer was significantly higher than that by direct excitation.The pseudo Stokes shift of the copolymer in the aggregation state is 215 nm.Moreover,P(TPE-alt-NI)also showed an interesting solvent sensitive fluorescence behavior: with the increase of water content in its tetrahydrofuran solution,its fluorescence intensity first decreases and then increases;while replacing water with n-heptane in tetrahydrofuran solution,the fluorescence intensity first increases and then decreases.We believe these fluorescence behaviors are the result of intramolecular charge transfer,energy transfer and solubility.2.A side-chain alternative copolymer P(DMPNI-alt-TPE)was designed and synthesized.Tetrastyrene(TPE)and 4-p-dimethylaminophenyl naphthalimide(DMPNI)with equal molar ratio were linked to styrene and maleimide respectively,and were introduced into the alternative copolymer as energy donor and energy acceptor respectively.The luminescent behavior of the copolymer was studied in a series of tetrahydrofuran solvents mixed with different water contents.The results showed that under the radiation at TPE group absorption wavelength,the copolymer emited the fluorescence of DMPNI group,and the pseudo Stokes shift is as high as 304 nm.And with the increase of water content in P(DMPNI-alt-TPE)tetrahydrofuran solution,the fluorescence intensity first decreases and then increases,and the luminescence wavelength first shifts red and then blue.Moreover,the fluorescence intensity generated by energy transfer is significantly higher than that by direct excitation of DMPNI group.In addition,due to the strong intramolecular charge transfer(ICT)effect of DMPNI group,the copolymers show obvious polarity sensitivity and different color fluorescence in different solvents,so that the pseudo Stokes shift can be adjusted from262 nm to 304 nm.Furthermore,the bulk film of P(DMPNI-alt-TPE)and the films mainly composed of polymethylmethacrylate or polyvinyl chloride doped with P(DMPNI-alt-TPE)were prepared.It was found that the three kinds of solid films could effectively absorb the harmful ultraviolet light to plants and emit the red light above600 nm which was beneficial to the growth of plants.Above study indicated that the copolymer had potential application as agricultural light conversion film.3.Two series of side-chain fluorescent alternative copolymers were designed and synthesized: copolymers P1,P2 and P3 with tetrastyrene(TPE)group as energy donor and 4-dimethylaminonaphthalimide(NI)as energy acceptor,whereas copolymers P4,P5 and P6 with tetrastyrene(TPE)group as energy donor and 4-dimethylaminophenyl naphthalimide(DMPNI)as energy acceptor.The ratios of energy donor and energy acceptor in these copolymers were adjusted by introducting non-emission monomer.The luminescent behaviors of two series of copolymers were studied in a series of tetrahydrofuran solvents mixed with different contents of water or n-hexane.The results showed that the double emission fluorescence and its luminescent color of the copolymer can be successfully controlled by adjusting the ratio of two chromophores and the aggregated degree of the copolymers.In different water/tetrahydrofuran or nhexane/tetrahydrofuran solutions,P1,P2,P3,P4 and P5 all showed single emission peaks contributed by corresponding energy receptor,while P6 showed double emission peaks.In tetrahydrofuran solution contained 40%,50% and 60% water,P6 emited white light with CIE coordinates of(0.314,0.357),(0.314,0.348)and(0.336,0.376)respectively,which indicated that P6 has potential application as a white-light display material.4.A fluorescent small molecule named NBDNI was designed and synthesized.The traditional ACQ blue chromophore 1,8-naphthalimide was transformed into a fluorescent molecule which can emit light both in solution and aggregaed state by simple amidation and Suzuki coupling reactions.NBDNI showed interesting multiple stimulation responses fluorescent behavios:(1)Polarity sensitivity.In the mixed solution with increasing water/tetrahydrofuran ratio,the luminescence intensity of NBDNI first decreased and then increased,and the luminescence wavelength first red shifted and then blue shifted.While in the mixed solution with increasing water/tetrahydrofuran ratio,the luminescence intensity of NBDNI continued to increase,and the wavelength continued to blue shift.And when NBDNI was dissolved in different polar organic solvents or doped in different polymers,it can emit fluorescence with different wavelengths.(2)Temperature sensitivity.In toluene,chloroform and dioxane,NBDNI shows obvious luminescence enhancement and blue shift with the increase of temperature.(3)p H response.After treatment with hydrochloric acid or trifluoroacetic acid,the solid-state NBDNI power,which originally emitted 550 nm green fluorescence,emited 454 nm blue fluorescence.Moreover,after treatment with trimethylamine,the acid-fumed NBDNI power turned to show 575 nm yellow fluorescence.And a reversible p H fluorescence test paper was prepared by drying the filter paper which soaked with NBDNI solution.Thus a simple fluorescence thermometer was prepared.(4)Mechanochromism.The solid-state NBDNI power,which originally emitted 550 nm green fluorescence,and turned to emit 589 nm redorange light after grinding.And its fluorescence color can be reverted by fuming with solvent.Furthermore,the structure-property relationship between the molecular structure and the above-mentioned stimulus responses were discussed based on the results of NMR and XRD.