Photoluminescence and Mechanism of Polyurea

Author:Cao Hong Yan

Supervisor:kong xiang zheng


Degree Year:2019





Organic light-emitting materials with adjustable emission wavelength have attracted great attention owing to their wide applications in optoelectronic devices,fluorescent probers and sensors.The development of new solid-state high-efficiency luminescent materials is therefore a hot spot of current researches.Compared with light-emitting small organic molecules,luminescent polymers have good film-forming property and synergistic amplification effect,and are advantageous in many apsects.Luminescent polymers have observed therefore,a rapid development in the last decades.It has become a vital branch of the field of polymer researches.For many traditional luminescent materials,luminescent emission is commonly decreased or even quenched in their concentrated solutions and at aggregated states.The phenomenon is commonly known as ACQ(Aggregation-Caused Quenching),which has significantly limited the application scope of these materials.In 2001,Tang and co-workers developed materials that show non-radiative decay via intramolecular rotation at low concentrations,and become strongly fluorescent on molecular aggregation.They proposed the concept of"Aggregation-Induced Emission"(AIE),based on this observation.Luminescent materials with AIE properties make use of the natural process of molecular aggregation to enhance their luminous efficiency in concentrated solution or at aggregate state.In recent years,some polymers without any type of any proverbial chromophores have been found to have strong fluorescence under suitable conditions.Most of these materials have AIE properties.Compared with the clear chemical structure and molecular weight of fluorescent small molecules,the structure of the polymer materials is more complicated and their molecular weight is usually polydisperse.And in addition,the interaction between the molecular chains of these polymers is also far more sophisticated.All these make the study much more difficult for the study on the structure-activity relationship of the luminescent polymers.Generally speaking,for any luminescent polymer,it can be either classified as ACQ or as AIE material.Luminescent polymers beyond these limitations are rarely reported.Developing new fluorescent polymer,of which the luminescent behavior is not limited by concentration with adjustable emission wavelength,is therefore high interesting and of great importance.Polyurea materials are rich in N and O atoms.However,studies on their fluorescent properties have rarely been reported.In this work,two types of polyureas(PU)are synthesized using toluene diisocyanate(TDI)and isophorone diisocyanate(IPDI)as the main monomers,respectively.Their luminescent properties are studied and the emission mechanisms are discussed.The main contents are as follows:(1)By precipitation polymerization using TDI as the only monomer,a PU(denoted as TPU0)was first synthesized through step-growth polymerization of TDI with toluene diamine(TDA),which was in-situ formed from the reaction of TDI with water.It was found that TPU0 is fluorescent not only in its dilute solution(10-4 mg/mL to 5 mg/mL)in dimethyl sulfoxide(DMSO)but also at concentrated solution(>5 mg/mL),and even at solid state.When excited with light of wavelength at 300 nm to 320 nm,a maximal emission was observed in UV region of 350 nm for the dilute solution.This maximal emission was attenuating with increased TPU0 concentration and practically disappeared when TPU0concentration reached 5 mg/mL.However,while for the solution at higher concentration(>5mg/mL),a maximal emission was detected at 440 nm when the solution was excited at 350nm instead of the light at 300 nm to 320 nm.This is to say that,with increased concentration around 5 mg/mL,the maximal emission red-shifted from 350 nm to blue light region at 440nm.The emission mechanisms of TPU0 were then explored by comparison of the fluorescent behavior of small molecule compounds of similar structure to TPU0.It was proposed that theπ-πinteraction between the benzene ring and the neighbor urea group was the main reason for its luminescence in dilute solutions.The form of TPU0 chains and their variation at different concentration were studied using different analyses,including FTIR,1H NMR,UV-Vis,and it was proposed that the aggregation of the polymer chains contributed mainly to it’s the emission in the visible region of 430 nm.The emission behavior of TPU0 was used to detect metal ions with TPU0 dissolved in DMSO solution or dispersed in aqueous solution of the metal ions to be detected.The emission was sharply attenuated with obvious selectivity towards ferric ions(Fe3+).A simple sensor by coating TPU0 on a paper substrate was made and used to detecte hydrogen peroxide(H2O2)based on the selective interaction of TPU0towards ferric ions(Fe3+)and ferrous ion(Fe2+).(2)In order to further explore the luminescence mechanism of TPU0,polyureas of different chemical structure were prepared.One was done by copolymerization of TDI with ethylene diamine(EDA),denotes as TPU2,indicating its chains contain one ethlene(two methylene,-CH2CH2-)group between two neihboring urea groups;and another by copolymerization of TDI with butene diamine(BDA),denotes as TPU4,because its chains are consisted of one butene(four methylene,-(CH2)4-)originated from BDA between two neihboring urea groups,alternatively with the toluylene unit(which is present in all PU prepared here).The fluorescence properties of TPU2 and TPU4,as solid powder and in solution of DMSO were studied.Both PUs have luminescence properties similar to those of TPU0.They both have bright fluorescence in their dilute and concentrated solution as well as at solid state.Furthermore,their emission is excitation-dependent.In particular,the maximal emission wavelength of TPU4 was seen extended to the yellow light region.In addition,two PUs of known structure different from PU2,namely NPU2 and NPPU2,were synthesizzed by copolymerization of TDI with N,N’-dimethylethylenediamine(DMEDA)and N,N’-dimethylethylenediamine(DPEDA),respectively.By comparing the fluorescence properties of the two polymer with those of TPU2,the luminescence mechanism of TPU was further illustrated.(3)Using isophorone diisocyanate(IPDI)as the main monomer,through its reaction with water,or copolymerization with BDA and 1,6-hexamethylene diamine(HDA)respectively,three polyureas,IPU0,IPU4 and IPU6,were prepared.These polymers do not contain any type of conventional chromophore.The structure was characterized by FTIR and 1H NMR.It was found that,under exitation of 330 nm light,these three PU materials have strong fluorescent emission in their DMSO solutions,and their fluorescence emission intensity gradually increased with increased concentration,and also with increased amount of a poor or no-solvent,a characteristic of AIE materials.Dispersed in water,IPU4 was used as a sensor to detecte ferric ions ions(Fe3+)based on its selective quenching effect on fluoreseence emission of IPU4.(4)All PU materials in this work are thermo-stable at 300oC,whereas polyethylene glycol(PEG),when heated at 220oC,is reported to change to carbon dot materials with fluorescent emission.TPU0 powder was dispersend in PEG400(Mn=400,other PEGs were also used)by simply blending,the mixture was heated at 220oC for 0.5 h to obtain Carbon dot-PU composites,and their luminescent properties were studied.The results revealed that the composite demonstrated different luminescence properties when dispersed in solvents of different polarity.In polar solvent,the luminescence behavior of the composites was similar to PEG carbon dots themselves,emitting blue light.However,in the non-polar solvent,the maximal emission was red-shifted.In tetrahydrofuran particularly,the composite showed bright green emission.In addition,the maximal emission of the composite,when dispersed in a non-polar solvent,gradually red-shifted with the concentration.The particle size test showed that a larger size was determined in non-polar solvent than in polar solvent for the composite,and the size was increased with the increasing concentration of the composite.