Study On Synthesis And Performances Of Functional Hyperbranched Polysiloxanes

Non-conjugated fluorescent polymers without containingπ-πelectron systems,possess the characteristics of good biocompatibility,environmental friendiness,low cytotoxicity to cells in the fields of bioimaging and fluorescence labeling.However,several drawbacks for these kinds of polymers are still remained,such as complex preparation process,narrow fluorescence range,and the absence of the unified understanding on luminous mechanism.This severely restricts the development and expansion of application domain of new light-emitting materials.Therefore,easily designing and developing non-conjugated light-emitting polymers with hightly fluorescent intensity,illuminating the relationship between structure and performance,and revealing luminous mechanism,have become the hotspot and difficulty in research of luminescent materials.Hyperbranched polysiloxanes（HBPSi）possess merits of conventional polysiloxanes and hyperbranched polymers.In recent years,they have received much attention due to their unique physicochemical properties and potential applications,and have been widely applied in several fileds,while few reports on HBPSi in research of fluorescent properties have been found.And,the synthesis of HBPSi mainly including hydrosilylation and hydrolysis-condensation still undergoes some defects,such as high cost of Pt/C catalyst and difficult to separate for the former,and difficult to control of hydrolysis and easy to gel.If the non-conjugated fluorescent hyperbranched polysiloxanes can be constructed using a straightforward approach,not only the preparation disadvantages of both the non-conjugated polymers and hyperbranched polysiloxanes can be simultaneously overcome,but also the light-emitting mechanism and breed of non-conjugated fluorescent polymers can be enriched.This will be of important significance to facilitate the design,synthesis and application of multifunctional organic fluorescent materials.For this purpose,this thesis designs and synthesizes a kind of non-conjugated fluorescent hyperbranched polysiloxanes with novel structure.Their structures are determined,photoluminescence properties are analyzed,and the aggregation-induced emission mechanism is investigated.This thesis focuses on the following investigations:（1）The HBPSi simultaneously containing unconjugated carbon-carbon double bonds and terminal hydroxyl groups is synthesized using triethoxyvinylsilane（A-151）and excessive neopentyl glycol（NPG）under catalyst of p-toluenesulfonic acid（p-TSA）.It is proved by GPC,¹H NMR,¹³C NMR,FTIR and GC that the resulting polymer has been successfully prepared.The polymer has significant UV absorption and excitation-dependent photoluminescent properties,and its UV absorption and fluorescence intensities both increase with its molecular weight and concentration;meanwhile,its average fluorescent lifetime(τ_avg)and absolute quntum yield（QY）are 4.88 ns and 3.68%,respectively.Studies show that double bonds and hydroxyl groups simultaneously contribute to the formation of blue fluorescence species.（2）To check the universality of the aforementioned synthetic method,the HBPSi simultaneously containing epoxy and hydroxyl gropus are synthesized using 3-glycidyloxypropyltrimethoxysilane（A-187）and excesive NPG,and it is indicated by several characterizations that the expected polymer has been triumphantly prepared,proving that the synthetic method possesses excellent universality.Similar to the above light-emitting behavior,the obtained polymers have concentration and excitation-dependent photoluminescence characteristics,and the brightest luminescence can be observed even in the 100%solid state,meanwhile itsτ_avg and QY are 4.30 ns and 4.61%,respectively.Our research shows that hydroxyl groups and ether bonds simultaneously play a key role in the generation of fluorescence.It is shown by dynamic light scattering（DLS）that the nanocluster is generated in ethanol solution,and its particle size distribution becomes broader and broader with increased concentration（1¹0 nm）,meanwhile its fluorescent intensity also enhances with concentration.To further improve the fluorescent properties,the obtained HBPSi are modified using water-soluble polyether,and the fluorescence intensity of the modified polymer further enhanced,and itsτ_avg and QY respectively increases to 8.89 ns and 7.30%,probably due to the increase of ether bond amount.DLS results indicate that its size distribution ranges from 20 to 1000 nm,suggesting the formation of clusters owing to the intermolecular aggregation.（3）Considering that the above synthesized HBPSi can only applied in organic solvent-based system.However,to expand their application domains,we synthesize the hydrosoluble HBPSi simultaneouslycontainingprimaryamineandhydroxylgroupusing（3-Aminopropyl）triethoxysilane（APTES）and excessive diols（NPG and MPD）.Similar to the above HBPSi,such polymers have concentration and excitation-dependent photoluminescence properties as well.Theτ_avg and QY of the polymer containing NPG moiety are,respectively,8.41 ns and 8.18%,while those of the polymer loading MPD moiety are 7.88 ns and 5.72%,respectively.Obviously,the polymer containing NPG moiety possesses more intense emission.This is because NPG have huger steric hindrance than MPD,and the intramolecular rotations is more easily restricted,which impedes the nonradiative channel,resulting in stronger photoluminescence.We also investigate hydrolysis of the polymer on its fluorescence property,and their hydrolyzates display more intense luminescence.For the hydrolyzates bearing NPG moiety,itsτ_avg and QY are,respectively,3.85 ns and 44.50%;while for that carrying MPD moiety,itsτ_avg and QY are 5.39 ns and 14.98%,respectively.The results show that the primary amine groups is not important in the generation of luminescent centers but rather that the aggregation of the terminal hydroxyl group plays a key role in forming blue-fluorescent species.（4）The two hydrosoluble HBPSi simultaneously containing aliphatic amine and terminal hydroxyl groups have been synthesized using tetraethoxysilane（TEOS）and polyols（DEG,NMDEA,TEA）,and several characterizations prove that the obtained polymers have been successfully prepared.Similar to the aforementioned HBPSi,such polymers have the concentration and excitation-dependent characteristics.For the polymer containing NMDEA moiety,itsτ_avg and QY are,respectively,1.02 ns and 5.79%,while for that containing DEG moiety,itsτ_avg and QY are 1.57 ns and 11.99%,respectively.This difference is owing to the different polymer structures.Throughout the existing literatures,the QY of 11.99%is the highest value in the current polymers containing nonconjugated chromophores.Furthermore,we also studied the effect of pH on photoluminescence,and found that the protonation of nitrogen significantly changes the fluorescence intensity,and the prominent molecule aggregation and the most intense photoluminescence can be observed at pH=7.0.In addition,the effects of different metal ions on luminescence are also investigated,and there is a strong selective quenching toward Fe³⁺.Therefore,such polymers are promising as Fe³⁺probes.