Synthesis Of Hyperbranched Fluorescent Polymers And Its Effect Of "Super Quenching" For Nitroaromatics

In recent years,explosives have seriously affected the safety of mankind and caused irreversible harm to the ecological environment.The fluorescent conjugated polymer as a"molecular conductor"that is,"a little contact,multi-point response",can produce fluorescence signal amplification,exciton can flow along the entire conjugate chain.Theoretically,compared with liner conjugated polymers,hyperbranched conjugated polymers have higher fluorescence intensity and quenching efficiency.The main contribution of this paper is exploring the"super-quenching"effect of the fluorescent hyperbranched conjugated polymer on nitroaromatics.Based on the"signal amplification"effect of conjugated polymers,we designed and synthesized six compounds on the basis of linear conjugated polymers.Among three compounds,three ones were synthesized firstly.On one part,it contained theπ-πconjugation,which based on pyrazine as the core and vinyl benzene for the side chain:Linear or hyperbranched monomer M₁（2,5-styrylpyrazine）,M₂（2,3,5,6-styrylpyrazine）.And their corresponding linear conjugated polymer P₁(（Poly（2,5-styrylpyrazine））and hyperbranched conjugated polymer P₂（poly（2,3,5,6-styrylpyrazine））.On the other part,it contained theσ-πconjugated,which was linear methylphenyl copolymersilane（PDMPS）.And three-dimensional hyperbranchedσ-πConjugated Polymers（3D-HPs）with polyhedral oligomeric silsesquioxane（POSS）as the core and PDMPS for the side chain.The characterization that synthesis of the compounds was successful by the measures of ¹H-NMR,Ft-IR and TGA.Through comparing their optical properties and the fluorescence quenching properties of TNT,the quenching constant(K_SV=6.84×10⁴M^-1)of the two-dimensional conjugated polymer P₂ was higher than that of the linear conjugated polymer P₁(K_SV=2.49×10⁴ M^-1).It shows that P₂ have a higher detection sensitivity.According to the fluorescence lifetime test,the fluorescence lifetime（τ=3.04 ns）of hyperbranched compounds did not change with the increasing of the concentration of TNT solution,indicating that the main quenching mechanism was static quenching.Optimizing the structure of the complex with TNT by the quantum chemistry theory calculation,which showed that the molecular spacing of them was about 2?.In the ground state,the electron density is concentrated on the fluorescent molecules.The excited electron density is mainly concentrated in the nitroaromatics.According to Marcus’s semi-empirical equation,three important influencing factors of linear or hyperbranched molecular structures were thermodynamic driving force(△G^O_M1=1.46 eV,△G^O_M2=2.33 eV),recombination energy（λ_i）and electronic coupling（V）.The results showed that the electron transfer rate(K_et)was positively correlated with the fluorescence quenching constant in the mechanism of PET on fluorescence quenching.And then,we guess that multi-dimensional hyperbranched conjugated polymer will have a stronger"super quenching"effect.In order to further study the"super quenching"effect of the fluorescent conjugated polymer sensor on nitroaromatics,we designed and synthesized three dimensional hyperbranchedσ-πconjugated polymers.The characterization that synthesis of the compounds was successful by the measures of ¹H NMR,Ft-IR and TGA,X-ray diffraction,nitrogen desorption and so on.With the POSS structure as the core,while increasing the permeability of nitroaromatic hydrocarbons,but also improve the thermal stability of the whole molecule(T_g=341 ^oC,800 ^o C,the weight loss of 53.1%).Flexible chains of polysilane act as fluorophores,which have efficient exciton transport in theσ-πconjugated backbone.Flexible chains of Polysilane also increase the solubility of the POSS structure in common organic solvents（THF,CH₂Cl₂,etc.）.Through studying their optical properties and the fluorescence quenching properties of TNT,the quenching constant(K_SV=2.83×10⁴ M^-1)of 3D-HPs.The lowest detection limit concentration of TNT is 1.76μM.The results show that compared with PDMPS(η_EP=82%)and two-dimensional conjugated polymer P(η_EP=70%),the 3D-HPs film has a higher fluorescence quenching efficiency(η_EP=95%).The positively charged inorganic silicon atoms attract N/O atoms of nitroaromatic effectively with the non-covalent bond formed.Furthermore,optimizing the structure of the complex with TNT by the quantum chemistry theory calculation,which showed the molecular front orbital energy level of polymers.Compared with PDMPS(E_LUMO=-0.841 eV),3D-HPs(E_LUMO=-0.692 eV)have greater electron transfer driving force.It is shown that the three-dimensionalσ-πconjugated polymer has the function borth of detecting gas phase and trace liquid nitroaromatics,which is a potential fluorescent sensor.In summary,this paper studies the fluorescent conjugated polymers with different dimensions.According to the fluorescence quenching mechanism,we have explored that the influencing factors of the"super quenching"effect.We design and synthesize novel conjugated polymers with reasonable guess.And then,we summarize the several regularity of the"super quenching"effect of the fluorescent conjugated polymer on nitroaromatics.This work is of certain practical significance on designing fluorescent sensing materials.