Synthesis And Characterization Of Fluorine-containing Polynitroazobenzene Derivatives

High-energy density materials（HEDMs）represent a significant class of advanced materials.How to design and synthesize energetic compounds with as high energy density as possible and the maximum possible chemical stability has been the focus of energetic materials community.However,for most HEDMs,the energy density and molecular stability have an inverse relationship.How to balance the relationship between energy and safety is a major challenge in developing new high-energy density compounds.Fluorine atom and nitro group are used as energy modification groups,and azobenzene with largeπconjugate system is used as basic ring framework to construct a variety of fluorine-containing polynitroazobenzene compounds in this paper.By comparing the performance parameters of fluorine-containing polynitroazobenzene derivatives,the influence law and mechanism of fluorine element on azobenzene energetic materials are explored.It provides a theoretical basis for balancing the relationship between energy and safety.The main study contents and results are as follows:（1）Six fluorine-containing polynitroazobenzene compounds（compounds 33～38）were designed and synthesized.The structures of the six compounds were determined by infrared,multinuclear nuclear magnetic resonance（1H NMR、13C NMR、19F NMR）and elemental analysis.The structures of compounds 35,36 and 38 were verified by X-ray diffraction（XRD）.The results of crystal showed that fluorine-containing polynitroazobenzene compounds have a high crystal density.The crystal density of the compound 34,36,38 is 1.848 g cm^-3（296 K）,1.982g cm^-3（173 K）,1.981 g cm^-3（100 K）,respectively.（2）The quantum chemical calculations are used to predict the solid phase formation enthalpy,detonation velocity and detonation pressure of six fluorine-containing polynitroazo-benzene.The true density,thermal properties,mechanical sensitivity and electrostatic spark sensitivity of the six compounds were measured by automatic true density tester,differential scanning calorimeter,Custer drop weight tester,pendulum tribometer and electrostatic spark sensitivity tester.The results showed that the true density of the six compounds range from1.823 g cm^-3 to 1.921 g cm^-3 at room temperature,and the highest true density of compound 36is 1.921 g cm^-3.The formation enthalpy ranges from-718.9 to 54.4 k J mol^-1,the theoretical detonation velocity is from 6500 to 8500 m s^-1,and the theoretical detonation pressure is from24 to 32 GPa.Compound 33 shows the best detonation performance(D=8318 m s^-1,P=30.41GPa).The thermal decomposition temperature of the six compounds exceeds 250℃.The impact sensitivity(H₅₀)is between 23 and 51 cm,and the friction sensitivity（P）is between 0and 100%.The electrostatic spark sensitivity(E₅₀)ranges from 0.18 to 1.10 J.（3）The molecular structure and detonation performance parameters of six fluorine-containing polynitroazobenzene compounds were compared to investigate the localization effect of fluorine atom and its influence rule and mechanism on detonation performance for azobenzene compounds.The results showed that fluorine atom is ortho-para directing group,and the selectivity for para-position is much higher than that for ortho-position.The introduction of fluorine atom will increase the density and detonation performance of the corresponding azobenzene compounds,but lead to the decrease of formation enthalpy for the corresponding compounds.Moreover,with the increase of the number of fluorine atoms,the density and enthalpy of formation for corresponding azobenzene compounds will also increase and decrease,but the detonation performance will show a certain downward trend with the excess of fluorine atoms.The change of fluorine atom position has little effect on the density,enthalpy of formation and detonation performance for corresponding azobenzene compounds.It is worth noting that the proximity of groups with the same electronegativity will slightly increase their compounds density.（4）The safety performance parameters of six fluorine-containing polynitroazobenzene compounds were compared to explore the influence rule and mechanism of fluorine atom on the safety performance of azobenzene compounds.The results showed that the thermal stability of the corresponding azobenzene compounds will decrease with the introduction of fluorine atom,further more with the increase of fluorine atom number at the nitro ortho site the thermal stability will gradually decrease.The mechanical sensitivity and electrostatic spark sensitivity of the corresponding azobenzene compounds will increase due to the introduction of fluorine atom,but the impact sensitivity will decrease if the planar structure with better symmetry can be formed after the introduction.Fluorine atoms and nitro appear alternately and the more evenly distributed fluorine-containing polynitroazobenzene compounds have better thermal stability,mechanical sensitivity and electrostatic spark sensitivity.The more adjacent groups of the same electronegativity for the polynitroazobenzene compounds will have the worse thermal stability,mechanical sensitivity and electrostatic spark sensitivity.