Synthesis, Characterization Of Conjugated Schiff Bases And Their Iron Salts On Microwave Absorption Performance

Conjugated Schiff base is a kind of two-dimensional planar compound with large π bond in the main chain.The main structure is composed of C=N double bond.The electron cloud of this kind of compound is uniformly distributed and its domain is strong,which can be modified by chemical or electrochemical methods to further expand the conjugation system.The absorbing material is a material capable of absorbing electromagnetic waves,with a strong absorption and loss of echo signals to reduce the possibility of detection,which can be applied to a wide range of different areas of the band.The conjugated Schiff base uses the planar structure of its conjugated electrons to change or enhance the conductive structure by means of doping or coordination,so as to achieve impedance matching and attenuation matching,finally achieve the purpose of absorbing wave.Based on this principle,three different systems of conjugated Schiff bases and their iron salts were designed and synthesized.The essence of the method was also explored by experimental and theoretical study.The main work is reflected in the following aspects:A total of 18 substances of 2-aminofluorene bis-Schiff bases（L₁ L₃）and their iron salts（C₁ C₃）,melamine tri-Schiff bases（L₄ L₆）and their iron salts（C₄ C₆）and polymer Schiff bases（L₇ L₉）and their iron salts（C₇ C₉）were designed and synthesized.The structure and characteristic were characterized by mass spectrometry（MS）,gel permeation chromatography（GPC）,infrared spectroscopy（IR）,thermogravimetric analysis（TD/DTG）,ultraviolet-visible spectroscopy（UV-Vis）,photoluminescence（PL）and other methods.The conductivity of these 18 substances were measured by a four-probe tester.The results of system I showed the laws of L₂ > L₃ > L₁ and C₂ > C₃ > C₁,system II showed L₄ >L₆ > L₅,C₄ > C₆ > C₅,and system III showed the law of L₇ > L₈ > L₉,C₇ > C₈ > C₉.These laws showed that the conductivity can be increased by 2 3 orders of magnitude after salt formation,and each system also showed the law of conjugate ability proportional to the conductivity.The absorbing ability of these 18 substances were measured by vector network analyzer.The results showed that the maximum reflection losses of Schiff bases in the three systems were L₂,L₄ and L₇,respectively,which were-3.157 dB（f = 10.4 GHz,d = 5mm）,-10.918 dB（f = 12.16 GHz,d = 4 mm）and-6.592 dB（f = 15.04 GHz,d = 5.5 mm）.While the maximum reflection loss of iron salts in the three systems were C₂,C₄ and C₇,respectively,which were-27.229 dB（f = 16.72 GHz,d = 5 mm）、-21.783 dB（f = 12.56 GHz,d = 5 mm）and-17.482 dB（f = 12.24 GHz,d = 3.5 mm）.The density functional theory（DFT）was used to optimize the geometric structure of the nine Schiff bases.The quantitative chemical structure-activity relationship was established by SPSS Statistics 19.0 software through the stepwise multiple linear regression method.The results show that the bond length of C=N,conductivity and some important atomic charge are the main factors affecting the microwave absorption performance.