Design,Synthesis,and Performance Characterization Of Chiral Schiff Bases/Organic Ionic Second-Order Nonlinear Crystal Devices

Laser technology and terahertz technology have great application prospects in civilian and military fields such as drug identification and explosives detection.At present,the crystal performance of nonlinear optics（NLO）is one of the core elements restricting the development of laser detection technology and terahertz detection technology.Compared with inorganic nonlinear crystal devices,organic NLO crystal devices have the advantages of stable dispersion curve,small absorption coefficient in the THz band,and flexible molecular design,which promotes organic NLO crystals to become the research hotspots of laser and terahertz detection related applications.At present,the controllability to obtain second-order nonlinear crystal devices with non-centrosymmetric structures and large nonlinear coefficients is still a hotspot and difficulty in the field of nonlinear materials.Based on the inherent asymmetry of chiral molecules,the degree ofπconjugate in organic molecules and the type and location of push-pull electron groups affecting the second-order polarization coefficient of organic molecules,etc.,this paper promotes the non-central symmetry of organic crystals due to the existence of optical pure chiral molecules by introducing optical pure chiral sources to organic molecules,and solves the problem of controllable preparation of crystal devices with non-central symmetry;By changing the degree of conjugateπorganic molecules,different substituents and heteroatoms are introduced to regulate the second-order nonlinear polarization intensity of crystal devices.The main experimental results are as follows:1.Ten chiral Schiff base molecules have been successfully synthesized by the reaction of optically pure chiral S-or R-diphenylethylenediamine with a series of aromatic aldehydes through Schiff base.Large crystals of six chiral Schiff base molecules L₁,L₃and L₆have been successfully obtained by solvent evaporation method.X-ray single crystal diffraction data show that S-L₁,R-L₁and S-L₆,R-L₆crystals synthesized by S-or R-diphenylethylenediamine and aldehyde are crystallized in the chiral space group（non-central symmetric space group）,which has the prerequisite for showing second-order nonlinear properties.The racemization of L₃occurred in the synthesis process,resulting in the accumulation of central-symmetric structure in L₃due to the equal amount of S-L₃and R-L₃molecules.The performance tests of L₁and L₆show that they have broad visi-NIR transmission band,good thermal stability,0.3 and 0.4 times KDP second-order nonlinear response,respectively.Therefore,the synthesized Schiff base crystals are potential new organic molecular type second-order nonlinear crystal device materials.2.Using optical pure chiral R-diphenylethylenediamine and a series of aromatic acid compounds through acid-base neutralization reaction to synthesize six kinds of chiral organic ionic compounds（D₁-D₆）,through solvent volatilization method,has successfully obtained large-size single crystals of chiral organic ion compounds D₄,D₅and D₆,X-ray single crystal diffraction data show that chiral organic ion crystals D4,D5 and D6 are crystallized in the chiral space group（non-central symmetric space group）,which has the prerequisite for showing second-order nonlinear performance.Chiral organic ionic compounds D₄,D₅and D₆all have good optical properties and thermal stability,and the second-order nonlinear responses are 0.008,0.08 and 0.5times KDP,respectively.