Design And Preparation Of Novel Benzothiazole Salt Organic Nonlinear Optical Materials

Nonlinear optical（NLO）materials have attracted considerable interest owing to their wide applications in frequency conversion,terahertz imaging,and optical data storage.Compared to inorganic materials,organic crystal materials exhibit superior nonlinear efficiency and ultrafast response time in electro-optic devices,and possess the advantages of synthetic diversity and design via theoretical simulation,suggesting them as ideal materials for nonlinear optical（NLO）applications.In this thesis,a new ion-type organic NLO material,namely 3,4-dimethoxystyrene-3-methylbenzo[b]thiophene-2-carboxylate p-toluene sulfonate（DOBT）,was designed,synthesized,and investigated.A study on DOBT,covering its design,synthesis,and characterizations,were systematically investigated,containing both theoretical simulations and experimental verifications.The main contents of this work include:（1）Design and simulations:A new D-π-A ion-type organic NLO material of DOBT was designed.3,4-dimethoxy group was introduced as an electron donor to enhance the solubility of DOBT in the solvents like methanol,which is crucial for promoting the crystal growth.Benzothiazole moiety serving as an electron acceptor for DOBT exhibits not only remarkable electron-withdrawing capability,but also enhanced resistance to hydrolysis against ambient conditions.DOBT crystal was simulated and calculated using first-principles and density functional theory.The simulated results revealed that the DOBT single crystal exhibits noncentrosymmetric structure,and it is categorized to the monoclinic system with space group P2₁.The first-order hyperpolarizabilityβ_max value of DOBT is 182×10^-30 esu,～34%higher than that(β_max=136×10^-30 esu)of 4-N,N-dimethylamino-4’-N’-methyl-stilbazolium tosylate（DAST）.These suggest that DOBT would be a high-performance NLO material.The electronic structure and optical property calculations of DOBT were also simulated,by which the contributions of various molecular groups to the crystal’s NLO properties were investigated.Moreover,it was confirmed that DOBT might be easily phase matching and efficient generation of THz waves.（2）Materials characterizations:The as-designed DOBT was further synthesized,and the purified products were characterized by HRMS,FTIR,NMR,elemental analysis,and XRD,respectively.All these analyses confirmed that the products indeed are the ion-type organic NLO material DOBT as we designed.Notably,DOBT single crystals were successfully grown by a slant plate method combined with natural evaporation,thus yielding the single crystals with a size of approximately 7×3×2 mm³.（3）Performance testing:The nonlinear response of DOBT crystals was experimentally measured.The Kurtz-Perry powder method was employed to measure the second harmonic generation（SHG）effect of the resultant DOBT.The experimental results revealed that DOBT exhibits excellent second-order nonlinear properties.Particularly,the SHG signal of DOBT crystal powder reached 3.4 V under the excitation of the laser at 1064 nm and environmental conditions,which is 6.9 times higher than that of DAST crystal powder（0.49 V）under the same measurement conditions.Additionally,DOBT shows high stability in air due to its resistance to water absorption and hydrolysis.All these further verified that DOBT designed in this work is a new and high-performance organic NLO material.