Quantum Chemistry And Molecular Dynamics Stimulation Study On Nonlinear Optical Properties Of Organic Molecular Materials

The nonlinear optics has got rapid development since the laser was invented in 1960 andthe nonlinear optical phenomenon can’t be studied without the help of materials with highnonlinear optical property. Due to the advantages of organic molecular nonlinear opticalmaterials, such as wider response wave band, high optical damage threshold, easiersynthesizing and modification, one has paid much attention to these materials. At present,the nonlinear optical properties of molecular materials are mainly dependent on the ab-initioquantum chemical theory. Compounds with different structures are designed and theirproperties are theoretically calculated. This method can exhibit a good tendency to describethe optical characteristics of molecules. Because the nonlinear optical properties are usuallyperformed under solvent and molecular aggregation induced by the interactions may occur,the solvent and aggregation effects can not be ignored during the theoretical research. Andthe isomerism, a common phenomenon in chemistry, includes geometrical and rotationalisomer and its influences on two-photon absorption have seldom been discussed so far,which deserves a deep investigation. Moreover, molecular dynamics simulation has beenrapidly developed in recent years, which can analyze the inter-molecular interactionsspecifically in solvents. This method shows many advantages in considering the nonlinearoptical properties for it can effectively simulate the experimental process and provide usefulguidelines in the molecular design and synthesis. Therefore, the combination of quantumchemistry and molecular dynamics simulation will a good assistant to exploremicro-mechanism of macroscopic natures and can perform well in studying molecularproperties, such as organic molecular aggregation and so on.The thesis is based on the quantum chemical theory and molecular dynamics simulation.Isomerism, solvent and aggregation effects on nonlinear optical properties of series oforganic molecules are investigated and the relationships of structure-property are analyzed and discussed. The main contents and conclusions are shown as follows.Ⅰ. Effects of isomerism on two-photon absorption of substituted benzenes with two pairsof donor-acceptorsThe effects of geometrical and rotational isomerism on TPA of several X-shaped moleculeshave been studied employing the quadratic response theory with both the B3LYP andCAM-B3LYP functionals. The solvent effects are taken into account by means of the PCMmethod. It is found that the three types of donor–acceptor substitutions which constitute thegeometrical isomers exhibit quite different TPA properties including the spectral profile, peakposition and intensity. The absorption property depends heavily on the choice of functional.The rotational isomerism induced by rotatable single bonds has been explored and ourcalculations indicate it has a certain effect on the absorption position and intensity.Ⅱ. Aggregation effects on two-photon absorption of charge-transfer molecular clustersThe aggregation effects on two-photon absorption properties of charge-transfer1,3-diamino-4,6-dinitro-benzene (DADB) clusters have been explored by quantum chemicalmethods in combination with molecular dynamics simulations. It can be seen that the TPAspectra of all chosen clusters vary from each other, for they are involved different types ofhydrogen bonds and different dihedrals, which is mainly caused by the distortions due to theaggregation. All dimers and trimers have large red-shifts compared with the spectrum of amonomer, namely their excited energies are reduced and the largest red-shift occurs in trimersas expected. A more important phenomenon is that the TPA activity of most aggregates isdecreased due to the formation of the aggregates.There are seven chapters in this thesis. The first chapter gives a brief introduction to thedevelopment process of nonlinear optics and the research progresses of two-photonabsorption property of organic molecular materials are also presented; The quantumchemical method to study the molecular structure and property is introduced in the secondchapter, including the Hartree-Fock method and the density functional theory; The theory ofmolecular dynamics simulation are introduced in chapter three, including algorithm and themain process, and several applications of MD simulation in nonlinear optics are alsopresented; In chapter four, the calculation methods about nonlinear optical properties areintroduced and the time-dependent perturbation theory and means to calculate the two-photon absorption cross-section are involved; In the fifth chapter, we discuss the effectsof isomerism on two-photon absorption of substituted benzenes with two pairs ofdonor-acceptors; In the sixth chapter, the aggregation effects on two-photon absorption ofcharge-transfer molecular clusters are presented; The summary and prospect are given in thelast chapter.