Theoretical Studies Of The Electronical Structures And Nonlinear Optical Properties In Nano-Molecules

As for the various applications from the Nonlinear Optical (NLO) materials, for example, optical computer, optical communication, optical process of information, the investigation of NLO is one of the hottest topics of the world at the present. Among these studies, the design and search of high-performance NLO materials are an important part. On the other hand, novel nanomolecules, for example fullerences, nanotube, have attracted lots of attentions, due to their unique electronic structures and properties. In the study of NLO materials, the physical parameter, the first hypoarizability is the microscopic origin of the second order of NLO materials, and is an important index for evaluating whether the NLO materials are high-performance or not. To explored novel nanomolecules with large NLO response, in this paper, we theoretically calculated the unique structures and properties of the representative nanomolecules, and revealed the relationships between the structures and properties.In this thesis, the main contributions are followings:1. Isomer structures of four endohedral fullerene dimers Na@C60C60@F with n-fold bond (n=1, 2, 5 and 6) covalent bond between two cages and long distance ion bond between Na and F are obtained for the first time by using density-functional theory (DFT). Further, the electronic properties of the endohedral dimers are first investigated. The dimers exhibit strong nonlinear optical (NLO) response—large static first hyperpolarizabilities (β0). The large first hyperpolarizabilitiy is up to 25169 au for [5+5], which is almost 110 times larger than that of NaF molecule (228 au). This provides a novel strategy for enhancing first hyperpolarizability by altering molecular structure. Moreover, it is found that the first hyperpolarizability and crucial charge transfer transition depend on the dimeric pattern, that is to say, modulating the first hyperpolarizability and crucial charge transfer transition by controlling the dimeric pattern.2. The proton transfer reaction in the complex H3N···HCl forming the ion pair NH4+Cl?, which is favored inside the C60 cage, is reported. The calculated result shows that the obtained NH4+Cl?@C60 is stable with the interaction energy to be -2.78 kcal/mol. Moreover, C60 cage plays the role as a catalyzer for the proton transfer. The catalysis of the C60 cage comes from two effects: i) the electrostatic inducement between C60 cage and endohedral molecules and ii) the confinement effect to compress endohedral molecular structures inside C60 cage. In addition, it is found that the confinement effect of the cage can cause the large blue shifts of N-H stretching vibrations in NH4+Cl?@C60 comparing with that in NH4+Cl·?··H2O complex.3. Lithium salt of substituted nanotube is suggested as a new kind of nonlinear optical (NLO) material. It is found that the combination of the lithiation effect and the donor substitution effect can greatly enhance the static first hyperpolarizability (β0). Specially, the unusual donor—acceptor(nanotube)—donor system Li-NT(6,0)-(NH2)2 has considerableβ0 to be 6.8 x105 au, due to its small transition energy and large transition moment. Further, the relationships betweenβ0 and the number of Li atoms m and the substituents R for Lim-NT(6,0)-(R)2 (m=1, 6 and R= -CN, -NH2) systems are explored.4. The structures of Li2 molecule doping boron nitride nanotube BNNT(n,0) (n=4, 5, 6, 7 and 8) (Li2@BNNT) are obtained. The natural bond orbital (NBO) charges show that the charge transfers are from Li2 molecule to BNNT, except for Li2@BNNT(8,0). Specially, the diameter effect of the tube on the dipole moment (μ0), polarizability (α0) and first hyperpolarizability (β0) has been studied systematically. Both the dipole moment and polarizability increase with the increasing of the tube diameter (n). Note that the static first hyperpolarizability (β0) is not monotonic with tube diameter, the order of first hyperpolarizability (β0) is 2716 (n=4) < 6274 (n=7) < 7939 (n=8) < 12392 (n=5) < 13918 au (n=6), which indicates that the choosing suitable tube diameter is important to obtain largeβ0 values. Moreover, it is found that the large diameter tube BNNT(8,0) can trap the excess electron of the Li2 molecule, which indicates that Li2@BNNT(8,0) is a novel electride compound. These results may widen the knowledge and potential applications of boron nitride nanotube in nonlinear optical (NLO) materials.In the thesis, the unique structures and properties of the representative nanomolecules are theoretically calculated, and the relationships between the structures and properties are revealed. The resules show that, the doped, modified novel nanomolecules have large first hyperpolarizabilit, and suggest that these novel nanomolecules can be potential high-performance NLO materials. Hence, the thesis plays some important role in the studies and explorations of novel NLO nanomaterials.