Theoretical Studies Of The Nonlinear Optical Properties Of The Electrides And Alkalides

In today's high-technology world, photonic materials are very importantin many fields, such as optical process of information, optical computer,optical communication, etc. The research of the nonlinear optical materials isan important field of its development. Our investigations find that the excesselectron is a new important influencing factor that can effectively increase thefirst hyperpolarizabilities of nonlinear optical materials. It provides a new ideato effectively increase the nonlinear optical response of nonlinear opticalmaterials, that is, the nonlinear optical response can be greatly increasedthrough introducing the excess electron into the system, which challenges theconventional method to design nonlinear optical materials. In this thesis, thestructures, molecular orbitals and nonlinear optical properties of thecompounds containing the excess electron are investigated by the quantumchemical methods-ab initio and density functional theory. The maincontributions are as followings:(1) The nonlinear optical properties of the dipole bound anion (H₂O)₃^ˉcontaining the excess electron are calculated. The calculational results showthat the water trimer anion owns the very large first hyperpolarizability, whichis more than 10⁷ au. The dipole-bound excess electron is the key factor in theextraordinary first hyperpolarizability of (H₂O)₃^ˉ species. In addition, theeffects of the applied electric field, diffuse basis set, bond function andelectron correlation on the first hyperpolarizability of the species with thedipole-bound excess electron are studied.(2) The nonlinear optical properties of (HCN)nLi and Li (HCN)n (n=1, 2,3) clusters with the electride characteristic are investigated in detail, includingthe effects of the applied electric field, diffuse basis set and electroncorrelation on them. These clusters have considerably large nonlinear opticalresponses ( β 0>103¹04au). In these clusters, because of the interactionbetween Li atom and (HCN)n cluster, the 2s valence electron of the Li atombecomes the loosely-bound excess electron. The excess electron from the Liatom plays an important role in the large first hyperpolarizability. Threeconclusions are obtained: 1) The polarizabilities and first hyperpolarizabilitiesof these clusters are large comparing with some traditional NLO matters. Suchlarge values are dominated by the Li atom. 2) The first hyperpolarizabilities of(HCN)n···Li are larger than those of Li···(HCN)n (n=1, 2, 3). 3) For (HCN)n…Li(n=1, 2, 3) system, the absolute values of the first hyperpolarizability decreasewith the length of the HCN chain. Oppositely, for Li…(HCN)n (n=1, 2, 3)system, the absolute values of the first hyperpolarizability increase with thelength of the HCN chain. The investigation shows that the electride compoundis a kind of promising high-performance nonlinear optical materials.(3) The nonlinear optical properties of the alkalide compoundsAdzH+Naˉ and AdzLi+Naˉ are studied. AdzH+Naˉ has very large firsthyperpolarizability to be 5.7675×104 au and the first hyperpolarizability ofthe analog AdzLi+Naˉ is 6.20681×105 au. We find that the Naˉ anion play acrucial role in the large first hyperpolarizability of this kind of compounds,which is rationalized by the two-level expression. In these systems, the chargevalue of the Naˉ anion has a large effect on the first hyperpolarizability ofthese compounds. The relationship is that the first hyperpolarizabilityincreases with the charge value of the sodium anion in these compounds. Twoapproaches can be used to enhance the charge value of the sodium anion. First,the charge value of Naˉ has an increase via changing complexants (the Adzcage replacing the (Me)3N). Second, the charge value of Naˉ has a furtherincrease when the Li+ cation replaces the H+ cation.(4) A new electride compound, Li@calix[4]pyrrole, is designed in theory,which resembles a cup-like shape. It may be a stable organic electride at roomtemperature. Its structure, spectra and nonlinear optical properties are obtained.This new compound has considerably large first hyperpolarizability to be 7326au, which is almost 20 times larger than that of calix[4]pyrrole ( β 0=390 au).When the Li atom is inserted in the calix[4]pyrrole molecule, the 2s electronfrom the Li atom is ejected out as the excess electron under the action of thelone pairs of four N atoms. It will decrease the excited energy in the crucialtransition and naturally results in a larger first hyperpolarizability. Thisinvestigation shows that the first hyperpolarizability can be greatly increasedby the alkali atom inserting into the polar cryptand molecule to form anelectride.(5) A new type of alkalide compounds, Li+(calix[4]pyrrole)Mˉ (M=Li,Na and K), is presented in theory. It has been shown by our calculations thatthe first hyperpolarizability is considerably large by means of the B3LYP, HFand MP2 methods. Comparing to the cryptand calix[4]pyrrole, the β 0value ofLi+(calix[4]pyrrole)Mˉ are enhanced by 20-60 times. We find that, comparingto Li+(calix[4]pyrrole)eˉ, the alkali atom aion is more effective to enhance thefirst hyperpolarizability of the molecule because it decreases the excitedenergy and also increases the oscillator strength of the main transition. Inaddition, we also find that, in these compounds, the β 0value depends upon theatomic number of the alkali anion, and can be enhanced by choosing the alkalianions with larger atomic numbers.