Structures And Large Nonlinear Optical Responses Of Alkali Metal Doped Molecules

Along with the development of science and technology, photonic materials are very important in many fields, such as optical process of information, optical computer, optical communication, etc. The research of the nonlinear optical materials is an important field of its development. The research of many types of nonlinear optical phenomena bring that the people find the better nonlinear optical materials. The nonlinear optical coefficients are main index of nonlinear optical materials, i.e. high performance nonlinear optical materials have larger first hyperpolarizability.In this thesis, the structures and nonlinear optical properties of the alkali metal doped molecules are investigated. The main contributions are as followings:(1) The static hyperpolarizabilities of the the charge transfer anion-radical salts M+TCNQ·– (M = Li, Na, K) are calculated at ROMP2 method. Three mian conclusions have been found:1) A monotonous dependence on the alkali atomic number of the first (β0) and second (γ0) hyperpolarizability is found. Theβ0 andγ0 values increase with increasing the alkali atomic number. The order of theβ0 values is 17086 (M = Li) < 21198 (M = Na) < 28485 au (M = K), while the order of theγ0 value is 1154000 (M = Li) < 1357000 (M = Na) < 1994000 au (M = K). 2) These anion-radical salts exhibit large first (β0) and second (γ0) hyperpolarizabilities, which are related to the ligand-to-metal charge transfer (LMCT) transitions. In the crucial transition (LMCT), the large difference of electron cloud distributions between HOMO and LUMO correlates to a long-range charge transfer from ligand TCNQ to alkali-metal M, which leads to the large difference of dipole moment between the ground state and the crucial excited state (Δμn0).(2) Doping two alkali atoms into TCNQ forms the radical ion pair salts M2·+TCNQ·– with excess electron. The formation of excess electron in a M2·+TCNQ·– salt can be considered to be divided into two steps. First, one valence s electron is transferred from M2 to TCNQ, and radical ion pair M2·+ and TCNQ·– are formed. Second, the valence s-electron remaining on the M2·+ is pushed out by lone pairs of the two nearby N atoms of TCNQ·– and becomes a diffused excess electron. However, the M+TCNQ·– salts have not electride characteristics. In the M+TCNQ·–, one valence s electron is transferred from M to TCNQ, and M+ and TCNQ·– are formed. No valence s-electron remaining on the M+ is pushed out by lone pairs of the two nearby N atoms of TCNQ·– to become a diffuse excess electron.Interestingly, an alkali atomic number dependence of the first hyperpolarizability is found in M2·+TCNQ·– (M = Li, Na, K). Theβ0 value increases with increasing the alkali atomic number in the order of 19203 (M = Li) < 24140 (M = Na) < 29065 a.u. (M = K). Specially, for the radical ion pair salts M2·+TCNQ·– with excess electron, the second hyperpolarizability is obtained for the first time. These complexes have large second hyperpolarizabilities (γ0) up to 7.9×106 au of K2·+TCNQ·– with alkali metal atoms, which is about 25 times larger than that of TCNQ without alkali metal atoms (3.2×105 au). It shows that the effect of alkali-metal doping on the second hyperpolarizability is conspicuous. In addition, theγ0 value of K2·+TCNQ·– is about 17 times larger than that of the organometallic complexσ-arylalkynyl trans-[Ru(4,4′-C≡CC6H4C≡CC6H4NO2)Cl(dppm)2] and about 9 times larger than that of the intramolecular charge transfer complexσ-arylvinylidene trans-[Ru(4-C=CHC6H4C≡CC6H4NO2)Cl(dppm)2]PF6, while the atom number of K2·+TCNQ·– is only about one-sixth of that ofσ-arylalkynyl orσ-arylvinylidene ruthenium(II) complex. ? Comparison of theγ0 values among these M2·+TCNQ·– (M = Li, Na, K) shows that theγ0 value increases with increasing the alkali atomic number in the order of 2213006 (M = Li) < 3136754 (M = Na) < 7905623 au (M = K). This means that doping the alkali atom with a larger atomic number is also effective for enhancing the second hyperpolarizability.(3) In this work, we designed and systematically studiedthe static and dynamic first hyperpolarizabilities of Li-doped cyclic polyamines (Li-[9]aneN3, Li-[12]aneN4, and Li-[15]aneN5) formed by the interactions of alkali-metal atoms with cyclic polyamine complexants, for the first time, and obtained the recordβ0 value of electride molecules.1) Doping alkali atoms into cyclic polyamines to form loosely bound excess electrons can effectively lower the transitioin energies of crucial excited states and increase the hyperpolarizabilities. Li-doped cyclic polyamines exhibit large static first hyperpolarizabilities (β0 = 52282 ~ 127617 au). Especially, with the same coordination number (four N atoms), theβ0 value of 6.5×104 au for Li-[12]aneN4 is about 9 times larger than that of 7.3×103 au for the corresponding Li@calix[4]pyrrole. The complexant [12]aneN4 is strongly deformed by the chemical doping with Li and exhibits more flexible feature than the complexant calix[4]pyrrole. Thus the higher flexible cyclic polyamines are better than the inflexible calix[4]pyrrole complexant in enhancing the first hyperpolarizability. Furthermore, theβ0 value of 1.3×105 au for Li-[15]aneN5 is about 6 times as large as that of 2.1×104 au for the organometallic complex trans-[Ru(4-C=CHC6H4NO2)Cl(dppe)2]PF6 and close to that of 1.7×105 au for a long dipolar donor-acceptor conjugated organic molecule. It shows that this type of alkalides could be a new member of the large family of nonlinear optical (NLO) materials with different types.2) Theβ0 value increases with increasing the petal number (n) in the order of 52282 (n = 3) < 65505 (n = 4) < 127617 au (n = 5). There is a substantial increase inβ0 due to replacing [9]aneN3 by [12]aneN4 and, particularly, by [15]aneN5, since the interaction between Li and cyclic polyamine and the deformation of the cyclic polyamines increase with the increase of the petal number (or cycle size).3) The frequency-dependentβvalues of the Li-doped cyclic polyamines are given. Results show that the frequency-dependentβ(-ω;ω, 0) andβ(-2ω;ω,ω) (atω= 0.005, 0.01 au) are larger than the corresponding staticβ0. Theβ(-ω;ω, 0) value increases with the increase of frequencyωvalue from 0.0000 to 0.01 au. The frequency-dependentβ(-ω;ω, 0) andβ(-2ω;ω,ω) all show the obvious dependence on the petal number (n) is similar to the case of staticβ0. As a result, our investigation may evoke one's attention to design new material with large NLO responses using the higher flexible complexants.(4) Cis-trans isomerization and spin multiplicity dependences on the static first hyperpolarizability for the two-alkali-metal-doped saddle[4]pyrrole compounds are found.1) For the singlet isomers, theβ0 value of 2.34×105 au for trans-3 is about 16 times enhanced as compared to that of 1.51×104 au for cis-1. For the triplet isomers, theβ0 value of 3.57×104 au for cis-2 is about 10 times enhanced as compared to that of 3.54×103 au for tran-4. These features show the effect of the cis-trans isomerization onβ0.2) For the cis isomers, theβ0 value of 3.57×104 au for triplet-2 is about 2 times larger than that of 1.51×104 au for singlet-1. For the trans isomers, theβ0 value of 2.34×105 au for singlet-3 is about 66 times larger than that of 3.54×103 au for triplet-4. Accordingly, the spin multiplicity significantly affects theβ0 value, especially in the trans isomers.3) In the trans-Li(saddle[4]pyrrole)Na, the high spin state is smaller than low spin state for NLO response, that is triplet-4 (3.54×103) < singlet-3 (2.34×105 au). This reason is that the change of spin multiplicity companies with the characteristic change between alkalide and electride for the trans-Li(saddle[4]pyrrole)Na. So, theβ0 value of the singlet-3 with alkalide characteristic is larger than that of the triplet-4 with electride characteristic.The result demonstrates that the cis-trans isomerization and spin multiplicity controls of the second-order NLO response are possible.