GONG Xue WEI Hang LUO Kai-Jun LI Quan

(Key Laboratory of Advanced Functional Materials, Sichuan Province Higher Education System,College of Chemistry and Material Science, Sichuan Normal University, Chengdu 610066, China)

1 INTRODUCTION

Nonlinear optics (NLO) is involved in the study on the nonlinearity produced under the action of strong coherent light and the application of nonlinearity. NLO materials are the important basic materials for the optoelectronic industry, and their potential applications in communication, data storage, image processing, all-optical switching, optical storage and memory system are worth much attention, and therefore research of new materials with strong nonlinear optical properties and high responding speed has become one of the important tasks in this field[1-9]. Molecules of organic complexes containing transition metals possess excellent nonlinear optical properties, and they integrate characters of inorganic and organic NOL materials together and are characterized by such advantages as ample structures, polyvalence of transition metals and easy modification of ligands, and attract increasing attention in respect of the study on the functional material[10-13]. The third-order optical nonlinearity does no imposition requirement on the structures of non-centrosymmetric molecules, the action of electron transition in molecules, and macro orderliness, so the prospect of the study on the third-order NLO materials is brilliant.

In recent years, the application of phosphorescent materials made of cyclometalated platinum complexes in organic light-emitting diode (OLED) attracts wide attention.

Related researches demonstrate that the introduction of heavy metal platinum into cyclometalated platinum complexes produced a strong spin-orbital conjugate action, and therefore complexes manifest better optical properties[14]. In the experimental synthesis, our research group introduced rigid camphor ring to the β-diketonate carbonyl ring, and till now 1～9 complexes have been synthesized as shown in Fig. 1[15]. Previous theoretical studies demonstrated that camphor-derived β-diketonate platinum complexes possessed good second-order nonlinear optical properties. The introduction of electron-donating group to the β-diketonate carbonyl ring and the introduction of electron-donating group to the ligand benzene ring would endow these complexes with better second-order nonlinear optical properties[16]. Thereby, in the current study we changed the property of substituent in β-diketonate carbonyl ring and the position of substituent in ligand to simulate the ultraviolet-visible absorption spectrum and properties of excited state of the complexes through theoretical calculation, to calculate their third-order nonlinear optical properties and discuss the influence of the introduction of substituent on the third-order nonlinear optical pr perties.

2 CALCULATION METHODS

At present, density functional theory (DFT) has become an effective method used to calculate the property of NLO materials containing transition metals. In the current study, the DFT B3LYP method[18]was used in the Gaussian 03 program[17].Considering such factors as relativistic effect of electrons of atomic kernel and more electrons of metal atoms in high periods, we adopted LANL2DZ groups and effective core potential[19]for metal atoms and adopted 6-31G* groups for non-metal atoms to optimize the structure of β-diketonate platinum complexes and calculate their frequencies to get their stable configurations. Based on the above result, TD-B3LYP method was adopted to simulate the UV-Vis spectrum of them through calculation.

The energy of a molecular system in a uniform electrostatic field may be evolved pursuant to Taylor series using finite field (FF) method by the following formula[20]:

In the above formula, the subscript is Cartesian coordinate. βijkrepresents the component of the tensor of the first-order hyperpolarizability (the second-order nonlinear optical properties), and γijklshows the component of the tensor of the secondorder hyperpolarizability (the third-order nonlinear optical properties). The total molecular energy under electronic fields of different intensities was calculated to get an equation set that was solved by the formula in reference[18]through FORTRAN language programming to get all components of nonlinear optical properties and the most stable γ values for all molecules at the field intensity of 0.003 a.u. Then βμ,the first-order hyperpolarizability in the direction of dipole moment was calculated using the following formulas:

In the above formulas, μx, μyand μzrepresent respectively the component of dipole moment in the direction of x, y and z; βirepresents the component of the first-order hyperpolarizability in the direction of i; βikkshows the component of the third-order tensor, and γxxxxis the component of the fourth-order tensor of the second-order hyperpolarizability. The third-order nonlinear optical coefficient γ was calculated by applying these components in the following formula.

3 RESULTS AND DISCUSSION

3.1 Electron absorption spectrum

After optimizing the structure, TD-B3LYP was adopted to perform calculation to simulate electron absorption spectra of 9 camphor-derived β-diketonate platinum complexes in Fig. 1, as shown in Figs. 2 and 3. Properties of the excited state, absorption wave-length λ, electronic excitation energy E,oscillator strength f, and the major contribution of orbital transition to absorption are shown in Table 1.The data of the calculated spectra deviated greatly from those measured in experiments, but the former may still be used in the qualitative study.

Table 1. Data of Electron Absorption Spectra of Camphor-derived β-diketonate Platinum Complexes

Fig. 1. Formula of the chemical structure for the targeted molecules

Fig. 2. UV-Vis absorption spectra of complexes 1～4

Fig. 3. UV-Vis absorption spectra of complexes 5～9

From Fig. 2, it may be known that the strongest absorption peak λmaxof camphor-derived β-diketonate molecules containing electron-donating group-CH3as substituent appeared at 266.5 nm, and the absorption peak associated with the lowest energy transition appeared at 408.8 nm. Taking the aforesaid as reference, the introduction of electrondonating group -C6H5and electron-drawing group-CF3caused the strongest absorption to shift to the red side of spectrum for 16.5 and 26.5 nm respectively, but the introduction of electron-donating group -C6H5increased remarkably the absorption strength, and the spectrum of compounds containing-C3F7was similar to that of compounds containing-CF3. The introduction of -C6H5imposed only a weak influence on the lowest energy absorption, and other two electron-drawing groups made the spectrum shift to the blue side of spectrum for 10 nm or so.

From Fig. 3, it may be known that in contrast with molecule 1, the introduction of 3 electron-donating groups (-C2H5, -C2H3, and -OC2H5) increased the strength of the strongest absorption. The introduction of -OC2H5and -C2H5caused red shift, and the introduction of -C2H3caused blue shift. The strongest absorbance of -OC2H5on pyridine ring was greater than that of -OC2H5on the benzene ring. The concurrent introduction of -OC2H5to the pyridine and benzene rings both lowered the absorbance instead. The introduction of -C2H3enlarged the conjugate plane to ease the transition, so that the lowest energy absorption shifted to the red spectrum for 11.1 nm. The introduction of -OC2H5caused the lowest energy absorption to shift to the blue spectrum.

3.2 Analyses of the frontier molecular orbital

Electron transition correlates closely to nonlinear optical properties, especially for the lowest energy transition. Fig. 4 shows the highest occupied molecular orbital (HOMO) and the lowest empty orbital(LUMO) of molecules 1～9; for all those molecules,camphor-derived β-diketonate ligand transited to benzene and pyridine rings, belonging to π → π*(LLCT), and electrons of the central metal atom transited to benzene and pyridine rings, belonging to n→π* (MLCT). From LUMO figures of molecules 3,4, 5 and 7, it may be seen that the electron cloud still distributed over the β-diketones. In molecules 3 and 4, it is obvious that the introduction of electrondrawing group limited the LLCT process; the stronger the electron-drawing ability, the greater the limitation of transition, which was disadvantageous to nonlinear optical properties. The comparison between molecules 5 and 7 revealed that the electron cloud distributing over β-diketones in LUMO of molecule 7 was thinner, indicating that the introduction of -OC2H5to the benzene ring was advantageous to electron transition. The introduction of electron-donating group enhanced remarkably the density of electron cloud over HOMO to increase further the transited charges, which was advantageous to nonlinear optical properties.

Fig. 4. Frontier molecular orbitals of the complexes

3.3 The third-order nonlinear optical properties

Table 2 demonstrates that the third-order nonlinear optical coefficient mainly consisted of components in the directions of x and y, i.e., was contributed by the molecular plane. Comparisons among complexes 1～4 revealed that the secondand third-order nonlinear optical properties of βdiketonate platinum complexes containing electron-donating substituent such as -CH3and -C6H5were better than those of β-diketonate platinum complexes containing electron-drawing substituent such as -CF3and -C3F7, and the second-order nonlinear optical properties of complexes containing substituent -C6H5were slightly weaker than those of complexes containing substituent -CH3, which may be attributed to the symmetry, whereas the thirdorder nonlinear optical properties of the former were intensified noticeably. Therefore, the introduction of electron-donating group to R helped to intensify the nonlinear optical properties of β-diketonate platinum complexes, and the introduction of electron-drawing group gave rise to the contrary effect. Analyses of data about molecules 5～7 showed that the secondand third-order nonlinear optical properties of complexes may be intensified remarkably by increasing the number of electron-donating substituent or regulating the position of the same substituent (the electron-donating ability of -OC2H5on pyridine ring was stronger than that of -OC2H5on the benzene ring). The introduction of -C2H3intensified the electron transition and enlarged the conjugate plane,giving rise to the best nonlinear optical properties. It may be expected that the concurrent introduction of benzene ring and vinyl group to R may produce molecules with excellent nonlinear optical properties.

Table 2. Dipole Moment (μ: Debye), Second-order (βμ: 103 a.u.)and Third-order Nonlinear Optical Properties (γ: 105 a.u.) of the Complexes

4 CONCLUSION

In this article, we report our study on UV-Vis spectrum and the third-order nonlinear optical properties of the targeted complex, and our results demonstrate that for 9 molecules under study, the lowest energy absorption was associated with the transition of camphor-derived β-diketonate ligand and the electrons of central metal atoms to the benzene and pyridine rings, belonging to LLCTMLCT mixed transition. The introduction of electron-drawing group to R would limit the electron transition and therefore would be disadvantageous to nonlinear optical properties, which was contrary to the introduction of electron-donating group. As compared with the introduction of electron-donating group to the pyridine ring, the introduction of electron-donating group to benzene ring would give rise to better molecular nonlinear optical properties.The enlargement of conjugate plane may intensify more effectively molecular nonlinear optical properties; in the current study, molecule 9 posses-sed the best nonlinear optical properties through the introduction of vinyl group.

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