有机电光晶体4-(4-二甲基氨基苯乙烯基)甲基吡啶对甲基苯磺酸盐的太赫兹光谱研究

采用色散校正的密度泛函理论(dispersion-corrected density functional theory,DFT-D2)对有机电光晶体4-(4-二甲基氨基苯乙烯基)甲基吡啶对甲基苯磺酸盐(4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate,DAST)进行结构优化和太赫兹光谱计算,通过逐步提高精度进行几何优化的方法寻找DAST收敛的基态稳定结构,获得与DAST初始结构相一致的基态稳定结构.在此结构的基础上,在0—4 THz范围的太赫兹计算光谱与实验测量结果一致,说明采用DFT-D2进行优化的合理性.重要的是,首次通过计算的太赫兹光谱对DAST在0—4 THz范围的太赫兹吸收峰的振动模式进行了详细归属.结果表明:1.12 THz处的振动是DAST阴阳离子的光学声子模式,1.46 THz和1.54 THz两处的振动主要与磺酸盐有关,而2.63 THz和3.16 THz两处的振动则分别源于阳离子的扭转振动和阴离子的转动.该结果不仅很好地说明了阴阳离子分别在太赫兹响应中的贡献,而且为今后通过取代阴阳离子基团获取具有更高二阶非线性效应的DAST衍生物的新合成提供了重要的参考和指导.本文结果说明密度泛函理论在太赫兹光子学上的重要应用,对探究有机电光晶体的太赫兹响应物理原理、性能控制等具有重要的指导价值.

Terahertz spectrum study of organic electro-optic crystal 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate

The ground-state structural optimization and the terahertz spectrum calculation of an organic electro-optical crystal of 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST) are performed using dispersion-corrected density functional theory (DFT-D2). DAST consists of an organic pyridinium salt (cation), one of the most efficient non-linear optical active chromophores and a sulfonate (anion) for enhancing the stability of the noncentrosymmetric macroscopic crystal. Such an organic crystalline salt DAST exhibits highly electro-optical and nonlinear optical coefficients, and it is an efficient emitter of THz pulses. The steady ground-state structure of DAST is obtained by a step-by-step optimization method with gradually increasing the convergence accuracy. The calculated terahertz spectra in 0-4 THz are in good agreement with experimental measurements, implying the reasonability of DFT-D2 method. Moreover, the vibration displacement vector diagrams for DAST molecular structure are obtained using Cambridge sequential total energy package animation simulation function. The results indicate that the phonon modes of DAST crystal at 1.12 THz are attributed to the optical phonon modes of the anion and cation, and DAST cation (organic pyridinium salt) and anion (sulfonate) undergo translational vibrations in their respective (benzene ring) plane. In contrast the vibrations at 1.46 THz and 1.54 THz are mainly related to the vibration of the sulfonate, among which 1.46 THz vibration is caused by the rotation of the sulfonate along the a axis, while 1.54 THz is due to the motion of the whole sulfonate along the c axis. And the vibrations at 2.63 THz and 3.16 THz originate from the torsional vibrations of cations and the rotation of anions, respectively. The results presented in this work clearly illustrate the contributions of the anion and cation of DAST in the THz responses. The mode assignments provide important reference and guidance for further synthesis of new DAST derivatives with larger electro-optical coefficients. In particular, our results suggest that DFT method is a powerful theoretical tool for studying the THz photonics and it is helpful not only for better understanding the mechanisms of the THz responses of organic electro-optic crystals, but also for controlling their performances.