过渡金属偶氮苯衍生物二阶非线性光学性质的密度泛函理论研究

TDDFT Study on Quadratic NLO Properties of Transition-metal Hybrid Azobenzene Chromophores

采用含时密度泛函理论方法(TDDFT)计算了IB族过渡金属及第5周期的金属杂化偶氮苯生色团的二阶非线性光学极化率。研究了金属杂化偶氮苯生色团的电子激发跃迁。结果表明, 与金属离子络合后的杂化偶氮苯生色团, 二阶非线性光学极化率明显改变, 是金属的推拉电子效应的结果。IB族金属的在原有机偶氮苯共轭体系的电荷转移方向的强拉电子效应, 增大了电荷转移范围, 使二阶非线性光学极化率增大。与Nb络合的杂化生色团, 沿原电荷转移相反的方向拉动电荷, 电荷转移跃迁局限于金属离子附近, 未能产生大的激发跃迁偶极矩, 体系的二阶非线性光学响应因而降低。激发跃迁能量是另一个主要影响因素, 同一类跃迁中, 金属杂化生色团的跃迁能量越低, 其二阶非线性光学响应就越大。络合Rh的生色团与Nb杂化生色团类似, 电荷转移跃迁范围都比较小, 同时由于Rh与羧基结合时几乎垂直羧基平面, 电荷同时沿X, Y方向转移跃迁, 属于二维电荷转移类型。

The quadratic NLO hyperpolarizabilities of IB group and some fifth-period metal hybrid chromophores of azobenzene dendrimers were calculated by TDDFT method, and its exciting states were also studied by this method, showing obvious changes in the NLO response of the hybrid chromophores, which are influenced by the electron push-pull effect of the metal irons. If the iron attracts electrons just along the conjugated azobenzene system and concords with the original charge-transfer transition (CT) direction, the charge-transfer scope is enlarged, and its quadratic NLO response is enhanced. On the contrary, if the iron acts as the electron donator on the opposite direction of the origin push-pull system, the charge-transfer transition is baffled, thus the NLO response decreases greatly. The excitation energy is another important factor for the second-order NLO response; the smaller the excitation energy is, the larger the hyperpolarizability will be. Owning to the vertical position of the Nb iron, the charge-transfer of the Nb hybrid chromophore processes both along the x and y axes, belonging to the two-dimensional charge-transfer transition model.