The linear and nonlinear optical (NLO) properties of a series of $\text{Λ}$-shaped derivatives containing a 4,5-dicyanopyrazine acceptor unit, N,N-dimethylamino donor groups and systematically enlarged π-conjugated linkers are investigated by means of UV/Visible and Hyper-Rayleigh scattering spectroscopies. Density functional theory calculations are also carried out to rationalize the magnitude and symmetry of the NLO responses. The results demonstrate that these compounds possess two low-lying excited electronic states close to each other in energy, which are accessible through one-photon optical transitions respectively polarized perpendicular and parallel to the two-fold molecular axis. These two states contribute additively to the first hyperpolarizability, which exhibits a dominant dipolar character. We also show that the NLO responses significantly deviate from Kleinman’s index permutation symmetry.

一系列的线性和非线性光学（NLO）特性 $\text{Λ}$4,5-二氰基吡嗪受体单元，N，N的异形衍生物通过UV /可见光和超瑞利散射光谱学研究了-二甲基氨基供体基团和系统扩大的π共轭连接基。还进行了密度泛函理论计算以合理化NLO响应的幅度和对称性。结果表明，这些化合物在能量上具有两个彼此靠近的低激发态电子态，可以通过分别垂直于和平行于两倍分子轴极化的单光子光学跃迁来访问它们。这两个状态对第一超极化性有附加作用，该超极化性表现出显着的偶极特性。我们还显示，NLO响应明显偏离Kleinman的索引排列对称性。