The linear and nonlinear optical (NLO) properties of a series of 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.