Second-order nonlinear optical properties of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"><mml:mtext>?</mml:mtext></mml:math>-shaped pyrazine derivatives

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