Structure Effect of Squarylium Cyanine Dyes on Third-Order Optical Nonlinearities in Ground and Excited States

A series of squarylium cyanine dyes with different substituents were synthesized and the third-order optical nonlinearities of their ground and excited states were investigated by backward degenerate four-wave-mixing. For the ground state, the molecular hyperpolarizability γ_g increases with the red-shift of the absorption peak λ^ab_max, of the squaraine with different substituents, whereas for the excited-state molecular hyperpolarizability γ_e, the nonlinear enhancement γ_e/γ_g decreases, which may indicate that in the excited state the electron accepting-donating ability of different substituents changes in the reverse order compared with the order in the ground state.