New symmetrical pi-conjugated molecules having large third-order optical nonlinearities

New symmetrical pi-conjugated molecules having large third-order nonlinear susceptibilities, χ(3), are reported. The χ(3) spectra of these molecules have been measured by a THG Maker-fringe method. The magnitudes of the three-photon resonant χ(3) reach 10−10 esu. The positions of the three-photon resonance lie at the lowest-energy peaks in the vibrational structure of each absorption spectra for these molecules.