(DICYANOMETHYLENE)PYRAN DERIVATIVES WITH C-2-NU SYMMETRY - AN UNUSUALCLASS ON NONLINEAR-OPTICAL CHROMOPHORES

A series of symmetric derivatives of ene)-2-methyl-6-(p-(dimethylamino )styryl)-4H-pyran (DCM laser dye) has been synthesized, and the nonlin ear optical properties and thermal decomposition temperatures of each of the chromophores has been measured. The symmetric chromophores all have larger hyperpolarizabilities than one would expect based on their absorption maxima. Estimations of their oscillator strengths based up on UV/vis spectra are all well above 1.0. It is proposed that these co mpounds possess two excited electronic states close to each other in e nergy, both contributing to the hyperpolarizability in an additive fas hion. Electro-optical absorption spectroscopy data and semiempirical c alculations on a model compound support this hypothesis. The implicati ons of these results with regarded to the tradeoff between nonlinearit y and transparency are discussed. The large dipole moments and high de composition temperatures of these chromophores make them attractive ca ndidates for use in electro-optic polymers.