Progress toward device-quality second-order nonlinear optical materials: 2. Enhancement of electric field poling efficiency and temporal stability bymodification of isoxazolone-based high mu beta chromophores

Isoxazolone (ISX) acceptor based chromophores terminated with hydroxyl func tional groups were synthesized, spin cast into optical quality 1-2 mu m thi ck films, poled in the presence of an electric field, and thermoset into po lyurethane networks to lock in the poling-induced acentric chromophore orde r. Alkyl groups were introduced into the chromophore backbone (pi-electron bridge) to increase chromophore solubility and to inhibit electrostatic int eractions between these "high mu beta" chromophores. Thermosetting polyuret hanes containing alkyl-substituted chromophores exhibited much (1.7-1.9 tim es) higher electrooptic coefficients than polymers containing unsubstituted chromophores. Chromophores with hydroxyl groups on only the donor end of t he molecule were processed into hardened "themoset" materials, exhibiting a dynamic thermal stability (temperature at which the macroscopic optical no nlinearity is first observed to decrease in a thermal ramping experiment) o f poling-induced dipolar order of similar to 80 degrees C, while chromophor es with hydroxyl groups on both the donor and acceptor ends display a dynam ic thermal stability of poling-induced acentric order of similar to 110 deg rees C. Optical loss in these materials originates from the scattering of i ncident light by chromophore aggregates, rather than from intrinsic (resona nt) optical absorption of the chromophores. Alkyl groups in the modified ch romophores serve as "bumpers" to sterically prevent aggregation of chromoph ores driven by strong, spatially anisotropic, intermolecular electrostatic interactions. Steric inhibition of aggregation serves not only to enhance p oling efficiency (and thus, electrooptic coefficients) but also to minimize scattering losses.