Progress toward Device-Quality Second-Order Nonlinear Optical Materials:  2. Enhancement of Electric Field Poling Efficiency and Temporal Stability by Modification of Isoxazolone-Based High μβ Chromophores

Isoxazolone (ISX) acceptor based chromophores terminated with hydroxyl functional groups were synthesized, spin cast into optical quality 1−2 μm thick films, poled in the presence of an electric field, and thermoset into polyurethane networks to lock in the poling-induced acentric chromophore order. Alkyl groups were introduced into the chromophore backbone (π-electron bridge) to increase chromophore solubility and to inhibit electrostatic interactions between these “high μβ” chromophores. Thermosetting polyurethanes containing alkyl-substituted chromophores exhibited much (1.7−1.9 times) higher electrooptic coefficients than polymers containing unsubstituted chromophores. Chromophores with hydroxyl groups on only the donor end of the molecule were processed into hardened “themoset” materials, exhibiting a dynamic thermal stability (temperature at which the macroscopic optical nonlinearity is first observed to decrease in a thermal ramping experiment) of poling-induced dipolar order of ∼80 °C, while chromophores with hydroxyl groups on both the donor and acceptor ends display a dynamic thermal stability of poling-induced acentric order of ∼110 °C. Optical loss in these materials originates from the scattering of incident light by chromophore aggregates, rather than from intrinsic (resonant) optical absorption of the chromophores. Alkyl groups in the modified chromophores serve as “bumpers” to sterically prevent aggregation of chromophores driven by strong, spatially anisotropic, intermolecular electrostatic interactions. Steric inhibition of aggregation serves not only to enhance poling efficiency (and thus, electrooptic coefficients) but also to minimize scattering losses.