Novel polyurethanes and small molecules for optical second harmonic generation

Abstract

The temperature variations in the chemical shift of proton resonances in 4-amino-4'-nitrodiphenylmethane in toluene solution have been measured. From these results, estimates have been made of the enthalpies and entropies of formation of solute-solvent complexes.

The depolarization of a poled, linear polyurethane was studies using thermally stimulated current (TSC). The TSC rises as the temperature continues to increase, reaches a maximium, and then drops sharply as the randomization of preferentially oriented dipoles becomes complete.

Eight polyurethanes in which chromophores having large quadratic hyperpolarizabilities are covalently attached or crosslinked to the backbone were synthesized. Advantage is taken of the fast kinetics of isocyanate/alcohol reactions to electrically pole the polymer during synthesis. This approach showed that melt polymerization provided particularly superior dipole alignment to the conventional method. The d33 values of poled polyurethanes were obtained shortly after poling and ca. 6 months after poling at room temperature. While the relative second harmonic (SH) intensity of poled, linear polyurethanes gradually or sharply decreased with time at room temperature or at 100 °C, the SH intensity of poled, crosslinked polyurethanes is extremely stable. No relaxation of SH intensity was observed as a function of time after 1 hour at 100 °C.

The compound 4-amino-4'-nitrodiphenylmethane can crystallize in two polymorphic forms, depending upon the solvent from which it is crystallized. The structures of the polymorphs were determined by single crystal x-ray diffraction. Both structures are noncentrosymmetric and one exhibits a modest efficiency of second harmonic generation (SHG), ca. 16xquartz. The other form is relatively inactive, with a maximium SHG efficiency of ca. 0.5 quartz.