Progress toward Device-Quality Second-Order Nonlinear Optical Materials. 4. A Trilink High μβ NLO Chromophore in Thermoset Polyurethane: A “Guest−Host” Approach to Larger Electrooptic Coefficients

A novel trilinkable thiophene-containing second-order nonlinear optical (NLO) chromophore [(HO) 3 FTC] was synthesized from a trilinkable donor bridge and a tricyanofuran electron acceptor (TCF). The TCF acceptor was modified with two butyls which greatly increased solubility and processability of the trihydroxy-functionalized chromophore and inhibited strong chromophore−chromophore interaction. A thermal stability study of (HO) 3 FTC indicates that the free hydroxyl group located close to the cyano acceptor causes the chromophore to decompose at a much lower temperature than FTC chromophores with no free hydroxyl groups. Significantly improved thermal stability of the chromophore in polyurethane film was obtained by masking the free hydroxyl groups with toluene diisocyanate. New polyurethane prepolymer synthetic schemes were designed and studied in detail to improve electric field induced dipole alignment. Enhancement of over 150% in poling efficiency was achieved by reducing the degree of chromophore attachment to polymer backbone before applying an electric poling field through a “guest−host” approach. It was found to be critical to allow isocyanate and hydroxyl cross-linkers to react at higher temperature for a longer time to form a partially cross-linked prepolymer before the −NCO masked trilink chromophore was added. By anchoring chromophores to a three-dimensional cross-linked polyurethane network at three points, the thermal stability of poling-induced electrooptic activity was enhanced by 33 °C.