Ultrahigh Electro-optic Coefficients, High Index of Refraction, and Long-term Stability from Diels-Alder Crosslinkable Binary Molecular Glasses

The development of organic electro-optic (EO) materials that concurrently possess a high electro-optic coefficient (r(33)), high index of refraction, and long-term or high-temperature stability of chromophore alignment has been a crucial goal. To address this challenge, we developed a crosslinkable EO system consisting of two chromophores, HLD1 and HLD2, which can be electric field poled and then thermally crosslinked in situ to form a stable EO material. This approach avoids the necessity for nonlinear optically inactive materials such as polymers or small molecule cross-linkers, thus resulting in high chromophore density (>5 x 10(20) molecules/cm(3)) and high index of refraction (n = 1.89 at 1310 nm) for HLD1/HLD2. Different ratios of HLD1 and HLD2 were evaluated to optimize poling efficiency and thermal stability of the poling-induced order. With 2:1 HLD1/HLD2 (wt/wt), a maximum r(33) of 290 +/- 30 pm/V was achieved in a cross-linked film. Thermal stability tests showed that after heating to 85 degrees C for 500 h, greater than 99% of the initial r(33) value was maintained. This combination of large EO activity, high index of refraction, and long-term alignment stability is an important breakthrough in EO materials. HLD1/HLD2 can also be poled without the subsequent cross-linking step, and even larger maximum r(33) (460 +/- 30 pm/V) and n(3)r(33) figure of merit (3100 +/- 200 pm/V) were achieved. Hyperpolarizabilities of HLD and control molecules were analyzed by hyper-Rayleigh scattering and computational modeling with good agreement, and they help explain the high acentric order achieved during poling.