Organic electro-optic (OEO) materials integrated into silicon-organic hybrid devices afford significant improvements in size, weight, power, and bandwidth performance of integrated electronic/photonic systems critical for current and next generation telecommunication, computer, sensor, transportation, and defense technologies. Improvement in molecular first hyperpolarizability (β), and in turn electro-optic activity, is crucial to optimizing device performance. Common hybrid density functional theory (DFT) methods, while attractive due to their computational scaling, often perform poorly for optical properties in systems with substantial intramolecular charge-transfer character, such as OEO chromophores. This study evaluates the utility of the long-range corrected (LC) DFT methods for computation of the molecular second-order nonlinear optical response. We compare calculated results for a 14-molecule benchmark set of OEO chromophores with the corresponding experimentally measured β and one-photon absorption energy, λmax. We analyze the distance dependence of the fraction of exact exchange in LC-DFT methods for accurately computing these properties for OEO chromophores. We also examine systematic tuning of the range-separation parameter to enforce Koopmans'/ionization potential theorem. This tuning method improves prediction of excitation energies but is not reliable for predicting the hyperpolarizabilities of larger chromophores since the tuning parameter value can be too small, leading to instabilities in the computation of βHRS. Additionally, we find that the size dependence of the optimal tuning parameter for the ionization potential has the opposite size dependence of optimal tuning parameter for best agreement with the experimental λmax, suggesting the tuning for the ionization potential is unreliable for extended conjugated systems.

A new organic electro-optic (EO) molecule was designed with two modifications aimed at increasing acentric order. The molecule is based on the well-known CLD donor-π bridge-acceptor template. The first structural modification introduces rigid aromatic fluorenyl and naphthyl site-isolation units (sterically bulky functional groups) to reduce aggregation. Site isolation units have been used in the past, but this is the first time that both the "front" and "back" of the CLD tetraene bridge have been modified with site-isolation units, and we had to introduce new synthetic methodology to do so. The second design element was the inclusion of cooperatively interacting aromatic dendron (HD) and fluoroaromatic dendron (FD) side groups to increase the acentric order. HD/FD units have previously been successfully used to increase EO performance, but we changed their location on the chromophore: they are attached to the donor and acceptor ends of the molecule to better match side chain ordering with the dipole moment of the molecule. Comparison chromophores were synthesized with alkyl (-MOM), hydroxyl (-OH), or HD units on the acceptor end of the molecule and either the traditional CLD bridge (T-bridge) or modified bridge (BB-bridge) for a family of eight chromophores. The HD/FD units increased glass transition temperature, T g , by 4−21 °C, and the bulky bridge modification increased T g by 27−44 °C, which is very beneficial as that results in extra thermal stability of the poling-induced acentric order. UV/vis absorbance spectroscopy shows that the site-isolation units reduce aggregation. Unfortunately, poor film formation of the neat materials precluded full chromophore evaluation in poling and r 33 experiments. The EO performance obtained for HD-BB-FD and HD-BB-OH was lower than expected, with r 33 /E p ≈ 1 nm 2 V −2 at 1310 nm. We found that blending in 25 wt % YLD124 improved film-forming and poling efficiency. Due to the effect of blending and improved site isolation, r 33 /E p improved to 2.1−2.3 nm 2 V −2 for 3:1 HD-BB-FD:YLD124, HD-BB-OH:YLD124, and HD-BB-MOM:YLD124, and r 33 as high as 351 pm V −1 was obtained with 3:1 HD-BB-MOM:YLD124. Chromophore blends were also evaluated in plasmonic organic hybrid (POH) phase modulators with slot lengths of 5−20 μm. In POH devices, r 33 was as high as 325 pm V −1 at 1260 nm and 220 pm V −1 at 1520 nm. Overall, the increase in acentric order afforded by the HD/FD interactions was found to be small and resulted in no increase in r 33 due to the reduced number density. Ultimately, the increase in r 33 /E p afforded by the site isolation and blending resulted in a modest increase in r 33 /E p relative to YLD124, but combined with the increased T g , the chromophore system is a significant improvement and points to an important design strategy.

Structure–function relationship study in a series of organic monolithic electro-optic materials has revealed the impact of donor and bridge molecular modification, leading to material with increased EO behavior and improved thermal stability.