1 research outputs found
Effect of Rigid Bridge-Protection Units, Quadrupolar Interactions, and Blending in Organic Electro-Optic Chromophores
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, <i>T</i><sub>g</sub>, by 4–21 °C, and the bulky bridge modification
increased <i>T</i><sub>g</sub> 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 <i>r</i><sub>33</sub> experiments.
The EO performance obtained for HD-BB-FD and HD-BB-OH was lower than
expected, with <i>r</i><sub>33</sub>/<i>E</i><sub>p</sub> ≈ 1 nm<sup>2</sup> V<sup>–2</sup> 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, <i>r</i><sub>33</sub>/<i>E</i><sub>p</sub> improved to 2.1–2.3 nm<sup>2</sup> V<sup>–2</sup> for 3:1 HD-BB-FD:YLD124, HD-BB-OH:YLD124, and HD-BB-MOM:YLD124,
and <i>r</i><sub>33</sub> as high as 351 pm V<sup>–1</sup> 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, <i>r</i><sub>33</sub> was as high as 325 pm V<sup>–1</sup> at 1260 nm and 220 pm V<sup>–1</sup> 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 <i>r</i><sub>33</sub> due to the reduced number density. Ultimately, the
increase in <i>r</i><sub>33</sub>/<i>E</i><sub>p</sub> afforded by the site isolation and blending resulted in a
modest increase in <i>r</i><sub>33</sub>/<i>E</i><sub>p</sub> relative to YLD124, but combined with the increased <i>T</i><sub>g</sub>, the chromophore system is a significant improvement
and points to an important design strategy