Efficient, Stable, and Scalable Push–Pull Heptamethines for Electro-Optics

Currently,
there is increasing interest in the use of push–pull
heptamethine chromophores containing the 2-dicyanomethylidene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran
(TCF) electron acceptor for electro-optic (EO) applications. Compared
with the benchmark push–pull tetraenes with a stronger trifluoromethyl-substituted
TCF acceptor (CF3-TCF), the TCF-based push–pull
heptamethines showed lower EO activities; however, they have better
chemical and thermal stability and higher synthetic efficacy. Herein
we report the facile synthesis and analysis of structure–property
relationship of dipolar heptamethines containing a strong electron
acceptor, bis­(4-fluorophenyl)-substituted TCF (FP-TCF). The FP-TCF
acceptor was synthesized through two consecutive multiple steps, one-pot
reactions to achieve a good overall yield of 50%, which is higher
than that of making CF3-TCF. The reduced π–π
stacking and fine-tuned bond-length alternation boost the nonlinear
optical activities of FP-TCF-based heptamethines in plasmon-coupled
EO polymer waveguides, leading to larger EO coefficients (up to 205%)
over those of the TCF-based chromophores. More importantly, we demonstrate
for the first time an ultralarge molecular first hyperpolarizability
of 8437 × 10–30 esu at 1304 nm for M1a-FP-ON, which exceeds the results of two best-performing but synthetically
demanding push–pull tetraene chromophores. The results mark
an important milestone in developing highly efficient push–pull
polymethines with good stability and synthetic scalability for photonic
applications.