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.