Electric Response of Twisted π-Conjugated 9,9'-Bifluorenylidene Dyes in Liquid Crystal Media

The synthesis of twisted 9,9'-bifluorenylidene dyes was accomplished through a palladium-catalyzed dual C-H activation and annulation transformation of the corresponding bis-biaryl alkyne precursor. This method enables efficient construction of the twisted 9,9'-bifluorenylidene framework with precise substitution patterns. The reaction conditions facilitated the formation of both cis- and trans-isomers, each exhibiting unique structural configurations that significantly influenced their physical properties. Notably, these isomers demonstrated distinct thermal behaviors, as evidenced by differential scanning calorimetry, with the cis-isomer showing a higher melting point indicative of more thermally stable molecular packing in the solid state compared to the trans-isomer. Furthermore, their solubilities varied markedly in the liquid crystal medium 4-cyano-4'-pentylbiphenyl (5CB), where the trans-isomer exhibited superior solubility and compatibility with the host matrix. At the same time, the cis-isomer tended to form microscopic aggregates due to limited solubility. Importantly, when incorporated into the liquid-crystalline environment, the trans-isomer exhibited reversible absorbance changes upon application and removal of an external alternating electric field. This behavior is attributed to the alignment of the trans-isomer's absorption transition dipole moment with the orientation of the liquid-crystalline host under the electric field, leading to reversible, repeatable modulations in optical absorption. These findings underscore the potential utility of the trans-isomer in electric-optic applications, where its responsive optical properties in liquid crystal media can be harnessed for devices such as smart windows and dichroic color filters.