Hexacatenar Columnar Liquid Crystals with a Perfluoronaphthalene-Core: Synthesis, Self-Organization, Charge Transport and Photoluminescence

Abstract

Columnar liquid crystals featuring one-dimensional charge transport and self-assembly are of considerable interest for organic electronic applications such as organic field-effect transistors (OFETs) and organic light-emitting diodes (OLEDs). Fluorination emerges as a powerful strategy to fine-tune the mesomorphic behavior through noncovalent interactions thereby optimizing stability, charge mobility, and device performance. In this work, we report the design and synthesis of a series of hexacatenar compounds (2_n series) constructed from an electron-deficient central perfluoronaphthalene core and two peripheral multi-tailed benzene units via butyllithium-mediated nucleophilic aromatic substitution (S_NAr). This palladium-free route affords the target compounds in 48−68% yield, circumventing the use of noble metal catalysts for cross-coupling. For comparative structure−property analysis, two control compounds were prepared: a nonfluorinated naphthalene-core analog (4₆) and a fluorinated naphthalene-core deprived of alkoxy chains (2H). The mesomorphic behavior was systematically investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and small- and wide-angle X-ray scattering (S/WAXS). All 2_n hexacatenars exhibit a hexagonal columnar (Col_hex) mesophase over a wide temperature range. Increasing the alkyl chain length gradually lowers the clearing point from 100 °C to 78 °C, whereas the incorporation of branched chains extends the mesophase stability down to room temperature, significantly broadening the mesogenic window. Notably, the nonfluorinated alkylated control compound (4₆) is not mesogenic, unequivocally demonstrating that fluorine-induced noncovalent interactionslikely involving F···H and F···π contactsserve as the essential driving force for mesophase formation. Beyond their mesogenic properties, the target compounds exhibit blue-violet photoluminescence both in solution and in the solid state, with a fluorescence quantum yield of up to 30% in cyclohexane, underscoring their favorable photophysical characteristics. SCLC measurements reveal that the chain topology, whether linear or branched, directly influences charge transport, resulting in either electron-dominated charge transport or ambipolar charge transport, with electron mobility ranging from 10⁻⁴ to 10⁻³ cm² V⁻¹ s⁻¹.The combined modulation of mesomorphism by fluorination and flexible chain engineering, together with the efficient, metal-free synthetic protocol, provides a robust molecular design strategy for optimizing polycatenar columnar liquid crystal materials for opto-electronic applications.