Room-Temperature Emissive Fluorinated Liquid Crystals for Application in Light Emitting Diodes

Noncovalent arene–fluoroarene π–π intermolecular interactions have been successfully exploited in the design of various intricate supramolecular assemblies and liquid crystalline materials. In particular, discotic liquid crystals containing fluorinated arenes (F-DLCs) have been found to foster stable columnar mesophases over broad temperature ranges and with higher clearing temperatures than their identical nonfluorinated arene counterparts. Furthermore, the incorporation of fluorinated aromatic cores into π-conjugated molecular systems enables tunability of their optical and electronic properties, which can be harnessed for advanced applications in display technologies, photonic devices, sensors, and organic electronics. In this study, both 1,2,3,4-tetrafluoro-6,7,10,11-tetraalkoxytriphenylene (1) and 9,10,11,12,13,14-hexafluoro-2,3,6,7-tetraalkoxybenzo[f]tetraphene (2), bearing 4 racemic 3,7-dimethyloctyloxy side-chains, were reacted with various aryl-lithium compounds (ArLi with Ar = naphthalen-1/2-yl (NA/NB), benzothiophen-2-yl (BT), triphenylen-2-yl (TP), pyren-1-yl (PY) and tetraphenylethylenyl (TPE)) to produce the corresponding trifluorotriphenylene (1Ar) and pentafluorobenzo[f]tetraphene (2Ar) monosubstituted derivatives, respectively. Single crystal structures of 3 model compounds without peripheral chain reveal F···H interactions and antiparallel π–π stacking and help confirming the substitution position of the aryl groups. The mesomorphic properties of these fluorine-containing pseudo discogens were investigated using polarized optical microscopy, differential scanning calorimetry, and small-/wide-angle X-ray scattering (S/WAXS). All the compounds systematically form a Colhex mesophase over broad temperature range, with the grafting of the lateral branched chains (3,7-dimethyloctyloxy) enabling room-temperature stability. Investigation of their photophysical properties revealed that triphenylene-cored 1TPE demonstrates significant aggregation-induced emission enhancement (AIEE, >64 times the initial emission intensity), while the larger-cored 2TPE exhibits weaker AIE and dual-state emission effects. Both compounds modified with a tetraphenylethylene unit, effectively overcome the aggregation-caused quenching typically observed in DLCs. Furthermore, a film of 2TPE was inserted in a simple OLED device, and the results demonstrate promising performance. Notably, the device exhibits efficiency and brightness levels that compare favorably with those reported for other organic conjugated molecules, highlighting the potential of this compound for optoelectronic applications. Thus, these F-DLCs obtained by Pd-free synthesis, exhibit good mesomorphic behavior, show outstanding fluorescent properties, and present a high potential for applications in organic light-emitting diodes (OLEDs) and related fields.