Mesomorphism Modulation of Perfluorinated Janus Triphenylenes by Inhomogeneous Chain Substitution Patterns

Abstract

Two isomeric series of compounds with “inverted” chains’ substitution patterns, 7,10‐dialkoxy‐1,2,3,4‐tetrafluoro‐6,11‐dimethoxytriphenylene and 6,11‐dialkoxy‐1,2,3,4‐tetrafluoro‐7,10‐dimethoxytriphenylene, labelled respectively p‐TPFn and m‐TPFn, and two non‐fluorinated homologous isomers, 3,6‐dibutoxy‐2,7‐dimethoxytriphenylene and 2,7‐dibutoxy‐3,6‐dimethoxytriphenylene, p‐TP4 and m‐TP4, respectively, were synthesized in three steps and obtained in good yields by the efficient transition‐metal‐free, fluoroarene nucleophilic substitution via the reaction of appropriate 2,2’‐dilithium biphenylenes with either perfluorobenzene, C6F6, to yield p‐TPFn and m‐TPFn, or o‐difluorobenzene, C6H4F2, for p‐TP4 and m‐TP4, respectively. The single‐crystal structures of p‐TPF4, m‐TPF4 and p‐TP4, unequivocally confirmed that the cyclization reactions occurred at the expected positions, and that the fluorinated molecules stack up into columns with short separation, a propitious situation for the emergence of columnar mesophases. The mesomorphous properties were found to be greatly affected by both chains’ length and positional isomerism: a Colhex phase is found for p‐TPF4 and m‐TPF4, but mesomorphism vanishes in p‐TPF6, and changes for the isomeric homologs m‐TPFn, with the induction for n ≥ 6 of a lamello‐columnar phase, LamColrec. As expected, both non‐fluorinated compounds are deprived of mesomorphism. These compounds emit blue‐violet colour in solution, independently of the chain ‘substitution pattern, and the absolute fluorescence quantum yields can reach up to 46%. In thin films, fluorescence is slightly redshifted.