Counterion Effects on the Columnar Mesophases of Triphenylene-Substituted [18]Crown-6 Ethers: Is Flatter Better?

The twisted lateral tetraalkyloxy ortho-terphenyl units in dibenzo[18]crown-6 ethers 1a-f were readily converted into the flat tetraalkyloxytriphenylene systems 2a-f by oxidative cyclization with FeCl3, in nitromethane. Reactions of the latter with potassium salts gave complexes KX center dot 2, which displayed mesomorphic properties. The aromatization increased both the clearing and melting points; the mesophase stabilities, however, were mainly influenced by the respective anions upon complexation with various potassium salts. In contrast, the alkyl chain lengths played only a secondary role. Among the potassium complexes of triphenylene-substituted crown ethers KX center dot 2, only those with the soft anions I- and SCN- displayed mesophases with expanded phase temperature ranges of 93 degrees C and 132 degrees C (for KX center dot 2e), respectively, as compared to the corresponding o-terphenyl-substituted crown ether complexes KI center dot 1e (Delta T=51 degrees C) and KSCN center dot 1 e (plastic crystal phase). Anions such as Br-, Cl-, and F- decreased the mesophase stability, and PF6- led to complete loss of the mesomorphic properties of KPF6 center dot 2 although not for KPF6 center dot 1. For crown ether complexes KX center dot 2 (X = F, Cl, Br, I. BF4, and SCN), columnar rectangular mesophases of different symmetries (c2mm, p2mg, and p2gg) were detected. In contrast to findings for the twisted o-terphenyl crown ether complexes KX center dot 1, the complexation of the flat triphenylene crown ethers 2 with KX resulted in the formation of organogels. Characterization of the organogel of KI center dot 2e in CH2Cl2, revealed a network of fibers.