Counterion effects on the columnar mesophases of triphenylene-substituted [18]crown-6 ethers: is flatter better?

Abstract

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 FeCl(3) in nitromethane. Reactions of the latter with potassium salts gave complexes KX.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.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.2e), respectively, as compared to the corresponding o-terphenyl-substituted crown ether complexes KI.1e (DeltaT=51 degrees C) and KSCN.1e (plastic crystal phase). Anions such as Br(-), Cl(-), and F(-) decreased the mesophase stability, and PF(6)(-) led to complete loss of the mesomorphic properties of KPF(6).2 although not for KPF(6).1. For crown ether complexes KX.2 (X=F, Cl, Br, I, BF(4), 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.1, the complexation of the flat triphenylene crown ethers 2 with KX resulted in the formation of organogels. Characterization of the organogel of KI.2e in CH(2)Cl(2) revealed a network of fibers.