Synthesis of poly(6-azulenylethynyl)benzene derivatives as a multielectron redox system with liquid crystalline behavior

A series of poly(6-azulenylethynyl)benzenes substituted with n-hexyloxycarbonyl chains at 1,3-positions in azulene rings, i.e., hexakis-, 1,2,4,5-tetrakis-, 1,3,5-tris-, and 1,4-bis(6-azulenylethynyl)benzene derivatives 1, 2, 3, and 4b, have been prepared by a simple one-pot reaction involving repeated Pd-catalyzed alkynylation of halogenated arenes with substituted 6-ethynylazulene and/or ethynylated arenes with substituted 6-bromoazulene under Sonogashira-Hagihara conditions. The redox behavior of these novel poly(6-azulenylethynyl)benzene derivatives was examined by cyclic voltammetry (CV), which revealed the presumed multielectron redox properties. Compound 4b exhibited a one-step, two-electron reduction wave upon CV, which revealed the formation of the dianion stabilized by two 6-azulenylethynyl substituents under electrochemical reduction conditions. Four 6-azulenylethynyl substituents on a benzene ring in a 1,2,4,5 relationship increased the electron-accepting properties because of the formation of a stabilized closed-shell dianionic structure, whereas 3 was reduced at more negative reduction potentials. In contrast to the multistep redox behavior of 2, compound 1 was reduced in one step at -1.28 V upon CV. Compound 1 showed a wide temperature range of columnar mesophases (Col(ho) and Col(ro)) from 77.3 degrees C to the decomposition temperature at ca. 270 degrees C. Compounds 2, 3, and 4b exhibited columnar mesomorphism (Col(ro)) with crystalline polymorphs for 2, unusual triple-melting behavior for 3, and both double-melting behavior and columnar mesomorphism (Col(ho)) for 4b. Therefore, the investigated systems exemplify a new principle for multielectron redox behavior with liquid crystalline properties.