Synthesis and Properties of Hexakis(6-octyl-2-azulenyl)benzene as a Multielectron Redox System with Liquid Crystalline Behavior

This paper describes the cyclotrimerization reaction of di(2-azulenyl)acetylenes (2a,b) catalyzed by Co₂(CO)₈ to produce hexa(2-azulenyl)benzene derivatives (1a,b). The cyclooligomerization of 2a and 2b utilizing CpCo(CO)₂ as a catalyst produced (η⁵-cyclopentadienyl)[tetra(2-azulenyl)cyclobutadiene]cobalt complexes (3a,b). The redox behavior of hexakis(6-octyl-2-azulenyl)benzene (1b), bis(6-octyl-2-azulenyl)acetylene (2b), and the cobalt complexes 3a and 3b along with 6-octyl-2-phenylazulene (19) was examined by cyclic voltammetry (CV). The reduction of compound 1b exhibited multiple-electron transfers in one step upon CV with a reduction potential similar to that of compound 19. However, the CVs of compounds 2b, 3a, and 3b were characterized by stepwise waves because of the reduction of each azulene ring. The mesomorphic behaviors of 1b, 2b, and 19 were also studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) techniques. A new series of azulene derivatives, 1b, 2b, and 19, substituted by a long alkyl chain at the 6-position shows mesomorphism with crystalline polymorphs. Compound 1b showed a large temperature range of hexagonal columnar mesophases (Col_ho) from 115.5 to 199.9 °C. Compound 2b has rectangular columnar (Col_ro), smectic E (S_E), and nematic (N) mesophases. Compound 19 exhibited an S_E mesophase.