Synthesis and electrochemical characterisation of some long chain 1,4,8,11,15,18,22,25-octa-alkylated metal-free and zinc phthalocyanines possessing discotic liquid crystalline properties

A series of long chain (C-10, C-12, C-15 and C-18) 3,6-dialkylated phthalonitriles, prepared in 4 steps from thiophene, have been converted into the title phthalocyanines as metal-free and zinc derivatives. The symmetric macrocycles were characterised using H-1 NMR, MALDI-TOF ms and solution phase electronic (UV-VIS) spectroscopy. Four ring-based redox processes for the metal-free macrocycles possessing shorter alkyl substituents could be identified in the potential range -1.8 to 1.2 V vs. Ag/Ag+ in dichloromethane or 1,2-dichloroethane. Reversibility became poorer with an increase in chain length of the eight alkyl substituents. The redox processes of the zinc complexes were not as well defined as those of the metal-free phthalocyanines. All compounds exhibited discotic thermotropic liquid crystal behaviour, which was studied using polarised optical microscopy and differential scanning calorimetry (DSC). Up to three different mesophases were detected. The zinc derivatives showed liquid crystalline behaviour at much higher temperatures than the metal-free compounds. The metal-free and zinc phthalocyanines with C-12 chains had the lowest crystalline to discotic liquid crystalline mesophase transition temperatures. Variable temperature UV-VIS spectra of thin films (ca. 1000 Angstrom thick) of the title compounds cast on glass were obtained for the compounds in the crystalline, meso- and isotropic liquid phase. The spectrum of a thin film of the isotropic liquid resembled the spectrum obtained from a solution of the same compound.