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 phthalon itriles, prepared in 4 steps from thiophene, have been converted into the t itle phthalocyanines as metal-free and zinc derivatives. The symmetric macr ocycles were characterised using H-1 NMR, MALDI-TOF ms and solution phase e lectronic (UV-VIS) spectroscopy. Four ring-based redox processes for the me tal-free macrocycles possessing shorter alkyl substituents could be identif ied 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 le ngth of the eight alkyl substituents. The redox processes of the zinc compl exes were not as well defined as those of the metal-free phthalocyanines. A ll compounds exhibited discotic thermotropic liquid crystal behaviour, whic h 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 temperature s than the metal-free compounds. The metal-free and zinc phthalocyanines wi th C-12 chains had the lowest crystalline to discotic liquid crystalline me sophase transition temperatures. Variable temperature UV-VIS spectra of thi n 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 sp ectrum obtained from a solution of the same compound.