Inter-ring interactions and peripheral tail effects on the discotic mesomorphism of 'free-base' and Co(II), Ni(II) and Cu(II) alkenyl(sulfanyl) porphyrazines

The self-organizing capability of a family of newly synthesized ‘free-base', H2(OASPz), and metal porphyrazines, M(OASPz) (OASPz2− = 2,3,7,8,12,13,17,18-octakis(alkenylthio)-5,10,15,20-tetraazaporphyrinato dianion, n = 4, 5, 6, 8; M = Co, Ni, Cu), into columnar, discotic liquid crystals, has been investigated by differential scanning calorimetry (DSC), optical microscopy and X-ray diffraction. It is found that of the metal-free porphyrazines only those with n = 4 or 5 show, as indicated by X-ray and optical microscopy studies, characteristic stable and discotic nematic mesophases. All the metallated porphyrazines show a discotic liquid crystalline behavior and those with the largest values of n present, consistent with the X-ray results, hexagonal columnar packing. Comparing the mesomorphic behavior of these compounds with those of the parent compounds peripherally substituted with saturated alkyl(sulfanyl)-chains it is seen that the effect of the increased stiffness at the periphery of the chains, induced in the title tetrapyrroles by the terminal double bonds, results in a decrease of the clearing temperature, Ti, and also of the mesophase temperature stability range, ΔT. This effect is larger in H2 and Ni than in Cu and especially in Co porphyrazines, where the axial coordinating capability of the Co seems to counterbalance the effect of this perturbation.