Metal ion mediated mesomorphism and thin film behaviour of amphitropic tetraazaporphyrin complexes

Abstract

Octa-substituted tetraazaporphyrins with amphiphilic 3,6-dioxaheptylthio and 3,6,9-trioxadecylthio chains and their metal complexes (Co(II), Ni(II), Cu(II), Zn(II)) were synthesised in order to compare their thermotropic and lyotropic mesomorphism as well as their thin film properties with related tetraazaporphyrins, phthalocyanines and triphenylenes, which have been reported previously. The Co, Ni, and Cu complexes melted into hexagonal columnar mesophases, whereas the Zn(II) complexes and the free-base porphyrins did not display mesomorphism. In fact, the zinc complexes did not crystallise on cooling, rather giving highly viscous isotropic oils at room temperature that solidified below −50 °C to glasses. This unusual behaviour may be caused by axial interactions between the zinc ion and the oxygen of the polyether groups. All 3,6,9-trioxadecylthio substituted derivatives solidified far below 0 °C giving rise to columnar hexagonal mesophases at room temperature for the Co(II), Ni(II), and Cu(II) complexes. The columnar hexagonal mesophases of the long chain derivatives spontaneously align homeotropically when sandwiched between two substrates, in contrast to the short chain analogues. Both the long chain and the short chain Co(II) complexes displayed a rather disordered hexagonal columnar packing as revealed by X-ray diffraction. Binary mixtures of the short chain derivatives with non-polar or polar organic solvents did not display additional lyotropic mesophases. However, the transition temperatures and enthalpies, as well as the texture of the hexagonal columnar mesophase were affected, assuming a solubility of the solvent in the thermotropic mesophase as shown for the Cu(II) derivative. All long chain derivatives were also soluble in water but, again, lyotropic mesomorphism was observed neither with water nor with organic solvents. The film forming properties of some derivatives were investigated by spin-coating and the Langmuir–Blodgett (LB) technique. Homogeneous films of the mesomorphic short chain copper complex were obtained by the spin-coating method. The films displayed a layer structure with edge-on orientation at ambient temperature, although they were of crystalline morphology, as shown by X-ray reflectivity measurements and polarised UV/VIS spectroscopy. Only monolayer films of short and long chain derivatives could be transferred by the LB method. Again, determination of the thickness of the monolayer by X-ray agreed with an edge-on orientation of the tetraazaporphyrins. In contrast, the extrapolated molecular areas on the water surface suggested a flat-on orientation of the macrocycle with the oligo(oxyethylene) chains being dissolved in the sub-phase.