Synthesis and phase behaviour of β-octaalkyl porphyrins

Abstract

A homologous series of three octakis-alkyl-substituted porphyrin (OAP) derivatives with alkyl chain lengths of C₈, C₁₀ and C₁₂ has been synthesised. For each compound, both the free-base (i.e. two H-atoms) and the fourfold coordinated zinc complex were investigated, where the latter for each chain length was found to exhibit liquid-crystalline mesophase behaviour. At low temperatures all three metal coordinated compounds formed a solid S phase with a structure based on a square packing of columns with a stacking periodicity of approx. 4.5 Å. At increased temperatures the square packed solid S phases undergo phase transitions to liquid-crystalline columnar phases based on centred rectangular lattices of C2/m symmetry, with the ratio of the lattice parameters, a/b, ranging between 1 and 1.732. The a/b ratio exhibited two discontinuous jumps with increasing temperature prior to melting to the isotropic phase for Zn-OAP-C₈, implying the existence of three columnar phases denoted Col_r1, Col_r2 and Col_r3. For the longer chain length compounds, the number of mesophases formed was reduced, first to two for Zn-OAP-C₁₀, then to one for Zn-OAP-C₁₂.