Synthesis and Characterization of Unsymmetrically β-Substituted Porphyrin Liquid Crystals:  Influence of the Chemical Structure on the Mesophase Ordering

The present study addresses the synthesis and characterization of a series of columnar liquid crystal compounds based on the natural porphyrin hemin. These compounds, which have different alkyl side chains (Alk = butyl, octyl, dodecyl, and hexadecyl) and either free base (M = 2H) or metalated (M = Zn, Cu) porphyrin cores, display hexagonal columnar mesophases over wide temperature ranges that cover room temperature. In the octyl, dodecyl, and hexadecyl free base compounds, the formation of a low-temperature mesophase (Colh1) on cooling from a high-temperature mesophase (Colh2) is marked by a slow transition. Furthermore, the X-ray diffraction pattern of the Colh1 mesophase indicates that intracolumnar noncovalent associations of three porphyrin molecules might be formed. As suggested by computer molecular modeling combined with the experimental data, the mobility of the propionic ester chains attached to the porphyrin core and the influence of metalation on porphyrin−porphyrin interactions may account for the mesophase structures encountered in the reported compounds.