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.

The conformational preferences and the self-associational behaviors of two hemin-derived porphyrin compounds, a tetramethyl ester and a liquid crystalline tetrakis(3,5-didodecyloxyphenyl)ester, have been studied by UV/vis and (1)H NMR spectroscopy in solution. Results indicate that the 3,5-didodecyloxyphenyl units play an important role in both the conformational and the self-associational behaviors of the mesomorphic tetraester. In the monomeric, nonassociated species, the two propionic 3,5-didodecyloxyphenyl esters establish mutual CH/pi interactions that restrict the fluctuative behavior of the chains. In the dimeric, self-associated species, intermolecular CH/pi interactions occur in addition to the pi-pi stacking of the porphyrin cores. The temperature-dependent addition of side CH/pi interactions to the pi-pi stacking of the porphyrin rings accounts for the observed tightening and for the slower dynamics of the dimeric structure. The relationship between the self-associational behavior and the mesomorphism of the hemin-derived porphyrin tetraesters is also discussed.

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