Synthesis and Mesogenic Properties of Lipophilic and Amphiphilic Tetraphenylporphyrins

Abstract

Liquid crystals, based on porphyrins, belong to the discotic mesogenes and are of considerable interest for use in optoelectronics and devices for display and storage of information. Porphyrins possess all the necessary properties for that, including synthetic versatility, thermal stability and a number of special photochemical properties. In comparison with widely studied class of phthalocyanines liquid crystals based on porphyrins have lower melting points. In this study we have synthesized homologous series of lipophilic symmetrical 5,15-di- and 5,10,15,20-tetrameso-arylsubstituted porphyrins containing long chain alkyl and acyl groups, their zinc and cobalt complexes, amphiphilic derivatives and studied their mesogenic properties. The series of 5,15-di- and 5,10,15,20-tetra-mesoarylsubstituted porphyrins differ by the quantity and the length of peripherical aliphatic substitutents, type of their attachment to the porphyrin meso-phenyl groups, nature of terminal group and presence of a metal. The chain length of lipophilic substituents was varied from C 8 to C 18 atoms. Studying of mesogenic properties has allowed us to reveal and generalize structural influence on the ability of porphyrins to form mesophases. We’ve used efficient synthetic procedures to obtain these meso-arylsubstituted porphyrins. We have suggested the new approach to the synthesis of the lipoporphyrins. According to this approach the residues of higher fatty acids and alcohols are incorporated into the molecules of benzaldehyde at the initial stages of synthesis which is followed by condensation with pyrrole. This method essentially simplifies the synthetic procedure, facilitates the purification of the target porphyrins, gives the chance to carry out synthesis in soft conditions and provides higher yields of lipoporphyrins. Synthesis of 5,15-dimeso-arylsubstituted porphyrins was carried out using dipyrrolylmethanes in 36-44% yields. 5,10,15,20-Tetra-mesoarylsubstituted porphyrins were synthesised by monopyrrole condensation in 33-45 % yields. Amphiphilic derivatives were prepared by chemical modification of periferical aliphatic residues. Zinc and cobalt complexes were obtained in 90-95% yields from porphyrin ligands. The mesomorphic behavior of the porphyrin derivatives was investigated by using optical polarizing microscopy and differential scanning calorimetry. The phase transition temperatures of all compounds in the course of heating are summarized. It has been shown that eight compounds among long chain meso-arylsubstituted porphyrins could form mesophase and exhibit thermotropic and lyotropic behaviour in nonpolar solvents. Five compounds in the series of 5,15-disubstituted and one compound among 5,10,15,20-tetra-mesoarylsubstituted porphyrins were amphotropic: the thermotropic and lyotropic mesomorphism in nonpolar solvents were observed. Two 5,10,15,20-tetra-meso-arylsubstituted porphyrins exhibited only thermotropic mesomorphism. Seven porphyrins derivatives were glassing mesogens. The mesophase in all compounds was characterized by nongeometric grain structure presumably associated with a column-type packing. As a result it has been established that discotic symmetrical long chain meso-arylsubstituted porphyrins exhibit thermotropic and lyotropic behaviour in nonpolar solvents, some of them are glassing mesogens. It is obvious, the mesophase range and glassing properties of these porphyrins are influenced, first of all, by the quantity and the chain length of lipophilic substituents, and also by the type of their attachment to the porphyrin meso-phenyl groups. We have shown, that replacement of donor alkoxy by acceptor alkanoyloxy groups influences on mesogenic properties of the porphyrins derivatives. Incorporation of coordinatively unsaturated metals (Zn II and Co II ) increases the number of mesogenic porphyrins derivatives in the series of 5,15-disubstituted porphyrins and considerably changes mesogenic properties in comparison with corresponding ligands. It has been shown, that liquid crystalline properties in the case of 5,15-disubstituted porphyrins with equal lengths of aliphatic substituents at the meso-positions and the corresponding metal complexes are more similar. Thus, we have expanded databank of discotic liquid crystals based on lipophilic porphyrins derivatives and their metal complexes. The mesomorphic properties of porphyrins indicate that they offer potential usage in optoelectronic field.