Novel Alkoxyaryl Substituted Porphyrins with Terminal Carboxymethyl and Carboxy Groups: Synthesis and Mesomorphic Properties

Abstract

Design of nanostructured materials based on amphiphilic porphyrins and their metal complexes is a new promising area.[1,2] Porphyrins have all the necessary properties, including the availability of precursors, simplicity of synthesis and functionalization, thermal stability and specific photochemical properties, and the amphiphilic structure is crucial for self-assembly of supramolecular assemblies through noncovalent interactions.[1–3] In this respect, liquid crystalline (LC) state is regarded as a way of achieving structural self-organization.[1] Usually, to obtain the LC properties of tetrapyrrolic compounds the macrocycle is functionalized by long chain alkyl substituents, what also improves the solubility in nonpolar solvents and promotes surface immobilization.[4] The introduction of metal usually leads to a change in mesomorphic properties.[4,5] It is proved that the LC-based porphyrins are promising for optoelectronics, device display and storage.[6–8] By now, a large number of mesogenic lipophilic porphyrins (both ligands and their metal complexes) with thermotropic and lyotropic mesomorphism have been described.[4–8] It has also been reported that amphiphilic porphyrin derivatives containing sulfogroups possess lyotropic mesomorphism.[9,10] We have demonstrated for cationic derivatives of 5,10,15,20-tetra-meso-aryl substituted porphyrins with terminal pyridinium groups that amphiphilic porphyrins possess thermotropic mesomorphism. At the same time a significant effect of the length of the aliphatic spacer and the metal nature on the liquid crystalline properties has been discovered.[11] In this work we have synthesized the structural analogs of the above mentioned cationic porphyrins – amphiphilic tetraalkoxyaryl porphyrins (1–5) and studied their mesomorphic properties. These compounds have long chain substituents in which the spacer methylene chains with a length of five or ten carbon atoms separate the carboxy or carboxymethyl groups from the porphyrin macrocycle. To obtain the 5,10,15,20-tetrasubstituted porphyrins 1 – 3 we used an improved method of monopyrrole condensation in which the maximum yields of porphyrins were achieved at the concentrations of benzaldehyde and pyrrole equal to 10-2 M.[12] Based on the relevant substituted 4-hydroxybenzaldehyde and pyrrole according to the developed approach to the synthesis of lipophilic porphyrins[13] we synthesized symmetrical meso-tetrakis(4-(methoxycarbonylalkyloxyphe