Synthesis of Amphiphilic meso-Aryl­porphyrins in Organic Solvents and Aqueous Micellar Medium

Abstract

Amphiphilic porphyrins find a remarkable place in chemistry of new materials due to their unique aromatic structure and spectral properties. Porphyrins and their metalloderivatives can be used as photoelectronic materials, sensors, electronics, photosensitized solar cells and organic light-emitting devices. Supramolecular synthetic methods and strategies rapidly develop, and porphyrins are particularly amenable to the design of complex and robust architectures because of their rigid framework. Architectural arrangement of the substituents plays an important role in such materials. Hydroxyl group side by side with carboxyl-group is often used to form supramolecular systems, and, at the other hand their asymmetry can provide interesting properties. Elaborations of the synthetic methods and obtaining of porphyrins in sufficient quantities are still in object of many researchers. In this study we have synthesized symmetrical and unsymmetrical amphiphilic tetraphenylporphyrins containing available hydroxylgroups and hydrophobic long chains. Two methods were explored – monopyrrole condensation in organic solvents mixture and in aqueous micellar medium. Initially both methods were used to produce meso-tetra(4-hydroxyphenyl) porphyrin. First method consists of the condensation of p-substituted benzaldehyde with pyrrole in mixed solvents (propionic acid, acetic acid and nitrobenzene). In this system nitrobenzene plays a role not only of the solvent, but of the efficient oxidant – the yields of porphyrins were doubled compared with those of the reaction system without nitrobenzene. The yields of this method were higher than from other general methods such as Adler method. Methodology of the isolation using mixed-solvents was fine-tuned. Yields were 39-43%. After that we have investigated the application of this method to the synthesis of the unsymmetrical amphiphilic porphyrins containing fatty alkyl residues. The aldehydes, 4-hydroxybenzaldehyde and 4-oxytetradecylbenzaldehyde, were condensed with pyrrole in ratio 3:1 and 1:3. Yields of the desired porphyrins were 10-13%, also the symmetrical porphyrins were obtained with sufficient yields 9-12%. The preparation of the meso-tetra(4-hydroxyphenyl)porphyrin using anionic sodium dodecyl sulfate micelles was the next step. Micelles act as potential wells, binding products more tightly than reactants, and may also catalyze condensation reactions. Choosing a micellar approach for the synthesis of amphiphilic porphyrins with long-chain substituents, we relied on the ability of these compounds to be embedded in micelles of various detergents, which are widely described in literature. By using this method meso-tetra(4hydroxyphenyl)porphyrin was obtained with 40% yield. In comparison with more hydrophilic aldehydes (like in the case of 4-hydroxybenzaldehyde), hydrophobic aldehydes gave much lower yields in micellar synthesis. For the first time we have proved a satisfiability of this method in condensation of the unsymmetrical aldehydes having hydroxylgroups and long alkyl substituents in phenyl rings. Yields of the porphyrins were 5-8% that is lower relatively the mixed solvents method. But it should be noted that monopyrrole condensation method in the micellar environment is easer in practice and more ecological. In the next stage of work it is planned to study the properties of these porphyrins, including liquid crystalline, and use them as syntons for subsequent functionalization of porphyrins.