One-flask synthesis of mono- and trifunctionalized 21-thia and 21-oxaporphyrin building blocks and their application in the synthesis of covalent and noncovalent unsymmetrical porphyrin arrays

Abstract

A rapid synthetic route has been developed to synthesize mono- and trifunctionalized 21-thia and 21-oxaporphyrin systems using simple precursors such as 2[α-(aryl)-α-hydroxymethyl] thiophene (thiophene mono-ol) and 2[α-(aryl)-α-hydroxymethyl] furan (furan mono-ol), respectively. Condensation of one equivalent of thiophene or furan mono-ol with two equivalents of aryl aldehyde and three equivalents of pyrrole under porphyrin forming conditions followed by column chromatography resulted in functionalized 21-thia or 21-oxaporphyrins. To synthesize monofunctionalized porphyrins, the mono-ol containing the functionalized aryl group was used. The functionalized aldehydes were used to synthesize trifunctionalized porphyrins. The mono-ol method is versatile and applicable to synthesize mono- and trifunctionalized 21-thia and 21-oxaporphyrins containing functional groups such as iodophenyl, ethynylphenyl, hydroxyphenyl, bromophenyl and pyridyl groups. The monofunctionalized porphyrin building blocks containing iodophenyl and ethynylphenyl groups were used further to synthesize four unsymmetrical covalent porphyrin dimers containing two different porphyrin cores such as N₃S−N₄, N₃O−N₄ and N₃S−N₃O bridged via diaryl ethyne group and one symmetrical phenylethyne bridged dimer containing two N₃S cores. A preliminary photophysical study on these dimers indicated a possibility of energy transfer from one subunit to another. We also demonstrated the use of trifunctionalized porphyrins in the synthesis of two noncovalent unsymmetrical porphyrin tetramers containing one N₃S and three N₄ porphyrin subunits.