Synthesis of unsymmetrical porphyrin triad containing three different porphyrin subunits assembled by covalent and non-covalent interactions

Abstract

Cis-21,23-dithiaporphyrin building block containing one iodophenyl and one pyridyl functional group at meso positions was synthesized by condensing unsymmetrical thiophene diol and symmetrical 16-thiatripyrrin under refluxing propionic acid conditions. The 21,23-dithiaporphyrin building block was coupled with mono-functionalized 21-thiaporphyrin building block containing meso-phenylethyne functional group under mild Pd(0) coupling conditions. The steady-state and time-resolved fluorescence studies support an efficient energy transfer in the singlet excited state from N₃S porphyrin subunit to N₂S₂ porphyrin subunit in the dyad. The N₃S- N₂S₂ porphyrin dyad was then treated with RuTPP(CO)(EtOH) in toluene at refluxing temperature and purified by column chromatography to afford a porphyrin triad containing N₃S, N₂S₂ and RuN₄ porphyrin subunits assembled using both covalent and non-covalent interactions. The photophysical properties showed the fluorescence quenching of N₃S and N₂S₂ porphyrin subunits in triad due to heavy ruthenium ion which was coordinated to meso-pyridyl 'N' of N₂S₂ porphyrin subunit of porphyrin triad.