Luminescence of porphyrins and metalloporphyrins. Part 11.—Energy transfer in zinc–metal-free porphyrin dimers

Abstract

A series of covalently linked Zn–H₂ porphyrin dimers has been synthesized. The linkage consists of a flexible alkoxy chain of variable length. Absorption spectroscopy shows that there is some exciton coupling between the two porphyrin rings, whilst fluorescence spectroscopy shows that the ZnP unit transfers singlet-state excitation energy to the H₂P. From time-resolved fluorescence studies it is concluded that the dimers exist in solution in different, non-equilibrating conformations. A partially closed form permits reasonably close approach of the two porphyrin rings so that Förster energy transfer is efficient. In a fully extended form the two porphyrins are held too far apart for energy transfer to compete effectively with non-radiative decay of the ZnP excited singlet state. Triplet energy transfer is not observed, owing to unfavourable orientations for exchange coupling, whilst redox ion intermediates are not seen, even in polar solvent.