Intramolecular excitation energy transfer in isomeric porphyrin anthracene dyads

Isomeric donor-acceptor (D-A) dyads in which an anthracene donor moiety is covalently linked, via a short ether bridge, to either ortho, meta or para position of one of the aryl groups of 5, 10, 15, 20-tetraphenylporphyrin have been synthesized and characterized by spectral and electrochemical methods. UV-visible and H-1 nuclear magnetic resonance data of these D-A systems suggest the presence of weak intramolecular pi-pi interaction between the porphyrin and the anthracene. Fluorescence from the anthracene subunit in each dyad is found to be quenched in comparison with the fluorescence of free anthracene. Excitation spectral data provide evidence for an intramolecular excitation energy transfer (EET) from the singlet anthracene to the porphyrin and the energy transfer efficiency is found to be dependent on the site of attachment (i.e. ortho > meta > para) of the donor to the acceptor. Detailed analysis of the data suggests that Forster's dipole-dipole mechanism does not adequately explain this energy transfer and that an electron-exchange-mediated mechanism can, in principle, contribute to the intramolecular EET in these short ether bridged dyads. Furthermore, arguments based on the thermodynamic considerations and also the solvent-dependent fluorescence data reveal that, while quenching of the fluorescence in the ortho isomer could be explained solely on the basis of EET, invoking an intramolecular electron transfer can rationalize the observed quenching in the meta and para isomers. Finally, a comparison is made of the EET reactions in these isomeric dyads with those observed for the previously reported porphyrin-based energy transfer systems which include a supramolecular, D-4-A pentad porphyrin bearing four anthracene donor subunits.