INTRAMOLECULAR EXCITATION-ENERGY TRANSFER IN ISOMERIC PORPHYRIN ANTHRACENE DYADS

Isomeric donor-acceptor (D-A) dyads in which an anthracene donor moiet y is covalently linked, via a short ether bridge, to either ortho, met a or para position of one of the aryl groups of 5, 10, 15, 20-tetraphe nylporphyrin have been synthesized and characterized by spectral and e lectrochemical 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. Fluorescen ce from the anthracene subunit in each dyad is found to be quenched in comparison with the fluorescence of free anthracene. Excitation spect ral data provide evidence for an intramolecular excitation energy tran sfer (EET) from the singlet anthracene to the porphyrin and the energy transfer efficiency is found to be dependent on the site of attachmen t (i.e. ortho > meta > para) of the donor to the acceptor. Detailed an alysis of the data suggests that Forster's dipole-dipole mechanism doe s not adequately explain this energy transfer and that an electron-exc hange-mediated mechanism can, in principle, contribute to the intramol ecular EET in these short ether bridged dyads. Furthermore, arguments based on the thermodynamic considerations and also the solvent-depende nt fluorescence data reveal that, while quenching of the fluorescence in the ortho isomer could be explained solely on the basis of EET, inv oking 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 anthrace ne donor subunits.