Intramolecular energy transfer in a protoporphyrin-(anthracene)(2) triad

(H-2)P-An(2), a molecular triad in which protoporphyrin(IX) has been covale ntly linked, through ester bridges at the two propionate side chains, to tw o anthracene donor subunits has been synthesized and fully characterized by FAB-MS, UV-Visible and H-1 NMR spectroscopic methods. A comparison of the UV-Visible and H-1 NMR features of (H-2)P-An(2) with those of the correspon ding individual model compounds (i.e. protoporphyrin(IX) dimethyl ester, H2 P and anthracene, An) reveal the presence of weak intramolecular ground-sta te interaction between the porphyrin and anthracene pi-planes. Quenching of fluorescence due to anthracene (but not porphyrin) part of the molecule ha s been observed in three different solvents. Excitation spectral data provi des evidence for an intramolecular excitation energy transfer (EET) from th e singlet anthracene to the porphyrin. Detailed analysis of the data sugges ts that Forster's dipole-dipole mechanism does not adequately explain this energy transfer but, an electron exchange mediated mechanism can, in princi ple, contribute to the intramolecular EET. The bi-exponential fluorescence decay observed in the time-resolved studies has been interpreted in terms o f the co-existence of "folded" and "extended" conformers of the molecule in solution. Finally, a comparison is made of the EET reactions of(H-2)P-An(2 ) with those observed for the previously reported porphyrin-based energy/el ectron transfer systems which include (tetraaryl)porphyrin-based isomeric p orphyrin-anthracene dyads and mesoporphyrin-based porphyrin-nitrobenzyl die ster triads.