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

(H-2)P-An(2), a molecular triad in which protoporphyrin(IX) has been covalently linked, through ester bridges at the two propionate side chains, to two 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 corresponding individual model compounds (i.e. protoporphyrin(IX) dimethyl ester, H2P and anthracene, An) reveal the presence of weak intramolecular ground-state interaction between the porphyrin and anthracene pi-planes. Quenching of fluorescence due to anthracene (but not porphyrin) part of the molecule has been observed in three different solvents. Excitation spectral data provides evidence for an intramolecular excitation energy transfer (EET) from the singlet anthracene to the porphyrin. Detailed analysis of the data suggests that Forster's dipole-dipole mechanism does not adequately explain this energy transfer but, an electron exchange mediated mechanism can, in principle, contribute to the intramolecular EET. The bi-exponential fluorescence decay observed in the time-resolved studies has been interpreted in terms of 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/electron transfer systems which include (tetraaryl)porphyrin-based isomeric porphyrin-anthracene dyads and mesoporphyrin-based porphyrin-nitrobenzyl diester triads.