Configuration of triporphyrin ethers probed by fluorescence measurements

The fluorescence spectra and lifetimes of triporphyrin ethers derived from hematoporphyrin, mesoporphyrin and protoporphyrin were examined, together with relative hydrophobicities estimated from reverse-phase high performance liquid chromatography (HPLC) elution times. The following data suggest a molecular arrangement with two of the three rings in a "folded configuration". The trimers display a greater fluorescence yield (phi f) than the corresponding diporphyrin ethers which contain only the two folded rings. The fluorescence lifetime data for the trimers are consistent with signals from both a folded ring pair (7-8 ns) and a free ring (14 ns). Reverse-phase HPLC studies indicate that the trimers are intermediate in hydrophobicity between the monomers and dimers. Preliminary data suggest that, for certain peripheral substitutions, the trimer configuration is superior to the dimer for photodynamic therapy.