INFLUENCE OF LINKING ALKYL CHAIN-LENGTH ON PHOTOINDUCED INTRAMOLECULAR ELECTRON-TRANSFER IN BIPYRIDINE-LINKED PORPHYRIN RUO2 CLUSTERS

In a series of covalently linked meso-tris(octyloxyphenyl)porphyrin-RuO2 composite clusters, the efficiency of photoinduced intramolecular electron transfer from the porphyrin to the covalently attached RuO2 cluster depends on the length of the alkyl chain (-(CH2)n-, n = 1, 4-7) separating the surfactant-like porphyrin from the bipyridine ligating site. The broadening of the porphyrin Soret band and the observation of two distinct triplet-state lifetimes upon complexation to RuO2 are best interpreted as arising from at least two distinct sets of conformers: (1) a family of folded conformers in which RuO2 is complexed to the porphyrin pi-electron system and (2) a family of more extended (noncomplexed) conformers. The complexed conformers exhibit broadened absorption spectra, decreased fluorescence quantum yields, and diminished triplet-state lifetimes compared to the noncomplexed conformers. Axial ligation of the zinc porphyrin to pyridine reduces intramolecular complexation between the porphyrin pi-electron system and the RuO2 cluster in ZnPB(n)R, resulting in unperturbed (narrow) absorption spectra, diminished fluorescence quenching, and a strong diminution of the relative amplitude of the short-lived porphyrin triplet state. However, even in pyridine, the fluorescence intensities, triplet yields, and singlet-state lifetimes of ZnPB(n)R are strongly decreased relative to the unbound porphyrin, whereas the porphyrin triplet-state lifetime is unaffected, suggesting intramolecular electron transfer as a major decay route of the porphyrin excited singlet state in the noncomplexed conformers.