Intramolecular photochemical electron transfer. 2. Fluorescence studies of linked porphyrin-quinone compounds

TLDR: In this article, a series of meso-tetratolylporphyrin-quinone molecules with diamide linkages with the two amides being separated by n methylene groups (n=2, 3, or 4) were studied.

Abstract: Systematic studies of absorption spectra and fluorescence spectra and lifetimes have been carried out on a series of meso-tetratolylporphyrins to which various molecular entities have been covalently attached via diamide linkages with the two amides being separated by n methylene groups (n=2, 3, or 4). The attached end groups include p-benzoquinone, hydroquinone, and dimethoxybenzene. These studies reveal the existence of at least two more or less distinct forms: a family of ''complexed'' conformers in which the end group is likely folded so as to interact with the porphyrin, and one or more ''extended'' conformers in which the porphyrin moiety is relatively unperturbed by the end group. The complexed conformers exhibit perturbations of spectra and diminished fluorescence lifetimes and quantum yields as compared with the extended conformer(s). Oxidation of the porphyrin-linked hydroquinone form to the quinone form does not significantly affect the absorption or fluorescence spectra but causes strong quenching of fluorescence and diminution of the fluorescence lifetimes. This quenching is interpreted primarily in terms of electron transfer from the lowest excited singlet state of the porphyrin to the quinone moiety. On the basis of the assumption that these shorter fluorescence lifetimes of the quinone relative to the hydroquinone are duemore » entirely to electron transfer, apparent electron-transfer rate constants k/sup et/ at room temperature range from <1 x 10/sup 8/ to <7 x 10/sup 9/ s-/sup 1/, depending on solvent and probably the specific geometry of the conformers. Quenching in both sets of conformers appears to be thermally activated and is strongly inhibited in frozen matrices. Parallel studies of porphyrin-quinone molecules with various methylene chains (n=2, 3, and 4) indicate that the geometry of the linkage is critical to the rate of electron transfer. A methylene chain with n = 3 appears to be optimum. 4 figures, 6 tables.« less