SYNTHESIS AND INTRAMOLECULAR ELECTRON-TRANSFER AND ENERGY-TRANSFER REACTIONS OF POLYYNE-BRIDGED OR POLYENE-BRIDGED DIPORPHYRINS

A series of geometrically restricted polyyne- or polyene-bridged diporphyrins 1-7 has been prepared by a convenient, one-pot, double porphyrin cyclization reaction in acceptable yields from the corresponding dialdehydes 1-7CHO. Polyene-bridged diporphyrins 8 and 9 were prepared by a double Horner-Emmons-Wadsworth reaction of formyl-substituted zinc-porphyrin monomer 12 with bis-phosphonate esters 13 and 14, respectively. The diporphyrins 1-9 were transformed into zinc-free-base and zinc-ferric hybrid diporphyrins. Photoinduced intramolecular electron transfer in the zinc-ferric hybrid diporphyrins as well as intramolecular excitation energy transfer in the zinc-free-base hybrid diporphyrins have been studied by picosecond time-resolved fluorescence spectroscopy. Rate constants through the polyene-bridges are larger than those through the polyyne-bridges in both reactions. The distance dependence of the electron-transfer rate was found to be quite small: the attenuation factors were determined to be 0.08 and 0.1 Angstrom(-1) for the polyene- and polyyne-bridged diporphyrins, respectively. The distance dependence of the energy-transfer rate was quite similar to that of the electron transfer and was explained better by the Dexter mechanism rather than by the Forster mechanism. These results clearly demonstrate that the linear pi-conjugated polyyne and polyene spacers enhance through-bond electronic coupling between the donor and acceptor.