ELECTRON-TRANSFER IN BIS-PORPHYRIN DONOR-ACCEPTOR COMPOUNDS WITH POLYPHENYLENE SPACERS SHOWS A WEAK DISTANCE DEPENDENCE

A series of phenylene-bridged bis-porphyrin adducts have been synthesized, containing one, two, or three phenyl bridges. Complete synthetic details are provided. For studies of photochemical electron transfer, mixed metals were incorporated, with zinc in one porphyrin macrocycle and Fe(III) (bis-imidazole) in the other macrocycle. When photoexcited, an electron is transferred from Zn to Fe(III). The rate of this process drops only slowly with distance: k-alpha exp(beta-r), with beta = 0.4 angstrom-1. This dependence can be predicted by a simple theory which assumes that the drop does not reflect increased distance, but rather reflects the break in conjugation which occurs at each phenyl juncture due to the biphenyl twist angle of ca. 50-degrees. Inefficient overlap in this angle results in a rate drop of ca. 6-fold per phenyl ring, in good agreement with the observed results.