The electronic factor (Delta) for photoinduced electron transfer (PET) in porphyrin-benzene-(bicyclo[2.2.2]octane)(n)- quinone (n = 0, 1, 2) systems (P-B-Q) is calculated by using the CI-CNDO/S method. An artificial potential is employed in order to bring the systems to the avoided crossing region. The effects of Zn coordination on the porphyrin ring and peripheral substitution are studied. We found that, without Zn, methyl substitution on meso positions diminishes significantly the decay coefficient of Delta with P-Q distance (alpha), P-tolyl substituents, on positions (beta, gamma, delta), have a lower effect on alpha. Zn coordination is effective only in the case of methylated P ring, where alpha is lowered.